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Tropical Storm GRACE Forecast Discussion Number 4
2015-09-06 10:40:07| National Hurricane Center (Atlantic)
Issued at 500 AM AST SUN SEP 06 2015 000 WTNT42 KNHC 060840 TCDAT2 TROPICAL STORM GRACE DISCUSSION NUMBER 4 NWS NATIONAL HURRICANE CENTER MIAMI FL AL072015 500 AM AST SUN SEP 06 2015 Deep convection has become better organized during the past several hours, with a more pronounced curved band developing to the west and south of the center. Recent microwave imagery, especially a 0331 UTC GCOM pass, showed a surprisingly well-organized inner core, with the low-level center located a little farther south than previous estimates. Subjective Dvorak estimates are T3.0/45 kt from TAFB and T2.5/35 kt from SAB, and the initial intensity is therefore raised to 40 kt. If Grace is as well organized as suggested by the microwave data, this intensity could be conservative. With the slight southward adjustment of the initial position, the estimated motion is now 270/11 kt. Grace is located due south of a mid-tropospheric anticyclone, and a ridge axis extends westward across the Atlantic to north of the Lesser Antilles. This pattern is expected to change little during the forecast period, which should keep Grace on a general westward path through day 5. The guidance is in generally good agreement, and the updated NHC track is shifted a little bit south of the previous forecast, mainly to account for the adjusted initial position. This solution is a bit south of the TVCN model consensus and leans closest to the GFS, HWRF, and the Florida State Superensemble. Grace will be moving through a low-shear environment for the next 36 to 48 hours, which is likely to support additional strengthening while it moves westward over the tropical eastern Atlantic. Between days 3 through 5, while Grace is moving over the central tropical Atlantic, vertical shear is forecast to increase to over 20 kt and veer from southwesterly to westerly by the end of the forecast period. Such a hostile environment should induce weakening beginning on day 3. The new NHC intensity forecast indicates continued gradual strengthening through 36 hours while the shear is low and shows a slightly higher peak intensity than the previous forecast. Weakening could be fast once the shear sets in, and the day 4 and 5 intensities are unchanged from before. FORECAST POSITIONS AND MAX WINDS INIT 06/0900Z 12.4N 28.5W 40 KT 45 MPH 12H 06/1800Z 12.6N 30.8W 45 KT 50 MPH 24H 07/0600Z 12.8N 33.6W 50 KT 60 MPH 36H 07/1800Z 13.1N 36.3W 55 KT 65 MPH 48H 08/0600Z 13.4N 39.1W 55 KT 65 MPH 72H 09/0600Z 14.0N 44.7W 50 KT 60 MPH 96H 10/0600Z 14.5N 50.5W 40 KT 45 MPH 120H 11/0600Z 15.5N 55.5W 35 KT 40 MPH $$ Forecaster Berg
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