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Post-Tropical Cyclone HERMINE Forecast Discussion Number 32
2016-09-05 10:57:21| National Hurricane Center (Atlantic)
Issued at 500 AM AST MON SEP 05 2016 000 WTNT44 KNHC 050857 TCDAT4 POST-TROPICAL CYCLONE HERMINE DISCUSSION NUMBER 32 NWS NATIONAL HURRICANE CENTER MIAMI FL AL092016 500 AM AST MON SEP 05 2016 Hermine remains a post-tropical cyclone with a cloud pattern more reminiscent of an extratropical low. However, moderate convection has been developing just north of the surface center in the northern semicircle during the past few hours within a region of increasing upper-level diffluence. Data from an earlier reconnaissance mission along with recent scatterometer data suggest that the intensity remains unchanged at 60 kt. The initial motion is a slow northward drift 010/03 kt. Water vapor imagery indicates that an upper-level low has developed east of the DelMarVa peninsula and is dropping to the south of Hermine's surface center. The latest model guidance is forecasting the surface and mid- to upper-level lows to rotate counterclockwise around each other for the next 24-36 hours before becoming vertically stacked by 36-48 hours. This cyclonic interaction has resulted in a significant westward shift in the global model tracks, especially by the UKMET and ECMWF models. As a result, the official forecast has been shifted a little to the west of the previous advisory track, but lies along the extreme eastern side of the guidance envelope in the event that the models shift back to the east since such complex interactions are difficult to forecast from cycle to cycle. The upper-level flow across Hermine is forecast by all of the global models to continue to become more difluent during the next 24 hours, which should aid in the development of deep convection near the low-level center. By 36-48 hours, the deep-layer vertical is expected to decrease from the current 45 kt to less than 10 kt, a pattern that is usually conducive for intensification. However, those favorable upper-level conditions will be negated by much cooler sea-surface temperatures of 23-25 deg C when Hermine will be north of the Gulf Stream at that time. The global models are in very agreement on both Hermine weakening and the 34-kt wind field steadily shrinking by 24 hours and beyond. Further weakening is expected through day 4, and both the ECMWF and GFS show Hermine dissipating by day 5 over the cold waters of the North Atlantic. The initial 50-kt wind radii have been adjusted based on data from the reconnaissance aircraft and an earlier ASCAT overpass. FORECAST POSITIONS AND MAX WINDS INIT 05/0900Z 37.7N 68.3W 60 KT 70 MPH...POST-TROPICAL 12H 05/1800Z 38.0N 69.1W 60 KT 70 MPH...POST-TROPICAL 24H 06/0600Z 38.6N 70.2W 55 KT 65 MPH...POST-TROPICAL 36H 06/1800Z 39.0N 70.4W 45 KT 50 MPH...POST-TROPICAL 48H 07/0600Z 39.7N 70.1W 40 KT 45 MPH...POST-TROPICAL 72H 08/0600Z 40.7N 67.7W 35 KT 40 MPH...POST-TROPICAL 96H 09/0600Z 43.0N 62.7W 30 KT 35 MPH...POST-TROPICAL 120H 10/0600Z...DISSIPATED $$ Forecaster Stewart
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