A September nor’easter? Seriously? Here’s what to expect later this week. - The Boston Globe

This storm is a lock to develop, but the weather models are divided on how this coastal system will track this far out.

With astronomical fall set to begin Tuesday, there’s no waiting for what could be the first significant storm of the season — a potent coastal system set to materialize midweek off the Mid-Atlantic coast and encroach on New England by Friday, possibly developing into a strong nor’easter.

What we know is that this storm is a lock to develop, but the atmospheric variables naturally have models divided on the probable storm track this far out. A complete miss is on the table, but so is a nor’easter that could impact us through much of the weekend.

Tropical storm in the mix?

You also might be asking, “What about a tropical storm or hurricane?” And it’s a good question. I’ll explain more on this in a bit, but the seas aren’t really warm enough for this storm to be considered “tropical,” so I doubt the National Hurricane Center will label it as such.

Whether New England sees scattered pockets of light rain or strong winds, heavy downpours, and coastal flooding will all come down to the key position and strength of high pressure north of New England by late Thursday into Friday.

Will it be more south and strong enough to bump this storm away? Or keep the door open for the storm to slam the region?

The Globe model reveals a closer, more impactful storm, while the large-scale GFS (Global Forecast System) shows the storm passing out to sea. Even the AI models have wide uncertainty at this point.

If the storm closes in on New England, the onshore winds paired with the higher tides from a full moon later this week could mean some flooding along the coast.

Are early fall nor’easters rare?

September is not usually a nor’easter-producing month; in fact, only about 1 percent of nor’easters on record develop during this stretch. However, this setup reveals a classic nor’easter pattern, with a surface front stalling off the Mid-Atlantic coast on the eastern side of a dipping jet stream (the storm highway). This is the recipe for a new low-pressure system to form, as upper-level winds on the eastern side of this jet stream are so strong that they actually pull up air from the surface and, in most cases, form a storm.

Take a look, below, at Thursday’s setup versus the traditional nor’easter setup:

We have to keep in mind that a nor’easter is characterized by winds blowing into the region from the northeast. Any deviation in the storm track can nix that label, but this system would still remain a “coastal storm.” Regardless of label, there is the potential for impacts along the New England coast. I mentioned the full moon, meaning higher tides will make the coast more vulnerable to flooding, but just how impactful remains to be seen as we narrow in on the storm track.

What the models are showing

Let’s break down the possible scenarios and what the weather models are saying.

I took a look at surface features and upper-level wind patterns; the jury is still very much out on how things will develop. It does appear that high pressure over southern Canada will be pretty strong, but depending on its position, it will either provide room for the storm to curl into the region or deflect this storm.

• Scenario 1: The European model (ECMWF) agrees with the Globe model and pulls the storm in tighter to the coast, which means strong winds, some coastal flooding, and pockets of heavy rain from Friday through a chunk of Saturday.

• Scenario 2: The GFS, ICON, and UKMet models all show the storm pulling farther away from the New England coast, mainly due to high pressure pressing farther south and east over southern Canada and the jet stream progressing more offshore as this storm forms.

• Scenario 3: This scenario isn’t shown, but high pressure could slide south into New England and keep the storm from really progressing north at all, forcing the storm to spin itself out over the Mid-Atlantic with only a few showers pushing into our region.

As of right now, I think we end up seeing a storm that passes nearby, but not as bullish as the European and the Globe models. I think that high pressure does build strong enough to limit impacts, but still — Friday and Saturday could turn interesting with some gusty winds up to 50 mph by Cape Cod, pockets of rain, and some coastal flooding incidents — technically making this event a mild nor’easter.

Of course, this forecast will change with five days to go, but here’s an idea of conditions for Friday — windiest by the coast, with rain up to 2 inches.

Could this storm be named?

The short answer is probably not. What’s important is that, regardless of title, some coastal flooding is likely. Sea surface temperatures need to be 80 degrees or warmer to fuel a developing tropical storm or hurricane. This storm should lack that heat, making it a coastal storm instead of an official tropical system.

A lot will change within the next couple of days, but we’ll keep you updated with the latest. Stay tuned!

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Ken Mahan can be reached at [email protected]. Follow him on Instagram @kenmahantheweatherman.