An upper level trough moved over northwest Montana with very cold air aloft. This interacted with the warmer and moist air over Flathead Lake to produce some cumulonimbus clouds.
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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 MSO wrote it up.
An upper level trough moved over northwest Montana with very cold air aloft. This interacted with the warmer and moist air over Flathead Lake to produce some cumulonimbus clouds. A waterspout extended out of one of these clouds over Skidoo Bay.
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 1, 10:55 AM MST, 1 hour earlier.
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MISSOULA MT
ISSUED BY NATIONAL WEATHER SERVICE GREAT FALLS MT
1155 AM MDT THU SEP 1 2011
UPDATED
.UPDATE...
THE WINTER WEATHER ADVISORY FOR GLACIER PARK IS BEING ALLOWED TO
EXPIRE...AS THE STEADY PRECIPITATION IS TAPERING OFF AND THE SNOW IS
BEGINNING TO CHANGE BACK TO RAIN. ISOLATED THUNDERSTORMS ARE ALSO
AFFECT LOCATIONS NEAR AND SOUTH MISSOULA. THESE STORMS WILL CONTINUE
TO MOVE TO THE SOUTHEAST OVER THE NEXT FEW HOURS. BRUSDA
&&
.AVIATION...
UPDATED 1800Z.
SCATTERED SHOWERS AND ISOLATED THUNDERSTORMS WILL AFFECT AREAS
AROUND MISSOULA AND BUTTE THROUGH MID AFTERNOON. EXPECT SOME IFR
CONDITIONS IN/NEAR MOUNTAIN PASSES...WITH MOUNTAINS BE OBSCURED AT
TIMES THROUGH 23Z. DRIER AIR WILL START TO MOVE INTO THE REGION
EARLY THIS EVENING...WITH WIDESPREAD VFR CONDITIONS DEVELOPING
DURING THE EVENING HOURS. BRUSDA
&&
.PREV DISCUSSION... /ISSUED 316 AM MDT THU SEP 1 2011/
DISCUSSION...A TROUGH OF LOW PRESSURE WILL CONTINUE TO SWING
THROUGH THE NORTHERN ROCKIES TODAY. THIS SYSTEM WILL CONTINUE TO
CAUSE WIDESPREAD PRECIPITATION...PRIMARILY ACROSS NORTHWEST
MONTANA. THIS SYSTEM APPEARS TO HAVE ENOUGH COLD AIR ASSOCIATED
WITH IT TO ALLOW SNOW LEVELS TO DROP DOWN TO 6500 FEET. THE HIGHER
TERRAIN ACROSS NORTHWEST MONTANA...ESPECIALLY GLACIER PARK...WILL
SEE SOME LIGHT SNOW ACCUMULATIONS THIS MORNING. ALSO AS THE
TROUGH AXIS MOVES THROUGH THE REGION THIS MORNING... ENOUGH
INSTABILITY WILL EXIST FOR THE DEVELOPMENT OF A FEW THUNDERSTORMS.
THESE STORMS COULD ENHANCE PRECIPITATION RATES ACROSS NORTHWEST
MONTANA THIS MORNING. TEMPERATURES TODAY ARE ANTICIPATED TO BE
AROUND 10 DEGREES BELOW NORMAL.
THE TROUGH WILL MOVE OFF TO THE EAST KEEPING THE NORTHERN ROCKIES
UNDER NORTHWESTERLY FLOW TONIGHT THROUGH SATURDAY NIGHT. THE FLOW
APPEARS TO REMAIN FAIRLY DRY. THE EXCEPTION IS A WEAK DISTURBANCE
MOVING ALONG THE CANADIAN BORDER ON FRIDAY. THIS FEATURE COULD
CAUSE A FEW SHOWERS ALONG THE NORTHERN PORTION OF THE CONTINENTAL
DIVIDE. TEMPERATURES FOR FRIDAY WILL WARM SLIGHTLY...BUT REMAIN
AROUND 5 DEGREES BELOW NORMAL.
THE START TO LABOR DAY WEEKEND SHOULD PROVE TO BE QUITE PLEASANT
ACROSS THE NORTHERN ROCKIES. WITH EXTENDED FORECAST MODELS BOTH IN
GOOD AGREEMENT ON BUILDING A LARGE AREA OF HIGH PRESSURE ACROSS
THE WESTERN UNITED STATES...CONFIDENCE IS HIGH THAT MOSTLY DRY
CONDITIONS WILL EXIST. WHILE TEMPERATURES WILL BEGIN SATURDAY IN
THE UPPER 70S AND LOW 80S ACROSS THE REGION...EACH DAY FOLLOWING
WILL BE A COUPLE DEGREES WARMER THAN THE PREVIOUS. EXPECT THIS
WARM AND DRY TREND TO LAST WELL INTO NEXT WEEK.
&&
NOTE: FLOOD WATCHES...WARNINGS AND ADVISORIES WILL NOT BE LISTED
BELOW.
.MSO WATCHES/WARNINGS/ADVISORIES...
MT...WINTER WEATHER ADVISORY UNTIL NOON MDT TODAY FOR EARLY SEASON
SNOW ACCUMULATIONS ON LOGAN PASS AND TRAILS IN GLACIER
NATIONAL PARK ABOVE 6500 FEET.
ID...NONE.
&&The national radar mosaic for the moment the first event began.
1 tornado. Local standard time, as filed.
A waterspout formed over Skidoo Bay and remained over water. No damage was reported.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Montana in 2011Every state in 2011This day in weather historyThe tornado archive for Montana, 2011Warnings in Montana, September 2011
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: 346528.