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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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