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Hurricane Helene Forecast Discussion Number 23

2018-09-13 04:38:12| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Wed Sep 12 2018 935 WTNT43 KNHC 130238 TCDAT3 Hurricane Helene Discussion Number 23 NWS National Hurricane Center Miami FL AL082018 1100 PM AST Wed Sep 12 2018 Helene's surface circulation has become rather difficult to find this evening in shortwave and enhanced infrared satellite imagery. Based on an earlier SSMI/S microwave pass, the center was removed well to the south of the convective mass. A blend of the Dvorak intensity estimates from TAFB and SAB, as well as a SATCON analysis of 72 kt, yields an initial intensity of 70 kt for this advisory. Global models and the ECMWF/GFS Decay SHIPS models show the southwesterly shear increasing significantly in 24 hours, however, the warm sea surface temperatures are expected to limit the weakening trend some. Beyond the 48 hour period, Helene is expected to traverse cooler waters while the harsh upper wind environment persists. Therefore, additional weakening is forecast at a faster rate. Toward the end of the period, the global models, as well as the Florida State Cyclone Phase Evolution analysis and forecast product, show Helene transitioning to a post-tropical extratropical cyclone as it moves toward Ireland and the United Kingdom. Helene is estimated to be moving northward, or 350/11 kt, within the mid-tropospheric southerly flow produced by an amplifying deep-layer trough over the central Atlantic. At the 48 hour period, the cyclone should turn to the north-northeast followed by a northeastward turn with an increase in forward motion toward the end of the forecast period. The large-scale models, and the HWRF hurricane model, are clustered fairly close together and indicate that Helene will pass near the Azores in about 72 hours. Interests in those islands should closely monitor the progress of Helene over the next several days. The NHC forecast lies in the middle of the guidance envelope and is nudged toward the TVCN multi-model consensus. FORECAST POSITIONS AND MAX WINDS INIT 13/0300Z 22.4N 36.9W 70 KT 80 MPH 12H 13/1200Z 24.4N 37.1W 60 KT 70 MPH 24H 14/0000Z 27.3N 37.0W 55 KT 65 MPH 36H 14/1200Z 30.7N 36.4W 55 KT 65 MPH 48H 15/0000Z 34.3N 35.3W 55 KT 65 MPH 72H 16/0000Z 40.1N 30.5W 50 KT 60 MPH 96H 17/0000Z 43.4N 22.4W 50 KT 60 MPH 120H 18/0000Z 48.7N 12.9W 45 KT 50 MPH...POST-TROP/EXTRATROP $$ Forecaster Roberts

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Tropical Storm Isaac Forecast Discussion Number 21

2018-09-12 22:58:43| National Hurricane Center (Atlantic)

Issued at 500 PM AST Wed Sep 12 2018 658 WTNT44 KNHC 122058 TCDAT4 Tropical Storm Isaac Discussion Number 21 NWS National Hurricane Center Miami FL AL092018 500 PM AST Wed Sep 12 2018 The satellite presentation of Isaac hasn't changed much in the past several hours, with the center exposed on the west side of a re-developing band of convection. On the last pass of the NOAA Hurricane Hunter aircraft, peak flight-level winds were 61 kt and SFMR values were about 50 kt. These data support an initial wind speed estimate of 50 kt. It is interesting to note that the surface center is estimated to be a tenth of a degree or two south of the well-defined swirl on satellite--a function of the fast forward speed and high shear. The shear is forecast to continue for the next day or two, and gradual weakening is anticipated, in line with the model guidance. Given the strong shear, it is possible that Isaac could degenerate into a wave near the Lesser Antilles. After that time, Isaac could escape the upper-level trough over the central Atlantic that has been shearing the cyclone, and find a more conducive environment. More of the model guidance is suggesting some reintensification of the cyclone in the western Caribbean Sea, with the biggest hold-out being the ECMWF. Continuity dictates that the forecast not change as much as the guidance, most of which is showing a hurricane at long range, so the compromise solution is no longer dissipating the cyclone and flat-lining the intensity at long range. It almost goes without saying that this is a low-confidence intensity forecast. Isaac has accelerated to near 17 kt over the past 12 hours, and the shorter-term motion is closer to 20 kt with the convection-less center. A mid-tropospheric high is forecast to persist for the next several days, which should keep the cyclone moving westward, albeit at a slightly slower pace. The model guidance has shifted a bit to the north on this cycle, but is mostly in response to the initial latitude. The NHC track forecast is moved northward as well but still lies on the south side of the consensus. At long range, it makes sense when forecasting a weaker system to stay on the left and faster side of the consensus, so that's where the new NHC prediction lies. 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 12/2100Z 15.4N 56.6W 50 KT 60 MPH 12H 13/0600Z 15.6N 59.0W 50 KT 60 MPH 24H 13/1800Z 15.8N 61.8W 45 KT 50 MPH 36H 14/0600Z 16.0N 64.8W 45 KT 50 MPH 48H 14/1800Z 16.0N 67.6W 40 KT 45 MPH 72H 15/1800Z 16.0N 72.5W 40 KT 45 MPH 96H 16/1800Z 16.5N 76.5W 40 KT 45 MPH 120H 17/1800Z 17.5N 80.5W 40 KT 45 MPH $$ Forecaster Blake

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Hurricane Florence Forecast Discussion Number 54

2018-09-12 22:42:13| National Hurricane Center (Atlantic)

Issued at 500 PM EDT Wed Sep 12 2018 677 WTNT41 KNHC 122042 TCDAT1 Hurricane Florence Discussion Number 54 NWS National Hurricane Center Miami FL AL062018 500 PM EDT Wed Sep 12 2018 Data from an Air Force Reserve hurricane hunter aircraft along with satellite imagery and various intensity estimates indicate that Florence has weakened instead of strengthening. However, while the hurricane hasn't strengthened in terms of peak winds, the inner-core and outer wind fields have continued to expand, resulting in an increase the cyclone's total energy, which will create a significant storm surge event. The upper-level outflow remains impressive and is still expanding except toward the south. Florence is moving toward the northwest or 315/14 kt. The new 12Z global and regional model runs have come into much better agreement on Florence moving steadily northwestward around a strong ridge located between Bermuda and the U.S. mid-Atlantic region for the next 48 hours or so. By late on day 2, Florence is forecast to approach the southern portion of the North Carolina coast, then slow down considerably and turn westward within collapsing steering flow, with a very slow westward motion near the coasts of North and South Carolina continuing into Friday and Saturday. Corrected-consensus models HCCA and FSSE remain very close to each other and are quite similar to the simple consensus model TVCA. Therefore, only a slight eastward shift was needed to the previous forecast track through 36 hours or so, mainly due to the more eastward initial position based on the reconnaissance fixes. At 48 hours and beyond, no significant changes were required to the previous advisory track, which still shows Florence moving slowly westward across South Carolina and western North Carolina on day 4, followed by a slow northward motion up the Appalachian mountain chain on day 5. A narrow window of opportunity remains during the next 24 hours or so for Florence to strengthen a little when the hurricane passes over the warmer SSTs and deeper warm water/higher upper-ocean heat content associated with the Gulf Stream, and low vertical shear conditions of 5-10 kt will aid in any strengthening process. However, significant strengthening is not anticipated due to Florence's large and expanding inner-core wind field. By 36 h and beyond, decreasing ocean heat content along with the slowing forward speed of Florence will likely produce cold upwelling beneath the hurricane, inducing a gradual weakening trend. When Florence moves over the shallow coastal shelf waters in 48-72 h, land interaction and more significant upwelling are anticipated, which should further enhance 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. Although the maximum winds are expected to weaken a little more, Florence 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 prolonged significant river flooding are likely over portions of the Carolinas and the southern and central Appalachians 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, 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 12/2100Z 30.9N 72.5W 105 KT 120 MPH 12H 13/0600Z 32.1N 74.1W 110 KT 125 MPH 24H 13/1800Z 33.4N 75.9W 110 KT 125 MPH 36H 14/0600Z 33.9N 77.1W 105 KT 120 MPH 48H 14/1800Z 34.0N 77.9W 100 KT 115 MPH...NEAR THE COAST 72H 15/1800Z 33.6N 79.2W 70 KT 80 MPH...NEAR THE COAST 96H 16/1800Z 34.0N 81.7W 30 KT 35 MPH...INLAND 120H 17/1800Z 35.6N 83.4W 20 KT 25 MPH...POST-TROP/INLAND $$ Forecaster Stewart

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Hurricane Helene Forecast Discussion Number 22

2018-09-12 22:41:43| National Hurricane Center (Atlantic)

Issued at 500 PM AST Wed Sep 12 2018 994 WTNT43 KNHC 122041 TCDAT3 Hurricane Helene Discussion Number 22 NWS National Hurricane Center Miami FL AL082018 500 PM AST Wed Sep 12 2018 The eye has continued to become less distinct in conventional imagery, but the overall convective pattern remains well organized and symmetric. Based on the latest Dvorak estimates, the initial intensity is lowered to 75 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 highly unfavorable, resulting in additional weakening. By the end of the forecast period, Helene should have acquired extratropical characteristics. The hurricane is moving toward the north or 350 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 about 3 days. Interests in those islands should closely monitor the progress of Helene over the next several days. FORECAST POSITIONS AND MAX WINDS INIT 12/2100Z 21.4N 36.7W 75 KT 85 MPH 12H 13/0600Z 23.1N 37.1W 70 KT 80 MPH 24H 13/1800Z 26.0N 37.2W 60 KT 70 MPH 36H 14/0600Z 29.0N 36.5W 55 KT 65 MPH 48H 14/1800Z 32.5N 35.5W 55 KT 65 MPH 72H 15/1800Z 39.0N 32.0W 50 KT 60 MPH 96H 16/1800Z 43.0N 24.0W 45 KT 50 MPH 120H 17/1800Z 47.5N 14.5W 40 KT 45 MPH...POST-TROP/EXTRATROP $$ Forecaster Avila

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Subtropical Storm Joyce Forecast Discussion Number 1

2018-09-12 22:40:13| National Hurricane Center (Atlantic)

Issued at 500 PM AST Wed Sep 12 2018 973 WTNT45 KNHC 122040 TCDAT5 Subtropical Storm Joyce Discussion Number 1 NWS National Hurricane Center Miami FL AL102018 500 PM AST Wed Sep 12 2018 The strong low pressure system which the NHC has been monitoring in the north Atlantic for a couple of days has developed a cyclonically curved band of deep convection near the center, and scatterometer data indicate that the winds are near 40 kt. Since the system is still vertically stacked with an upper-low, it is then classified as a subtropical cyclone at this time. However, the cyclone is gaining organization while it is acquiring tropical characteristics. The NHC forecast calls for Joyce to transform into a tropical system in about 12 to 24 hours. Since the ocean is warm, some slight strengthening is possible during the next 3 days. After that time, Joyce will be over cooler waters and should then be absorbed by a larger extratropical low. Joyce has been moving southwestward or 225 degrees at 5 kt, steered by the northerly flow around the subtropical high which has been steering Florence and to west of the mid-latitude trough which is forcing Helene to recurve. In a couple of days, global models indicate that the steering pattern will change as the trough amplifies, and Joyce should then turn toward the northeast with an increase in forward speed. FORECAST POSITIONS AND MAX WINDS INIT 12/2100Z 34.3N 41.9W 40 KT 45 MPH 12H 13/0600Z 33.7N 42.6W 40 KT 45 MPH...TROPICAL STORM 24H 13/1800Z 33.0N 43.5W 40 KT 45 MPH 36H 14/0600Z 32.5N 43.5W 45 KT 50 MPH 48H 14/1800Z 32.5N 43.5W 50 KT 60 MPH 72H 15/1800Z 34.0N 41.0W 50 KT 60 MPH 96H 16/1800Z 37.0N 35.0W 40 KT 45 MPH 120H 17/1800Z...ABSORBED $$ Forecaster Avila

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