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Tropical Storm BERTHA (Text)


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TROPICAL STORM BERTHA DISCUSSION NUMBER  18
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL032014
500 AM EDT TUE AUG 05 2014

Data from an Air Force Reserve reconnaissance aircraft indicate
that Bertha's winds have decreased below hurricane strength, which
is logical given that little deep convection now exists near the
center. The initial intensity has been lowered to 55 kt, which
could generous given that only 45-kt surface winds have been
reported thus far. However, the recon aircraft has not yet completed
its entire mission, so I do not want to bring the intensity
too quickly in the event stronger winds are found east of the center
and/or deep convection redevelops.

The initial motion estimate is an uncertain 015/19 kt. Bertha
briefly accelerated to about 21-22 kt after losing its deep
convection a few hours ago, but recon fix data suggests that the
cyclone's motion has settled down to around 19 kt now. Overall, the
official track forecast and philosophy remains basically unchanged.
The NHC model guidance remains in very good agreement on Bertha
continuing its northeastward trek around the western periphery of a
deep-layer subtropical ridge for the next 24 hours or so, and then
accelerate eastward after getting caught up in the mid-latitude
westerlies by 36-48 hours. The official forecast track is similar to
but slightly west of the previous advisory track to account for the
more westward initial position, and closely follows the consensus
model TVCA.

Bertha is forecast to experience increasing vertical wind shear
over the next two days, with the shear reaching 50 kt or more by
24-36 hours. The combination of the strong shear and decreasing SSTs
should induce steady weakening and cause extratropical transition
by 36 hours or so. However, not as much weakening is expected with
such strong shear conditions due to the expected infusion of
baroclinic energy associated with a strong mid-latitude trough that
is forecast to capture Bertha by 48 hours, which will help maintain
the cyclone's intensity. The intensity forecast during the
extratropical phase closely follows input from the NOAA Ocean
Prediction Center.

FORECAST POSITIONS AND MAX WINDS

INIT  05/0900Z 33.4N  72.9W   55 KT  65 MPH
 12H  05/1800Z 35.9N  70.8W   55 KT  65 MPH
 24H  06/0600Z 38.6N  66.9W   50 KT  60 MPH
 36H  06/1800Z 41.3N  62.0W   50 KT  60 MPH...POST-TROP/EXTRATROP
 48H  07/0600Z 44.1N  56.4W   50 KT  60 MPH...POST-TROP/EXTRATROP
 72H  08/0600Z 48.5N  44.0W   45 KT  50 MPH...POST-TROP/EXTRATROP
 96H  09/0600Z 49.3N  28.0W   45 KT  50 MPH...POST-TROP/EXTRATROP
120H  10/0600Z 50.3N  14.5W   45 KT  50 MPH...POST-TROP/EXTRATROP

$$
Forecaster Stewart

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Page last modified: Wednesday, 31-Dec-2014 12:09:09 UTC