A slow-moving warm front sparked multiple rounds of isolated severe thunderstorms across the Middle Atlantic on Thursday, August 6th, 2020. The first moved through the greater DC metro area during the early morning hours.
Every event with a place to put it. Tap one for its entry below.
Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.
The episode as NWS LWX wrote it up.
A slow-moving warm front sparked multiple rounds of isolated severe thunderstorms across the Middle Atlantic on Thursday, August 6th, 2020. The first moved through the greater DC metro area during the early morning hours. The second was a supercell that moved across the southern Shenandoah River Valley and central Virginia Piedmont, causing significant damage and unusually large hail for so late in the season.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS Baltimore/Washington wrote its last area forecast discussion before the first event at Aug 5, 8:26 PM EST, 6 hours 34 minutes earlier.
Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 926 PM EDT Wed Aug 5 2020 .SYNOPSIS... A cold front stalled just to the south of the region today will begin to slowly push back north across the region through the end of the week. Brief high pressure builds in for the first half of the weekend, before unsettled conditions return later Sunday into the beginning of next week. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/... Weak front stretches from southern Maryland to central and southern Virginia this evening. Even though one storm briefly became stronger moving off the Blue Ridge, the main concern through tonight will be hydro related. A weak shortwave tracking to the northeast is interacting with the front and a deeper moisture discontinuity and instability gradient to spark showers and storms this evening. Training storms have been observed in central and southern Virginia, and it's a possibility in our forecast area too, especially since the propagation vectors also parallel the Blue Ridge and motions will be slow. That said, none of the individual heavy rain forecast parameters or advection/dynamic features are screaming excessive rainfall. There is a CAM signal of localized 2-3" totals...though they could certainly be overdone since so far, convection in our area has been weaker outside of the one stronger storm. One hour flash flood guidance is around 1-1.5 inches across north central Virginia/Blue Ridge vicinity (where radar and model guidance suggests the highest coverage), so am very carefully considering a Flash Flood Watch. Will be monitoring trends over the next hour or so to make this determination. General thinking is that any activity should weaken as it advects northward later tonight, but may be sustained in some form by the shortwave. Have capped PoPs at 50% for now since coverage is uncertain. Thunder should be less common, though a few storms could occur mainly south and east of the DC metro area. Lower clouds and perhaps some fog will also develop, especially closer to the stalled front. Low temperatures will range through the 60s to lower 70s. && .SHORT TERM /6 AM THURSDAY MORNING THROUGH FRIDAY NIGHT/... The boundary will continue to lift northward throughout the day on Thursday, and likely stall somewhere over central VA into southern MD. With upper-level energy approaching from the west, and the surface frontogenesis over the area, showers and storms are likely on Thursday afternoon evening near/along this frontal boundary. Moist air will be funneling back into the region south of this boundary, and given the orientation of the frontal boundary compared to the mid-upper level flow, flooding could become an issue (more on that in the hydrology section below). The weak frontal boundary will continue slowly migrating northward Thursday night through Friday night as it gradually falls apart. However, a shortwave will be moving overhead. Thus, despite the diminished surface forcing, the increased upper level support should result in more showers and thunderstorms through the end of the week. The risk of clouds and rain will continue to keep temps lower, with highs in the low-mid 80s overall. Despite the cooler temps, dew points in the lower 70s will make it feel quite humid. The high moisture content will continue to result in a risk of isolated flooding from thunderstorms. && .LONG TERM /SATURDAY THROUGH WEDNESDAY/... A stalled boundary that has been near the area starts to shift southeastward away from the region on Saturday. A few showers and thunderstorms will continue to be possible on Saturday but the main threat area should be focused over central Virginia and southern Maryland closer to the boundary. The front should finally shift away from our area by Sunday and high pressure brief builds over our region. A westerly flow aloft combined with a weak northerly flow at the surface should help limit temperatures this weekend to the mid to upper 80s with over night lows in the 60s and lower 70s. Another boundary is forecast to move into our area and stall during the early parts of next week. Multiple shortwaves will lead to continued chances for showers and thunderstorms both Monday and Tuesday. A westerly flow aloft may limit the coverage of storms as dry air gets pulled into the system. Temperatures should trend upwards into the lower 90s as we remain mainly on the warm side of the boundary. && .AVIATION /01Z THURSDAY THROUGH MONDAY/... Upper level disturbance is kicking off some convection in central Virginia near a stalled front this evening. This obviously gives CHO the best chance of seeing a storm, but coverage is in question, so didn't put specific impacts in the TAF for now. Remnant showers (low risk of a rogue storm) may continue northeastward across the metro areas late tonight into early Thursday, but again, coverage and intensity are in question and thus did not try to get too specific. A little higher certainty exists for low cloud development close the stalled front. BWI/MTN are least likely to be affected, with MVFR at DCA/IAD/MRB and IFR at CHO. These clouds may end up lingering into Thursday afternoon, though this is less certain. Additional showers and storm are likely at times late Thursday through Friday as the front remains nearby...though exact timing and placement are highly uncertain. Just included a broad brush mention in the extended TAFs for now. Additionally, morning fog possible each morning, associated with moisture from recent rains. CHO would be the best candidate for this. VFR conditions expected this weekend and into early next week but showers and thunderstorms may cause brief periods of restrictions. && .MARINE... Light winds expected through Friday, outside of gusty winds in thunderstorms. Can't completely rule out a rogue storm over the lower Potomac into the central Bay tonight as activity works northward from central Virginia. However, the greatest chance for showers and storms is Thursday and Friday, as the frontal boundary to our south this evening slowly pushes northward, and an upper trough moves overhead into Friday. No Small Craft Advisories expected at this time for this weekend. && .HYDROLOGY... Thursday afternoon and evening are growing a bit more concerning over central/southern MD and northeast VA, mainly southeast of I-95. Several things of note for Thursday afternoon. First, a stalled /slow-moving boundary will be situated over central VA/southern MD. This boundary will be roughly parallel to the WSW mid-upper level flow, which will favor training storms along the front. PWATs, while not exceedingly impressive, should be close to 2 inches near/south of the boundary. Given the recent heavy rainfall from Isaias, FFG values are quite low over central/southern MD (1-1.5 inches in an hour). Many areas saw 4-6 inches of rain Tuesday, with some even seeing up to 7-8 inches. The 12z run of the HREF show the potential for several inches of rain over this region in roughly a 6 hour period Thursday afternoon/evening. This would easily exceed FFG if realized. WPC has introduced the slight risk of excessive rainfall over this area tomorrow as a result of all of this. Monitor the latest forecast for updates. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && SYNOPSIS...RCM NEAR TERM...ADS/CJL SHORT TERM...CJL LONG TERM...JMG AVIATION...ADS/CJL/JMG MARINE...ADS/CJL/JMG HYDROLOGY...CJL
Hazardous Weather Outlook National Weather Service Baltimore MD/Washington DC 251 AM EDT Thu Aug 6 2020 ANZ530>543-DCZ001-MDZ011-013-014-016>018-508-VAZ052>057-070700- Chesapeake Bay north of Pooles Island MD- Chesapeake Bay from Pooles Island to Sandy Point MD- Chesapeake Bay from Sandy Point to North Beach MD- Chesapeake Bay from North Beach to Drum Point MD- Chesapeake Bay from Drum Point MD to Smith Point VA- Tidal Potomac from Key Bridge to Indian Head MD- Tidal Potomac from Indian Head to Cobb Island MD- Tidal Potomac from Cobb Island MD to Smith Point VA- Patapsco River including Baltimore Harbor- Chester River to Queenstown MD-Eastern Bay- Choptank River to Cambridge MD and the Little Choptank River- Patuxent River to Broomes Island MD- Tangier Sound and the inland waters surrounding Bloodsworth Island-District of Columbia-Southern Baltimore-Prince Georges- Anne Arundel-Charles-St. Marys-Calvert-Southeast Harford- Prince William/Manassas/Manassas Park-Fairfax- Arlington/Falls Church/Alexandria-Stafford-Spotsylvania- King George- 251 AM EDT Thu Aug 6 2020 This Hazardous Weather Outlook is for the Maryland portion of the Chesapeake Bay, Tidal Potomac River, and adjacent counties in central Maryland and northern Virginia as well as the District of Columbia. .DAY ONE...Overnight A Flash Flood Watch is in effect for the rest of tonight. Thunderstorms may produce scattered instances of flooding. .DAYS TWO THROUGH SEVEN...Thursday through Tuesday A Flash Flood Watch is in effect through Thursday evening for the outlook area. Thunderstorms may produce scattered instances of flooding. Isolated severe thunderstorms are also possible, primarily on Thursday. Scattered thunderstorms may produce isolated instances of flooding Friday. .SPOTTER INFORMATION STATEMENT... Spotter activation is not expected at this time. MDZ003>006-501>507-VAZ025>031-036>040-050-051-501>508-WVZ050>053- 055-501>506-070700- Washington-Frederick MD-Carroll-Northern Baltimore- Extreme Western Allegany-Central and Eastern Allegany- Northwest Montgomery-Central and Southeast Montgomery- Northwest Howard-Central and Southeast Howard-Northwest Harford- Augusta-Rockingham-Shenandoah-Frederick VA-Page-Warren-Clarke- Nelson-Albemarle-Greene-Madison-Rappahannock-Orange-Culpeper- Northern Fauquier-Southern Fauquier-Western Highland- Eastern Highland-Western Loudoun-Eastern Loudoun- Northern Virginia Blue Ridge-Central Virginia Blue Ridge- Hampshire-Morgan-Berkeley-Jefferson-Hardy-Western Grant- Eastern Grant-Western Mineral-Eastern Mineral-Western Pendleton- Eastern Pendleton- 251 AM EDT Thu Aug 6 2020 This Hazardous Weather Outlook is for eastern West Virginia, northern and central Virginia, and central and western Maryland with the exception of Garrett County. .DAY ONE...Overnight A Flash Flood Watch is in effect for the rest of tonight for eastern and southern portions of the outlook area. Thunderstorms may produce scattered instances of flooding. .DAYS TWO THROUGH SEVEN...Thursday through Tuesday A Flash Flood Watch is in effect through Thursday evening for eastern and southern portions of the outlook area. Thunderstorms may produce scattered instances of flooding. Isolated severe thunderstorms are also possible, primarily on Thursday. Scattered thunderstorms may produce isolated instances of flooding Friday. .SPOTTER INFORMATION STATEMENT... Spotter activation is not expected at this time.
The Sterling radar (KLWX), 32 km away, was scanning. The volume nearest the first event began at 07:57 UTC.
4 thunderstorm wind. Local standard time, as filed.
Multiple trees were blown down along Clara Barton Parkway near the MD/DC line.
Waugh Chapel Road was closed due to trees blown into the roadway between Seneca Drive and Strawberry Lake Way.
Trees were blown down on MD-175 Annapolis Road between Gambrills Road/Dairy Lane and Claffy Avenue.
A tree was blown down on MD-2 Solomons Island Road just north of US-50 John Hanson Highway.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Maryland in 2020Every state in 2020This day in weather historyWarnings in Maryland, August 2020
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 901061, 901064, 901066, 901067.