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Posted at 10:30 AM ET, 05/04/2011

What is Washington, D.C.’s weather like in May? Breaking down norms and extremes


Flowers and momentos at the Vietnam War Memorial near Memorial Day. By CWG photographer Kevin Ambrose.
After April’s showers comes May’s flowers, right? Yes -- and more rain as well. While warmth typically continues to increase its grip on the region, there are still volatile mixes with winter’s remnant cold air masses to the north that create energetic storms with plenty of rain. In fact, May is typically D.C.’s wettest month of the year.

May is also the final month of meteorological spring (Mar.-May), and by the end of it -- when 80-degree days are the norm -- summer’s presence is usually widely known.

Keep reading for more in this secondinstallment of a year-long series of posts detailing specific statistics on climate norms, extremes, and averages for the each month in Washington, D.C…

PRECIPITATION

May’s average rainfall is 3.99” (previously 3.82” from 1971-2000)*. As noted, this ranks first among the months at D.C. Whereas April jumped several spots in the new 1981-2010 averages, May retained its previous rank as number 1.


May rainfall, in inches, at Washington, D.C. since records began. (See sources for data)
The maximum rainfall during any May on record was the 10.69”, which fell in both 1899 and 1953. There have been four Mays (on record), most recently in 2008, with over 10” of rain. The least amount of rain that has fallen during the month was 0.41” in 1939.

The maximum daily rainfall in May is 3.24” and it fell on the 5th in 1953. One other day, all the way back in 1900 also saw a 3”+ rainfall. In total, there have been 17 occurrences of daily totals over 2”. Compared to April, average days with at least a trace of rain are slightly higher, 15.6 vs. 15.2. Disruptive rains of .1” or more are also a bit more frequent with 7.3 compared to 6.5 in April.


Change in .1” rainfall days as an average of each 30-year period since the beginning of records at Washington, D.C. (See sources for data)
While we’ve seen noteable changes (mostly upward) in temperature trends at D.C. since the records began, precipitation changes are often not as significant. If looking at the “disruptive” rainfall days, those over .1” in one day, we see very little variation overall in 30-year averages going back to 1871. Rain is of course a bit more random than temperature, so in some ways it is more interesting to see these trends hold relatively steady.

TEMPERATURES


May temperature averages at Washington, D.C. since records began. Red line is the 1971-2000 temperature average provided by the NWS. (See sources for data)
May’s average monthly temperature is 65.6 (per NWS 1971-2000 norms**), a gain of nearly 10 degrees from April. The monthly average has been as high as 73 in 1991, and as low as 59.2 in 1907 as well as 1882. The month starts off with temperature averages above 70 and finishes just above 80.

Highs are typically in the 70s and 80s, 41% and 29% of the time respectively. The coldest high temperature in May was 46 on the 5th in 1917. The warmest high temperature was 99 in 1991.

Nighttime lows are on the way up too, with lows mainly in the 50s (51%) and 60s (31%). There have been no freezes at D.C. past April until the next cold season. May’s coldest temperature fell just short of freezing back on May 11, 1906 with a reading of 33. More recently, the coldest DCA has seen was 34 in 1947. The warmest low temperature in May was 78 on the 30th in 1991.


Number of occurrences of a high temperature equalling 80 degrees or greater on each day in May during the 30-year period from 1981-2010. The 80-degree average arrives on the 30th. (See sources for data)
This is the month the 80-degree average high shows up (right at the very end). Once that average arrives, it lasts until mid-September. At the beginning of the month our odds of seeing an 80-degree day are about 16.7%, and by the end of the month it’s just over 50%.

Though 90+ in April is not super uncommon, May is the typical start of the 90-degree day season at D.C., coming in at 1.6 days per year out of the broader average of 36.4. 1991 was the clear standout in this regard, with a week-long stretch of 90+ to finish the month catapulting it to the warmest May on record.

DATA

30-year (1981-2010) Averages / Overall (All history) Averages...

Highs 70 or above: 23.3 / 22.6
Highs 80 or above: 10.6 / 10.4
Highs 90 or above: 1.6 / 1.6
Highs below 70: 7.7 / 8.4
Lows below 50: 4.8 / 7.1
Lows 60 or above: 10.3 / 9.3
Days with a trace of rain: 15.6 / 15.9
Days with .1” of rain or more: 7.3 / 7.2
Days with .5” of rain or more: 2.9 / 2.6
Days with 1” of rain or more: 0.9 / 0.8

Days with thunder: 4.9 / NA
Days with hail: 0.1 / NA
-This count is done through records on Weather Underground, and does not include the entire historical period.

All-Time Records (High / Low)...

Highs 70 or above: 30 (1953, 1969) / 14 (2003)
Highs 80 or above: 21 (1911, 1944) / 2 (1924)
Highs 90 or above: 11 (1991) / 0 (54 years)
Highs below 70: 17 (2003) / 1 (1969)
Lows below 50: 19 (1967) / 0 (1887, 1953, 1991, 1998)
Lows 60 or above: 23 (1944) / 0 (1901, 1924)
Days with a trace of rain: 25 (1943) / 6 (1880)
Days with .1” of rain or more: 15 (1933, 2003) / 2 (6 years)
Days with .5” of rain or more: 8 (1889, 2009) / 0 (7 years)
Days with 1” of rain or more: 5 (2008) / 0 (71 years)

SOURCES

Daily climate records for National Airport (DCA), 1871-present
- See selected and additional statistics compiled for this post (Excel)

DCA May Daily Norms and Records

DCA Monthly Temperatures

DCA Monthly Precipitation

NWS Baltimore/Washington climate information

NOTES

*All averages, unless otherwise noted, refer to the 30-year period of 1981-2010.

**All instances of monthly temperature average, daily temperature average or daily precipitation average refer to the 1971-2000 climate normals as presented by the National Weather Service. NWS smooths some averages without daily data in a method described here. Monthly rainfall is a simple average and new 1981-2010 data is used here. NWS data on temperature averages should update shortly and will be reflected in these posts.

ABOUT THIS SERIES

While numerous local climatology statistics including daily, monthly, and seasonal normals and records are available from National Weather Service Baltimore/Washington, there are myriad other pieces of information which help frame the story. Rather than attempt to recreate what is already available, this series of posts will act as a companion to most information presented there.

This is the second installment of a series of living documents. Through early 2012, each month will be broken down in a similar or evolving way. As input is taken from the community, some items may be added to or changed in older postings. These postings will be updated as necessary to reflect changes when needed. Other articles may also become part of this series.

Feel free to share thoughts or additional data you may be interested in seeing.

Learn more about each month:

-April Weather in Washington, D.C.
-May Weather in Washington, D.C. (viewing)

Article last updated: May 4, 2011.

By  |  10:30 AM ET, 05/04/2011

Categories:  Local Climate, Latest

 
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