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TROPICAL STORM ERIKA DISCUSSION NUMBER   2
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL052015
500 AM AST TUE AUG 25 2015

The cloud pattern of Erika has not changed much overnight, with the
convection located near the center and in the southern semicircle.
The latest Dvorak estimates are T2.5/35 kt from TAFB and SAB, and
the initial intensity remains 40 kt based on the earlier buoy data.

Erika will be moving over warming SSTs with light to moderate shear
for the next 36 to 48 hours, and the NHC forecast shows steady
intensification during this time. After that, Erika could encounter
increasing westerly shear due to interaction with an upper-level low
near Hispaniola and there is the potential for land interaction with
the Greater Antilles. The HWRF and LGEM models show more
strengthening during this time, while the GFDL and SHIPS models are
less aggressive. There is also disagreement among the global models.
The GFS weakens Erika to a trough by 5 days while the ECMWF has
trended stronger and the UKMET continues to show a more robust
cyclone. The NHC intensity forecast has been adjusted upward a bit
at days 4 and 5 and is close to or a bit below the IVCN intensity
consensus. Due to the large spread in the guidance, the intensity
forecast at days 3-5 is of even lower confidence than usual.

The initial motion estimate of 275/17 is based largely on
continuity, given the difficulty locating the center with infrared
imagery. The track guidance is in good agreement through 36 hours,
as Erika should be steered westward to west-northwestward south of
the subtropical ridge. At 48 hours and beyond, there are two
distinct camps of track guidance. The UKMET, HWRF, and GFDL
show a deeper system that moves northwestward, while the GFS, ECMWF,
and GEFS ensemble mean show a weaker cyclone and a more west-
northwestward motion. The NHC forecast has been nudged a bit
to the right this cycle and remains close to the ECMWF and GEFS
ensemble mean. This track is to the right of the GFS but well left
of the TVCA multi-model consensus. Given the divergence in the
guidance, confidence in the track forecast late in the period is
low.

Based on the new forecast, Tropical Storm Watches have been issued
for portions of the Lesser Antilles.

FORECAST POSITIONS AND MAX WINDS

INIT  25/0900Z 14.6N  49.4W   40 KT  45 MPH
 12H  25/1800Z 15.2N  51.9W   45 KT  50 MPH
 24H  26/0600Z 16.0N  54.8W   50 KT  60 MPH
 36H  26/1800Z 16.9N  57.9W   55 KT  65 MPH
 48H  27/0600Z 17.6N  61.0W   60 KT  70 MPH
 72H  28/0600Z 19.3N  66.2W   60 KT  70 MPH
 96H  29/0600Z 21.5N  71.0W   65 KT  75 MPH
120H  30/0600Z 24.0N  75.0W   70 KT  80 MPH

$$
Forecaster Brennan

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