* Frost advisory for Fauquier, western Loudoun, northern Montgomery and Frederick counties, 2 a.m. to 9 a.m. Tuesday *

The fall equinox is one day away, but it already feels like Halloween. It seems pumpkin spiced latte season has come early.

Since Saturday, temperatures more commonly experienced in late October have greeted Washingtonians, with some of the coldest weather during mid-to-late September since about 1930.

The temperature averaged over the weekend (57.8 degrees) ranked as the sixth-coldest on record for the two-day period (Sept. 19 and 20). In fact, it was the coldest on those two days since 1929.

The weekend weather resembled typical conditions for this time of year in Portland, Maine, according to the Southeast Regional Climate Center.

Many Capital Weather Gang readers are taking the chill in stride.

“Loving this pleasant September surprise gift. Awesome weather for outdoors,” @anu2000m tweeted.

“My dog has been bugging me to go outside 5 times already, and I can’t complain!,” @notyourdearabby tweeted.

A few, however, are not quite ready for it.

“I prefer a more gradual transition in temperatures,” @MrTinDC tweeted.

The last three mornings, including Monday, slipped into the 40s in Washington while afternoon temperatures held in the 60s both weekend days.

Temperatures in several of our outlying areas in Loudoun, upper Montgomery, and Frederick counties even plummeted into the 30s with scattered reports of frost.

On Tuesday, temperatures are predicted to settle in the 40s for a fourth straight morning in Washington before they start to moderate during the second half of the week. Patchy frost is once again possible in our outlying areas west of the Beltway.

In perspective

Let’s consider how chilly these temperatures are historically:

  • Saturday’s low of 49 degrees in Washington was the coolest on that date since 1959, when it was 48. It was the first time below 50 that early in September since Sept. 16, 2011, when the temperature also dipped to 49.
  • Sunday’s low of 46 degrees was the coldest Sept. 20 since a 45-degree reading on the same date in 1937. It hasn’t been colder so early in the season since.
  • Monday’s low of 48 degrees was the coldest Sept. 21 since 1962, when it was 44 degrees.

The streak of three straight days with lows in the 40s is the earliest to occur in September in Washington since 1959 when there was such a streak during the Sept. 18-20 period.

If the streak in the 40s extends to four days, it will tie for the 11th longest on record during September in observations dating to 1871, last occurring in 1950. The longest streak of low temperatures below 50 in September is eight days, set in 1875.

The high temperatures since Saturday have also been notably cool. Saturday’s high of 67 degrees was the coolest on Sept. 19 since it was 66 in 1990 and tied for the seventh coolest temperature on record for the date.

On balance, temperatures have been about 10 to 12 degrees cooler than normal since the weekend, pushing September’s overall temperature below normal. The weekend tied for the third chilliest two-day period in the first 20 days of September since 1945 and temperatures were comparable to a typical weekend in late October.

Washington has a chance to have its first cooler-than-normal September since 2017. This year is a major departure from 2019 and 2018 which featured the third and seventh warmest September on record.

The cause of the chill and looking ahead

The chilly weather is the result of a steep dip in the jet stream, which is more common a bit deeper into the fall. It has been unusually chilly not only in Washington but also throughout the Northeast. Scores of cities set record lows Saturday and Sunday. Several locations in the Adirondacks and interior New England dipped into the 20s and were under freeze warnings.

Warmer weather is in the forecast for the second half of the week, with highs well into the 70s to near 80 and nighttime lows rising into the 50s to near 60.

But long-range models are hinting that the jet stream will take another big plunge in nine or 10 days, with even colder air sweeping over the region than the present.