Area classified as under slight risk of severe storms Friday. Part of this area may be upgraded to moderate risk says SPC. (NWS Storm Prediction Center (SPC))

Presently, the National Weather Service Storm Prediction Center places this region under a “slight risk” of severe thunderstorms but notes “even a categorical moderate risk might be warranted” - which is rare in our region (frequency once or twice per year on average)

Details regarding the timing and severity of storms are subject to change. Forecasting severe thunderstorms is difficult and conditions can vary significantly over a small region. Severe weather may not develop where you live even though the risk is elevated. Here’s my latest thinking...

Timing: A shower or two - perhaps a rumble of thunder - is possible during the first half of Friday, as a warm front lifts through the region. But the highest potential for significant severe weather is during the late afternoon to mid-evening timeframe.

From I-81 to I-95: Between 2 and 9 p.m. (west to east)

Immediate D.C. beltway region: Between 3:30 and 9:30 p.m.

From I-95 to the Chesapeake Bay: Between 4 and 11 p.m. (west to east)

We will attempt to refine this timing tomorrow.

Possible storm impacts: The most likely effects from storms that develop will be heavy rain, gusty winds, dangerous lightning and (somewhat less likely than the preceding) small hail. In decreasing order of likelihood, the following dangerous weather conditions could develop:

* Damaging winds over 60 mph (likely in some areas)

* Flash flooding (possible in a few areas)

* Large hail over 0.75” in diameter (possible in a few areas)

* Small tornadoes (EF0 or EF1, possible in a few areas)

* Large tornadoes (EF2 or higher, unlikely but not out of the question)

As the National Weather Service’s Storm Prediction Center states:



Forecast weather map at 8 p.m. Friday (National Weather Service)

A measure of instability, known as CAPE (convective available potential energy) will probably surge into the 1,000-2,000 range - sufficiently high to support strong to severe thunderstorms. A caveat here is that if it remains cloudy during much of the day Friday, instability may not reach these levels which would dampen the severe weather threat some.

Jest stream (upper level winds) at 8 p.m. Friday

Helpful link: Wind shear and thunderstorm development (Haby’s Hints)

The combination of instability (assuming the sun comes out and it heats up) and wind shear is chief cause of concern for the severe storm potential.

How real is the tornado risk? Typically when tornadoes strike our region, they are small and spread out - affecting relatively few people and resulting in modest damage. But there are exceptions like the College Park (2001) and La Plata (2002) tornadoes.

Top: Areas where supercell parameter is greater than one Friday evening are countoured, indicating elevated potential for rotating thunderstorms. Bottom: Probability significant tornado parameter will exceed one. (NWS Storm Prediction Center)

Note that the highest supercell parameter values (indicating the potential for violent, rotating thunderstorms) displayed on the right (top) are over southern Pennsylvania as are the highest probabilities of the significant tornado parameter (STP) exceeding one. Historically, a majority of significant tornadoes (F2 or greater damage) have been associated with STP values greater than 1.

Related: Washington D.C. area’s worst five tornado events

Right now, our thinking is that straight line winds are the bigger threat in the D.C./Baltimore areas. We may well see the development of a large squall line with embedded bow segments which contain localized damaging winds. Sometimes, small tornadoes develop in individual thunderstorms ahead of a larger line or embedded within the line.

Have there been similar setups to this in the past? CWG’s Ian Livingston notes some similarities with the weather pattern of September 24, 2001 when the College Park tornado occurred. Surface (archive map from 2001) and upper air (archive map from 2001) features are somewhat comparable.

On the WJLA weather blog, meteorologist Chad Merrill draws comparisons with a thunderstorm outbreak on June 2, 1998. This outbreak produced an F4 tornado in Frostburg, Maryland.