The first drops arrived around sunset on Friday, and for a time on Sunday it seemed the rain would never end. When all was done, most of the D.C. area had picked up about two to four inches of rain. The drenching clinched Washington’s wettest year on record with exclamation.

This much rain was enough for renewed flooding and other assorted troubles that accompany repeated downpours.

The 3.44 inches of rain that poured down on Washington between Friday and Sunday marked its highest three-day amount on record for the winter months of December through February. It pushed 2018′s total to 64.22 inches in the city, two feet above the long-term average.

Such a storm deserves a proper dissection. Let’s start with how much it rained.

The heaviest rainfall was concentrated in the heart of the metro region, with widespread totals of 2.5 to 3.5 inches and even higher amounts just west of the District. The heaviest rains were somewhat intermittent, cycling through in waves Saturday and Saturday night. Rain fell at varying intensities for 45 straight hours between late Friday afternoon and early Sunday afternoon.

What caused such massive rain totals?

A triad of meteorological factors help explain why it rained so much.

  1. A highly amplified jet-stream pattern locked over the East Coast, which was overtopped by a blocking high pressure to the north. 
  2. A developing coastal low-pressure zone at the surface and additional enhancement from a slow-moving low-pressure zone at high altitudes.
  3. Record-setting amounts of moisture streaming over Washington from the Caribbean and the Atlantic’s Gulf Stream. 

The interaction of these elements is shown in the satellite image below.

With multiple corridors of highly saturated air intersecting our region, the stage was set for a historic rain event.

The water vapor satellite imagery here highlights the location and amount of mid-atmospheric moisture. The entire East Coast pattern developed around a meandering upper-level low in the middle and upper atmosphere (labeled here as “Slow Moving Upper Vortex”). On the east side of this low, winds out of the south drew a massive plume of tropical moisture northward over the Washington region. This area is labeled “Deep Moisture Conveyor Belt.”

At the same time, a low-pressure system at the surface developed over the Hampton Roads region (labeled “Surface Low”). In combination with another surface low over eastern Kentucky (not shown), it drew in a plume of low-level, moist air off the Atlantic Ocean. This separate branch is labeled “Low Level Atlantic Moisture,” and it flowed over Washington beneath the deeper, tropical plume.

Additional details are fleshed out in the diagram below. It is primarily created by meteorologists for forecasters, so it is a bit complex. Mostly take note that our immediate area was targeted for dangerous flooding Saturday night, shown by our inclusion of the orange, scalloped line.

First, note the low-pressure system over Virginia’s Hampton Roads area and the second low-pressure zone over Kentucky.

The storm over Kentucky was fading and transferring its energy to the coastal low. A stationary frontal zone extended between the two lows (purple line with scallops and triangles). That east-west-oriented front focused persistent bands of moderate to heavy rain over our region as deep moisture impinging from the south was lifted by the front.

Also shown is an invasion of moisture-rich air off the Atlantic, from the east. The heavy green line denotes the leading edge of unusually moist air for this time of year. (It set a record at Dulles International Airport’s weather balloon site.) Moisture was propelled westward on a current called a “low-level jet stream.” Here, its core flow approached 55 to 60 mph about a mile above the ground. This moisture current was being drawn inland from the combined effect of low-pressure systems.

Although we did not have any cold air to work with this time, this kind of setup is a great way to maximize precipitation locally, as we saw this weekend. In many ways it is similar to how big winter storms do their damage, including 2010′s blockbuster Snowmageddon.

The next graphic shows what happened at jet-stream level in the upper atmosphere. This is around commercial-aircraft height, or about 30,000 feet above sea level.

The key player here was the giant upper-level low-pressure system slowly churning east. Winds circulating counterclockwise around this center overspread the Mid-Atlantic from the south. Again, this helped tap tropical moisture, and the airflow also contained an embedded pocket of extra-speedy wind, called a “jet streak” (outlined in a heavy blue line).

Air rises in the “left exit region” (A) and sinks in the “right exit region” (B) of these jet streaks (because of unbalanced airflow). This kind of upper-level pattern helped accelerate those low-level winds off the Atlantic.

An important feature here is the fanning apart of upper-level winds on the east side of the lumbering upper-level low. This phenomenon has a nerdy name: difluence. In the case of the weekend, as air escaped horizontally over the Mid-Atlantic, more had to be drawn up from below to fill the void. This went to work for nearly 48 hours over the Washington region, drawing copious amounts of moist air upward before delivering it back to us in the form of rain.

The records

We already mentioned how this kind of storm shares similarities to Snowmageddon, which is about as big a snowstorm as our region can see, based on history. And it does appear that in a similar light, this one dropped about as much rain as you can get out of a storm in winter (not including March, which tends to bring juicier storms) locally.

A total of 2.55 inches in one day is a very rainy day any time of year. In winter, when less moisture is available in the atmosphere, it’s close to as much as the sky can squeeze out in 24 hours. The 2.55-inch total for Saturday demolished the record for Dec. 15 of 1.38 inches in 1901. That total was good enough for fourth-wettest day in December and the fifth-wettest in meteorological winter (December to February).

Even more, the storm total of 3.44 inches of rain in Washington is higher than any other three-day total on record for the city during climatological winter. This means we can think of it as the wettest storm to hit the region in December through February.

For comparison, Washington’s “wettest” winter storms that produced snow occurred on the Presidents' Day weekend of 2003, with the equivalent of 2.85 inches of liquid (or melted snow), and in January 1922, when 2.81 inches were recorded during the Knickerbocker snowstorm.

This record winter rainstorm developed from numerous, interacting elements, involving the entire depth of the weather-bearing atmosphere. And it’s another remarkable example of a highly amplified, distorted jet-stream pattern that has triggered numerous heavy-rain events and helped make 2018 Washington’s wettest year on record.