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Tropical Storm Karen Discussion Number  11
NWS National Hurricane Center Miami FL       AL122019
500 PM AST Tue Sep 24 2019

The satellite appearance of Karen has improved over the past
several hours, with a large convective band wrapped about halfway
around the center in the western semicircle and an outer band in
the southeastern semicircle.  However, surface observations,
Doppler radar data, and reports from an Air Force Reserve Hurricane
Hunter aircraft indicate that the low-level circulation is elongated
from south to north and that multiple vorticity centers are present.
The lowest pressures and the strongest winds are at the southern
end of the elongation near the eastern end of the convective band,
and this is the center used for this advisory.  The Hurricane Hunter
aircraft reported 53 kt flight-level winds at 850 mb and data
suggesting a central pressure near 1005 mb, so the initial intensity
is increased to 40 kt.

The initial motion is again 360/7, with some uncertainty due to the
multiple vorticity centers and an apparent re-formation of the
center closer to the convection between 15Z-18Z.  Karen remains in a
complex steering environment that includes a low- to mid-level ridge
to the east and northeast, Tropical Storm Jerry to the north-
northwest, and a large mid- to upper-level trough extending from
near Jerry to eastern Cuba.  These weather systems should steer
Karen generally north-northeastward for the next couple of days.
This motion should bring the center of Karen near or over Puerto
Rico and the Virgin Islands during the next few hours and then into
the open Atlantic tonight.  During the 72-120 h period, the
large-scale models forecast a low- to mid-level ridge to build near
Karen, which will significantly slow its forward motion.  The track
guidance is in somewhat better agreement that this ridge will cause
Karen to turn westward near the end of the forecast period,
although there remains a lot of uncertainty about when and how
fast.  The new forecast track keeps Karen a little farther south
before this turn occurs, and thus the 120-h position is a little to
the south of the previous forecast.

Karen is moving into an area of weaker shear, and should remain in
that environment for the next 2-3 days.  Thus, strengthening is
expected, with the main limiting factor being the current poor
organization of the circulation.  From 72-120 h, the storm is
expected to start entraining dry air, which could limit
intensification despite the otherwise favorable shear and sea
surface temperature environment.  In addition, several of the
global models suggest that another round of shear could affect
Karen near the 120 h point.  If this occurs, the cyclone could end
up weaker than forecast in this advisory.  The new intensity
forecast has minor changes from the previous forecast, and it again
lies between the weaker dynamical models and the stronger
statistical-dynamical models.


Key Messages:

1.  Karen is expected to bring heavy rainfall, flash floods and
mudslides to Puerto Rico and the U.S. and British Virgin Islands
today, where a tropical storm warning is in effect.  The rainfall
and potential flooding will likely continue on Wednesday even as the
center of Karen moves away from Puerto Rico and the Virgin Islands.


FORECAST POSITIONS AND MAX WINDS

INIT  24/2100Z 18.0N  65.8W   40 KT  45 MPH
 12H  25/0600Z 19.6N  65.5W   40 KT  45 MPH
 24H  25/1800Z 21.9N  64.9W   45 KT  50 MPH
 36H  26/0600Z 24.2N  64.2W   50 KT  60 MPH
 48H  26/1800Z 25.9N  63.5W   50 KT  60 MPH
 72H  27/1800Z 27.2N  61.9W   55 KT  65 MPH
 96H  28/1800Z 27.0N  62.5W   60 KT  70 MPH
120H  29/1800Z 26.5N  65.5W   60 KT  70 MPH

$$
Forecaster Beven

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