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Tropical Storm GRACE Forecast Discussion Number 11
2015-09-08 04:52:53| National Hurricane Center (Atlantic)
Issued at 1100 PM AST MON SEP 07 2015 000 WTNT42 KNHC 080252 TCDAT2 TROPICAL STORM GRACE DISCUSSION NUMBER 11 NWS NATIONAL HURRICANE CENTER MIAMI FL AL072015 1100 PM AST MON SEP 07 2015 After being devoid of any significant convection near the center for almost 6 hours, a small burst of deep convection with cloud tops colder than -80 deg C has developed since 0000 UTC just south of and over the previously exposed low-level circulation center. This recent convective development is the reason for keeping Grace as a 35-kt tropical storm. This intensity is supported by a Dvorak current intensity estimate of T2.5/35 kt from TAFB and a couple of 32-kt ASCAT-B wind vectors. The initial motion estimate is 275/17 kt. There is essentially no change to the previous forecast reasonings over the past couple of days. Grace is expected to gradually weaken and become more vertically shallow, and be steered quickly westward by the moderate low-level easterly trade wind flow on the south side of a deep-layer ridge. The official track forecast is just an update and extension of the previous advisory track, and lies close to the dynamical model consensus, TVCA. With increasing southwesterly to westerly vertical wind shear and an abundance of dry mid-level air ahead of Grace, steady weakening is expected due to these hostile environmental conditions. The only saving grace is the cyclone's robust low-level circulation, which could force periodic bursts of deep convection for the next 24 hours or so, similar to the most recent convective development. However, by 36 hours and beyond, westerly shear in excess of 25 kt should cause the circulation to weaken significantly due to less frequent and shorter duration convective bursts. Degeneration into a remnant low is expected by 48 hours, followed by dissipation in 96 hours. However, dissipation could occur sooner, similar to the ECMWF model solution. The new NHC intensity forecast closely follows the IVCN consensus model. FORECAST POSITIONS AND MAX WINDS INIT 08/0300Z 14.0N 39.2W 35 KT 40 MPH 12H 08/1200Z 14.1N 41.8W 35 KT 40 MPH 24H 09/0000Z 14.3N 45.1W 30 KT 35 MPH 36H 09/1200Z 14.7N 48.4W 30 KT 35 MPH 48H 10/0000Z 15.1N 51.8W 25 KT 30 MPH...POST-TROP/REMNT LOW 72H 11/0000Z 16.0N 58.8W 20 KT 25 MPH...POST-TROP/REMNT LOW 96H 12/0000Z...DISSIPATED $$ Forecaster Stewart
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