A llama at the Goodstone Inn in Middleburg, Va.,  on May 22. (Erin via Flickr)

So far this May, D.C. has had a record number of rainy days (19 of 23) and the coolest high temperatures since 1882. This May’s afternoon highs are running an incredible 15 degrees cooler than last May’s. And we still haven’t hit 80 degrees.

The Southeast Regional Climate Center says D.C.’s recent temperatures most resemble San Francisco’s.

Why has D.C.’s weather been so awful this month?

There’s a very simple explanation. The storm track has been stuck over the Mid-Atlantic, continuously cycling weather system after weather system through the region — like a conveyor belt.

Animation showing jet stream taking different shapes and forms this May, but almost always flowing over the D.C. area.
Animation showing jet stream taking different shapes and forms this May, but almost always flowing over the D.C. area.

The jet stream, the high altitude river of air along which storms usually track, has pointed at and run through the Mid-Atlantic throughout the month.

High altitude wind speed, difference from normal this May. (NOAA)
The difference from normal wind speed  at high altitudes (300 mb or roughly 30,000 feet) this May, which approximates where the jet stream has been flowing. (NOAA)

It is perfectly normal for the D.C. area to be caught in the jet stream’s meandering flow during May. In fact, May is D.C.’s wettest month on average, because the jet routinely zips through the region. But usually, this so-called atmospheric super highway is not fixed on the region for weeks on-end, but on the move.

The jet stream is powered by temperature contrasts, flowing fastest where warm and cool air collide. So when much of southern and central United States start to turn summery as May wears on, its average position shifts toward the northern U.S. Once that shift occurs, D.C. tends to have more warm, sunny days, and fewer cool, damp days.

Difference from normal pressure (or heights) at altitude of about 18,000 feet. (NOAA)
Difference from normal pressure (or heights) at altitude of about 18,000 feet. Lower pressures are associated with more storminess. (NOAA)

This May, however, the usual northern jog of the jet stream hasn’t yet occurred in the Mid-Atlantic, although it’s right around the corner. While it’s tempting to try to find an explanation for this, there isn’t one.

The specific configuration and position of the jet stream in any given year at a given location is somewhat random. Last year, it moved north of the Mid-Atlantic at the very beginning of May and highs were in the 80s for almost the entire month. Very little rain fell.

Sometimes certain atmospheric patterns like El Nino can stack the deck for certain jet stream configurations, but there’s no clear association between El Nino and cool, wet Mays in D.C.

This May’s relentless rainy, cool pattern is best attributed to bad luck, in my view.

Capital Weather Gang asked its followers to send in their best haiku to describe the Washington area as it experiences the most consecutive days with rain in its recorded history. (Ashleigh Joplin/The Washington Post)

If you’re curious just how rainy, cloudy and cool this month has been, I’ve compiled some very telling statistics below …

The rain

  • D.C. has recorded at least a trace of rain on 19 of 23 days in May and, if we include late April, 23 of the last 27 days.  5.04 inches of rain has fallen since the beginning of May, and 5.68 inches since April 27.
  • From April 27 to May 11, D.C. logged measurable rainfall on 15 straight days, the longest streak on record by five days.
  • This May, measurable rain (at least 0.01 inches) has fallen on 18 days, most on record month-to-date (three days ahead of 1953, the May with next most rainy days).  If measurable rain falls on three more days this month, that would mark the most rainy days on record in May, passing 2003’s 20 days.

The clouds

  • D.C. has had overcast conditions at noon 77 percent of the time this May, the highest fraction on record and way above the normal of 32 percent.

(Iowa Environmental Mesonet)

The cold

  • This May’s average high temperature is just 66 degrees, the second coldest on record month-to-date.  It is an amazing 15 degrees cooler than last year’s average month-to-date high of 81 degrees.  Month-to-date, the only cooler year on record  is 1882 (average high of 65.5 degrees).
  • All 22 days have had highs below 80. 1935 is the only other year not to have an 80-degree day this deep into May. (1935 finally hit 80 on May 26, but it’s unlikely this year will go that deep given the forecast for highs in the 80s on Wednesday, May 25).
  • We’ve had 15 (of 22) days with highs below 70, tied for the most on record with 1954.
  • We’ve had five days (of 22) with highs below 60, tied for 4th most on record.