OSEAU, Dominica -- Leaders across much of the Caribbean closed schools and government offices on Wednesday and urged people to stay at home as Tropical Storm Isaac swept toward the region, threatening to soon become a hurricane and perhaps eventually to menace South Florida.

The storm was 210 miles (340 kilometers) east of the French Caribbean island of Guadeloupe on Wednesday morning with maximum sustained winds of 45 mph (75 kph). Isaac was moving west at 19 mph (31 kph) and was expected to become a hurricane by Thursday, according to the U.S. National Hurricane Center.

The current storm track put Isaac as a category 1 hurricane over Florida Bay north of the Keys at 8 a.m. Monday. South Florida and the Treasure Coast could
expect tropical storm force winds in this scenario.

CBS12.com HURRICANE TRACKER

In Dominica, one of the first islands in the storm's path, Prime Minister Roosevelt Skerrit urged people to stay home from work on Wednesday.

"I want us all to be safe," he said. "I don't want lives to be lost. I have listened to the advice of the experts and so I am asking all to stay indoors."

In Puerto Rico, Gov. Luis Fortuno declared a state of emergency and activated the National Guard. He also canceled classes and closed government agencies on Wednesday. The U.S. Coast Guard ordered all commercial vessels bigger than 200 gross tons to leave the port or obtain permission to remain in port.

The Liat airline said it expected to cancel flights to and from Dominica by Wednesday afternoon, and American Eagle has already canceled all its flights, according to Benoit Bardouille, CEO of the island's Air & Seaport Authority. The fast ferry that runs to Guadeloupe and Martinique also will temporarily suspend service, he said.

Disaster Coordinator Don Corriette warned of landslides and asked people in low-lying areas to seek shelter if needed.

The storm's center was expected to move over the Leeward Islands on Wednesday evening, and forecasters said it is expected to hit the Dominican Republic, Haiti and Cuba as a hurricane later in the week.

Isaac also poses a possible threat to Florida during next week's Republican National Convention in Tampa.

Tropical storm warnings are in effect for Puerto Rico, the U.S. and British Virgin Islands and a swath of islands across the Caribbean including Martinique, Dominica, Guadeloupe, St. Martin, St. Kitts, Nevis, Antigua, Barbuda, Montserrat, Anguilla, Saba, St. Eustatius, St. Maarten, Culebra and Vieques.

A hurricane watch is in effect for Puerto Rico, Vieques, Culebra, the U.S. and British Virgin Islands and the south coast of the Dominican Republic.

CBS12.com HURRICANE TRACKER

FROM THE NATIONAL HURRICANE CENTER:

TROPICAL STORM ISAAC DISCUSSION NUMBER   5
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL092012
500 AM AST WED AUG 22 2012

SATELLITE IMAGERY INDICATES A LARGE CENTRAL DENSE OVERCAST FEATURE HAS DEVELOPED OVER THE LOW-LEVEL CENTER...WHICH INCLUDES SOME COLD CLOUD TOPS OF -82 TO -86C VERY NEAR THE CENTER. HOWEVER...THE SHARP
INCREASE IN INNER-CORE CONVECTION HAS NOT YET TRANSLATED INTO ANY SIGNIFICANT INTENSIFICATION. AN AIR FORCE RESERVE RECONNAISSANCE AIRCRAFT INVESTIGATING ISAAC THIS MORNING HAS ONLY FOUND THE CENTRAL PRESSURE TO BE 1003 MB AND 850 MB FLIGHT-LEVEL WINDS OF 47 KT...WHICH EQUATES TO ABOUT A 40-KT INTENSITY ESTIMATE. SFMR WINDS IN VERY HEAVY RAIN HAVE BEEN AS HIGH AS 61 KT. THE BIAS-ADJUSTED SFMR WINDS HAVE BEEN RUNNING AS HIGH AS 54 KT...BUT EVEN THOSE VALUES APPEAR TO BE INFLATED. UNTIL THE AIRCRAFT COMPLETES ITS MISSION... THE INITIAL INTENSITY WILL REMAIN AT 40 KT BASED ON THE 1003 MB PRESSURE-WIND RELATIONSHIP.

THE INITIAL MOTION ESTIMATE IS 270/15 KT. THE INITIAL POSITION IS A BLEND OF THE RECON FIXES AND SATELLITE POSITIONS...AND CONTINUITY WITH PREVIOUS FORECAST TRACK. THERE IS NO SIGNIFICANT CHANGE TO THE
PREVIOUS FORECAST TRACK OR REASONING. THE EAST-WEST ORIENTED SUBTROPICAL RIDGE TO THE NORTH OF ISAAC ALONG 30N LATITUDE IS FORECAST BY ALL OF THE GLOBAL MODELS TO REMAIN INTACT ACROSS FLORIDA AND THE SOUTHEASTERN U.S. THROUGH 72 HOURS OR SO. AFTER THAT...A BROAD MID- TO UPPER-LEVEL TROUGH CURRENTLY LOCATED OVER THE CENTRAL U.S. FROM THE GREAT LAKES SOUTHWARD TO THE CENTRAL GULF COAST IS EXPECTED TO DIG SOUTHWARD OVER THE NEXT 24 HOURS BEFORE MOVING EASTWARD. THIS SHOULD RESULT IN AN EROSION OF THE RIDGE ACROSS THE EASTERN GULF OF MEXICO AND THE SOUTHEASTERN U.S. INCLUDING FLORIDA AND THE NORTHWESTERN BAHAMAS. THIS DEVELOPING WEAKNESS IN THE RIDGE SHOULD ALLOW ISAAC TO TURN NORTHWESTWARD ACROSS EASTERN AND CENTRAL CUBA BY DAYS 4 AND 5. THE NHC MODEL GUIDANCE IS IN EXCELLENT AGREEMENT ON THIS EVOLVING SCENARIO WITH THE ECMWF BEING THE LEFTMOST OF THE MODELS...AND THE GFDL AND HWRF BEING THE RIGHTMOST OF THE MODELS. GIVEN THE RUN-TO-RUN CONSISTENCY OF THE GFS AND UKMET MODELS...MORE WEIGHT HAS BEEN PLACED ON THOSE SOLUTIONS...ESPECIALLY SINCE THEY HAVE A STRONGER AND VERTICALLY DEEPER CYCLONE DEPICTED IN THE MODELS.

SHEAR ANALYSES FROM THE GFS-BASED SHIPS INTENSITY MODEL AND UW-CIMSS INDICATE BOTH THE DEEP-LAYER AND MID-LEVEL SHEAR VALUES HAVE DROPPED BELOW 10 KT...WHICH HAS FINALLY ALLOWED DEEP CONVECTION TO DEVELOP AND PERSIST OVER THE LOW-LEVEL CENTER. THE SHEAR IS FORECAST TO GRADUALLY DECREASE TO VALUES BELOW 5 KT BY 24 HOURS... WHICH SHOULD ALLOW ISAAC TO STRENGTHEN TO A HURRICANE BY 48 HOURS. AT 72 AND 96 HOURS...THE INTENSITY HAS BEEN DECREASED DUE TO EXPECTED LAND INTERACTION WITH HAITI AND EASTERN CUBA. AFTER 96 HOURS...HOWEVER...ISAAC SHOULD BE MOVING BACK OVER THE VERY WARM WATER BETWEEN CUBA AND FLORIDA. WITH SEA-SURFACE TEMPERATURES
EXPECTED TO BE NEAR 30C AND THE GFS AND UKMET MODELS DEPICTING A EXTENSIVE UPPER-LEVEL OUTFLOW REGIME COMPLETE WITH DUAL OUTFLOW CHANNELS TO THE NORTHWEST AND SOUTHEAST...STRENGTHENING APPEARS
LIKELY. THE OFFICIAL INTENSITY REMAINS ON THE CONSERVATIVE SIDE DUE TO UNCERTAINTY IN HOW LONG AND HOW MUCH ISAAC WILL INTERACT WITH HISPANIOLA AND CUBA...AND IS CLOSE TO A BLEND OF THE SHIPS... DECAY-SHIPS...AND LGEM STATISTICAL-DYNAMICAL INTENSITY MODELS.

FORECAST POSITIONS AND MAX WINDS

INIT  22/0900Z 15.5N  57.3W   40 KT  45 MPH
 12H  22/1800Z 15.9N  59.7W   45 KT  50 MPH
 24H  23/0600Z 16.3N  62.8W   55 KT  65 MPH
 36H  23/1800Z 16.8N  65.7W   65 KT  75 MPH
 48H  24/0600Z 17.3N  68.4W   75 KT  85 MPH
 72H  25/0600Z 18.8N  73.2W   65 KT  75 MPH...OVER WATER
 96H  26/0600Z 21.8N  77.1W   65 KT  75 MPH...OVER WATER
120H  27/0600Z 24.8N  80.0W   80 KT  90 MPH
UPDATED: South Florida in the 5-day cone for Tropical Storm Isaac