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Tropical Storm Isaac Forecast Discussion Number 20

2018-09-12 16:59:38| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Wed Sep 12 2018 829 WTNT44 KNHC 121459 TCDAT4 Tropical Storm Isaac Discussion Number 20 NWS National Hurricane Center Miami FL AL092018 1100 AM AST Wed Sep 12 2018 GOES-16 1-minute satellite imagery along with NOAA Hurricane Hunter data show that the center of Isaac has become fully exposed on the northwestern side of a shrinking area of deep convection. There is still a lot of wind there, however, with the NOAA plane observing SFMR and adjusted flight-level winds to support 50 kt as an initial intensity. An upper-level trough is dropping southward over the central Atlantic, which has increased the shear this morning over Isaac. This high shear is forecast to remain for the next day or two, and it is possible that Isaac will degrade into a tropical wave during that time, although the area of tropical-storm-force winds will likely not go away for a while. In a couple of days, while the wind shear could decrease over the east-central Caribbean Sea, there might not be much of a system left to take advantage of the more conducive conditions. As a compromise, the official forecast shows a slow weakening, in the direction of the model consensus, and continues to show dissipation after 96 hours in line with the GFS/ECMWF guidance. I wouldn't pay much attention to the long range forecast for now, given the uncertainties in Isaac surviving during the next day or two. The low-level center of Isaac has recently accelerated, moving westward at 15 kt, as it decoupled from the slower mid-level flow. Assuming the system stays vertically coherent, a large mid-tropospheric high is forecast to persist for the next several days. This should keep the system moving westward at a somewhat slower pace throughout the forecast period. There isn't much spread in the guidance, and the official forecast is close to the previous one after accounting for the initial position. The long-range track forecast highly depends on the intensity forecast, so it should be considered low confidence. Key Messages: 1. Isaac is expected to remain at tropical storm intensity when it moves across the Lesser Antilles on Thursday, and tropical storm warnings are in effect for Martinique, Dominica, and Guadeloupe. Tropical storm watches are in effect for Montserrat, St. Kitts and Nevis, Antigua, and Saba and St. Eustatius. Interests on those islands should follow any advice given by their local officials. 2. Life-threatening flash flooding is possible with Isaac. The storm is expected to produce total rainfall accumulations of 2 to 4 inches with isolated amounts up 8 inches across Martinique, Dominica and Guadeloupe. Rainfall amounts of 1 to 2 inches with isolated amounts to 4 inches are expected across Puerto Rico and the southern United States Virgin Islands. FORECAST POSITIONS AND MAX WINDS INIT 12/1500Z 15.0N 54.7W 50 KT 60 MPH 12H 13/0000Z 15.1N 56.7W 50 KT 60 MPH 24H 13/1200Z 15.3N 59.4W 45 KT 50 MPH 36H 14/0000Z 15.3N 62.6W 45 KT 50 MPH 48H 14/1200Z 15.4N 65.6W 40 KT 45 MPH 72H 15/1200Z 15.5N 71.0W 40 KT 45 MPH 96H 16/1200Z 15.5N 75.0W 35 KT 40 MPH 120H 17/1200Z...DISSIPATED $$ Forecaster Blake

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Hurricane Florence Forecast Discussion Number 53

2018-09-12 16:56:38| National Hurricane Center (Atlantic)

Issued at 1100 AM EDT Wed Sep 12 2018 779 WTNT41 KNHC 121456 TCDAT1 Hurricane Florence Discussion Number 53 NWS National Hurricane Center Miami FL AL062018 1100 AM EDT Wed Sep 12 2018 An Air Force Reserve hurricane hunter aircraft investigating Florence this morning has found no change in the hurricane's peak intensity of 115 kt, even though the central pressure had decreased a few millibars down to 943 mb. However, the aircraft data do indicate that Florence's inner-core wind field has expanded, with the 50-kt wind radii now extending outward up to 100 n mi to the northeast. Florence still has a very distinct eye in satellite imagery, but cloud top temperatures have been waxing and waning in the eyewall region, with slight downward trend noted in the past hour or so. In contrast, the upper-level outflow remains impressive and continues to expand everywhere except to the south. Florence is now moving toward the northwest or 305/13 kt. There has been no significant change to the NHC model guidance, including the corrected-consensus models HCCA and FSSE, which are now virtually on top of each other and the simple consensus model TVCA. As a result, no changes were required to the previous NHC track. The shortwave trough over the southern Plains seen in water vapor imagery could end up being a significant factor as it rounds the narrow ridge over the Tennessee Valley and is expected to erode the ridge along the U.S. mid-Atlantic coast on days 3-5. At this time, little change has been made to the NHC track forecast, which remains very close to the aforementioned consensus aids through 72 hours. On the current forecast track, the center of Florence is expected to be near the coasts of southern North Carolina and northern South Carolina in 48 to 72 hours and then drift westward to west- southwestward in weak steering flow. There is still a narrow window of opportunity for Florence to strengthen a little when the cyclone moves over the warmest SSTs and highest upper-ocean heat content while the shear will be the lowest between 0600-1200 UTC tomorrow morning. After that, decreasing ocean heat content along with the slowing forward speed of Florence should cause at least some cold upwelling beneath the hurricane, which should induce a gradual weakening trend. Once Florence reaches the shallow coastal shelf waters in 72 h, land interaction and more significant upwelling are expected, further enhancing the weakening process. The NHC intensity forecast remains near the higher statistical guidance through 48 hours, then follows the trend of the decay SHIPS model after that time. While Florence's maximum winds are expected to weaken a little, it is still expected to remain a dangerous major hurricane as it approaches the coast. The threat to life from storm surge and rainfall will not diminish, and these impacts 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. All interests in these areas should complete preparations and follow any advice given by local officials. 2. Life-threatening, catastrophic flash flooding and significant river flooding is likely over portions of the Carolinas late this week into 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, and a Hurricane Warning is in effect. Strong winds could also spread inland into portions of the Carolinas. 4. Large swells affecting Bermuda and portions of the U.S. East Coast will continue this week, resulting in life-threatening surf and rip currents. FORECAST POSITIONS AND MAX WINDS INIT 12/1500Z 29.8N 71.3W 115 KT 130 MPH 12H 13/0000Z 31.1N 73.1W 120 KT 140 MPH 24H 13/1200Z 32.6N 75.1W 125 KT 145 MPH 36H 14/0000Z 33.5N 76.5W 120 KT 140 MPH 48H 14/1200Z 33.8N 77.4W 105 KT 120 MPH...NEAR THE COAST 72H 15/1200Z 33.6N 78.4W 85 KT 100 MPH...NEAR THE COAST 96H 16/1200Z 33.6N 80.4W 45 KT 50 MPH...INLAND 120H 17/1200Z 34.7N 82.8W 25 KT 30 MPH...INLAND $$ Forecaster Stewart

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Hurricane Helene Forecast Discussion Number 21

2018-09-12 16:33:12| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Wed Sep 12 2018 130 WTNT43 KNHC 121433 TCDAT3 Hurricane Helene Discussion Number 21 NWS National Hurricane Center Miami FL AL082018 1100 AM AST Wed Sep 12 2018 The eye has become less distinct, but the overall convective pattern remains well organized and symmetric. Based on the latest Dvorak estimates, the initial intensity is kept at 80 kt. Most of the guidance show a significant increase in the shear in 24 hours, but the SSTs are not changing much for the next 2 days or so, thus gradual weakening is forecast. After that time, both shear and SSTs will become belligerently unfavorable, resulting in additional weakening. By the end of the forecast period, Helene should have acquired extratropical characteristic. The hurricane is moving toward the north-northwest or 340 degrees at 12 kt, embedded in the southerly flow to the east of the digging trough over the central Atlantic. Since the trough is forecast to amplify, Helene will be forced to turn to the north-northeast and northeast with an increase in forward speed. The global models remain in fairly good agreement on the current forecast track of Helene, and they all bring the cyclone near the Azores in 3 or 4 days. Interests in those islands should closely monitor the progress of Helene over the next several days. FORECAST POSITIONS AND MAX WINDS INIT 12/1500Z 20.3N 36.5W 80 KT 90 MPH 12H 13/0000Z 22.0N 37.2W 70 KT 80 MPH 24H 13/1200Z 24.5N 37.5W 65 KT 75 MPH 36H 14/0000Z 27.5N 37.2W 60 KT 70 MPH 48H 14/1200Z 31.0N 36.5W 50 KT 60 MPH 72H 15/1200Z 37.7N 33.2W 45 KT 50 MPH 96H 16/1200Z 42.5N 26.0W 45 KT 50 MPH 120H 17/1200Z 47.5N 17.5W 45 KT 50 MPH...POST-TROP/EXTRATROP $$ Forecaster Avila

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Hurricane Florence Forecast Discussion Number 52

2018-09-12 10:53:36| National Hurricane Center (Atlantic)

Issued at 500 AM EDT Wed Sep 12 2018 564 WTNT41 KNHC 120853 TCDAT1 Hurricane Florence Discussion Number 52 NWS National Hurricane Center Miami FL AL062018 500 AM EDT Wed Sep 12 2018 The eye of Florence remains very distinct in infrared satellite imagery this morning. There has been little change to the cloud top temperatures surrounding the eye overnight, however, the overall cloud shield and central dense overcast has become slightly more symmetric. An Air Force Reserve Hurricane Hunter aircraft that was in the hurricane until just after midnight measured a peak 700-mb flight-level wind of 130 kt, and a SFMR wind of 107 kt in the northeast eyewall. A blend of these data and recent subjective and objective satellite intensity estimates supports an initial wind speed of 115 kt. Florence will be moving over sea surface temperatures of around 29C and through an area of low vertical wind shear during the next day or so. These conditions favor some strengthening, but eyewall replacement cycles could cause some fluctuations in intensity during that time. After that time, an increase in southwesterly shear, upwelling, and interaction with land is likely to lead to some weakening, however, Florence is expected to remain a dangerous major hurricane as it approaches the coastline. The new NHC intensity forecast is near the higher statistical guidance through 48 hours, then follows the trend of the decay SHIPS model after that time. Florence is moving west-northwestward or 300 degrees at 15 kt. There has been no change to the track forecast or reasoning for the first 36-48 hours, as Florence will continue to be steered west-northwestward to northwestward around the southwestern portion of a mid-level ridge centered northeast of Bermuda. By late Thursday, a mid-level ridge is forecast to begin building over the east-central United States, which is expected to cause Florence to slow down significantly by 48 hours. The track guidance is in good agreement through the first couple of days of the forecast period, and the NHC track forecast again brings the center of the hurricane very close to the coasts of North and South Carolina within 48 hours. Later in the period, the dynamical model spread increases but most of the guidance has continued its southward shift, and now take Florence southwestward near the coast of South Carolina by day 4. The NHC track has been adjusted southward at days 4 and 5, and is a little north of the consensus out of respect for continuity, however, the GFS, ECMWF, and the ECMWF ensemble mean is south of the NHC track forecast, and additional southward adjustment may be warranted in future advisories. It is important for users to realize that significant impacts extend well away from the center of Florence, and serious hazards such as a dangerous storm surge and flooding rains will cover a large area regardless of exactly where the center 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. All interests from South Carolina into the mid-Atlantic region should complete preparations and follow any advice given by local officials. 2. Life-threatening, catastrophic flash flooding and significant river flooding is likely over portions of the Carolinas and Mid-Atlantic states from late this week into 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, and a Hurricane Warning is in effect. Strong winds could also spread inland into portions of the Carolinas. 4. Large swells affecting Bermuda and portions of the U.S. East Coast will continue this week, resulting in life-threatening surf and rip currents. FORECAST POSITIONS AND MAX WINDS INIT 12/0900Z 29.0N 70.1W 115 KT 130 MPH 12H 12/1800Z 30.3N 72.1W 120 KT 140 MPH 24H 13/0600Z 32.0N 74.4W 125 KT 145 MPH 36H 13/1800Z 33.2N 76.1W 120 KT 140 MPH 48H 14/0600Z 33.8N 77.3W 105 KT 120 MPH...NEAR THE COAST 72H 15/0600Z 33.8N 78.2W 85 KT 100 MPH...NEAR THE COAST 96H 16/0600Z 33.6N 79.7W 45 KT 50 MPH...INLAND 120H 17/0600Z 34.2N 82.7W 25 KT 30 MPH...INLAND $$ Forecaster Brown

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Post-Tropical Cyclone Paul Forecast Discussion Number 16

2018-09-12 10:37:06| National Hurricane Center (East Pacific)

Issued at 200 AM PDT Wed Sep 12 2018 641 WTPZ43 KNHC 120837 TCDEP3 Post-Tropical Cyclone Paul Discussion Number 16 NWS National Hurricane Center Miami FL EP182018 200 AM PDT Wed Sep 12 2018 For the first time since 14 August, the northeast Pacific has no tropical cyclones. Paul has lacked organized deep convection for over 12 hours and has therefore been declared a remnant low. The initial intensity is still 30 kt, based on ASCAT data from around 0500 UTC which showed a few wind vectors between 25 and 30 kt, but weakening should resume soon due to the lack of deep convection. The remnant low is moving west at around 8 kt, and a west to west-southwest motion with a decrease in forward speed is expected for the next few days until dissipation occurs later this week. This is the last NHC advisory on Paul. For additional information on the remnant low please see High Seas Forecasts issued by the National Weather Service on the web at https://ocean.weather.gov/shtml/NFDHSFEPI.shtml. FORECAST POSITIONS AND MAX WINDS INIT 12/0900Z 22.3N 127.4W 30 KT 35 MPH...POST-TROPICAL 12H 12/1800Z 22.4N 128.7W 25 KT 30 MPH...POST-TROP/REMNT LOW 24H 13/0600Z 22.4N 130.3W 25 KT 30 MPH...POST-TROP/REMNT LOW 36H 13/1800Z 22.4N 131.3W 25 KT 30 MPH...POST-TROP/REMNT LOW 48H 14/0600Z 22.3N 131.8W 25 KT 30 MPH...POST-TROP/REMNT LOW 72H 15/0600Z 21.8N 132.4W 20 KT 25 MPH...POST-TROP/REMNT LOW 96H 16/0600Z...DISSIPATED $$ Forecaster Zelinsky

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