Storms exit D.C. area, ushering in cooler air after four days of triple-digit heat

8:05 p.m. — Storms exit, heat relief arrives

Other than areas right along the bay, the main line of storms has pushed east of the region and the National Weather Service has discontinued the severe thunderstorm and flood watches as well as the heat advisory.

The storms were generally less intense than feared, although there were some scattered reports of downed trees, with the greatest concentration around Baltimore, where there were also reports of flooding.

Temperatures have fallen into the 70s behind the storms, offering welcome relief from four straight days reaching 101 degrees or higher, for the first time on record.

We could have a few more downpours overnight but the risk of severe weather has diminished.

This is the last update in this article.

6:50 p.m. — Intense storms from Bowie to Waldorf

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Storms that passed through the District without much punch have gained strength over Prince George’s and Charles counties. The most intense storms are between Bowie and Waldorf where radar shows very heavy rain, frequent lightning and strong wind gusts. This activity is pushing eastward at 25 mph and should pass through Anne Arundel County between 7 and 8 p.m.

An additional area of strong storms is south of La Plata and is set to pass through Southern Maryland over the next hour or so.

We’ll post a final update around 8 p.m.

6 p.m. — Storms are mostly below severe limits while sweeping east of Interstate 95

Storms stretch from Baltimore through the District and south to Fredericksburg, Va., and continue to progress eastward at around 30 mph. Warnings that were issued for Fairfax County, the District and locations just to the south were allowed to expire at 6 p.m., but a severe thunderstorm warning was issued for the area around Fredericksburg until 6:30 p.m. That’s where the most intense storm activity is and where some damaging gusts are most probable over the next 30 minutes or so.

This line of storms will push east of I-95 through Washington’s eastern suburbs over the next hour.

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We’ll post another update around 6:45 p.m. or so

5:35 p.m. — Severe thunderstorm warning issued for Fairfax County and most of Beltway area until 6 p.m.

Radar-estimated wind speeds show the thunderstorm sweeping across Fairfax County toward the Beltway gaining some strength. This has prompted a severe thunderstorm warning until 6 p.m. Strong winds are headed toward the area between Potomac and Burke and will sweep inside the Beltway within 10 minutes and the District in the next 10 to 20 minutes.

5:20 p.m. — Storms rapidly approaching immediate D.C. area

Radar shows a solid line of storms extending from Frederick and northern Montgomery southward into Central Virginia. It is progressing eastward at around 30 mph. The storms are most intense south and southwest of Manassas through southern Fauquier County, where there’s a severe thunderstorm warning through 6 p.m. for damaging winds up to 60 mph.

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To the north, storms with downpours and lightning are moving into western Fairfax and western Montgomery counties. They’re not severe but could intensify some as they head east. They should reach the Beltway between 5:45 and 6 p.m.

4:40 p.m. — Line of storms developing to west, approaching Warrenton and Leesburg

Severe thunderstorm warnings are in effect for much of Loudoun and central and northern Fauquier counties as a line of storms stretches from just south of Hagerstown to south of Charlottesville. The storms, which should reach Leesburg and Warrenton in the next 30 minutes, could contain damaging wind gusts up to 60 mph. They’re headed east at around 35 mph and should reach Montgomery and Fairfax counties a little after 5 p.m. and reach the Beltway area between 5:30 and 6 p.m.

We’ll post another update between 5:15 and 5:30 p.m.

Original article

The D.C. area is in the final throes of one of its worst heat waves on record, with Wednesday reaching 100 or higher for a record-tying fourth straight day. It could go out with a bang, as strong to severe thunderstorms threaten Wednesday afternoon through evening.

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A cold front heading east is on a collision course with the entrenched heat and humidity and is poised to spawn numerous storms, some of which could become quite intense.

Given the threat, the National Weather Service has issued a severe thunderstorm watch until 10 p.m. “Locally damaging wind gusts” up to 70 mph are possible, the Weather Service said.

A watch means ingredients are in place for possible severe storms, but it’s not a guarantee. Stay weather-aware, and if a severe thunderstorm warning is issued for your location, seek shelter.

In addition to strong winds, storms could also produce downpours and frequent lightning. The potential for heavy rain prompted the Weather Service to also issue a flood watch that is in effect until midnight.

“Thunderstorms this afternoon and evening could have intense rain rates exceeding 2 inches per hour,” the Weather Service wrote. “If locally repetitive or prolonged thunderstorms develop, flash flooding could occur in flood-prone urban and suburban areas.”

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By midafternoon, as temperatures climbed to near 100, storms were already popping up near the Blue Ridge and were expected to progress eastward during the late afternoon into evening.

Because some storms may move slowly, areas may flood despite the drought that overtook much of the region in recent weeks.

Storms will eventually help push out the record-setting heat.

The storm potential in brief

• Storm timing: Greatest storm coverage should be through 9 p.m., moving from west to east.
• Rainfall potential: Amounts will be highly variable but should average between 0.5 and 1.0 inches. Locations that receive repeated downpours could see as much as 2 to 4 inches in a few hours, hence the flood threat.
• Wind threat: Scattered damaging wind gusts could bring down trees and utility lines and lead to power outages.
• Flooding: Creeks and streams prone to flooding and areas of poor drainage will be most susceptible.

Storm discussion

Wednesday’s storm threat comes with the approach of a much-awaited cold front and end of a severe heat wave.

An initial batch of storms is forecast by high-resolution models to develop over the mountains to our west, and clusters or short arcs of storms will push east. Any earlier-afternoon storms may initially target our northern area first, then fill in a bit later to the south, with the focus locally probably during the evening.

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In the upper levels, a disturbance in the upper flow is preconditioning the air to rise, possibly more vigorously and more widespread than in days past.

Our area may be put under a severe thunderstorm watch by the Weather Service’s Storm Prediction Center. We’re in a Level 2 out of 5 risk zone for severe storms.

The flood threat stems from several factors. First, there is the possibility later today of repeated rounds of storms within a few hours. Second, the deep moisture content of the atmosphere is very large. Third, the deep, warm air mass favors efficient development of torrential downpours, even in storms that don’t meet official severe criteria for wind and hail. Fourth, for those areas that have gotten drenched in the past few days, the threshold to trigger flash flooding is lower today.

The other biggest threat will be wind damage, from straight-line, localized blasts called downbursts. In this type of air mass setting, rain-choked cells can suddenly let loose their torrents, and this drags the air downward. Some of that rain chills from evaporation, further accelerating it downward; these bubbles impact the surface in a violent outburst. Some gusts could reach 60 to 80 mph.

Heat streak continues

Before 10 a.m. Wednesday, temperatures had risen past 90 for a fifth day in a row and 29th time this summer, following an overnight minimum of 83. The afternoon high temperature of 101 degrees marked the fifth instance of triple-digit heat this summer in the District, tying 1997 and 2011 for the fifth-most 100-degree days during a calendar year.

The consecutive record highs of 101, 102 and 104 Sunday, Monday and Tuesday were the hottest since the start of the Dust Bowl. The high of 101 Wednesday makes this only the third time there have been four consecutive highs at or above the century mark; the previous instances were in 1930 and 2012. It is the first time on record the high has reached at least 101 on four straight days.

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Tuesday’s average temperature of 92 degrees — the average of the day’s high and low — was the fifth-highest of any day since records began in the 1872.

Over the past four weeks, it has reach at least 95 degrees on 17 days — matching the most of any four weeks on record.

This summer is the hottest on record to date and the number of days reaching at least 90, 95, 98 and 100 all rank among the most observed.

Once the cold front pushes through on Wednesday night, temperatures will be more moderate, but still expect summerlike highs in the 80s to near 90 for the next few days.

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