512 
 WTNT42 KNHC 242034
 TCDAT2
 
 TROPICAL STORM GASTON DISCUSSION NUMBER   9
 NWS NATIONAL HURRICANE CENTER MIAMI FL       AL072016
 500 PM AST WED AUG 24 2016
 
 Gaston is being affected by southwesterly vertical shear associated
 with a strong mid- to upper-level trough and cut-off low seen in
 water vapor imagery near 26n 51w.  The shear has caused the
 low-level center to become partially exposed while much of the
 deep convection has been shunted to eastern half of the circulation.
 In spite of the degraded satellite presentation, dropsonde data
 from the unmanned NASA Global Hawk aircraft investigating Gaston
 support keeping the intensity at 60 kt.  In fact, additional
 observations from the ongoing mission might reveal that the system
 is even a little stronger than this estimate.
 
 Gaston is in for a prolonged period of strong southwesterly shear
 for the next 36 to 48 hours, with the shear possibly peaking around
 30 kt during that time.  The considerable shear is expected to
 induce weakening, and it is possible that more weakening could occur
 than indicated in the forecast despite the cyclone's moving over
 warmer waters.  Once Gaston's interaction with the trough lessens in
 about 2 days, the cyclone should reach 29 deg C water when the
 shear diminishes.  This should give Gaston an opportunity to
 re-intensify for at least a couple of days before the models
 indicate an increase in shear at the end of the forecast period.
 The intensity forecast is reduced relative to the previous one and
 is a little below the multi-model consensus through 48 hours, but
 then reverses and is above the consensus aids from 72-120 hours.
 
 Gaston's heading is now definitively northwestward, and the initial
 motion estimate is 315/14.  A continued northwestward motion is
 likely for the next few days as Gaston moves between one cell of the
 subtropical ridge centered over the eastern Atlantic and the cut-off
 low to the northwest.  After 72 hours, Gaston should approach a more
 significant weakness in the Atlantic subtropical ridge around 60w
 and slow while turning north-northwestward to northward.  There are
 substantial model differences again this cycle, with the ECMWF
 indicating a weaker subtropical ridge.  The weaker ridge makes
 Gaston more vulnerable to the mid-latitude westerly flow over
 the North Atlantic, which results in an earlier recurvature.
 However, the bulk of the guidance has a stronger ridge, and thus
 have solutions that go much farther west.  No major changes have
 been made to the previous forecast in the short term but the track
 has been adjusted much farther to the left after 72 hours, in the
 direction of but not as far left as the multi-model consensus.
 
 FORECAST POSITIONS AND MAX WINDS
 
 INIT  24/2100Z 17.4N  40.6W   60 KT  70 MPH
  12H  25/0600Z 18.8N  42.3W   55 KT  65 MPH
  24H  25/1800Z 21.0N  44.5W   45 KT  50 MPH
  36H  26/0600Z 23.5N  46.6W   45 KT  50 MPH
  48H  26/1800Z 25.7N  49.2W   50 KT  60 MPH
  72H  27/1800Z 28.3N  54.6W   65 KT  75 MPH
  96H  28/1800Z 30.1N  57.4W   80 KT  90 MPH
 120H  29/1800Z 31.6N  58.2W   85 KT 100 MPH
 
 $$
 Forecaster Kimberlain/Franklin