Issue Date: Monday, August 23rd, 2021
Issue Time: 9:05 AM MDT

Summary:

As forecasted, yesterday was calm as hot and dry conditions settled into the state. There were a handful of isolated showers in the Grand Valley and Southwest Slope overnight and early morning, but only Trace – 0.05 inches was reported. The main weather story was temperatures kicking back up after a cooler Friday and Saturday. The Southeast Plains reached over 100 degrees in La Junta, Lamar, and Granada.

No flooding was reported on Sunday. On Friday, the USGS published a storymap detailing the many debris flows that occurred on the Grizzly Creek burn scar through Glenwood Canyon after this summer’s heavy rain, as well as some historical flows after fires in 1994 and 2002.

For rainfall estimates in your area, check out the State Precipitation Map below.

Click Here For Map Overview

Issue Date: Sunday, August 22nd, 2021
Issue Time: 9:50 AM MDT

Summary:

Yesterday saw early morning storms along the Northeast Plains due to a shortwave, however storms dissipated early with daytime heating and the general northeast storm movement exiting the state. As the day progressed, isolated showers developed in the Northwest Slope, Central and Northern Mountains, as another shortwave disturbance traversed the state from the west. Limited instability kept precipitation totals low for most of the region, but up to 0.31 inches fell over the Grizzly Creek burn scar near Glenwood Springs.

Storms became more organized in the late afternoon along the Front Range Mountains and spilling into the Urban Corridor. An intense storm in Boulder dropped between 0.23-0.37 inches across the north side of town. Some unofficial reports on twitter suggest that rain fell in just a few minutes, and a CoCoRaHS remark at a station in West Boulder reported that the 0.26 observed was “extreme heavy rain for 3 minutes.” A MesoWest gauge in town reported 0.30 inches in 5-minutes (time series plot below) corroborating the reports of a short-lived, intense storm. This amount of rain in Boulder is between a 2-year and 5-year rainfall event (20%-50% chance of occurrence in any given year) according to Precipitation Frequency Estimates from NOAA Atlas 14. The fast moving storm traveled northeast and dropped over half an inch of rain in Longmont as well.

Overnight, a thunderstorm developed along the Palmer Ridge and Northeast Plains which brought over half an inch to locations along its path. A PWS in Kirk in Yuma County reported 0.55 inches from the cell in 2 hours and CoCoRaHS observer in Liberty in Kit Carson County reported 0.93 inches.

No flooding was reported on Saturday. For rainfall estimates in your area, check out the State Precipitation Map below.

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Issue Date: Saturday, August 21st, 2021
Issue Time: 9:25 AM MDT

Summary:

Late Thursday and early Friday’s cold frontal passage ushered in cooler and dryer air, effectively shutting down the heavy precipitation observed over the last few days. It also helped to clear out the air from some of the smoke and haze from western fires and general summer ozone pollution. Temperatures felt more like fall, with highs in the 70s and 80s for most of the state – only the Southeast Plains and Raton Ridge, which didn’t benefit from much rain over the last few days, remained in the 90s. The map below shows the 24-hour maximum temperature from yesterday at all NWS stations across Colorado.

There were still some very early morning residual showers in the Northeast Plains, and as of 7:30 yesterday morning flooding was still being reported after Thursday night’s extreme rainfall in Holyoke and the surrounding area. The flood warning issued overnight for the area expired at noon.

As mentioned in the FTB yesterday, there was the potential for an overnight disturbance along the Raton Ridge, Southeast Plains, and Palmer ridge last night. This materialized and allowed for a line of showers to develop, producing modest rainfall as showers moved north-northeast. A gauge in Colorado City reported 0.10 inches just after midnight, and 0.01-0.03 inches were reported along the I-70 corridor of the Palmer Ridge and Eastern Plains.

For rainfall estimates in your area, especially over the last few days, check out the State Precipitation Map below!

Click Here For Map Overview

Issue Date: Friday, August 20th, 2021
Issue Time: 10:50 AM MDT

Summary:

Yesterday’s weather across Colorado had it all – heavy rain, severe thunderstorms including large hail and tornadoes, and snow! There’s a lot to cover this morning, starting out west:

The Western Slopes and Grand Valley remained under a plume of monsoonal moisture and northwest flow spinning around the upper level low, resulting in a second day of widespread, long-duration rainfall. Heavy rain caused a debris flow in Uravan in Montrose County on the Southwest Slope, reported yesterday afternoon by CDOT.

COLORADO HIGHWAY 141 HAD ALTERNATING TRAFFIC DUE TO MUDSLIDE CLEAN-UP BETWEEN COUNTY ROAD Y11 AND COUNTY ROAD Q12.

A CoAgMet weather station in Bedrock, 7 miles southeast of the reported debris flow location, reported 2.05 inches in 24-hours yesterday. For this part of the state, 2.05 inches in a day is between a 25-year and 50-year Average Recurrence Interval (or 2%-4% chance of occurrence in any given year). CDOT has reported that CO-141 since reopened.

While no flooding was reported, the last two days of rain in Grand Junction have broken daily records at the airport. A total of 0.47 was reported on the 18th, and 0.68 inches at the time of the following tweet from the Grand Junction WFO. Since then, a total of 0.79 inches was reported at the airport for a 2-day total of 1.26 inches. This falls between a 2-year and 5-year rainfall event for Grand Junction.

The @GJAirport has officially set new daily rainfall records for both August 18th and 19th! Yesterday we received 0.47″ which broke the old record of 0.26″ set in 1989. Today we’ve received 0.68″ (so far!) which breaks the old record of 0.60″ set in 1925! ☔ #cowx

— NWS Grand Junction (@NWSGJT) August 19, 2021

Other notable rainfall totals along the Western Slopes and Grand Valley include:

• 1.66 in Paonia
• 1.60 in Hotchkiss
• 1.56 in Olathe and Redstone
• 1.25 in Montrose
• 1.21 in Carbondale (“the most significant rain event in the 13 years of data collection at this station.”)
• 1.15 in Nucla
• 0.92 in Ridgway

Moving towards the high county in the Northern and Central mountains – a cold-frontal passage associated with the low-pressure system resulted in August snow in the high elevations! The Boulder WFO shared the following tweet, showing snow at Rocky Mountain National Park and Vail Pass. Snow was also reported yesterday at Wolf Creek Pass, Berthoud Pass, and Storm Peak in Steamboat Springs. Trail Ridge Road in Rocky Mountain National Park was closed due to white out conditions.

Say it ain’t snow! ❄️
Yesterday, some of our mountain areas received some summer snowfall. (Left: Alpine Visitor Center in @RockyNPS) (Right: Vail Pass on I-70) #COwx pic.twitter.com/wHAH1wesFC

— NWS Boulder (@NWSBoulder) August 20, 2021

I-70 through Glenwood Canyon remained closed for the entire day yesterday due to the threat of flooding and debris flows. The Glenwood Springs area received over 1.15 inches of precipitation yesterday in the form of long-duration rain most of the day and overnight. Thankfully, no flooding or debris flows were reported along I-70 due to the Grizzly Creek burn scar.

Continuing to the Urban Corridor, the eastward movement of the low-pressure system met with daytime heating ahead of the cold-front to produce two swaths of severe thunderstorms across the northern and southern Denver Metro Area. First up north, a severe-thunderstorm and tornado warned storm traversed the northwest Denver suburbs from Golden to Brighton, producing up to 2.00 inch hail in Golden, a funnel-cloud in Arvada, and 1.95 inches of rain in Brighton! To the south, a similar setup moved southwest to northeast across Castle Rock to Bennet, with over 1.00 inch hail reported all along the storm track, a funnel-cloud in Parker, and over an inch of rain in just an hour in Castle Pines. An additional round of rainfall occurred over the Urban Corridor overnight from the passage of the cold front, boosting 24-hour precipitation totals.

Minor street flooding was reported in Denver, including the Cherry Creek bike trail being overtopped. The Cherry Creek River at Champa gauge briefly reached just below “Action” stage yesterday, peaking at 5.75 feet as seen in the hydrograph below. Other small streams and rivers across the Urban Corridor had similar responses to the heavy rain.

Other notable rainfall totals from the Urban Corridor include:

• 1.28 inches in Fort Lupton
• 1.10 inches Greenwood Village
• Another 1.10 inch observation in Boulder
• 1.05 inches in Highlands Ranch
• 0.87 inches in Littleton
• 0.71 inches in Arvada

Finally, on the Northeast Plains a supercell thunderstorm followed by training thunderstorms over the same area produced heavy rain and flooding in Holyoke and nearby towns. The severe thunderstorm also produced up to 66 mph wind gusts, 0.25-1.00 inch hail, and a tornado on the Colorado-Nebraska border. The following flood reports were made by the public or law enforcement.

SEVERAL HOUSES HAD WATER RISE UP TO THE FRONT DOORS.
SIGNIFICANT STREET FLOODING INCLUDING MAIN STREET IN AND NEAR HOLYOKE.
SIGNIFICANT FLASH FLOODING 6 MILES EAST OF HOLYOKE ON U.S. HIGHWAY 6.
THREE 6 INCH RAIN GAUGES ON HIS FARM OVERFLOWED.
WIDESPREAD FLOODING OF ROADS, DITCHES, AND FIELDS. AT LEAST ONE ROAD CROSSING FRENCHMAN CREEK ARE IMPASSABLE. FRENCHMAN CREEK AND OTHER STREAMS HAVE SIGNIFICANT FLOODING.

A CoAgMet Station in Holyoke, which was directly impacted by the supercell (tweet below), received 3.26 inches of rain in a single hour! This amounts to between a 100-year and 200-year ARI for this area. The series of storms over this area ultimately dropped 8.11 inches of rain in Holyoke yesterday (as seen in the time series plot below), with 8.06 of that falling within a 12-hour period. The 12-hour, 1000-year ARI (or 0.1% chance of occurrence in any given year) for Holyoke is 6.48 inches, so the 8.06 inches observed from this storm well exceed the 1000-year return interval!

Our station southeast of Holyoke recorded 3.26″ in one hour between 5-6 pm. Flooding being reported, and more heavy rain moving into the area. #cowx @NWSBoulder pic.twitter.com/wrsHRzJ27X

— CoAgMET (@ColoradoMesonet) August 20, 2021

Other notable rainfall totals in the Northeast Plains (although they seem paltry in comparison) include:

• 4.23 in Wray
• 1.95 in Ovid
• 1.70 in Big Springs
• 1.59 in Haxtun
• 1.20 in Iliff
• 0.98 in Yuma

The San Luis Valley, Southeast Mountains, Raton Ridge, and Southeast Plains stayed largely dry yesterday. For rainfall estimates in your area, check out the State Precipitation Map below.

Click Here For Map Overview