Rare September nor’easter could bring coastal flooding, gusty winds to the Northeast

By Meteorologist Mary Gilbert, CNN

(CNN) — A potent storm could develop into a rare September nor’easter later this week, raising concerns for coastal flooding, wind damage and rain in parts of the Northeast.

A quick refresher: A nor’easter is a strong coastal storm that gets its name from the direction its punishing winds come from — northeast. Nor’easters typically develop off the East Coast between Georgia and southern New England and are most common from mid-fall to mid-spring. The storms are typically major snowmakers in the winter, but that’s not a factor this time.

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Right now, the potential nor’easter is just a twinkle in the atmosphere’s eye. It’s forecast to form off the coast of the Carolinas or Virginia sometime Wednesday or Thursday.

It will likely strengthen from that point and peak in strength this weekend. But where exactly the storm tracks as it heads north is still in question.

Regardless of its track, some coastal flooding issues are guaranteed, even when the storm is in its early stages. More widespread impacts depend heavily upon how closely it hugs the East Coast.

There are two main scenarios we’re monitoring now.

Scenario 1: Closer to the coast, stronger impacts

Some weather forecast models are depicting a storm that beefs up very quickly from mid-to-late week as it slides up the East Coast to linger near the coasts of New Jersey and New York’s Long Island this weekend.

A stronger storm closer to the coast means powerful, gusty winds would hit nearby areas, including Boston and the rest of southern New England, New York City and even parts of the Mid-Atlantic.

It’s too early to say exactly how strong these gusts could become, but some up to 50 mph are possible, especially Friday into Saturday. These winds will also rough up seas considerably and intensify coastal flooding, pushing tides 2 to 3 feet higher than normal.

Rain is likely, but it’s difficult to pinpoint if there will be any flooding concerns. Some areas may not be able to soak up even moderate rain as readily as normal. Parts of the Northeast and Mid-Atlantic have been soaked in recent weeks and dangerous flooding struck earlier this month in New Jersey and Connecticut.

The storm could then loiter just off the coast into next week. The longer it’s out there, the longer impacts will linger.

The nor’easter would also hover over warmer than normal water off the coast for an extended time in this scenario. Nor’easters are not warm-water fueled tropical systems like hurricanes, but if they hang out over warm water long enough, they can become hybrid systems — fed by both the ocean heat and energy in the atmosphere — or fully tropical.

This nor’easter is unlikely to become tropical, but it’s not impossible. If it did, the impacts wouldn’t change much; the National Hurricane Center would just have to keep an eye on it.

Scenario 2: Out to sea, limited impacts

Other forecast models predict a potent storm, but keep it farther off the East Coast.

Rough seas and similar levels of coastal flooding are still likely with this scenario, but the wind and rain threats would not be as intense as in scenario one.

The timing of the storm’s development and its generally northeast track is similar in this scenario, but where it could linger is different.

The most noticeable impacts along the coast south of New England would likely occur through Friday.

The storm could remain off the coast of southern New England through at least Sunday, meaning coastal flooding issues could persist there, especially around the Massachusetts Cape.

Fall chill and the full moon will drive coastal flooding

Coastal flooding is a lock this week, even if the nor’easter stays farther offshore, with two other factors driving the threat.

The first is a strong area of high pressure set up over eastern Canada, sending crisp and chilly air to the Northeast. High temperatures in the region are forecast to only climb into the 60s into the weekend as a result.

As the nor’easter takes shape off the Mid-Atlantic coast on Wednesday or Thursday, air will be squeezed between the fledgling storm and that high pressure farther north, increasing wind speeds along the East Coast.

The second factor at play: a full moon. Tide levels will climb higher each day until they hit their monthly peak around Saturday’s full moon. Higher water levels and stronger winds are the exact combination for serious coastal flooding.

Coastal areas from Virginia to New Jersey could start seeing some flooding at high tide as early as Tuesday evening, and some gauges could push into major flood stage by Thursday morning.

Flooding issues will likely peak late this week or early this weekend for many spots, and could be slow to ease next week if the nor’easter lingers close to the coast.

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