[Tropical] Tropical Cyclone Discussion

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WTNT45 KNHC 181449
TCDAT5

Tropical Storm Odette Discussion Number   4
NWS National Hurricane Center Miami FL       AL152021
1100 AM AST Sat Sep 18 2021

Strong shear of about 30 kt from the west-southwest continues to
push all of Odette's deep convection well to the east of the
surface center.  The center itself lacks some definition, with
multiple low-level swirls rotating around a common point.  Odette
is well on its way through the extratropical transition process,
with cold air advection noted on the west side of the circulation,
and a band of overcast clouds on the north side of the developing
warm front.  We're awaiting some new ASCAT data, which should
arrive soon, and for now the initial intensity remains 40 kt based
on continuity.

Odette is being picked up by the mid-latitude westerlies, and it is
moving a little faster toward the northeast, or 055/15 kt.  The
cyclone should turn to the east-northeast and accelerate further
later today, but then a significant slow down is expected on
days 3 through 5 when Odette detaches from the mid-latitude flow
and meanders to the southeast of Newfoundland.  In contrast to
yesterday, the global models are in much better agreement on this
scenario, and the updated NHC track forecast has been hedged in the
direction of the GFEX consensus aids, since the GFS and ECMWF
global models should have a good handle on the behavior of an
extratropical cyclone.

The global models vary slightly on when extratropical
transition will be complete, but the GFS- and ECMWF-based
SHIPS diagnostics indicate that it should happen by this evening
or tonight.  Baroclinic forcing will likely be the main contributor
to Odette's expected strengthening over the next few days, and the
intensity models, including the GFS and ECMWF global models, have
been consistent in showing the peak winds reaching about 55 kt in
36-48 hours.  The post-tropical cyclone is expected to occlude and
lose its upper-air support in about 48 hours, which should lead to
gradual weakening through the end of the 5-day forecast period.

Odette's wind field is expected to expand significantly during the
next few days while the system becomes a strong post-tropical
cyclone.  The post-tropical cyclone is forecast to affect portions
of Newfoundland with strong winds and heavy rainfall Sunday and
Monday.  Please refer to products from the Canadian Hurricane
Centre for additional information on potential impacts in
Newfoundland.


FORECAST POSITIONS AND MAX WINDS

INIT  18/1500Z 38.5N  67.3W   40 KT  45 MPH
 12H  19/0000Z 40.0N  64.2W   45 KT  50 MPH
 24H  19/1200Z 41.8N  59.9W   50 KT  60 MPH...POST-TROP/EXTRATROP
 36H  20/0000Z 42.7N  56.4W   55 KT  65 MPH...POST-TROP/EXTRATROP
 48H  20/1200Z 42.8N  53.1W   55 KT  65 MPH...POST-TROP/EXTRATROP
 60H  21/0000Z 42.2N  50.5W   50 KT  60 MPH...POST-TROP/EXTRATROP
 72H  21/1200Z 41.5N  49.1W   45 KT  50 MPH...POST-TROP/EXTRATROP
 96H  22/1200Z 41.3N  48.7W   40 KT  45 MPH...POST-TROP/EXTRATROP
120H  23/1200Z 42.7N  48.8W   35 KT  40 MPH...POST-TROP/EXTRATROP

$$
Forecaster Berg
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