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Tropical Storm Sally Forecast Discussion Number 7
2020-09-13 11:42:02| National Hurricane Center (Atlantic)
Issued at 500 AM EDT Sun Sep 13 2020 000 WTNT44 KNHC 130941 CCA TCDAT4 Tropical Storm Sally Discussion Number 7...Corrected NWS National Hurricane Center Miami FL AL192020 500 AM EDT Sun Sep 13 2020 Corrected for inland status at 60 and 72 hourrs A large burst of convection has formed near the center of Sally this morning, and Air Force Reserve Hurricane Hunter data indicate that the storm is strengthening. Peak flight-level winds were up to 57 kt at 925 mb, with believable SFMR winds of up to 45 kt, and pressure falling a couple of mb to 1001. The initial wind speed is set to 45 kt for this advisory. While northwesterly shear has been affecting Sally, the global models suggest that the shear will decrease later today as it moves under a narrow ridge. Models all respond to these changing conditions by showing intensification, but they disagree on the rate of change. It is an extremely tricky forecast because of how this might happen close to landfall, but guidance is generally 5-10 kt higher than the last cycle so the intensity forecast has been raised by those amounts up through landfall. It is worth noting that rapid intensification is a possibility if the system develops a more solid inner core on Monday, and stronger solutions like the HWRF model can't be dismissed. Sally is moving west-northwestward at a faster speed this morning. The biggest change to note is that almost every model has shifted westward (faster) due to narrow ridging developing across the northern Gulf Coast between the storm and an exiting mid-latitude trough. The flow gets quite weak near and after landfall, so the cyclone should slow down on Tuesday/Wednesday and turn northward. After that time, the next trough should induce a faster northeastward motion by day 5. The new track is shifted westward between 30-45 n mi near the coast of Louisiana, and is still east of the GFS and ECMWF models. Users are reminded to not focus on the exact details of the track or intensity forecasts, as the average NHC track error at 48 h is around 80-90 miles and the average intensity error is around 10-15 mph. In addition, wind, storm surge, and rainfall hazards will extend far from the center. Unfortunately, confidence is increasing that Sally's expected slow forward speed near the Gulf Coast will exacerbate the storm surge and heavy rainfall threats. KEY MESSAGES: 1. An extremely dangerous and life-threatening storm surge is now expected, and a Storm Surge Warning is in effect for areas outside the southeastern Louisiana Hurricane and Storm Damage Risk Reduction System from Port Fourchon Louisiana to the Mississippi/Alabama border. Residents in these areas should follow any advice given by local officials. 2. Hurricane conditions are expected by early Tuesday from Grand Isle, Louisiana to Ocean Springs, Mississippi, including Metropolitan New Orleans, with tropical storm conditions likely by Monday. Preparations should be rushed to completion in those areas. 3. Tropical Storm Sally is expected to produce flash flooding across southern and central Florida and prolong existing minor river flooding across Central Florida through Monday. Significant flash flooding and minor to isolated major river flooding is likely across portions of the Central Gulf Coast from Monday through the middle of the week. FORECAST POSITIONS AND MAX WINDS INIT 13/0900Z 27.0N 84.0W 45 KT 50 MPH 12H 13/1800Z 27.7N 85.4W 50 KT 60 MPH 24H 14/0600Z 28.3N 87.2W 60 KT 70 MPH 36H 14/1800Z 28.7N 88.7W 70 KT 80 MPH 48H 15/0600Z 29.2N 89.7W 85 KT 100 MPH 60H 15/1800Z 30.1N 90.0W 70 KT 80 MPH...INLAND 72H 16/0600Z 31.0N 90.0W 50 KT 60 MPH...INLAND 96H 17/0600Z 32.8N 88.6W 30 KT 35 MPH...INLAND 120H 18/0600Z 34.5N 84.5W 20 KT 25 MPH...INLAND $$ Forecaster Blake
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