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TROPICAL STORM ERIKA DISCUSSION NUMBER  12
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL052015
500 PM AST THU AUG 27 2015

The center has been wobbling during the day, but the mean
motion is estimated to be 285/13.  In the mean, a west-northwestward
track to the south of a subtropical ridge is likely to continue
for the next couple of days.  Later in the forecast period, the
cyclone should turn to the right along the southwestern and western
periphery of the ridge.  There remains considerable spread in the
track model guidance at days 3 to 5, partly due to differences in
model-predicted intensities at those time frames.  The official
track forecast is in good agreement with the latest dynamical model
consensus.  The NOAA Gulfstream-IV is currently conducting a
synoptic surveillance mission around Erika to provide the numerical
models with a better depiction of the storm's environment.  These
data will be reflected primarily in the 00Z run of the GFS.

Vertical shear is expected to be fairly strong for the next couple
of days and that, along with the interaction with land, should
preclude significant strengthening for the next 48 hours or so.
Beyond that time, the shear is forecast to relax somewhat, and
this could allow for intensification assuming that the cyclone is
not too disrupted by the mountainous land mass of Hispaniola.
Because of the marginal upper-level wind environment and potential
interaction with land over the next few days, there is unusually
high uncertainty in the forecast intensity, especially at days 3 to
5.

The biggest short-term threat posed by Erika is very heavy rainfall
over portions of the Leeward Islands, which should spread over the
Virgin Islands and Puerto Rico tonight and early Friday.  These
rains could produce flash floods and mud slides.  More than 12
inches of rain has fallen in Dominica, with reports of fatalities
in that island.



FORECAST POSITIONS AND MAX WINDS

INIT  27/2100Z 16.6N  64.0W   40 KT  45 MPH
 12H  28/0600Z 17.9N  66.4W   45 KT  50 MPH
 24H  28/1800Z 19.2N  69.2W   45 KT  50 MPH
 36H  29/0600Z 20.7N  72.0W   45 KT  50 MPH
 48H  29/1800Z 22.1N  74.7W   45 KT  50 MPH
 72H  30/1800Z 24.9N  78.8W   55 KT  65 MPH
 96H  31/1800Z 27.0N  80.0W   65 KT  75 MPH
120H  01/1800Z 29.5N  80.5W   75 KT  85 MPH

$$
Forecaster Pasch

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