Caribbean-On-Line's Caribbean Hurricane Blog



Tropical Storm Isaac Discussion Number 17, Watched and Warnings

Published on September 11, 2018 8:20 PM | Comments

It is a weakening storm, but you should still be prepared - watches and warnings are up for Guadeloupe, Martinique, Dominica, Antigua, Montserrat
and St. Kitts and Nevis.

Tropical Storm Isaac Intermediate Advisory Number 17A NWS National Hurricane Center Miami FL AL092018 800 PM AST Tue Sep 11 2018

...ISAAC LOSING SOME ORGANIZATION AS IT MOVES QUICKLY WESTWARD...


SUMMARY OF 800 PM AST...0000 UTC...INFORMATION
----------------------------------------------
LOCATION...14.6N 52.2W
ABOUT 610 MI...980 KM E OF THE LESSER ANTILLES
MAXIMUM SUSTAINED WINDS...70 MPH...110 KM/H
PRESENT MOVEMENT...W OR 270 DEGREES AT 17 MPH...28 KM/H
MINIMUM CENTRAL PRESSURE...996 MB...29.42 INCHES


WATCHES AND WARNINGS
--------------------
CHANGES WITH THIS ADVISORY:

None.

SUMMARY OF WATCHES AND WARNINGS IN EFFECT:

A Hurricane Watch is in effect for...
* Guadeloupe
* Martinique
* Dominica

A Tropical Storm Watch is in effect for...
* Antigua
* Montserrat
* St. Kitts and Nevis

A Hurricane Watch means that hurricane conditions are possible
within the watch area.

A Tropical Storm Watch means that tropical storm conditions are
possible within the watch area.

Interests elsewhere in the Leeward Islands should monitor the
progress of Isaac as additional watches or warnings will likely be
issued tonight.

For storm information specific to your area, please monitor
products issued by your national meteorological service.


DISCUSSION AND OUTLOOK
----------------------
At 800 PM AST (0000 UTC), the center of Tropical Storm Isaac was
located near latitude 14.6 North, longitude 52.2 West. Isaac is
moving toward the west near 17 mph (28 km/h), and this general
motion is expected to continue for the next few days. On the
forecast track, Isaac is anticipated to move near or over the
central Lesser Antilles on Thursday, move into the eastern Caribbean
Sea Thursday night, and move into the central Caribbean Sea by the
weekend.

Maximum sustained winds are near 70 mph (110 km/h) with higher
gusts. Isaac is expected to be near hurricane strength when it
moves through the central Lesser Antilles, with some weakening
forecast afterward on Friday and Saturday.

Tropical-storm-force winds extend outward up to 105 miles (165 km)
from the center.

The estimated minimum central pressure is 996 mb (29.42 inches).


HAZARDS AFFECTING LAND
----------------------
RAINFALL: Isaac is expected to produce total rainfall accumulations
of 3 to 5 inches with isolated amounts near 10 inches across
Martinique, Dominica, and Guadeloupe, with 1 to 2 inches anticipated
across the remaining Windward and Leeward Islands.

STORM SURGE: A storm surge of 2 to 4 feet above normal tide levels
is possible near and to the north of where the center moves through
the Lesser Antilles. Near the coast, the surge will be accompanied
by large and destructive waves.

WIND: Hurricane conditions are possible within the hurricane watch
area by Thursday morning, with tropical storm conditions possible
overnight Wednesday in both the hurricane and tropical storm watch
areas.

SURF: Swells generated by Isaac will begin to affect portions
of the Lesser Antilles on Wednesday afternoon. These swells are
likely to cause life-threatening surf and rip current conditions.
Please consult products from your local weather office.

Tropical Storm Isaac Discussion Number 17 NWS National Hurricane Center Miami FL AL092018 500 PM AST Tue Sep 11 2018

Microwave and visible satellite data show that the center of Isaac
is partially exposed on the western edge of the central dense
overcast. Perhaps Isaac is a little weaker than earlier, but a
blend of the latest subjective and objective estimates still gives
an uncertain initial wind speed of about 60 kt. We should have a
better idea of the maximum winds by tomorrow morning when a NOAA
Hurricane Hunter enters the storm, or if we get lucky with a
scatterometer pass this evening.

Isaac is barely holding on in the face of increasing shear, which
is much more evident in the 300-500 mb layer than a conventional
analysis at 200 mb. However, convection could redevelop near the
center while the storm moves over warming SSTs into a more moist and
unstable environment east of the Lesser Antilles. These
conflicting factors lead to a forecast of little change in the winds
of Isaac until the cyclone moves into the eastern Caribbean. I
should mention that the short-term trend on satellite makes me think
the intensity forecast is too high over the next 36 hours. Yet,
given our limited skill in forecasting intensity changes in marginal
environments, plus the 5 or 10 kt of initial wind speed uncertainty,
this is the course of least regret. We can see if this trend
becomes more consistent before making substantial changes on the
next advisory. Model guidance is very divergent in the long range
for Isaac, with the regional hurricane and statistical models
showing intensification, while the global models show continued
weakening. The global models have done fairly well with this
cyclone, so the forecast will lean more heavily on their weaker
solutions.

The latest initial motion estimate is 270/15, a little faster than
before. Isaac is forecast to move westward at roughly the same
speed for the next few days due to a mid-level ridge over the
central Atlantic Ocean. Model guidance is tightly clustered on this
cycle, with the UKMET now coming into good agreement with the rest
of the guidance. The only change to this forecast is to accelerate
Isaac roughly 1 kt over the course of the prediction, which over the
course of a 5-day forecast results in a sizable westward shift.
This forecast could still be too slow, since the GFS and ECMWF are
a bit ahead of the new NHC track prediction.

Key Messages:

1. Isaac is expected to be at or near hurricane intensity when it
approaches the Lesser Antilles on Thursday, and hurricane and
tropical storm watches have been issued for several islands.
Interests in those areas should follow any advice given by their
local officials, and warnings will likely be issued this evening.

2. Interests elsewhere in the Lesser Antilles should continue to
monitor Isaac during the next couple of days.



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