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Post-Tropical Cyclone HERMINE Forecast Discussion Number 26
2016-09-03 22:57:11| National Hurricane Center (Atlantic)
Issued at 500 PM EDT SAT SEP 03 2016 000 WTNT44 KNHC 032057 TCDAT4 POST-TROPICAL CYCLONE HERMINE DISCUSSION NUMBER 26 NWS NATIONAL HURRICANE CENTER MIAMI FL AL092016 500 PM EDT SAT SEP 03 2016 Hermine has the look of an extratropical cyclone this afternoon, with most of the deep convection situated well northeast of the center and dry, subsiding air wrapping around the southern semicircle. This descending air resulted in a band of very strong winds over portions of the Outer Banks of North Carolina today, and those winds along with SFMR data from the aircraft support an initial intensity of 60 kt for this advisory. Hermine has an expansive wind field, with 34-kt winds extending about 200 miles from the center. Hermine is still expected to interact with a shortwave trough in the next couple of days over warm SSTs, which should result in some intensification, although the global models are a little less bullish this cycle. The NHC forecast continues to show Hermine at or near hurricane intensity for the next 72 hours, with slow weakening expected thereafter. After moving due eastward earlier today, the initial motion is now back toward the east-northeast at around 10 kt. Hermine should turn northward and slow to a crawl as it merges with the upper-level trough, and the NHC track shows only 2-3 kt of forward speed from 24 through 96 hours. The spread in the track model guidance has increased this cycle, with the UKMET and GFS now showing more of a westward motion and are slower to begin moving Hermine northeastward. The ECMWF has trended eastward and is much faster, taking Hermine south of Cape Cod in about 4 days, while the GFS and UKMET are still centered offshore of New Jersey at that time. Given the spread, and the possibility of looping motions during the interaction with the upper trough, confidence in the details of the track forecast remains quite low. The new NHC track tries to maintain some continuity with the last one, but lies to the right of the previous one in the first 12-24 hours due to the more easterly motion of Hermine today. The NHC forecast is close to the multi-model consensus through 72 hours. After that time, the NHC track is faster than the new consensus but slower than the previous forecast, reflecting the increasing spread at those times and low confidence in any particular solution. KEY MESSAGES: 1. The slow motion and large wind field associated with Hermine will result in a long duration of hazardous conditions along much of the mid-Atlantic coast extending into southern New England through the holiday weekend and into midweek. 2. Small changes in the meandering track of Hermine could result in large differences in the impacts along the mid-Atlantic coast during the next several days. In addition, there could be multiple occurrences of tropical storm conditions in some locations within the warning area during this time. 3. Although Hermine has become a post-tropical cyclone, NHC will continue to issue its full suite of advisory and warning products as long as the system remains a significant threat to land areas. 4. P-surge, the model that drives the Potential Storm Surge Flooding Graphic, is designed for a wind field typical of a tropical cyclone. The wind field of Hermine is very poorly represented by the P-surge model and as a result, Flooding Graphics prior to today's 11 am EDT advisory understated the inundation risk from the Carolinas northward. The NWS is using the GFS ensemble system instead of P-surge for ongoing issuances of the Potential Storm Surge Flooding Graphic to provide a more realistic depiction of the threat. 5. The Prototype Storm Surge Watch/Warning Graphic accounts for the current wind structure of Hermine, and therefore accurately identifies those areas at risk for life-threatening storm surge. This graphic will also continue to be produced for Hermine. FORECAST POSITIONS AND MAX WINDS INIT 03/2100Z 36.2N 73.3W 60 KT 70 MPH...POST-TROPICAL 12H 04/0600Z 36.8N 72.0W 60 KT 70 MPH...POST-TROPICAL 24H 04/1800Z 37.2N 71.3W 65 KT 75 MPH...POST-TROPICAL 36H 05/0600Z 37.7N 71.6W 65 KT 75 MPH...POST-TROPICAL 48H 05/1800Z 38.0N 72.0W 65 KT 75 MPH...POST-TROPICAL 72H 06/1800Z 38.8N 71.9W 60 KT 70 MPH...POST-TROPICAL 96H 07/1800Z 40.0N 71.0W 55 KT 65 MPH...POST-TROPICAL 120H 08/1800Z 41.0N 68.5W 50 KT 60 MPH...POST-TROPICAL $$ Forecaster Brennan
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