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TROPICAL STORM ERIKA DISCUSSION NUMBER  13
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL052015
1100 PM AST THU AUG 27 2015

The center of Erika is not easy to locate tonight, and it
appears that a few smaller swirls are rotating within a larger
gyre. In fact, one of these small swirls moved near St. Croix
producing tropical storm force wind gusts during the past few
hours on the island. Due to the lack of an inner core, the initial
position is based on a mean center of circulation. Despite the poor
organization, the reconnaissance plane currently in Erika was able
to measure 700 mb flight-level winds of 59 kt well to the southeast
of the alleged center. Based on the SFMR, these winds are not at the
surface, and the initial intensity is kept at 40 kt. The central
pressure is not falling, which is another indication that Erika is
not strengthening. The NHC forecast calls for no change in intensity
during the next 36 hours, given the fact that cyclone will be
moving through a very hostile shear environment, and will also feel
the effects of land. Once in the Bahamas, however, the upper-level
flow is expected to become more favorable, and if Erika survives, it
has the opportunity to strengthen some. The NHC forecast is very
close to the intensity consensus and is similar to the previous one.

The best estimate of the initial motion is toward the west or 270
degrees at 15 kt, and this estimate is highly uncertain. Erika
should begin to turn toward the west-northwest during the next
several hours around the periphery of the western Atlantic
subtropical ridge, and should reach the Central Bahamas between 36
and 48 hours. By then, the cyclone will be located on the
southwestern edge of the ridge, and should begin to turn to the
northwest with decreasing forward speed. Most of the track
guidance, including the ECMWF and the GFS global models, show a
tropical cyclone approaching southeast Florida in about 3 days and
moving northward near or over the east coast of Florida during the
latter portion of the forecast period.  There is unusually high
uncertainty in this forecast, especially at days 3 to 5, given that
the cyclone has to recover from shear and from the effects of
land for this to occur.

The greatest short-term threat posed by Erika continues to be
very heavy rainfall over portions of the northern Leeward Islands
tonight, and over the Virgin Islands and Puerto Rico on Friday.
These rains could produce flash floods and mud slides.


FORECAST POSITIONS AND MAX WINDS

INIT  28/0300Z 16.6N  65.3W   40 KT  45 MPH
 12H  28/1200Z 18.2N  67.4W   40 KT  45 MPH
 24H  29/0000Z 19.7N  70.2W   40 KT  45 MPH
 36H  29/1200Z 21.1N  73.2W   40 KT  45 MPH
 48H  30/0000Z 22.5N  75.5W   45 KT  50 MPH
 72H  31/0000Z 25.2N  79.0W   55 KT  65 MPH
 96H  01/0000Z 27.3N  80.2W   65 KT  75 MPH
120H  02/0000Z 29.5N  80.7W   75 KT  85 MPH

$$
Forecaster Avila

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