[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT43 KNHC 140849
TCDAT3

Hurricane Lee Discussion Number  36
NWS National Hurricane Center Miami FL       AL132023
500 AM AST Thu Sep 14 2023

Lee's satellite presentation has been a little deceiving early this
morning, as earlier reconnaissance flight-level and dropsonde
surface wind data indicated that the hurricane's center was located
about 15-20 n mi to the west of the satellite eye feature.  On the
last couple of passes of the Air Force Reserve C-130 through the
center, the highest 700-mb wind measured was 95 kt, and peak SFMR
readings from both the Air Force and NOAA were 70-75 kt.  Accounting
for some undersampling, Lee's initial intensity is reduced slightly
to 85 kt.

The initial motion remains 350/8 kt.  A faster northward motion,
with some wobbles, is expected during the next 2-3 days as Lee
moves between a mid-tropospheric high over the west-central Atlantic
and a deep-layer trough over the eastern United States.  As noted
in earlier forecasts, a slight bend to the west toward the Gulf
of Maine is likely in 48-60 hours when Lee interacts with the tail
end of the trough over the Mid-Atlantic states.  A turn toward the
north-northeast and then northeast is forecast on days 3 and 4,
bringing Lee's center across Atlantic Canada.  The NHC track
forecast is nearly on top of the previous forecast, largely due to
consistent and tightly clustered model guidance.

The latest shear analyses suggest that moderate southwesterly shear
has begun to affect Lee, and this is confirmed by the offset of
the aircraft fixes relative to the satellite eye feature.  The
shear is forecast to increase further in 24-36 hours, which will
cause Lee to ingest drier and more stable air into its circulation.
In addition, sea surface temperatures along Lee's future path drop
off significantly after 36 hours.  Therefore, continued gradual
weakening is forecast, and the NHC intensity forecast is at or
slightly above the intensity consensus aids for much of the
forecast period.  Extratropical transition is likely to begin on
Friday as Lee interacts with a frontal boundary moving off the east
coast of the United States, and a good chunk of the guidance
suggests the transition could be complete by 60 hours.  In order to
maintain continuity with the previous forecast, Lee is now shown to
be a fully post-tropical cyclone by 72 hours.  Regardless of its
designation, Lee will remain a large and dangerous cyclone while it
approaches eastern New England and Atlantic Canada into the weekend.


KEY MESSAGES:

1. Tropical storm conditions, heavy rainfall, and high surf are
beginning to impact Bermuda, and a Tropical Storm Warning is in
effect for the island.

2. Hurricane conditions and coastal flooding are possible in
portions of eastern Maine, southern New Brunswick, and western Nova
Scotia on Saturday, and a Hurricane Watch is in effect for that
area.  Heavy rainfall in these areas may produce localized urban
and small stream flooding from Friday night into Saturday night.
Tropical storm conditions are possible elsewhere across New England
and Atlantic Canada within the Tropical Storm Watch areas.

3. There is the potential for life-threatening storm surge flooding
in portions of southeastern Massachusetts, including Cape Cod and
Nantucket, late Friday and Saturday, where a Storm Surge Watch is
in effect.


FORECAST POSITIONS AND MAX WINDS

INIT  14/0900Z 29.1N  68.1W   85 KT 100 MPH
 12H  14/1800Z 30.7N  68.2W   80 KT  90 MPH
 24H  15/0600Z 33.3N  67.7W   80 KT  90 MPH
 36H  15/1800Z 36.3N  66.9W   75 KT  85 MPH
 48H  16/0600Z 39.5N  66.6W   70 KT  80 MPH
 60H  16/1800Z 42.5N  66.7W   60 KT  70 MPH
 72H  17/0600Z 45.2N  65.4W   50 KT  60 MPH...POST-TROP/EXTRATROP
 96H  18/0600Z 50.2N  58.1W   40 KT  45 MPH...POST-TROP/EXTRATROP
120H  19/0600Z 54.6N  45.8W   40 KT  45 MPH...POST-TROP/EXTRATROP

$$
Forecaster Berg
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