Strong rising air associated with the nose of a 50 knot mid-level south-southwest jet combined with near record high moisture levels and an unstable atmosphere triggered wet multicellular thunderstorms over Lemhi County by late afternoon on September 5th. Flash flooding and power outages were the main impacts.
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The episode as NWS MSO wrote it up.
Strong rising air associated with the nose of a 50 knot mid-level south-southwest jet combined with near record high moisture levels and an unstable atmosphere triggered wet multicellular thunderstorms over Lemhi County by late afternoon on September 5th. Flash flooding and power outages were the main impacts. To get an understanding how moist the air mass associated with this southwest surge was, the Boise weather balloon sounding recorded the 2nd highest moisture level (precipitable water values) on record on the evening of the 5th.
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 Missoula, MT wrote its last area forecast discussion before the first event at Sep 5, 2:14 PM MST, 1 hour 50 minutes earlier.
Area Forecast Discussion National Weather Service Missoula MT 314 PM MDT Thu Sep 5 2013 .DISCUSSION... Showers and thunderstorms will move across the Northern Rockies from time to time tonight through Friday. Considering that a lower pressure circulation off the Pacific Northwest coast continues to slowly drift eastward toward the region, severe thunderstorms will begin to be a concern. Damaging winds are possible with storms that develop this afternoon and evening over Idaho and Clearwater counties in Idaho, particularly the Highway 95 corridor from Riggins to Grangeville and vicinity. A high atmospheric moisture content over the region will increase the precipitation potential for each storm that develops. In anticipation of heavy rain potential, a flash flood watch continues for portions of western Montana and north central Idaho, including old wildfire burn scars, through this evening. The severe weather and heavy rain potential will continue through Friday, however the details on where flooding and/or severe weather will be concentrated remains to be seen. During this weekend, showers and thunderstorms will continue to be present, however the focus of activity will be over west central and northwest Montana. The closed low pressure circulation will slowly migrate north and east, eventually clearing the Northern Rockies. Cool temperatures will be evident as the low pressure moves overhead, particularly over the higher terrain. Monday through Thursday, high pressure will develop over the Northern Rockies, bringing dry and warm conditions back to the region. Daytime temperatures may warm back to the low 90s by Wednesday. && .AVIATION... Showers and thunderstorms will be present near Northern Rockies airfields from time to time during the next 24 hours. These showers and storms will be capable of producing brief wind gusts and heavy rain. Heavy rain will temporarily reduce visibility, lower ceilings, and obscure terrain. It is difficult to time specific cells, however most instances of deteriorated conditions will last no longer than 1-2 hours. && .MSO WATCHES/WARNINGS/ADVISORIES... MT...None. ID...None. && Follow us on Facebook and Twitter! https://www.facebook.com/US.NationalWeatherService.Missoula.gov https://www.twitter.com/NWSMissoula
The Missoula radar (KMSX), 183 km away, was scanning. The volume nearest the first event began at 23:06 UTC.
3 heavy rain, 2 thunderstorm wind, 2 flash flood. Local standard time, as filed.
Although not under the main multicellular thunderstorm cores, the Indianola RAWS located at 3,529 ft MSL still reported 0.39 of rain in one hour. Considering what other weather stations in the area reported, this amount could have fallen in a much quicker timescale.
The Kriley Creek RAWS at 5,200 ft. MSL reported a hurricane force wind gust associated with a wet multicellular thunderstorm. It also reported a 69 mph gust during the 4:51 to 5:51 pm MDT hour. Doppler radar data shows that this gust probably occurred after 5:30 pm MDT. It also showed a wind shift from a west-southwest to a north-northeast direction as the storm passed to the northeast between the 5:51 and 6:51 pm MDT observations. This potentially could be classified as a wet microburst. Power and phone outages were reported in North Fork.
A washout near Cramer Creek closed Salmon River Road/NF - 30 for a time. Nearby weather stations in Lemhi county reported anywhere from 0.39 up to 0.58 in less than 30 minutes with these storms.
The Salmon RAWS, located at 5,100 ft. MSL, reported 0.58 of rainfall in one hour. According to radar storm echo trends, the rain could have fallen in less than 30 minutes time.
The Salmon RAWS located 3 miles southwest of the city of Salmon at 5,100 feet reported a strong wind gust to 69 mph that occurred sometime between 5:36 and 6:00 pm MDT. Weather spotters reported trees down and a power outage in Salmon. A television news station reported on Facebook that over 5,000 people were affected. A person from the public in Salmon commented on this post that power was still out in parts of Salmon at 9 am MDT on September 6th.
The Kriley Creek RAWS, located at 5,200 ft MSL, reported 0.52 of rain in one hour. Radar data reveals that the rain could have fallen much quicker in approximately 30 minutes or less.
Very wet thunderstorms triggered flash flooding in the Kriley Creek drainage. This debris flow traveled over the northbound lane of highway 93 shutting it down for a period. The BLM road Kriley Gulch Rd was closed in the Kriley Creek drainage due to another separate washout.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Idaho in 2013Every state in 2013This day in weather historyWarnings in Idaho, September 2013
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: 474055, 474179, 474229, 474054, 474180, 474053, 474232.