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Tropical Storm BERTHA Forecast Discussion (Text)


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000
WTNT43 KNHC 020254
TCDAT3

TROPICAL STORM BERTHA DISCUSSION NUMBER   5
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL032014
1100 PM AST FRI AUG 01 2014

While some convection developed near the estimated center during the
past few hours, the circulation of Bertha remains disorganized. In
fact, the Hurricane Hunter aircraft has been unable to find a center
at 5000 ft this evening. However, surface observations suggested
that there was still a small closed surface circulation when Bertha
moved between Martinique and Dominica a few hours ago. The estimated
center position is on the southwestern edge of the convective
canopy, consistent with the 16 kt of southwesterly shear analyzed by
the SHIPS model. The initial intensity remains 45 kt based on
several SFMR winds of 40-45 kt reported by the aircraft east and
northeast of the center.

While the SHIPS model shows the shear decreasing on Saturday, Bertha
will continue moving through a dry environment during the next 24 to
36 hours. There is also the potential for land interaction with
Puerto Rico and Hispaniola during this time, which could disrupt the
small circulation. Given all of these factors, little change in
intensity is expected through 36 hours. After that time, if Bertha
survives, the environment is expected to become more favorable for
intensification with warming SSTs, increasing moisture, and lower
vertical shear. Much of the intensity guidance shows Bertha reaching
hurricane strength in 72 to 96 hours, and the official forecast
follows suit. The new NHC intensity forecast has been adjusted
upward from 48 to 96 hours, but remains a little below the IVCN
intensity consensus. Note that it is possible that the combination
of shear, dry air, and land interaction could cause Bertha to
degenerate to a tropical wave during the next 36 hours, followed by
possible regeneration when the system reaches the more favorable
environment later in the forecast period.

The initial motion estimate is 290/19, as Bertha is being steered
west-northwestward by a deep-layer subtropical ridge over the
western Atlantic. The ridge will begin to erode after 24 hours as a
mid- to upper-level trough moves through the eastern United States,
which should allow Bertha to turn northwestward and then northward
by 3 days. After that time, Bertha is expected to complete
recurvature and accelerate northeastward into the north Atlantic.
The track model guidance remains in good agreement on this general
scenario, however, there is a fair bit of spread in how sharply
Bertha will recurve. The GFS and GEFS ensemble mean lie on the
eastern edge of the guidance envelope, with the ECMWF, HWRF, and
GFDL showing a more gradual turn and a track a little farther west.
Through 36 hours the new NHC track is an update of the previous one.
After that time, the official forecast has been nudged toward the
left, but lies a little to the right of the middle of the guidance
envelope.

FORECAST POSITIONS AND MAX WINDS

INIT  02/0300Z 15.7N  63.0W   45 KT  50 MPH
 12H  02/1200Z 16.9N  65.5W   45 KT  50 MPH
 24H  03/0000Z 19.0N  68.6W   45 KT  50 MPH
 36H  03/1200Z 21.4N  71.2W   45 KT  50 MPH
 48H  04/0000Z 24.0N  73.2W   50 KT  60 MPH
 72H  05/0000Z 30.0N  73.5W   60 KT  70 MPH
 96H  06/0000Z 35.5N  68.0W   65 KT  75 MPH
120H  07/0000Z 40.0N  59.0W   60 KT  70 MPH

$$
Forecaster Brennan



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Page last modified: Saturday, 02-Aug-2014 02:55:07 UTC