Forecast rainfall this week from the National Weather Service.

* Flash flood watch through late Tuesday night *

From flash drought to flash flood, this July is a story of weather extremes in Washington.

A week ago, no measurable rainfall had fallen in Washington during July, a record 16-day dry spell so deep into the month. One week later, the heavens have unloaded more than eight inches of water on the city, and it is one of the wettest Julys on record.


Some of the heaviest rain is probably still to fall. A water hose in the sky, known as an atmospheric river, will blast the Mid-Atlantic, including the D.C.-Baltimore urban corridor, for the next 48 to 72 hours. An additional three to five inches or more of rain may come down, which, if it materializes, would make this the wettest July on record in Washington.

Problematically, the ground is saturated and rivers and streams are swollen, so they cannot take on much more water before flooding occurs. “A potentially dangerous, even life threatening, situation is setting up for much of the Mid-Atlantic,” the National Weather Service Middle Atlantic River Forecast Center tweeted.

The waves of rain will be on and off, but, especially in the afternoon and evening hours, corridors are likely to form over which heavy storms track repeatedly in a process known as “training.” It’s within these training corridors that serious flooding could occur. Unfortunately, they cannot be predicted with much lead time.

Outside the training zones, bursts of heavy of rain are still possible, but they may come through so quickly that water doesn’t build up enough for flooding.

The National Weather Service has placed our region in its moderate-risk zone for excessive rainfall on Monday and Tuesday, signaling a 20 to 50 percent chance of flooding within 25 miles of any location.


(National Weather Service)

Tuesday is perhaps the day of greatest concern. “Things get worse as moisture plume moves a bit more inland and aims at Washington D.C.,” the National Weather Service wrote in a discussion. “The model signal is stronger for heavy rainfall.”

In the near term, flooding is most likely near streams, creeks and areas with poor drainage. Motorists should, when it’s raining heavily (or heavy rain has just occurred), avoid routes near creeks and streams and never attempt to cross flooded roads. The water level is difficult to judge, and when motorists become stranded, they endanger themselves and the first responders forced to rescue them.

Later this week, there is also the growing concern about at least moderate river flooding, if heavy rains continue.


(National Weather Service)

Discussion

The atmospheric river spilling over our region is depicted in Monday morning’s satellite image, using what’s called the water vapor channel. This image depicts regions of moist, middle- and upper-level flow using gray and white shades. We’ve outlined the atmospheric river zone using red lines.


(NOAA, adapted by CWG)

Note how this river in the sky is rooted deep in the Caribbean as it surges northward across the entire Mid-Atlantic. The Weather Service indicates that the flow of moisture into the region is 2½ to three standard deviations above normal, signifying that the conveyor belt of tropical air into the region is quite rare.

A stalled weather front, embedded within this tropical plume, is encouraging all of this moist air to rise. The front is shown below.


(National Weather Service)

The tiny arrows represent the deeper flow within the atmospheric river. The green scalloped region highlights the area that has experienced repeated “training” or passage of storm cells, over the same regions, since Sunday.

This general setup is expected to continue through Wednesday. Not depicted is a broad pocket of rising air, high above the front, that is also helping to lift the nearly saturated air, and connected to the larger jet stream pattern. That pattern features an unseasonably strong trough of low pressure stalled over the Great Lakes and Tennessee Valley. This trough is not expected to begin breaking down and lifting northward until Wednesday — thus shutting off the tap to the atmospheric river.

In the mean time, the axis of the atmospheric river and the front are expected to waffle west and east of the D.C. region. But how much and when these elements shift are difficult to pin down, even in the forecast models.

If the front shifts away, it should draw the most intense focus of rising, moist air with it. This could relax the rainy conditions for several hours in our region. The breaks in the rain, however, are a double-edged sword; in quiescent times, the atmosphere will destabilize, thus fueling the next round of storms.

There are additional impulses of energy moving through the jet stream flow, subtle and difficult to identify, that have been retriggering (and will continue to initiate) episodes of storms. These include small pockets of rising air called shortwaves and small areas of spin streaming through in the middle atmosphere.

The bottom line is that a very wet corridor or “super highway” for rain cells overlies the entire Mid-Atlantic, but the timing and location of the heavy rain episodes will be very difficult to identify.

To add to our problems, there is a non-trivial risk of severe storms this afternoon and evening. It’s a low-level threat (deemed marginal by the Storm Prediction Center), and widespread severe activity is not anticipated. And it’s contingent on getting enough breaks in the rain to allow filtered sunlight to destabilize the atmosphere. There is sufficient wind shear (change in wind speed with altitude, and wind direction across the front) for a low risk of an isolated, weak tornado.

The rainfall, thus far, in historical context


The 8.14 inches that have fallen this month rank as 10th most in July on record, but we’d move into the No. 1 spot with three more inches, passing the 11.06 inches from 1945.

Month to date, the 8.14 inches that have fallen rank fourth most on record.

Peter Mullinax, a meteorologist for Planalytics, tweeted that Washington has now received at least eight inches of rain in back-to-back Julys for the second time on record. Last year, 9.15 inches fell, ranking as the seventh-wettest July.

Some of the heaviest rain in recent days has focused along the west side of the Chesapeake Bay in northern Calvert and southern Anne Arundel counties, where severe flooding has occurred. Capital Weather Gang winter weather expert Wes Junker, who lives in northern Calvert County, measured over a foot of rain Saturday and Sunday alone.

Baltimore has received 9.55 inches of rain this month, which ranks as fourth-most on record for July. It needs only an additional 1.5 inches to break the July record from 1889.