High water in Washington, May 20. (Wolfkann/Flickr)

The rain never seemed to stop but failed to cool us off. May ranked as the sixth-wettest and third-warmest in Washington’s recorded history, which dates to 1872.

The soggy month included a record seven-consecutive-day-stretch in which each day at least 0.25 inches fell, May 13 to 19. The downpours flooded the Potomac River, sending the water level to its highest point in four years.

The humid weather pattern also set the stage for devastating flash flooding in Frederick and Ellicott City, Md., on May 15 and 27. In both locations, the saturated sky unloaded a half-foot of rain in just a few hours.

In Washington, the 8.73 inches that fell was more than double the normal amount for May, which is historically the wettest month of the year — averaging 3.99 inches. In fitting fashion, the month ended with a deluge on its final evening when a vigorous thunderstorm unleashed 1.29 inches of rain.


As wet as it was, it was also abnormally warm. After a March to April stretch that felt more like winter, we ran right into 90-degree heat the first week of May. The month’s average temperature of 72.4 degrees was 6.4 degrees above normal and the warmest since 2015’s 73.2-degree average, the warmest on record.


The radical flip in temperature was sufficient to bring the overall spring temperature very close to average. Precipitation for spring was almost four inches above normal, ending any drought concerns.

May weather extremes and records

Our hottest and coolest weather conditions for the entire month occurred in the first two days with a cool low of 49 degrees on May 1 and a hot high of 91 the next day. We experienced measurable rainfall almost 50 percent of the days in May and had cloud cover close to 75 percent of the time.


Here are the May records set at our nearby airports:

Reagan National:

  • May 2: Record high temperature of 91 bests 89 from 1930.

Dulles:

  • May 2: Record high temperature of 88 ties 2010 and 1992.
  • May 4: Record high temperature of 90 ties 2001.
  • May 12: Record high temperature of 90 bests 89 from 1991 and 1982.

Baltimore-Washington International:

  • May 2: Record high temperature of 90 bests 88 from 1951, 1930, 1913.
  • May 3: Record high temperature of 92 bests 91 from 1913.
  • May 4: Record high temperature of 91 bests 90 from 2001.

May weather pattern

A sprawling and hot high-pressure zone dominated much of the United States in stark contrast with the cool-to-cold upper-level trough pattern in March and April. Nationally, the month is likely to rank as the warmest May on record.


May forecast evaluation

A month ago, we anticipated May would “turn the corner” from a cool pattern to a warmer one — which worked out well. However, our estimate of a dry pattern continuing was incorrect. By mid-May, the new soggy pattern generated lots of instability, and May lived up to its status of being our wettest month of the year.

The transition from a drier pattern (La Niña) to a wetter one (El Niño) will continue to cause consternation in month-to-month forecasting as we’ll probably see both wet and dry spells, but at least the drier periods shouldn’t be durable enough to generate significant drought concerns in the summer.

Our average temperature estimate of 68 to 70 degrees, a few degrees above normal, was not quite warm enough, as the average worked out to be 72.2 degrees.

Our precipitation estimate of 2 to 4 inches was woefully short of the actual 8.73 inches.

Overall, our temperature forecast was in the ballpark, but precipitation was way wrong, so we earn a grade of C (down from last month’s A+).

January-through-May temperature and precipitation rankings

Our very wet May has propelled 2018 to the fourth wettest of the 2000s. This has certainly put any drought and associated severe heat concerns  into the back seat.  The warm temperatures in May have advanced this year to the seventh-warmest of the 2000s.