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TROPICAL STORM ERIKA DISCUSSION NUMBER  15
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL052015
1100 AM AST FRI AUG 28 2015

Morning visible imagery shows that the broad low-level center of
Erika is exposed to the west of the main convective area due to the
effects of 20-25 kt of westerly vertical wind shear.  An Air Force
Reserve Hurricane Hunter aircraft has reported a large area of
40-45 kt surface wind estimates from the Stepped Frequency
Microwave Radiometer and 850 mb flight-level winds as high as 55 kt,
so the initial intensity remains 45 kt.  The central pressure based
on the aircraft reports is 1008 mb.

Now that the center is somewhat easier to locate, the initial motion
is a somewhat more confident 285/16.  A generally west-northwestward
motion is expected for the next 48 hours or so as Erika moves around
the southwestern periphery of the subtropical ridge. After that
time, a northwestward and northward turn and a decrease in forward
speed are forecast as Erika moves between the ridge and a mid- to
upper-level trough over the Gulf of Mexico.  The track guidance
continues to show some spread based on the forecast strength of
Erika.  The models with a stronger cyclone, such as the GFDL and
GFS, show a faster northward turn and are on the eastern side of the
guidance envelope.  On the other hand, the ECMWF forecasts a weaker
storm and lies on the left side of the envelope.  An extra
complication is that passage over Hispaniola is likely to disrupt
the circulation, with the possibility that the center could reform
somewhere northwest or west of the island.  Overall, the guidance
envelope has shifted a little to the west since the previous
advisory, and the new forecast track shows a similar nudge through
96 hours.  The guidance is also showing a faster forward speed than
six hours ago, so the new track is also a little faster than the
previous track.

Westerly to southwesterly vertical wind shear is forecast to
continue or increase during the next 12 to 24 hours.  This,
combined with land interaction, suggests that the cyclone should
weaken, and there is a chance the system could degenerate to a
tropical wave while crossing Hispaniola.  Assuming the cyclone
survives, the shear should decrease some after 36 hours, which
could allow some strengthening up to the time of possible landfall
on the Florida Peninsula.  The new intensity forecast is an update
of the previous forecast, and like the previous forecast it is low
confidence.

The greatest short-term threat posed by Erika continues to be
very heavy rainfall over portions of the Dominican Republic and
Haiti today and tonight. These rains could produce flash floods and
mud slides.


FORECAST POSITIONS AND MAX WINDS

INIT  28/1500Z 17.7N  69.4W   45 KT  50 MPH
 12H  29/0000Z 19.3N  71.4W   40 KT  45 MPH...INLAND
 24H  29/1200Z 20.8N  74.5W   40 KT  45 MPH...OVER WATER
 36H  30/0000Z 22.3N  77.3W   40 KT  45 MPH
 48H  30/1200Z 23.5N  79.2W   45 KT  50 MPH
 72H  31/1200Z 26.0N  81.5W   50 KT  60 MPH...INLAND
 96H  01/1200Z 28.5N  82.0W   40 KT  45 MPH...INLAND
120H  02/1200Z 30.5N  82.0W   35 KT  40 MPH...INLAND

$$
Forecaster Beven

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