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New England Deluge of May 2006: Ensemble Forecast Evaluation
By
Richard H. Grumm
National Weather Service Office
State College, PA 16803
and
Walt Drag
National Weather Service Office
Taunton, MA 02780
1. INTRODUCTION New England exceeded 5 inches over
large portions of three aforementioned
Heavy rains impacted portions of New States. Several localized areas of 10-15
England on 13-15 May 2006. The inches of rain were observed. The storm
persistent rain produced locally heavy delivered heavy rain, intermittently, over
rain and flooding over portions of a period that spanned about 100 hours.
Massachusetts, New Hampshire, and The initial heavy rains affected the
southeast Maine. As shown in Figure 1, Connecticut River valley with flooding
the 5-day total rainfall in east-central on 12 May. Areas of Connecticut
Figure 1 Spotter report rainfall collected by the National Weather service office in Taunton, Massachusetts.
Contour interval is 2 inches.
received 5 to 8 inches of rainfall with ensemble forecast system (SREF) did
lesser amounts outside the flood area. indicate a significant rain event. Overall,
The rain continued over the 13 and 14th in this event, recognition of a long
adding to the overall rainfall duration event and the pattern conducive
accumulation. Heavy rains were for heavy rains may have been as
observed in portions of Massachusetts important as ensemble prediction system
and New Hampshire over this two day (EPS) precipitation forecasts.
period. A final slug of 1.5 inches of rain
brought more flooding late on the 15th This type of event, though rare is not
and early on the 16th in northeastern uncommon, a heavy rain event affect
Massachusetts. portions of New England on 7-8 October
2005. This event brought heavy rains
The National Centers for Environmental and flooding to portions of New
Predictions (NCEP) short-term ensemble Hampshire (Grumm 2005) key findings
forecast system (SREF) will be used to associated with this event included:
evaluate forecasts of this event. It is
difficult to conceive a model predicting a) A strong, slow moving frontal zone,
typically oriented in a north-south
the observed rainfall amounts. However, configuration.
it will be shown that the short-range
Figure 3 As in Figure 1 except 00-hour forecast valid at 2100
Figure 2 SREF21 forecasts of 500 hPa heights initialized on UTC 13 May 2006.
2100 UTC 11 May 2006 valid at 1200 UTC 13 May 2006
showing a) spaghetti plots of 5460 and 5760 m contours by
member and the ensemble mean (black) with the spread
about the mean and b) the ensemble mean its departure from
normal in standard deviations from normal.
The May 2006 event was not directly
b) Heavy rainfall occurring in the warm associated with a tropical system.
moist air along the axis of the strong
low-level jet.
However, satellite imagery in this event
and moisture forecasts in the SREF
c) A large anticyclone was present over imply a tropical moisture connection
eastern Canada. during the event. A similar connection
with tropical moisture was observed in
d) The event is observed with
precipitable water anomalies which are
the southeast Maine heavy rain event of
well above normal. These highly 20-22 October 1996 (Konrad 2001). In
anomalous values were due to the fact, the 1996 event ingested tropical
interaction with a frontal system with the moistures associated with hurricane
remnants of a tropical storm. Lilli. A key feature, not noted as
e) This was a common transition season
important in the October 2005, event
heavy rain event type with the added was an anomalous blocking ridge and
punch of tropical moisture. strong low-level anticyclone to the north
and east. The ingredients found to be
important to forecasting heavy rain
Figure 4 As in Figure 2 except MSLP (hPa) forecasts valid at Figure 5 As in Figure 3 except precipitable water (mm).
2100 UTC 13 May 2006.
(Doswell et al 1996) were present with All MREF and SREF data were archived
this event including abundant, moisture, in real-time along with all the NAM and
lift, and a boundary. GFS model data. All images were
recreated after the event using GrADS
This paper will document the heavy rain (Doty and Kinter 1992). Standard
event of 14-16 May 2006. The focus is displays of “spaghetti” and “spread” are
on the performance on the NCEP SREF shown. The spread is normally shown
in predicting the overall pattern with gray shading. The spaghetti plots
associated with the event; the ingredients show distinct contours for each
conducive to producing heavy rains; and parameter (Sivillo et al 1997). Lower
the quantitative precipitation forecast panels for some fields show the
produced by the EPS. This document ensemble mean with color shading
will not provide details about the showing the ensemble mean in standard
catastrophic flooding, including the deviations from normal, referred to as
historic floods along the Merrimac standardized anomalies (Grumm and
River. Hart 2001;Hart and Grumm 2001).
Precipitation displays show the critical
2. METHODS thresholds over a time period, normally
24 hours and 0.50 inches of quantitative
precipitation forecast (QPF). Upper
Figure 6. SREF forecasts initialized at 0900 UTC 10 May 2006 showing a) precipitable water anomalies and b) 850
hPa winds. Precipitable water shows spaghetti plots and dispersion about the mean and the ensemble mean with the
departures from normal. Winds show wind speed (kts) and the U and V wind anomalies in standard deviations from
normal.
panels normally show the probabilities Figures 2 & 3 show forecasts and
and the consensus of the specified “ensemble verification” on 13 May
thresholds. Lower panels show the 2006. Key features with this event
position of the specified threshold for included an anomalous high latitude
each EPS member and shading shows ridge over eastern North America and an
the ensemble mean QPF. anomalous upper-level trough over the
east-central United States. The forecasts
The spaghetti plots follow the standard of positive height anomalies from
ensemble concepts outlined by Sivillio et 11/2100 UTC under forecast the
al (1997). This paper is a good reference intensity of the ridge over eastern
point for those learning to use Canada. The anomalies were on the
ensembles. order of 2-3 standard deviations (SDs)
above normal but were observed to be
Forecast data shown here will primarily on the order of 3-5SDs above normal.
from the NCEP SREF. All dates and The trough and the anomalies in the
times will be in the format of day and trough were closer to the observed
hours. Such that 0900 UTC 14 May values.
2006 would be displayed at 14/0900.
The blocking pattern, with the high-
3. RESULTS latitude ridge and the trough formed the
western end of an omega block over the
i. Overview of the salient features western Atlantic and eastern North
America. This block produced a strong
surface anticyclone (Fig. 4) which, as
will be shown, supported a persistent
frontal boundary which was the focus of
a multi-day rain event over New
England. This boundary had normal to
slightly above normal precipitable water
(PWAT: Fig. 5) which persisted through
out the event.
ii. SREF Forecasts 10 May
SREF forecasts of precipitation and the
features associated with the event will be
the focus of this section. It would be
exhaustive to show all forecasts so a
limited suite of data is presented.
Figure 6 shows the SREF forecasts from
10/0900 UTC valid at 13/0900 UTC of Figure 7. SREF forecasts initialized at 0900 UTC
PWAT and 850 hPa winds. The winds 10 May 2006 showing accumulated precipitation
show the U (east-west) and V (north- for the 36-hour period ending at 2100 UTC 13 May
2006. Upper panels shows the probability of
south) wind anomalies respectively. In
exceeding 1 inch and the lower panels shows the
addition to the slightly above normal ensemble mean (shaded) and each members 1.0
PWAT over south-central New England, inch contour.
an anomalous southeasterly 850 hPa jet spaghetti plots showed a large spatial
was forecast over the region. As will be variation in the area of heaviest rain
shown, this anomalous jet verified and which led to the lower mean
persisted for an extended period of precipitation forecast and the relatively
time. low probability (60% or less) of
exceedance.
The precipitation forecasts for the period
extending from 12/0900 UTC through Forecasts from 10/2100 UTC were
13/2100 UTC is shown in Figure 7. similar and for brevity they are not
These data show only 50-60% of the shown.
members forecasting 1 inch or more of
rainfall over central New England along
the boundary. The highest probability of iii. SREF forecasts from 11 May
1 inch or more was over portions of
Connecticut and Massachusetts.
However, the 1 inch contour in the
Figure 8 As in Figure 6 except SREF forecasts initialized at 2100 UTC 11 May 2006.
Figure 8 shows the SREF PWAT and
850 hPa wind forecasts initialized at
11/2100 UTC valid at 13/0900 UTC.
These data show the same concept from
earlier forecasts, with a persistent
boundary over south-central New
England. Along this boundary, above
normal PWAT was present along with
above normal southeasterly flow. Most
forecast periods covering the period of
rain showed this persistent feature in
the moisture fields as well as the
persistent strong southeasterly flow.
The 11/2100 UTC precipitation forecasts
are shown in Figure 9. These data show
a 30-50% chance of 1 inch or more QPF
in the Connecticut Valley for the 24 hour
period ending at 14/0000 UTC. The
lower panel shows most SREF members
focused the 1.00 inch contour over New
England.
Figure 9. As in Figure 6 except for SREF initialized
iv. SREF forecasts from 12 May at 2100 UTC 11 May and forecast of 24-hour
accumulations for the period ending at 0000 UTC 14
May 2006.
Figure 10 SREF 850 hPa wind forecasts initialized at Figure 11. As in Figure 9 except valid at 0000 UTC 14
2100 UTC 12 May 2006 valid at 0900 UTC 13 May May 2006.
2006. Upper panel shows winds and U-wind anomalies
and lower panel shows 850 hPa winds and the V-wind
anomalies.
SREF forecasts from 12 May 2006
continued to show the same general
conditions as those produce in earlier
forecasts. This included the persistent
boundary in the PWAT field over south-
central New England and the anomalous
low-level jet.
This strong low-level jet is shown in
Figures 10 and 11. In these forecasts,
some areas of negative U-wind
anomalies in excess of -3SDs below
normal were present at 13/0900 UTC
and a broader area of -3SD easterly wind
anomalies were present at 14/0000 UTC.
Though not shown, this area of intense
southeasterly flow maintained itself and Figure 12. As in Figure 8 except SREF forecasts
intensified from 13/0900 UTC through initialized at 2100 UTC 12 May showing 1 inch
14/0000 UTC. The SREF’s forecast a or more in 24-hours for the 24 hour period
persistent anomalous flow over the ending at 0300 UTC 14 May.
boundary. These conditions may have favored training over and along the cool
side of the boundary.
Figure 13. SREF accumulated precipitation for Boston, MA from SREF forecasts initialized at
0900 UTC 10 May 2006. Gray lines show the instantaneous 3-hour amounts and the colored lines
show the accumulated amounts. Sums by type are provided on the left hand side.
Figure 14. As in Figure 12 except SREF forecasts initialized at 0900 UTC 11 May
2006.
sufficient precipitation relative to what
The SREF precipitation forecasts was observed.
continued to show low precipitation
amounts relative to what was observed v. SREF point forecasts
(Fig. 12). To its credit, the SREF had the
correct area but was unable to generate
Figure 15. As in Figure 11 except initialized at 21Z 11 May 2006.
Figure 16. As in Figure 11 except initialized at 0900 UTC 12 May 2006.
A series of SREF point forecasts are Figure 13 shows the QPF from the SREF
presented for the environs about Boston, for forecasts initialized at 10/0900 UTC.
Massachusetts. These data put the These data show a prolonged period of
forecast quantitative precipitation in a precipitation. The interval of the
context not easily seen in plan view maximum accumulations varies widely
maps. from member-to-member. The mean
QPF is 1.51 inches and at least 1
member forecast over 3.60 inches of
QPF. Nearly 33% of the members were Heavy rains impacted portions of New
clustered with light amounts generally England on 13-15 May 2006. The
less than 1 inch. persistent rain produced locally heavy
rain and flooding over portions of
Figure 14 shows the SREF forecasts Connecticut, Massachusetts, New
initialized at 11/0900 UTC. Overall, Hampshire, and southeast Maine. Heavy
there was an increase in the QPF rainfall, most of it falling over a 100
forecasts. The maximum was 5.40 hour period, caused widespread flooding
inches and the mean was 3.14 inches. across much of eastern Massachusetts
There still were some dry members and and southern New Hampshire of small
the timing of when the accumulations streams and main stem rivers. Major
would occur varied. This would impact flooding of a number of small streams
the appearance of plan view displays. and main stem rivers was observed in
portions of Massachusetts and New
Figure 14 shows the SREF plume from Hampshire. States of Emergency were
forecasts initialized at 11/2100 UTC. declared for portions of Massachusetts
The mean QPF is 2.76 inches with a and New Hampshire. Several locations
maximum of 5.15 inches. The driest observed the worse flooding since the
member still had 0.83 inches of QPF. 1938 Hurricane and the great rain and
Similar to previous forecasts, the snowmelt floods in March 1936. This
forecasts implied a long duration event. storm produced rainfall amounts similar
But the time of accumulation varied to those observed in the 20-21 October
from member-to-member. The varying 1996 event (Storm Series Report #96-7).
interval of when the QPF would The flooding was comparable to and in
accumulate made making probability some locations worse than that observed
charts difficult. in the October 1996 event (NWS
Figure 15 shows SREF forecasts Taunton).
initialized at 12/0900 UTC. The overall
maximum precipitation is lower; the As shown in Figure 1, the 7-day total
mean was higher than previous SREF rainfall in east-central New England
cycles. Similar to previous cycles, there exceeded 5 inches over large portions of
was considerable variation in the time of three aforementioned States. The overall
when the largest accumulations would pattern conducive to producing heavy
occur. In this example, several members rains was well forecast; however, model
show rapid accumulations on the 12th and ensemble QPF’s were too low for
and early on the 13th while two members this event.
show rapid accumulations after 14/0000
UTC. In this time period, there was a The relatively poor SREF QPF suggests
clustering of members between 3.8 and there are still some limitations in using
4.6 inches of QPF. Note that 7 members ensemble prediction data to get the QPF
arrived at such high numbers but the got “correct”. Though no single model
there with the QPF falling at widely appeared to offer a better solution and
varied times. some SREF members did forecast 2-3
inches of rainfall. All deterministic
4. CONCLUSIONS members should be considered part of an
ensemble whenever practical to capture For this event, SREF precipitation
the local and regional maximum. In this forecasts appeared to under forecast the
case, no deterministic solutions were rainfall; when viewed in plan view over
presented. Though the forecasts discrete time intervals; with this event
contained the SREF RSM, NAM, and and thus under estimated the potential
the two WRF set of members. for flooding. However, the features were
well forecast. The persistent moisture
Though the QPF’s were not particularly boundary with above normal PWAT
accurate 1 , the SREF’s produced and the persistent strong southeasterly
excellent forecasts of a pattern flow were well forecast by the SREF.
conducive to heavy rain, and they Thus, the models captured a potential
correctly forecast this pattern to persist heavy rain signal but were generally
for a prolonged period of time. The unable to capture the timing and
boundary outlined by the SREF PWAT intensity of the rainfall in the
contours and the anomalies associated precipitation forecasts.
with these forecasts proved quite
accurate. This boundary played a critical The plume diagrams, shown only for
role in the areas receiving heavy Boston, revealed that the individual
precipitation. The ability to forecast this members of the SREF system did
persistent pattern was a SREF strength forecasts a significant and long
and provided useful guidance to duration rainfall event. Given the
forecasters familiar with heavy rainfall resolution of the members, the fact that
patterns. Admittedly, the PWAT several members forecast 4-5 inches of
anomalies, though present were not QPF is quite remarkable. Clearly, timing
significantly above normal. They issues led to the uncertainty, but taken
remained about 1 SD above normal over the long course of this event, the
throughout the event and in the QPF forecasts and the probability of
forecasts. significant rainfall was quite well
depicted by the plumes. This signal was
Persistent southeasterly flow was a harder to see in the plan view maps.
continual problem and threat. Due to Heavy rainfall, most of it falling over a
southeasterly flow, there were both 100 hour period, caused widespread
anomalous southerly and easterly winds. flooding across much of eastern
Anomalies were on the order of 2-3SDs Massachusetts and southern New
for several days. The strong deep Hampshire of small streams and main
southeasterly flow is another signal stem rivers. Major flooding of a number
associated with heavy rains in the of small streams and main stem rivers
eastern United States. This feature and was observed in portions of
significant positive PWAT anomalies Massachusetts and New Hampshire.
were associated with heavy rains of States of Emergency were declared
October 2005. portions of Massachusetts and New
Hampshire. Several locations observed
the worse flooding recorded since the
1 1938 Hurricane and the great rain and
It is unrealistic to expect a 32km based forecast
suite to predict rainfall in the 6-20 inch ranges. snowmelt floods in March 1936. This
Forecasts of 3-5 inches are in and of themselves storm produced rainfall amounts similar
quite respectable.
to those observed in the 20-21 October amounts of moisture are
1996 event (Storm Series Report #96-7). present and strong
The flooding was comparable to and in southeasterly flow is present,
some locations worse than that 1996 the features and the anomalies
event (NWS Taunton). may be signals to help
anticipate a heavy precipitation
The plume diagrams clearly pointed out event.
several significant problems when c. When properly viewed, the
displaying forecast data, most can be SREF system appears capable
attributed to uncertainty in weather of forecasting large heavy rain
forecasting in general; but the salient events.
points are:
a) the timing of when the 5. ACKNOWLEDGEMENTS
accumulations would occur
varied, NECP for continued access to the SREF
b) this impacted the appearance of data.
plan view displays over
discrete intervals 6. REFERENCES
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