A cold front slid south across much of Wyoming as a slow-moving upper level low tracked south of the state. This upper level low wrapped moisture into central Wyoming which combined with the colder air to produce heavy snow across Natrona County and portions of Fremont County.
Every event with a place to put it; 4 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 RIW wrote it up.
A cold front slid south across much of Wyoming as a slow-moving upper level low tracked south of the state. This upper level low wrapped moisture into central Wyoming which combined with the colder air to produce heavy snow across Natrona County and portions of Fremont County. The heaviest snow amounts of 16 inches to nearly two feet fell on Casper Mountain. Snowfall in and around Casper generally ranged from six to ten inches. Six inches of snow was also measured at Jeffrey City.
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 Riverton, WY wrote its last area forecast discussion before the first event at May 9, 1:37 AM MST, 3 hours 23 minutes earlier.
AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RIVERTON WY 237 AM MDT SAT MAY 9 2015 .SHORT TERM...TODAY THROUGH MONDAY NIGHT SHOWERS CONTINUE ACROSS EASTERN AND CENTRAL WYOMING THIS MORNING AS THE AREA OF LOW PRESSURE LIFTS NORTH INTO COLORADO TODAY. SHOWER COVERAGE IS EXPECTED TO INCREASE THROUGH THE DAY. SNOWFALL EXPECTED OVER THE CENTRAL AND NORTHERN MOUNTAINS...WITH A RAIN AND SNOW MIX COMMON ACROSS THE EASTERN AND CENTRAL BASINS. THE PRECIPITATION MIX WILL CONVERT TO SNOWFALL OVER THE BASIN LOCATIONS THIS EVENING...WITH SIGNIFICANT SNOWFALL POSSIBLE OVERNIGHT. ALL REMAINING WINTER STORM WATCHES ACROSS NATRONA AND JOHNSON COUNTIES HAVE BEEN UPGRADED TO WARNINGS. NATRONA COUNTY WILL BE THE HOT SPOT FOR SNOW ACCUMULATION...WITH EVEN NATIONAL GUIDANCE ANTICIPATING SIGNIFICANT SNOWFALL OVER THE AREA. JOHNSON COUNTY IS EXPECTED TO SEE SIGNIFICANT SNOWFALL AS WELL...ESPECIALLY ACROSS THEIR SOUTHERN HALF OF THE ZONE. TRAVEL IS EXPECTED TO BE IMPACTED BY THIS LATE SEASON STORM...AS WELL AS THOSE WITH OUTDOOR AND AGRICULTURAL INTERESTS. THE AREA OF LOW PRESSURE WILL CONTINUE TO LIFT NORTHEAST...AND MOVE INTO EAST CENTRAL NEBRASKA BY SUNDAY AFTERNOON. THIS PLACEMENT OF THE LOW PRESSURE WILL TAKE THE SYSTEM FAR ENOUGH EAST TO HELP DRAG SOME OF THE MOISTURE OUT OF THE FORECAST AREA. SNOWFALL WILL TAPER OFF FROM WEST TO EAST...WITH NATRONA AND JOHNSON COUNTIES TENTATIVELY HANGING ON TO SNOWFALL THROUGH SUNDAY EVENING. HOWEVER...THE BEST ACCUMULATIONS WILL FALL DURING THE EARLIER PORTION OF THE DAY. THE SIGNIFICANT SNOWFALL IS GENERALLY EXPECTED TO WEAKEN BY SUNDAY MORNING WITH DEVELOPING NORTHWEST FLOW EXPECTED OVER THE FORECAST AREA. SOME WEAK PUSHES OF MOISTURE WILL KEEP LIGHT SHOWER ACTIVITY ACROSS SOUTHWESTERN WYOMING LATE SUNDAY AND INTO MONDAY. A TRANSITORY RIDGE IS EXPECTED TO SET UP LATE MONDAY WITH DRY CONDITIONS AND LIMITED WARMING. THE AXIS OF THE RIDGE OF HIGH PRESSURE WILL MOVE INTO CENTRAL WYOMING BY MONDAY EVENING...ALLOWING SOME SOUTHWEST FLOW TO DEVELOP ACROSS THE WEST. THIS WILL PERMIT SOME ADDITIONAL MOISTURE TO MOVE OVER THE AREA MONDAY EVENING...MAINLY IMPACTING THE WESTERN MOUNTAINS. .LONG TERM...TUESDAY THROUGH SATURDAY A COMPACT UPPER LOW OVER THE PACIFIC NORTHWEST/NORTHERN CALIFORNIA AREA TUESDAY IS EXPECTED TO OPEN UP AND LIFT NORTHEAST ACROSS THE NORTHERN ROCKIES IN THE TUESDAY NIGHT/WEDNESDAY TIME FRAME. THE 00Z ECMWF AND GEM HAVE A MORE SOUTHERLY TRACK WHICH RESULTS IN A BETTER CHANCE OF CONVECTION. EITHER WAY IT APPEARS THE BEST CHANCE WILL BE ACROSS THE WESTERN AND NORTHERN SECTIONS OF THE FORECAST AREA. TEMPERATURES WILL BE MORE SEASONAL BY TUESDAY AND WEDNESDAY WITH SNOW LEVELS GENERALLY ABOVE 9500 FEET. IN WAKE OF THE SYSTEM WEDNESDAY...A MAINLY DRY DAY IS EXPECTED THURSDAY...BUT CANNOT RULE OUT SOME CONVECTION MAINLY OVER THE MOUNTAINS. A STRONGER UPPER LOW/TROUGH SHOULD IMPACT THE AREA FRIDAY INTO NEXT WEEKEND. MODELS SHOW THE ASSOCIATED PACIFIC COLD FRONT PUSHING ACROSS THE AREA FRIDAY NIGHT/SATURDAY MORNING. THIS SYSTEM WILL GIVE THE ENTIRE AREA A CHANCE FOR CONVECTION FRIDAY AND SATURDAY. HAVE GONE CONSERVATIVE WITH ONLY SLIGHT VALLEY/CHANCE MOUNTAIN POPS FOR BOTH FRIDAY AND SATURDAY. CURRENTLY THE TRACK OF THIS SYSTEM IS FARTHER NORTH...AND WARMER THAN THE CURRENT MOTHERS DAY WEEKEND SNOW STORM. PROJECTED 700MB TEMPERATURES WOULD KEEP PRECIPITATION MAINLY AS RAIN IN THE VALLEYS...BUT MOUNTAIN PASSES COULD BE SLICK AGAIN WITH SNOWFALL. && .AVIATION.../12Z ISSUANCE/ EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES WIDESPREAD MVFR/IFR CONDITIONS IN -RA WITH SNOW LEVELS BETWEEN 6-7KFT WILL CONTINUE TO DETERIORATE EARLY THIS MORNING. WIDESPREAD IFR CONDITIONS WITH AREAS OF LIFR WILL PREVAIL ACROSS THE AREA BY 15Z SATURDAY INTO SATURDAY EVENING. SNOW LEVELS ARE EXPECTED TO LOWER THROUGH THE DAY TO BETWEEN 5-6KFT SATURDAY AFTERNOON. THE HEAVIEST PRECIPITATION IS EXPECTED TO SHIFT IN THE VCNTY AND EAST OF INTERSTATE 25 AFTER 06Z SUNDAY WITH GRADUAL IMPROVEMENT FROM THE NORTHWEST SUNDAY MORNING. WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES SCT-NMRS MVFR/IFR SHRA/SHSN WILL CONTINUE WITH THIS ACTIVITY MOST WIDESPREAD VCNTY AND SOUTH OF A KPNA-KAFO LINE. SNOW LEVELS WILL BE MOSTLY BTWN 7-8KFT THIS MORNING. A LARGER SURGE OF MOISTURE IS EXPECTED TO PUSH INTO SW WYOMING BY 15Z SATURDAY WHEN WIDESPREAD IFR CONDITIONS ARE EXPECTED VCNTY AND SOUTH OF A KPNA-KAFO LINE. DOWNSLOPE NE WINDS WILL RESULT IN IMPROVING CONDITIONS VCNTY AND NORTH OF KJAC SATURDAY AFTERNOON. SNOW LEVELS WILL LOWER TO MOST VALLEY FLOORS SATURDAY AFTERNOON...WITH THE HEAVIEST PRECIPITATION SHIFTING FURTHER SOUTH AND EAST...VCNTY AND SOUTH OF A KLND-KRKS-KEMM LINE WITH IFR/LIFR SNOW PERSISTING IN THIS AREA THROUGH EARLY SUNDAY MORNING. && .FIRE WEATHER... A LATE SEASON WINTER STORM WILL IMPACT THE AREA TODAY...SPREADING SNOWFALL ACROSS CENTRAL AND EASTERN WYOMING. SNOWFALL WILL BEGIN ACROSS CENTRAL WYOMING...AND WILL BE MOST SIGNIFICANT ACROSS THE CENTRAL MOUNTAINS AND ADJACENT FOOTHILL LOCATIONS. SNOWFALL WILL INTENSIFY AND COVERAGE WILL INCREASE THROUGH THE NIGHT...WITH SIGNIFICANT SNOWFALL ANTICIPATED TO MOVE ACROSS EASTERN WYOMING. WINTER STORM WARNINGS HAVE BEEN POSTED FOR MUCH OF EASTERN WYOMING AND THE CENTRAL MOUNTAINS AND FOOTHILLS. ADDITIONAL WINTER WEATHER ADVISORIES HAVE BEEN ISSUED FOR ADDITIONAL AREAS OF CENTRAL WYOMING...AND ALSO ACROSS THE EASTERN SLOPES OF THE NORTHWESTERN MOUNTAINS. WITH THIS SYSTEM DEVELOPING...FIRE BEHAVIOR SHOULD REMAIN LIMITED DUE TO ONGOING MOISTURE. && .RIW WATCHES/WARNINGS/ADVISORIES... WINTER STORM WARNING FROM 6 AM THIS MORNING TO 9 AM MDT SUNDAY WYZ018-019. WINTER STORM WARNING FROM NOON TODAY TO NOON MDT SUNDAY WYZ009>011-020-022. WINTER WEATHER ADVISORY FROM 6 AM THIS MORNING TO 9 AM MDT SUNDAY WYZ007-016-017. WINTER STORM WARNING UNTIL 9 AM MDT SUNDAY WYZ015. WINTER WEATHER ADVISORY FROM 6 AM THIS MORNING TO MIDNIGHT MDT TONIGHT WYZ002-003. && SHORT TERM...BRANHAM LONG TERM...WM AVIATION...AEM FIRE WEATHER...BRANHAM
HAZARDOUS WEATHER OUTLOOK NATIONAL WEATHER SERVICE RIVERTON WY 300 AM MDT SAT MAY 9 2015 WYZ001>020-022>030-101200- ABSAROKA MOUNTAINS-BIGHORN MOUNTAINS SOUTHEAST- BIGHORN MOUNTAINS WEST-CASPER MOUNTAIN-CODY FOOTHILLS- EAST SWEETWATER COUNTY-FLAMING GORGE- GREEN MOUNTAINS AND RATTLESNAKE RANGE-JACKSON HOLE-LANDER FOOTHILLS- NATRONA COUNTY LOWER ELEVATIONS-NORTH BIG HORN BASIN- NORTHEAST JOHNSON COUNTY-OWL CREEK AND BRIDGER MOUNTAINS- ROCK SPRINGS AND GREEN RIVER-SALT RIVER AND WYOMING RANGES- SOUTH LINCOLN COUNTY-SOUTHEAST BIG HORN BASIN- SOUTHEAST JOHNSON COUNTY-SOUTHWEST BIG HORN BASIN-STAR VALLEY- TETON AND GROS VENTRE MOUNTAINS-UPPER GREEN RIVER BASIN- UPPER GREEN RIVER BASIN FOOTHILLS-UPPER WIND RIVER BASIN- WIND RIVER BASIN-WIND RIVER MOUNTAINS EAST-WIND RIVER MOUNTAINS WEST- YELLOWSTONE NATIONAL PARK- 300 AM MDT SAT MAY 9 2015 ...WINTER STORM WARNINGS AND WINTER WEATHER ADVISORIES FOR PORTIONS OF CENTRAL WYOMING THIS WEEKEND... THIS HAZARDOUS WEATHER OUTLOOK IS FOR WESTERN AND CENTRAL WYOMING. .DAY ONE...TODAY AND TONIGHT SIGNIFICANT SNOWFALL EXPECTED ACROSS PORTIONS OF CENTRAL AND EASTERN WYOMING TODAY AND TONIGHT. SNOWFALL WILL BEGIN OVER THE MOUNTAINS WITH IMPACTS FELT AT THE MOUNTAIN PASS LEVEL. SNOW LEVELS WILL FALL THROUGH THE MORNING AND AFTERNOON...WITH WIDESPREAD BASIN SNOWFALL EXPECTED. POSSIBLE GUSTY WIND THIS AFTERNOON WILL COMBINE WITH SNOW TO CREATE LIMITED VISIBILITIES ON ROADWAYS...ESPECIALLY MOUNTAIN PASSES. MOTORISTS SHOULD MONITOR CURRENT ROAD CONDITIONS BEFORE TRAVELING. SNOW WILL CONTINUE OVERNIGHT BEFORE WEAKENING SUNDAY. PLEASE VISIT THE WINTER STORM PRODUCTS FOR ADDITIONAL INFORMATION. .DAYS TWO THROUGH SEVEN...SUNDAY THROUGH FRIDAY SUNDAY...SNOWFALL WILL STILL BE SIGNIFICANT THROUGH EARLY SUNDAY MORNING WITH ADVERSE ROAD CONDITIONS ANTICIPATED. SNOWFALL SHOULD WEAKEN THROUGH THE LATE MORNING AND AFTERNOON HOURS. MONDAY THROUGH WEDNESDAY...PATCHY FOG AND TEMPERATURES BELOW FREEZING ARE LIKELY EARLY MONDAY CREATING SOME ICY ROADS TO START THE DAY. OTHERWISE...ISOLATED AFTERNOON AND EVENING THUNDERSTORMS ARE POSSIBLE EACH DAY. .SPOTTER INFORMATION STATEMENT... SPOTTERS ARE ENCOURAGED TO REPORT SNOWFALL AMOUNTS. THANK YOU. TEAM RIVERTON
The national radar mosaic for the moment the first event began.
4 winter storm. Local standard time, as filed.
Snowfall of 12 inches was estimated at the Townsend Creek SNOTEL.
Trained spotters measured anywhere from 16 to 22.5 inches of new snow on Casper Mountain.
Trained spotters measured 4 to 10.5 inches of snow around Casper. The highest amounts of snow were in south Casper, near the foothills of Casper Mountain.
A cooperative weather observer measured six inches of snow at Jeffrey City.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Wyoming in 2015Every state in 2015This day in weather historyWarnings in Wyoming, May 2015
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: 583584, 570010, 570009, 570008.