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Tropical Storm GRACE Forecast Discussion Number 7
2015-09-07 04:50:45| National Hurricane Center (Atlantic)
Issued at 1100 PM AST SUN SEP 06 2015 000 WTNT42 KNHC 070250 TCDAT2 TROPICAL STORM GRACE DISCUSSION NUMBER 7 NWS NATIONAL HURRICANE CENTER MIAMI FL AL072015 1100 PM AST SUN SEP 06 2015 A partial ASCAT-B overpass at 2346 UTC, along with several passive microwave passes, indicates that Grace has a well-developed, compact low-level circulation that extends upward into the mid-levels of the cyclone. However, both conventional and microwave imagery also indicate that the inner-core convection has been significantly disrupted by the entrainment of dry mid-level air into the western and southern quadrants of the circulation, and has penetrated into the cyclone's center. The initial intensity of 45 kt, which could be generous, is based on Dvorak current intensity estimates of T3.0/45 kt from both TAFB and SAB, which is supported by a 40-kt ASCAT-B wind vector. Grace's motion continues to be 280/13 kt. The small tropical cyclone is expected to move a little north of due west throughout the forecast period due to moderate low- to mid-level easterly flow on the south side of a large subtropical ridge located to the north of Grace. The NHC model guidance is tightly packed around the previous advisory track, so the new forecast is just an update and extension of the previous advisory, and lies close to the consensus model, TVCN. Grace's fairly robust circulation should be able to mix out any dry air intrusions for the next 24 hours or so while the vertical wind shear remains low and, therefore, a return of inner-core deep convection is expected later tonight. Shortly after that time, however, the vertical shear is forecast by both the GFS and ECMWF models to increase to 25-30 kt from a westerly direction and for the mid-level humidity to decrease to near 40 percent, a combination that will halt any intensification and induce a steady weakening trend despite warm SSTs of near 28 deg C beneath the cyclone. The official intensity forecast is a little below the previous advisory forecast, and is similar to the intensity consensus model, IVCN. It is worth noting that the GFS and ECMWF models show significant weakening of the cyclone in 4 to 5 days, with possible dissipation occurring by 120 h. FORECAST POSITIONS AND MAX WINDS INIT 07/0300Z 13.3N 32.5W 45 KT 50 MPH 12H 07/1200Z 13.5N 34.7W 45 KT 50 MPH 24H 08/0000Z 13.8N 37.7W 50 KT 60 MPH 36H 08/1200Z 14.0N 40.7W 45 KT 50 MPH 48H 09/0000Z 14.5N 44.0W 40 KT 45 MPH 72H 10/0000Z 15.3N 50.3W 40 KT 45 MPH 96H 11/0000Z 16.1N 56.5W 35 KT 40 MPH 120H 12/0000Z 17.2N 62.0W 30 KT 35 MPH $$ Forecaster Stewart
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