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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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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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Tropical Storm Nicholas Forecast Discussion Number 5
2021-09-13 10:50:28| National Hurricane Center (Atlantic)
Issued at 400 AM CDT Mon Sep 13 2021 000 WTNT44 KNHC 130850 TCDAT4 Tropical Storm Nicholas Discussion Number 5 NWS National Hurricane Center Miami FL AL142021 400 AM CDT Mon Sep 13 2021 Radar data from Brownsville shows that the center of Nicholas is on the southwestern side of a large area of deep convection over the western Gulf of Mexico. While southwesterly shear continues to affect the storm, the radar presentation has recently improved, with what could be the start of a partial eyewall forming in the northern quadrant. The initial wind speed remains 50 kt based on earlier aircraft flight-level winds of 59 kt, believable SFMR values up to 50 kt, along with radar winds at 5000 ft near 60 kt. The storm is moving north-northwestward at about 12 kt. Nicholas is forecast to turn northward soon into a weakness in the subtropical ridge. The track prediction is only nudged slightly westward from the previous one through landfall, consistent with recent model guidance. Thereafter, there isn't good agreement among the models on how quickly the tropical cyclone will move northeastward out of Texas. Generally the models are faster this cycle, which seems believable given the large northward re-formation earlier likely exposing Nicholas to stronger mid-latitude flow. Thus the new NHC forecast is trended faster as well, but remains behind the model consensus. Obviously the forward speed is important to the heavy rainfall forecast, and this trend will be one to watch. Nicholas should continue to strengthen up until landfall due primarily to the very warm Gulf waters and the recent inner-core improvements. Moderate southwesterly shear and some dry air are the main inhibiting factors and will hopefully keep the strengthening in check. However, it is possible that Nicholas could become a hurricane before landfall, and that's the reason for the hurricane watch area. Nicholas should weaken after landfall, diminish into a tropical depression within a couple of days, and degenerate into a remnant low in about 3 days. No significant changes were made to the previous NHC wind speed prediction. 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 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 today, and could be near hurricane intensity at landfall. Tropical storm conditions are expected along portions of the middle Texas coast beginning by this afternoon, with hurricane conditions possible from Port Aransas to Freeport this afternoon and tonight. 4. Tropical storm conditions are expected along portions of the northeastern coast of Mexico and the coast of south Texas beginning during the next few hours. FORECAST POSITIONS AND MAX WINDS INIT 13/0900Z 25.5N 96.6W 50 KT 60 MPH 12H 13/1800Z 27.3N 96.8W 60 KT 70 MPH 24H 14/0600Z 29.2N 96.5W 45 KT 50 MPH...INLAND 36H 14/1800Z 30.6N 95.8W 35 KT 40 MPH...INLAND 48H 15/0600Z 31.5N 94.5W 30 KT 35 MPH...INLAND 60H 15/1800Z 32.1N 93.0W 25 KT 30 MPH...INLAND 72H 16/0600Z 32.5N 91.5W 20 KT 25 MPH...POST-TROP/REMNT LOW 96H 17/0600Z...DISSIPATED $$ Forecaster Blake
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Tropical Storm Nicholas Forecast Discussion Number 4
2021-09-13 06:31:23| National Hurricane Center (Atlantic)
Issued at 1130 PM CDT Sun Sep 12 2021 000 WTNT44 KNHC 130431 TCDAT4 Tropical Storm Nicholas Special Discussion Number 4 NWS National Hurricane Center Miami FL AL142021 1130 PM CDT Sun Sep 12 2021 The center of Nicholas has re-formed about 150 n mi to the north-northwest of the previous feature we were tracking, as confirmed by aircraft reconnaissance and Brownsville radar, and this large change necessitates a special advisory. The initial wind speed is set to 45 kt, a blend of the reduced 56-kt flight level winds and 45-50 kt SFMR values recently found by the plane. Because of the re-formation, the track forecast has been accelerated to indicate a landfall about 12 hours sooner than the last advisory. The intensity forecast is about 5 kt stronger in the short term, but actually ends up near the same intensity at landfall as the last advisory because of less time over water. The track forecast is blended with the previous one after landfall, and the intensity forecast is lower after landfall because it is farther inland. No changes to the watches or warnings are required at this time. Key Messages: 1. Periods of heavy rainfall are expected to impact portions of the Texas and Louisiana coasts 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 on Monday, 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, with hurricane conditions possible from Port Aransas to Freeport 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 13/0430Z 24.8N 96.3W 45 KT 50 MPH 12H 13/1200Z 26.0N 96.7W 50 KT 60 MPH 24H 14/0000Z 28.0N 96.6W 55 KT 65 MPH 36H 14/1200Z 29.4N 96.3W 45 KT 50 MPH...INLAND 48H 15/0000Z 30.2N 95.6W 35 KT 40 MPH...INLAND 60H 15/1200Z 30.8N 94.9W 30 KT 35 MPH...INLAND 72H 16/0000Z 31.3N 94.2W 25 KT 30 MPH...INLAND 96H 17/0000Z 31.7N 92.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 120H 18/0000Z...DISSIPATED $$ Forecaster Blake
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Tropical Storm Nicholas Forecast Discussion Number 3
2021-09-13 04:59:47| National Hurricane Center (Atlantic)
Issued at 1000 PM CDT Sun Sep 12 2021 000 WTNT44 KNHC 130259 TCDAT4 Tropical Storm Nicholas Discussion Number 3 NWS National Hurricane Center Miami FL AL142021 1000 PM CDT Sun Sep 12 2021 Nicholas is not yet a well-organized tropical cyclone, with little evidence of convective banding features on satellite imagery. Observations from the Air Force Hurricane Hunter aircraft indicate that the center is still not very well-defined, and appears to have reformed again, this time farther south. Data from the aircraft indicate that the central pressure has not changed much since earlier today, and the maximum winds remain near 35 kt. This is above the latest Dvorak intensity estimates from TAFB and SAB. Nicholas will be traversing very warm waters during the next day or so, and within a moist, unstable atmosphere. These factors would favor strengthening. However, moderate southwesterly shear associated with an upper-level trough over northern Mexico could be an inhibiting factor for strengthening. In spite of the shear, the GFS model forecasts the system to strengthen while it nears the Texas coast. The official intensity forecast is at the upper end of the current model guidance. Given the uncertainties in the future strength of Nicholas, a Hurricane Watch is in effect for a portion of the Texas coast. The current location of Nicholas is quite uncertain since the aircraft-reported center position is within a broad area of light winds and is well south of most of the deep convection. Although the fixes show little motion this evening, based on the model predictions, it appears likely that the center will shift or re-form significantly northward as early as Monday morning. This is at least partially accounted for in the latest NHC track forecast, but some northward adjustments are possible over night. The storm is expected to move toward a weakness in the subtropical ridge during the next 36 hours or so. The track guidance and the model consensus has shifted eastward somewhat since the earlier advisory package. This has necessitated an eastward shift in the official track, and a northeastward extension of the watches and warnings. Key Messages: 1. Periods of heavy rainfall are expected to impact portions of the Texas and Louisiana coasts 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 Freeport 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 13/0300Z 22.5N 95.5W 35 KT 40 MPH 12H 13/1200Z 24.7N 96.5W 45 KT 50 MPH 24H 14/0000Z 27.0N 96.5W 50 KT 60 MPH 36H 14/1200Z 28.6N 96.1W 55 KT 65 MPH...NEAR THE COAST 48H 15/0000Z 29.9N 95.4W 45 KT 50 MPH...INLAND 60H 15/1200Z 30.4N 94.8W 40 KT 45 MPH...INLAND 72H 16/0000Z 31.0N 94.0W 30 KT 35 MPH...INLAND 96H 17/0000Z 31.7N 92.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 120H 18/0000Z...DISSIPATED $$ Forecaster Pasch
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