As the ridge of high pressure moved into a more favorable position near the Southern Plains, eastern Utah was able to access the deeper monsoon moisture. This led to increased coverage of showers and thunderstorms.
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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 GJT wrote it up.
As the ridge of high pressure moved into a more favorable position near the Southern Plains, eastern Utah was able to access the deeper monsoon moisture. This led to increased coverage of showers and thunderstorms. Storms were slow moving and produced heavy rain which led to some flooding in portions of Grand County.
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 17, 2:51 PM MST, 24 minutes earlier.
Area Forecast Discussion National Weather Service Grand Junction CO 351 PM MDT Thu Aug 17 2023 .SHORT TERM...(Tonight through Friday night) Issued at 351 PM MDT Thu Aug 17 2023 Moisture levels remained anomalously high across the forecast area today. Dew points were running in the mid 40s to lower 50s across the region and the 12Z/GJT sounding held 0.96" precipitable water (PW). High pressure continued to dominate the southern Plains while a quasi-stationary closed low was spinning over the eastern Pacific off the southern California coast. With few clouds to limit solar insolation this morning, surface warming resulted in thunderstorm formation over the Abajo mountains by 1730Z. Since then, storms continued to develop over the La Sal and San Juan Mountains, the eastern Tavaputs and Roan Plateaus, the Flat Tops and portions of the central Colorado mountains. Unlike yesterday at this time, today's storms were moving northward as the axis of high pressure ridge over the area shifted east over the past 24 hours. Slow storm movement combined with deep moisture raises the threat for excessive rainfall and the potential for localized flash flooding. Models indicated a weak short wave trough will push across northeast Utah and northwest Colorado this evening which is likely to keep shower/thunderstorm activity going well into the evening. Otherwise, diurnal cooling will bring a slow diminishment of moist convection during the night though isolated showers may linger through sunrise Friday. Southerly flow aloft increases a bit on Friday as the eastern Pacific low moves toward the coast sharpening the pressure gradient overhead. Meanwhile, PW levels will remain high with values in the 1 to 1.2" range for much of the area which is close to 150% of normal. As mentioned in the previous discussion, jet divergence will add dynamic lift to diurnally driven instability which should enhance storm coverage and intensity. Though storm motion should increase slightly compared to today, the potential for excessive rainfall and localized flooding will continue. Increased clouds and showers are expected to bring slightly cooler temperatures tonight through Friday morning, though readings will continue to run just a little above normal. .LONG TERM...(Saturday through Thursday) Issued at 351 PM MDT Thu Aug 17 2023 Saturday's forecast and beyond will be keeping an eye to Hilary and its impact on moisture volume here on the West Slope. Presently, deterministic models are keeping the richest moisture too far west for our benefit. However, a bit of recycled moisture will likely get tapped for some afternoon convection on Saturday, mostly on terrain and short lived beyond sunset. A few rich PWATS are showing up in ensemble forecasts over northwest CO on Saturday afternoon, this could make for a few stronger showers there. A formidable high pressure center baking the Plains pushes back westward enough to cut things off on Sunday and likely allow those summer temperatures to return. Mountain areas under the stronger high pressure will see temperatures climb 5-10 degrees above August averages Sunday and Monday. Meanwhile desert valleys will still be 5 degrees above average for the weekend and starting the work week. Height falls from Hilary making its way ashore out west will shift some moisture east enough for a stray shower or two along the Divide and on the San Juans Tuesday. This will begin another uptick for monsoonal moisture. Wednesday and Thursday both look active with afternoon convection. As previously discussed here, some attention to an easterly wave will be addressed in coming packages thanks to the potential for a strong push of moisture. Models seem to be struggling with the next wave coming ashore out west and the impacts on CONUS pressure heights. Suffice to say, the persistent high pressure pattern we've endured over the last several weeks will continue to deliver warm conditions under sunny skies, punctuated with a few cool spots under storm activity. Recyclable moisture is not under threat from an extended dry spell either. So, stray afternoon convection will stick around in the extended period, even without a good monsoonal push. && .AVIATION...(For the 18Z TAFS through 18Z Friday afternoon) Issued at 1141 AM MDT Thu Aug 17 2023 Showers and thunderstorms will fire on terrain mostly south of I-70 this afternoon. Gusty winds, small hail, and heavy downpours can be expected with these storms. Winds will remain terrain driven outside of storms. Storms will remain anchored to their source terrain with a few drifting out over valleys and diminishing quickly. Convection is expected to taper by midnight with a few clouds hanging around the overnight hours and VFR conditions returning by daybreak. && .HYDROLOGY... Issued at 351 PM MDT Thu Aug 17 2023 A stream of subtropical moisture will continue to move northward across the region through the end of the week. Elevated moisture levels combined with slow storm movement pose a threat for localized excessive rainfall each afternoon/evening through Friday. Areas with unstable soils in steep terrain, or where soils are already saturated from rain earlier in the week are at greatest risk. Models have evolved since earlier in the week, with the consensus now indicating that the subtropical moisture plume will shift west of the area as high pressure expands westward this weekend. && .GJT WATCHES/WARNINGS/ADVISORIES... CO...None. UT...None. && SHORT TERM...NL LONG TERM...LTB AVIATION...LTB HYDROLOGY...NL/TGJT
Hazardous Weather Outlook National Weather Service Grand Junction CO 317 AM MDT Thu Aug 17 2023 COZ001>014-017>023-UTZ022>025-027>029-180930- 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- 317 AM MDT Thu Aug 17 2023 This hazardous weather outlook is for eastern Utah and western Colorado. .DAY ONE...Today and Tonight Moisture will spread further northward as the flow shifts to the south. This will result in scattered to numerous showers and thunderstorms this afternoon and evening with potential for heavy rain and localized flash flooding in the stronger storms. Central and southern areas will be favored. .DAYS TWO THROUGH SEVEN...Friday through Wednesday Deep moisture will increase through Friday with a disturbance resulting in convection continuing through late Friday night. Potential exists for stronger storms capable of heavy rain, localized flash flooding, strong gusty winds and small hail. A drying trend is possible for the weekend as high pressure expands westward, shifting the moisture plume to the west. This will result in a downturn in storm activity this weekend before moisture returns early next week. .SPOTTER INFORMATION STATEMENT... Spotter activation will not be needed.
The national radar mosaic for the moment the first event began.
1 flood. Local standard time, as filed.
Heavy rain from thunderstorms over the Mill Creek drainage resulted in a spillover on Powerhouse Dam in Grand County. Radar estimated 0.80 to 1.10 inches of rain fell over the North Fork Mill Creek drainage.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Utah in 2023Every state in 2023This day in weather historyWarnings in Utah, August 2023
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: 1113842.