* Flash flood watch in effect for the entire region 2 a.m. Saturday through 2 a.m. Sunday *
The worst conditions are most likely late Saturday afternoon and into the evening.
Basic information:
- Storm timing and coverage: 6 a.m. Saturday to 6 a.m. Sunday for widespread showers and thunderstorms (nearly 100 percent chance of measurable rain at a given location)
- Likely timing of most hazardous-intense weather: 4-10 p.m. Saturday
- Storm progression: Southwest to northeast
- Rainfall forecast: 1-2″ generally, locally higher amounts possible
- Most likely storm hazards: Locally heavy rain, localized flash flooding; lightning, strong wind gusts
- Possible storm hazards: Widespread flash flooding, small stream and river flooding, isolated severe wind gusts, small tornadoes, small hail
- Unlikely storm hazards: Widespread damaging wind; strong to violent, long-track tornadoes; giant hail
- General forecast confidence: Low to Medium – Biggest uncertainties are (1) where the most intense rain and flash flooding will set up, and (2) likelihood of severe thunderstorms remaining just south and east of the immediate D.C. region
Pattern overview
This morning a stationary, west-east oriented front is draped just south of Washington, D.C. and we remain on the cool side of this boundary. With the approach of a strong upper-level disturbance, a low pressure center will form along this front tonight, and intensify through the day on Saturday (shown below). As Gulf moisture overspreads the system, heavy rains will develop north of and along the front.
Through early afternoon on Saturday, our region will remain on the cool, rainy side of the warm front. However, as the storm lifts northeast and intensifies, the warm front will move north. Portions of our region may break into the storm’s warm sector, where the most unstable air resides. This scenario is most likely for the southern and eastern suburbs of the D.C. region. Heavy showers and thunderstorms, some strong to severe, will become widespread through the warm sector after perhaps a brief intermission from rain (a few hours of dry weather are possible).
The core of the low pressure center may should pass right over the D.C. region tomorrow night. From sustained uplift of very moist air, the heavy rain and flood threat will continue through Saturday night and into early Sunday morning. Strong upper-level dynamics including a jet streak will contribute to widespread, heavy rain. Moist air forced to flow up mountain slopes will also enhance rain totals over the Appalachians.
On Sunday morning, the storm fully occludes (the cold front sweeps around and combines with the warm front) and lifts into coastal New England. A dry slot should work its way into our region after sunrise, leading to rapid clearing.
The severe thunderstorm threat
With strong upper-level dynamics comes wind shear (increase in wind speed with height); where the shear coincides with the warm sector delineates the greatest likelihood of severe storms. CWG’s Ian Livingston notes that this type of upper-air pattern is conducive to severe weather in the Mid Atlantic. Additionally, low-level turning of winds in the vicinity of the warm front will enhance directional wind shear. This favors development of rotating storm cells (both supercells and bow echoes).
The Storm Prediction Center had placed the entire D.C. region in a slight Risk zone for severe thunderstorms on Saturday (15% chance of severe storms within 25 miles of a location).
It’s likely that the Tidewater region and southern Maryland will reside deeper in the cyclone’s warm sector – thus the threat of severe storms may be greatest to our south and east. However, the threat over D.C. is non-negligible, until we get a better handle on the exact track of the low, the extent of the warm sector, and the northward limit of the warm front.
High pressure to our northeast will likely prevent the warm front from lifting quickly northward, and in fact there is some suggestion that a weak wedge or cool damming event may unfold. Solid overcast, heavy rain and northeast winds can all contribute to the development of a cool wedge. If this is the case, most of the immediate D.C. region may never break into the warm sector.
The flash flood threat
As noted by CWG’s winter weather expert Wes Junker (who spent many years forecasting heavy precipitation events at NOAA’s Hydrometeorological Prediction Center), the setup described above portends several inches of rainfall across portions of the Mid Atlantic. As Wes explains, “The question is where. That’s always tricky, as models often have problems pinpointing the exact location of where the convection might organize into a system. My guess is that the heaviest rain will be north of the city, possibly north of the Mason Dixon Line.”
Junker also notes that total precipitable water in the warm sector will be greater than two standard deviations above normal.
Another problem is the legacy of a very rainy month, which has left large portions of the ground close to saturation. Our area is primed for runoff problems – leading to flash flooding, small streams overtopping their banks, and potentially river flooding concerns.
The figure below presents the latest rain accumulation guidance from the NOAA Weather Prediction Center (WPC). This shows the expected rain totals from Friday morning through Monday morning, with the bulk of the rain falling Saturday and Saturday night.
Widespread 1-2” of rain is anticipated, with a 3.5” maximum over the Pennsylvania Appalachians. The map does not portray locally higher amounts, on the order of 4-5”, due to organized convective systems. It is exceedingly difficult to determine a priori where these might set up, and how long they will last. They may tie to local terrain, or to a portion of the warm front. Part of the convective process may include repeated passage of storm cells over the same locations (echo training).
Stay tuned for updates on this evolving situation later today and on Saturday.