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Tropical Storm Karen Forecast Discussion


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000
WTNT42 KNHC 222037
TCDAT2

Tropical Storm Karen Discussion Number   3
NWS National Hurricane Center Miami FL       AL122019
500 PM AST Sun Sep 22 2019

The convective structure of Karen has continued to lose organization
this afternoon, with the center becoming exposed to the north and
northwest of the primary convective band.  An Air Force
Reconnaissance aircraft investigating the storm has found a
well-defined circulation, and flight-level and SFMR winds to
support an intensity of around 30 kt.  The plane, however, has
not fully sampled the storm so the initial intensity remains
35 kt which is between the wind speeds indicated in earlier ASCAT
data and the recent in situ observations.

The upper-level environment ahead of Karen is forecast to remain
unfavorable with the global models and SHIPS guidance predicting an
increase in northeasterly shear by Monday.  This is expected to
prevent the storm from strengthening, and Karen may even have
trouble maintaining tropical cyclone status while it moves over the
eastern Caribbean Sea.  After the system moves north of Puerto Rico
around midweek, it is forecast to reach more favorable upper-level
conditions, and the NHC intensity forecast again calls for
strengthening later in the forecast period.  The new intensity
forecast, however, is slightly more conservative than the previous
one at days 3-5, and is a little above the latest intensity
consensus (IVCN) model).

The tropical storm is moving west-northwestward or 295/11 kt.  The
track forecast philosophy is unchanged from earlier today.  Karen
should turn northwestward, and then northward during the next day or
so as it begins to respond to a weakness in the subtropical ridge
over the western Atlantic.  This motion should bring the center of
Karen over Puerto Rico or the Virgin Islands on Tuesday. By late in
the forecast period, the dynamical models build a strong
deep-layer ridge over the southeast United States that extends
northeastward over the western Atlantic.  This pattern is likely to
cause Karen to slow its northward progression by days 4 and 5.  The
track guidance remains in very good agreement, and only slight
adjustments to the previous official forecast were required.

Key Messages:
1.  Tropical-storm-force winds and heavy rainfall will continue to
affect the southern Windward Islands through tonight.

2.  Karen is forecast to approach Puerto Rico, and the U.S. and
British Virgin Islands on Tuesday, and a tropical storm watch has
been issued.  Tropical-storm-force winds, heavy rainfall, and flash
flooding is possible on these islands.


FORECAST POSITIONS AND MAX WINDS

INIT  22/2100Z 12.9N  62.7W   35 KT  40 MPH
 12H  23/0600Z 13.7N  63.8W   35 KT  40 MPH
 24H  23/1800Z 14.9N  64.8W   35 KT  40 MPH
 36H  24/0600Z 16.3N  65.4W   35 KT  40 MPH
 48H  24/1800Z 18.0N  65.9W   35 KT  40 MPH
 72H  25/1800Z 21.5N  65.7W   40 KT  45 MPH
 96H  26/1800Z 24.3N  65.5W   45 KT  50 MPH
120H  27/1800Z 26.0N  66.3W   50 KT  60 MPH

$$
Forecaster Brown