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 904 
 WTNT41 KNHC 290837
 TCDAT1
 
 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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