Tropical System Isaac

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ISAAC remains a tropical storm this morning with peak winds estimated at 50 mph. It is gaining organization and is expected to become a hurricane in the next day or so.

All indications are that high pressure filling in behind Hurricane Florence will push far enough south to keep Isaac from turning north. Isaac is expected to move due west and arrive at the eastern Caribbean islands, possibly including those devastated last hurricane season, late Wednesday or Thursday.

The challenge in the forecast is how strong Isaac will be when it reaches the islands. Since Florence is now forecast to stay near or over the Southeast U.S., the air flowing out of that strong hurricane will create an upper level wind flow which could be detrimental to Isaac. The system labeled POTENTIAL DISTURBANCE may also contribute to these somewhat hostile upper-level winds.

The National Hurricane Center is forecasting some weakening of Isaac as it approaches the islands, but it is too far in the future to be sure whether that will happen or by how much. For now, it is best to assume a hurricane will be approaching the islands on Wednesday.

1. Isaac is a small hurricane and uncertainty in the forecast is
higher than normal. Although Isaac is forecast to begin weakening
by Tuesday when it approaches the Lesser Antilles, it is still
expected to be at or near hurricane intensity when it reaches the
islands.

2. Interests in the Lesser Antilles should continue to monitor Isaac
during the next few days.

HURRICANE ISAAC is still forecast to be a threat to the eastern Caribbean Islands – the Lesser Antilles – this week. The weather should deteriorate in the islands Wednesday night, with the storm passing through the islands on Thursday. If Isaac stays to the right side of the cone, it could affect Puerto Rico and the surrounding islands after that. The forecast for Isaac is unusually uncertain, however.

Since Isaac is a small hurricane, it has the ability to intensify quickly, but also to weaken quickly if the atmospheric conditions become hostile. The upper winds that will impact the track of Isaac are forecast to be marginally hostile in part thanks to Hurricane Florence to the north. Those winds should keep Isaac from dramatically intensifying, but it is still forecast to reach the islands at or near hurricane strength.

The bottom line is that the eastern Caribbean islands should prepare for a hurricane arriving Thursday, but another Hurricane Maria is not in the cards.

After Isaac moves into the Caribbean late Thursday, its is most likely to weaken and move west, but that it uncertain. The models diverge somewhat, but most show a weaker Isaac at that time. It is too early to know for sure what factors might steer Isaac, though usually weaker storms are swept along to the west.

Here are the Key Messages from the National Hurricane Center concerning Isaac:

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 couple of days. Watches will likely be required for portions of the Lesser Antilles later today.

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.

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.