[Tropical] Tropical Cyclone Discussion

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WTNT45 KNHC 070839
TCDAT5

Hurricane Dorian Discussion Number  56
NWS National Hurricane Center Miami FL       AL052019
500 AM AST Sat Sep 07 2019

Dorian is gradually becoming less organized.  The cyclone is feeling
some effects of southwesterly wind shear with most of its deep
convection located to the north and east of the estimated center.
The initial intensity is lowered a little to 75 kt, which is in good
agreement with the latest Dvorak CI-numbers from TAFB and SAB.  A
pair of ASCAT passes from several hours ago indicate that the wind
field has expanded significantly, and tropical-storm-force winds
have been observed over far southeastern Massachusetts.

Dorian is racing northeastward, with the latest initial motion
estimated to be 050/22 kt.  The hurricane is well embedded in the
fast mid-latitude flow, and it should continue to move quickly
northeastward toward Nova Scotia later today.  After the cyclone
passes Nova Scotia, it is forecast to move through Newfoundland and
Labrador before turning east-northeastward over the far north
Atlantic.  The NHC track forecast is largely an update of the
previous one and not far from the various consensus aids.

Dorian is expected to gradually weaken due to even stronger
southwesterly wind shear and much colder SSTs to the north of
the Gulf Stream current.  The combined influences of these cold
waters and an approaching mid- to upper-level trough should cause
Dorian to lose its tropical characteristics in about 24 hours.
However, the post-tropical cyclone will likely still be producing
hurricane-force winds as it moves through portions of eastern
Canada.  The global models are in good agreement that the
post-tropical cyclone should slowly weaken and ultimately become
absorbed by another extratropical low in 3 to 4 days.  The NHC
intensity forecast is in best agreement with the GFS and ECMWF
models, which typically handle large extratropical lows better that
than the intensity models that are made for tropical cyclones.


Key Messages:

1. Regardless of whether it is a hurricane or a post-tropical
cyclone, Dorian is expected to have a significant impact in portions
of eastern Canada beginning later today. Dangerous storm surge
impacts are likely in portions of the Gulf of St. Lawrence,
southwestern Newfoundland and eastern Nova Scotia. Hurricane-force
winds are also likely in Nova Scotia, Prince Edward Island and
Newfoundland later today and tonight. Refer to information from the
Canadian Hurricane Centre for more information on these hazards.


FORECAST POSITIONS AND MAX WINDS

INIT  07/0900Z 39.7N  68.1W   75 KT  85 MPH
 12H  07/1800Z 42.8N  64.9W   70 KT  80 MPH
 24H  08/0600Z 46.9N  61.6W   65 KT  75 MPH...POST-TROP/EXTRATROP
 36H  08/1800Z 50.0N  58.0W   55 KT  65 MPH...POST-TROP/EXTRATROP
 48H  09/0600Z 52.5N  52.0W   45 KT  50 MPH...POST-TROP/EXTRATROP
 72H  10/0600Z 57.5N  36.0W   40 KT  45 MPH...POST-TROP/EXTRATROP
 96H  11/0600Z...DISSIPATED

$$
Forecaster Cangialosi
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