After a hot start to September, a cold weather system brought widespread low elevation rain and snow accumulations above 6000 feet across the Northern Rockies impacting back-country hunters, wildfire crews and travel across mountain passes. The large degree of change coupled with the somewhat unseasonable timing of this event produced greater than normal impacts.
Every event with a place to put it; 1 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.
After a hot start to September, a cold weather system brought widespread low elevation rain and snow accumulations above 6000 feet across the Northern Rockies impacting back-country hunters, wildfire crews and travel across mountain passes. The large degree of change coupled with the somewhat unseasonable timing of this event produced greater than normal impacts.
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 15, 3:21 AM MST, 2 hours 39 minutes earlier.
Area Forecast Discussion
National Weather Service Missoula MT
421 AM MDT Fri Sep 15 2017
...EXCEPTIONALLY COLD TODAY FOR WESTERN MONTANA NEAR THE
CONTINENTAL DIVIDE...MOUNTAIN SNOW CONTINUES...
.DISCUSSION...
A weather system over the Northern Rockies will continue to
produce low elevation rain and high elevation snow through
early Saturday. Hazardous winter driving conditions due to light
snow accumulations on roadways are anticipated above 6000 feet
elevation, particularly for mountain passes along the Continental
Divide. For some areas of the Northern Rockies, such as the
Highway 95 corridor in north central Idaho and Kootenai River
Country in northwest Montana, may not see much precipitation. For
locations relatively closer to the Divide, melted precipitation
accumulation will be greater than 1 inch. The presence of
precipitation and/or high relatively humidity across the region
will continue to squash activity on area wildfires.
Conditions dry across the Northern Rockies for much of the
upcoming weekend, with a brief warm-up to near normal temperatures
on Sunday afternoon. Clearing skies for parts of the region will
allow temperatures to cool near freezing both Saturday and Sunday
morning. Residual moisture from recent rain/snow will allow
middle elevation valleys to develop fog on Sunday morning.
Another long-lasting, cold, wet weather system is expected to
arrive over the Northern Rockies early Monday and keep
precipitation and cold temperatures in the forecast for much of
next week. Snow levels will once again dip below 6000 feet at
times, likely impacting high elevation passes and area travelers,
hunters, and wildland firefighters.
In the longer term, there is little to suggest that the onslaught
of cool Fall weather systems will cease in the next 1 to 2 weeks.
&&
.AVIATION...Light rain showers will remain over KMSO and KSMN for
much of the day today, occasionally reducing visibility and
ceilings. KBTM is currently reporting snow (1000Z), and will
continue off and on until around 15/1700, then turning to a mix of
rain/snow through the evening. Visibility and lowering ceilings
are the main impacts, as the runways are warm enough to melt the
falling snow. KGPI and air terminals in north-central Idaho should
remain dry. Fog may become an issue for area terminals tomorrow
morning, as skies are expected to clear, which will allow for fog
development, due to the precipitation that has fallen.
&&
.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Storm Warning until 6 AM MDT Saturday ABOVE 6000 FEET for
the Bitterroot/Sapphire Mountains...Butte/Blackfoot
Region...Potomac/Seeley Lake Region.
ID...Winter Storm Warning until 6 AM MDT Saturday ABOVE 6000 FEET for
the Eastern Lemhi County.
&&The national radar mosaic for the moment the first event began.
1 heavy snow. Local standard time, as filed.
Sixteen to eighteen inches of heavy snow was reported in the foothills southeast of Leadore. Visibility reduced to one half mile or less at Gilmore Summit on Highway 28 due to snow during the morning and afternoon hours on the 15th.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Idaho in 2017Every state in 2017This day in weather historyWarnings in Idaho, September 2017
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: 718404.