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Hurricane Helene Forecast Discussion Number 20

2018-09-12 10:35:06| National Hurricane Center (Atlantic)

Issued at 500 AM AST Wed Sep 12 2018 538 WTNT43 KNHC 120835 TCDAT3 Hurricane Helene Discussion Number 20 NWS National Hurricane Center Miami FL AL082018 500 AM AST Wed Sep 12 2018 Earlier this morning, Helene looked quite ragged in IR imagery. The eye became poorly defined and cloud tops were steadily warming. However, beginning around 0500 UTC, a new burst of convection began wrapping around the previously open western portion of Helene's inner core. Objective and subjective Dvorak current intensity estimates range from 77 to 91 kt, and the initial intensity is therefore set at 80 kt as a blend of all of these data. Despite the fact that Helene appears to be on the upswing at the moment, the cyclone is still passing over sub-26 deg C waters and all of the intensity guidance shows weakening for the next 24 h. Beyond that time, there is still some spread among the intensity models, though it has decreased since the last advisory. Helene will begin interacting with a mid-latitude trough over the north-central Atlantic, which will result in an increase in shear over the hurricane. However, Helene will also begin to move over warmer waters and could get some baroclinic support from the upper-level trough. Overall the intensity guidance is a little lower than before, and now the NHC intensity forecast is near the middle of the intensity guidance envelope. Once again, no meaningful change has been made to the NHC track forecast which continues to closely follow the HFIP Corrected Consensus. Helene is moving north-northwestward, and should gradually turn northward, and eventually northeastward, as the cyclone is steered between a mid-level ridge to the east and the aforementioned mid-latitude trough to northwest. The global models remain in fairly good agreement on the track of Helene, especially through 72-96 h, when Helene is forecast to pass near the Azores, and interests in those islands should closely monitor the progress of Helene over the next several days. FORECAST POSITIONS AND MAX WINDS INIT 12/0900Z 19.2N 35.7W 80 KT 90 MPH 12H 12/1800Z 20.7N 36.6W 75 KT 85 MPH 24H 13/0600Z 22.9N 37.2W 65 KT 75 MPH 36H 13/1800Z 25.5N 37.3W 55 KT 65 MPH 48H 14/0600Z 28.9N 36.9W 55 KT 65 MPH 72H 15/0600Z 36.0N 34.3W 50 KT 60 MPH 96H 16/0600Z 41.0N 28.5W 50 KT 60 MPH 120H 17/0600Z 45.5N 19.0W 50 KT 60 MPH $$ Forecaster Zelinsky

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Tropical Storm Isaac Forecast Discussion Number 19

2018-09-12 10:34:06| National Hurricane Center (Atlantic)

Issued at 500 AM AST Wed Sep 12 2018 274 WTNT44 KNHC 120834 TCDAT4 Tropical Storm Isaac Discussion Number 19 NWS National Hurricane Center Miami FL AL092018 500 AM AST Wed Sep 12 2018 Isaac is a poorly organized tropical storm. Geostationary satellite and microwave images indicate that the center of the cyclone is located near the western edge of the main area of deep convection. This asymmetric cloud pattern is due to about 20 kt of west-northwesterly shear. The satellite intensity estimates have been decreasing, and an average of the Dvorak CI-numbers supports lowering the initial wind speed slightly to 50 kt. The NOAA Hurricane Hunters are scheduled to investigate Isaac later this morning, and that data should provide a better assessment of the storm's intensity and structure. Isaac continues to move due westward at about 13 kt. The track forecast reasoning remains unchanged. A fairly strong mid-level ridge to the north of Isaac should steer the system westward at about the same forward speed during the next several days. The models are now in fairly good agreement, and the NHC track forecast is just a little north of the previous one to come in line with the latest guidance. Based on this forecast, Isaac is expected to cross portions of the Leeward and Windward Islands in about 36 hours. Isaac is feeling the effects of wind shear, and the global models suggest that the shear will likely remain moderate to strong during the next couple of days. These hostile winds combined with nearby dry air will likely cause Isaac to gradually weaken during the next several days. The intensity models are in fair agreement, and this forecast is close to the model consensus, except a little lower than that aid at 96 hours. The latest runs of the GFS and ECMWF models show Isaac opening up into a trough by day 5, and the NHC forecast follows those models. Key Message: 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. FORECAST POSITIONS AND MAX WINDS INIT 12/0900Z 14.5N 53.5W 50 KT 60 MPH 12H 12/1800Z 14.8N 55.7W 50 KT 60 MPH 24H 13/0600Z 15.0N 58.6W 45 KT 50 MPH 36H 13/1800Z 15.2N 61.5W 45 KT 50 MPH 48H 14/0600Z 15.4N 64.8W 40 KT 45 MPH 72H 15/0600Z 15.3N 70.6W 40 KT 45 MPH 96H 16/0600Z 15.2N 75.0W 35 KT 40 MPH 120H 17/0600Z...DISSIPATED $$ Forecaster Cangialosi

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Tropical Storm Isaac Forecast Discussion Number 18

2018-09-12 04:59:36| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Tue Sep 11 2018 374 WTNT44 KNHC 120259 TCDAT4 Tropical Storm Isaac Discussion Number 18 NWS National Hurricane Center Miami FL AL092018 1100 PM AST Tue Sep 11 2018 A slew of recent microwave passes have revealed that Isaac's center is outrunning the deep convection by about a degree due to strengthening westerly shear. With the degradation in structure, Dvorak estimates have fallen to T3.0/3.5 from both TAFB and SAB. The initial intensity is being set at 55 kt to match the CI number (3.5), but I wouldn't be surprised if the actual intensity is closer to the final-T number. Isaac is moving quickly westward, or 270/14 kt. Ridging to the north should keep the cyclone on a similar speed and path for much of the forecast period. The only main difference among the track models is Isaac's future speed, and the NHC official forecast continues to favor the faster models--the GFS, ECMWF, and HCCA models. Since the forecast thinking is unchanged, the updated NHC forecast is not too different from the previous one. Isaac's center is expected to move across the Lesser Antilles near Martinique and Dominica on Thursday. Vertical shear is expected to increase and turn more northwesterly during the next 36 hours as Isaac moves closer to the base of an upper-level trough which extends northeast of the Leeward Islands. Therefore, deep convection is expected to remain decoupled from the low-level center, and Isaac's structure could degrade further over the next day or two. The cyclone's maximum winds are also expected to decrease, and the NHC official forecast is close to the HCCA, Florida State Superensemble, and ICON intensity consensus aids. Even though the statistical-dynamical models and the HWRF restrengthen Isaac by days 4 and 5 when the shear decreases, there may not be much left of the cyclone for any strengthening to occur. Both the GFS and ECMWF global models show Isaac weakening to a remnant low or opening up into a trough by days 4 or 5, which is the scenario favored by the NHC official forecast. As such, dissipation is now shown in the forecast at day 5. Key Message: 1. Isaac is expected to remain at tropical storm intensity when it moves across the Lesser Antilles on Thursday, and tropical storm warnings have been issued 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. FORECAST POSITIONS AND MAX WINDS INIT 12/0300Z 14.5N 52.3W 55 KT 65 MPH 12H 12/1200Z 14.6N 54.7W 50 KT 60 MPH 24H 13/0000Z 14.7N 57.4W 50 KT 60 MPH 36H 13/1200Z 14.9N 60.4W 45 KT 50 MPH 48H 14/0000Z 15.1N 63.7W 45 KT 50 MPH 72H 15/0000Z 15.2N 69.7W 40 KT 45 MPH 96H 16/0000Z 15.0N 74.5W 35 KT 40 MPH 120H 17/0000Z...DISSIPATED $$ Forecaster Berg

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Hurricane Florence Forecast Discussion Number 51

2018-09-12 04:54:04| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Tue Sep 11 2018 176 WTNT41 KNHC 120254 TCDAT1 Hurricane Florence Discussion Number 51 NWS National Hurricane Center Miami FL AL062018 1100 PM AST Tue Sep 11 2018 Florence's eye became a little ragged in appearance on satellite images a couple of hours ago, but recently it has become better defined. There has also been a little cooling of the tops surrounding the eye. These features suggest that Florence is maintaining its intensity. The current intensity is kept at 120 kt, which is a little above the adjusted flight-level winds from the Air Force Hurricane Hunters and a little below the latest SATCON estimate from UW-CIMSS. Over the next couple of days, the hurricane is expected to move over a warm ocean and through an environment of low vertical shear. This would allow some additional strengthening until about 48 hours. From that time and up to landfall, the global models suggest some increase in shear which would cause some weakening. However, Florence is still likely to remain a dangerous major hurricane when its center crosses the coast. The official intensity forecast is a blend of the Decay-SHIPS and ICON intensity model consensus. The initial motion estimate is west-northwestward, or 300/15 kt. A mid-level ridge to the northeast should continue to steer Florence on a west-northwestward to northwestward heading until the hurricane nears the coast. The steering pattern from 72 hours and beyond becomes more complicated and uncertain. The latest GFS model run shows a mid-level ridge building over the east-central United States in 3-4 days. This temporarily blocks the forward progress of the hurricane and forces a southwesterly turn in the model run. Later in the period, the ridge rebuilds to the north of Florence allowing the system to move inland. The official track forecast is somewhat to the left of the previous NHC track, but to the right of the latest consensus predictions. It should be noted that, due to increased model spread, there is substantial uncertainty in the 3-5 track forecast. 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 has been issued for a part of this area. Damaging winds could also spread well inland into portions of the Carolinas and Virginia. 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 12/0300Z 28.4N 68.7W 120 KT 140 MPH 12H 12/1200Z 29.6N 70.8W 125 KT 145 MPH 24H 13/0000Z 31.4N 73.4W 135 KT 155 MPH 36H 13/1200Z 32.9N 75.5W 130 KT 150 MPH 48H 14/0000Z 33.8N 76.8W 115 KT 130 MPH 72H 15/0000Z 34.3N 78.2W 95 KT 110 MPH...INLAND 96H 16/0000Z 34.3N 79.7W 45 KT 50 MPH...INLAND 120H 17/0000Z 34.9N 82.5W 25 KT 30 MPH...INLAND $$ Forecaster Pasch

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Hurricane Helene Forecast Discussion Number 19

2018-09-12 04:33:34| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Tue Sep 11 2018 516 WTNT43 KNHC 120233 TCDAT3 Hurricane Helene Discussion Number 19 NWS National Hurricane Center Miami FL AL082018 1100 PM AST Tue Sep 11 2018 Satellite imagery indicates that Helene is gradually weakening, likely due to cool SSTs of around 25.5 deg C beneath the cyclone. The eye has become a little less distinct and cloud tops associated with the inner-core convection have warmed. The initial intensity has been lowered to 85 kt, based on a blend of Final-T and CI numbers from TAFB. Continued weakening is likely for the next 36 to 48 h since Helene will remain over marginal SSTs, and the intensity guidance is in good agreement. After that time, the cyclone will reach somewhat warmer waters and will begin to interact with an upper-level trough to the northwest. While this interaction will cause an increase of shear over Helene, it could also provide some baroclinic support to help maintain Helene's intensity before it undergoes extratropical transition. Some of the dynamical models suggest that reintensification is possible through this period, while the statistical models quickly weaken the cyclone. For now the official intensity forecast continues to show little change in intensity from 72-120 h and remains close to the various intensity consensus aids, but confidence in this portion of the forecast is lower. Virtually no change has been made to the official track forecast. Helene is currently moving northwestward, and a steady turn northward, and eventually northeastward is expected as the cyclone is steered between a mid-level ridge to the east and a trough to west. All of the global models are in good agreement, with the forward speed of the cyclone being the only notable difference from model to model. Since I have no reason to depart from the consensus at this point, the NHC forecast remains very close to HCCA through day 5. FORECAST POSITIONS AND MAX WINDS INIT 12/0300Z 18.0N 35.4W 85 KT 100 MPH 12H 12/1200Z 19.3N 36.3W 75 KT 85 MPH 24H 13/0000Z 21.2N 37.3W 65 KT 75 MPH 36H 13/1200Z 23.6N 37.7W 60 KT 70 MPH 48H 14/0000Z 26.6N 37.7W 55 KT 65 MPH 72H 15/0000Z 33.7N 35.9W 50 KT 60 MPH 96H 16/0000Z 40.5N 31.0W 50 KT 60 MPH 120H 17/0000Z 45.0N 23.0W 50 KT 60 MPH $$ Forecaster Zelinsky

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