A subtropical storm could drench the East Coast this weekend

The odds are rising for a storm to form off the Southeast coast later this week before working its way northward this weekend, soaking the Mid-Atlantic and Northeast. While it’s uncertain whether the system will qualify for a name, it’s expected to bring gusty winds, rough surf, rip currents and shoreline flooding along the coast and heavy rain over a swath that could extend well inland.

A very wet weekend is possible from the Carolinas through the Northeast.

The biggest wild cards at this point are how far inland the rain and wind will spread and whether the disturbance will gradually acquire subtropical characteristics. If it does, it could earn the name Ophelia.

There are also two other systems to watch in the Atlantic. Nigel, a Category 1 hurricane, may strengthen briefly as it whirs over the open Atlantic Ocean. Then there’s a tropical wave set to roll off the coast of Africa on Wednesday. It will probably slowly become a tropical depression, the precursor to a tropical storm. For now, it’s expected to stay over the tropical Atlantic while working westward.

An East Coast system to watch

A stationary front is draped across the Florida peninsula; waves of moisture riding along the front have contributed to flare-ups of thunderstorms. Eventually, the thunderstorm activity along the front is forecast to gel into a more concentrated area of low pressure. That surface low will exist off the Florida-Georgia coastline toward Friday into Saturday.

At the same time, an approaching shortwave, or high-altitude pocket of cold air, low pressure and spin nestled within a weak dip in the jet stream, will swing out of the northwest and slip over the surface low. That will intensify it.

Because this storm may be energized by the warm waters of the Gulf Stream in addition to mid-latitude weather features, it may become what’s known as a “subtropical storm.” The National Hurricane Center says there’s about a 30 percent chance of that happening.

Uncertainty in how far inland rainfall will extend

Confidence is high that heavy rain will affect the coastline, arriving first in the Carolinas on Friday into Friday night and then spreading northward, affecting parts of the Mid-Atlantic and Northeast on Saturday into Sunday. Models have varied in their projections of how far inland the rain will spread. Some places might see 2 to 3 inches or more by Sunday, but models conflict on how far west meaningful precipitation will extend.

The European model has a sharper cutoff in the western extent of the rainfall, while the American model shows more widespread precipitation.

It will take another day or two to have more confidence in rainfall predictions.

It’s also too soon to predict specific wind speeds, but the strongest gusts will probably occur near the coast. And the winds will create rough surf and perhaps coastal flooding, particularly around high tide.

Other systems to watch

On Tuesday morning, Hurricane Nigel had winds of 85 mph and was 690 miles east-southeast of Bermuda. It was moving northwest at 13 mph.

Nigel is an “annular” cyclone, meaning it consisted of one large eye surrounded by a single band of convection, or downpours and storminess. Annular cyclones tend to be more resistant to quick fluctuations in strength, so it’s likely that Nigel will plateau in intensity for several days.

It’s expected to pass harmlessly northwest of the Azores, and then largely avoid Europe this weekend.

Forecasters are monitoring another tropical wave over western Africa. It’s rolling westward. The National Hurricane Center estimates a 70% chance that it will develop into a tropical depression or storm as it passes over the eastern and central tropical Atlantic.