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TROPICAL STORM JOAQUIN DISCUSSION NUMBER   6
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL112015
500 AM EDT TUE SEP 29 2015

The low-level center of Joaquin remains exposed on the northwestern
edge of the deep convection due to about 20 kt of shear. A pair of
ASCAT passes around 01-02Z showed peak winds of around 35 kt, and
that remains the intensity for this advisory, in agreement with the
latest TAFB and SAB Dvorak estimates. The ASCAT data were also used
to adjust the initial 34-kt wind radii.

After moving quickly southwestward earlier, the center of Joaquin
appears to be moving more slowly westward at about 4 kt. The track
forecast remains highly uncertain, and if anything, the spread in
the track model guidance is larger now beyond 48 hours due to
uncertainty in the evolution of the synoptic-scale flow and the
structure of Joaquin. In particular, the global models are having
trouble with the evolution of a deep layer trough over the
southeastern U.S. late in the forecast period, with added
uncertainty about the structure of Joaquin and how it might interact
with the trough. The latest runs of the UKMET and ECMWF bring
Joaquin farther southwest early in the period and then show a
northward and northeastward motion, respectively, ahead of the
trough at days 4 and 5. The GFS shows a weaker Joaquin being
absorbed into a frontal zone off the U.S. east coast in 3 to 4 days,
while the HWRF and GFDL continue to show a faster northward and
then northwestward track. In the first 48 hours, the new NHC track
is a little to the south of the previous one accounting for the
initial position and motion and a trend toward the multi-model
consensus. At days 3 through 5, the official forecast is a little to
the right of and slower than the previous one given the large
spread in the track guidance. Needless to say, confidence in the
details of the track forecast, especially beyond 48 hours, is
extremely low.

The SHIPS model output shows moderate to strong northerly shear
continuing over the cyclone for the next 24 to 36 hours, which
should only allow for slow strengthening. After that time, the shear
decreases a bit and the cyclone could strengthen a little more while
over warm waters. However, there is a lot of spread in the intensity
guidance as well. As noted above, the latest run of the GFS
dissipates Joaquin in 3 to 4 days. On the other hand, the ECMWF and
UKMET show the cyclone deepening considerably in the short term,
while the HWRF and GFDL show more strengthening later in the period.
Given the uncertainty in Joaquin's structure and track, the new NHC
forecast remains conservative, and is closest to the SHIPS model and
below the intensity consensus.

FORECAST POSITIONS AND MAX WINDS

INIT  29/0900Z 26.6N  70.6W   35 KT  40 MPH
 12H  29/1800Z 26.6N  71.3W   40 KT  45 MPH
 24H  30/0600Z 26.7N  72.2W   40 KT  45 MPH
 36H  30/1800Z 26.8N  72.8W   45 KT  50 MPH
 48H  01/0600Z 27.1N  73.2W   50 KT  60 MPH
 72H  02/0600Z 28.5N  73.5W   55 KT  65 MPH
 96H  03/0600Z 32.0N  73.0W   55 KT  65 MPH
120H  04/0600Z 36.5N  73.0W   55 KT  65 MPH

$$
Forecaster Brennan

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