An eastward-moving upper level trough and associated area of surface low pressure moved across northwest Oregon and southwest Washington September 17-18, resulting in fairly widespread showers and thunderstorms beginning late in the afternoon of the 17th through most of the day on the 18th. One severe thunderstorm developed southeast of Salem, OR during the overnight hours of Sept.
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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 PQR wrote it up.
An eastward-moving upper level trough and associated area of surface low pressure moved across northwest Oregon and southwest Washington September 17-18, resulting in fairly widespread showers and thunderstorms beginning late in the afternoon of the 17th through most of the day on the 18th. One severe thunderstorm developed southeast of Salem, OR during the overnight hours of Sept. 17-18, and moved northward across the eastern Willamette Valley and Cascade Foothills before eventually weakening over northeast sections of the Portland metro area. This storm produced hail to near 1 inch in diameter, wind damage, and locally heavy rain with minor street flooding. Other thunderstorms developed over southwest Washington during the afternoon and evening of the 18th, producing heavy rainfall and some local flooding.
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 Portland, OR wrote its last area forecast discussion before the first event at Sep 16, 8:15 PM PST, 4 hours earlier.
Area Forecast Discussion...UPDATED National Weather Service Portland OR 915 PM PDT Wed Sep 16 2020 Updated aviation discussion .SYNOPSIS...Roughly another 24-36 hours of smoky/hazy conditions are expected across the Willamette Valley and adjacent lower terrain areas, but increasing chances for precipitation and more wind should finally allow for an improvement in air quality Thursday night and Friday. Chances for showers and thunderstorms should increase from south to north Thursday afternoon/evening, with gusty and erratic winds, and at least small chances for a few severe storms, followed by showery conditions in most areas Friday into Saturday underneath an upper low that will move ashore. A return to drier weather is then expected later this weekend into the middle part of next week. && .SHORT TERM...Today through Friday night... Large scale upper level trof offshore of the Pacific Northwest continues to edge closer to the coast this afternoon, as numerous embedded short wave impulses rotate around the common center. Several periodic lightning episodes have been noted well offshore with several of these impulses over the past couple of days. Another such disturbance resulted in persistent showers/tstorms over the northwest PQR CWA overnight, and yet another subsequent weaker impulse is currently moving into southwest Washington per water vapor imagery. So, for most of the tonight period, we will be in between individual waves, resulting more of the same sensible weather that we have seen for the past week: lots of smoke trapped in the valleys with visibility between 1-3 miles and extremely poor air quality. There will also be a risk of dense fog formation again tonight/Thursday morning across the lower elevations of the Willamette Valley and adjacent rivers and favored areas, including much of the Portland/Salem/Eugene metro areas. However, seeing as how fog was much later to form and less expansive last night than previous nights (and also less than expected for last night at this time yesterday), we have opted to not hoist an advisory for the fog tonight at this time, and let the evening shift monitor and issue if necessary. Similar conditions should persist through early afternoon Thursday, with any small chances for very light precip tonight and Thursday morning confined to the Coastal Range and areas westward. Big changes finally start to evolve Thursday afternoon. A lead shortwave trof will rotate north-northeastward around the eastern periphery of the large scale trof, and will result in increasingly strong forcing for large-scale ascent initially over the southern PQR CWA, but rapidly spreading northward through Thursday evening. Most hi-res guidance paints fairly decent instability developing in advance of this wave (anywhere from 500 to 2000 j/kg depending on the model and location) along with strong south flow above the inversion layer and 40-50 knots of deep layer shear withing the cloud-bearing layer. Obviously, this kind of kinematic parameter space would be supportive of organized strong to severe thunderstorms, with a primary threat of strong and gusty winds with storm motions to the north between 35-45 mph. It remains to be seen if we will realize the full potential that appears to be within the realm of possibilities, but regardless of whether storms reach severe levels, gusty and erratic winds, lightning and locally heavy downpours are expected with any of the stronger storms from late Thursday afternoon well into Thursday night. While those elements may have detrimental effects on firefighting efforts (especially the winds), beneficial effects should be rain and wind assisting in mixing out the extremely poor air quality. The upper level low will then come ashore later Thursday night/Friday, and should be centered somewhere around Eugene by late Friday afternoon per latest short range deterministic and ensemble guidance. This will keep chances for showers around through the day Friday and into Friday night for most areas, with additional rainfall amounts and higher humidity. There will also be at least some non-zero risk of a thunderstorm or two on Friday/Friday evening underneath the core of the upper low and coldest temps aloft. Miller .LONG TERM...Saturday through Tuesday... Medium range deterministic and ensemble guidance is fairly consistent on taking the upper level low/trof axis and associated thermal trof aloft east of the Cascades by later Friday night/Saturday, which will bring an end to precip chances/showers from west to east during the day Saturday, although we wouldn't be surprised to see some showers persist in into Saturday night in some orographic areas favored in west/northwest flow. Thereafter, a switch to onshore northwest flow for the weekend as weak high pressure builds into the Pacific Northwest is expected. Onshore flow aloft is expected to persist into mid week, but no major weather systems are expected at this point through the remainder of the 7 day forecast window. Temps should return to near or perhaps slightly above seasonal norms. Miller/Kranz && .AVIATION...Southerly flow aloft will continue as low pressure system off the Oregon coast slowly drifts towards the coastline. Expect fog and stratus to continue to affect coastal terminals. Smoky conditions will continue to improve, but expect LIFR cigs to develop overnight. A mix of conditions is then likely in the afternoon with passing showers. Smoke will continue to impact areas inland, especially within the Willamette Valley. Expect IFR/LIFR vsbys to persist, with pockets of fog developing overnight. Smoke cigs will remain a mix of MVFR to VFR this evening, but should lower below IFR Thursday morning. Conditions look to slowly improve Thursday afternoon as showers move across the interior, but expect conditions to remain a mix of MVFR and VFR. There is a chance for isolated thunderstorms after 18Z, with the best chances over the Cascades and Foothills. Refer to http://weather.gov/zse for detailed regional aviation weather and hazard information. KPDX and APPROACHES...Smoke will continue to affect flight and ground operations the next 24 hours. Expect IFR vsbys below 1 SM and cigs below 1500 ft with light winds. Conditions may improve Thursday afternoon as passing showers move across the terminal. /64/DDH && .MARINE...A low pressure system off the Oregon coast will rotate weak surface features north across the waters as the low drifts closer to the coastline through the end of the week. Generally southerly winds around 10 kt or less are expected through Friday with seas remaining around 3 to 4 feet. A weak front may cross the waters late Friday before winds shift to be more northerly by late Saturday as high pressure builds over the waters. Another weak front will likely approach the outer waters late Sunday but appears to dissipate. Therefore no significant concerns through the weekend. Looking beyond the forecast periods, a significant wind system appears to develop late Tuesday night. It stems from a 970mb to 980mb low centered in the Gulf of Alaska with a strong cold front draping south across the waters Tuesday night. Preliminary data shows strong gales are possible. It's too early to say anything definitive about this set-up given the long timeframe, and it certainly bears watching. DDH/JBonk && .PQR WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. PZ...None. && Interact with us via social media: www.facebook.com/NWSPortland www.twitter.com/NWSPortland
The Portland,OR radar (KRTX), 89 km away, was scanning. The volume nearest the first event began at 08:14 UTC.
14 heavy rain, 5 thunderstorm wind, 2 hail, 1 flood. Local standard time, as filed.
A 6-8 inch diameter tree branches blown down.
This is the Eugene Airport ASOS report.
A video of street flooding was posted on social media.
Lots of precipitation.
Lots of rain fall from the public.
Station EW7604 in Salem.
Station FW8313 in Salem.
Station 2711D 4 miles WSW of Wheatland.
Station AU762 in Salem.
A CoCoRaHS report of significant rainfall.
Rainfall total of 1.67 inches. Also measured a peak wind gust of 47 mph.
Rainfall amounts of 1.86 inches.
This is an 8 hour rainfall total.
Pea sized hail and rain reported.
This is the 16 hour rain total.
This is a 24 hour precipitation total.
Report of several small to medium size tree branches blown down.
Photos of a 4-8 inch diameter tree limbs blown down and an uprooted tree.
An empty tractor trailer was blown over.
Multiple trees snapped about 15-20 feet above the surface. Multiple power lines down and a metal shed severely damaged.
Penny to nickel size hail accumulated on the ground. Report from social media.
Hail up to the size of pennies accumulated up to 2 inches deep on the ground.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Oregon in 2020Every state in 2020This day in weather historyWarnings in Oregon, September 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: 922977, 922969, 922987, 922974, 922975, 922981, 922979, 922983, 922978, 922976, 922985, 922986, 922982, 922980, 922970, 922968, 922984, 922971, 922972, 922973, 922961, 922962.