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Hurricane Sally Forecast Discussion Number 20
2020-09-16 08:43:26| National Hurricane Center (Atlantic)
Issued at 130 AM CDT Wed Sep 16 2020 000 WTNT44 KNHC 160643 TCDAT4 Hurricane Sally Special Discussion Number 20 NWS National Hurricane Center Miami FL AL192020 130 AM CDT Wed Sep 16 2020 Justification for this special advisory is to increase the initial intensity and the 12-h forecast intensity at landfall. Observations from an Air Force Hurricane Hunter aircraft and Mobile Alabama WSR-88D Doppler radar data indicate that Sally has continued to strengthen this morning. Radar data show that Sally's eye has become better defined and Doppler velocities in the northern eyewall have reached average values of at least 110 kt between 5000-6000 ft ASL, which supports an equivalent surface wind speed of about 89 kt. The reconnaissance aircraft has recently measured peak 700-mb flight-level winds of 103 kt, which equates to about 93 kt using a standard 90 percent adjustment factor, while peak SFMR surface wind speed have been 85 kt. In addition, a recent dropsonde in the northeast eyewall measured an average wind speed of 113 kt in the lowest 150 meters, which equates to about a 94-kt surface wind. However, those winds appeared to be possibly contaminated by wind gusts. Based on the above data, the initial intensity has been increased to 90 kt. Some additional slight strengthening is possible until landfall occur, and Sally could peak at 95 kt. Rapid weakening will occur after the center moves inland, and the system should become a remnant low in a couple of days. This is consistent with the latest model guidance. Radar and aircraft center fixes indicate that Sally's motion is north-northeastward, or 030/02 kt. No changes were made to the previous track forecast. Sally should continue to move north-northeastward this morning, and then turn northeastward with a gradual increase in forward speed by tonight. That motion will then continue for the next day or so. Then, as Sally approaches the westerly flow at higher latitudes, the cyclone should turn toward the east-northeast with a slight further increase in forward speed until becoming a dissipating remnant low near the southeast U.S. coast in 2-3 days. The official forecast is close to the latest corrected dynamical model consensus, HCCA, prediction. KEY MESSAGES: 1. Historic, life-threatening flash flooding due to rainfall is likely through Wednesday along and just inland of the coast from the Florida Panhandle west of the Apalachicola River to the Alabama/Mississippi border. Widespread moderate to major river flooding is forecast across the Florida Panhandle and southern Alabama. Significant flash and urban flooding, as well as widespread minor to moderate river flooding, is likely across inland portions of Mississippi and Alabama, and into Georgia and the western Carolinas this week. 2. Life-threatening storm surge is expected along portions of the coastline from Alabama to the western Florida Panhandle, including Mobile Bay. 3. Hurricane conditions are expected this morning and then continue into this afternoon within portions of the Hurricane Warning area along the Mississippi and Alabama coastlines and the western Florida Panhandle. FORECAST POSITIONS AND MAX WINDS INIT 16/0630Z 29.9N 87.8W 90 KT 105 MPH 12H 16/1200Z 30.3N 87.5W 95 KT 110 MPH 24H 17/0000Z 31.2N 86.8W 40 KT 45 MPH...INLAND 36H 17/1200Z 32.0N 85.7W 30 KT 35 MPH...INLAND 48H 18/0000Z 32.9N 83.9W 25 KT 30 MPH...INLAND 60H 18/1200Z 33.6N 81.5W 20 KT 25 MPH...INLAND 72H 19/0000Z 34.0N 79.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 96H 20/0000Z...DISSIPATED $$ Forecaster Stewart
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