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Tropical Storm Isaac Discussion Number 19
NWS National Hurricane Center Miami FL AL092018
500 AM AST Wed Sep 12 2018
Isaac is a poorly organized tropical storm. Geostationary satellite
and microwave images indicate that the center of the cyclone is
located near the western edge of the main area of deep convection.
This asymmetric cloud pattern is due to about 20 kt of
west-northwesterly shear. The satellite intensity estimates have
been decreasing, and an average of the Dvorak CI-numbers supports
lowering the initial wind speed slightly to 50 kt. The NOAA
Hurricane Hunters are scheduled to investigate Isaac later this
morning, and that data should provide a better assessment of the
storm's intensity and structure.
Isaac continues to move due westward at about 13 kt. The track
forecast reasoning remains unchanged. A fairly strong mid-level
ridge to the north of Isaac should steer the system westward at
about the same forward speed during the next several days. The
models are now in fairly good agreement, and the NHC track forecast
is just a little north of the previous one to come in line with the
latest guidance. Based on this forecast, Isaac is expected to cross
portions of the Leeward and Windward Islands in about 36 hours.
Isaac is feeling the effects of wind shear, and the global models
suggest that the shear will likely remain moderate to strong during
the next couple of days. These hostile winds combined with nearby
dry air will likely cause Isaac to gradually weaken during the next
several days. The intensity models are in fair agreement, and this
forecast is close to the model consensus, except a little lower than
that aid at 96 hours. The latest runs of the GFS and ECMWF models
show Isaac opening up into a trough by day 5, and the NHC forecast
follows those models.
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.
FORECAST POSITIONS AND MAX WINDS
INIT 12/0900Z 14.5N 53.5W 50 KT 60 MPH
12H 12/1800Z 14.8N 55.7W 50 KT 60 MPH
24H 13/0600Z 15.0N 58.6W 45 KT 50 MPH
36H 13/1800Z 15.2N 61.5W 45 KT 50 MPH
48H 14/0600Z 15.4N 64.8W 40 KT 45 MPH
72H 15/0600Z 15.3N 70.6W 40 KT 45 MPH
96H 16/0600Z 15.2N 75.0W 35 KT 40 MPH