FTB 07-20-2021: Heavy Rainfall Threat Expands Over Western Colorado

Issue Date: Tuesday, July 20th, 2021
Issue Time: 9:30AM MDT

LOW flood threat has been issued for the Northern Mountains, Central Mountains, San Juan Mountains and portions of the Northwest Slope, Southwest Slope and Grand Valley

There’s a very strong east/west gradient in moisture today over the state with the Continental Divide acting as the barrier between the two air masses. Over western Colorado, the upper-level High continues to pull a very strong PW plume northward on its west side (green dashed oval), which is forecast to linger for the next week or so and bring beneficial moisture to the area. PW at Grand Junction was measured at 1.05 inches again this morning, which is well above average for this time of year. Northward progression of the plume should allow for more storm development over the higher elevations of the Central and Northern Mountains today, which includes the elevated regions within the Northwest and Southwest Slope. No strong shortwaves are forecast to move through the flow, but some weak mid-level energy may aid in more widespread coverage of rainfall when compared to yesterday. Slower steering flows (10-15mph) and some training storms will increase the localized, heavy rainfall threat this afternoon and evening, so a Low flood threat has been issued. The more intense storms that develop (likely south) may also produce some strong outflow winds along with some hail.

To the east, quite a different story. PW at Denver was measured at 0.58 inches with a noticeable dry layer around 500mb. That translates to precipitation chances being very low east of the Divide. Best chance for isolated storms and measurable rainfall will be over the northern Front Range and southern Southeast Mountains. Other than that, it should remain dry and hot over eastern Colorado today.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. If there is a threat, hover over the threat areas for more details, and click on burn areas to learn more about them. For Zone-Specific forecasts, scroll below the threat map.

Zone-Specific Forecasts:

Northern Mountains, Grand Valley, Northwest Slope, Southwest Slope, Central Mountains, San Luis Valley & San Juan Mountains:

Coverage and intensity of storms should pick up this afternoon with the moisture plume moving northwards. The majority of the storms today will be capable of producing 0.50 inches in a half hour or so, which will elevate the flood threat for recent burn areas. Best chance for a low-end severe storm or two will be south over the San Juan Mountains where some better instability looks to build. The main threat from the stronger storms that develop will be outflow gusts and small hail.

Max 1-hour rain rates up to 1.10 inches will be possible today over the mountains and elevated plateaus, so a Low flood threat has been issued. Threats over the steeper terrains today include enhanced runoff into smaller creeks (bankfull conditions), mud flows and debris slides. A couple storms may move off the plateaus and into the lower valleys, but storms will likely quickly dissipate and produce more wind than rainfall. A couple weak showers may linger over the Northwest Slope overnight.

Primetime: 12:30PM to 11PM

Front Range, Southeast Mountains, Palmer Ridge, Raton Ridge, Urban Corridor, Northeast Plains & Southeast Plains:

Mostly dry and hot conditions are forecast today. Highs will reach into the 90Fs again across the lower elevations and into the 80Fs over the mountains. Best chance for rainfall will be over northern Front Range (Larimer County) or Southeast Mountains/Raton Ridge intersect. These storms are expected to be isolated and will likely produce more virga/wind/cloud cover than rainfall. Max 1-hour rain rates up to 0.25 inches (south) and 0.50 inches (north) are possible. A couple sprinkles may also be possible over the western Palmer Ridge. Flooding is NOT forecast today.

Primetime: 2PM to 8:30PM