A thunderstorm produced very heavy rain late in the evening over Sylvan Pass. S.
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 RIW wrote it up.
A thunderstorm produced very heavy rain late in the evening over Sylvan Pass. A slide of mud and earth oozed onto U.S. Highway 16 about 6 miles west of Yellowstone National Park's east entrance. Four feet of mud and debris trapped sixteen people in four vehicles for three hours. The debris flow was estimated to contain 30,000 tons of mud and rock and was in some cases 10 feet deep. The east entrance of Yellowstone National Park was closed for four days following the mudslide. No injuries occurred.
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 Jul 18, 1:02 PM MST, 5 hours 58 minutes earlier.
AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RIVERTON WY 202 PM MDT SUN JUL 18 2004 .TONIGHT THROUGH WEDNESDAY... WV IMAGERY INDICATING UPPER LEVEL RIDGE AXIS OVER THE SOUTHERN AND CENTRAL ROCKIES. MONSOON MOISTURE PLUME ON BACK SIDE OF RIDGE FORM THE SOUTHWEST US ON NORTH ACROSS THE GREAT BASIN...WITH SHORT WAVE MOVING NORTH INTO SOUTHERN IDAHO. ALSO...SMALLER SCALE WAVE JUST WEST OF YNP...MOVING NORTH. CONVECTION HAS DEVELOPED ACROSS SOUTHEAST IDAHO...NORTHERN UTAH AND WESTERN COLORADO WHERE BEST INSTABILITY AND SOME DYNAMICS ARE LOCATED. ISOLD TSTMS HAVE MADE IT INTO WESTERN WYOMING. GOING WITH SCT POPS ACROSS THE WESTERN 1/3 OF CWFA THIS EVENING DUE TO SOME DYNAMICS AND BEST INSTABILITY...AND ISOLD POPS ELSEWHERE. SHORT WAVE ENERGY ROTATES AROUND UPPER LEVEL RIDGE ON MONDAY ACROSS MONTANA...AND RIDGE FLATTENS TUESDAY AND WEDNESDAY. POPS WILL BE MAINLY ISOLD AND AFTERNOON AND EVENING POPS...WITH SOME SCT POPS ACROSS THE MOUNTAINS FOR MONDAY AND TUESDAY. STORMS WILL MAINLY BE INSTABILITY DRIVEN. WILL BE DRY ACROSS THE NORTH ON WEDNESDAY AS MOISTURE IS PUSHED SOUTH. MAINLY ISOLD AFTERNOON AND EVENING POPS ACROSS THE SOUTH ON WEDNESDAY. .WEDNESDAY NIGHT THROUGH SATURDAY... MONSOON MOISTURE CONTINUES TO WORK ITS WAY NORTH WEDNESDAY THROUGH FRIDAY WITH A SERIES OF WEAK SHORT WAVE LOW PRESSURE TROUGHS ROTATING THROUGH THE WEST AND NORTH SIDES OF THE MAIN UPPER LEVEL HIGH PRESSURE RIDGE. THE SHORT WAVE ENERGY...COMBINED WITH THE MOISTURE AND DAYTIME HEATING SHOULD SUPPORT AFTERNOON AND EVENING CONVECTION. HAVE INCREASED CHANCE OF PRECIPITATION THROUGH FRIDAY EXCEPT IN THE NORTH WHERE MOISTURE WILL BE MORE LIMITED. WITH CLOUD COVER AND PRECIPITATION AROUND EXPECT COOLER DAY TIME HIGHS AND HAVE COOLED HIGHS SLIGHTLY WHILE RAISING OVERNIGHT LOWS SLIGHTLY ALSO... DUE TO INCREASED CLOUD COVER LIMITING RADIATIONAL COOLING. && .RIW WATCHES/WARNINGS/ADVISORIES...NONE. && MC/C.BAKER
HAZARDOUS WEATHER OUTLOOK NATIONAL WEATHER SERVICE RIVERTON WY 505 AM MDT SUN JUL 18 2004 WYZ001>020-022>030-191105- 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- 505 AM MDT SUN JUL 18 2004 THIS HAZARDOUS WEATHER OUTLOOK IS FOR WESTERN AND CENTRAL WYOMING. .DAY ONE...TODAY AND TONIGHT SCATTERED AFTERNOON AND EVENING THUNDERSTORMS ARE EXPECTED THROUGHOUT SOUTHWEST WYOMING AND AREAS WEST OF THE CONTINENTAL DIVIDE...WITH ONLY ISOLATED THUNDERSTORM ACTIVITY EAST OF THE CONTINENTAL DIVIDE. THESE STORMS MAY PRODUCE DANGEROUS LIGHTNING...PERIODS OF HEAVY RAIN...HAIL AND BRIEF GUSTY WINDS. .DAYS TWO THROUGH SEVEN...MONDAY THROUGH SATURDAY. ISOLATED AFTERNOON AND EVENING THUNDERSTORMS WILL DEVELOP OVER WESTERN AND CENTRAL WYOMING MONDAY AND TUESDAY. THESE STORMS MAY PRODUCE DANGEROUS LIGHTNING...PERIODS OF HEAVY RAIN...HAIL...AND BRIEF GUSTY WINDS. THERE WILL BE A SLIGHT CHANCE OF THUNDERSTORMS OVER PARTS OF CENTRAL AND WESTERN WYOMING WEDNESDAY THROUGH FRIDAY. .SPOTTER INFORMATION STATEMENT... SPOTTERS ARE ENCOURAGED TO REPORT HAIL SIZES...WIND GUSTS OVER 60 MPH OR HEAVY RAINFALL. THANKS. CF/C.BAKER
The national radar mosaic for the moment the first event began.
1 debris flow. Local standard time, as filed.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Wyoming in 2004Every state in 2004This day in weather historyWarnings in Wyoming, July 2004
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: 5411422.