(This story has been updated.)

Wednesday update: While available weather records indicate Monday’s event was the heaviest one-hour total since at least 1936 in Washington, it is possible the heavy rainfall event on Sept. 13, 1934 was slightly more extreme. As noted below, the Sept. 14, 1934 edition of The Washington Post reported 3.25 inches fell in an hour that day, which is slightly less than the 3.3 inches that fell Monday. But Capital Weather Gang reader Eric Peterson (eric654) presented evidence that the 1934 hourly total was actually 3.42 inches. We checked the records for heaviest hourly rainfall in a hard copy edition of The Climatic Handbook For Washington, D.C. published in 1949 and it also reports the hourly rainfall from the 1934 event was 3.42 inches, which is slightly heavier than the 3.3 inches in Monday’s event.

Original post from Tuesday

Monday morning’s deluge in Washington was exceptional in its intensity and for the amount of water it dispensed in a short time.

In just an hour, about a month’s worth of rain drowned the District, a staggering 3.3 inches falling at Reagan National Airport.

This hourly output was Washington’s highest since at least 1936 (National Airport is the city’s official weather observing site)

“According to data from the Iowa Mesonet ... the 3.30” recorded between 8:52-9:52 AM yesterday was Washington DC’s highest hourly precip report in records dating back to 1936," the Maxar Weather Desk, a consulting group based in Gaithersburg, Md., tweeted.

Exploring Washington weather records dating back farther, to 1871, there appears to be no heavier one-hour rainfall. A weather system on Sept. 13, 1934 dispensed 3.25 inches in a single hour which, at the time, was more “than any previous 60-minute period in the 63-year-old records of the Weather Bureau” according to the Sept. 14, 1934 edition of The Washington Post.


Estimates of rainfall rates on Monday from NASA's Global Precipitation Measurement Core Observatory. (NASA)

As Monday’s torrent raged, the first-ever flash flood emergency was declared for the city as well as nearby Arlington and Alexandria, which suffered damaging downpours.

At one point Monday, it was raining at a rate of 5.04 inches per hour.

The severity of the precipitation over such a quick window of time had about a 0.5 percent chance of happening in any given year.

But just how much water came down? We crunched the numbers, and the answer is: 3 billion gallons. And that’s only within the perimeter of Washington proper.

(If you include all of Virginia and Maryland in this calculation, the volume balloons to nearly half a trillion gallons, according to meteorologist Ryan Maue, who ran a similar analysis. Meteorologist Ryan Miller estimated over a billion gallons fell in Arlington alone.)

How much water is 3 billion gallons?

That amount of water would be enough to fill 27 million whirlpool bathtubs. It could also fill 1.1 million hot air balloons (would that make them water balloons?).

If hot air balloons aren’t your thing, picture this: All that water could also fill the White House — floor to ceiling — 103 times.

All the water that fell on Washington on Monday would weigh the equivalent of 102 million President Trumps. Stacked end on end, that would be enough presidents to stretch 120,000 miles — exactly halfway the distance to the moon!

Monday’s water could also fill 48,000 rail cars on the Metro. The rainfall weighed the equivalent of 317,000 Metro carriages.

If the rainfall was distributed equally across everyone living within the District, each person would receive 35,000 pounds of it. That’s probably nine or 10 times the weight of your car! It’s also roughly the same as a residential swimming pool. Consider enough water fell for everyone to fill their own swimming pool, and it’s easy to see why Monday’s rain was so disastrous.

Now picture the Potomac River. We reviewed information about the river’s rate of drainage. If the Potomac was to drain directly into Washington, D.C., it would take more than nine and a half hours for it to pass through the volume of water that fell from the sky Monday.

The cloudburst also unleashed enough water to outweigh 136 Washington Monuments. That amount of water could sustain exactly half of the world’s population for a day. It would also be enough to water the Mall for a little over 19 years.

The rainfall totals in inches

If you prefer your rainfall amounts presented in the conventional way, here’s a map showing rain gauge totals in inches reported to the National Weather Service:


Mapped rainfall totals Monday from the National Weather Service.

In a stripe from Frederick, Md., to Alexandria, Va., three to four inches of rain was widespread, with isolated totals exceeding five inches.

A zoomed-out view of totals, drawn from rain gauges and radar estimates, shows the heavy rainfall extended into Southern Maryland as well.


Rainfall between 11 a.m. Sunday and 11 a.m. Tuesday. (National Weather Service Mid-Atlantic River Forecast Center)

A vast moisture supply in the sky

For so much rain to fall to the ground, exceptional amounts of moisture were required and indeed present in the sky.

We know this based on weather balloon and model estimates of PWATs — or Precipitable Water Indices. These are measures of how much moisture is contained in the atmosphere. They answer the question: If the air above expelled all of its water, how much would fall?

PWATs Monday exceeded two inches — near record territory for the date. (Rainfall totals can exceed the PWAT value, like they did Monday, if the moisture supply is continually replenished.)


European model simulation of precipitable water on Monday morning at 8 a.m. (WeatherBell.com)

Given how much water was squeezed out of the air, we can do a quick back-of-the-envelope calculation to see just how much air must have been processed by the storms to begin with. The result? It’s likely at least 1,300 cubic miles of moisture-rich, juicy air passed through these storms, just like air fueling a vehicle engine.

The oppressive humidity Washington experienced Monday was characteristic of Florida weather this time of year. So imagine all the air sitting on top of an area four times the size of Walt Disney World. That’s basically what went into fueling Monday’s intense drenching over the District.