* Flash flood watch this afternoon and evening *

Radar courtesy MyRadar | © OpenStreetMap contributors

3:50 p.m. - Showers and storms increasing and may converge on immediate Washington area this evening.

Showers and storms which have mainly remained north and south of Washington through mid-afternoon are now encroaching on the immediate area from both directions. They’re forming along the outflow boundaries or cool wakes from earlier storms and moving into a zone with high moisture levels. This means it may be a very wet late afternoon and evening for some areas.

Not everyone will see storms, and models show less rain west of town compared to the east, but those that do can expect very heavy downpours that could trigger flooding and locally strong winds.

Real-time storm updates will be posted, as needed, in our PM Forecast update which will be posted between 4 and 5 p.m.

Original article from late this morning

Rick Vaughan awoke to a sudden cloudburst Wednesday morning that dumped nearly 3 inches of rain in an hour at his home, between Chantilly and Dulles. The complex of storms and associated downpours caused a sinkhole near Manassas and led to several nearby high water rescues.

Vaughan, who lives in the community of Oak Hill, ended up with 3.61 inches of rain from the event, which was not predicted. The deluge marked a surprise opening act of a five-day stretch during which significant amounts of rain could fall over the Washington region.

The area is particularly prone to flooding because of saturated soils from previous heavy rainfall this month from Tropical Storm Isaias and other storms.

A flash flood watch is in effect for the Washington region this afternoon and evening, for the potential of one to two inches of rain with isolated amounts up to four inches. The National Weather Service has also placed the region in a moderate risk zone for excessive rainfall and flooding.

“[T]he environmental signal for heavy local rainfall rates is pretty exceptional,” the Weather Service wrote in a discussion for a corridor roughly between Fredericksburg and Philadelphia, encompassing 18 million people.

Because of previous rainfall that has soaked the ground — nearly 4 inches so far in Washington during August — it will take only 1 to 1.5 inches of rain in an hour to trigger flooding, according to the Weather Service. Models show the potential for rates of 2 to 3 inches per hour where the heaviest rain falls.

Widely scattered showers and storms could begin as soon as early afternoon today, but they are expected to increase in coverage and intensity during the evening, before decreasing after midnight. Because the distribution of storms may be disorganized and chaotic, heavy rain and/or flooding is not expected to occur everywhere. But storms that develop are forecast to be intense and slow-moving.

In addition to heavy rainfall, some storms will also produce dangerous lightning and the potential for a few localized damaging wind gusts.

The approaching front triggering today’s heavy rainfall is forecast to bring the chance of more slow-moving downpours at times through the weekend as it stalls in the region. However, the front’s exact placement may meander, meaning the zone where showers and storms concentrate may shift.

While flooding rains will be an ever present threat through Sunday, there are also likely to be extended dry intervals. The chance of rain will vary day-to-day and require fine-tuning.

What’s driving the heavy rain potential?

Today’s setup features a cold front moving south into the D.C.-Baltimore area, then stalling for the next day or so. This boundary will serve as a convergent focus for humid, low-level winds. Combine this with a very unstable, tropical-like atmosphere, and we have a recipe for thunderstorms that have the potential to produce very heavy rain.

The winds in the middle and upper atmosphere today are very weak and disheveled, so storm cells are expected to be slow movers or even become stationary in spots. They may also congeal and merge into longer-lived, persistent complexes.

Typically, we look to wind shear — a strong increase in winds with altitude — to facilitate storm complex organization, and echo training. The shear will be absent today. However, stubborn “outflow boundaries” or pools of cool, downdraft air may focus repeated thunderstorm development along the frontal boundary.

It is nearly impossible, within the broad outlook area for moderate flood risk, to identify towns, cities or portions of counties that will experience a flash flood, a priori. Today will require diligent monitoring of the radar, for small areas where the rain rates are becoming excessive.

As we saw during early morning hours today in parts of Northern Virginia, the flash flood threat will not necessarily taper after sunset. The most likely period is the afternoon and evening hours, when instability that fuels storms will be strongest.