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Tropical Storm Bill Forecast Discussion

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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.


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