Post-Tropical Cyclone Idalia Discussion Number 28
NWS National Hurricane Center Miami FL AL102023
1100 AM AST Sat Sep 02 2023
Although the center of Idalia has moved to the southeast of
Bermuda, the island is experiencing the cyclone's strongest winds,
which are located on the northwestern side of the frontal boundary.
Deep convection persists to the northeast of the center, but the
system is still extratropical due to the presence of attached
fronts. The initial wind speed remains 50 kt based on continuity.
Idalia's initial motion is eastward, or 100/8 kt, and a relatively
slow eastward motion is expected to continue today. A shortwave
trough is forecast to move off the coast of New England and cut off
from the mid-latitude flow, which should cause Idalia to accelerate
toward the northeast and then north during the next few days.
Another slow down is likely on days 4 and 5 a couple of hundred
miles southeast of Nova Scotia when Idalia becomes collocated with
the cut-off low. The NHC track forecast is a bit east of the
previous forecast, but not as far east as the HCCA aid.
Even though Idalia is producing what could be considered organized
deep convection, global models fields are fairly unanimous that the
cyclone will not shed its associated frontal boundaries. Since the
most likely scenario is that Idalia will remain an extratropical
cyclone, a transition to a tropical or subtropical cyclone is no
longer indicated in the official forecast during the short term
(although it still can't be completely ruled out). Little change
in strength is anticipated during the next day or two, but gradual
weakening is forecast toward the end of the forecast period when
Idalia goes through another occlusion and fills south of Nova
Scotia.
Even though Idalia is no longer explicitly forecast to transition
to a tropical or subtropical cyclone, NHC advisories will continue
until tropical storm conditions diminish on Bermuda and the
Tropical Storm Warning is discontinued (most likely some time
Sunday morning).
KEY MESSAGES:
1. Tropical storm conditions will continue in Bermuda through early
Sunday.
2. Swells from Idalia are forecast to cause life-threatening surf
and rip current conditions along portions of the east coast of the
United States through Labor Day weekend.
FORECAST POSITIONS AND MAX WINDS
INIT 02/1500Z 31.6N 63.5W 50 KT 60 MPH...POST-TROP/EXTRATROP
12H 03/0000Z 31.7N 62.5W 50 KT 60 MPH...POST-TROP/EXTRATROP
24H 03/1200Z 32.9N 60.6W 50 KT 60 MPH...POST-TROP/EXTRATROP
36H 04/0000Z 34.7N 58.8W 50 KT 60 MPH...POST-TROP/EXTRATROP
48H 04/1200Z 36.8N 57.4W 45 KT 50 MPH...POST-TROP/EXTRATROP
60H 05/0000Z 39.0N 56.9W 45 KT 50 MPH...POST-TROP/EXTRATROP
72H 05/1200Z 40.8N 57.4W 40 KT 45 MPH...POST-TROP/EXTRATROP
96H 06/1200Z 42.3N 58.7W 35 KT 40 MPH...POST-TROP/EXTRATROP
120H 07/1200Z 44.3N 58.6W 30 KT 35 MPH...POST-TROP/EXTRATROP
$$
Forecaster Berg
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