Significant severe weather possible in the Mid-Atlantic on Sunday


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10:35 p.m. update: As we watch a broken line of thundershowers move into the area this evening – which may well fall apart by the time they reach the District – we remain concerned about the potential for severe storms Sunday. Not much has changed since this morning (per the discussion below). We still favor two possible rounds of showers and storms – one in the morning and one (more likely) late in the afternoon and into the evening. The afternoon and evening storms are the ones most likely to become severe.

Writes the National Weather Service office serving our region:

POTENTIAL EXISTS FOR A WIDESPREAD AND/OR SIGNIFICANT SEVERE EVENT SUNDAY AFTERNOON AND EVENING WITH ALL SEVERE HAZARD TYPES POSSIBLE INCLUDING DAMAGING WINDS…LARGE HAIL…FLASH FLOODING AND ALSO ISOLATED TORNADOES.

We will have updates at 6 a.m. Sunday, again late in the morning, and then during the afternoon as storms develop and/or watches/warnings are issued.

Original post, from 10:50 a.m. this morning:


Storm Prediction Center “Day 2″ outlook for Sunday. (SPC)

The threat of a severe weather outbreak continues to present itself for Sunday, with all types of severe storms possible. This includes damaging winds, tornadoes, and hail. Heavy rain and lightning is also a concern.

In its overnight package, the NWS Storm Prediction Center highlighted an elevated risk stretching into the region.

NWS Baltimore/Washington is also characterizing Sunday as a “critical weather day.”


Storm Prediction Center “Day 2″ outlook probabilities. The “hatched” area indicates an increased likelihood of widespread severe weather.

While SPC’s highest threat area focuses on the Moderate Risk zone over parts of Ohio, West Virginia, and Kentucky, the 30% “hatching” (10% or greater probability of significant severe weather within 25 miles of a point) for our region is about as strong of a signal from them as you’ll see a day out. It’s right on the cusp of a Moderate Risk.

Below is SPC’s most recent technical discussion for the area. I’ve added emphasis to a few spots and some notes in brackets:

AN MCS [organized storm complex] IS LIKELY TO BE ONGOING SOMEWHERE NEAR WV SUN MORNING…AND IT IS UNCLEAR WHETHER IT WILL PERSIST INTO VA AND MD DURING THE DAY. EVEN IF IT DOES NOT REMAIN INTACT…OUTFLOW BOUNDARIES…AS WELL AS INCREASING CONVERGENCE WITH THE LEE SURFACE TROUGH [wind shift zone near the mountains] SHOULD PROVIDE ADDITIONAL FOCUS FOR SEVERE STORMS BY LATE AFTERNOON. SHEAR PROFILES WILL BECOME QUITE FAVORABLE FOR SUPERCELLS WITH TIME…AND PLENTY OF INSTABILITY WILL BE PRESENT TO SUPPORT THEM. LARGE HAIL WILL BE LIKELY…AND A COUPLE TORNADOES WILL ALSO BE POSSIBLE GIVEN CELLULAR STORM MODE AND MODESTLY-LOOPING HODOGRAPHS. THERE IS A STRONG CONVECTIVE SIGNAL IN SEVERAL MODELS ACROSS MD…ERN VA AND DE NEAR 00Z [evening].

There’s little question that the threat tomorrow is on the high side for this region. This is largely thanks to an abnormally strong low pressure — both in the mid-levels and at the surface — for late July. A strong punch of jet stream winds is likely to aid any storm development that occurs.


Today’s 12z NAM showing a strong jet streak at 500mb (18,000 feet) pushing into the region late in the day. This will aid any storm development and increase the risk for supercells as well as damaging wind threats. (College of Dupage)

With these strong winds aloft, isolated rotating supercell thunderstorms capable of large hail and tornadoes are possible, as are bowing lines with the risk for widespread damaging winds.

There is plenty of uncertainty as to the evolution of the threat at this time. Possible morning rain and storms, the potential for cloud cover to remain in the area during the day, boundaries shifting the highest threat south or north, and mistiming of storm development can all hinder reaching potential.

While the greatest threat currently appears to occur in the early afternoon to evening window, we probably won’t have a true handle until tomorrow morning.

Regardless, this is the type of event to be aware of in advance. We’ll have more details on this if needed today, and Jeff Halverson will take a detailed look at the risks early tomorrow.

Ian Livingston is a forecaster/photographer and information lead for the Capital Weather Gang. By day, Ian is a defense and national security researcher at a D.C. think tank.
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Ian Livingston · July 26