A vigorous upper level disturbance combined with a moist flow from the eastern Pacific brought periods of heavy snow and strong winds to the Snowy and Sierra Madre ranges.
Every event with a place to put it; 2 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 CYS wrote it up.
A vigorous upper level disturbance combined with a moist flow from the eastern Pacific brought periods of heavy snow and strong winds to the Snowy and Sierra Madre ranges.
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 Cheyenne, WY wrote its last area forecast discussion before the first event at Apr 17, 4:59 PM MST, 5 hours 1 minute earlier.
AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHEYENNE WY 559 PM MDT SUN APR 17 2011 .UPDATE...UPDATE TO AVIATION. IN ADDITION...LOWERED PROB. OF PRECIPITATION EAST OF THE LARAMIE RANGE THIS EVENING INTO WESTERN NEBRASKA WITH OBS SHOWING DECREASING CLOUDS AND RAIN SHOWER ACTIVITY DISSIPATING. AFTER MIDNIGHT TONIGHT...PRECIP WILL INCREASE IN COVERAGE ONCE AGAIN AS A PACIFIC FRONT MOVES INTO THE REGION. ADDED PATCHY FOG TO THE SOUTHERN LARAMIE RANGE INCLUDING THE I80 SUMMIT LATE TONIGHT WITH MODELS SHOWING SURFACE WINDS SHIFTING INTO THE SOUTHEAST...MAY NEED TO ADD FOG TO PORTIONS OF WESTERN NEBRASKA BY MONDAY MORNING AS WELL. TT .AVIATION...00Z TAF ISSUANCE. ANOTHER TRICKY AVIATION FORECAST EXPECTED FOR TONIGHT AS A PACIFIC COLD FRONT BEGINS TO INFLUENCE SOUTHEAST WYOMING AND WESTERN NEBRASKA TONIGHT AND INTO MONDAY MORNING. FOR NOW...VFR CONDITIONS WILL PREVAIL THIS EVENING FOR MOST OF THE AREA EXCEPT FOR SOME MVFR CIGS AROUND 2K FEET AGL AT KCDR. CONDITIONS ARE EXPECTED TO DETERIORATE AFTER MIDNIGHT TONIGHT WITH WIDESPREAD MVFR CONDITIONS AND EVENTUALLY IFR CIGS DUE TO EASTERLY UPSLOPE FLOW AND RAIN SHOWER ACTIVITY...MIXED WITH SNOW AT TIMES...MOVING INTO THE REGION. THE BEST CHANCE OF IFR DUE TO LOW VIS AND LOW CIGS BELOW 1K FEET AGL WILL BE KAIA...KCDR...AND KRWL THROUGH SUNRISE MONDAY. RAIN IS EXPECTED TO BECOME WIDESPREAD ON MONDAY ALTHOUGH WINDS ARE EXPECTED TO BECOME RELATIVELY LIGHT COMPARED TO THE WINDS THIS WEEKEND. && .PREV DISCUSSION... /ISSUED 231 PM MDT SUN APR 17 2011/ UPDATE... LATEST MEDICINE BOW PROFILER WINDS SHOWING LOWEST GATE WINDS CONTINUE TO DECREASE THIS AFTERNOON...DOWN TO 40KTS FROM A PEAK OF 50KTS WHEN THE WINDS WERE HITTING WARNING CRITERIA ACROSS THE SOUTHERN LARAMIE RANGE. AREA MESONET OBSERVATIONS SHOWING A DOWNWARD TREND IN WIND SPEEDS AS WELL. THEREFORE...WILL GO AHEAD AND CANCEL THE HIGH WIND WARNING WITH THIS FORECAST PACKAGE AND WILL DEAL WITH ANY FURTHER WIND ISSUES WITH SPS STATEMENTS. UPDATES HAVE BEEN SENT. CLAYCOMB PREV DISCUSSION... /ISSUED 148 PM MDT SUN APR 17 2011/ SHORT TERM...TONIGHT THROUGH TUESDAY NIGHT ACTIVE PATTERN TO CONTINUE OVER THE SHORT TERM AS A SERIES OF FAST MOVING SHORTWAVES INTERACT WITH A STALLED SURFACE BOUNDARY OVER SOUTHEAST WYOMING. CURRENTLY...STATIONARY FRONT LOOKS TO BE LYING ACROSS SOUTH CENTRAL MONTANA...ACROSS NORTHEASTERN WYOMING AND THEN EXTENDING SOUTHWARD INTO THE PANHANDLE AND SOUTHWESTERN NEBRASKA THIS AFTERNOON. MODELS DID A PRETTY GOOD JOB FORECASTING THIS FEATURE NORTHEAST YESTERDAY BY THE WAY. STRONG WINDS THAT MATERIALIZED EARLIER THIS MORNING ARE STARTING TO SHOW SIGNS OF WEAKENING...BUT ARE STILL QUITE STRONG. THESE WINDS WERE NOT FORECAST VERY WELL ON MOS GUIDANCE OR THE LOCAL WIND STUDY. HEIGHT DIFFERENCES ON THE WRKHGT PRODUCT WERE ONLY IN THE 30 TO 40 DM RANGE AT 850MBS AND A LITTLE STRONGER AT 700MB. THIS WIND EVENT LOOKS MORE LIKE THE DOWNWARD MOTIONS FORCING STRONGER WINDS DOWN TO THE SURFACE THAT WE SAW ON A FEW OCCASIONS LAST MONTH. THIS WAS PICKED UP WELL ON THE WRF TIME-CROSS SECTION PRODUCT THAT CLEARLY SHOWS THESE WINDS BEING FORCED DOWN TO THE SURFACE. THE FORECAST CONTINUES THESE STRONG WINDS UNTIL THE 21Z TIME FRAME. WE MAY BE ABLE TO CANCEL THE WIND WARNING EARLY. WILL LET THE WARNING RIDE THOUGH FOR ANOTHER COUPLE HOURS BEFORE LIFTING THE HEADLINE. STARTING TO SEE SOME LIGHTNING STRIKES WITH SHOWERS NORTH AND WEST OF CHEYENNE HERE EARLY THIS AFTERNOON. CELLS ARE MOVING FAIRLY FAST...SO DO NOT THINK MANY LOCATIONS WILL SEE A LOT OF RAINFALL...MAYBE JUST A FEW HUNDREDTHS. OF MORE SIGNIFICANCE WITH THESE CELLS...THEY ARE BRINGING HIGHER WINDS DOWN TO THE SURFACE. AS EVIDENCED HERE AT KCYS...LIGHT RAIN STARTED AND WINDS PICKED UP TO ALMOST 50KTS WITH THE SHOWERS. ANOTHER REASON I WILL KEEP THE HIGH WIND WARNING GOING FOR ANOTHER FEW HOURS. FRONT SAGS SOUTH TONIGHT TO NEAR THE WYOMING/COLORADO BORDER BY EARLY MONDAY MORNING. THIS SHOULD HELP DECREASE WINDS AND INCREASE PRECIPITATION COVERAGE...ESPECIALLY ACROSS OUR NORTHERN ZONES. MONDAY LOOKS LIKE A PRETTY WET DAY ON TAP. BOTH THE GFS AND ECMWF SHOWING ANYWHERE FROM .1 TO POSSIBLY .5 INCHES OF RAINFALL FOR MONDAY. WE CERTAINLY COULD USE THE RAINFALL. AS FOR SNOWS...LOOKS LIKE THE ATMOSPHERE IS GOING TO REMAIN FARLY WARM WITH 700MB TEMPERATURES STILL ABOVE FREEZING ACROSS SOUTHEAST WYOMING. MAJORITY OF THIS PRECIP LOOKS TO FALL AS RAIN. COLDER TEMPERATURES MOVE INTO THE AREA TUESDAY WITH 700MB TEMPERATURES FALLING TO -6 TO -8C TUESDAY AFTERNOON. AT THAT TIME...WE SHOULD SEE MAINLY ALL SNOW IN THE HIGHER PEAKS OF THE SNOWY AND SIERRA MADRE RANGES WHERE 4-8 INCHES OF SNOW COULD FALL. AS FOR THE REST OF US...IT LOOKS LIKE WE COULD GET A DUSTING DURING THE DAY TUESDAY. LONG TERM...WEDNESDAY THROUGH SUNDAY UNSETTLED WEATHER WILL CONTINUE THROUGHOUT THE EXTENDED PERIOD. A WEAK UPPER RIDGE WILL BE OVER THE CWA ON WED WHICH WILL RESULT IN MAINLY DRY CONDITIONS...BUT A UPPER LEVEL DISTURBANCE WILL MOVE ACROSS THE FORECAST AREA LATE IN THE DAY AND WILL PRODUCE RAIN AND SNOW SHOWERS ACROSS THE AREA. A SERIES OF DISTURBANCES IN THE SW FLOW ALOFT WILL MOVE ACROSS THE CWA FROM WED NIGHT THROUGH FRI NIGHT. THESE DISTURBANCE WILL PRODUCE RAIN SHOWERS DURING THE AFTERNOON AND EVENINGS...WITH SNOW SHOWERS FROM LATE EVENING INTO THE EARLY MORNING. A WEAK UPPER RIDGE WILL MOVE OVER THE AREA ON SAT AND THIS WILL RESULT IN A DECREASE IN THE CHANCE OF SHOWERS. SAT NIGHT AND SUN WILL BE DRY BUT SW FLOW WILL REMAIN ALOFT AS UPPER TROUGH DEEPENS CONSIDERABLY OVER THE WESTERN U.S. HIGH TEMPERATURES WILL BE A FEW DEGREES BELOW NORMAL ON WED...AROUND 5 DEGREES ABOVE NORMAL FOR THUR...AND AROUND NORMAL FOR FRI THROUGH SUN. FIRE WEATHER... DESPITE STRONG WINDS...FIRE WEATHER CONCERNS WILL BE MINIMIZED DUE TO CONTINUED RAIN AND SNOW FALL OVER THE NEXT SEVERAL DAYS. WETTING RAINS LOOK TO BE A PRETTY GOOD BET FOR MONDAY AS A COLD FRONT STALLS OUT NEAR THE WYOMING/COLORADO BORDER AND IS EXPECTED TO MEANDER NORTH AND SOUTH THROUGH MUCH OF THE WEEK. HYDROLOGY... A FLASH FLOOD WATCH REMAINS IN EFFECT FOR SOUTHWEST CARBON COUNTY THROUGH FRIDAY AFTERNOON DUE TO A POTENTIAL DAM FAILURE AT THE LITTLE ROBBER DETENTION DAM...13 MILES NORTH OF BAGGS. REPAIRS TO THE DAM WILL NOT BE DONE UNTIL THAT TIME. RIVER LEVELS ON THE NORTH PLATTE RIVER DOWNSTREAM OF THE GUERNSEY RESERVOIR WILL REMAIN STEADY LEVELS TODAY...WITH SLIGHT RISES TONIGHT AND MONDAY...BEFORE RESUMING A SLOW FALL THROUGH THE MIDDLE OF THE WEEK. THE GAGE SITES AT HENRY AND MITCHELL NEBRASKA WILL REMAIN IN THE ACTION STAGE UNTIL FURTHER NOTICE. ADDITIONAL RELEASES FROM GUERNSEY RESERVOIR WILL CONTINUE...WITH MOST OF THE WATER DIVERTED TO IRRIGATION CANALS. && .CYS WATCHES/WARNINGS/ADVISORIES... WY...FLASH FLOOD WATCH UNTIL 6 PM FRIDAY FOR WYZ111. NE...NONE. && SHORT TERM... LONG TERM/AVIATION...
HAZARDOUS WEATHER OUTLOOK NATIONAL WEATHER SERVICE CHEYENNE WY 237 PM MDT SUN APR 17 2011 WYZ101>119-182045- CONVERSE COUNTY LOWER ELEVATIONS-NIOBRARA COUNTY- NORTH LARAMIE RANGE-FERRIS/SEMINOE/SHIRLEY MOUNTAINS- SHIRLEY BASIN-CENTRAL LARAMIE RANGE AND SOUTHWEST PLATTE COUNTY- EAST PLATTE COUNTY-GOSHEN COUNTY-CENTRAL CARBON COUNTY- NORTH SNOWY RANGE FOOTHILLS-SOUTHWEST CARBON COUNTY- SIERRA MADRE RANGE-UPPER NORTH PLATTE RIVER BASIN-SNOWY RANGE- LARAMIE VALLEY-SOUTH LARAMIE RANGE-SOUTH LARAMIE RANGE FOOTHILLS- CENTRAL LARAMIE COUNTY-EAST LARAMIE COUNTY- 237 PM MDT SUN APR 17 2011 THIS HAZARDOUS WEATHER OUTLOOK IS FOR EAST CENTRAL WYOMING... SOUTH CENTRAL WYOMING AND SOUTHEAST WYOMING. .DAY ONE...THIS AFTERNOON AND TONIGHT STRONG WEST TO SOUTHWEST WINDS WILL CONTINUE THROUGH EARLY THIS EVENING. GUSTS UP TO 55 MPH WILL OCCUR THROUGH 6 PM. ISOLATED THUNDERSTORMS WILL DEVELOP THIS AFTERNOON AND EARLY EVENING OVER MUCH OF SOUTHEAST WYOMING. SMALL HAIL WILL BE POSSIBLE WITH THESE THUNDERSTORMS. SNOW SHOWERS ARE EXPECTED FOR THE HIGHER ELEVATIONS WHERE A FEW INCHES OF SNOW MAY ACCUMULATE. .DAYS TWO THROUGH SEVEN...MONDAY THROUGH SATURDAY COOLER TEMPERATURES WITH SCATTERED RAIN AND SNOW SHOWERS ARE FORECAST FOR MUCH OF THE WEEK. .SPOTTER INFORMATION STATEMENT... SPOTTER ACTIVATION WILL NOT BE NEEDED TODAY AND TONIGHT. && THIS PRODUCT...ALONG WITH OTHER WEATHER...HYDROLOGIC...AND CLIMATE INFORMATION IS AVAILABLE ON THE WEB AT... HTTP://WEATHER.GOV/CHEYENNE NEZ002-003-019>021-054-055-095-096-182045- DAWES-BOX BUTTE-SCOTTS BLUFF-BANNER-MORRILL-KIMBALL-CHEYENNE- NORTH SIOUX-SOUTH SIOUX- 237 PM MDT SUN APR 17 2011 THIS HAZARDOUS WEATHER OUTLOOK IS FOR THE WESTERN NEBRASKA PANHANDLE. .DAY ONE...THIS AFTERNOON AND TONIGHT WEST TO NORTHWEST WINDS WILL INCREASE TO 15 TO 25 MPH WITH GUSTS TO 40 MPH TODAY. THE WINDS WILL DIMINISH THIS EVENING. ISOLATED THUNDERSTORMS WILL DEVELOP THIS AFTERNOON AND EARLY EVENING. .DAYS TWO THROUGH SEVEN...MONDAY THROUGH SATURDAY COOLER TEMPERATURES WITH SCATTERED RAIN AND SNOW SHOWERS ARE FORECAST FOR MUCH OF THE WEEK. .SPOTTER INFORMATION STATEMENT... SPOTTER ACTIVATION WILL NOT BE NEEDED TODAY AND TONIGHT. && THIS PRODUCT...ALONG WITH OTHER WEATHER...HYDROLOGIC...AND CLIMATE INFORMATION IS AVAILABLE ON THE WEB AT... HTTP://WEATHER.GOV/CHEYENNE
The national radar mosaic for the moment the first event began.
2 winter storm. Local standard time, as filed.
SNOTEL site at Old Battle (Elev. 10,000 ft) estimated 21 inches of snowfall.
SNOTEL site at North French Creek (Elev. 10,130 ft) estimated 21 inches of snowfall.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Wyoming in 2011Every state in 2011This day in weather historyWarnings in Wyoming, April 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: 295953, 295955.