Operational Use of the
Dvorak Technique at the
NHC
JACK BEVEN
NATIONAL HURRICANE
CENTER
WHERE AMERICA’S CLIMATE AND WEATHER SERVICES BEGIN
Operational Background
• The Tropical Analysis and Forecast Branch
(TAFB) of the NHC performs manual subjective
Dvorak location and intensity estimates.
• The TAFB also performs Hebert-Poteat
subtropical cyclone technique intensity estimates.
• NHC philosophy is that these estimates are
supposed to be as independent of other data
sources as possible (not 100% possible!).
• Intensity estimates from the Advanced Dvorak
Technique (ADT) are used mainly to supplement
to the manual estimates.
Operational Background
• The Hurricane Specialists Unit (HSU, the TC
forecasters) of the NHC integrates the Dvorak
analyses with other data sources (when
available).
• Even when they are the only data source, the
HSU forecasters are free to employ the Dvorak
analyses in whatever way they feel is
appropriate. This applies to both real-time
forecasting and post-analysis.
Operational Background
• Full Dvorak analyses are normally made every
six hours at the synoptic times, with locations
provided at intermediate synoptic times.
• Full Dvorak analyses can be made at non-
synoptic times if necessary.
• The TAFB uses the 1984 version of the Dvorak
Technique.
• No changes have been made to the wind and
pressure basic calibration of the technique.
• No changes have been made to the basic Dvorak
procedures on the technique flow charts.
Departures from Dvorak (1984)
• Cloud systems centers that do not meet the
convective criteria can be tracked by providing
a location and an intensity estimate of “too
weak to classify”.
• When measuring infrared eye patterns in step
2C, occasionally a BD enhancement color
surrounds the eye that is too narrow to use for
the eye number. In these cases, the color shade
is not used to determine the eye number, but is
used to determine the eye adjustment.
Step 2C - BD Color Used For Eye Adjustment Can
Differ From Color Used For Eye Number
Black (B) completely
surrounds the eye. However,
the B ring is less than 0.5
degrees thick. So, it cannot
be used for the eye number.
The eye number uses Light
Gray (LG) for a 5.0, while
the eye adjustment is
determined by a Warm
Medium Gray (WMG) Jeanne (2004)
embedded in B (+1.0) -
CF=6.0
WMG eye embdd in LG/B LG 5.0 1.0 6.0 0.0 6.0
Departures from Dvorak (1984)
• In step 2C/page 36, there is a rule for the eye
adjustment for large or elongated eyes in infrared
imagery. The TAFB does not use this rule.
• For step2C/visible eye patterns and step 2D/visible
CDO pattern, the TAFB interpolates between the
distances/sizes in the tables to produce Central
Feature numbers at 0.5 T-number resolution.
• For step 6/Pattern T-Numbers, the TAFB does not
strictly follow the rule that the Pattern T-Number
must be within one column (0.5 T-Numbers) of the
Model Expected T-number.
Departures from Dvorak (1984)
• The TAFB uses a modified set of the Dvorak (1984)
constraints on the allowed Final T-Number
changes (step 7/8) based on Lushine (1977). This
deals with when the analyst can allow a looser
constraint.
Modified FT Constraints now in
Original FT Constraints for use for developing storms above
storms with T≥4.0 (Dvorak): T1.5 (24 hr or more after the initial
T1) (Lushine 1977):
1.0 T-numbers over 6 hr
1.5 T-numbers over 12 hr
2.0 T-numbers over 18 hr
2.5 T-numbers over 24 hr
Departures from Dvorak (1984)
• For step 10/Forecast Intensity, the TAFB uses a
combination of Dvorak (1984) and rules from the
version of the technique published in 1995.
• For systems that spend a significant amount of
time over land and then re-emerge over water, the
TAFB re-starts Dvorak analyses using the
observed Data T-Number and Pattern T-Number.
Dvorak Error Distribution
These errors are from comparison to aircraft-based best track data.
Images courtesy of Brown and Franklin
2010 ADT validation results
ADT 8.1.1 CI vs. Best Track Wind
8.0
7.5
7.0
6.5
Alex
6.0
Bonnie
Colin
5.5
TD-5
5.0 Danielle
CI Number
Earl
4.5
Fiona
4.0 Hermine
Igor
3.5 Karl
Matthew
3.0
Paula
2.5 Richard
Tomas
2.0
1.5
1.0
20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0 110.0 120.0 130.0 140.0 150.0 160.0 170.0
Best Track Wind (kt)
2010 ADT validation results
ADT 8.1.1 Final T-Number vs. Best Track Wind
8.0
7.5
7.0
6.5
Alex
6.0
Bonnie
Colin
5.5
TD-5
5.0 Danielle
Final T-number
Earl
4.5
Fiona
4.0 Hermine
Igor
3.5 Karl
Matthew
3.0
Paula
2.5 Richard
Tomas
2.0
1.5
1.0
20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0 110.0 120.0 130.0 140.0 150.0 160.0 170.0
Best Track Wind (kt)
2010 ADT validation results
ADT 8.1.1 CI vs. Best Track MSLP
8.0
7.5
7.0
6.5
Alex
6.0
Bonnie
Colin
5.5
TD-5
5.0 Danielle
CI Number
Earl
4.5
Fiona
4.0 Hermine
Igor
3.5 Karl
Matthew
3.0
Paula
2.5 Richard
Tomas
2.0
1.5
1.0
1020.0 1010.0 1000.0 990.0 980.0 970.0 960.0 950.0 940.0 930.0 920.0 910.0
Best Track MSLP (mb)
2010 ADT validation results
ADT 8.1.1 Final T-Number vs. Best Track MSLP
8.0
7.5
7.0
6.5
Alex
6.0
Bonnie
Colin
5.5
TD-5
5.0 Danielle
Final T-Number
Earl
4.5
Fiona
4.0 Hermine
Igor
3.5 Karl
Matthew
3.0
Paula
2.5 Richard
Tomas
2.0
1.5
1.0
1020.0 1010.0 1000.0 990.0 980.0 970.0 960.0 950.0 940.0 930.0 920.0 910.0
Best Track MSLP (mb)