Tropical Storm Isaac Briefing - 0815 hrs - Aug 23 2012
Bottom Line: Prepare for the possibility of tropical storm force winds from the period Sunday through Monday, with also a possibility of hurricane force winds Sunday night. More specific timing may be provided in the formal Monroe County briefings beginning this afternoon. We recommend you revisit your hurricane preparedness kits for your home and office, and ensure you have all the supplies you will need. The weather over the Keys will remain favorable for preparation activities through Saturday evening. For marine preparations, winds should remain 15 knots or less through Friday night. Caution is advised for small vessel operations Saturday on the Atlantic waters (including inside the reef). Hazardous conditions for small craft will begin Saturday night.
Tropical Storm Isaac has been a difficult tropical cyclone to assess overnight. Reconnaissance aircraft found a rather large, disorganized weather system with a broad low pressure center with multiple points of rotation. This indicates that Isaac is going to take more time to organize, although some forecast models have indicated this lack organization. This also causes erratic motion, as the true "core" of Isaac will tend to bounce around within the broad envelope of the storm.
While the motion overnight has been westerly, it's likely the center will wobble northwest and settle on a general west-northwest path.
Strong high pressure over the western Atlantic will keep Isaac on a general west-northwest path towards the Dominican Republic and Haiti, and conditions do favor strengthening to hurricane strength before any possible landfall. There has been no change to the future steering pattern, to be dominated by a trough of low pressure moving into the southeast United States and Gulf of Mexico this weekend, which will break the high pressure ridge and allow Isaac to turn to the northwest. Needless to say, the error cone has not shifted one bit over the Florida Keys and remains firmly in place.
Intensity forecasts will continue to be low confidence as Isaac tangles with the mountainous terrain of the Dominican Republic, Haiti, and possibly eastern Cuba. Simply put, the more land Isaac travels over, the weaker Isaac will be. Per the latest NHC forecast, Isaac is expected to emerge from the north coast of Cuba Saturday night as a tropical storm, but restrengthen to category 1 strength near the Florida Keys by Sunday night.
If Isaac manages to avoid some of the land, then Isaac will be stronger. As of the 5 am NHC official forecast, there is a 29% chance that Isaac will be hurricane strength late Sunday night, when it could be nearest the Florida Keys. The chance of Isaac being a major (Cat 3+) hurricane at that time remains very low at 2%. The wind probabilities continue to climb for our area and are as follows:
Between now and Monday night (over the next 5 days):
Chance of Tropical Storm Force Winds: 35% at Key West; 39% at Marathon, 36% at Miami
Chance of Hurricane Force Winds: 6% at Key West, 5% at Marathon, 2% at Miami
For military interests, the chance of 50-knot winds at Key West is 14% and 8% at Miami.