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Tropical Storm HERMINE Forecast Discussion Number 24
2016-09-03 10:55:35| National Hurricane Center (Atlantic)
Issued at 500 AM EDT SAT SEP 03 2016 000 WTNT44 KNHC 030855 TCDAT4 TROPICAL STORM HERMINE DISCUSSION NUMBER 24 NWS NATIONAL HURRICANE CENTER MIAMI FL AL092016 500 AM EDT SAT SEP 03 2016 Satellite, radar and surface observations continue to show that the circulation of Hermine is elongated with most of the convection well removed from the area of lowest pressure. It appears that Hermine has already begun the process of extratropical transition. Based on earlier satellite-derived winds, the initial intensity remains 50 kt. These winds are occurring over water well removed from the center in the southeast quadrant. As indicated in the previous NHC discussion, during the next day or two, Hermine is expected to undergo a complex interaction with a mid- to upper-level baroclinic trough that is developing over the eastern United States. After that time, the dynamical models forecast the upper trough to cut off directly over the surface cyclone, and the surface cyclone could regain some tropical cyclone characteristics, even though it would be under the upper-level low. By then, the strongest winds are expected to be closer to the center. Nevertheless, the dynamical guidance forecasts Hermine to strengthen during this evolution regardless of its final structure, and the NHC forecast is basically an update of the previous one. Given the uncertainty in the structure and evolution, the forecast keeps the cyclone as post-tropical after 24 hours. The initial motion is difficult to estimate given that Hermine's circulation is elongated, but cyclone appears to be moving toward the east-northeast or 060 degrees at 18 kt. The cyclone is forecast to turn more toward the northeast in about 24 hours, and then turn northward with a significant decrease in forward speed as interacts with the upper-level trough, and the steering currents weaken. After day 4, the cyclone should then move eastward with the mid-latitude flow. The track forecast is a blend between the previous NHC one and the multi-model consensus TVCN. KEY MESSAGES: 1. Hermine is expected to become a post-tropical cyclone while still producing hazardous winds and storm surge over land. NWS policy allows NHC to write advisories on and issue tropical storm watches and warnings for post-tropical cyclones, when the system continues to pose a significant threat to life and property. NHC and the NWS Eastern Region have decided that this option will be invoked for Hermine. After Hermine becomes a post-tropical cyclone, NHC will continue to issue its full suite of advisory and warning products for as long as the system remains a significant threat to land. 2. There is still considerable uncertainty as to how many of the characteristics of a tropical cyclone Hermine will have while it is off of the coast of the Mid-Atlantic and New England States. Regardless of its structure, Hermine is expected to be a vigorous storm with a large wind field that will cause wind, storm surge and surf hazards along the coast. FORECAST POSITIONS AND MAX WINDS INIT 03/0900Z 35.4N 76.5W 50 KT 60 MPH...ON THE COAST 12H 03/1800Z 36.5N 74.2W 50 KT 60 MPH...OVER WATER 24H 04/0600Z 37.3N 72.8W 50 KT 60 MPH...POST-TROPICAL 36H 04/1800Z 38.0N 72.5W 55 KT 65 MPH...POST-TROPICAL 48H 05/0600Z 38.5N 72.3W 60 KT 70 MPH...POST-TROPICAL 72H 06/0600Z 39.0N 72.0W 65 KT 75 MPH...POST-TROPICAL 96H 07/0600Z 39.5N 71.5W 55 KT 65 MPH...POST-TROPICAL 120H 08/0600Z 40.5N 70.0W 45 KT 50 MPH...POST-TROPICAL $$ Forecaster Avila
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Post-Tropical Cyclone GASTON Forecast Discussion Number 47
2016-09-03 10:38:06| National Hurricane Center (Atlantic)
Issued at 500 AM AST SAT SEP 03 2016 000 WTNT42 KNHC 030838 TCDAT2 POST-TROPICAL CYCLONE GASTON DISCUSSION NUMBER 47 NWS NATIONAL HURRICANE CENTER MIAMI FL AL072016 500 AM AST SAT SEP 03 2016 Gaston has been devoid of deep convection for more than 12 hours, and is now a swirl of stratocumulus clouds. Since the cyclone is moving over 22-23 deg C waters and embedded in northwesterly shear of about 35 kt, the likelihood that any new convection would reform is slim. Given this, Gaston is being declared a post-tropical remnant low on this advisory. Without any new convection redeveloping the vortex is expected to spin down rather quickly, and global models show Gaston opening up into a trough in about 18 hours. The initial motion estimate is 065/15. The post-tropical cyclone should accelerate east-northeastward at the base of the mid- latitude westerlies today, and then turn northeastward ahead of a trough digging over Atlantic Canada prior to dissipation on Sunday. This is the last advisory issued on Gaston by the National Hurricane Center. Additional information on this system can be found in High Seas Forecasts issued by Meteo France under WMO header FQNT50 LFPW and available on the web at http://www.meteofrance.com/previsions-meteo-marine/bulletin. FORECAST POSITIONS AND MAX WINDS INIT 03/0900Z 40.5N 26.9W 30 KT 35 MPH...POST-TROP/REMNT LOW 12H 03/1800Z 42.4N 24.4W 25 KT 30 MPH...POST-TROP/REMNT LOW 24H 04/0600Z...DISSIPATED $$ Forecaster Kimberlain
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Tropical Storm HERMINE Forecast Discussion Number 23
2016-09-03 05:00:41| National Hurricane Center (Atlantic)
Issued at 1100 PM EDT FRI SEP 02 2016 000 WTNT44 KNHC 030300 TCDAT4 TROPICAL STORM HERMINE DISCUSSION NUMBER 23 NWS NATIONAL HURRICANE CENTER MIAMI FL AL092016 1100 PM EDT FRI SEP 02 2016 Radar imagery and surface observations indicate that the circulation of Hermine has become elongated from east-northeast to west-southwest over the past few hours. This is likely in response to the tropical cyclone encroaching on a developing frontal boundary that extends from eastern North Carolina eastward into the Atlantic. However, buoy reports from the Atlantic southeast of the center remain in the 40-45 kt range, so the initial intensity remains 45 kt. During the forecast period, Hermine is expected to undergo a complex interaction with a mid- to upper-level baroclinic trough that is developing over the eastern United States. During the first 36-48 hours, the cyclone is likely to start extratropical transition as it tries to merge with the frontal boundary. From 48-96 hours, the dynamical models forecast the upper trough to cut off directly over the surface cyclone, and as this happens they forecast the surface cyclone to acquire a structure that resembles a tropical cyclone with the strongest winds close to the center. This suggests the possibility that Hermine could regain some tropical cyclone characteristics even though it would be under the upper-level low. For all of this complexity, the dynamical guidance forecast Hermine to strengthen during this evolution regardless of its final structure, so the new intensity forecast is an update of the previous forecast. Given the uncertainty in the structure and evolution, the forecast keeps the cyclone as post-tropical after 24 hours. The initial motion is 055/19 as Hermine is now embedded in deep-layer southwesterly flow ahead of the above mentioned baroclinic trough. During the next 24-36 hours, the cyclone should decelerate and gradually turn more toward the north. The dynamical models agree that the surface center should make at least a partial cyclonic loop from 48-96 hours as it moves under the upper-level low. After 96 hours, there is spread in the guidance, as the GFS shows a very slow motion while the ECMWF/CMC/UKMET move the system somewhat faster toward the east-northeast. The new forecast track shows a little more bend back toward the west than the previous track, then it is a little slower to move the system to the east-northeast later in the period. It should be noted that the GFS and ECMWF both bring the center of Hermine closer to the coast than the current forecast, and if this trend continues it may require some adjustment to the track in later advisories. KEY MESSAGES: 1. Hermine is expected to become a post-tropical cyclone while still producing hazardous winds and storm surge over land. NWS policy allows NHC to write advisories on and issue tropical storm watches and warnings for post-tropical cyclones, when the system continues to pose a significant threat to life and property. NHC and the NWS Eastern Region have decided that this option will be invoked for Hermine. After Hermine becomes a post-tropical cyclone, NHC will continue to issue its full suite of advisory and warning products for as long as the system remains a significant threat to land. 2. There is considerable uncertainty as to how many of the characteristics of a tropical cyclone Hermine will have while it is off of the coast of the Mid-Atlantic and New England States. Regardless of its structure, Hermine is expected to be a vigorous storm with a large wind field that will cause wind, storm surge and surf hazards along the coast. FORECAST POSITIONS AND MAX WINDS INIT 03/0300Z 34.1N 78.4W 45 KT 50 MPH...INLAND 12H 03/1200Z 35.5N 76.0W 45 KT 50 MPH...INLAND 24H 04/0000Z 36.8N 73.7W 50 KT 60 MPH...OVER WATER 36H 04/1200Z 37.8N 72.6W 55 KT 65 MPH...POST-TROPICAL 48H 05/0000Z 38.4N 72.8W 60 KT 70 MPH...POST-TROPICAL 72H 06/0000Z 38.5N 73.0W 65 KT 75 MPH...POST-TROPICAL 96H 07/0000Z 39.5N 71.5W 60 KT 70 MPH...POST-TROPICAL 120H 08/0000Z 40.5N 70.0W 50 KT 60 MPH...POST-TROPICAL $$ Forecaster Beven
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Tropical Storm GASTON Forecast Discussion Number 46
2016-09-03 04:39:05| National Hurricane Center (Atlantic)
Issued at 1100 PM AST FRI SEP 02 2016 000 WTNT42 KNHC 030238 TCDAT2 TROPICAL STORM GASTON DISCUSSION NUMBER 46 NWS NATIONAL HURRICANE CENTER MIAMI FL AL072016 1100 PM AST FRI SEP 02 2016 A 2222 UTC ASCAT-B pass indicated that Gaston was producing maximum winds of 30-35 kt in its southeastern quadrant. The initial intensity is therefore lowered to 35 kt, but Gaston won't be a tropical storm for much longer. The last bit of deep convection dissipated around 1800 UTC, so the cyclone is likely to be declared post-tropical Saturday morning. The remnant low is likely to maintain 30-kt winds until it dissipates in 36 hours, based on guidance from the global models. The initial motion is 070/14 kt. The remnant low is expected to turn northeastward and accelerate away from the Azores during the next 24 hours ahead of an approaching cold front. The track models agree on this scenario, and the NHC forecast remains close to the TVCN multi-model consensus. FORECAST POSITIONS AND MAX WINDS INIT 03/0300Z 40.1N 28.7W 35 KT 40 MPH 12H 03/1200Z 41.1N 26.5W 30 KT 35 MPH...POST-TROP/REMNT LOW 24H 04/0000Z 43.3N 23.1W 30 KT 35 MPH...POST-TROP/REMNT LOW 36H 04/1200Z...DISSIPATED $$ Forecaster Berg
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Tropical Storm HERMINE Forecast Discussion Number 22
2016-09-02 22:46:06| National Hurricane Center (Atlantic)
Issued at 500 PM EDT FRI SEP 02 2016 000 WTNT44 KNHC 022045 TCDAT4 TROPICAL STORM HERMINE DISCUSSION NUMBER 22 NWS NATIONAL HURRICANE CENTER MIAMI FL AL092016 500 PM EDT FRI SEP 02 2016 While the cloud pattern of Hermine has become elongated north and east of the center, and the central convective tops have warmed, numerous surface observations of 40-kt winds along the South Carolina coast support maintaining an initial intensity of 45 kt for this advisory. While the center of Hermine remains over land for the next 12 to 18 hours, little change in intensity is expected, as the strongest winds should remain over the coastal waters of the Carolinas. After the center moves offshore, intensification is expected through a combination of diabatic and baroclinic processes. The global models continue to show Hermine interacting with a potent upper-level shortwave trough in 2-3 days, and the system is expected to restrengthen to near hurricane force in 48 to 72 hours. As the upper-level forcing moves away, slow weakening is expected later in the period. The NHC intensity forecast is above the tropical cyclone guidance suite and is based largely on global models. The initial motion estimate is 055/17, as Hermine is currently embedded in deep-layer southwesterly flow ahead of a positively- tilted mid-latitude trough. During the next 36 to 48 hours the cyclone should gradually slow down and then turn more poleward as the shortwave trough amplifies over the central Appalachians and mid-Atlantic states. This trough interacts with and superimposes itself on top of Hermine between 48 and 72 hours. Not surprisingly, there are differences in the details of how the model guidance handles the motion of Hermine during this time, with the ECMWF and ECMWF ensemble mean along the western side of the guidance by 72 hours, and the GFS, HWRF, and UKMET farther to the east. Note that the interaction between Hermine and the upper trough could result in some looping motions that are not captured by the 12 and 24 hour spacing of the official NHC forecast points. Late in the forecast period, a northeastward motion is expected as the upper trough slides eastward. Spread continues in the guidance at those times as well, with the HWRF joining the ECMWF on the left side of the guidance envelope at those times. Overall, the new NHC forecast has been adjusted a bit to the left and is close to the middle of the guidance envelope. This track is near the GEFS ensemble mean and a blend of the GFS and ECMWF. Needless to say, small changes in the eventual track of Hermine could result in large changes in impacts along the mid-Atlantic coast. The NHC forecast shows Hermine as a post-tropical cyclone in 36 hours, but model fields do not show it undergoing a full extratropical transition, as it instead appears to acquire a warm seclusion structure with a strong low-level vortex underneath an upper-level low. There is a possibility that the system could regain some tropical characteristics in 3-5 days, but this remains uncertain. It is important to remind users that Hermine is expected to remain a dangerous cyclone through the forecast period regardless of the details of its structure. Based on the latest forecast, tropical storm warnings have been expanded northward along the U.S. mid-Atlantic coast to Sandy Hook, New Jersey. KEY MESSAGE: 1. Hermine is expected to become a post-tropical cyclone while still producing hazardous winds and storm surge over land. NWS policy allows NHC to write advisories on and issue tropical storm watches and warnings for post-tropical cyclones, when the system continues to pose a significant threat to life and property. NHC and the NWS Eastern Region have decided that this option will be invoked for Hermine. After Hermine becomes a post-tropical cyclone, NHC will continue to issue its full suite of advisory and warning products for as long as the system remains a significant threat to land. FORECAST POSITIONS AND MAX WINDS INIT 02/2100Z 33.0N 80.5W 45 KT 50 MPH...INLAND 12H 03/0600Z 34.5N 77.9W 45 KT 50 MPH...INLAND 24H 03/1800Z 36.1N 75.0W 50 KT 60 MPH...OVER WATER 36H 04/0600Z 37.1N 73.1W 55 KT 65 MPH...POST-TROPICAL 48H 04/1800Z 37.8N 72.5W 60 KT 70 MPH...POST-TROPICAL 72H 05/1800Z 38.4N 72.8W 65 KT 75 MPH...POST-TROPICAL 96H 06/1800Z 39.7N 71.0W 60 KT 70 MPH...POST-TROPICAL 120H 07/1800Z 40.5N 69.5W 50 KT 60 MPH...POST-TROPICAL $$ Forecaster Brennan
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