[Tropical] Tropical Cyclone Discussion

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WTNT42 KNHC 250852
TCDAT2

Tropical Storm Karen Discussion Number  13
NWS National Hurricane Center Miami FL       AL122019
500 AM AST Wed Sep 25 2019

Karen's convection has increased significantly and become better
organized since the previous advisory. The well-defined low-level
circulation center that previously had been fully exposed now has
intense convection with cloud tops of -85C to -90C almost completely
encircling the center at times. During the last leg of an earlier
Air Force Reserve reconnaissance mission, an 850-mb flight-level
wind of 46 kt and SFMR surface winds of 37 kt were observed in the
southeastern quadrant, suggesting that Karen may have weakened
slightly during an earlier convective hiatus. However, the recent
increase in deep convection over the center, along with a pronounced
increase in the Doppler velocities and the development of a smaller
radius of maximum winds of less than 10 nmi above 20,000 ft, suggest
that Karen has likely strengthened. For now, however, the intensity
is being maintained at 40 kt. The secondary mid-level circulation
that had developed north of Puerto Rico several hours ago has since
weakened based on San Juan Doppler radar data and satellite images.

Now that deep convection has redeveloped, resulting in a stronger
and deeper the vortex column, the motion has steadied off in a
northerly direction of 360/12 kt over the past 6 hours. A northward
motion is forecast to continue this morning, followed by a turn
toward the north-northeast by this afternoon. A north-northeastward
to northeastward motion accompanied by a gradual decrease in forward
speed is expected in the 12-48 hour period. By 72 hours, steering
currents are forecast to collapse and Karen is expected to stall or
make a clockwise loop. By 96 and beyond, the global models forecast
that a ridge will build eastward from the southeastern United States
to Bermuda, forcing Karen in a slow westward to west-southwestward
direction. The new NHC track forecast is very close to the previous
advisory track, and lies close to the simple consensus track model
TVCN, which is about midway between the corrected-consensus models
FSSE located to the north and NOAA-HCCA to the south.

The intensity forecast remains somewhat perplexing with the
dynamical global and regional models showing no strengthening for
the next 4 days, followed by weakening and even dissipation by day
5, whereas the GFS- and ECMWF-based statistical-dynamical models
show slow but steady strengthening during the forecast period with
Karen becoming a hurricane by 120 hours. The main reasons for global
models weakening the cyclone is due to the low- and upper-level
circulations decoupling in about 3 days, followed by very dry
mid-level overspreading of the low-level circulation, shutting
off convective development. The problem with that scenario is that
Karen will be moving into very low vertical shear conditions and
underneath an upper-level anticyclone by 36 hours, which favors
strengthening since the cyclone will also be sitting over 29 deg C
water of considerable depth. For now, the new official intensity
forecast remains a compromise between the weak dynamical model
solutions and the stronger SHIPS intensity forecasts, which is a
little above the intensity consensus models IVCN, HCCA, and FSSE.

Key Messages:

1.  Karen will continue to produce rainfall causing flash floods and
mudslides across Puerto Rico, Vieques and the U.S. and British
Virgin Islands today even as the center moves away from the region.


FORECAST POSITIONS AND MAX WINDS

INIT  25/0900Z 20.5N  65.2W   40 KT  45 MPH
 12H  25/1800Z 22.3N  64.5W   45 KT  50 MPH
 24H  26/0600Z 24.5N  63.9W   45 KT  50 MPH
 36H  26/1800Z 26.4N  63.2W   50 KT  60 MPH
 48H  27/0600Z 27.3N  62.5W   55 KT  65 MPH
 72H  28/0600Z 27.1N  61.9W   55 KT  65 MPH
 96H  29/0600Z 26.3N  64.4W   55 KT  65 MPH
120H  30/0600Z 25.9N  67.8W   55 KT  65 MPH

$$
Forecaster Stewart
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