ZCZC MIATCDAT2 ALL TTAA00 KNHC DDHHMM Tropical Storm Bill Discussion Number 4 NWS National Hurricane Center Miami FL AL022021 500 AM AST Tue Jun 15 2021 Bill has developed an impressive convective shear pattern to be so far north in latitude. AMSU-A temperature data around 0117 UTC indicated that Bill has a marginal mid-level warm core, suggesting that the cyclone might be a hybrid low pressure system. A late-arriving ASCAT scatterometer pass indicated surface winds up to 42 kt in the southeastern about 40 nmi from the center, plus an expansion of the 34-kt wind field in that quadrant as well. Convection has continued to increase since the time of that earlier ASCAT data as indicated by a 0600Z Dvorak satellite data-T estimate of 3.0/45 kt from TAFB. Therefore, the initial intensity has been increased to 45 kt. Bill's motion is more rapidly toward the northeast now, or 055/27 kt. The latest track forecast is nearly identical to the previous advisory. Bill is embedded within southwesterly flow ahead of an approaching deep-layer trough and associated cold front, and this steering pattern is expected to persist until Bill dissipates as an extratropical low near eastern Newfoundland in 36 h or less. The new NHC track forecast lies down the middle of the very tightly packed guidance model suite. Despite strong southwesterly deep-layer vertical wind shear in excess of 30 kt, Bill has managed to maintain deep convection while passing over marginal sea-surface temperatures (SST) of 26.5 deg C. However, the cyclone will be moving over SSTs near 20 deg C by 12 h and over sharply colder waters thereafter while the wind shear concurrently increases. As a result, Bill is expected to transition to an extratropical low pressure system and possible gale area at or shortly after the 12-h period, followed by dissipation by 36 h when the cyclone will be near eastern Newfoundland. The new intensity forecast is similar to the previous advisory, and closely follows a blend of the IVCN and HCCA consensus models. FORECAST POSITIONS AND MAX WINDS INIT 15/0900Z 38.5N 67.2W 45 KT 50 MPH 12H 15/1800Z 41.0N 62.9W 45 KT 50 MPH 24H 16/0600Z 44.7N 57.6W 40 KT 45 MPH...POST-TROP/EXTRATROP 36H 16/1800Z 48.6N 52.6W 35 KT 40 MPH...POST-TROP/EXTRATROP 48H 17/0600Z...DISSIPATED $$ Forecaster Stewart NNNN
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Page last modified: Friday, 31-Dec-2021 12:09:05 UTC