10:40 p.m. — Shower and storm coverage increasing as flood watch begins

There has been a notable increase in showers and storms across the region late this evening. The flood watch for our area begins at 11 p.m. and runs until 2 p.m. Saturday.

We’re already seeing what this air mass is capable of in and around Charlottesville. A flash flood warning has been issued for that area until at least 1 a.m. as 3-5 inches of rain has fallen in a short period with rain ongoing.

If you come across high water: turn around, don’t drown.

Pretty significant flooding off 5th street SW in Charlottesville, VA. #vawx pic.twitter.com/h3g0v7T788

3:50 p.m. — Excessive rainfall risk upgraded to Level 3 of 4 locally

In their afternoon update, the Weather Prediction Center has upgraded D.C. and much of the immediate area to a moderate risk, Level 3 of 4, for excessive rainfall tonight. The same general area is also now under a moderate risk for Saturday. The upgrade comes amid increasing confidence of the potential for flooding rain.

“For now have aligned the moderate misk area up with the general multi-model consensus, which also incorporates the more elevated 1 and 3 hour [precipitation] exceedance probabilities,” they wrote.

2:50 p.m. — Flood watch issued for entire region, except Southern Maryland late tonight into Saturday

Anticipating the potential heavy rainfall, the National Weather Service has issued a flood watch for much of the area from 11 p.m. tonight through 2 p.m. Saturday.

“Average rainfall amounts around 1 to 3 inches are expected, but localized amounts around 4 to 7 inches in heavier showers and thunderstorms,” the Weather Service writes. “Rainfall amounts around 1 to 3 inches within an hour or two are possible in areas where the heaviest rainfall occurs. The best chance for the heaviest rainfall will be overnight into Saturday morning.”

A Flood Watch is in effect for portions of the area. See map for more details. pic.twitter.com/n7eaHuIjyz

Zones most vulnerable to flooding include those that often flood during heavy rain events, including near creeks and streams and low-lying and poor drainage areas. The northern part of the District, southern Montgomery County and northern Prince George’s County are also more prone to flooding because of the excessive rainfall they experienced last Saturday night.

Remember never to attempt to drive across a flooded road as the water level is difficult to judge. Turn around, don’t drown.

Original article from 2 p.m.

Humid conditions continue to swamp the area Friday as our next heavy rainmaker approaches for tonight and Saturday.

The Weather Service has our region under a Level 2 of 4 risk for excessive rainfall. Although the severe weather threats other than flooding are low, an isolated damaging wind gust or brief and weak tornado can’t be ruled out, given proximity to frontal boundaries.

The Weather Service is calling for rainfall rates of one to two inches per hour, or possibly slightly higher at times tonight. The threat for excessive rainfall persists into at least Saturday morning, with the Level 2 of 4 risk also up for that time-frame, primarily near and east of Interstate 95.

There are still questions as to timing and location of the heaviest rainfall. However, the date on the calendar is a classic, coming on the third anniversary of a significant rainfall event that spawned a flood emergency in D.C. and was ignited by a similar setup.

Onset of rain and storms: Although subject to change, the majority of showers and storms probably arrive during the following time windows:

Storms may start advancing into the area around sunset, but it may take until a few hours after that for many spots to see steadier rainfall. Flooding potential increases with time, especially in urban areas and in any place that see storms that pass repeatedly over the same zone or “train."

Rain and storm duration: While thunderstorms may be heaviest overnight, areas of light to moderate rain could continue into Saturday afternoon, ending first (early afternoon) in the northwest part of the area and last (toward late afternoon) in the southeast.

Rainfall potential: While totals may vary considerably throughout the region, about an inch is most likely on average, with isolated totals of at least 3 or 4 inches. While weather models are predicting the potential for a high-impact event for someone in the region, if and where that occurs is a bit murky because of uncertainty about the ultimate location of the front and where any low pressure forms along it.

Variability in the rainfall forecast for Washington highlights this well, as shown below. We’ve also included the rough maximum locally for each model in parentheses:

High-resolution NAM: 3.73 inches (5 inches in southern Maryland)

High-resolution Canadian: 1.63 inches (3 inches south of Annapolis)

Low-resolution NAM: 1.20 inches (5 inches south of Baltimore)

European (ECMWF): 1.29 inches (2 inches south of D.C. and near Baltimore)

American (GFS): 0.87 inches (2 inches around Baltimore)

The meteorological setup for the next 24 to 36 hours will feature a frontal boundary stalled just south of the area, along which has pooled (and will continue to accumulate) very high humidity levels.

The location of this front may vacillate north and south; it is essentially caught in a tug-of-war between southwesterly winds rounding the Bermuda High over the Atlantic, and northeasterly flow associated with a cool dome of high pressure to our north.

Temperature contrast between these dueling air masses, and an approaching wave of low pressure aloft, will conspire to trigger the development of an area of low pressure on the frontal boundary this evening. By early Saturday (shown in the graphic above), the low is likely to be positioned over north-central Virginia.

Even weak low pressure will enhance the convergence and lift of very moist air over the Mid-Atlantic. A bit of a boost in that lift will come from the trailing zone featuring a pocket of fast flow in the jet stream. This “jet streak” will be passing just north of the Mason-Dixon.

There will be a lot of water vapor, with an atmosphere almost fully saturated. The saturation values of water vapor increase steeply in a warmer atmosphere, such as what we have been experiencing, and some model guidance suggests these values may push record levels for this date in July.

The forecast map of total water vapor content (called precipitable water) for Saturday morning is shown below. This comes from the NAM weather model. Areawide values of 2 to 2.5 inches are expected (magenta and gray shades). These values are similar to the flood event on this day three years ago.

High water vapor content is the raw material of substantial rain. Slightly unstable air and uplift associated with the low- and upper-level dynamics will trigger concentrated updrafts within convective cells — bands or waves of which may pass repeatedly over the same locations.

We expect the rain to pick up in coverage and intensity after midnight, persisting through the early afternoon Saturday, then gradually tapering from northwest to southeast.

The swath of heaviest rain accumulation depends heavily on the exact positioning of the front, and the track and intensity of the low pressure along it. The diagram below, which shows the National Weather Service estimate (based on a blend of all model guidance), suggests a widespread one to two inches in the immediate D.C. area. This seems reasonable.

These types of areawide projections cannot capture the extremely localized amounts that often arise from individual storms, which the models have a hard time resolving. Some spots could quickly pick up three to four inches or even more — so the risk of flash flooding is a concern. This is especially the case in areas hit hard by heavy rain in the past couple of weeks, including urban centers.

We anticipate that the NWS will issue a flood watch later today for the overnight and first part of Saturday.