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Tropical Storm BERTHA Forecast Discussion Number 18
2014-08-05 10:59:32| National Hurricane Center (Atlantic)
Issued at 500 AM EDT TUE AUG 05 2014 000 WTNT43 KNHC 050859 TCDAT3 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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