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| #707317 (Received by flhurricane at: 5:02 AM 03.Aug.2014) TCDAT3 TROPICAL STORM BERTHA DISCUSSION NUMBER 10 NWS NATIONAL HURRICANE CENTER MIAMI FL AL032014 500 AM AST SUN AUG 03 2014 The overall convective pattern of Bertha has changed little since the previous advisory. The center position has been difficult to determine and is based on a blend of the past motion and available reconissance data. An Air Force Reserve reconnaissance was able to close off an 850 mb center around 0200 UTC, and a dropsonde released to the southeast of the alleged center indicated a surface wind of 210 degrees at 21 kt, which implies a west wind of at least 10 kt somewhere in the southern quadrant. These data are the basis for keeping Bertha as a tropical cyclone for this advisory. Also during the outbound leg, the aircraft found rain-contaminated SFMR surface winds of 53, 57, and 48 kt in the southeastern quadrant. After adjusting those wind speeds due to rain enhancement yields a consecutive 3-bin average speed of about 42 kt, which justifies maintaining the intensity at 40 kt. The initial motion estimate is 310/18 kt. The NHC model guidance remains in excellent agreement with Bertha gradually moving northward and then northeastward around the periphery of a deep-layer subtropical ridge located along 28N latitude and ahead of an broad mid-level trough that is located over the eastern United States through 72 hours. After that, the cyclone is expected to become embedded in the mid-latitude westerlies and accelerate northeastward and east-northeastward as it becomes extratropical or merges with a frontal system in 96-120 hours. The NHC track forecast is essentially just an update of the previous advisory, only slightly faster, and follows the trend of the consensus model TVCA. It is possible that Bertha could briefly open up into a wave this morning due to strong northwesterly shear of at least 25 kt. However, the environment ahead of the cyclone for about the next 48 hours is expected to become increasingly more conducive for at least gradual strengthening to occur, especially at 36-48 hours when the vertical shear is expected to decrease to near 10 kt while SSTs and mid-level moisture reach their peak. By 96 hours, extratropical transition over sub-23C SSTs and/or merger with a frontal system is expected. The ECMWF indicates frontal merger in about 72 hours, but this appears be premature based on other available model guidance and input from the NOAA Ocean Prediction Center (OPC). The new NHC intensity forecast is consistent with the previous advisory, which now calls for Bertha to reach hurricane strength by 72 hours. However, this could occur sooner than indicated given that the most favorable oceanic and atmospheric conditions become juxtaposed by 48 hours or so. Intensity forecasts during the extratropical phase are based on input from OPC. FORECAST POSITIONS AND MAX WINDS INIT 03/0900Z 20.9N 71.1W 40 KT 45 MPH 12H 03/1800Z 22.8N 72.7W 45 KT 50 MPH 24H 04/0600Z 25.6N 74.0W 50 KT 60 MPH 36H 04/1800Z 28.8N 74.0W 55 KT 65 MPH 48H 05/0600Z 32.0N 72.7W 60 KT 70 MPH 72H 06/0600Z 37.8N 66.8W 65 KT 75 MPH 96H 07/0600Z 42.5N 58.0W 60 KT 70 MPH...POST-TROP/EXTRATROP 120H 08/0600Z 46.0N 47.3W 50 KT 60 MPH...POST-TROP/EXTRATROP $$ Forecaster Stewart |
