A very moist air mass where thunderstorms developed in a slow moving westerly flow resulted in heavy rainfall and flash flooding.
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The episode as NWS GJT wrote it up.
A very moist air mass where thunderstorms developed in a slow moving westerly flow resulted in heavy rainfall and flash flooding.
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 Grand Junction, CO wrote its last area forecast discussion before the first event at Aug 12, 10:50 AM MST, 3 hours 10 minutes earlier.
Area Forecast Discussion National Weather Service GRAND JUNCTION CO 1150 AM MDT Sat Aug 12 2017 Updated aviation section .SHORT TERM...(Today through Sunday) Issued at 314 AM MDT Sat Aug 12 2017 IR satellite showing some clouds hanging around this morning with a few showers noticed on radar but nothing too impressive. Not much change to going forecast as PWATs remain between .75 to 1 inch and a weak upper level jet remains above us. Some turning of the winds can be seen at 700mb later on this afternoon indicating a weak wave will move through the area, looking to favor northern portions of the CWA. However, in this moist, fairly unstable regime, convection could fire just about anywhere or form over mountains and drift into adjacent valleys. Convection will fire around noon with coverage increasing though the afternoon and evening hours. Biggest concerns are the usual: dangerous lighting, wind gusts of 40 to 50 mph, small hail, and heavy downpours from time to time. Temps will remain right where they should be this time of year. Overnight, the GFS keeps our area quite active while the NAM just about shuts all convection down. As the GFS tends to overdo convection, started dropping PoPs after midnight through Sunday morning. More of the same on Sunday as another wave moves over Utah and Colorado allowing another round of convection to fire. This trend looks to continue through midweek before some drying kicks in as described below. .LONG TERM...(Sunday night through Friday) Issued at 314 AM MDT Sat Aug 12 2017 Longwave trough digging across the western U.S. will drive the upper level ridge over the region eastward with southwest flow becoming established early in the week. Moisture surging northward along the leading edge will provide the fuel for more widespread showers and thunderstorms. Activity peaks on Monday with slow drying trend into midweek and beyond. Should see a few stronger storms on Tuesday as moisture gradient presses eastward and trough moves in. Convection will be diminished beyond Tuesday with only isolated mountain storms anticipated. Temperatures will be running near normal through Tuesday with slowly warming expected through the end of the week. && .AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon) Issued at 1150 AM MDT Sat Aug 12 2017 Afternoon thunderstorms are starting to develop over higher terrain, and over NW Colorado. This trend will continue as storms move west to east. An area of showers and tstorms is expected to move from central Utah eastward this evening, bringing a continuation of convection through the overnight hours. ILS breakpoints will be reached at times, and potentially MVFR temporarily in stronger storms if they target the airports. && .GJT WATCHES/WARNINGS/ADVISORIES... CO...None. UT...None. && SHORT TERM...TGR LONG TERM...MDA AVIATION...JAM
Hazardous Weather Outlook National Weather Service GRAND JUNCTION CO 525 AM MDT Sat Aug 12 2017 COZ001>014-017>023-UTZ022>025-027>029-131200- Lower Yampa River Basin-Central Yampa River Basin- Roan and Tavaputs Plateaus-Elkhead and Park Mountains- Upper Yampa River Basin-Grand Valley-Debeque to Silt Corridor- Central Colorado River Basin-Grand and Battlement Mesas- Gore and Elk Mountains/Central Mountain Valleys- Central Gunnison and Uncompahgre River Basin- West Elk and Sawatch Mountains-Flat Tops- Upper Gunnison River Valley-Uncompahgre Plateau and Dallas Divide- Northwest San Juan Mountains-Southwest San Juan Mountains- Paradox Valley/Lower Dolores River- Four Corners/Upper Dolores River-Animas River Basin- San Juan River Basin-Southeast Utah-Eastern Uinta Mountains- Eastern Uinta Basin-Tavaputs Plateau-Arches/Grand Flat- La Sal and Abajo Mountains-Canyonlands/Natural Bridges- 525 AM MDT Sat Aug 12 2017 This hazardous weather outlook is for eastern Utah and western Colorado. .DAY ONE...Today and Tonight Similar conditions are in place today allowing more convection to fire this afternoon and evening. Convection will start around noon and increase through the afternoon and evening hours. Expect some heavy rain, gusty winds, and small hail from the stronger cells. Most activity will die down after sunset but a few stray showers will linger into the overnight. .DAYS TWO THROUGH SEVEN...Sunday through Friday Little change for Sunday through Tuesday as plenty of moisture will lend themselves for more afternoon convection. Winds will shift Wednesday onwards which will limit showers and storm formation bringing dry conditions back into the forecast. .SPOTTER INFORMATION STATEMENT... Spotter activation will not be needed.
Special Weather Statement National Weather Service Grand Junction CO 111 PM MDT SAT AUG 12 2017 UTZ023-121945- Eastern Uinta Mountains UT- 111 PM MDT SAT AUG 12 2017 ...SIGNIFICANT WEATHER ADVISORY FOR NORTH CENTRAL UINTAH AND CENTRAL DAGGETT COUNTIES UNTIL 145 PM MDT... At 111 PM MDT, Doppler radar was tracking a strong thunderstorm 16 miles south of Manila, moving east at 5 mph. Dime size hail and wind gusts up to 50 mph will be possible with this storm. This storm will remain over mainly rural areas of north central Uintah and central Daggett Counties. This includes U.S. Highway 191 in Utah between mile markers 372 and 383. LAT...LON 4074 10972 4084 10968 4082 10937 4064 10950 TIME...MOT...LOC 1911Z 289DEG 5KT 4077 10963 JAM
Special Weather Statement National Weather Service Grand Junction CO 251 PM MDT SAT AUG 12 2017 COZ002-013-122130- Flattops CO-Central Yampa River Basin CO- 251 PM MDT SAT AUG 12 2017 ...SIGNIFICANT WEATHER ADVISORY FOR NORTHEASTERN RIO BLANCO COUNTY UNTIL 330 PM MDT... At 250 PM MDT, Doppler radar was tracking a strong thunderstorm 10 miles east of Meeker, or 34 miles south of Craig, moving southeast at 15 mph. Half inch hail and wind gusts up to 50 mph will be possible with this storm. Locations impacted include... Buford. PRECAUTIONARY/PREPAREDNESS ACTIONS... Heavy rainfall is also occurring with this storm, and may lead to localized flooding. Do not drive your vehicle through flooded roadways. This storm may intensify, so be certain to monitor local radio stations and available television stations for additional information and possible warnings from the National Weather Service. && LAT...LON 4000 10783 4014 10769 3997 10730 3976 10751 TIME...MOT...LOC 2050Z 307DEG 14KT 4003 10769 JDC
The Grand Junction radar (KGJX), 85 km away, was scanning. The volume nearest the first event began at 21:00 UTC.
1 flash flood. Local standard time, as filed.
Heavy rainfall, with radar estimates of nearly two inches of total rain within about an hour on the Roan Plateau, produced flash flooding several feet deep which came down drainages and ran across farm fields before ending up in Piceance Creek.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Colorado in 2017Every state in 2017This day in weather historyWarnings in Colorado, August 2017
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: 707699.