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Tropical Update – September 24, 2019

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.


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

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