When Isaias got free of the complicating effects of the Caribbean islands, it strengthened quickly into a hurricane. It’s now charging into the Southern Bahamas on a path through the island chain. The water is warm and the structure of the storm is conducive for strengthening.
It is forecast to encounter some somewhat-hostile upper winds, which should moderate its ability to get super strong, but the moderating effect of those winds has so far been less than forecast.
A stronger storm is taller in the atmosphere, and is therefore steered by higher-level winds. It is often the case that the upper winds will cause a strong system to turn north sooner than if the east-to-west lower winds were the greater influence. Isaias is no exception.
A more abrupt turn to the north means a track farther away from South Florida, meaning the odds favor the worst of the effects of the storm passing to the east. This combined with the lopsided nature of this hurricane - with the worst weather on the east side - is giving us optimism that the most dangerous effects will stay offshore.
Some gusty squalls with heavy tropical rain are still forecast to affect South Florida tomorrow. Some gusts could reach 60 mph. Local flooding is also possible if an especially intense band of rain rotates over the same areas. Tides will be running high as the ocean water is pushed toward the shore by Isaias.
Unfortunately, this stronger version of Isaias is forecast to come very close to Nassau, and may directly impact Abaco and/or Grand Bahama Island, where recovery efforts from Hurricane Dorian are sputtering due to the COVID-19 pandemic.
The current thinking is that the winds will pick up in Nassau later today, peaking overnight. The weather in South Florida gets noticeably breezy tonight with gusty downpours rotating through tomorrow. After Isaias heads north, it will leave behind tropical moisture, so thunderstorms will be in the forecast each day going forward.
It is very important that everybody in South Florida monitor the forecasts today to be sure that Isaias tracks as forecast and indeed stays offshore. Ironically, if it ends up weaker and comes up the left side of the cone, it could still directly affect the Florida peninsula, which would make the weather over land significantly worse that with this offshore track.
The current National Hurricane Center forecast is for Isaias to peak as a Category 2 hurricane in the Central Bahamas, which means that preparations there should be made for a Category 3 to allow for the intrinsic uncertainties in the forecasts. This means full hurricane preparations in the Bahamas.
The dry air caused by Saharan Dust over the Florida peninsula, which has limited our rain for the last several days, will still be in place when Isaias arrives in the neighborhood. Upper-level winds are forecast to be blowing more or less from west to east toward the storm, which will try to inject some drying into the circulation. That should make the distribution of bad weather in the storm quite asymmetric – with the worst weather to the east, away from South Florida.
The orientation of the steering flow would seem to favor Isaias coming closer to the Florida peninsula near the Palm Beaches or the Treasure Coast before speeding north. The storm is forecast to be near or over eastern North Carolina late Monday and over or near the Northeast coast on Tuesday.
Slight, unforecastable differences in the angle of the track can mean a landfall or an offshore miss whenever a storm track is parallel to the coast. Everybody along the East Coast north of the Carolinas needs to stay aware of the latest forecasts over the next few days.
Because of the storm’s fast movement once it’s north of Florida, there will not be much time to react if the track shifts such that action is required for safety.
Elsewhere in the Atlantic, a tropical disturbance near Africa has a brief window of time to develop, but it will stay on that side of the ocean. Another disturbance east of the Caribbean island has a slight chance to develop in the next few days, as it turns north into the Atlantic. It may have to be watched next week.