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Tropical Storm Isaac Forecast Discussion Number 29

2018-09-14 22:51:59| National Hurricane Center (Atlantic)

Issued at 500 PM AST Fri Sep 14 2018 779 WTNT44 KNHC 142051 TCDAT4 Tropical Storm Isaac Discussion Number 29 NWS National Hurricane Center Miami FL AL092018 500 PM AST Fri Sep 14 2018 It has been a very interesting flight of the NOAA Hurricane Hunter aircraft this afternoon. Dropsonde and SFMR data indicate that the maximum winds are around 35 kt again, and the pressure has fallen to about 1002 mb. However, the system is tilted southward with vertical height due to northerly shear, and scatterometer and Hurricane Hunter tail-Doppler radar data indicate the system is elongated from northeast to southwest, albeit closed. Note that the plane was unable to sample the southwestern quadrant well because of strong convection and intense lightning, so the flight-level winds showing no circulation were unrepresentative. The intensity forecast hasn't gotten any easier with Isaac. Vertical shear is expected to decrease a bit more, which might diminish the effects of dry air seen on water vapor images around the cyclone. However, the GFS and ECMWF continue to insist that Isaac will degenerate into a wave within the next 2 or 3 days, which is almost unanimously supported by their respective ensemble members. This is the solution that the official forecast follows. It should be noted that none of the global models show pressures this low now with Isaac, except the UKMET, and that model is the one that holds onto the circulation the longest. The reality of the situation is that the long-term future of Isaac is cloudy, and the best advice is just to follow NHC advisories every 6 hours to see if we gain any confidence. Isaac is moving westward at about 12 kt. A ridge over the southwestern Atlantic should steer the cyclone westward over the weekend then west-northwestward early next week as Isaac reaches a weaker portion of the ridge. Just like yesterday, a stronger system would likely turn more to the west-northwest earlier, moving closer to Jamaica. Since the system is forecast to be weaker, the forecast lies on the westward and faster side of the guidance, and should also be considered low confidence since it depends on the intensity. Interests in Jamaica should monitor the progress of the Isaac. FORECAST POSITIONS AND MAX WINDS INIT 14/2100Z 15.3N 68.2W 35 KT 40 MPH 12H 15/0600Z 15.4N 70.1W 35 KT 40 MPH 24H 15/1800Z 15.4N 72.2W 35 KT 40 MPH 36H 16/0600Z 15.7N 74.2W 35 KT 40 MPH 48H 16/1800Z 16.2N 76.3W 30 KT 35 MPH 72H 17/1800Z 17.5N 80.0W 25 KT 30 MPH 96H 18/1800Z...DISSIPATED $$ Forecaster Blake

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Tropical Storm Joyce Forecast Discussion Number 9

2018-09-14 22:42:31| National Hurricane Center (Atlantic)

Issued at 500 PM AST Fri Sep 14 2018 959 WTNT45 KNHC 142042 TCDAT5 Tropical Storm Joyce Discussion Number 9 NWS National Hurricane Center Miami FL AL102018 500 PM AST Fri Sep 14 2018 Convection associated with Joyce has increased since the last advisory, with a vigorous band now near the center in the northern semicircle. In response, satellite intensity estimates have edged upward, and the initial intensity is increased to 40 kt as a blend between the TAFB subjective Dvorak estimates and the CIMSS satellite consensus. The cyclone now has good cirrus outflow in the northern semicircle as an upper-level low seen in water vapor imagery passes south of the cyclone. The storm is starting its forecast turn toward the east and the initial motion is now 125/5. As Tropical Storm Helene to the east-northeast of Joyce moves away, Joyce is expected to turn northeastward with an increase in forward speed during the next 24 h, followed by an eastward turn after 48 h. The global models continue to trend faster, and after 48 h there has been a southward shift compared to the previous guidance. Therefore, the new forecast track will be notably faster than the previous track and the 72 h point will be shifted southward. The new track is still a little to the north of the various consensus models. Southwesterly upper-level winds are still forecast to increase over the cyclone during the next day or two, and the forecast track takes Joyce over decreasing sea surface temperatures. This combination should lead to a weakening trend, especially after 24 h. The new intensity forecast is adjusted upward a little from the previous one based on the initial intensity, but it still calls for Joyce to dissipate by 96 h in agreement with most of the global models. It is possible that Joyce could strengthen a little more during the first 12 h while the upper-level low helps keep the stronger shear away from the storm. FORECAST POSITIONS AND MAX WINDS INIT 14/2100Z 31.4N 44.1W 40 KT 45 MPH 12H 15/0600Z 31.6N 43.1W 40 KT 45 MPH 24H 15/1800Z 32.6N 41.1W 40 KT 45 MPH 36H 16/0600Z 33.9N 38.7W 35 KT 40 MPH 48H 16/1800Z 35.0N 35.6W 30 KT 35 MPH 72H 17/1800Z 35.5N 28.5W 30 KT 35 MPH 96H 18/1800Z...DISSIPATED $$ Forecaster Beven

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Tropical Storm Helene Forecast Discussion Number 30

2018-09-14 22:35:59| National Hurricane Center (Atlantic)

Issued at 500 PM AST Fri Sep 14 2018 001 WTNT43 KNHC 142035 TCDAT3 Tropical Storm Helene Discussion Number 30 NWS National Hurricane Center Miami FL AL082018 500 PM AST Fri Sep 14 2018 The 40-45 kt of southwesterly vertical shear has really taken its toll on Helene's cloud pattern. A severe tilt toward the northeast is evident in conventional and microwave imagery. Despite the sheared structure of the cyclone, the initial intensity is held at 60 kt due to the earlier scatterometer pass showing that value in the northeast quadrant, and a 1603 UTC SATCON analysis that yields an intensity estimate of 58 kt. Little change in strength is forecast during the next 12 hours as Helene remains over 26 to 27C sea surface temperatures. Beyond that brief period, the cyclone should weaken a bit as it completes extratropical transition while passing just to the northwest of the Azores Islands and traversing much cooler waters. The European models and the Canadian still show Helene being absorbed by a larger baroclinic system in 4 days, as the cyclone quickly approaches Ireland and the United Kingdom. The GFS, which earlier indicated a different solution resulting in Helene taking a more north- northeastward to northwest track around the periphery of a larger, complex non-tropical low, now shows Helene merging with a front, similar to the European and Canadian scenario. A series of recent microwave passes reveal that Helene's surface circulation has been tracking quite a bit to the left of the previous forecasts. Consequently, an adjustment to the best track was made to conform more with the microwave overpasses. The initial motion is estimated to be northward, or 010/16 kt. It is also quite possible that there maybe some binary involvement between Helene and Tropical Storm Joyce that could be influencing a motion just to the left of track. In any event, a north-northeast to northeast turn is expected over the weekend as the cyclone accelerates within the mid-latitude westerlies. The NHC forecast is again adjusted to the left through 36 hours, due to the much welcomed microwave images, and is in best agreement with TVCN multi-model consensus. Helene is expected to be a post-tropical cyclone when it approaches Ireland and the United Kingdom in a few days. Interests in those locations should consult products from their local meteorological service for information about potential impacts from Helene. Local forecasts and warnings for the UK can be found on the website of the UK Met Office at https://www.metoffice.gov.uk/. Local forecasts and warnings for Ireland can be found on the website of Met Eireann at https://www.met.ie/. FORECAST POSITIONS AND MAX WINDS INIT 14/2100Z 34.1N 36.8W 60 KT 70 MPH 12H 15/0600Z 36.4N 36.0W 60 KT 70 MPH 24H 15/1800Z 38.9N 33.7W 55 KT 65 MPH 36H 16/0600Z 41.0N 30.0W 50 KT 60 MPH 48H 16/1800Z 43.3N 25.2W 50 KT 60 MPH...POST-TROP/EXTRATROP 72H 17/1800Z 49.5N 12.9W 50 KT 60 MPH...POST-TROP/EXTRATROP 96H 18/1800Z...DISSIPATED $$ Forecaster Roberts

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Hurricane Florence Forecast Discussion Number 61

2018-09-14 16:49:57| National Hurricane Center (Atlantic)

Issued at 1100 AM EDT Fri Sep 14 2018 326 WTNT41 KNHC 141449 TCDAT1 Hurricane Florence Discussion Number 61 NWS National Hurricane Center Miami FL AL062018 1100 AM EDT Fri Sep 14 2018 Florence's satellite signature remains very impressive even though the eye is now located just inland over southeastern North Carolina near Cape Fear. However, land interaction has taken its toll on the inner-core circulation, and the previously well-defined eye in radar imagery has shrunk and become filled with rain echoes. The advisory intensity has been lowered to 70 kt based on earlier reports from an Air Force Reserve reconnaissance aircraft, NOAA Doppler weather radar velocity data from Wilmington and Morehead City, and nearby surface observations. The central pressure of 958 mb is based on observations from a NOAA NOS site in Wrightsville Beach and a Weatherflow private station in Federal Point. Another Air Force Hurricane Hunter aircraft will be sampling the portion of Florence's circulation over water during the next few hours. Florence has turned west-southwestward and the motion estimate is now 245/03 kt. Florence is expected to remain embedded within a weak steering flow regime within a weakness in the subtropical ridge, resulting in a slow westward motion for the next 36-48 hours across South Carolina. As a shortwave trough currently located over the eastern Great Lakes to Maryland moves eastward away from the region over the next 2 days, the ridge will begin to build back in and shift eastward, allowing Florence or its remnants to gradually turn northward over the weekend, and then move northeastward into the mid-latitude westerlies as an extratropical low. The new NHC forecast track is similar to the previous advisory, and is close to the middle of the model guidance envelope. Wind data from the earlier aircraft mission, along with Doppler radar velocity data and surface observations, indicate that Florence has weakened. Additional slow weakening is expected today as the center of Florence moves farther inland, with more rapid weakening forecast over the weekend as Florence moves westward across South Carolina. The new NHC intensity forecast is a little lower than the previous advisory, and closely follows the trend of Decay SHIPS model. Although coastal storm surge flooding will gradually subside later today, it cannot be emphasized enough that another serious hazard associated with slow-moving Florence is and will be extremely heavy rainfall. More than 14 inches of rain has already fallen in many areas across southeastern North Carolina, and more rain is still to come, which will cause disastrous flooding that will spread inland through the weekend. Key Messages: 1. A life-threatening storm surge is already occurring along portions of the North Carolina coast and will continue through today and tonight. This surge is also likely along portions of the South Carolina coast. 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 while it moves inland. 3. Damaging hurricane-force winds are occurring along portions of the North Carolina coast and are expected to spread to portions of the South Carolina coast later today. Strong winds could also spread inland into portions of the Carolinas over the next couple of days. 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 14/1500Z 34.0N 78.0W 70 KT 80 MPH...INLAND 12H 15/0000Z 33.9N 78.9W 65 KT 75 MPH...INLAND 24H 15/1200Z 33.8N 79.7W 55 KT 65 MPH...INLAND 36H 16/0000Z 33.9N 80.8W 35 KT 40 MPH...INLAND 48H 16/1200Z 34.7N 82.0W 25 KT 30 MPH...INLAND 72H 17/1200Z 37.7N 83.0W 25 KT 30 MPH...POST-TROP/INLAND 96H 18/1200Z 41.2N 78.4W 20 KT 25 MPH...POST-TROP/INLAND 120H 19/1200Z 44.4N 68.7W 20 KT 25 MPH...POST-TROP/INLAND $$ Forecaster Stewart

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Tropical Storm Joyce Forecast Discussion Number 8

2018-09-14 16:42:30| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Fri Sep 14 2018 716 WTNT45 KNHC 141442 TCDAT5 Tropical Storm Joyce Discussion Number 8 NWS National Hurricane Center Miami FL AL102018 1100 AM AST Fri Sep 14 2018 Joyce has again changed little in organization since the last advisory, with several small bands of convection to the east and northeast of the partly exposed center. The initial intensity remains 35 kt in agreement with the latest satellite estimates from TAFB and the CIMSS satellite consensus. The initial motion is now 180/7. Joyce is currently being steered by the larger and stronger Helene to its east, and a southward to southeastward motion is expected for the next 12 h or so. As Helene passes to the east-northeast of Joyce, Joyce should turn eastward and northeastward with a faster forward speed due to the approach of a mid-latitude trough from the west. The global models have again trended toward a faster northeastward motion after 24 h, and the NHC track forecast has been adjusted to be faster and a little farther to the east. There is no change to the intensity forecast philosophy from the previous advisory. Southwesterly upper-level winds are forecast to increase over the cyclone within the next day or two, which should prevent Joyce from strengthening. The NHC forecast calls for little change in wind speed over the next 36 h, then shows weakening as the shear increases further. The global models mostly dissipate Joyce by day 4, but the system could lose its deep convection and become post-tropical before that time. FORECAST POSITIONS AND MAX WINDS INIT 14/1500Z 31.6N 44.6W 35 KT 40 MPH 12H 15/0000Z 31.3N 44.3W 35 KT 40 MPH 24H 15/1200Z 32.0N 42.9W 35 KT 40 MPH 36H 16/0000Z 33.2N 41.0W 35 KT 40 MPH 48H 16/1200Z 34.6N 38.4W 30 KT 35 MPH 72H 17/1200Z 36.0N 32.0W 30 KT 35 MPH 96H 18/1200Z...DISSIPATED $$ Forecaster Beven

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