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| #938491 (Received by flhurricane at: 10:57 AM 04.Sep.2018) TCDAT2 Tropical Storm Gordon Discussion Number 9 NWS National Hurricane Center Miami FL AL072018 1000 AM CDT Tue Sep 04 2018 After an earlier disruption of the inner-core convection due to some southerly wind shear, Gordon has since developed a band of deep convection very near the well-defined center noted in both NOAA GOES-16 high-resolution satellite imagery and coastal NOAA WSR-88D Doppler radars. In addition, there has been a significant increase in lightning activity during the past couple of hours in the inner core, and Doppler radar velocities have increased to more than 45 kt at 25,000 ft in the band of convection near the center. NOAA and Air Force Reserve reconnaissance aircraft further indicate that Gordon has maintained a small radius of maximum winds (RMW) of 10-15 nmi, and that SFMR surface winds of 53-55 kt exist in the northeastern quadrant. These data support an intensity of 55 kt. The initial motion estimate is now toward the northwest at a slightly slower forward, or 305/13 kt. There is basically no change to the previous NHC forecast track reasoning. Reconnaissance fixes continue to fall along the previous forecast track, and the latest model guidance remains tightly packed and on top of the previous NHC forecast. As a result, there is high confidence that Gordon will make landfall along the north-central Gulf of Mexico coast in about 18 hours. After landfall, steering currents are forecast to weaken, resulting in Gordon slowing down considerably, which will enhance the heavy rainfall potential. By Friday, an approaching cold front and associated mid-/upper-level trough are expected to break down the subtropical ridge, allowing Gordon to turn northward and eventually turn northeastward on Saturday and merge with the cold front. The new official track forecast is near the model consensus and essentially on top of the previous NHC track. The aforementioned inner-core changes that have been occurring could be a harbinger that Gordon is finally starting to intensify. Intense lightning activity inside the small RMW is an indication of strong updrafts, which can enhance the strengthening process. Although the GFS and ECMWF models continue to forecast northwesterly shear of 10-15 kt, the explicit upper-level wind fields in the GFS, ECMWF, and UKMET models indicate that the 200-mb flow is forecast to become southeasterly and diffluent across Gordon in the next 12 h or so, which would act to enhance the development of inner-core convection. As result, the official intensity forecast maintains continuity and shows Gordon becoming a hurricane before landfall, which is above the guidance through that time. Once Gordon moves inland, the small circulation should spin down fairly quickly, and that portion of the forecast closely follows the various consensus-model intensity guidance. By 120 h, Gordon`s circulation is expected to merge with a cold front over the Mississippi Valley region. Key Messages: 1. Gordon is expected to bring life-threatening storm surge and hurricane conditions to portions of the central Gulf Coast where a Storm Surge Warning and Hurricane Warning are in effect. Residents in these areas should listen to advice from their local officials. 2. Heavy rainfall from Gordon will affect the western Florida Panhandle, southwest Alabama, southern and central Mississippi, much of eastern Louisiana, and southern Arkansas, where totals could reach as high as 12 inches. This rainfall will cause flash flooding in portions of these areas. FORECAST POSITIONS AND MAX WINDS INIT 04/1500Z 28.5N 86.8W 55 KT 65 MPH 12H 05/0000Z 29.8N 88.5W 65 KT 75 MPH 24H 05/1200Z 31.5N 90.2W 35 KT 40 MPH...INLAND 36H 06/0000Z 32.8N 91.6W 25 KT 30 MPH...INLAND 48H 06/1200Z 33.7N 92.7W 20 KT 25 MPH...INLAND 72H 07/1200Z 35.4N 94.3W 20 KT 25 MPH...POST-TROP/REMNT LOW 96H 08/1200Z 37.7N 93.8W 20 KT 25 MPH...POST-TROP/REMNT LOW 120H 09/1200Z 40.9N 90.7W 20 KT 25 MPH...POST-TROP/EXTRATROP $$ Forecaster Stewart |
