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Tropical Storm BERTHA Forecast Discussion Number 4
2014-08-01 22:48:57| National Hurricane Center (Atlantic)
Issued at 500 PM AST FRI AUG 01 2014 000 WTNT43 KNHC 012048 TCDAT3 TROPICAL STORM BERTHA DISCUSSION NUMBER 4 NWS NATIONAL HURRICANE CENTER MIAMI FL AL032014 500 PM AST FRI AUG 01 2014 Earlier observations from an Air Force Reserve Hurricane Hunter aircraft and recent surface observations from Martinique indicate that Bertha still has a closed circulation, although the area of westerly winds south of the center is rather small. The aircraft- reported pressures were 1006-1007 mb, and the Martinique data suggests a current central pressure of 1007 mb. Based on this and the aircraft wind data, the initial intensity remains 45 kt. New convection is currently developing in this bands near the center, with a more solid area of convection farther east. The initial motion is now 290/21. Bertha continues to be steered west-northwestward by the flow around the Atlantic subtropical ridge and this should continue for the next 36-48 hours. After that time, the cyclone is expected to turn northward into a break in the ridge caused by a deep-layer trough over the eastern United States. This motion should be followed by recurvature into the westerlies over the Atlantic between Bermuda and New England, and then by a northeastward motion over the open North Atlantic. While the track guidance has nudged a little to the left since 6 hours ago, it remains in good agreement with this scenario. The new forecast track is therefore tweaked just a little to the left of the previous track, and it is a little faster than the previous track after recurvature. Bertha continues to experience about 15 kt of southwesterly vertical wind shear, and a combination of water vapor imagery and microwave total precipitable water shows abundant dry air near the storm. The forecast track calls for Bertha to interact with one or two upper- level troughs during the next 48 hours or so, which should cause some shear and dry air entrainment to continue. This, combined with the current lack of organization, suggests little change in strength should occur during that time. This part of the new intensity forecast is unchanged from the previous forecast. Subsequently, Bertha is expected to move into an environment of less shear and greater moisture. The intensity guidance responds to this by forecasting significant intensification, with several models showing Bertha becoming a hurricane during recurvature. Based on this, the latter part of the intensity forecast is nudged upward from the previous forecast, although it is still weaker than most of the guidance. An alternative scenario remains possible: that a combination of shear, dry air entrainment, and land interaction causes Bertha to degenerate to a tropical wave during the next 48 hours, followed by possible regeneration in the 72-120 hours when the system reaches the more favorable environment. FORECAST POSITIONS AND MAX WINDS INIT 01/2100Z 14.9N 61.2W 45 KT 50 MPH 12H 02/0600Z 16.0N 63.8W 45 KT 50 MPH 24H 02/1800Z 17.8N 66.9W 45 KT 50 MPH 36H 03/0600Z 19.9N 69.6W 45 KT 50 MPH 48H 03/1800Z 22.4N 72.0W 45 KT 50 MPH 72H 04/1800Z 27.5N 74.0W 50 KT 60 MPH 96H 05/1800Z 33.5N 71.0W 55 KT 65 MPH 120H 06/1800Z 38.5N 62.0W 60 KT 70 MPH $$ Forecaster Beven
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