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Tropical Storm Karen Discussion Number   9
NWS National Hurricane Center Miami FL       AL122019
500 AM AST Tue Sep 24 2019

During the last pass through Karen's center around 0430 UTC, the Air
Force reconnaissance aircraft reported some believable SFMR winds of
34-35 kt in the southeastern quadrant and an extrapolated central
pressure of 1006 mb. The 925-mb flight-level height had also
decreased by almost 20 meters in about 2 hours, an indication that
the lower pressure estimate was legitimate. Reflectivity and
velocity data from the San Juan NOAA WSR-88D Doppler radar also
enunciate that Karen's inner-core has become better defined over
the past few hours. Satellite intensity estimates at 0600 UTC were
T2.5/35 kt from TAFB, and T2.4/34 kt and 40 kt from UW-CIMSS ADT and
SATCON, respectively. Based on the recon and satellite intensity
data, Karen has re-strengthened to tropical storm status.

The initial motion is now northward or 360/06 kt. Karen's forecast
track hinges heavily on the future intensity and associated vertical
structure of the cyclone. The GFS, GFS-Legacy, UKMET, and HRWF
dynamical models take a much weaker and more vertically shallow
cyclone northeastward after 48 hours and either continue with that
motion through day 5 or dissipate the system. In contrast, the ECMWF
and many of its stronger ensemble members stall Karen around days
3-4 and then turn the somewhat stronger and deeper cyclone westward
to west-southwestward to the south of a building ridge. Given that
Karen is forecast to be stronger and vertically deeper than the
weaker models, the current track forecast leans more toward the
stronger ECMWF and ECMWF-Ensemble model solutions. The new NHC
forecast is to the right of the previous advisory track and slower,
especially on days 3-5, but the new track does not extend as far
east as the weaker GFS, UKMET, and HWRF models or the consensus
models that incorporate those three models.

None of the dynamical models, including the HWRF and HMON hurricane
models, show much in the way of strengthening once Karen moves
north of Puerto Rico and Virgin Islands except for the ECMWF model,
despite the very low vertical wind shear conditions that the cyclone
will be moving into by 24 hours and beyond. Given Karen's decent
wind field and vertical structure, the expected low shear
conditions, moist mid-level environment, and SSTs of more than 29
deg C for the next 48 hours or so, forecasting at least slow but
steady strengthening seems to be quite reasonable. By days 3-5, the
mid-level environment dries out significantly, which the weaker
models seem to be keying on. However, if Karen strengthens as
currently expected, then the cyclone's robust circulation should
be able to mix out any dry air intrusions, allowing for at least
additional modest intensification to occur in the 72-120 hour
period. The official intensity forecast is a little above the
previous advisory, and is basically an average of the weaker
dynamical models and the stronger GFS- and ECMWF-based
statistical-dynamical models Decay-SHIPS and LGEM.

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  24/0900Z 16.8N  65.8W   35 KT  40 MPH
 12H  24/1800Z 18.1N  65.9W   35 KT  40 MPH
 24H  25/0600Z 20.1N  65.5W   40 KT  45 MPH
 36H  25/1800Z 22.6N  64.7W   45 KT  50 MPH
 48H  26/0600Z 24.6N  64.0W   50 KT  60 MPH
 72H  27/0600Z 27.3N  62.7W   55 KT  65 MPH
 96H  28/0600Z 28.0N  63.0W   60 KT  70 MPH
120H  29/0600Z 27.7N  65.5W   60 KT  70 MPH

$$
Forecaster Stewart

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