National Weather Service rainfall forecast through Sunday afternoon. (WeatherBell.com)

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(This post, originally published at 12:08 p.m., was updated at 2 p.m. to incorporate flash flood watch, and at 5:20 p.m. to update European model rainfall projection.)

A long duration, heavy rain event has become increasingly likely for the Washington region Friday through Saturday night, possibly lingering into early Sunday. Several inches of rain could fall and areas of flooding could develop.

It will probably not rain continuously and some areas will get hit harder than others. But anyone with outdoor plans Friday afternoon through Saturday evening should begin thinking about a Plan B.

This is a very unusual storm system to develop in late July — in some ways resembling a winter Nor’easter. But because it is the middle of summer and the air is much more humid, this storm system will be able to generate significantly more precipitation than a winter storm. Some models predict pockets of 6 to 7 inches of rain. (This is the equivalent of 60 to 70 inches of snow but winter storms in Washington won’t ever produce this much snow because not as much moisture is available when temperatures are colder.)


European model rainfall forecast through 2 p.m. Sunday. (WeatherBell.com)

Not only will the storm have the potential to produce extremely heavy rain, but it will also be slow moving — nearly slowing to a halt along the Mid-Atlantic coast. For all of these reasons, 2 to 4 inches is a conservative rainfall forecast, and some areas could receive 3 to 6 inches. Isolated amounts exceeding 6 inches are certainly not out of the question.

Heavy storms may line up along a corridor and hit the same areas repeatedly — a worrisome phenomenon known as training. Areas that experience training will be most prone to flooding.

On Thursday afternoon, National Weather Service issued a flash flood watch for the region between Friday afternoon and Saturday afternoon, noting the potential for three or more inches of rain.


(National Weather Service, adapted by CWG)

The weather disturbance coming through at high altitudes is unusually strong (for stats geeks: 4 standard deviations below the norm) and will contain a deep pool of chilled air. When that cold pool moves over our region Saturday, temperatures may well become unusually chilly for late July, dipping into the 60s Saturday afternoon.


European model showing extremely low pressures and a pool of cold air at high altitudes over the Mid-Atlantic Saturday. (WeatherBell.com)

Timeline

The most likely period for heavy rain is between late Friday afternoon and Saturday morning. There is the chance of another period of heavy rain late in the day Saturday. Nailing down timing specifics is challenging and subject to change, but here’s our best assessment of what to expect:

  • Thursday night into Friday morning: Hit or miss showers and perhaps a thunderstorm are possible, but some areas stay dry. Temperatures in the 70s to around 80 by noon Friday.
  • Friday afternoon and evening: Widespread showers and storms develop, likely to produce very heavy rain. Temperatures back into the 70s.
  • Friday night: Continuing waves of showers and storms with heavy downpours. Temperatures falling to around 70.
  • Saturday: Periods of rain, possibly heavy at times with embedded thunder. Much like a winter storm, a dry slot may affect parts of the region for a time, temporarily lessening the rain. Temperatures near 70, falling into the 60s, and becoming breezy.
  • Saturday night: Breezy with intermittent showers — especially east and northeast of Washington. Temperatures falling through the 60s, maybe even into the 50s in some areas.
  • Sunday morning: Showers taper off, becoming partly sunny and breezy. Temperatures in the 60s.

Forecast rain amounts

Averaging all of the forecasts together, 2 to 4 inches seems like the most reasonable projection for the region, but models and the atmospheric setup can certainly support higher and lower amounts, depending on where the heaviest rain cells set up.

Forecast totals through 1 p.m. Sunday:

  • National Weather Service: 2 to 4 inches (3.4 inches in Washington)
  • GFS model: 2 to 4 inches (3.9 inches in Washington)
  • NAM model: 1 to 5 inches (3.4 inches in Washington)
  • Canadian model: 1 to 4 inches (2.6 inches in Washington)
  • European model: 2 to 7 inches (3.7 inches in Washington)

GFS model rainfall forecast through 2 p.m. Sunday.

The meteorology of the storm

The weather pattern expected to develop across our region Friday through Saturday is more reminiscent of a winter set up, complete with a strong jet stream wave aloft and a coastal low pressure system. The jet stream wave in particular will be strengthening as it crosses our region and may stall, causing the surface low to become stationary along the Delmarva, thus prolonging the period of rain. Gusty winds will attend the rain and, as mentioned, air temperatures will be remarkably cool for this time of the year.

The weather pattern setup is shown in the diagram below, valid at 8 p.m. Friday.


Weather map, effective 7 p.m. Friday evening. (National Weather Service)

A stationary front sets up through the D.C. — Baltimore corridor, and a wave of deepening low pressure develops nearly overhead. High pressure over the Great Lakes feeds cool air from the north into this system, while a surge of warm and humid air approaches the storm from North Carolina and Virginia.

By Saturday morning (8 a.m.), the low has crept eastward to the Delmarva, leaving our region on the cool, cloudy backside with heavy rain continuing.

The upper level flow, shown below, is a bit more complicated but very potent for summertime.


(Twisterdata.com)

A strong wave of low pressure in the jet stream (red “L”) will approach from Ohio on Friday night, intensifying and acquiring what we call “negative tilt”. What this means is that high-level airflow over the Mid Atlantic will begin to fan outward (a process called difluence), effectively drawing up a large quantity of saturated air from below — rising like a chimney (shown in the next diagram). This is expected to boost heavy rain production over a widespread area. Additionally, an approaching 115 mph maximum in the jet stream (light purple region in diagram) will further enhance the dynamic and widespread uplift of air.

The amount of moisture feeding into this storm on its southeast side is very high, with precipitable water (a quantity that “sums up” the available water vapor in the atmospheric column) exceeding two inches (purple region in diagram below). That’s a lot of moisture being drawn into a storm that will be moving very slowly, and intensifying the whole time.


(Twisterdata.com)

Taken together, the scenario is a classic setup for widespread heavy rain, with significant area-wide flooding potential, from Friday afternoon through Saturday evening.

Part of our task is to try to identify the regions with greatest rain accumulation, and historically this is challenging. This is partly due to the likely convective nature of the rains — that is, bouts of heavy showers (with some thunder) embedded in a steadier rain shield. In general, a band of up to 3 to 4 inches accumulation is possible to the north and west of the low center, across northern and central Md., through D.C. and northern Virginia.

We base this on an analogy with wintertime, coastal storms, where the band of heaviest snow sets up in the so-called “deformation zone” along a strong front that sets up between 5,000-10,000 feet above the ground. However, some model guidance suggests heavy rain band placement elsewhere, including central Va or near the Mason-Dixon Line. The band’s placement should come into sharper focus over the next 24 hours.

What about the possibility of strong to severe thunderstorms? The National Weather Service Storm Prediction Center (SPC) has placed the region immediately south and east of D.C. in its “slight risk” zone, with “marginal” risk from D.C. northward. We think this is reasonable given the predicted location of the front and low pressure track. The further north of D.C. one lives, the less likely will be severe storms, as this will coincide with the storm’s cooler and more stable air.

Locations just south of D.C. have an increased risk of periodic, localized, torrential rains in embedded thunderstorm cells — with a low risk of strong to damaging wind gusts. We note that treefall may become problematic in any thunderstorm, given the likelihood that soils will saturate in the heavy rain.