Issue Date: Thursday, July 22nd, 2021
Issue Time: 9:45AM MDT

— A MODERATE flood threat has been issued for the San Juan Mountains, Northern Mountains, Central Mountains, Southeast Mountains and Front Range
— A LOW flood threat has been issued for the San Luis Valley and portions of the Northwest Slope, Grand Valley, Southwest Slope, Palmer Ridge and Raton Ridge

There has been little change in the atmosphere overhead over the past 24 hours, with a significantly above normal amount of monsoonal moisture continuing to rotate around the Four Corners. As shown in the visible satellite image, below, dissipating overnight showers and storms have left pockets of cloud cover mainly over the higher terrain, but this should continue to dissipate quickly with the July sunshine. PW at Grand Junction this morning measured at a whopping 1.18 inches, which is within 10% of its daily record for this time of year. Denver PW was also high at 1.07 inches. With this plentiful amount of moisture and strong heating expected in the morning hours, numerous to widespread showers and thunderstorms are expected again for basically the entire Colorado higher terrain above ~8,000 feet. Storms produced very high rainfall rates yesterday, up to 1.0inch per 20 minutes warranting a continuation of the MODERATE flood threat. Model guidance shows more potential for storms to make into the lower elevations of the Grand Valley today, so a LOW threat has been extended there. This is consistent with a nearly moist adiabatic Grand Junction sounding with a mid-level lapse rate below 7C/km. On the other hand, guidance suggests less storm activity in the foothills and plains east of the Continental Divide due to a slightly stronger atmospheric cap in place and a switch to easterly steering flow keeping storms anchored closer to the higher terrain. But otherwise, a widespread MODERATE flood threat continues for the higher terrain where max 1-hour rainfall up to 2.0 inches will cause isolated flash flooding, debris slides and mud flows. Up to 3.4 inches of 24-hour precipitation is expected through tomorrow morning. Small creek and tributary flooding will be possible in areas with multiple rounds of heavy rainfall.

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:

San Juan Mountains, Southwest Slope, Northwest Slope, Grand Valley, Northern Mountains, Central Mountains, Raton Ridge, San Luis Valley, Southeast Mountains, Front Range and Palmer Ridge:

Numerous to widespread showers and thunderstorms especially over the higher terrain above 8,000 feet. Max 1-hour rainfall up to 2.0 inches with max 24-hour rainfall (through tomorrow morning) up to 3.4 inches. Flash flooding, debris slides and mud flows will all be possible with the stronger storms and/or multiple short duration storms. Furthermore, small creek and tributary flooding will also be possible especially in regions that receive multiple rounds of heavy rainfall. This threat exists mainly over the higher terrain.

Primetime: 12PM through 11PM

Urban Corridor, Northeast Plains and Southeast Plains:

Hot with isolated to widely scattered showers and storms mainly closer to the foothills. If sufficiently strong outflow boundaries off the higher terrain make it far enough east, a strong storm could produce up to 2.0 inches per hour of rainfall especially along the I-25 corridor. However, at this time, this is unlikely and does not warrant a flood threat. Lower elevations of eastern Colorado are expected to stay hot and dry today.

Primetime: 4PM through 10PM

Issue Date: Wednesday, July 21st, 2021
Issue Time: 9:50AM MDT

— A MODERATE flood threat has been issued for the San Juan Mountains and portions of the Northern Mountains, Central Mountains and Front Range
— A LOW flood threat has been issued for the Northwest Slope, Grand Valley, Southwest Slope, San Luis Valley, Southeast Mountains and a portion of the Urban Corridor

The High continues to pull sub-tropical moisture northward on its west side, and its more southwest to northeast orientation today has also begun to push higher moisture to the east as well. PW values within this plume (shown in blue below) are greater than 1 inch, which is a good proxy for the flood potential from storms that develop. Specifically, PW at Grand Junction was measured at 1.15 inches, and better moistening within the surface layer from the last two days of rainfall was noted. This should allow rainfall rates to become more efficient today and raise the flood threat. Over Denver, PW has nearly doubled since yesterday to 1 inch, and it should rise a bit more throughout the day.

Subtle shortwaves were noted moving through the flow (orange “X’s”), which will help produce lift out in front of them and cause more widespread coverage of the rainfall producing storms. As far as steering flow, a slight uptick in steering flow was noted (north), but training storms are expected to increase the precipitation totals and still increase the local flood threat. Slightly slower steering flows over the central and southern mountains should allow for another round of heavy rainfall today. A Moderate flood threat has been issued for this area with a Low flood threat issued for most of western Colorado and the eastern high terrains. The westerly steering flow will likely push some of these storms into the southern Urban Corridor late this afternoon into this evening where they could drop a quick 1 inch of rainfall in 30 to 45min, so the Low flood threat has been extended into this area. Flood threats today include road flooding, local stream flooding and mud flows/debris slides over the steeper terrains. There are elevated flood threats also issued for the recent burn areas, so head over the Fire Burn Forecast (top of page).

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, San Luis Valley & San Juan Mountains:

Another day of widespread rainfall is forecast for these regions with the higher accumulations again expected over the elevated terrains. There should be a little bit higher instability north today, which will allow some more vigorous storms to develop. Instability south is forecast to be similar to yesterday. With slower steering flows (south) and training storms (north), the flood threat returns.

Max 1-hour rain rates up to 1.10 inches will be possible and the majority of storms will likely dump between 0.60 and 1 inch of rainfall in 30 to 45 minutes. Therefore, a LOW flood threat has been issued. Further south, isolated storm totals up to 1.80 inches will be possible with max 1-hour rainfall rates up to 1.25 inches. A MODERATE flood threat has been issued for these regions. Flood threats today include road flooding, local stream flooding and mud flows/debris slides over the steeper terrains. Even the San Luis Valley could see some stronger storms this evening with max 1-hour rain rates up to 0.75 inches possible. Some light showers may linger near the western border through early tomorrow morning, but most storms should begin to dissipate just before midnight.

Primetime: 1PM to 2AM

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

Since the moisture has moved east, thunderstorm and rainfall activity are expected to pick up in coverage and intensity this afternoon and evening. The southern Front Range and Central Mountains could see max 1-hour rainfall rates up to 1.25 inches with isolated totals up 1.50 inches possible. This could lead to local stream flooding as well as mud flows and debris slides. A MODERATE flood threat has been issued. A LOW flood threat has been issued for the northern Front Range and Southeast Mountains where max 1-hour rain rates may reach just over 1 inch. Some activity may spill into the adjacent southern Urban Corridor with westerly steering flows, so the LOW flood threat has been extended into the area where this would be most likely. Some outflow boundaries could initiate some additional storms over the Northeast Plains this evening (north) capable of moderate rainfall (up to 0.75 inches), but the Raton Ridge and Southeast Plains are expected to stay dry.

Primetime: 1PM to Midnight

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

— A 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

Issue Date: Monday, July 19th, 2021
Issue Time: 9:50AM MDT

— A MODERATE flood threat has been posted for parts of the San Juan Mountains
— A LOW flood threat has been posted for parts of the Southwest Slope and Grand Valley

A plume of monsoonal moisture continues to stream unusually far north, as shown in the visible satellite image below. Meanwhile, substantial drying is occurring on the eastern side of the upper-level ridge guiding the monsoonal surge. This has created a strong gradient in moisture this morning. Grand Junction PW came in at 1.05 inches, while Denver’s is only 0.65 inches. Model initializations this morning suggest the southwest corner of Colorado is seeing PW in the 1.0-1.2 inch range, which is much above normal. Additionally, dewpoint temperatures are 3-5F higher this morning. For example, Cortez and Durango have dewpoints of 55F or higher, which is noteworthy given their elevation of ~6,000 feet or higher. While some drying will erode moisture a bit over the eastern San Juan Mountains, we expect plenty of moisture to hang around the far southwest part of the state to support at least one if not several rounds of heavy rainfall this afternoon and evening. Yesterday’s rainfall totals were not overly impressive, but a look at individual storm cores showed intensity up to 0.4 inches in 10 minutes. Instability today will be higher, up to 1,400 J/kg. This, coupled with veering southerly steering flow over time should limit cloud cover to the south of the region (due to lower terrain in northern New Mexico), and support a longer period of instability. Thus, a MODERATE flood threat is warranted for a climatologically preferred but narrow region on the west side of the San Juans where up to 2.0 inches of rainfall is possible through tomorrow morning. Mud flows and debris slides are possible given the steep terrain here. Further north, a LOW flood threat extends through the Uncompahgre Plateau, but drier air north of there is expected to limit rainfall efficiency.

Elsewhere, very limited rainfall is expected though isolated to widely scattered storms, possibly strong, will again be possible along the KS border. However, strong steering flow of 25mph or greater should limit point rainfall so flooding is not expected here.

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:

San Juan Mountains, Southwest Slope, Grand Valley and Central Mountains:

Scattered to numerous showers and thunderstorms especially over the higher terrain of the San Juan Mountains where max 30-min rainfall up to 0.9 inches and max 1-hour rainfall up to 1.4 inches is expected. Total rainfall up to 2.0 inches is possible through tomorrow morning. A MODERATE flood threat has been posted for parts of the region for isolated flash flooding, debris slides and mud flows with the stronger storms. Reminder that mud flows and debris slides can occur well after the rainfall ends. A LOW flood threat extends slightly to the north where max 1-hour rainfall up to 1.1 inches is possible. Elsewhere, widely scattered storms are expected with max 1-hour rainfall up to 0.7 inches.

Primetime: 1PM through 11PM

Northeast Plains and Southeast Plains:

Isolated to widely scattered showers and thunderstorms possible this afternoon and evening. Max 1-hour rainfall up 1.5 inches, which is just below flood threat thresholds. However, nuisance street/field ponding is possible under the strongest cells. Gusty winds and hail will be possible with the strongest storms especially in eastern areas along the KS and NE borders.

Primetime: 2PM through 9PM

Northwest Slope, Grand Valley, Northern Mountains, Front Range, Palmer Ridge and Urban Corridor:

Hot with an isolated shower or storm possible especially over the higher terrain and foothills. Max 1-hour rainfall up to 0.6 inches possible. Flooding is NOT expected today.

Primetime: 1PM through 9PM