Winter is coming … in May.

An extraordinarily strong blast of cold air for the time of year will spill over the Washington region late Friday into the weekend, potentially setting records while bringing frost to some and even a small chance of wet snowflakes.

The wintry blast will put an exclamation mark on a cool, dreary weather pattern that began in mid-April, during which the overwhelming majority of days have seen below-normal temperatures and rain. Measurable rain has fallen on 12 of the past 14 days, totaling more than four inches.

During the upcoming polar plunge, to follow yet another bout of rain, temperatures will sink 20 to 25 degrees below normal. If we manage to see snowflakes, it will be one of the latest instances in recorded history and only the 4th time in the month of May.

Washingtonians who have put away their heavy coats may regret that decision.

Record-low temperatures and frost

The drop in temperatures will come suddenly.

After temperatures climb into the 60s early Friday afternoon, the Arctic cold front will arrive by evening, with temperatures falling through the 50s around sunset. Overnight, they should eventually plunge into the mid- to upper 30s. For perspective, Washington’s average last instance of low temperatures below 40 is April 13.

Saturday will be downright blustery, with highs struggling to make 50 in the colder parts of the area and only 50 to 55 elsewhere. Washington’s coldest high on record for May 9 of 52 degrees, set in 1877, could be challenged. Wind chills will be in 30s and 40s for much of Saturday, as gusts reach 30 mph. It could be the coolest May day in more than a decade.

The highs predicted are more typical of late February and early March, whereas the average high for May 9 is 74 degrees.

Winds will relent some on Saturday night, but this is when temperatures probably fall to their lowest levels, ranging through the 30s. Areas west of the Beltway could see an unusually late frost or freeze.

The National Weather Service writes there is “a threat of frost/freeze conditions for most locations west of the metros as temperatures fall into the low to middle 30s,” while noting “[s]ensitive vegetation will certainly be at risk, especially in the sheltered valleys.”

Inside the Beltway and to the east, lows in the upper 30s to near 40 are more likely, but it’s not out of the question that a few pockets even within this zone see patchy frost if skies clear and winds calm, promoting ideal radiational cooling.

Areas near the Interstate 81 corridor have the highest probability of freezing temperature, but places even closer to the metro area aren’t out of the woods.

Dulles International Airport could see freezing temperatures early Sunday and challenge the date’s record low of 31 (last set in 1983); however, cold of this magnitude so this late in the season is not unprecedented there.

Dulles’s latest freeze on record occurred on May 22, 2002, and, as recently as in 2013, it occurred on May 14. This is why, if you live well north and west of the Beltway, it’s normally not safe to put in sensitive plants until this late into spring. Dulles’s average last freeze is around April 20.

After the potentially frosty start on Sunday, while much cooler than normal, it will be sunny and pleasant with highs in the low 60s.

Snow

A historically late appearance of snow in the region is somewhat of a long shot but cannot be ruled out Friday night and Saturday. There will actually be two opportunities for flakes.

The first chance on Friday night after the cold front passes if low pressure along it forms quickly enough to draw moisture back over the incoming Arctic air. A few models show the rain briefly mixing with and changing to snow late Friday (after 9 or 10 p.m.) into very early Saturday morning as temperatures fall into the 30s. The window for snow to fall is narrow and, because temperatures will have risen into the 60s earlier on Friday, the atmosphere will have to cool unusually fast for a change from rain to snow to materialize.

Everything would have to come together just right for snow in the immediate D.C. area (inside the Beltway), so such chances are low. Even during the heart of winter these ingredients often fail to gel under similar circumstances. Areas toward I-81 and especially into the mountains have a better chance to see snow, although it’s not a lock even there.

Temperatures would probably be too warm for snow to stick, although there’s a remote chance of a bit of slush on the grass if conditions line up perfectly.

The second chance to see flakes is on Saturday morning as a fast-moving disturbance high in the atmosphere swings by. It could generate a few showers that, especially in the colder parts of our region, could include snow. Any snow during the day Saturday would almost certainly not accumulate.

If flakes are observed Friday night and/or Saturday, it would mark the second-latest occurrence on record in Washington. May 10 is the latest Washington has seen snowflakes in recorded history, way back in 1906, when a trace was recorded (a trace of snow means flakes were observed in the air but did not stick to the ground).

In the extremely unlikely event measurable snow occurs it would mark the first occasion during May on record. April 28 is the latest Washington has seen accumulating snow; 0.5 inches fell on that date in 1898. Washington weather records date back to 1871.

The mountains of Western Maryland and eastern West Virginia have the highest chance to see some rare May snow accumulation.

Robert Leffler, a retired National Weather Service climatologist who closely monitors and forecasts weather in the West Virginia high country, wrote that “there is the potential for a record setting, late-spring 4-to-8 inches of snow” around Canaan Valley (elevations above 3,000 feet).

Will this pattern change?

The cold spring weather pattern is the opposite of what the region experienced during winter, which ranked among the warmest and least snowy on record.

The frequent rains and cool temperatures have intensified cabin fever for many Washingtonians stuck at home during the coronavirus pandemic. However, the conditions have had the beneficial effects of somewhat suppressing pollen levels and contributing to the cleanest air the region has seen in decades.

There are signs that this chilly pattern will finally relent in about a week, near the midpoint of May.