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| #707540 (Received by flhurricane at: 5:00 AM 04.Aug.2014) TCDAT3 TROPICAL STORM BERTHA DISCUSSION NUMBER 14 NWS NATIONAL HURRICANE CENTER MIAMI FL AL032014 500 AM EDT MON AUG 04 2014 Convection has been pulsing near the center, but the overall convective cloud pattern of Bertha has changed little since the previous advisory due to moderate northwesterly wind shear. Earlier Air Force Reserve reconnaissance wind data around 0500 UTC supported a 55-kt intensity, especially since the pressure had decreased 3 mb down to 1004 mb. However, a NOAA research aircraft has just started its investigation of the cyclone, and surface wind data support increasing the intensity to 60 kt along with a central pressure fall to about 999 mb. Bertha is now moving northward or 350/14 kt. There remains no significant change to the previous track forecast or reasoning. The latest NHC model guidance remains tightly clustered and in excellent agreement on Bertha continuing to move around the western periphery of the Bermuda-Azores high pressure ridge for the next 36 hours or so, followed by a northeastward acceleration into the mid-latitude westerlies ahead of a deep-layer trough and frontal system currently approaching the U.S. east coast. The official forecast track is just a tad to the left of the previous advisory, and is close to the consensus model TVCA. The 72-, 96-, and 120-hour positions are based on a blend of input from the NOAA Ocean Prediction Center. On the forecast track, Bertha is expected to pass about midway between the U.S. coast and Bermuda on Tuesday. The northwesterly vertical wind shear that has been plaguing Bertha for the past few days is now decreasing and is gradually backing around to a southwesterly direction. In fact, the global models indicate that Bertha will move underneath a 200 mb ridge axis in 18-24 hours, at which time we should see the cyclone reach hurricane status. That intensity should be short-lived since the shear is forecast to increase to more than 30 kt by 48 hours and beyond, which should induce at least steady weakening. The intensity, however, has been held higher than normal due to Bertha's expected transition to an extratropical cyclone with possible baroclinic energy keeping the system fairly strong. Extratropical transition is expected by 72 hours, but it could occur sooner due to 50 kt or more of shear and SSTs around 20C by that time. The NHC intensity forecast is similar to the previous advisory, remaining above the consensus models ICON and IVCN, and is close to a blend of the SHIPS and LGEM intensity models. FORECAST POSITIONS AND MAX WINDS INIT 04/0900Z 26.1N 73.6W 60 KT 70 MPH 12H 04/1800Z 28.6N 73.7W 60 KT 70 MPH 24H 05/0600Z 31.9N 72.9W 65 KT 75 MPH 36H 05/1800Z 35.0N 70.8W 65 KT 75 MPH 48H 06/0600Z 37.7N 67.0W 60 KT 70 MPH 72H 07/0600Z 43.4N 57.3W 55 KT 65 MPH...POST-TROP/EXTRATROP 96H 08/0600Z 47.5N 47.5W 50 KT 60 MPH...POST-TROP/EXTRATROP 120H 09/0600Z 49.5N 37.5W 45 KT 50 MPH...POST-TROP/EXTRATROP $$ Forecaster Stewart |
