A subtle pulse of monsoon moisture worked its way northward across western Colorado. This, paired with the already present abundance of moisture, resulted in scattered showers and thunderstorms across the region.
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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.
A subtle pulse of monsoon moisture worked its way northward across western Colorado. This, paired with the already present abundance of moisture, resulted in scattered showers and thunderstorms across the region. The stronger storms produced heavy rain, some of which led to localized debris flows in Pitkin and San Juan counties.
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 Jul 20, 10:30 PM MST, 34 minutes earlier.
Area Forecast Discussion National Weather Service Grand Junction CO 1130 PM MDT Tue Jul 20 2021 .SHORT TERM...(Tonight through Wednesday night) Issued at 109 PM MDT Tue Jul 20 2021 Water vapor imagery pretty much sums up the story for our anticipated weather pattern over the next several days. Deep push of north to northeast marching monsoonal moisture will bring daily showers and thunderstorms. Afternoon focus will be over the higher terrain with valley drift occurring in the late afternoon and evening hours. Weak steering flow will result in slow moving cells, with heavy rain likely from the stronger storms. Precipitable water values were around an inch on the 12Z soundings from Grand Junction and Flagstaff with over two inches down in Phoenix. As we start to see more widespread convection and heavier rainfall, soils will start to saturate in some areas (hard to believe after the months of dry weather) with potential for flooding/flash flooding increasing. Looking for triggers in these deeper monsoon pushes can be challenging, especially if we stick to the standard pressure levels. However, cross sections can help find a few hidden triggers that may aid in convective development and there's some weak support anticipated to drag across the northern zones Wednesday morning. This should be enough to trigger a few showers and possibly a thunderstorm or two tomorrow morning somewhere between Meeker, Craig and Steamboat Springs (at least that's the plan right now). Slight exhale from mid-morning into the early afternoon, before we start building another round of diurnally driven convection Wednesday afternoon and evening. Since most of the forecast area will just be getting started with more robust rainfall potential on Wednesday, we're still holding off on any type of hydro watches, however. our burn scars will continue to be monitored closely for short duration watches and/or warnings as needed. Temperatures will continue to moderate somewhat for afternoon highs with values dropping into the "near normal" range, but mild to somewhat warm nights will stick around with sfc dew points in the 50s and generally muggy conditions (muggy for us) sticking around. .LONG TERM...(Thursday through Tuesday) Issued at 109 PM MDT Tue Jul 20 2021 Not much change from previous model runs as increased moisture remains across the forecast area. Afternoon and evening showers and thunderstorms will be common, more so down south, with some heavy rain mixed in thanks to the monsoonal push. Forecast PWATs continue to run in the 1+ inch range over much of the area with some spots seeing values running up to 1.3 inches. This is a lot of available moisture for daytime heating, favorable orographics, and any little wiggles that will create increased instability. With PWAT values that high, and remaining high day after day, flash flooding risks will increase especially over areas that have seen continued rainfall or recent burn scars. For now, areas along and south of I- 70 are favored for the precip, sorry northern valleys, with the San Juans really looking to see the brunt of heavier precip. Through the long-term period, there will be days with more widespread convection and some days with a bit less but the guidance does show precip for every day. The high pressure system will wobble and move around as disturbances to the north deflect the ridging and force the center of the high to shift the east. When this occurs, the high will tap into moisture from the Gulf of Mexico and stream it up into the CWA for the weekend. Some drying does look to occur Monday and Tuesday according to the GFS while the EC keeps precip going. Suffice to say, we'll see plenty of precip through the period which is good news for the drought, bad news for possible flash flooding. If travelling along the I-70 corridor through this period, please stay up-to-date on the Grizzly Creek burn scar as heavy rain over the scar may force occasional road closures from time to time. Temperatures will range from near average to a bit below depending on cloud cover and precipitation. && .AVIATION...(For the 06Z TAFS through 06Z Wednesday night) Issued at 1130 PM MDT Tue Jul 20 2021 Scattered storms will continue overnight and into Wednesday morning, focused on the eastern tier of the forecast area. Skies will remain scattered to broken through the night with generally light terrain driven winds, with occasionally breezy conditions due to lingering outflow winds. Storms will redevelop over high terrain after 18Z Wednesday with drift into valleys after 21Z with potential for gusty outflow winds and heavy rain. Conditions will generally remain VFR with brief reductions to MVFR in heavier showers. && .HYDROLOGY... Issued at 109 PM MDT Tue Jul 20 2021 Deep monsoonal moisture is expected to bring heavy rain producing showers and thunderstorms to much of Eastern Utah and Western Colorado over the next 7 to 10 days. Storms will be slow moving with some training of echoes anticipated. Burn scars will be especially susceptible to heavy rainfall. We will continue to monitor things closely as we work through the week and as grounds become more saturated each day. && .GJT WATCHES/WARNINGS/ADVISORIES... CO...None. UT...None. && SHORT TERM...JDC LONG TERM...TGR AVIATION...TGJT HYDROLOGY...JDC
Hazardous Weather Outlook National Weather Service Grand Junction CO 229 AM MDT Tue Jul 20 2021 COZ001>014-017>023-UTZ022>025-027>029-211300- 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- 229 AM MDT Tue Jul 20 2021 This hazardous weather outlook is for eastern Utah and western Colorado. .DAY ONE...Today and Tonight Monsoonal moisture will fuel scattered to numerous showers and thunderstorms across the region today. Storms will be efficient rain producers with heavy rain possible in addition to gusty outflow winds. Isolated showers may linger overnight. .DAYS TWO THROUGH SEVEN...Wednesday through Monday Daily scattered to numerous showers and thunderstorms can be expected through the weekend. Moderate to heavy rain and gusty winds will be the primary threats with storms. Temperatures will trend cooler than normal in response to increased clouds and showers. .SPOTTER INFORMATION STATEMENT... Spotter activation is not expected today or tonight, however spotters are encouraged to relay rainfall amounts or any observed flooding to the National Weather Service in Grand Junction.
PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GRAND JUNCTION CO
1152 PM MDT TUE JUL 20 2021
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0925 PM DEBRIS FLOW 3 NE SHOSHONE 39.61N 107.15W
07/20/2021 GARFIELD CO DEPT OF HIGHWAYS
NUMEROUS PHOTOS AND WEBCAM IMAGES OF DEBRIS
FLOW OVER I-70. WESTBOUND LANES BLOCKED AND
REQUIRE DEBRIS REMOVAL CLEANUP.
&&
MACPRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GRAND JUNCTION CO
1158 PM MDT TUE JUL 20 2021
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0530 PM FLASH FLOOD 37 NNE CASTLE VALLEY 39.13N 109.17W
07/13/2021 GRAND UT NWS STORM SURVEY
NWS STORM SURVEY FOUND EVIDENCE OF A MAJOR
FLASH FLOODING EVENT IN THE WESTWATER
DRAINAGE VALLEY. WATER WAS ESTIMATED TO BE 6
TO UP TO 12 FEET DEEP BASED ON EROSION OF
THE ARROYO WALLS. NUMEROUS INSTANCES OF DRY
ARROYO CROSSINGS WASHED OUT WITH MAJOR
TRANSPORT OF SEDIMENT AND DEBRIS DOWNVALLEY.
&&
MACThe national radar mosaic for the moment the first event began.
2 debris flow. Local standard time, as filed.
A mudslide resulted in the closure of Colorado Highway 133 just north of Redstone at mile marker 55. The road opened to one lane later that morning before crews finished cleanup operations.
A significant debris flow of mud, rocks and branches occurred on Cinnamon Pass in the San Juan Mountains. The exact time of the slide is unknown.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Colorado in 2021Every state in 2021This day in weather historyWarnings in Colorado, July 2021
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: 965674, 965681.