829 
 WTNT44 KNHC 121459
 TCDAT4
 
 Tropical Storm Isaac Discussion Number  20
 NWS National Hurricane Center Miami FL       AL092018
 1100 AM AST Wed Sep 12 2018
 
 GOES-16 1-minute satellite imagery along with NOAA Hurricane Hunter
 data show that the center of Isaac has become fully exposed on the
 northwestern side of a shrinking area of deep convection.  There is
 still a lot of wind there, however, with the NOAA plane observing
 SFMR and adjusted flight-level winds to support 50 kt as an initial
 intensity.
 
 An upper-level trough is dropping southward over the central
 Atlantic, which has increased the shear this morning over Isaac.
 This high shear is forecast to remain for the next day or two, and
 it is possible that Isaac will degrade into a tropical wave during
 that time, although the area of tropical-storm-force winds will
 likely not go away for a while.  In a couple of days, while the
 wind shear could decrease over the east-central Caribbean Sea, there
 might not be much of a system left to take advantage of the more
 conducive conditions.  As a compromise, the official forecast
 shows a slow weakening, in the direction of the model consensus,
 and continues to show dissipation after 96 hours in line with the
 GFS/ECMWF guidance.  I wouldn't pay much attention to the long
 range forecast for now, given the uncertainties in Isaac surviving
 during the next day or two.
 
 The low-level center of Isaac has recently accelerated, moving
 westward at 15 kt, as it decoupled from the slower mid-level flow.
 Assuming the system stays vertically coherent, a large
 mid-tropospheric high is forecast to persist for the next several
 days.  This should keep the system moving westward at a somewhat
 slower pace throughout the forecast period.  There isn't much spread
 in the guidance, and the official forecast is close to the previous
 one after accounting for the initial position.  The long-range track
 forecast highly depends on the intensity forecast, so it should be
 considered low confidence.
 
 Key Messages:
 
 1. Isaac is expected to remain at tropical storm intensity when it
 moves across the Lesser Antilles on Thursday, and tropical storm
 warnings are in effect for Martinique, Dominica, and Guadeloupe.
 Tropical storm watches are in effect for Montserrat, St. Kitts and
 Nevis, Antigua, and Saba and St. Eustatius.  Interests on those
 islands should follow any advice given by their local officials.
 
 2. Life-threatening flash flooding is possible with Isaac.  The
 storm is expected to produce total rainfall accumulations of 2 to 4
 inches with isolated amounts up 8 inches across Martinique,
 Dominica and Guadeloupe. Rainfall amounts of 1 to 2 inches with
 isolated amounts to 4 inches are expected across Puerto Rico and the
 southern United States Virgin Islands.
 
 
 FORECAST POSITIONS AND MAX WINDS
 
 INIT  12/1500Z 15.0N  54.7W   50 KT  60 MPH
  12H  13/0000Z 15.1N  56.7W   50 KT  60 MPH
  24H  13/1200Z 15.3N  59.4W   45 KT  50 MPH
  36H  14/0000Z 15.3N  62.6W   45 KT  50 MPH
  48H  14/1200Z 15.4N  65.6W   40 KT  45 MPH
  72H  15/1200Z 15.5N  71.0W   40 KT  45 MPH
  96H  16/1200Z 15.5N  75.0W   35 KT  40 MPH
 120H  17/1200Z...DISSIPATED
 
 $$
 Forecaster Blake