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TROPICAL STORM ERIKA DISCUSSION NUMBER   7
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL052015
1100 AM AST WED AUG 26 2015

Deep convection associated with Erika has increased during the past
24 hours, but the overall organization of the tropical cyclone has
not changed very much.  Reconnaissance aircraft data indicate that
the center is located near the northwestern edge of the thunderstorm
activity due to moderate northwesterly shear.  The aircraft has
measured believable SFMR winds of around 40 kt this morning, and
the initial intensity is set at that value.

Erika is forecast to pass through an environment of moderate to
strong westerly vertical wind shear during the next two to three
days.  The shear will be caused by an upper-level low that is
expected to remain near eastern Cuba through Friday.  The upper low
is forecast to weaken on Saturday, which should produce a more
conducive upper-level wind pattern over the Bahamas. The NHC
intensity forecast calls for little change in strength through 72
hours, which is in line with the latest statistical guidance.  After
that time, strengthening is indicated due to the expected more
favorable upper-level environment.  The official forecast lies
between the more robust HWRF/GFDL and lower statistical guidance.
An alternative forecast scenario, supported by the GFS model, is
that Erika weakens to a tropical wave due to the shear and
interaction with the Greater Antilles.  The amount of strengthening
on days 4-5 will be dependent in part on how Erika responds to the
the preceding unfavorable shear.

The initial motion estimate is 280/15 kt.  Erika is expected to
move westward to west-northwestward during the next several days
to the south of a subtropical ridge over the western Atlantic.
The track guidance is in good agreement through much of the forecast
period, with the exception of the GFDL model that takes a stronger
storm northwestward much sooner.  The new NHC track is essentially
an update of the previous advisory and is close to a consensus of
the ECMWF, GFS, HWRF, and UKMET.  This is also in good agreement
with the Florida State Superensemble.  One should remember to not
focus on the exact forecast track, especially at the long range
where the average NHC track errors during the past 5 years are about
180 miles at day 4 and 240 miles at day 5.


FORECAST POSITIONS AND MAX WINDS

INIT  26/1500Z 16.1N  57.6W   40 KT  45 MPH
 12H  27/0000Z 16.7N  59.9W   45 KT  50 MPH
 24H  27/1200Z 17.6N  63.0W   45 KT  50 MPH
 36H  28/0000Z 18.7N  65.9W   45 KT  50 MPH
 48H  28/1200Z 19.8N  68.7W   45 KT  50 MPH
 72H  29/1200Z 22.0N  73.7W   45 KT  50 MPH
 96H  30/1200Z 24.4N  77.7W   55 KT  65 MPH
120H  31/1200Z 26.5N  80.5W   65 KT  75 MPH...INLAND

$$
Forecaster Brown

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