Dynamic rainfall patterns in the nation's capital

 

A September day in the nation's capital

The nation’s capital has experienced no shortage of extreme weather over the last two decades. Most notable are its wildly divergent rainfall totals with three of the wettest and four of the driest months on record since 2005. That includes DC’s driest August last month when a paltry 0.2” was observed at National Airport. Official weather records in the nation’s capital date back to 1871.

DC’s driest months over the last 20 years include August 2025, February 2009, March 2006, and September 2005. To add to that, Washingtonians have experienced another eight months over the same 20-year time period that finished among the Top 10 driest months on record.

The easiest way to offset prolonged periods of unusually dry weather would be for a decaying tropical system to bring widespread and beneficial rainfall. A good example of this occurred in the fall of 2005. Following DC’s driest September, there was a hard pivot to wetter than average weather. The nation’s capital experienced its wettest October on record in 2005 with 9.41” of rain. That was largely the result of record-setting rainfall on October 7 and October 8 with a two-day rainfall total of 7.30”. This heavy rain was the result of the remnants of Tropical Storm Tammy and a cold front. 

The nation’s capital experienced its wettest November in 2018. Not only did 2018 see DC’s wettest November, but also had its sixth wettest May, fourth wettest July and fifth wettest September. Not surprisingly, 2018 finished as DC’s rainiest year with a record annual total of 24 days that had at least an inch of rain.

In addition, the liquid equivalent of snow is measured as rainfall. The snowiest winter season in the nation’s capital was in 2009-2010. It included DC’s snowiest December (16.6 inches) and two snowstorms that rank among DC’s Top 10 largest. The first occurred on December 18-19, 2009 with 16.4 inches. Another winter storm, dubbed “Snowmageddon,” occurred on February 5-6, 2010, left 17.8 inches at National Airport. DC’s previous record for snowiest winter was during the 1898-1899 season. 

Large swings between very wet and very dry months have been a frequent occurrence in the nation’s capital over the last 20 years. No single event can be attributed to a changing climate. However, climate change is increasing both the intensity and frequency of extreme weather events, according to the Fifth National Climate Assessment (November 2023).

NOAA updates its 30-year climate averages every decade and the current 30-year period for the nation’s capital is for 1991-2020. Since the 2010s were generally a wetter decade than the 1980s, many of DC’s monthly rainfall averages increased. For example, May had been DC’s wettest month of the year for the 30-year period from 1981-2010 (3.99”), but DC now has two months that average over 4” of rain (June and July).

It’s worth noting that 2025 has an annual rainfall deficit of 1.03” since January 1. If 2025 were to finish with below average rainfall in the nation’s capital, then it would be the third consecutive year. Washington, D.C. hasn’t had three consecutive drier than average years since a six-year period that ended in 2002.