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Tropical Storm Dorian Discussion Number  12
NWS National Hurricane Center Miami FL       AL052019
500 AM AST Tue Aug 27 2019

Dorian passed over Barbados a few hours ago and it is now very near
the Windward Islands.  Data from the Air Force Hurricane Hunters
earlier this morning suggest that the storm has held steady in
intensity with a blend of the flight level and SFMR winds
supporting a wind speed of 45 kt.  The Caribbean composite
radar data show that Dorian remains a very compact system and that
it still lacks a well-defined inner core.

Dry air in the mid-levels of the atmosphere around Dorian has been
causing the storm to remain in a fairly steady state during the
past day or so despite relatively low wind shear conditions and warm
SSTs.  The models show this dry air persisting in the vicinity of
the system while it tracks across the Caribbean during the next day
or two, so it seems likely that Dorian will continue to only
gradually intensify during that time.  When Dorian nears Hispaniola
in 36 to 48 hours, there will likely be some increase in wind shear
and those less favorable winds aloft and the interaction with the
landmasses of Puerto Rico and Hispaniola should cause considerable
weakening.  Once the system moves north of the Greater Antilles, the
environmental conditions become more favorable for intensification,
and most of the intensity guidance responds by showing an increase
in winds.  However, there remains considerable uncertainty for this
part of the forecast because the future intensity of Dorian will be
quite dependent on how much land interaction there is with the
mountainous island of Hispaniola.  The NHC intensity forecast lies
near the upper end of the guidance in the short term, while the
system is over the Caribbean Sea, but near the middle of the
guidance envelope after that.

The storm is moving west-northwestward at 11 kt steered by a
subtropical ridge to the north.  This general motion should
continue for another 12 to 24 hours taking Dorian across the eastern
Caribbean Sea.  After that time, a slight turn to the northwest is
likely when the cyclone nears the eastern portion of a cut off mid-
to upper-level low and moves toward a weakness in the ridge.  This
should take Dorian across the Dominican Republic and the Mona
Passage and over the Bahamas in the 3 to 4 day time period.  A ridge
is likely to rebuild to the north of Dorian when it is forecast to
be over the Bahamas and that could cause it to turn slightly to the
left by the end of the forecast period.  The new NHC track forecast
is largely unchanged while Dorian is in the Caribbean, but it has
been shifted slightly to the north thereafter. Users are reminded
not to focus on the details of the extended track forecast as the
average 5-day track error is around 200 miles.


Key Messages:

1. Dorian is expected to bring tropical storm conditions to portions
of the Lesser Antilles during the next several hours, where
tropical storm watches and warnings are in effect.  Rainfall of 3 to
6 inches is expected from Martinique to St. Vincent with isolated
totals as high as 10 inches possible.

2. Tropical storm conditions are expected and hurricane conditions
are possible in Puerto Rico on Wednesday, where a tropical storm
warning and hurricane watch have been issued. Rainfall of 2 to 4
inches with maximum totals of 6 inches are possible across Puerto
Rico and St. Croix.

3. Hurricane and Tropical Storm Watches have been issued for
portions of the Dominican Republic.

4. While uncertainty remains high, wind and rain impacts are
possible in the Bahamas and Florida later this week and this
weekend. Residents in these areas should monitor the progress of
Dorian and ensure that they have their hurricane plan in place.


FORECAST POSITIONS AND MAX WINDS

INIT  27/0900Z 13.5N  60.7W   45 KT  50 MPH
 12H  27/1800Z 14.4N  62.3W   50 KT  60 MPH
 24H  28/0600Z 15.7N  64.5W   55 KT  65 MPH
 36H  28/1800Z 17.0N  66.5W   60 KT  70 MPH
 48H  29/0600Z 18.5N  68.2W   60 KT  70 MPH
 72H  30/0600Z 21.7N  71.4W   50 KT  60 MPH
 96H  31/0600Z 24.7N  75.1W   60 KT  70 MPH
120H  01/0600Z 27.1N  79.6W   60 KT  70 MPH

$$
Forecaster Cangialosi

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