* Severe thunderstorm watch for Southern Maryland through 11 p.m. *

For the forecast through Mother’s Day, see: Cloudy with much cooler temperatures Sunday

10 p.m. update

Intense storms continue to push southeast through Southern Maryland, but have exited the rest of the metro region where the severe thunderstorm and tornado watches, in effect earlier, have been discontinued.

The storms in Southern Maryland should exit over the next hour.

As the storms passed through the immediate area, and even after, they put on quite a lightning show. Here are a few terrific scenes posted to Twitter:

This will be the last update of the night. See our PM Update for the forecast through tomorrow and have a great night.

8:55 p.m. update

An intense area of storm stretches from Annapolis southwest to around Clinton. This line of activity is expected to push east-southeast through Southern Maryland over the next hour, prompting a severe thunderstorm warning for that entire area through 10:15 p.m.

It’s a generously-sized warning box and not everyone will experience severe weather, but especially parts of northern Charles and Calvert counties may experience some pockets of damaging winds and hail over the next hour or so.

Our next update will come around 9:45-10:00 p.m., or earlier if conditions warrant.

Original post

Temperatures are soaring this summerlike Saturday. Humidity is up, and there’s a cold front nearing our region. These are common ingredients for a recipe of strong to severe storms, and we continue to watch that risk for late today and through this evening.

Severe storms today will be likeliest closer to the Maryland-Pennsylvania border, but remnants could impact the D.C. region, especially this evening. We can’t rule out an isolated storm popping up locally before that either. Everything in mind, it does seem that our best chance of rain and rumbles in the immediate area comes in the hours after sunset.

Storm dashboard

  • Approximate window when storms are most likely:
    • 6 to 10 p.m. Loudoun, Frederick, and Howard counties
    • 7 to 11 p.m. Montgomery County and the District
    • 8 p.m. to 12 a.m. Fairfax and Prince George’s counties and to the southeast
  • Storm duration: 30 to 60 minutes in any one location
  • Chance of measurable rainfall in any location:
    • 60 percent in northern Maryland.
    • 50 percent in immediate area (within one county from D.C.).
    • 40 percent in southern suburbs.
  • Storm motion: Northwest to southeast
  • Likely storm effects: Downpours, lightning
  • Possible storm effects: Damaging winds, small to medium hail
  • Very small chance of: Large hail, isolated tornado
  • Rainfall potential: Averaging 0.1 to 0.5 inches, but locally higher amounts possible

The D.C. region will continue to experience a hot day with humidity levels becoming a tad on the uncomfortable side this afternoon. These conditions will also breed plenty of buoyant air, to the point that late day thunderstorms may fire. The storms are expected to be confined to frontal boundary sliding slowly south out of Pennsylvania, as shown below. A weak wave of low pressure will develop along the front, in the vicinity of Harrisburg and slide east overnight.

Winds aloft are also concurrently increasing, out of the west, as this surface low takes shape. This will increase the wind shear across our region throughout the rest of the day. The combination of buoyant air, wind shear, and the approach of a frontal boundary sets the stage for potentially severe weather.

Accordingly, the National Oceanic and Atmospheric Administration Storm Prediction Center has set the outlook for the immediate D.C. region for a “slight risk” of severe thunderstorms (Level 2 on a 5-point threat scale; shown below). For Washington and its immediate suburbs, there is a 15 percent damaging wind probability, along with 15 percent for hail and 2 percent for tornado. The risk of damaging straight-line wind climbs to 30 percent in the small enhanced outlook zone.

Across north-central Maryland (north of Montgomery County), and including Baltimore, the risk has been elevated to enhanced (Level 3).

One factor that may mitigate widespread severe activity near Washington is a westerly wind in the low levels of the atmosphere, as revealed by the morning weather balloon launch at Dulles International Airport. This means airflow approaching Washington will have descended the Appalachians, which tends to dry the air and suppress uplift needed to initiate storms. Additionally, forecast models suggest that the atmosphere may actually attempt to stabilize a bit through mid-afternoon (caused by air descending from a higher altitude), which could inhibit convective cloud growth.

Those forecast models show a band of showers and thunderstorms confined to the immediate front, along the Mason-Dixon Line, through early evening (animation shown below). After 7 to 8 p.m., this band sags southeast, as the cold front advances southward, and activity eventually begins to impact Loudoun, Frederick, Montgomery and Howard counties. By roughly 10 p.m. or so, activity could be reaching Washington. However, convective intensity seems likely to wane deeper into the evening, as we lose the destabilizing influence of the sun.

In summary, we believe the main convective threat will stay north of the D.C. region this afternoon.

If you have plans for this afternoon, timing may work in our favor today; if things end up as expected, once the band of storms moves in, it will probably be evening. Available buoyant energy will be waning. The approximate likelihood of storms decreases from 60 percent across northern Maryland, down to 50 percent in the immediate D.C. region and 40 percent farther south. This is not to say that a stray storm or won’t become severe, locally. However, widespread severe activity appears to be on the less probable side. The greatest threat will be isolated damaging wind gusts and pea- to nickel-size hail.

We will closely monitor the situation through the evening, for any watches issued, and perhaps eventually some warnings as well. Again, we cannot rule out that an isolated intense cell or two will pop up late this afternoon ahead of the main storm band. If that happens, prepare to take quick action as it could be rough. These front-running cells, if they form, are the most likely to have larger hail and even potentially a tornado.

Through midnight and into the early morning, isolated to scattered showers continue, and a cooler airflow from the northeast becomes established. Temperatures may struggle to reach 70 on Sunday, with overcast conditions and spotty showers. We’ll be on the cool side of the front, as a wave of low pressure tracks eastward away from our region. More on that in our morning forecast.

Updates from earlier that have become outdated

8:30 p.m. update

Storms have now pushed east of Interstate 95 across the region. From just northeast of the District through Severna Park they are intense. A severe thunderstorm warning has been issued for northern Prince George’s County and much of Anne Arundel County, including Annapolis, through 9:30 p.m. This storm could produce 60 mph winds and hail.

Earlier, the storm that charged through Baltimore unloaded quite a bit of hail. Here are a few pics:

8:10 p.m. update

While the most concentrated storm activity is from around northern Anne Arundel County and Baltimore to the northeast, where storms warnings are in effect, storms have significantly increased in coverage and intensity around the immediate D.C. area in the last 30 minutes.

A severe thunderstorm warning remains in effect for Alexandria, the southern part of the District and central Prince George’s County through 8:30 p.m. This storm has produced pea-sized hail.

Meanwhile, additional storms have formed to the north and south, and may contain heavy downpours and lightning.

Much of this activity should push east of Interstate 95 and the Beltway by 9 p.m. Because the storms have extended well south of Washington, the severe thunderstorm watch has been expanded into southern Maryland, which may have strong storms in the 9 p.m. hour.

7:50 p.m. update

A severe thunderstorm warning has been issued for central Fairfax County, including Fairfax City, Arlington, Falls Church, Alexandria, the District (from downtown south), and central Prince George’s County through 8:30 p.m.

An intense storm that has developed around Annandale is moving east around 35 mph and could produce some isolated damaging wind gusts and hail. Seek shelter.

7:40 p.m. update

Widely scattered showers and storms have developed around the immediate D.C. area, but they are not severe.

The much more intense activity is around Interstate 70 in Howard County north-northeast toward the Pennsylvania line, where storms are numerous and some warnings are in effect.

It’s possible the immediate D.C. area escapes severe weather or that it remains rather isolated, but it’s worth remaining vigilant this evening.

We’ll post another update between 8:30 and 8:45 p.m., or earlier, if severe weather develops closer.

6:40 p.m. update

The closest storms to the region are around Hagerstown and these should move into and through Frederick County, perhaps focused in the northern half of the county, over the next hour. They are not severe but could strengthen some.

Latest radar and modeling trends suggest storms may be quite hit or miss around the immediate metro area and more concentrated in our farther northern and eastern areas. One of the big uncertainties about storms this evening is the overall coverage.

We’ll post our next update around 7:30-7:45 p.m. or as storms develop closer to the region, whichever comes first.

5:30 p.m. update

The severe thunderstorm watch previously issued for Loudoun and Frederick (Md.) counties has been extended east and now includes Fairfax County, Prince George’s County, Montgomery County, and the District.

To the east, a tornado watch is in effect through 1 a.m. for Howard and Anne Arundel counties and locations to the north and east.

Strong to severe thunderstorms are erupting in southwest and south central Pennsylvania and are moving east/southeast. In addition, new storms may erupt closer to the immediate metro area as the evening wears on.

In both the severe thunderstorm and tornado watch zones, storms capable of producing damaging winds and large hail are possible. In the tornado watch zone, in particular, the National Weather Service says “a couple intense tornadoes are possible.” This is a situation where storms are likely to be hit or miss…not everyone will see storms, but storms that form may be intense.

Based on radar and short-term modeling, the main time frame to watch for storms is between 7 and 11 p.m. this evening. If you’re headed out this evening, stay weather-aware and pay attention to any warnings issued.

If a severe thunderstorm warning is issued, seek shelter.

If a tornado warning is issued, go to an interior room in the lowest level of a strong building, away from windows.

We’ll issue our next update around 6:30-6:45 p.m. or as storms develop closer to the region, whichever comes first.

3:10 p.m. update

The National Weather Service has issued a severe thunderstorm watch for Frederick and Loudoun counties and locations to the northwest through 11 p.m. Storms which form in this zone could produce damaging winds up to 70 mph and hail to the size of golf balls.

Some storms are starting to fire off in eastern Ohio and southwest Pennsylvania. Those storms should increase in coverage over the next few hours as they head east-southeast.

Short-term modeling indicates we probably won’t see too much storm activity in the Washington region through at least 6 p.m. and whatever storms form may take until after dark to arrive. But stay tuned for additional updates as it is possible that the severe thunderstorm watch will be extended east later this afternoon or early this evening.