Unseasonably cold air spilled into the region resulting in strong northeast winds.
Every event with a place to put it; 3 filed by county or forecast zone are drawn at its center, in a lighter ring. 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 MSO wrote it up.
Unseasonably cold air spilled into the region resulting in strong northeast winds.
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 7, 1:55 AM PST, 6 hours 5 minutes earlier.
Area Forecast Discussion
National Weather Service Missoula MT
355 AM MDT Mon Sep 7 2020
...Wind, cold and mountain snow today...
.DISCUSSION...As an upper level trough deepens over western
Montana and north central Idaho today, it will push a Canadian
cold front across the Continental Divide. This will result in
wind, precipitation and much cooler temperatures for much of the
region. Little has changed with the overall forecast. The front
is currently banking up against the Continental Divide in
northwest Montana with some breezy northeast winds already
spilling into Kalispell.
The main concern with this system continues to be wind. The front
looks to spill over the divide in earnest over the next few
hours, with peak winds across northwest Montana expected this
morning, spreading southward into west central Montana by mid day
and into southwest Montana and central Idaho this afternoon and
early evening. Winds gusting 30 to 40 mph will be possible for
most areas, with gusts of 50 mph possible along the Highway 93
corridor from Eureka to Polson, over Flathead Lake and also across
southwest Montana and Lemhi County in Idaho. The most recent
forecast guidance has trended wind potential stronger in the
vicinity of Butte and bears watching as we move into the later
afternoon and evening. Damage to trees and powerlines is possible
where stronger winds develop. In addition, dangerous conditions
are expected on Flathead Lake due to large waves developing,
particularly on the southern end of the lake. Dock damage is
possible.
Precipitation will accompany the front, with the main focus of
precipitation expected along the Continental Divide and into
southwest Montana. Those across northwest Montana and central
Idaho will largely be missed by this system in terms of moisture.
Snow levels will quickly drop through the day, lowering to between
5000 to 6000 feet this evening. Higher elevations near the
Continental Divide should expected accumulating snow of generally
1 to 3 inches. Some of the high valleys across southwest Montana,
such as Butte and Georgetown Lake will likely see a dusting of
snow by this evening. Travel over mountain passes such as
MacDonald and Homestake could become difficult for a time late
this afternoon and early evening due to snow and blowing snow
making for poor visibility.
This system exits the region rather quickly late tonight into
Tuesday morning. The much colder airmass combined with clear skies
will make for abnormally cold temperatures for this time of year,
with a widespread freeze looking likely across much of western
Montana and some of the higher valleys of central Idaho.
High pressure builds back into the region beginning Wednesday,
and continuing through the rest of the week. This will allow
temperatures to warm back up above seasonal normals by Thursday
and Friday with generally dry conditions expected.
&&
.AVIATION...A Canadian cold front will surge across the
Continental Divide this morning, bringing a burst of gusty
northeast to east winds for all airfields today. Widespread gusts
between 25 to 35kts are anticipated, with some locations across
northwest Montana, southwest Montana and Lemhi County seeing
potential gusts to 45 kts. Peak winds are expected to reach KGPI
between 07/1200z and 07/1600z, KMSO between 07/1600z and 07/1900z,
KBTM between 07/2000z and 08/0200z and KSMN between 07/2300z and
08/0400z. The front will also be accompanied by an area of
precipitation, which will cause lowered ceilings and visibility,
especially across western Montana. The focus of heaviest
precipitation will be near the Continental Divide. As snow levels
lower to roughly 6000 feet, the higher terrain will become
obscured at times.
&&
.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Weather Advisory from 9 AM this morning to midnight MDT
tonight ABOVE 6000 FEET for the Bitterroot/Sapphire
Mountains...Butte/Blackfoot Region...Potomac/Seeley Lake
Region.
Wind Advisory from noon today to midnight MDT tonight
Butte/Blackfoot Region.
High Wind Warning from 9 AM this morning to 8 PM MDT this
evening Flathead/Mission Valleys.
Winter Weather Advisory until 6 PM MDT this evening ABOVE 6000
FEET for the West Glacier Region.
Wind Advisory until 6 PM MDT this evening Kootenai/Cabinet
Region...West Glacier Region.
Wind Advisory from 11 AM this morning to 8 PM MDT this evening
Missoula/Bitterroot Valleys...Potomac/Seeley Lake Region.
ID...Wind Advisory from noon today to midnight MDT tonight Eastern
Lemhi County.
Winter Weather Advisory from noon today to midnight MDT tonight
ABOVE 6000 FEET for the Eastern Lemhi County...Western
Lemhi County.
&&PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE MISSOULA MT
800 AM MDT MON SEP 7 2020
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0755 AM NON-TSTM WND GST 4 N CRESTON 48.24N 114.12W
09/07/2020 M58 MPH FLATHEAD MT MESONET
MESONET STATION FW5910 KALISPELL.
&&
ALPRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE MISSOULA MT
829 AM MDT MON SEP 7 2020
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0810 AM NON-TSTM WND GST 4 N CRESTON 48.24N 114.12W
09/07/2020 M68 MPH FLATHEAD MT MESONET
MESONET STATION FW5910 KALISPELL.
&&
ALThe national radar mosaic for the moment the first event began.
1 strong wind, 1 wildfire, 1 high wind. Local standard time, as filed.
Downed trees were reported along Highway 12 from Lowell to the Montana border. Sporadic power and phone outages occurred at Lowell. Wind gusts were generally 40 to 45 mph.
Winds gusting to 45 mph caused trees to fall on power lines, and this in turn sparked several wildfires near Orofino. These fires spread quickly in the low humidity and windy environment. Later named the Sunnyside Complex, an initial damage assessment included 13 homes, 31 outbuildings, and 27 vehicles destroyed. Highway 12 was also closed for a time.
Appropriately 40 mature Ponderosa pine and Douglas fir trees were blown down along 4th of July Creek Road northeast of Carmen. Several campers were trapped by the falling trees. In addition, around 20 cottonwood and aspen trees were either uprooted or had tops snapped out along Carmen Creek. Isolated tree damage was noted around Salmon. Power and phone outages occurred because of downed trees.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Idaho in 2020Every state in 2020This day in weather historyWarnings in Idaho, 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: 910779, 910780, 910771.