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					III. Results

       Section 3.1 of this chapter will present the results of the work conducted to

obtain the final dataset that led to the selection of individual cases for further study.

Section 3.2 will give in-depth, detailed case analyses of the landfall and transition of

Hurricanes Bob, Gloria, and Belle, with an historical perspective resulting from

available data for Hurricanes Connie and Diane in section 3.3.



3.1 Focusing of Tropical Cyclone Dataset from Initial to Final Size

       The results of the initial 41-storm dataset containing landfalling and transitioning

tropical cyclones producing > 100 mm (4 in) of rainfall over the Northeast US during the

time period 1950–2001 are presented in Table I. A modified version of this dataset

(Table II) was produced to take into account the lack of UPD data past 1998. This

dataset weathered the loss of three more recent storms (Floyd, Gordon, and Allison) that

will likely receive increased attention in future investigations of this nature. The final

subset of nine storms containing precipitation distributions that were potentially

influenced by coastal frontogenesis is presented in Table III. The most scrutiny will be

reserved for a small selection of these cases (presented shortly) in an attempt to quantify

the role of synoptic and mesoscale processes on observed precipitation distributions.



3.2 Case Studies

3.2.1 Hurricane Bob

3.2.1.1 Overview




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       Hurricane Bob was the only tropical cyclone to hit the US in 1991 (16–29

August 1991), and the first to strike the northeastern US since Hurricane Gloria in 1985.

The impacts of Bob were quite severe, with 17 reported deaths and two billion dollars of

damage occurring as a result of the storm.

       Bob originated from a large disturbance that slowly intensified to tropical

depression status by 0000 UTC 16 August, while centered approximately 330 km east of

the Bahamas. Over the next couple of days, Bob began to turn toward the north and

northeast, propagating parallel to the mid-Atlantic coast (Fig. 3.1). When the system

was located approximately 165 km east-southeast of Norfolk, Virginia, maximum

sustained intensity as a category 3 hurricane on the Saffir–Simpson scale was achieved.

Bob slowly weakened as it accelerated to the northeast, eventually moving over Block

Island, Rhode Island, and making landfall in Newport, Rhode Island, as a category 2

hurricane. After initial landfall, Bob continued on its northeast path, passing over Rhode

Island and Massachusetts, before eventually making a second landfall as a tropical storm

near Rockland, Maine. Bob then became extratropical as it crossed over the Gulf of St.

Lawrence around 1800 UTC on 20 August and continued recurving over the North

Atlantic (Pasch and Avila 1992).



3.2.1.2 Precipitation Distributions

       Examination of the tropical cyclone track versus total precipitation (Fig. 3.1)

shows the progression of the system parallel to the east coast of the US, with a

pronounced left-of-track shift in the distribution of precipitation as Bob propagates

northeastward through the New England region. Maximum storm total accumulations




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are located in a southwest–northeast oriented band extending from western Long Island,

New York, through central Connecticut and Massachusetts, and northeastward into

southern New Hampshire and extreme western portions of Maine.

       The daily UPD precipitation plots and topography (Fig. 3.2a) for the 24-h period

ending at 1200 UTC 19 August display a broad and rather intense precipitation shield

well in advance of the passage of the center of circulation. Heavy precipitation values

extend northward from Long Island, New York, to southeastern areas of New

Hampshire. Precipitation totals for the next 24-h period (Fig. 3.2b) show the axis of

heaviest precipitation (although noticeably weaker than the previous 24 h) oriented

through central New England, with an extension northward into Maine, closely

following the track of the system. Smaller areas of enhanced precipitation are

distributed throughout the region, including an area southwest of Boston, Massachusetts,

and signatures tied to elevated terrain in northern New Hampshire and western Maine.

       A finer-resolution NWS precipitation analysis (Fig. 3.3) identifies a southwest–

northeast oriented maximum across central New England, which extends to coastal

locations of Maine. Precipitation accumulations approaching 250 mm (10 in) are clearly

defined, and orographic signatures can be identified in northern New Hampshire, near

the vicinity of Mount Washington.



3.2.1.3 Synoptic-Scale Analysis

       Investigation of analyses containing 200 hPa geopotential height, wind speed and

divergence, show that approximately 24 h before initial landfall on the southern New

England coast (1800 UTC 18 August), the upper-tropospheric flow pattern was




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dominated by a positively tilted large–amplitude trough over the upper Great Lakes

region, and a ridge over the western Atlantic ocean (Fig. 3.4a). Coupled upper-level jet

maxima located in southwesterly flow over the Northeast and southern Canada has

produced a broad area of divergence associated with an implied thermally direct

ageostrophic circulation about the equatorward entrance region of the broad jet

maximum. The area containing significant upper-level divergence, which is conducive

to the most vigorous upward motion, is located in the equatorward entrance region of the

southernmost jet streak, immediately north of the center of the tropical cyclone and

impinging upon portions of southern New England.

          Progression of time by 12 h (Fig. 3.4b) reveals marked changes in the overall

structural characteristics of the environment at this level. The positively tilted trough

located over the upper Great Lakes has sharpened and propagated slightly

southeastward, while the western Atlantic ridge has amplified due to implied diabatically

induced upper-level outflow from convection associated with the tropical cyclone. The

resultant effect of these two changes has caused an increase in the magnitude of the

geopotential height gradient over the northeast, and produced a single, focused, jet streak

with maximum wind speeds on the order of 60 m s-1. The increase in the maximum

speed of the jet streak is accompanied by a more vigorous implied thermally direct

ageostrophic circulation, identified as an increased divergence maximum in the

equatorward entrance region of the jet, near the New Jersey shoreline. The next two

consecutive time periods (Figs. 3.4c,d) show a rotation of the trough to a more neutral

tilt and detachment of the southern portion of the trough over the Ohio Valley, while

Atlantic ridging continues to occur. A split flow structure between the main flow




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located to the north and the detaching trough to the south, teamed with Atlantic ridging,

has resulted in a strong confluence region and geopotential height gradient over southern

Canada. Jet streak intensity has increased to 65 m s-1 on 1200 UTC 19 August and

undergoes slight weakening to 60 m s-1 by 1800 UTC. Propagation of the jet to the

northeast is a slow and gradual process, resulting in the stagnation of a vigorous implied

thermally direct ageostrophic circulation over areas experiencing maximum precipitation

in southern New England.

       An analysis of geopotential height and absolute vorticity at the 500 hPa level for

0600 UTC 19 August (Fig. 3.5a) shows a rather potent trough digging into the Great

Lakes region in association with the fracturing of the southern branch of a northward–

displaced trough. The mid-tropospheric reflection of the tropical cyclone is evident

along the Mid-Atlantic coast, manifested as a negatively tilted orientation of the

geopotential height field along the periphery of a western Atlantic ridge. A rather

pronounced area of confluence is evident over the Northeast in association with the

intersection of flows created by the Great Lakes trough, the tropical cyclone and the

Atlantic ridge.

       Progression through the next two time periods (1200 and 1800 UTC 19 August;

Figs. 3.5b,c) shows the tropical cyclone propagating northward into the northeastern

region, pivoting around the eastern periphery of the southern portion of a trough that has

been cutoff from the main flow to the north. Major interaction does not occur with the

trough located over the Ohio Valley; instead, the tropical cyclone becomes attached to

the eastward propagating positively tilted trough in Canada (identified by interaction

between vorticity centers). The western Atlantic ridge experiences continued




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reinforcement, acting to efficiently increase the magnitude of the geopotential height

gradient between itself and the tropical cyclone, thereby inducing an enhanced southerly

flow. The confluence region created by the intersection of flows continually shifts with

the movement of the tropical cyclone, and can be identified as it propagates into

southern Canada. Cyclonic vorticity advection over the Northeast increases with the

approach of Bob to the coast, and becomes increasingly important to induced vertical

motions as interaction with the mid-latitude trough occurs in northern New England and

southern Canada (Fig. 3.5c).

       Examination of 850 hPa geopotential height, potential temperature, and wind,

shows a remnant baroclinic zone located in southern Canada (Fig. 3.6a) associated with

a recently departed low-pressure system (not shown) off the east coast of Newfoundland.

The juxtaposition of the building Atlantic ridge and the tropical cyclone along the

eastern seaboard has increased the geopotential height gradient, producing

intensification in the southerly flow (over southern New England) at low levels to ~15

m s-1. The increased southerly flow has not produced significant WAA due to a lack of

interaction with the baroclinic zone thus far.

       Evolution through the next 18 h (Figs. 3.6b–d) shows a continued increase in the

geopotential height gradient between the Atlantic ridge and the mobile tropical cyclone,

producing further intensification of the low-level flow to ~ 30 m s-1 over southern New

England. The enhanced flow has acted to rotate the baroclinic zone cyclonically, while

intensification of the frontal baroclinic zone has occurred across northern New England

as a result of the flux of warm air on the east side of the tropical cyclone. Warm-air




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advection has also increased slightly, and can be observed in the northeastern quadrant

of the cyclone as the low-level jet supplies warm air to eastern areas of New England.

       Examination of 925 hPa geopotential heights, equivalent potential temperature,

and wind for 0600 UTC 19 August (Fig. 3.7a) reveals an enhanced geopotential height

gradient between the ridge and the tropical cyclone, resulting in the intensification of the

low-level southeasterly jet to ~20 m s-1. The advection of high θe (warm and moist) air

by the southeasterly wind has resulted in the introduction of values in excess of 355 K

along the Mid-Atlantic coast, with somewhat lower (but still relatively high) values

encroaching on the New England region well in advance of the tropical cyclone. This

southeasterly flow provides a continuous supply of warm, moist air to the vertical branch

of the implied thermally direct circulation, thereby establishing conditions conducive for

heavy precipitation. A gradual strengthening and reorientation of the core winds

associated with the low-level jet occurs over the next 18 h (Figs. 3.7 b–d). The

positioning of the jet deviates from the east/northeast quadrant to the southeast quadrant

of the cyclone based upon the fact that the strength of the ridge is not sufficient to

maintain the orientation of the geopotential height gradient (Figs. 3.7b,c). Enhancement

of easterly wind speeds to ~20 m s-1 around the north side of the storm are encroaching

on the southern New England region, but lack the intensity observed further offshore.

The modified offshore low-level jet possesses speeds in excess of 30 m s-1 from a

southerly/southwesterly direction, resulting in the advection of highest θe values (350+

K) across the southeastern areas of New England. Even though maximum θe values

have been transported southeast of New England, an appreciable amount of southerly

flow has resulted in the penetration of warm, moist tropical air in excess of 345 K




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through central portions of the region. Figure 3.7d illustrates the reconfiguration of the

strongest geopotential height gradient to a southwest–northeast orientation, and the

resultant effect of the low-level flow in efficiently advecting the main axis of warm,

moist air out to sea during the latter stages of the evolution.



3.2.1.4 Surface Analysis

       Approximately 2 h before the landfall of Bob, the center of circulation was

located just south of the western tip of Long Island, New York (Fig. 3.1). Surface plots

for this time (Fig. 3.8) show a northerly flow of ~10–13 m s-1 along the eastern slopes of

the Berkshire Mountains in Massachusetts, continuing south through central Connecticut

along a streamline bound for the center of circulation. The intersection of enhanced

easterly flow on the order of 15 m s-1 and the cyclone–induced northerly flow has

produced a weak convergence zone (i.e., a coastal front) in eastern Connecticut, western

portions of Rhode Island, and southern Massachusetts. Temperature and dewpoint

gradients across the frontal zone are rather subtle, and on the order of 3–4oC/100 km and

2oC/100 km, respectively. Inferences pertaining to the mesoscale frontal zone are

subjective in nature due to the limitations imposed by a critical hole of surface data in

the eastern Connecticut region. Inspection of an interpolated analysis containing

contours of θ and shaded θ gradients, as well as contours of θe and shaded θe gradients

for this time (Figs. 3.9a,b) shows that weakly enhanced θ and θe gradients (albeit weaker

than less reliable adjacent oceanic calculations) have developed in the aforementioned

region while in the presence of a convergent wind.




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       Progression of time by 1 h (Fig. 3.10) shows that the cyclone induced northerly

flow has remained unchanged, while the enhanced easterly flow has intensified slightly

to 18–20 m s-1. The position of the coastal front has shifted slightly eastward following

the propagation of the system to a location near Block Island, Rhode Island, and is

weakly apparent in analyses of θ and θe gradients in eastern Connecticut and Rhode

Island due to the small-scale and subtle nature of the feature (Figs. 3.11a,b). Weakening

of the feature continues into the next hourly time period (1800 UTC 19 August), and is

identified as progressively weaker θ and θe gradients (Figs. 3.12a,b). The analysis

procedure used to resolve the thermal gradient is objective and should be interpreted

with caution due to inaccuracies arising from smoothing processes. Surface

observations for 1800 UTC show a northward movement of the coastal front from

central Rhode Island, to north of Boston, Massachusetts, as Bob makes landfall in

southern Rhode Island (Fig. 3.13). The thermal boundary possesses temperature and

dewpoint gradients on the order of 2–3oC/50 km and is collocated with an area of

enhanced precipitation southwest of Boston, Massachusetts. Northeastward propagation

of the cyclone during subsequent hours produces a brief cyclonic rotation and rapid

dissipation of the very weak coastal front (not shown).

       Previously mentioned in the presentation of precipitation analyses was the

identification of what appeared to be significant orographic modification in northern

New Hampshire and western Maine. Unfortunately, a lack of surface data in northern

portions of New England inhibits the analysis of conditions important to possible

orographic enhancement observed in the area.




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3.2.2 Hurricane Gloria

3.2.2.1 Overview

       Hurricane Gloria (16 September–2 October 1985) developed from an African

easterly wave, reaching tropical depression status near the Cape Verde Islands on 16

September. Hurricane status was achieved on 22 September approximately 400 km east

of the Lesser Antilles. Gloria then made a slow northward turn and passed northeast of

the Lesser Antilles and eventually north of the Bahamas, in route to the mid-Atlantic

region of the United States. A minimum central pressure of 919 hPa was reached

approximately 1500 km southeast of Cape Hatteras, North Carolina, at 0120 UTC on 25

September. The storm then passed over the Outer Banks of North Carolina and

weakened while rapidly accelerating parallel to the eastern seaboard (Fig. 3.14),

eventually making landfall on Long Island, New York. Eight deaths and total damage of

$900 million along the east coast of the US were the end results when Gloria finally lost

tropical characteristics as it propagated into the Maine region (Case 1986).



3.2.2.2 Precipitation Distributions

       A plot of storm track and total precipitation (Fig. 3.14) shows the progression of

Gloria parallel to the east coast of the US, passing over the Outer Banks of North

Carolina in transit to landfall on Long Island, New York. Maximum precipitation

accumulations are highlighted in a relatively narrow band that extends from northeastern

North Carolina to the Catskill region of New York State. The distribution of

precipitation possesses a preferential left-of-track shift when compared to the track of

the system.




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       Examination of a plot containing 24-h precipitation accumulations and

topography for 26 September (1200–1200 UTC) (Fig. 3.15a) shows the northern fringes

of the precipitation shield working into coastal locations of the Mid-Atlantic US. The

vast majority of the rainfall during the event occurs over the next 24-h period (ending

1200 UTC 27 September) when significant amounts fall in areas of eastern Virginia,

stretching northward into eastern Pennsylvania and south central portions of New York

(Fig. 3.15b). The most extreme rainfall amounts are focused in an area from

southeastern Virginia north into Maryland, Delaware, eastern Pennsylvania, and western

New Jersey. Lighter precipitation amounts exhibiting embedded areas of possible

orographic enhancement were observed as Gloria quickly accelerated and raced through

the northeast, exiting in northern Maine (Fig. 3.15c). Precipitation signatures indicative

of orographic modulation are evident along the eastern slopes of the Catskill and

Berkshire Mountains, as well as the mountainous regions of northern New Hampshire.

       A finer-resolution NWS precipitation analysis for storm total accumulations (Fig.

3.16) shows the heaviest values occurring in a band that extends from eastern Virginia,

northward through eastern areas of Pennsylvania into the southern tier of New York.

Maximum rainfall on the order of 150–250 mm is embedded within the larger-scale

band, in areas west of the New Jersey border, stretching south through Maryland and

Virginia. Signatures of precipitation enhancement due to orographic processes are

consistent with UPD analyses discussed previously.



3.2.2.3 Synoptic-Scale Analysis




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       Inspection of 200 hPa geopotential height, wind speed and divergence for 1800

UTC 26 September (Fig. 3.17a) shows a significantly amplified geopotential height

pattern, possessing a large, broad trough that encompasses the entire northern tier of the

US, and a strong ridge over the western Atlantic Ocean. A positively tilted short-wave

feature propagating through the base of the trough has acted to enhance the geopotential

height gradient between itself and the ridge to the east, thus resulting in a significant and

expansive jet streak centered near Lakes Erie and Ontario that is on the order of 60 m s-1.

The combination of upper-level effects from an implied thermally direct ageostrophic

circulation in the equatorward entrance region of the jet and the tropical cyclone has

produced a broad area of upper-level divergence along the eastern seaboard, stretching

from the Carolina region to southern areas of New England.

       Progression through the next 18 h (Figs. 3.17b–d) shows ridging over the western

Atlantic downstream of a rather substantial trough to the northwest, resulting in a

significant increase of the geopotential height gradient, and an intensification and

reconfiguration of the jet maximum to 75 m s-1 over eastern Canada. The ridging

occurring over the western Atlantic most likely can be attributed to diabatically induced

upper-level outflow associated with convection from Gloria. The rapid increase in jet

intensity and the highly amplified nature of the flow have produced a vigorous

divergence maximum that slowly propagates northward along the eastern seaboard and

through northern New England, resulting in conditions conducive to upward motion over

the region of heaviest precipitation for an extended period of time.

         An analysis of 500 hPa geopotential height and absolute vorticity for 0000 UTC

27 September (Fig. 3.18a) shows a positively tilted double vorticity short-wave center




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propagating through the large-scale flow near the western Great Lakes, stretching south

to eastern portions of the Midwest, while a strong ridge is evident over the Atlantic

Ocean. Nestled between these two mid-latitude features is the mid-tropospheric

reflection of Gloria (located near coastal areas of the Carolinas), which is causing minor

distortion of the geopotential height field (manifested as an eastward bulge) along the

western periphery of the ridge. Six hours later (Fig. 3.18b), increased north Atlantic

ridge building is observed, manifested as a northwestward bulge of the high over coastal

locations of Maine. The northward vorticity center associated with the Great Lakes

short wave is showing signs of being sheared by the increased southwesterly flow over

the ridge. Cyclonic vorticity advection associated with the northward movement of the

tropical cyclone is impinging upon regions of the mid-Atlantic, providing support for

ascent and precipitation.

       Interaction of the tropical cyclone with the approaching trough is evident as the

short wave over the Ohio Valley has become negatively tilted, and the vorticity fields

have merged (Fig. 3.18c). Cyclonic vorticity advection has continued to press

northward, promoted by the enhanced flow generated between the cyclone and the

building ridge. Six hours later (1800 UTC), the northern portion of the Ohio Valley

trough has been sheared off to the northeast over the continually building ridge (Fig.

3.18d). A minor eastward bulge on the western side of the high, a northward spreading

of CVA, and a less recognizable vorticity center associated with Gloria are evident as the

system quickly propagates northward into northern New England.

       A signature of the intense ridge over the western Atlantic Ocean is the most

noteworthy feature in the analysis of geopotential height, potential temperature, and




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wind at 850 hPa for 0000 UTC 27 September (Fig. 3.19a). An enhanced geopotential

height gradient between the ridge and the tropical cyclone has produced flow on the

order of 25 m s-1 on the eastern side of the cyclone, with 20 m s-1 easterly flow wrapping

around the northern side of the system and encroaching on coastal locations of the Mid-

Atlantic. A southwest–northeast oriented baroclinic zone associated with the trough

over the Great Lakes and eastern regions of the Midwest can be identified from the

southeastern US into southeastern sections of Canada. This baroclinic zone is a distinct

entity that exhibits very little interaction with the approaching tropical cyclone.

       A continued increase in the geopotential height gradient between the Atlantic

ridge and the cyclone has produced an enhanced southeasterly low-level jet on the order

of 30 m s-1 (Figs. 3.19b–d). The nose of this jet is wrapped around the northern side of

the cyclone during passage along the Carolinas (Fig. 3.19b), producing a significant

increase in easterly flow along the Mid-Atlantic coast. Interaction of the tropical

cyclone with the approaching baroclinic zone does not fully occur until 1200 UTC 27

September (Fig. 3.19c) when the enhanced southeasterly flow intersects the temperature

gradient, causing WAA in extreme northern portions of New England. Gloria continues

its northward propagation and shifts the main axis of enhanced southerly flow to the east

side of the cyclone, penetrating into southern Canada (Fig. 3.19d). The baroclinic zone

has been deformed into an “S” shape due to the advective effects of the cyclonic

circulation; however, any WAA is well displaced into Canada and out of the domain

considered for this case study.

       Examination of the 925 hPa geopotential height, equivalent potential

temperature, and wind reveal similar flow characteristics previously described at the 850




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hPa level. Figure 3.20a shows a readily identifiable ridge and enhanced geopotential

height gradient between the ridge and the cyclone along the east coast of the US. The

rather pronounced low-level jet possesses wind speeds on the order of 28 m s-1, with 20

m s-1 flow on the north side of the cyclone impinging upon Mid-Atlantic coastal

locations. The core of highest θe air (~355 K) is for the most part symmetrically

distributed in accordance with the geopotential height contours of the cyclone; however,

advection of warm, moist air of tropical origin (~350 K) is beginning to occur on the

north side of the cyclone near coastal locations experiencing a maximum in easterly

onshore flow.

        Maintenance of the enhanced easterly flow is achieved as northward propagation

into Maryland and New Jersey occurs (Fig. 3.20b). A tongue of high θe air has

propagated northward over the Northeast in advance of the system, and possesses 340 K

air, while even higher values (approaching 355 K) are being advected into eastern

Virginia and Maryland. The reanalysis wind field produces a signature of possible

coastal frontogenesis near the Chesapeake Bay area associated with the intersection of

southeasterly onshore flow and inland northeasterly flow (investigated in more detail

later in this chapter).

        The enhanced southeasterly flow approaching 20 m s-1 continues to advect the

tongue of warm, moist air farther northward throughout the Northeast in advance of the

passage of the tropical cyclone (Fig. 3.20c). Immediately north of the center of

circulation, in eastern Pennsylvania, the wind field continues to indicate a possible

coastal front, manifested as the intersection of 10 m s-1 northerly inland flow and 10

m s-1 easterly onshore flow. The wind shift is positioned in an area possessing a




                                            67
significant θe gradient, resulting from the intersection of westward–advected warm,

moist air, and a midlatitude air mass associated with the upper-tropospheric trough. The

progression of Gloria into interior sections of the Northeast results in the continued

northward penetration of high θe air by the southeasterly flow, and a more circular inland

flow that does not indicate the presence of a coastal front signature (Fig. 3.20d).



3.2.2.4 Surface Analysis

       A surface plot for 0600 UTC 27 September roughly places the center of

circulation near the North Carolina coast (Fig. 3.21). A cyclone-induced northerly

inland flow exists along eastern areas of Pennsylvania, central Maryland and eastern

Virginia. A broad area of easterly onshore flow around the north side of Gloria extends

along coastal and immediate inland locations, stretching from Virginia to New Jersey. A

convergence zone formed in situ by the intersection of these two flows extends from the

Chesapeake Bay area northward to the border of New Jersey and Pennsylvania.

Temperature and dewpoint gradients across the coastal front are approximately 4oC/80

km and 6oC/80 km, respectively, within the most intense section located near

Chesapeake Bay. A slightly weaker gradient exists across northern regions of the

boundary, where gradients of temperature and dewpoint across western New Jersey and

eastern Pennsylvania are on the order of 3–4oC/100 km and 3–4oC/100 km, respectively.

An analysis of θ and θe gradients for the same time (Fig. 3.22) further verifies the

location of the coastal front, identifying enhanced θ gradients of 2–3 x 10-4 K (100 km)-1

and θe gradients of 10–12 x 10-4 K (100 km)-1 under the influence of convergent wind




                                             68
flow in the regions previously discussed, and collocated with the band of heaviest

precipitation.

       Three hours later (0900 UTC), a boundary extending northward through the

center of Maryland and into eastern Pennsylvania is sustained by the intersection of a 15

m s-1 northerly inland flow and a 10–15 m s-1 easterly onshore flow (Fig. 3.23).

Temperature and dewpoint gradients possess similar magnitudes as those discussed

earlier, with θ and θe gradients (Fig. 3.24) identifying the position of the north–south

oriented coastal front.

       The convergent coastal front boundary slowly encompasses less areal coverage

and steadily intensifies as Gloria moves north along the east coast (Figs. 3.25 and 3.26).

Overall, the baroclinicity intensifies due to the interaction of a midlatitude baroclinic

zone with the baroclinicity produced by the tropical cyclone, amounting to θ and θe

gradients of 3–4 x 10-4 K (100 km)-1 and 12–14 x 10-4 K (100 km)-1 respectively (Figs.

3.27 and 3.28). The enhanced baroclinic zone is occurring in the presence of a relatively

convergent wind flow that can be initially identified from northern Maryland through

eastern Pennsylvania (Fig. 3.25), and is experiencing a cyclonic rotation to a position in

northern New Jersey and northeastern Pennsylvania (Fig. 3.26). Directional wind shear

slowly ceases over the next 2 h, resulting in significant weakening and dissipation of the

coastal front feature near the Pocono Mountains (not shown).

       Investigation of surface observations during the passage of the cyclone through

the northeast US shows that easterly flow was maintained along the north side of the

tropical cyclone during much of the translational period. This easterly onshore flow

intersected the windward slopes of topographic barriers in southern New York (Fig.




                                             69
3.25), Massachusetts, Vermont and New Hampshire (not shown), producing areas of

orographic precipitation modification.



3.2.3 Hurricane Belle

3.2.3.1 Overview

       Hurricane Belle (6–10 August 1976) developed from an easterly wave

propagating off of the African continent on 28 July. Belle attained tropical depression

status on 6 August approximately 450 km east-northeast of the Bahamas, and proceeded

to strengthen and quickly move northward parallel to the east coast of the United States

(Fig. 3.29). Belle reached hurricane status during the late afternoon of 7 August,

achieving a minimum central pressure 24 h later of approximately 958 hPa. Landfall

occurred in the vicinity of Jones Beach, New York, at 0500 UTC 10 August, as a much

weaker system. Belle then continued propagating to the north-northeast through central

New England before succumbing to dissipative forces and losing its tropical

characteristics over the New Hampshire and Maine region. Belle was the deadliest and

costliest Atlantic hurricane to strike the mainland US in 1976 (Lawrence 1977).



3.2.3.2 Precipitation Distributions

       The examination of an analysis displaying tropical cyclone track versus storm

total precipitation (Fig. 3.29) shows the axis of heaviest precipitation located in a narrow

band exhibiting a preferential left-of-track shift as Belle progresses northward. The

linear band of precipitation stretches from coastal locations of Maryland and Delaware




                                            70
north into central locations of New England, with two localized precipitation maxima

observed in southeastern Massachusetts and northern Maine.

         Dissection of the total precipitation accumulations into separate daily analyses

shows a plume of precipitation along the Mid-Atlantic coast extending northeastward

into southern New England (Fig. 3.30a), well in advance of the northward–propagating

center of circulation located east of the Outer Banks of North Carolina (Fig. 3.29).

Maximum accumulations are observed during the following 24 h (Fig. 3.30b) as Belle

penetrates inland, producing a relatively linear and intense area of rainfall from eastern

New Jersey north along the eastern slopes of the Berkshire and Green Mountains.

Embedded within the broad precipitation shield that encompasses a significant portion of

the Northeast are both large and small regions of heavier rainfall that are evident in

central and extreme northern areas of Maine, and near Mt. Washington, New Hampshire.

Lighter, residual rainfall accumulations associated with the northward movement of the

system into Canada dominate the last 24-h period across the extreme northern New

England region (Fig. 3.30c).

       The NWS precipitation analysis shows a narrow and intense linear band of

precipitation extending from southern Maryland northward into central Vermont (Fig.

3.31). Accumulations throughout this broad area of precipitation are on the order of 75–

100 mm, while smaller regions embedded within the larger precipitation shield possess

amounts that approach 100–180 mm. An area of heavy precipitation detached from the

main axis of precipitation is observed south of Boston, Massachusetts, and in northern

Maine, consistent with UPD analyses discussed previously.




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3.2.3.3 Synoptic-Scale Analysis

       Inspection of 200 hPa geopotential height, wind speed and divergence for 0600

UTC 9 August (Fig. 3.32a) shows a split-flow structure in the geopotential height field

exhibiting an amplified southern branch of the flow across the US, and a more zonal

westerly flow across Canada. A weak trough is evident over the Great Lakes region

stretching south into the southeastern states, with a ridge visible across the western

Atlantic Ocean. A confluent region formed by the intersection of southwesterly flow

over the ridge and westerly flow across Canada has produced a pronounced jet

maximum of approximately 50 m s-1 over the Newfoundland area, stretching

southwestward into central New England. A broad area of divergence and vigorous

upward motion is located in the New England region, and is likely associated with a

thermally direct ageostrophic circulation about the equatorward entrance region of the jet

maximum.

       Six hours later (1200 UTC), minor building of the western Atlantic ridge has

resulted in a slight tightening of the geopotential height gradient between the ridge and

the trough located over the Great Lakes region (Fig. 3.32b). The overall structure and

intensity of the jet maximum has been relatively unchanged by the subtle geopotential

height field adjustment; however, a broader and less focused area of upper-level

divergence produced by the introduction of tropical cyclone induced outflow has become

evident along the east coast. Examination of the following 12 h (Figs. 3.32c,d) shows an

increase in western Atlantic ridging, with a slight westward retrogression of the ridge,

and a northeastward lifting of the northern portion of the Great Lakes trough. The

overall intensity of the jet maximum does not change appreciably; however, a




                                             72
reconfiguration of the jet core to the southwest in association with the tightening of the

geopotential height gradient between the trough and the ridge has placed the maximum

upper-level divergence in a band oriented from Maryland, north through central

Vermont, near the area of maximum precipitation.

       Examination of the 500 hPa geopotential heights and absolute vorticity

approximately 24 h before landfall (0600 UTC 9 August; Fig. 3.33a) confirms the split-

flow structure containing a weakly amplified southern branch of flow across the US,

which is detached from the main flow in Canada. A weak trough located over the Great

Lakes is identified by a maximum of vorticity, while the beginning stages of interaction

between the tropical cyclone and the vorticity maximum are evident across the Carolina

region. The western Atlantic ridge is firmly entrenched at this time, exhibiting an

enhanced geopotential height gradient between itself and the trough, thereby intensifying

the southerly flow along the eastern seaboard.

       Continued northward movement of the tropical cyclone along the east coast

produces a progressive increase in the degree of interaction with the isolated short-wave

trough slowly lifting to the north across southern Canada (Figs. 3.33b–d). The ridge

located over the western Atlantic Ocean shows signs of amplification and retrogression

westward as time progresses. An eastward bulge in the geopotential height field

produced by the northward propagation of the tropical cyclone along the western

periphery of the ridge is weak due to the magnitude of the ridge; thereby maintaining a

north–south orientation of the region of increasing geopotential height gradient along the

east coast. The intensified geopotential height gradient results in the enhancement of




                                            73
southerly flow along the east coast, subsequently producing an increase of CVA into the

southern New England region.

       An analysis of 925 hPa geopotential height, equivalent potential temperature, and

wind for 0600 UTC 9 August (Fig. 3.34a) shows the reflection of the tropical cyclone

located east of the Carolina region. An enhanced geopotential height gradient between

the ridge over the western Atlantic and the tropical cyclone is already evident at this

time, accompanied by enhanced southeasterly flow on the order of 15 m s-1 some

distance offshore. A core of warm, moist high θe air (approaching 350 K) is located

within the area of maximum low-level flow in the northeastern quadrant of Belle, while

the orientation of the ridge adjacent to the tropical cyclone has resulted in a persistent

southerly flow that is acting to transport a tongue of higher θe air (~335 K) into eastern

portions of New England.

        As the tropical cyclone continues to move northward along the coast (Fig.

3.34b), the building ridge remains firmly in place and does not become extremely

deformed in the presence of the circulation. Consequently, a further increase in the

magnitude of the north-south oriented region of geopotential height gradient occurs,

resulting in an increase of the low-level jet to 18 m s-1. The highest θe air is located in

the northeastern quadrant of Belle and is close to impinging upon areas of southern New

England, while weaker transport of tropical air induced by the southerly flow between

the two systems has occurred well in advance of the center of circulation, extending into

northern portions of Maine.

       Twelve hours prior to the landfall of Belle, the geopotential height gradient

between the western Atlantic ridge and the tropical cyclone continues to strengthen,




                                             74
resulting in a southeasterly low-level jet on the order of 20–23 m s-1 (Fig. 3.34c). The

location of the most pronounced low-level flow is displaced slightly offshore, with the

leading edge of the jet wrapping around the northeastern side of Belle and advecting

warm, moist air into central and eastern regions of the northeast US. Closer to landfall,

the orientation of the low-level jet becomes less southeasterly and proceeds to become

more southerly as a slight weakening of the high over the western Atlantic Ocean is

observed (Figs. 3.34d,e). This subtle reorientation of the low-level jet advects the main

axis of tropical air across eastern portions of New England; however, an appreciable

amount of flow exhibiting an easterly component on the north side of the system is able

to maintain a significant flux (although less pronounced than to the east) into interior

portions of New England.



3.2.3.4 Surface Analysis

       Inspection of a manual surface analysis close to the time of landfall on Long

Island, New York, reveals a weak thermal boundary (~3oC/100 km) in the presence of

convergent flow along a north–south line extending through eastern Connecticut (Fig.

3.35), coincident with an area of observed heavy precipitation. The subtle coastal front

is a very short-lived feature that is not well represented in analyses containing θ and θe

gradients due to a critically low number of observations in the area, and a close

proximity to questionable interpolated analyses over the open ocean (not shown).

       A surface plot immediately after the initial penetration of Belle into interior

sections of the Northeast (Fig. 3.36) shows that the air mass encompassing the area has

become rather homogeneous, with little evidence remaining of the subtle coastal front




                                             75
boundary in northwestern Connecticut. The positioning of the tropical cyclone center is

such that an enhanced easterly flow around the northern periphery of the cyclonic

circulation is beginning to impinge upon the southern extension of the Berkshires

located in extreme northern areas of Connecticut. Intersection of the low-level flow with

the topographic barrier farther north is inferred from multiple reports of due easterly

surface winds in Massachusetts and Vermont. Movement of the cyclonic circulation into

central Massachusetts by 1100 UTC 10 August (Fig. 3.37) simultaneously repositions

the easterly flow on the north side of the system to southern areas of New Hampshire

and Vermont, resulting in flow mechanically forced upward along the eastern slopes of

the Green Mountains. A signature of a weak inland coastal front along the Vermont and

New Hampshire border is manifested as the intersection of northwesterly wind over

Vermont and southeasterly wind over New Hampshire; however, the presence of the

coastal front cannot be further verified due to a lack of data in the region (the coastal

front is not analyzed on Figs. 3.37 and 3.38). The observed easterly wind flow continues

to produce upsloping along the windward facing slopes of the high terrain of the

Northeast as Belle continues to move in a northward direction through northern New

England (Fig. 3.38).



3.3 The Events Surrounding the Passage of Hurricanes Connie and Diane

3.3.1 Overview

       Within a nine-day period (12–20 August 1955), Hurricanes Connie and Diane

made landfall in the Mid-Atlantic region of the US. Poleward propagation of the two

tropical systems produced copious amounts of precipitation throughout the region. The




                                             76
subsequent collocation of heavy precipitation associated with the two events combined

to produce major flooding in the southern New England region. Nearly five billion

dollars of property damage, 180 deaths and 200 partial or total dam failures were

reported.



3.3.2 Hurricane Connie: 12–14 August 1955

       Hurricane Connie made landfall in Cape Lookout, North Carolina, on the

morning of 12 August 1955. Subsequent to landfall, Connie slowly propagated

northwestward. Precipitation fell along both sides of the storm track (Fig. 3.39), with

the heaviest accumulations in excess of 250 mm located in a narrow band extending

from the Mid-Atlantic region northward into western New England (Fig. 3.40).

       The 500 hPa geopotential heights and absolute vorticity analyses (Figs. 3.41a,b)

show a northward-displaced flow passing through southern portions of Canada with a

short-wave feature lifting northeastward over a ridge anchored over the western Atlantic

Ocean. Minimal interaction between the tropical cyclone and the short-wave trough is

evident from the geopotential height and vorticity fields, resulting in relative isolation of

Connie from the main westerly flow. Parallel positioning of the geopotential height and

absolute vorticity contours along the Mid-Atlantic States suggests that CVA is very

weak to nonexistent throughout the event.

       Upper-level western Atlantic ridging has led to delayed propagation of an

intensifying jet core to the north and east, allowing the maximum upper-level divergence

associated with the equatorward entrance region of the jet to stagnate over the areas of

heaviest precipitation in the Mid-Atlantic and southern New England regions for an




                                             77
extended period of time (Figs. 3.42a,b). Examination of the lower levels shows that at

850 hPa (Figs. 3.43a,b) the main baroclinicity is displaced well to the north in Canada.

An increase in the geopotential height gradient between the western Atlantic ridge and

the tropical system has resulted in an increase of the low-level southeasterly jet to ~20

m s-1 over the Mid-Atlantic and southern New England regions. The 925 hPa analysis

(Figs. 3.44a,b) reinforces the observation that an intensification of the geopotential

height gradient has occurred. In response to the increase of the geopotential height

gradient, both the speed of the low-level southeasterly jet and the advection of high θe air

into the Mid-Atlantic and southern New England region have increased, efficiently

feeding the lower branch of the inferred thermally direct ageostrophic circulation

associated with the upper-level jet. Low-level wind analyses (not shown) suggest the

occurrence of coastal frontogenesis along an area stretching from eastern Maryland to

Pennsylvania, which will be investigated in more detail shortly.

       A surface analysis for 0330 UTC 13 August (Fig. 3.45a) shows Connie centered

in the vicinity of extreme southeastern Virginia. An intense baroclinic zone draping

southwest to northeast across central Maryland and southeastern Pennsylvania is

collocated with convergent flow, producing an area of coastal frontogenesis and

enhanced precipitation over the northwestern shores of the Chesapeake Bay (refer back

to Fig. 3.40 as necessary for coastal front locations). By 0630 UTC (Fig. 3.45b) the

baroclinicity and convergent flow have continued to propagate northeastward pushing

into extreme southeastern Pennsylvania. The coastal front continues its northeastward

movement through 0930 UTC (Fig. 3.45c), until it shows signs of weakening across

east-central Pennsylvania by 1230 UTC (Fig. 3.45d). The weakening can be attributed




                                             78
to a slight rotation of the baroclinic zone, such that it has become oriented quasi-parallel

to the wind flow. A shift in the wind direction from northeasterly to easterly between

0930 UTC and 1230 UTC occurs in southern New England, resulting in flow

perpendicular to the Berkshire and Green Mountain ranges.

       Figure 3.40 contains a plot of fine-resolution precipitation accumulations and

approximate coastal front locations analyzed every three hours. A general swath of

extremely heavy precipitation extends from the Chesapeake Bay area northeastward into

central New England. The movement of the coastal front can be seen to coincide with

the axis of heaviest precipitation. Daily precipitation analyses for the 24 h period (1200–

1200 UTC) ending on 12 and 13 August (not shown) indicate that the heavy total

accumulations located in eastern areas of Virginia were not due entirely to the coastal

front, but were partially induced by the approach of the tropical cyclone to coastal

locations of the Mid-Atlantic.



3.3.3 Hurricane Diane: 17–20 August 1955

       Hurricane Diane made landfall on the morning of 17 August 1955 in Carolina

Beach, North Carolina. Initial precipitation distributions (Fig. 3.46) position the greatest

accumulations to the right of track, occurring through the central Virginia area. Slow

northward post-landfall movement is evident, with a preference for a slow right turn and

eventual acceleration to the northeast. The precipitation shield exhibits a shift to a left-

of-track orientation as the right turn takes place, leaving southern New England to bear

the brunt of the heaviest precipitation, which accumulated to 400–450 mm (Fig. 3.47).




                                             79
       A large negatively tilted trough near the center of Hudson Bay at 500 hPa (Figs.

3.48a,b) exhibits a short-wave feature propagating through the extreme southeastern

extent of the trough, while a ridge is positioned over the western Atlantic Ocean. A

marked influence on the propagation of Diane can be inferred from the large-scale flow,

manifested by the increased translational speed and the change from northward to

northeastward movement. Interaction between Diane and the short wave is revealed by

the partial merger of the respective vorticity fields surrounding the two features, with the

short wave acting as the catalyst for the right turn and movement of Diane parallel to the

southeastern New England coast.

       Upper-level amplification of the western Atlantic ridge has acted to intensify and

reconfigure a jet over Newfoundland into a more potent and compressed jet core on the

order of 45 m s-1 (Figs. 3.49a,b). Maximum upper-level divergence associated with the

equatorward-entrance region of the developing jet has positioned itself in a broad area

that is collocated with the belt of heavy precipitation over southern New England.

       Examination of the lower levels shows that at 850 hPa (not shown) the main axis

of baroclinicity is still contained well to the north in association with the trough over

Hudson Bay. The tropical system maintains a cutoff signature in the geopotential height

field along the Mid-Atlantic, albeit weak, and a brief intensification in the

south/southwesterly low-level jet occurs over southern New England. The 925 hPa

analysis (Figs. 3.50a,b) reveals an increase in the geopotential height gradient between

the reinforced Atlantic ridge and Diane, producing an acceleration of the low-level

southerly/southwesterly jet to ~18–20 m s-1 and a significant increase in the transport of




                                             80
very warm and moist high θe air into the southern New England region (approaching 360

K).

       A surface analysis for 0630 UTC 19 August (Fig. 3.51a) shows Diane centered in

southeastern Pennsylvania. A slight compression of the isotherms exists over southern

New England and is collocated with convergent flow in extreme southern portions of the

region to form a coastal front. The coastal front is formed and maintained by the

intersection of tropical-cyclone induced northeasterly inland flow and southerly onshore

flow. The coastal front remains relatively stationary over the next 3 h (Fig. 3.51b). A

slight progression of the front to the north and east occurs by 1230 UTC as Diane passes

over Long Island, New York (Fig. 3.51c), while the precipitation at this time is driven

predominantly by the collocation of and interaction between convergent flow and a

small-scale baroclinic zone. At 1530 UTC (Fig. 3.51d) Diane has moved toward the

eastern tip of Long Island, New York, pushing the coastal front even farther to the north

and east, impinging upon the Boston, Massachusetts, area. Toward the end of the

analysis period, the temperature gradient coinciding with the coastal front has weakened

to ~3oC/100 km, resulting in the slow dissipation of the front as Diane moves eastward,

south of Rhode Island. The heaviest precipitation was found to reside in a narrow band

north of the coastal front, as exhibited in a finer-resolution precipitation analysis with

approximate coastal front locations analyzed every 3 h (Fig. 3.47).




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