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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.
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