A stationary front draped south of New Hampshire moved back north as a warm front allowing for a hot and unstable airmass to move into Southern New Hampshire south of the boundary. In addition, a moderately strong jet led to a good shear environment that allowed developing daytime thunderstorms to become severe.
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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 GYX wrote it up.
A stationary front draped south of New Hampshire moved back north as a warm front allowing for a hot and unstable airmass to move into Southern New Hampshire south of the boundary. In addition, a moderately strong jet led to a good shear environment that allowed developing daytime thunderstorms to become severe.
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 Gray, ME wrote its last area forecast discussion before the first event at Aug 23, 9:50 AM EST, 3 hours 25 minutes earlier.
Area Forecast Discussion National Weather Service Gray ME 1050 AM EDT Sun Aug 23 2020 .SYNOPSIS... Above normal temperatures will continue as a frontal boundary meanders north and south across the area. The front will also bring chances for showers and thunderstorms into next week, although the prospect of widespread beneficial rainfall remains low. Some thunderstorms this afternoon and evening could contain strong gusty winds and hail. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 1035 AM...Notable adjustments come to POPs for today as I adjust based on timing of the current cluster of convection crossing S of the intl border attm. This should move through and shift S a bit thru about 18Z or so. About the time this dies off and exits east should start seeing instability convection developing over srn and central NH, where boundaries were left over from Saturday's storm. Once again these storms have decent mid-lvl shear to work with and could produce some rotating updrafts, but it will probably be limited to a few at best. Also, added hazard of today will be some potential for heavy rain and back building producing a small chance for for flash flooding. Otherwise it looks to be a warn and humid day across much of the area, with highs 85-90 in central and srn NH. 735 AM...Minor update to drop the dense fog advisory for the mid coast and capital district of Maine. Prev disc... 405 AM...Minor ESTF update to add dense fog advisory to mid coast Maine where I expect low visibilities to be most widespread and persistent. Prev Disc... At 06z...a wavy frontal boundary extended from the Gulf of Maine through central New England to the Ottawa Valley. GOES infrared imagery showed a few high and mid clouds across the area... otherwise light winds and mainly clear skies were allowing areas of stratus and fog to develop across the forecast area. NWS Doppler Radar mosaic showed an area of convection along and north of the baroclinic zone over southeast Ontario associated with warm air advection and a weak impulse. Extrapolation and available short range guidance has this activity entering far northern New Hampshire by 13z. For today...the boundary will work its way back north across the area. Daytime heating will yield CAPES AOA 1500 j/kg across much of the region. Convection will become more widespread by afternoon with modified soundings suggesting multicell and a few line segments with damaging wind gusts the primary threat. However...closer to the boundary there should be sufficient shear for a few supercells and the possibility of a brief spinup. Highs today will range from the upper 70s and lower 80s in the mountains through central and mid coast Maine where more widespread clouds and convection will limit heating. Elsewhere...mid 80s to around 90 will be the rule. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM MONDAY/... Convection should quickly fizzle again this evening with loss of heating. Overnight...we'll see mainly clear skies and light winds contribute to areas of stratus and fog developing. Lows ranging from 50s in the mountains to lower and mid 60s elsewhere. On Monday....the boundary remains draped across the area with the prospect of another round of showers and thunderstorms and the potential for a few strong to severe storms with wind again the primary threat. High temperatures will be similar to Sunday. && .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/... The upper level pattern will consist of a broad ridge stretching from California to the Midwest and the southeastern states. More meridional flow over Canada will continue to thread energy through a mean trough over eastern Canada which dips down into the northeastern states. A closed low will travel southeast within the trough and eventually stall over the Gulf of Saint Lawrence. On Tuesday we will be in a warm sector with the associated surface low well to our north. There will be antecedent cloud cover due to moderate theta-e advection. An increasing LLJ will support stratus and fog from Portland to the Mid Coast and north to include the lower Kennebec Valley during the morning hours. A cold front will move through during the morning and early afternoon before pushing offshore. The timing of the FROPA as well as the expected cloud cover will tamp down the possibility of storms. That said, partly sunny skies should develop over parts of southern NH and southwest ME, and this is where stronger storms could develop, with damaging winds being the main concern. All storms will have the capability of localized heavy rainfall and gusty winds. Highs will vary Tuesday from north to south, with readings in the lower 70s north to mid/upper 80s south in the stronger WAA. Tuesday night will be cooler with drier air allowing temperatures to drop into the upper 40s north to mid/upper 50s south. This is where lows will linger for several days. Highs will also be cooler and in the mid 60s north to upper 70s south through the Saturday. PWATs get down to about a third of an inch over the mountains Wednesday morning. PWATs will be slightly higher for Gray, where the minimum for that date is 0.45". With high pressure building in, residual clouds over the mountains and gusty NW winds will be what will keep us from tanking. Thursday morning will also be chilly but moisture will be starting to return at that point. After Tuesday's cold front we will stay mostly dry until late Thursday as short wave energy crosses the area and a warm front nears from the south. IVT shows a tropical connection as moisture from the tropical cyclones affecting Louisiana snakes its way around the Bermuda high and towards New England. At this time it looks like most of the precip will be to our south, but we should see scattered showers at least over portions of NH. Another WAA regime and FROPA set up looks to bring us a rainy weekend. && .AVIATION /15Z SUNDAY THROUGH THURSDAY/... Short Term /Through Monday/...Areas of MVFR and lcl IFR in cigs and fog...improving to VFR aft 12z with sct MVFR and brief IFR developing in -SHRA/TSRA. Aft 00z Mon...Areas of MVFR and lcl IFR in cigs and fog once again...improving to VFR aft 12z Mon with sct MVFR and brief IFR developing in -SHRA/TSRA. Long Term...MVFR and local IFR conditions are likely Tuesday in -TSRA as a cold front moves across the area. Improvement to VFR should occur after 18Z Tuesday. VFR expected Wednesday and most of Thursday before a chance of showers return to southern sections. Gusty NW winds develop behind the frontal passage Tuesday and continue into Wednesday. There may be a period of LLWS early Wednesday morning with a 40-50 kt LLJ just above. Local IFR will also be possible in dense valley and ground fog early Tuesday morning. && .MARINE... Short Term /Through Monday/...Winds/seas stay below SCA levels. Vsby restrictions at times in fog, haze and scattered showers and thunderstorms. Long Term...Winds may approach 25 kts for the outer waters late Tuesday/early Wednesday with a frontal passage. Waves may approach 5 ft over the outer waters next weekend. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...None. && SYNOPSIS... NEAR TERM...Cempa/Schwibs SHORT TERM...Schwibs LONG TERM...Hanes AVIATION... MARINE...
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</html>The Boston radar (KBOX), 95 km away, was scanning. The volume nearest the first event began at 18:13 UTC.
11 thunderstorm wind, 1 hail. Local standard time, as filed.
Quarter size hail associated with a severe thunderstorm was reported by the public in Windham.
Damaging outflow winds associated with a severe thunderstorm downed numerous trees and wires throughout Swanzey. Trees and wires reported down on Oliver Hill Rd, Taff road and others.
Damaging outflow winds associated with a severe thunderstorm downed trees and wires on Jaffrey Rd. in Marlboro.
Damaging outflow winds associated with a severe thunderstorm downed trees and wires in Troy, blocking Central Square Rd.
Damaging outflow winds from a severe thunderstorm downed trees and wires in Fitzwilliam.
Damaging wind gusts caused by a severe thunderstorm downed trees and wires on Easton Rd and Loudon Rd in Pittsfield.
Damaging outflow winds associated with a severe thunderstorm downed multiple trees in Deerfield.
Damaging wind gusts caused by a severe thunderstorm downed trees and wires south of Epping Center.
Damaging wind gusts caused by a severe thunderstorm downed multiple trees and wires south on West Hollis St. in Nashua.
Damaging wind gusts caused by a severe thunderstorm blew over a tent where many people gathered for an outdoor event. At least 6 people where injured with minor injuries being reported by media outlets.
Damaging wind gusts caused by a severe thunderstorm downed trees and wires in Stratham.
Damaging wind gusts caused by a severe thunderstorm downed trees and wires on Oxbrook Rd. in Charlestown.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in New Hampshire in 2020Every state in 2020This day in weather historyWarnings in New Hampshire, 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 2026-09-30. Each event’s entry at NCEI: 918663, 918668, 918709, 918672, 918726, 918770, 918664, 918758, 918760, 918761, 918759, 918766.