658 
 WTNT44 KNHC 122058
 TCDAT4
 
 Tropical Storm Isaac Discussion Number  21
 NWS National Hurricane Center Miami FL       AL092018
 500 PM AST Wed Sep 12 2018
 
 The satellite presentation of Isaac hasn't changed much in the past
 several hours, with the center exposed on the west side of a
 re-developing band of convection.  On the last pass of the NOAA
 Hurricane Hunter aircraft, peak flight-level winds were 61 kt and
 SFMR values were about 50 kt.  These data support an initial wind
 speed estimate of 50 kt.  It is interesting to note that the
 surface center is estimated to be a tenth of a degree or two south
 of the well-defined swirl on satellite--a function of the fast
 forward speed and high shear.
 
 The shear is forecast to continue for the next day or two, and
 gradual weakening is anticipated, in line with the model
 guidance.  Given the strong shear, it is possible that Isaac could
 degenerate into a wave near the Lesser Antilles.  After that time,
 Isaac could escape the upper-level trough over the central Atlantic
 that has been shearing the cyclone, and find a more conducive
 environment. More of the model guidance is suggesting some
 reintensification of the cyclone in the western Caribbean Sea, with
 the biggest hold-out being the ECMWF.  Continuity dictates that the
 forecast not change as much as the guidance, most of which is
 showing a hurricane at long range, so the compromise solution is no
 longer dissipating the cyclone and flat-lining the intensity at
 long range.  It almost goes without saying that this is a
 low-confidence intensity forecast.
 
 Isaac has accelerated to near 17 kt over the past 12 hours, and the
 shorter-term motion is closer to 20 kt with the convection-less
 center. A mid-tropospheric high is forecast to persist for the next
 several days, which should keep the cyclone moving westward, albeit
 at a slightly slower pace.  The model guidance has shifted a bit to
 the north on this cycle, but is mostly in response to the initial
 latitude.  The NHC track forecast is moved northward as well but
 still lies on the south side of the consensus.  At long range, it
 makes sense when forecasting a weaker system to stay on the left
 and faster side of the consensus, so that's where the new NHC
 prediction lies.
 
 
 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  12/2100Z 15.4N  56.6W   50 KT  60 MPH
  12H  13/0600Z 15.6N  59.0W   50 KT  60 MPH
  24H  13/1800Z 15.8N  61.8W   45 KT  50 MPH
  36H  14/0600Z 16.0N  64.8W   45 KT  50 MPH
  48H  14/1800Z 16.0N  67.6W   40 KT  45 MPH
  72H  15/1800Z 16.0N  72.5W   40 KT  45 MPH
  96H  16/1800Z 16.5N  76.5W   40 KT  45 MPH
 120H  17/1800Z 17.5N  80.5W   40 KT  45 MPH
 
 $$
 Forecaster Blake