Official Discussion issued by the National Hurricane Center

Isaac (AL092018) DATA RELEASED: 9/10/2018 11:00:00 PM UTC

Copy of official data

Tropical Storm Isaac Discussion Number  14
NWS National Hurricane Center Miami FL       AL092018
1100 PM AST Mon Sep 10 2018

Several microwave images that arrived after the issuance of the last
advisory revealed that the low-level center of Isaac has raced ahead
of the main convective mass, and the system is poorly vertically
aligned. The system also recently passed very near NOAA buoy 41041,
which confirmed that the center is on the western side of the
central dense overcast. The maximum winds observed by the buoy were
only about 30 kt, with a minimum pressure of near 1001 mb. Due to
the western adjustment of the center location, the latest Dvorak
classification from TAFB is lower, and a blend of the TAFB final-T
and CI-numbers supports a lower initial intensity of 60 kt. It is
worth noting that this estimate is more uncertain than normal due to
limitations of the Dvorak technique, and most of the other intensity
estimates are higher. Given the high pressure and light winds at the
buoy, I am inclined to favor the lower estimate at this time.

Confidence in the track forecast is slightly higher now, and the
UKMET is the only outlier that does not bring Isaac westward into
the Caribbean within the next 72 h or so. There is still some
disagreement on the speed of Isaac, especially once it crosses the
Leeward Islands, and this seems largely related to the cyclone's
intensity. The ECMWF and GFS depict a weakening storm that simply
continues westward, while the HWRF and CTCI show strengthening as
Isaac slows down and turns more west-northwestward. The NHC track
forecast for now continues to favor the typically more reliable
global models and shows a steady westward motion through day 5. The
updated NHC forecast is a little faster than the previous advisory,
due primarily to the westward adjustment of the initial position,
and remains close to HCCA.

A slight adjustment has been made to the intensity forecast, but
confidence in the intensity forecast remains quite low. Due to the
current structure of Isaac, it seems less likely that the tropical
storm will intensify more than what is shown by the intensity
guidance over the next 24-48 h. The NHC forecast has therefore been
lowered slightly, but it is still near the top end of the guidance,
and calls for Isaac to be a hurricane when it approaches the
Leeward Islands later this week. It is also worth noting that the
small size of Isaac could make it more susceptible to sudden
short-term changes in intensity that are nearly impossible to
forecast. The intensity spread is tremendous by the end of the
forecast period, with multiple models showing Isaac becoming a major
hurricane around day 4 or 5, and others showing outright
dissipation. The NHC forecasts have been favoring a weaker solution
thus far due to an expected increase in shear beginning in a couple
of days, and I see no reason to make a dramatic change at this
point.

Key Messages:

1. Isaac is expected to be at or near hurricane intensity when it
approaches the Lesser Antilles. However, confidence in the forecast
is lower than normal.

2. Interests in the Lesser Antilles should continue to monitor
Isaac during the next few days. Watches will likely be required for
portions of the Lesser Antilles tomorrow.

FORECAST POSITIONS AND MAX WINDS

INIT  11/0300Z 14.5N  46.9W   60 KT  70 MPH
12H  11/1200Z 14.5N  49.0W   60 KT  70 MPH
24H  12/0000Z 14.6N  51.7W   65 KT  75 MPH
36H  12/1200Z 14.8N  54.3W   65 KT  75 MPH
48H  13/0000Z 15.0N  56.9W   65 KT  75 MPH
72H  14/0000Z 15.4N  62.4W   60 KT  70 MPH
96H  15/0000Z 15.5N  68.0W   55 KT  65 MPH
120H  16/0000Z 15.5N  72.5W   50 KT  60 MPH

$$
Forecaster Zelinsky