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Tropical Storm Nicholas Forecast Discussion Number 7
2021-09-13 22:46:13| National Hurricane Center (Atlantic)
Issued at 400 PM CDT Mon Sep 13 2021 000 WTNT44 KNHC 132045 CCA TCDAT4 Tropical Storm Nicholas Discussion Number 7...Corrected NWS National Hurricane Center Miami FL AL142021 400 PM CDT Mon Sep 13 2021 Corrected third paragraph typo Earlier reconnaissance aircraft flight-level wind data, along with recent Doppler radar velocity data from Brownsville and Corpus Christi, indicate that the inner-core wind field has still not consolidated into a single low-level wind center. High-resolution 1-minute GOES-16 visible satellite imagery, radar data, and reconnaissance wind data all indicate at least three small but very tight swirls revolving counter-clockwise around a mean center. An eye feature has tried to form on multiple occasions, only to dissipate after less than half an hour. An Air Force Reserve Hurricane Hunter aircraft sampled the most of the northeastern quadrant of Nicholas' larger circulation this morning and afternoon, and measured 63-67-kt 850-mb flight-level winds, which roughly equals 53-54-kt surface winds in that quadrant; the aircraft also found SFMR surface winds of 50-51 kt in the same area. In addition, Doppler velocity data from Corpus Christi and Brownsville have been indicating average velocities of 59-60 kt between 9,000-10,000 ft near the center, which also equates to about 53-54-kt surface winds. Based on these wind data, the advisory intensity has been increased to 55 kt. The initial motion estimate is 015/10 kt. The new NHC model guidance has come into better agreement on Nicholas moving toward the north- northeast until landfall occurs, now that the 12Z ECMWF model has made a significant eastward shift closer to the previous and current GFS and HWRF model solutions. After landfall, Nicholas is expected to move around the northwestern periphery of a deep-layer subtropical ridge that is oriented east-to-west across the central and eastern Gulf of Mexico. The latest guidance has continued to trend more eastward through 24 hours, followed by a more southward or right-of-track trend thereafter. As a result, the new NHC track forecast has followed suit, and has also been shifted a little to the right of the previous advisory track, and lies between the tightly packed consensus models to the west and the GFS model to the east. Doppler velocity data from the Houston WSR-88D radar has shown a large swath of hurricane-force wind speeds in the northeastern quadrant of Nicholas' circulation above 12,000 ft during the past couple of hours, with brief appearances of average velocities of 80-100 kt at high altitudes. Thus, there is an abundance of large-scale cyclonic vorticity available for another burst of intense convection to tap into, which could allow Nicholas to approach hurricane strength by landfall. This would most likely occur tonight during the convective maximum period near landfall where increased frictional convergence along the coast could aid in the development of convection on the west side of the circulation. After landfall, rapid weakening is expected owing to land interaction, strong southwesterly vertical wind shear in excess of 30 kt, and entrainment of mid-level dry air from the southern Plains. As a result of these negative conditions, Nicholas is forecast to weaken to tropical depression by late Tuesday and degenerate into a remnant low on Wednesday. Key Messages: 1. Heavy rainfall will impact portions of southeastern Texas, Louisiana, and southern Mississippi 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, along with isolated major river flooding across smaller river basins and urban areas. 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 this evening, and could be near hurricane intensity at landfall. Tropical storm conditions are expected along portions of the middle and upper Texas coasts this evening and tonight, with hurricane conditions possible from Port Aransas to San Luis Pass. FORECAST POSITIONS AND MAX WINDS INIT 13/2100Z 27.4N 96.4W 55 KT 65 MPH 12H 14/0600Z 28.7N 96.1W 60 KT 70 MPH...ON THE COAST 24H 14/1800Z 29.9N 95.3W 40 KT 45 MPH...INLAND 36H 15/0600Z 30.4N 94.4W 30 KT 35 MPH...INLAND 48H 15/1800Z 30.9N 92.9W 25 KT 30 MPH...INLAND 60H 16/0600Z 31.1N 91.4W 20 KT 25 MPH...INLAND 72H 16/1800Z 32.0N 90.6W 20 KT 25 MPH...POST-TROP/REMNT LOW 96H 17/1800Z...DISSIPATED $$ Forecaster Stewart
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