Strengthening to a category-1 hurricane overnight, Isaias will bring strong winds and storm surge to much of the Bahamas today. Thereafter, risk has decreased for much of the Florida peninsula, while risk is increasing for hurricane impacts from the Carolinas through the Mid-Atlantic over the next five days.
Key regional impacts from Isaias:
- Bamahas: category 1-2 sustained wind speeds up to 100 mph with peak storm surge heights of 6-9 feet on Friday.
- Florida: meaningfully reduced risk with tropical storm force winds likely the worst outcome
- Southeast US / Carolinas: much higher risk today of a direct landfall as a category-1 hurricane, possibly stronger
- Mid-Atlantic / Northeast US: increasing concern for tropical storm force winds in the middle of next week.
The latest National Hurricane Center forecast is highlighted below. As a reminder, the NHC cone of uncertainty represents a 67% likelihood the hurricane will verify within the cone, based on forecast skill of the last five years. Examining today’s cone and potential ramifications of verification:
- Down the middle of the cone would be the worst case scenario for the quantity of regions impacted by Isaias, notably the Southeast, Mid-Atlantic and Northeast.
- Track on the left hand side is more problematic for Florida and the Southeast.
- Farther to the right hand side represents the best case scenario, with only the Bahamas seeing direct impacts from Isaias.
Before commencing today’s discussion, here are the outcomes of yesterday’s questions regarding Isaias:
How much does the mountains of Hispaniola disrupt the circulation of Isaias?
- Not as much as forecast, as Isaias emerged more quickly than anticipated as a hurricane last night.
Does Isaias intensify slowly or more rapidly over warmer than average sea surface temperatures between Hispaniola and the Bahamas?
- Intensification over the warmer waters has been more rapid so far than forecasted. A track not making landfall over the Florida peninsula also raises the intensity forecast through the weekend.
Will Isaias make direct landfall on the Florida peninsula, or skirt along the coastline, raising the risk for Southeast and Mid-Atlantic US interests?
- This is the key question for today, discussed further next.
Bahamas Impact from Isaias
A key headline of this morning’s update is that Isaias is stronger than all of the 24 hour intensity forecasts from yesterday. As a result, rather than a weak category-1 hurricane for the Bahamas, it now looks like Isaias will intensify to category-2 strength with maximum sustained winds of 100 mph.
With a more intense Isaias than forecast yesterday for the Bahamas, a mounting issue is the potential for damaging storm surge inundating several population centers in the Bahamas. The islands of New Providence (Nassau, Freeport), Grand Bahama and Abaco, as well as Andros and Exuma are all at risk for peak surge heights of 6-9 feet with the current forecast.
The trend in the forecast over the several days has been favorable for lower impacts to Florida. Whereas the three-day forecast cone of uncertainty was centered on south Florida earlier this week, the animation of the NHC forecast shows the significant decrease in risk as the forecast trends eastward through the latest update on Friday morning.
With the eastward shift in the forecast, the strongest peak gusts now expected along the east coast of Florida are at tropical storm strength.
Carolinas: Much Higher Risk with Direct Category-1 Landfall Increasingly Likely
The consensus of the majority of weather models this morning portray the likelihood of a landfall on southern coast of North Carolina, with the mean expectation from the National Hurricane Center of 85 mph maximum sustained winds by Sunday evening into Monday morning.
The expectation on intensity continues to carry some uncertainty. While conditions are favorable for intensification through Saturday, increased wind shear from a cold front across the eastern US is forecast to slowly decrease the intensity prior to landfall. Yet, models performed poorly with the intensity forecast over the last 24 hours, so there is some risk for slightly stronger outcomes for the Carolinas if the cold front and associated increase in wind shear is delayed.
Mid-Atlantic & Northeast US
Day over day, risk is increasing for some impacts from Isaias further north along the Eastern Seaboard. The overnight European ensemble model has at least 60% of the 51 individual models impacting the coastline north of the Carolinas. This can be seen by the yellow shading in the longer term track forecast below. The probabilities are a significant increase from the 25-30% likelihood from yesterday’s model guidance. The potential impacts to the northeastern US and Canada will be an item to monitor closely moving forward.
The next scheduled update for Hurricane Isaias is planned for Saturday, August 1.
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Official statements from the NHC and U.S. National Weather Service, and those of emergency management agencies supersede this update, and should be closely monitored concerning matters of personal safety.