On spring’s final day, a front dividing spring and summerlike air may act as a focus for a round of heavy showers and storms, particularly as the evening turns to night.

While the atmosphere isn’t as humid as it was Tuesday, when storms unloaded around an inch of rain in the District, a disturbance moving along the front may concentrate and intensify an area of storms aimed at the region around sunset.

Hit-or-miss showers and storms are possible before that through Wednesday afternoon into the early evening but are unlikely to be severe. We’re most concerned about activity that may target the region when it’s turning dark, which could produce a few pockets of damaging winds and flash flooding. It is the focus of the storm dashboard and discussion below.

Storm dashboard

Approximate arrival time for storms:

  • West of Beltway: 6 to 9 p.m.
  • Around Beltway: 7 to 10 p.m.
  • East of Beltway: 8 to 11 p.m.

Storm duration: 45 minutes

Chance of measurable rainfall in any location: 60 percent west to 40 percent east

Storm motion: West to east

Possible storm effects: Heavy rain, lightning

Possible storm effects: Damaging wind gusts, flash flooding

Very small chance of: An isolated tornado

Rainfall potential: Average 0.1 to 0.5 inches, highest west, but highly variable. Localized amounts of 2 to 3 inches possible.

Discussion

Wednesday presents a forecasting challenge in terms of a stalled frontal boundary, draped across the greater Washington region. The front is somewhat diffuse, shallow and does not have great impetus to move. It is located across Northern Virginia. Some of the models suggest that a weak wave of low pressure will form over western Pennsylvania on Wednesday afternoon; increasing southerly flow ahead of this low may drive the front northward, through the District, and into the Baltimore region, by evening.

Overcast conditions across much of the D.C. region have kept temperatures down, as well as the amount of instability (buoyant energy) available to power thunderstorms. The atmosphere is on the way to destabilizing further south and west of Washington. The National Weather Service Storm Prediction Center (SPC) recognizes this region as the most possible for any widespread severe storms. It has issued a slight risk; indeed, the immediate D.C. region is on the very northern edge of this risk zone.

If the frontal boundary advances north through the mid-to-late afternoon, the District-proper and even our northern tier counties could break into a more unstable air mass. This would extend the slight severe threat further north, into the evening. The greatest severe threat, area-wide, will be isolated to scattered strong, to severe, wind gusts (microbursts). An isolated, weak tornado cannot be ruled out, given the tendency of these frontal boundaries to generate a bit of spin at low levels and feeding that spin into a thunderstorm cell.

Wind shear (increase in winds with altitude) should be on the weak side, meaning storms will struggle a bit to organize into more widespread, long-lived and intense clusters or lines.

We are also under slight risk for some spotty flash flooding, particularly Wednesday evening, as shown in the graphic below. Like Tuesday, there is very significant, deep atmospheric moisture in place. The frontal boundary may serve to focus, and re-focus, waves of convective showers and thunderstorms. It’s nearly impossible to say where this might set up, hours in advance. Any single storm cell can generate 0.5 to 1.0 inches of rain in a short time, but the potential is there for some spots to receive 2 to 3 inches or more. Models suggest our far western areas, toward Interstate 81, have the highest rainfall potential. And these areas are under a flash flood watch.

The weak wave of low pressure coupled with a high altitude disturbance should transit our region, out of Pennsylvania, overnight, driving a cool front through. This may give us a brief reprieve from heavy weather Thursday. Alas, the cool front comes back north as a warm front Friday and Saturday, when we will once again be concerned about significant storminess.