Dangerous thunderstorm set up for D.C. area Thursday; storms possible tonight as well

Ingredients are coming together for a possible large outbreak of severe thunderstorms in the D.C. area Thursday, capable of producing damaging winds, multiple tornadoes, and flash flooding.  Late this afternoon and overnight some severe storms are possible as well, but Thursday’s severe weather risk is particularly serious.

UPDATE, 1:55 p.m.: The National Weather Service has issued a flash flood watch for the region from midnight tonight through 8 p.m. Thursday.

Storms this afternoon and evening

setupIf you’ve been outside today, you can feel the hot and humid air seeping into the region.  This is due to a warm front that has lifted to our north, and is now positioned in southern Pennsylvania.  This afternoon and evening, this front, which stretches back to near Chicago, will serve as a focus for developing thunderstorms.  (The National Weather Service says there is a high risk for storms with damaging wind gusts in northern Illinois and Indiana.)

Locally, any thunderstorms that develop this afternoon and early this evening are expected to be isolated, with odds in the 20-30 percent range.  However,storms that develop could produce very heavy rain and a few damaging wind gusts.

Overnight, a more organized area of thunderstorms – the leftovers of potentially violent storms in the Ohio Valley – may impact the area (50 percent chance).  The timing and intensity of any overnight storms is uncertain, so stay tuned for updates.

High res NAM model simulates storms over region in 2 a.m. to 3 a.m. timeframe Thursday morning.  This is just one model and may not reflect reality. (WeatherBell.com)

High res NAM model simulates storms over region in 2 a.m. to 3 a.m. timeframe Thursday morning. This is just one model and may not reflect reality. (WeatherBell.com)

Note: we think the odds of a derecho in the D.C. area overnight are low, but a cluster of storms with heavy rains, dangerous lightning and strong winds are possible.

The National Weather Service Storm Prediction Center (SPC) has placed our region under a slight risk of severe thunderstorms through Thursday morning.

(National Weather Service Storm Prediction Center)

Thunderstorm risk outlook through Thursday morning (National Weather Service Storm Prediction Center)

Thursday’s severe weather risk

deep-low

Model representation of unusually strong area of low pressure passing just north of the District Thursday afternoon

The severe weather ingredients that may gel Thursday are rare for this region and the time of year.  A strengthening and unusually powerful cold front and low pressure center for mid-June – more characteristic of March – will plow through the region.  As it clashes with a hot, summertime airmass, an explosive line of thunderstorms may fire and plow through the region.

SPC has placed the region under a moderate risk of severe storms, which is highly unusual for this region, especially more than 24 hours ahead of the threat.  Only five such outlooks have been issued (more than one day in advance) in the last 13 years within about 50 miles of the D.C. area.

Thunderstorm risk for Thursday (National Weather Service Storm Prediction Center)

Thunderstorm risk for Thursday, after 7 a.m. into the afternoon and evening (National Weather Service Storm Prediction Center)

Southerly flow ahead of the inbound cold front will pump in very unstable air, essentially fuel for thunderstorms.  A measure of instability, known as Convective Available Potential Energy (CAPE), is forecast to be very high (near 3,000).

High res NAM model simulation of CAPE early Thursday afternoon (WeatherBell.com)

High res NAM model simulation of CAPE early Thursday afternoon (WeatherBell.com)

A particular concern is strong turning of the winds with altitude in the atmosphere – or wind shear, due to southerly winds ahead of the cold front at ground level contrasted with roaring winds more from the westerly direction at high altitudes.

The amount of wind shear between 0-6 km (20,000 feet) altitude.  Supercell (rotating) thunderstorms become more common as shear reaches 25-40 knots and greater.  In this map, level are greater than 40 knots over the D.C. area. (National Weather Service Storm Prediction Center)

The amount of wind shear between 0-6 km (20,000 feet) altitude. Supercell (rotating) thunderstorms become more likely as shear reaches 25-40 knots and greater. In this map, level are greater than 40 knots over the D.C. area. Thursday midday. (National Weather Service Storm Prediction Center)

“[Wind] shear will be very supportive of updraft rotation…and thus supercells capable of producing large hail…damaging winds…and isolated tornadoes can be expected,” SPC writes. “At this time…the greatest tornado risk appears to exist within a zone from Northern Virginia/Maryland/Delaware northward into central and eastern Pennsylvania and New Jersey.”

Greg Forbes, The Weather Channel’s severe weather expert, has raised his tornado potential scale, TOR:CON, to 5 out of 10 for the D.C. area indicating a moderate to high probability of tornado activity.

Writes the normally cautious National Weather Service office in Sterling, Va., serving Washington, D.C. and Baltimore:

“The atmosphere across the Mid-Atlantic will sufficiently destabilize and be primed for what could be a significant severe weather event during the afternoon and early evening.”

In addition to damaging winds and tornadoes, flash flooding is the other concern, especially in light of recent heavy rains.

“This antecedent condition combined with the expectation for heavy downpours from thunderstorms may lead to flash flooding in some areas,” NWS writes. “A flash flood watch may become necessary.”

Important: As conducive as the atmosphere may be for severe weather tomorrow, not everyone will experience hazardous weather conditions and it’s possible some ingredients do not come together.  For example, if the front speeds up and comes through earlier in the day rather than late in the day, storms would not reach their full potential.

Preparedness

In light of the severe storm threat both tonight and tomorrow, consider taking the following actions:

* Prepare for possible power outages: Charge portable devices and phones, have flashlights ready to go, have fresh batteries, fill your car with gas, and have a plan in case outages are extended (lasting days).

* Move loose outdoor items inside or secure

* Clear storm drains and gutters for possible flash flooding

* Stay up-to-date on the latest storm warnings.  Follow our blog, Twitter feed, and Facebook page.  Also consider subscribing to our Twitter feed dedicated to *only* storm warnings: @DCWeatherAlerts.  Other options: get a weather radio and/or download a smartphone app that will sound an alert if warnings are issued, such as IMAP Weather Radio.

Advice from Chris Strong, warning coordination meteorologist for the NWS office in Sterling, serving Washington, D.C. and Baltimore:

My advice to people is to be on alert, be able to get NWS weather warnings quickly, and when you get the warnings, inside to your lowest floor, with as many walls between you at the outside as possible. If you stay alert, and are ready to act, you’ll be ok.

Also on Capital Weather Gang

D.C. area forecast: Thunderstorm, flooding threat looms through Thursday