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Tropical Storm Nicholas Forecast Discussion Number 2
2021-09-12 23:01:41| National Hurricane Center (Atlantic)
Issued at 400 PM CDT Sun Sep 12 2021 000 WTNT44 KNHC 122101 TCDAT4 Tropical Storm Nicholas Discussion Number 2 NWS National Hurricane Center Miami FL AL142021 400 PM CDT Sun Sep 12 2021 Visible satellite imagery, scatterometer wind data, and earlier reconnaissance aircraft observations indicate that the circulation of Nicholas is elongated from northwest to southeast. In fact, visible satellite imagery and the aircraft data has shown that there have been several low-level swirls rotating about a mean center. This is not surprising since the tropical cyclone is still in its formative stage. The Air Force plane did not find winds any stronger than they did this morning and the ASCAT data revealed peaks winds of around 30 kt. Given the typical undersampling of the scatterometer instrument, the earlier aircraft data, and peak one-minute wind observations of 31 kt from NOAA buoy 42055 earlier today, the intensity remains 35 kt for this advisory. Nicholas will be moving over the warm waters of the western Gulf of Mexico during the next day or so, and this combined with a moist, unstable atmosphere favors strengthening. The primarily inhibiting factor appears to be moderate south-southwesterly vertical wind shear caused by an upper-level trough over northern Mexico. The trough is forecast to move westward and weaken during the next day or so, which could allow for a more favorable upper-level wind pattern later tonight and Monday. The NHC intensity forecast again calls for strengthening while the system moves toward the northwest Gulf coast, but the main uncertainty regarding the intensity forecast is how much time the cyclone will spend over the Gulf waters. The GFS and HWRF models, which depict a track farther east, show significantly more strengthening than the UKMET and ECMWF models which show a weaker tropical cyclone moving inland over northeastern Mexico or southern Texas much sooner. The NHC intensity forecast is similar to the previous advisory, but indicates a faster rate of strengthening during the next 12-24 hours. Although not explicitly shown in the intensity forecast, Nicholas could approach hurricane strength when it nears the northwest Gulf coast, especially if it moves to the right of the NHC forecast track and spends more time over water. Due to this uncertainty a Hurricane Watch has been issued a for a portion of the Texas coast. The NHC forecast is in best agreement with the SHIPS and HFIP corrected consensus model, but is not as high as the latest HWRF. The center of Nicholas appears to have re-formed farther north since this morning and the initial motion estimate is again a somewhat uncertain 340/12 kt. The track forecast reasoning has not changed from this morning. Nicholas should move north-northwestward to northward during the next day or so around the western portion of a mid-level ridge that is located near the southeast U.S. coast. The latest runs of the various dynamical models have shown typical variability, but the overall guidance envelope has not changed too much through the first 36 hours. The GFS has been the most consistent model and its 12Z run was fairly close to the previous NHC track forecast. Therefore, the NHC track leans along the right side of the guidance envelope between the HWRF and GFS, which are a little to the right of the consensus aids. Due to the acute angle of approach of Nicholas to the coast, users are reminded to not focus on the exact forecast track as small changes in the heading of the cyclone could result in differences in both the location and timing of landfall. Regardless of where Nicholas makes landfall, storm surge, wind, and rainfall impacts are likely over a large portion of northeastern Mexico and Texas coastal areas. After landfall, a slower north-northeastward motion is forecast, and by 72 hours the cyclone is forecast to be located between a couple of mid-level ridges, which will likely result in weaker steering currents and an even slower northeastward motion. By day 5, the global model guidance suggest that the low-level circulation will become an open trough so dissipation is indicated at that time. Key Messages: 1. Periods of heavy rainfall are expected to impact portions of the Texas and Louisiana coasts today through the middle of the week. Significant rainfall amounts are possible, potentially resulting in areas of considerable flash and urban flooding, especially in highly urbanized metropolitan areas. Isolated minor to 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 San Luis 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 late Monday and early Tuesday, and could be near hurricane intensity if it moves to the right of the forecast track and remains over water longer. Tropical storm conditions are expected along portions of the middle Texas coast beginning Monday afternoon, with hurricane conditions possible from Port Aransas to Sargent late Monday and Monday night. 4. Tropical storm conditions are expected along portions of the northeastern coast of Mexico and the coast of south Texas beginning Monday morning. FORECAST POSITIONS AND MAX WINDS INIT 12/2100Z 22.8N 95.5W 35 KT 40 MPH 12H 13/0600Z 24.4N 96.3W 45 KT 50 MPH 24H 13/1800Z 26.7N 96.9W 55 KT 65 MPH 36H 14/0600Z 28.7N 96.8W 55 KT 65 MPH...INLAND 48H 14/1800Z 30.4N 96.3W 35 KT 40 MPH...INLAND 60H 15/0600Z 31.2N 95.8W 30 KT 35 MPH...INLAND 72H 15/1800Z 31.7N 95.2W 25 KT 30 MPH...INLAND 96H 16/1800Z 31.9N 94.7W 20 KT 25 MPH...POST-TROP/REMNT LOW 120H 17/1800Z...DISSIPATED $$ Forecaster Brown
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