The Washington PostDemocracy Dies in Darkness

Showers sweep through, decreasing late-day storm threat

The early afternoon showers depleted available energy for storms later on

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Radar courtesy MyRadar | © OpenStreetMap contributors

2:50 p.m. — Showers and storms sweep off to the east; late-day storm threat decreases

After some heavy showers and even a strong storm or two southeast of Washington, much of the rain has moved off into southern Maryland and is about to head over the bay.

Radar does not show much threatening in their wake nor do short-term computer models. The exception to this might be northern Maryland, which was missed by the initial showers. It’s possible it may get scraped by some showers and storms sweeping eastward along the Mason-Dixon Line.

But, for much of the area, while we can’t rule out a few pop-up showers and storms through the evening, it should be mostly tranquil. The early afternoon showers and storms used up a lot of the available energy, and prevalent cloud cover should prevent the atmosphere from destabilizing much in the hours ahead.

Unless severe storms develop in the region, this will be our last update for this article. Our PM forecast update will post late this afternoon, by 5 p.m.

12:20 p.m. — Early blooming showers and storms already arriving

Some showers and storms have developed ahead of schedule and are quickly moving into the Route 15 corridor (from Leesburg to Warrenton). These are generally not severe, so mainly expect some brief downpours and perhaps a bit of lightning with this activity as it sweeps eastward over the next 60 to 90 minutes. However, the activity could intensify a bit — especially after crossing Interstate 95 around 1 p.m.

This activity may use up some of the atmosphere’s available energy, reducing the odds of additional storms later.

We’ll reassess the late-afternoon storm threat after these come through.

Original article from late morning

Muggy air is spreading over the Washington region, displacing the low humidity we enjoyed on the Fourth of July. The arrival of this warm, moist air sets the stage for possibly intense thunderstorms Tuesday afternoon into the early evening.

The National Weather Service Storm Prediction Center has placed our region in a Level 2 out of 5 risk zone for severe storms, noting the potential for “damaging gusts” and “isolated large hail.”

Any storms that affect the area should move through quickly, reducing the risk of flooding. However, some of the areas that were deluged Saturday night (i.e., saturated ground in the northern part of the District and southern Montgomery and northern Prince George’s counties) might again contend with high water if heavy storms pass through.

How flooding rain deluged District and northern suburbs Saturday night

Short-range computer models suggest the best chance for storms is between about 3 and 6 p.m., with precipitation sweeping from west to east.

As of late morning, showers and storms were present from the Ohio Valley into West Virginia and were generally pushing east-southeast in the general direction of the Washington region.

Storm risk at a glance

Storm timing: While subject to change, storms should arrive and exit the following areas in the following windows:

  • Interstate 81 (Hagerstown to Front Royal): 2 to 4 p.m.
  • Route 15 (Frederick to Leesburg to Warrenton): 2:45 to 4:45 p.m.
  • Interstate 95 (Baltimore to D.C. to Fredericksburg): 3:30 to 5:30 p.m.
  • Route 301 (Bowie to La Plata): 4:15 to 6:15 p.m.

Storms should be moving quickly, lasting in any one area about 30 to 45 minutes. Note that some widely scattered showers and storms are possible after the initial round but should decrease in coverage and intensity after dark.

Storm coverage: Scattered — any individual area has about a 60 percent chance of measurable rain.

Storm hazards:

  • Likely: Torrential rain, lightning, gusty winds (up to 30-40 mph).
  • Possible: Damaging winds (up to 60-70 mph), small hail.
  • Very small chance: Large hail, flooding, tornado.

Rain potential: In areas hit by storms, 0.25 to 0.5 inches is most likely, with isolated totals up to 1 to 2 inches.

Discussion

Today’s potential severe weather setup features a warm front moving through the region (as shown in the map below), ushering in a more humid air mass on southerly winds. Additionally, an upper-level disturbance in the jet stream flow will move into the Mid-Atlantic from the Ohio Valley.

That disturbance has been organizing scattered showers and thunderstorms across West Virginia, and those may keep solid to broken cloud cover around the D.C. region throughout the early and midafternoon.

The degree of storm severity will depend on the extent to which the atmosphere becomes destabilized over the next several hours. Any persistent breaks in the clouds will enable sunshine to drive up surface temperatures, which is key to destabilization.

Picture-perfect weather for a spectacular fireworks show in Washington

There is sufficient wind shear (increase in wind speed with altitude) to help storm cells become more intense, should they blossom, and organize the cells into clusters and bowing lines.

The suite of high-resolution models all suggest rather early (1 to 2 p.m.) triggering of storms in the Blue Ridge, with those storms then sweeping through the metros as early as mid- to late afternoon.

As the simulated radar fields below show, the line marches through the D.C. region. The warm frontal boundary may help organize and intensify this complex.

With these types of fast-moving complexes, there is potential for a swath of damaging wind gusts — and that is likely to be the biggest severe-weather risk this afternoon and early evening.

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