An upper level low pressure system deepened off of the west coast while an area of high pressure was located east of the divide. This resulted in a strong pressure gradient across western Montana that produced strong winds across the region.
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.
An upper level low pressure system deepened off of the west coast while an area of high pressure was located east of the divide. This resulted in a strong pressure gradient across western Montana that produced strong winds across the region.
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 May 13, 1:52 PM MST, 5 hours 8 minutes earlier.
AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE MISSOULA MT 252 PM MDT FRI MAY 13 2011 .DISCUSSION...SOUTHWESTERLY FLOW ALOFT AS ALLOWED FOR SOME MOISTURE TO STREAM INTO THE NORTHERN ROCKIES TODAY. THIS MOISTURE COMBINED WITH AN ATMOSPHERE THAT IS DESTABILIZING SHOULD ALLOW FOR THE DEVELOPMENT OF A FEW SHOWERS AND THUNDERSTORMS THIS AFTERNOON AND EVENING...ESPECIALLY ACROSS LEMHI COUNTY INTO SOUTHWEST MONTANA. THE SHOWER ACTIVITY IS EXPECTED TO DECREASE AFTER SUNSET. THE LARGE LOW PRESSURE SYSTEM SITTING OFF THE COAST OF OREGON WILL BE MOVING TO THE SOUTHWEST TONIGHT INTO TOMORROW. THIS TRANSITION WILL CAUSE THE UPPER LEVEL FLOW TO BECOME MORE SOUTHEASTERLY. THIS FLOW PATTERN WILL KEEP THE BEST MOISTURE AND INSTABILITY IN THE OREGON AND WASHINGTON AREA. BUT A STALLED BOUNDARY WILL BE DRAPED OVER LEMHI COUNTY UP INTO NORTH CENTRAL IDAHO. THIS BOUNDARY WILL BE THE FOCUS POINT FOR SHOWER AND THUNDERSTORM DEVELOPMENT ON SATURDAY AFTERNOON. AT THIS MOMENT THESE THUNDERSTORMS DO NOT APPEAR TO BE SEVERE...BUT WILL HAVE TO BE MONITORED. ALSO WITH THE MOVEMENT OF THE UPPER LEVEL LOW MOVING...THE SURFACE PRESSURE GRADIENT WILL TIGHTEN CAUSING GUSTY EASTERLY WINDS OVER WESTERN MONTANA. THE INTERACTION OF THE EASTERLY WINDS AND NORTH TO SOUTH MOUNTAIN RANGES COULD POTENTIALLY CAUSE DOWNSLOPING WINDS. SATURDAY NIGHT THROUGH SUNDAY NIGHT...FOR NORTH CENTRAL IDAHO AND EXTREME NORTHWEST MONTANA...MOIST AND UNSTABLE AIR WILL CONTINUE TO EXIST OVER THE AREA. THUNDERSTORMS DEVELOPING OVER THE COLUMBIA RIVER BASIN MAY GRAZE THE WEST ENDS OF IDAHO AND CLEARWATER COUNTIES DURING SATURDAY EVENING. SOME OF THESE STORMS WILL HAVE THE POTENTIAL FOR BRIEF GUSTY WINDS...PEA SIZE HAIL...AND BRIEF HEAVY RAIN. THUNDERSTORM ACTIVITY WILL BECOME MORE WIDESPREAD...HOWEVER LESS INTENSE...DURING SUNDAY AFTERNOON. FOR MUCH OF WESTERN MONTANA AND LEMHI COUNTY IN IDAHO...GUSTY EASTERLY WINDS ARE ANTICIPATED TO REMAIN THROUGH SATURDAY EVENING. ON SUNDAY...AFTERNOON AND EVENING THUNDERSTORM POTENTIAL INCREASES AS MOISTURE AND INSTABILITY MOVE IN FROM THE SOUTHWEST. REGION WIDE...ALL OF THE AFOREMENTIONED THUNDERSTORM ACTIVITY WILL PRECEDE A COOL PACIFIC STORM SYSTEM TO AFFECT THE NORTHERN ROCKIES FOR MUCH OF THE NEXT WEEK. MONDAY THROUGH FRIDAY...A PACIFIC STORM SYSTEM WILL EXIST OVER THE NORTHERN ROCKIES FOR MUCH OF NEXT WEEK. RELATIVELY COOL AND SHOWERY CONDITIONS WILL BE PRESENT DURING THIS TIME. THERE IS SOME UNCERTAINTY OF HOW THIS SYSTEM WILL EXIT THE REGION LATE NEXT WEEK...WITH SOME MODEL GUIDANCE SUGGESTING A WARMING TREND BEGINNING THURSDAY. NO MAJOR CHANGES HAVE BEEN MADE TO THE PREVIOUS FORECAST DURING THE FAR EXTENDED PERIODS. && .AVIATION...CONVECTIVE SHOWERS ARE STARTING TO FORM LATE THIS AFTERNOON AND COULD LEAD TO ISOLATED THUNDERSTORM DEVELOPMENT...PARTICULARLY ALONG AND SOUTH OF INTERSTATE 90. AIRFIELDS KSMN AND KBTM WILL MOST LIKELY BE IMPACTED BY THIS WEATHER...MAINLY AFTER 13/2200Z. IN ADDITION...EASTERLY SURFACE WINDS WILL INCREASE THE REST OF TODAY AND REMAIN ELEVATED TONIGHT. EAST WINDS WILL PEAK AGAIN SATURDAY IN WESTERN MONTANA AND BECOME GUSTY. THUNDERSTORMS ARE POSSIBLE ONCE AGAIN SATURDAY AFTERNOON BUT WILL BE SHUNTED INTO NORTH CENTRAL IDAHO...AND KSMN IS LIKELY TO BE IMPACTED. ANY STORMS THAT DEVELOP WILL MOVE FROM SOUTHEAST TO NORTHWEST. && .HYDROLOGY...WITH SNOW MELT RUNOFF CONTINUING TO AID IN SMALL STREAM AND MAIN STEM RIVER RISES...MORE SPECIFIC FLOODING CONCERNS ARE NOW OCCURRING. A FEW MAIN STEM RIVERS IN NORTHWEST MONTANA ARE IN DANGER OF APPROACHING FLOOD STAGE. THESE INCLUDE THE YAAK RIVER NEAR TROY...THE FISHER RIVER NEAR LIBBY...AND THE THOMPSON RIVER NEAR THOMPSON FALLS. PLEASE SEE THE FLOOD RELATED HEADLINES THAT HAVE BEEN ISSUED FOR FURTHER INFORMATION ON THESE RIVERS. WHILE ALL MAIN STEM RIVERS WILL CONTINUE TO SEE AN INCREASE IN THEIR FLOWS THROUGH THE WEEKEND...SMALL STREAMS AND CREEKS ACROSS ALL OF WESTERN MONTANA AND CENTRAL IDAHO WILL BE THE MOST SENSITIVE TO THE ADDITIONAL SNOWMELT BEING ADDED. MANY COULD SEE MINOR FLOODING BY THE END OF THIS WEEKEND. IT IS ALSO IMPORTANT TO REMEMBER THAT ALL AREA RIVERS AND STREAMS WILL BE RUNNING HIGH AND FAST FOR QUITE SOME TIME. THOSE WITH RECREATION INTERESTS DURING THIS WEEKENDS PLEASANT WEATHER SHOULD EXERCISE CAUTION IF PLANNING TO BE NEAR AREA RIVERS. PLEASE CONTINUE TO MONITOR THE FORECAST AS FURTHER HYDROLOGIC HEADLINES MAY BE ISSUED. && .MSO WATCHES/WARNINGS/ADVISORIES... MT...NONE. ID...NONE. && SHORT TERM...SMITH LONG TERM....ZUMPFE AVIATION...DE ROSA
The national radar mosaic for the moment the first event began.
3 high wind. Local standard time, as filed.
An 18 inch diameter live tree was blown down onto a car in the University district near downtown Missoula.
Numerous trees and power lines were down across the Missoula valley, confirmed by the Missoula Sheriff office. Power lines and trees down in the city of Missoula on Orange street, Broadway, Pine and Rattlesnake drive. Rattlesnake drive blocked at Lolo and Greenough streets due to downed trees and power lines.
Numerous downed trees in the Rattlesnake area of Missoula. A tree had been uprooted and fallen on a house, only minor damage reported. |A row of 9 pine trees, 40 feet tall, were uprooted and fell on a car and camper. A 100 year old tree, 60 feet tall fell on a garage damaging the roof and a car next to the garage.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Montana in 2011Every state in 2011This day in weather historyWarnings in Montana, May 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: 306258, 306265, 306266.