A nor’easter is about to hit the Northeast in May. That’s very unusual.

Nor’easters are powerful storms that form near the East Coast, driven by the strong temperature contrasts that arise when cold, continental air collides with warm, moist air over the Atlantic.

People living along the Eastern Seaboard may be familiar with these storms, but they are much less common during late spring because the temperature differences that form them tend to be weaker.

This makes the nor’easter about to drench the Mid-Atlantic and Northeast starting late Wednesday a rarity for May. The storm is expected to bring heavy rain and wind to millions through Friday, including some areas that recently experienced flooding.

According to historical climate data, there have been only two May nor’easters in the past 35 years — one in 2008 and another in 1990. The storms are named for the northeasterly winds they bring and are known for their potential to cause winter blizzards.

Helping to fuel this nor’easter is a heat dome over Iceland, which drove temperatures in that country near the Arctic Circle to nearly 80 degrees (26.6 degrees Celsius).

Because of that pattern, unseasonably cold air was displaced southward, leading to March-like temperatures and now heavy rain and strong winds in the Northeast.

What this nor’easter will bring

A nor’easter during late spring won’t bring snow, unless you’re on Mount Washington in New Hampshire, but will instead come with heavy rain, strong winds, rough seas and cold temperatures.

Heavy rain

Rainfall amounts of 1 to 3 inches will be common across the Mid-Atlantic and Northeast from Wednesday to Friday.

Flood watches are in effect through Wednesday night for southwestern Pennsylvania, including Pittsburgh, the panhandles of West Virginia and Maryland, and the Blue Ridge Mountains of Virginia, where saturated soils could make conditions more susceptible to flash flooding. These adverse conditions will be caused by a separate weather system that will help form the nor’easter, not the nor’easter itself.

Rain will be particularly heavy in New York on Thursday, where flight delays are a likelihood, with downpours arriving in Boston during the afternoon and lasting through the night. Heavy rain will also hit the Litchfield Hills of Connecticut and Berkshires of Massachusetts, where isolated flooding can’t be ruled out.

Nor'easter-driven downpours in the Mid-Atlantic and Northeast through Friday may cause isolated flooding.

The nor'easter will bring strong wind gusts of up to 60 mph to coastal parts of New England on Thursday, causing rough seas, blowing around loose items and perhaps small tree branches.

Strong winds and rough seas

Wind gusts of 40 to 60 mph will buffet coastal areas from near Atlantic City to Portland, Maine, from Wednesday night into Friday. Some of the gustiest conditions will occur in Nassau County on Long Island and across Cape Cod and Nantucket, Massachusetts, where loose items may be blown around and perhaps small tree branches downed.

Waves of 10 to 15 feet will occur just offshore from Long Island to Portsmouth, New Hampshire, from Thursday into early Friday, leading to rough seas and dangerous beach conditions.

Cool temperatures

Daytime temperatures will struggle to reach 50 degrees in parts of the interior Northeast on Thursday, conditions more typical of late March.

A handful of low daytime-high-temperature records are expected to be broken, such as in Poughkeepsie and Albany in New York; Bridgeport and Hartford in Connecticut; Worcester in Massachusetts; and Concord and Manchester in New Hampshire.

An early nor’easter forecaster

Some of the earliest observations of nor’easters come from Benjamin Franklin, known for his deep curiosity in science and experimentation with lightning, electricity and the Gulf Stream.

In the fall of 1743, Franklin, of Philadelphia, had hoped to watch a lunar eclipse, but a sky full of clouds and rain blocked his view.

However, Franklin’s brother, living in Boston, reported being able to see the eclipse, with clouds rolling in sometime thereafter.

With this knowledge, Franklin began a research effort that included gathering weather reports from newspapers along the entire Atlantic seaboard.

His investigation led him to conclude that storms move northeastward near the Eastern Seaboard. Though he initially thought they moved at 100 mph, modern meteorological knowledge informs us that their speed typically ranges from 30 to 50 mph.

Just as Franklin observed 282 years ago, rain from this week’s nor’easter will arrive first in Philadelphia before it reaches Boston.