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TROPICAL DEPRESSION ELEVEN DISCUSSION NUMBER   4
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL112015
ISSUED BY THE NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
500 PM EDT MON SEP 28 2015

The low-level center has become well exposed due to continued
northwesterly shear of 20 kt or more, with the mid-level
center displaced well to the southeast of the low-level center.
Flight-level and SFMR-observed wind data from an Air Force
reconnaissance aircraft showed that the maximum winds remain near
30 kt.  This is also consistent with the latest Dvorak estimates
from SAB and TAFB.

The center of the depression has been moving well to the left of
the previous track, with the initial motion estimated to be 270/05
kt.  In addition to the more westward initial motion, there have
been significant changes in the model guidance, with the latest
ECMWF forecast taking the cyclone much farther south and west than
the GFS in 3-5 days.  This appears to be the result of enhanced
mid-level ridging to the north of the cyclone in the ECMWF at that
time frame.  This has necessitated a significant southwestward
shift of the official forecast, although for continuity the new NHC
track does not take the cyclone as far to the southwest as the
ECMWF.  Given the large spread now seen in the dynamical track
guidance, confidence in the track forecast is rather low.

The strong shear over the system is likely to continue for at least
the next 12-24 hours and given the large displacement between the
low- and mid-level centers, no intensification is anticipated in
the short term.  The dynamical guidance indicates some relaxation
of the shear in a day or two, so the official forecast shows the
cyclone strengthening into a tropical storm later on Tuesday.  This
is close to the intensity model consensus through 48 hours, and a
little below it thereafter.  Given the large uncertainty in the
track forecast, there is also uncertainty as to what kind of
environment the cyclone will encounter during the forecast period,
which also leads to low confidence in the official intensity
forecast.

FORECAST POSITIONS AND MAX WINDS

INIT  28/2100Z 27.5N  70.2W   30 KT  35 MPH
 12H  29/0600Z 27.7N  71.0W   30 KT  35 MPH
 24H  29/1800Z 27.9N  72.0W   35 KT  40 MPH
 36H  30/0600Z 28.3N  72.8W   40 KT  45 MPH
 48H  30/1800Z 28.6N  73.4W   45 KT  50 MPH
 72H  01/1800Z 30.0N  74.0W   45 KT  50 MPH
 96H  02/1800Z 33.0N  74.0W   45 KT  50 MPH
120H  03/1800Z 38.5N  73.5W   45 KT  50 MPH

$$
Forecaster Pasch/Hayes

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