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Hurricane Florence Forecast Discussion Number 58
2018-09-13 22:52:22| National Hurricane Center (Atlantic)
Issued at 500 PM EDT Thu Sep 13 2018 207 WTNT41 KNHC 132052 TCDAT1 Hurricane Florence Discussion Number 58 NWS National Hurricane Center Miami FL AL062018 500 PM EDT Thu Sep 13 2018 The satellite and radar presentations of Florence have changed little this afternoon with a 20-25 nmi wide eye waxing and waning as intrusions of dry air have occasionally eroded the southern and eastern eyewall. Despite the occasional ragged appearance of Florence's eye, reconnaissance aircraft data indicate that the central pressure has remained steady at 955 mb. The last reports from the Air Force Reserve hurricane hunters support an intensity of 85 kt, and this is corroborated by average Doppler velocity values of near 105 kt at 2500-3000 ft ASL. A report of a 10-minute average wind of 59 kt and a gust to 74 kt was recently received from the Cape Lookout C-MAN station (CLKN7). The 59-kt 10-minute wind speed is roughly equivalent to a 65-kt 1-minute wind. Florence has continued to slow down, and radar fixes over the past couple hours suggest that Florence has possibly stalled due to a re-organization of the eye/eyewall. Smoothing through the fixes over the past 6 h yields an initial motion estimate of 295/04 kt. There is no change to the previous forecast reasoning. The ridge to the north and east of Florence remains intact over the Atlantic Ocean, but water vapor imagery and special upper-air observations indicate that a shortwave trough has weakened that portion of the ridge along the U.S. mid-Atlantic coast, resulting in a weakening of the steering currents. The ridge is forecast by all of the latest model guidance to remain intact, albeit weak, for the next 72 hours, which will nudge Florence on a slow westward to west- southwest track into central South Carolina. On days 4 and 5, Florence is expected to become an extratropical low as it interacts with a front while moving northward and northeastward along the Appalachian Mountains. The official forecast track is similar to, but slightly south of the previous advisory track through 72 hours, with little change indicated on days 4 and 5. This scenario closely follows the simple consensus model TVCN/TVCA, which is north of the corrected-consensus models HCCA and FSSE, which are heavily weighing the southernmost model, ECMWF, which keeps Florence over or near the Atlantic through about 48 hours. Radar data indicate that Florence may be developing an outer eyewall. If this trend continues, then little change to the intensity is likely until landfall occurs in about 24 hours due in part to the low vertical wind shear conditions and the warm, deep waters of the Gulfstream current. Florence is expected to weaken after landfall, but the rate of weakening may be tempered somewhat due to much of the hurricane's circulation remaining over the warm waters of the Atlantic Ocean and Gulfstream. Once Florence moves farther inland over central South Carolina, rapid weakening of the inner-core wind field should occur due to land interaction and the cyclone's slow forward speed of 5 kt or less. However, intense rainbands are expected to continue developing over the Atlantic waters and keep moving along the coast and inland, likely producing strong wind gusts through Saturday night. Aircraft and satellite wind data show that Florence remains a large hurricane. Life-threatening storm surge, heavy rainfall, and damaging wind will cover a large area regardless of exactly where the center of Florence moves. Key Messages: 1. A life-threatening storm surge is highly likely along portions of the coastlines of South Carolina and North Carolina, and a Storm Surge Warning is in effect for a portion of this area. The greatest storm surge inundation is expected between Cape Fear and Cape Hatteras, including the Neuse and Pamlico Rivers and western Pamlico Sound. 2. Life-threatening, catastrophic flash flooding and prolonged significant river flooding are likely over portions of the Carolinas and the southern and central Appalachians through early next week, as Florence is expected to slow down as it approaches the coast and moves inland. 3. Damaging hurricane-force winds are likely along portions of the coasts of South Carolina and North Carolina beginning this evening, and a Hurricane Warning is in effect. Strong winds could also spread inland into portions of the Carolinas. 4. Large swells affecting Bermuda, portions of the U.S. East Coast, and the northwestern and central Bahamas will continue this week, resulting in life-threatening surf and rip currents. FORECAST POSITIONS AND MAX WINDS INIT 13/2100Z 33.7N 76.2W 85 KT 100 MPH 12H 14/0600Z 34.1N 77.2W 85 KT 100 MPH 24H 14/1800Z 34.2N 78.2W 75 KT 85 MPH...INLAND 36H 15/0600Z 33.9N 79.0W 50 KT 60 MPH...INLAND 48H 15/1800Z 33.8N 79.9W 35 KT 40 MPH...INLAND 72H 16/1800Z 34.8N 82.3W 25 KT 30 MPH...INLAND 96H 17/1800Z 37.9N 82.6W 20 KT 25 MPH...POST-TROP/INLAND 120H 18/1800Z 42.7N 76.3W 20 KT 25 MPH...POST-TROP/INLAND $$ Forecaster Stewart
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Subtropical Storm Joyce Forecast Discussion Number 5
2018-09-13 22:52:22| National Hurricane Center (Atlantic)
Issued at 500 PM AST Thu Sep 13 2018 206 WTNT45 KNHC 132052 TCDAT5 Subtropical Storm Joyce Discussion Number 5 NWS National Hurricane Center Miami FL AL102018 500 PM AST Thu Sep 13 2018 Joyce has an exposed center this afternoon with no substantial deep convection anywhere nearby. The initial intensity remains 35 kt, consistent with the Hebert-Poteat ST2.5 classification from TAFB as well as the appearance of the very tight low-level center as seen in the GOES-16 visible satellite imagery. As Joyce still remains embedded within an upper-level trough, the system retains the subtropical storm label. Joyce has now been devoid of deep convection for almost a day, due to the dry stable air combined with strong tropospheric vertical shear. As the system is not expected to undergo extratropical transition, without deep convection gradual weakening is anticipated until dissipation in about four days. The official intensity forecast is a blend of the IVCN intensity consensus scheme along with the GFS and ECMWF global model guidance. This new forecast indicates a reduction from the previous advisory, and in fact, Joyce could become a post-tropical cyclone soon if deep convection does not reappear shortly. Joyce is moving toward the southwest at about 5 kt, as it is situated slightly to the west of an upper-level low. In a couple days, a new short wave trough will dive equatorward west of Joyce and begin advecting Joyce toward the northeast. The new NHC track forecast is a bit to the left (southwest) of the previous advisory and is based upon a blend of the HWRF hurricane model and the GFS and ECMWF global models. FORECAST POSITIONS AND MAX WINDS INIT 13/2100Z 33.6N 44.2W 35 KT 40 MPH 12H 14/0600Z 33.1N 44.6W 35 KT 40 MPH 24H 14/1800Z 32.3N 44.7W 35 KT 40 MPH 36H 15/0600Z 32.4N 44.0W 35 KT 40 MPH 48H 15/1800Z 33.2N 43.1W 30 KT 35 MPH 72H 16/1800Z 36.0N 39.5W 25 KT 30 MPH 96H 17/1800Z...DISSIPATED $$ Forecaster Landsea
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Tropical Storm Helene Forecast Discussion Number 26
2018-09-13 22:48:20| National Hurricane Center (Atlantic)
Issued at 500 PM AST Thu Sep 13 2018 615 WTNT43 KNHC 132048 TCDAT3 Tropical Storm Helene Discussion Number 26 NWS National Hurricane Center Miami FL AL082018 500 PM AST Thu Sep 13 2018 Helene has maintained deep convection this afternoon though only in the northern semicircle. Dvorak estimates from TAFB and SAB are continuing to drop, and a blend of these with the CIMSS SATCON supports lowering the intensity to 55 kt. Strong tropospheric vertical shear along with only lukewarm waters are likely contributing toward Helene's weakening. The shear should further increase during the next two days, though the sea surface temperatures will rebound up to 27C along with an increase in low-level moisture. Helene is expected to be either slowly weakening or steady state during this time. Beginning in about three days, Helene will commence baroclinic transition, and it is expected to become an extratropical cyclone by 96 hours. The baroclinic forcing should preclude any additional weakening through the remainder of the forecast period, and the official intensity forecast is nearly the same as the last advisory. This prediction is based on a blend of the stronger HWRF/COAMPS dynamical models and the weaker SHIPS/LGEM statistical schemes for the next couple of days, and is based on a blend of the GFS/ECMWF/UKMET global models at the extended lead times. Helene is moving northward at about 12 kt between the subtropical ridge to its east and a deep-layer trough to its west. The system should accelerate and turn toward the northeast over the next few days as it rounds the subtropical ridge and starts getting picked up by the mid-latitude westerlies. The official track forecast is based upon the tightly clustered variable consensus technique (TVCN) and is nearly the same as the previous forecast. FORECAST POSITIONS AND MAX WINDS INIT 13/2100Z 26.3N 37.0W 55 KT 65 MPH 12H 14/0600Z 28.8N 36.7W 50 KT 60 MPH 24H 14/1800Z 32.3N 35.8W 50 KT 60 MPH 36H 15/0600Z 35.5N 34.2W 50 KT 60 MPH 48H 15/1800Z 37.9N 31.7W 50 KT 60 MPH 72H 16/1800Z 40.6N 24.4W 50 KT 60 MPH 96H 17/1800Z 44.0N 17.5W 50 KT 60 MPH...POST-TROP/EXTRATROP 120H 18/1800Z 51.0N 7.5W 50 KT 60 MPH...POST-TROP/EXTRATROP $$ Forecaster Landsea
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Tropical Storm Isaac Forecast Discussion Number 25
2018-09-13 22:44:21| National Hurricane Center (Atlantic)
Issued at 500 PM AST Thu Sep 13 2018 085 WTNT44 KNHC 132044 TCDAT4 Tropical Storm Isaac Discussion Number 25 NWS National Hurricane Center Miami FL AL092018 500 PM AST Thu Sep 13 2018 The center of Isaac has been mostly exposed during the past several hours as temporary bursts of convection fire just east of the center. Assuming some spin down of the circulation after the plane left, the initial wind speed is set to 35 kt. Since there haven't been any reports of sustained tropical-storm-force winds in the Lesser Antilles and the center is moving away, all watches and warning have been lowered. However, there remains a threat of flash flooding due to a large area of rain that has yet to move through the Lesser Antilles. Strong shear and mid-level dry air is forecast to persist for the next day or so, which should cause Isaac to drop back to tropical depression status on Friday. However, the shear is forecast to weaken by the weekend, and moisture values are expected to rise as the system gets deeper into the Caribbean Sea. These conditions would normally allow for some restregthening, but the initial character of the system is pretty weak, and it is uncertain whether there will be much left of Isaac to take advantage of the more conducive conditions. Model guidance has been inconsistent on the future of Isaac, which speaks to the lack of predictability of this situation. The bulk of the guidance show Isaac staying weak or degenerating into a tropical wave over the next few days, so the official forecast follows that consensus. At this point it is just best to keep an eye on the long-range forecast on each advisory to see if there are any changes. Isaac is moving westward at about 15 kt. A mid-level ridge is forecast to persist over the southwestern Atlantic, steering Isaac westward for the next few days, with a turn to the west-northwest possible at long range. The future track is very uncertain at long range because it is dependent on the intensity. If Isaac degenerates into a wave, the remnants will likely move south and west of the guidance mean, and if the cyclone is stronger, it will probably move slower and more to the right. The new NHC forecast assumes that Isaac will stay weak, and therefore is on the left and faster side of the guidance. FORECAST POSITIONS AND MAX WINDS INIT 13/2100Z 14.9N 63.2W 35 KT 40 MPH 12H 14/0600Z 14.9N 65.4W 35 KT 40 MPH 24H 14/1800Z 14.9N 68.2W 30 KT 35 MPH 36H 15/0600Z 14.9N 70.8W 30 KT 35 MPH 48H 15/1800Z 15.0N 73.0W 30 KT 35 MPH 72H 16/1800Z 16.2N 77.4W 30 KT 35 MPH 96H 17/1800Z 17.5N 81.5W 30 KT 35 MPH 120H 18/1800Z...DISSIPATED $$ Forecaster Blake
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Hurricane Florence Forecast Discussion Number 57
2018-09-13 16:56:19| National Hurricane Center (Atlantic)
Issued at 1100 AM EDT Thu Sep 13 2018 236 WTNT41 KNHC 131456 TCDAT1 Hurricane Florence Discussion Number 57 NWS National Hurricane Center Miami FL AL062018 1100 AM EDT Thu Sep 13 2018 The satellite and radar presentations of Florence have improved somewhat this morning, with a 20-25 nmi wide eye closing off in the radar data from the Morehead City and Wilmington WSR-88D Doppler weather radars. However, an Air Force Reserve reconnaissance aircraft investigating the hurricane this morning has not yet found any flight-level or SFMR winds to support more than about 80 kt at the surface thus far, even though the pressure has decreased to 955 mb. The initial intensity has only been lowered to 90 kt, given that there are peak Doppler velocity values up to 110 kt with average values of 95-97 kt at 15,000 ft in the northern eyewall region, an area of the hurricane that the reconnaissance aircraft has not yet sampled. The upper-level outflow pattern remains quite impressive. Florence has been gradually slowing down this morning, and the initial motion estimate is now 315/09 kt. The subtropical ridge to the northeast and east of Florence is now well-established between Bermuda and the U.S. mid-Atlantic region and extends westward into Virginia and the central Appalachians. This large-scale feature is expected to keep the hurricane moving northwestward today, followed by a turn toward the west at a much slower speed on Friday as the ridge to the north of Florence weakens due to a weak shortwave trough dropping slowly southward from the Ohio Valley. On days 3-5, Florence is forecast turn toward the northwest and north around the western periphery of the subtropical ridge, and move across western South Carolina on Sunday, across western North Carolina and eastern Tennessee on Monday, and then move up the spine of the Appalachians as an extratropical low after the cyclone merges or interacts with a frontal system. The new NHC forecast track is similar to the previous one, and lies close to a blend of the consensus models TVCA, HCCA, and FSSE. Florence is currently approaching the Gulfstream current, and the hurricane is forecast to move over warmer and deeper waters in 6-12 hours, which could allow for some slight strengthening. Just prior to landfall in about 24 hours, Florence is expected to weaken some due to upwelling of the shallow coastal waters. After landfall occurs, rapid weaning of the stronger inner-core wind field is expected to due land interaction and Florence's slow forward speed of 5 kt or less. However, intense rainbands are expected to develop over the Atlantic waters and keep moving along the coast and inland, likely producing strong wind gusts through Saturday night. Aircraft and satellite wind data show that Florence remains a large hurricane. Life-threatening storm surge, heavy rainfall, and damaging wind will cover a large area regardless of exactly where the center of Florence moves. Key Messages: 1. A life-threatening storm surge is now highly likely along portions of the coastlines of South Carolina and North Carolina, and a Storm Surge Warning is in effect for a portion of this area. The greatest storm surge inundation is expected between Cape Fear and Cape Hatteras, including the Neuse and Pamlico Rivers and western Pamlico Sound. 2. Life-threatening, catastrophic flash flooding and prolonged significant river flooding are likely over portions of the Carolinas and the southern and central Appalachians through early next week, as Florence is expected to slow down as it approaches the coast and moves inland. 3. Damaging hurricane-force winds are likely along portions of the coasts of South Carolina and North Carolina as soon as this evening, and a Hurricane Warning is in effect. Strong winds could also spread inland into portions of the Carolinas. 4. Large swells affecting Bermuda, portions of the U.S. East Coast, and the northwestern and central Bahamas will continue this week, resulting in life-threatening surf and rip currents. FORECAST POSITIONS AND MAX WINDS INIT 13/1500Z 33.4N 75.5W 90 KT 105 MPH 12H 14/0000Z 33.9N 76.6W 95 KT 110 MPH 24H 14/1200Z 34.2N 77.8W 85 KT 100 MPH...NEAR THE COAST 36H 15/0000Z 34.1N 78.7W 65 KT 75 MPH...INLAND 48H 15/1200Z 34.0N 79.6W 45 KT 50 MPH...INLAND 72H 16/1200Z 34.3N 81.8W 25 KT 30 MPH...INLAND 96H 17/1200Z 36.9N 83.0W 25 KT 30 MPH...INLAND 120H 18/1200Z 40.3N 79.2W 20 KT 25 MPH...POST-TROP/EXTRATROP $$ Forecaster Stewart
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