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Tropical Storm Nicholas Forecast Discussion Number 6
2021-09-13 16:36:32| National Hurricane Center (Atlantic)
Issued at 1000 AM CDT Mon Sep 13 2021 000 WTNT44 KNHC 131436 TCDAT4 Tropical Storm Nicholas Discussion Number 6 NWS National Hurricane Center Miami FL AL142021 1000 AM CDT Mon Sep 13 2021 Doppler radar data from Brownsville and Corpus Christi, Texas, along with reconnaissance aircraft flight-level wind data, indicate that Nicholas' inner-core structure has undergone some radical changes since the previous advisory. The earlier near-eyewall pattern dissipated a few hours ago, and has been replaced with what appears to be an ongoing reformation of a new center abut 90 nmi north-northeast of the old center. The aircraft recently found a pressure of 1000 to 1002 mb with the dissipating original center, while Doppler radar velocity data show a pronounced mid- to upper-level circulation forming farther north as previously mentioned. The Air Force Reserve Hurricane Hunter aircraft is going to be concentrating its reconnoiter mission in Nicholas' northeastern quadrant to see if reformation of the low-level center is occurring beneath the mid-level circulation noted in radar data. The initial wind speed remains 50 kt based on recent aircraft 850-mb flight-level winds of 59 kt, which equates to about 47 kt equivalent surface winds. The initial motion is an uncertain 005/10 kt due to the erosion and ongoing reformation of the center. Despite the recent inner-core erosion, the latest NHC model guidance remains in fairly good agreement on Nicholas moving northward through a weakness in the subtropical ridge. The GFS model has been performing exceptionally well with predicting the recent erosion and more northward reformation of the Nicholas' center. The new NHC track forecast is similar to but slightly east or right of the previous advisory track, and lies along the eastern edge of the track consensus model envelope, which is to the left of the GFS track prediction. Although the inner-core convective pattern has been disrupted, recent trends in the radar data suggest that a new center should reform farther northeast into the convective cloud shield. The latest GFS- and ECMWF-based SHIPS shear analyses indicate that westerly shear of near 20 kt is allegedly affecting Nicholas. However, water vapor satellite imagery suggests that the upper-level shear vector is actually from a south-southwesterly direction, which is more along than across the cyclone's forward motion, thus reducing the magnitude and negative effects of the vertical wind shear. Therefore, strengthening is still expected until landfall as Nicholas continues to move over slightly warmer Gulf waters. It is possible that Nicholas could become a hurricane just before landfall, and that's the reason for the northeastward extension of the hurricane watch area. Nicholas should rapidly weaken after landfall due to increased frictional effects, strong southwesterly shear, and entrainment of mid-level dry air, resulting in degeneration into a tropical depression by late Tuesday and a remnant low on Wednesday. Otherwise, no significant changes were made to the previous advisory intensity forecast. Key Messages: 1. Heavy rainfall will impact portions of the Texas and Louisiana coasts through the middle of the week. Significant rainfall amounts are possible, potentially resulting in areas of life-threatening flash and urban flooding, especially in highly urbanized metropolitan areas. Minor to isolated moderate river flooding is also expected. 2. There is the danger of life-threatening storm surge inundation along the coast of Texas from Port Aransas to Sabine Pass. Residents in these areas should follow any advice given by local officials. 3. Nicholas is forecast to approach the middle Texas coast as a strong tropical storm by this evening, and could be near hurricane intensity at landfall. Tropical storm conditions are expected along portions of the middle Texas coast beginning this afternoon, with hurricane conditions possible from Port Aransas to San Luis Pass by late afternoon through tonight. 4. Tropical storm conditions are expected along portions of the coast of south Texas into the afternoon. FORECAST POSITIONS AND MAX WINDS INIT 13/1500Z 26.4N 96.8W 50 KT 60 MPH 12H 14/0000Z 27.5N 96.7W 60 KT 70 MPH 24H 14/1200Z 29.1N 96.3W 45 KT 50 MPH...INLAND 36H 15/0000Z 30.2N 95.4W 30 KT 35 MPH...INLAND 48H 15/1200Z 31.0N 94.0W 25 KT 30 MPH...INLAND 60H 16/0000Z 31.5N 92.5W 20 KT 25 MPH...INLAND 72H 16/1200Z 32.0N 91.1W 20 KT 25 MPH...POST-TROP/REMNT LOW 96H 17/1200Z...DISSIPATED $$ Forecaster Stewart
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