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TROPICAL STORM ERIKA DISCUSSION NUMBER   6
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL052015
500 AM AST WED AUG 26 2015

Convection has increased near and to the east of the estimated
center position of Erika during the past few hours. Subjective and
objective Dvorak estimates remain T2.5/35 kt, and that is the
intensity for this advisory. The central pressure of 1003 mb is
based on a dropsonde from a NOAA aircraft that reported 1004 mb and
25 kt of wind south of the apparent surface center.

The intensity forecast remains very uncertain. Despite the recent
improvement in the convective pattern, the statistical guidance
shows only very gradual strengthening during the forecast period
due to 15 to 25 kt of westerly to northwesterly shear. Another
major negative factor in the SHIPS model is the weakening of the
vortex in the GFS forecast. The HWRF is also very slow to
strengthen Erika in the short term, but continues to show
intensification late in the period. The latest ECMWF run shows a
more robust cyclone in its latest forecast, while the GFS, as
mentioned above, weakens the vortex to a wave in 2 to 3 days. Given
this uncertainty, the NHC forecast remains close to the intensity
consensus and shows very slow intensification in the first 2
to 3 days followed by steadier strengthening at days 4 and 5.

The initial motion estimate is 280/16. Erika will be steered
westward to west-northwestward over the next 2 to 3 days by the
Atlantic subtropical ridge to the north, and the track model
guidance is in good agreement on this scenario. After that time,
the track model guidance shows increasing spread with a lack of run
to run consistency. For example, the latest run of the GFS takes a
much weaker Erika west-northwestward while the new ECMWF run is
stronger with Erika with a track farther to the right. Overall, the
track guidance envelop has shifted a little to the left this cycle,
with the majority of the aids showing a west-northwestward to
northwestward motion as the cyclone nears the western edge of the
ridge by day 5. The NHC forecast follows this trend and is close to
the HWRF and a little to the right of the ECMWF and GEFS ensemble
mean at days 3 through 5. This track is left of the TVCA consensus,
which has been pulled to the right by the outlier GFDL model this
cycle. Given the uncertainty, this is a good time to remind users
that average NHC track forecast errors over the past 5 years are 180
miles at day 4 and 240 miles at day 5.

New tropical storm warnings have been issued for Puerto Rico, the
Virgin Islands, and several other islands in the northeastern
Caribbean by their respective Meteorological Services.

FORECAST POSITIONS AND MAX WINDS

INIT  26/0900Z 16.1N  56.0W   35 KT  40 MPH
 12H  26/1800Z 16.5N  58.4W   40 KT  45 MPH
 24H  27/0600Z 17.2N  61.5W   40 KT  45 MPH
 36H  27/1800Z 18.1N  64.5W   45 KT  50 MPH
 48H  28/0600Z 19.2N  67.2W   45 KT  50 MPH
 72H  29/0600Z 21.4N  72.4W   50 KT  60 MPH
 96H  30/0600Z 24.0N  76.5W   60 KT  70 MPH
120H  31/0600Z 26.0N  79.5W   70 KT  80 MPH

$$
Forecaster Brennan

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