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TROPICAL STORM ERIKA DISCUSSION NUMBER  16
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL052015
500 PM EDT FRI AUG 28 2015

The low-level center of Erika continues to be be located west of
the convection due to the impact of 25-30 kt of westerly vertical
wind shear.  Satellite imagery and reports from an Air Force
Reserve Hurricane Hunter aircraft indicate that the circulation has
been losing definition during the day and it is now barely
closed, at best.  However, clusters of strong convection continue in
the eastern semicircle, and the initial intensity is held at 45 kt
on the premise that the strong winds observed earlier are still
present. The central pressure of 1009 mb is based on observations
from the Dominican Republic.

Erika has been moving more westward for the past several hours and
the initial motion is 280/17.  There is no change to the forecast
philosophy from the previous advisory.  A generally west-
northwestward to 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.  While the track guidance is generally in good agreement, it
should be noted that the guidance has been consistently forecasting
a west-northwestward turn that has so far failed to occur.  On the
other hand, there is still a chance for a center to re-form
farther to the north during the passage over Hispaniola.  So while
the forecast track has been shifted west of the previous advisory,
it still lies near the northern edge of the guidance envelope for
the first 48 hours to maintain some continuity with the previous
forecast. Additional adjustments may be necessary depending on how
Erika evolves during the next 12-18 hours.

The forecast intensity has been significantly changed to show a
much weaker cyclone, with Erika now forecast to become a
tropical depression in about 24 hours, followed by some slight
intensification at 48 and 72 hours.  This represents a compromise
between two scenarios.  The first is the increasing likelihood that
Erika will degenerate to a tropical wave during the passage over
Hispaniola, a scenario supported by the GFS, UKMET, and ECMWF
models. The second acknowledges that the shear is still expected to
subside after 36 hours and we're are not quite prepared to rule out
tropical storm impacts in Florida.  The SHIPS and LGEM support this
possibility.

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.

Although this would normally be an appropriate time for a tropical
storm watch for portions of southern Florida, following typical
timelines, we have elected to wait until we see what's left of
Erika after it passes Hispaniola.  There is a significant chance
that no watches or warnings for Florida will be required.


FORECAST POSITIONS AND MAX WINDS

INIT  28/2100Z 17.9N  71.2W   45 KT  50 MPH
 12H  29/0600Z 19.5N  73.5W   40 KT  45 MPH
 24H  29/1800Z 21.3N  76.5W   30 KT  35 MPH...INLAND
 36H  30/0600Z 22.7N  79.3W   30 KT  35 MPH...OVER WATER
 48H  30/1800Z 24.0N  81.0W   35 KT  40 MPH
 72H  31/1800Z 27.0N  82.5W   35 KT  40 MPH
 96H  01/1800Z 29.0N  82.5W   30 KT  35 MPH...INLAND
120H  02/1800Z 31.0N  82.5W   25 KT  30 MPH...INLAND

$$
Forecaster Beven

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