The Washington PostDemocracy Dies in Darkness

A soggy December put an exclamation mark on Washington’s wettest year ever recorded

Washington, Dulles and Baltimore each broke all-time precipitation records in 2018

Madeline Poe walks through the rain down a cobblestone alley on Dec. 16 in Alexandria, Va. (Matt McClain/The Washington Post)

Water was still falling from the sky in the moments before midnight Monday night, a fitting close to Washington’s wettest year in recorded history.

Washington’s grand total of 66.28 inches of rain (and melted snow) in 2018 blew away the prior record of 61.33 inches from 1889 and was 26.54 inches (67 percent) above the normal amount of 39.74 inches. This amount of rain is equivalent to average amounts in the wettest cities in the U.S. - mostly along the Gulf Coast - including New Orleans and Mobile, Ala.

Dulles’s 66.74 inches, just slightly more than Washington, edged past the 65.67 inches in 2003, the previous record holder.

Baltimore’s 2018 precipitation total was even higher. Its 71.82 inches obliterated the prior record of 62.66 inches from 2003.

Washington received almost as much precipitation in 2018 (66.28 inches) as it did in 2016 (31.7 inches) and 2017 (35.6 inches) combined (67.3 inches). Every month except January, March and October had above-normal rainfall. Five months (May, July, September, November and December) ranked among the top 10 wettest on record.

At least an inch of rain fell on 24 days in 2018, the most on record in Washington, besting 21 days in 1878.

Not only was it exceptionally wet but also abnormally warm. Washington’s average temperature of 59.8 degrees in 2018 ranked as the ninth-warmest on record (since 1872). It was 1.6 degrees above normal and the sixth-warmest year of the 2000s.

December 2018 recap

Warm and wet, December was a microcosm of 2018. The total precipitation of 5.82 inches was 2.77 inches above normal and ranked eighth-wettest on record. The average temperature of 43.5 degrees was 3.8 degrees above normal and the 19th-warmest on record.

No snow fell, but it came close. Accumulating snow fell as nearby as Prince William County on Dec. 9, and the 11.5 inches that fell in Richmond gave that city its sixth-snowiest December on record.

The snow shutout followed 1.4 inches that fell in November. Only one other winter on record (1888-89) in Washington has had at least an inch of snow in November and then none in December.

The month was somewhat split as the cold November pattern lingered into the first third of December before the pattern turned mild through the holidays. Consequently, the month’s coldest days clustered early on, with the warmest weather later.

An enhanced southern branch of the jet stream fueled by a developing El Niño event continued to increase precipitation in storms that came up from the south.

A heavy rainstorm Dec. 14 to 16 broke daily records for Dec. 15 in Washington, Dulles and Baltimore with 2.55 inches, 2.49 inches and 2.24 inches falling respectively. The 3.44 inches of rain that poured down over the three days in Washington marked its highest three-day amount on record for the winter months of December through February.

The science behind the record-breaking winter rainstorm

It was this extreme rain event that catapulted 2018′s rainfall total to the highest on record in Washington.

Reviewing our December forecast

A month ago, we posted the following outlook for December:

Temperatures: We predict temperatures to run near to below normal. It has a chance to become the second colder-than-normal December in a row. The average temperature should end up somewhere from 38 to 41 degrees (the 30-year normal is 39.7 degrees).
Rain: We project a continuation of wetter than normal conditions, with enough rain to make 2018 the wettest year on record. The total amount of rain and melted frozen precipitation should reach 3.5 or more inches (the 30-year normal is 3.05 inches).
Snow: November’s snow was above normal and, with December’s 30-year average of only 2.4 inches, I believe we can do it again. We have a chance of snow this weekend and, even if that misses, should have additional chances later this month. My call is for four inches or more for December.

It didn’t turn out well.

Our temperature forecast was wrong, as the average temperature of 43.5 degrees was well above our predicted ranged of 38 to 41 degrees.

We did well with the rain forecast, correctly calling for more than 3.5 inches.

The snow forecast, however, was a big bust. We called for four inches and got shut out. We literally missed being correct by just 50 miles, as Fredericksburg received that amount of snow in the storm that slid just to our south on Dec. 9.

Given the flawed temperature and snowfall forecast, we must give ourselves a failing grade. Monthly outlooks are hard.

Jason Samenow and Ian Livingston contributed to this report.