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Tropical Storm NORBERT Forecast Discussion Number 4
2014-09-03 10:56:19| National Hurricane Center (East Pacific)
Issued at 200 AM PDT WED SEP 03 2014 000 WTPZ44 KNHC 030856 TCDEP4 TROPICAL STORM NORBERT DISCUSSION NUMBER 4 NWS NATIONAL HURRICANE CENTER MIAMI FL EP142014 200 AM PDT WED SEP 03 2014 The convective cloud pattern of Norbert has improved significantly since the previous advisory. A pronounced CDO feature has developed with two distinct convective bands wrapping into the center. A 0432 UTC AMSU microwave indicated that a small mid-level eye feature could be trying to form. Satellite intensity estimates from TAFB and SAB were a consensus T3.0/45 kt at 0600 UTC, and UW-CIMSS ADT values have been steadily increasing and are now T3.7/59 kt. Based on these intensity estimates, the continued improvement in the cloud pattern, and the small radius of maximum winds, the advisory intensity is conservatively set at 50 kt. The initial motion estimate is still northwestward or 305/07 kt. Norbert is forecast to move in a general northwestward direction around the southwestern periphery of a deep-layer subtropical ridge that stretches westward across northern Mexico and Baja California. This motion, accompanied by a slight decrease in forward speed, is expected to persist throughout the forecast period. The NHC model guidance has shifted slightly to the right again, but the size of the guidance envelope has decreased, showing much less spread on this cycle. The official forecast track is similar to the previous advisory track through 36 hours, and then is somewhat to right of the previous forecast after that. The combination of the small radius of maximum winds, 29.5C SSTs, a moist mid-level environment, and vertical shear decreasing to around 10 kt suggests that Norbert should at least intensity at the typical rate of 20-25 kt per day for the next 36 hours or so. However, if thew shear decrease more than currently expected, then rapid intensification during the next 24 hours is a very distinct possibility. By 96 hours and beyond, Norbert will be moving over SSTs less than 26.5C, which should induce slow weakening. NHC intensity forecast is above the previous forecast, and closely follows the SHIPS intensity model. FORECAST POSITIONS AND MAX WINDS INIT 03/0900Z 19.6N 107.6W 50 KT 60 MPH 12H 03/1800Z 20.0N 108.5W 55 KT 65 MPH 24H 04/0600Z 20.6N 109.4W 65 KT 75 MPH 36H 04/1800Z 21.4N 110.1W 75 KT 85 MPH 48H 05/0600Z 22.3N 110.9W 75 KT 85 MPH 72H 06/0600Z 24.0N 112.7W 65 KT 75 MPH 96H 07/0600Z 25.1N 115.1W 60 KT 70 MPH 120H 08/0600Z 26.2N 117.0W 55 KT 65 MPH $$ Forecaster Stewart
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