Issue Date: Saturday, September 4th, 2021
Issue Time: 9:20AM MDT

— A LOW flood threat has been posted for parts of the Palmer Ridge, Southeast Plains, Raton Ridge and Southeast Mountains

Although much drier air has begun to infiltrate into western Colorado, significant moisture will remain over the next 12-18 hours east of the Continental Divide. This will support another round of potentially heavy rainfall this afternoon and evening, although coverage will be lower than seen on Friday. The water vapor image, below, shows a pronounced disturbance draped across northern Colorado. This disturbance will move southeast today, causing a weak surface low to develop along the NM/CO border. Precipitable Water (PW) has dropped to 0.74 inches at Grand Junction and 0.69 inches in Denver this morning and will continue to drop across most of the state. However, along the southeast Colorado border with KS, OK and NM, PW is greater than 1 inch and will hold steady or even slightly increase as the aforementioned surface low causes easterly flow to advect moisture back into the state. This morning’s sounding from Dodge City, KS, showed a PW of 1.86 inches, which is near the all-time September record here. At least some of this will be drawn into CO this afternoon. Morning sunshine will boost temperatures, allowing for instability up to 1,600 J/kg in a narrow axis along and just east of I-25, south of Colorado Springs. Storm motion will be slightly faster over the Palmer Ridge, which will mitigate point rainfall here. Slower storm motion towards the NM border will allow for higher rainfall accumulations, warranting a LOW flood threat for this limited area. In addition to heavy rainfall, severe weather, in the form of large hail over 1 inch and damaging winds, will also be possible with the strongest storms.

Elsewhere, mainly dry weather and seasonably warm temperatures are expected, setting up a beautiful late summer Saturday (or early fall, depending on your preference of meteorological versus astronomical bookkeeping).

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:

Palmer Ridge, Southeast Plains, Northeast Plains, Raton Ridge, Southeast Mountains:

Becoming partly cloudy with scattered showers and thunderstorms developing by mid-afternoon. Max 1-hour rainfall up to 1.2 inches (west) and 1.6 inches (east) supports a LOW flood threat south of I- 70 for isolated flash flooding, along with debris slides and mud flows over steeper terrain.

Severe weather will also be possible with the stronger storm cells.

Storms will begin to dissipate by sunset, but isolated cells could persist into the late evening.

Primetime: 1PM through 11PM

Southwest Slope, Grand Valley, Northwest Slope, Central Mountains, Northern Mountains, San Juan Mountains, San Luis Valley, Front Range, Urban Corridor:

Seasonably warm today with an isolated shower possible over the higher terrain of the Central Mountains, San Juan Mountains and southern Front Range. Max 1-hour rainfall up to 0.2 inches. Flooding is NOT expected today.

Issue Date: Friday, September 3rd, 2021
Issue Time: 10:25AM MDT

— A LOW flood threat has been issued for the Urban Corridor, Northeast Plains, Palmer Ridge & Southeast Plains
— This is an overnight flood threat

The upper-level trough will continue to approach from the west today and turn this morning’s southwesterly flow aloft more westerly by this afternoon. This should continue to scour out any remaining surface moisture over the western portion of the state and limit the scattered, high-based storm coverage to the high terrains and elevated plateaus. As the trough approaches, the upper-level winds are also expected to pick up just north of the border. This should keep storms moving relatively quickly that develop over the Northern/Central Mountains and Front Range. Together, these two forecast elements should effectively limit the flood threat over the area.

For the most part, as the mountain storms are steered into the adjacent plains with the westerly flow, they will likely be moving too quickly to cause more than nuisance flooding issues. However, a couple training storms may be possible over the Palmer Ridge where some better surface moisture can hang on with increased dynamics, so a LOW flood threat has been issued for the area. As the storms continue to move east and southeast and into higher moisture over the plains, the convection is expected to “line out” and become more widespread. With decent shear over the area, the storm complex is anticipated to intensify, and storms may be capable of producing severe weather which includes large hail, damaging winds and dangerous lightning. Due to the anticipated increased of rain rates over the area with the higher moisture, a LOW flood threat has been issued.

Finally, another moisture axis looks like it will set up over the Southeast Plains today and lasts through the overnight hours. With possible boosted lift from the low-level jet tonight across the southern border and enhanced convergence along this boundary, training storms may develop over the Southeast Plains tonight. With dew points around 60F or slightly above, the storms would have the potential to cause local heavy rainfall and flooding issues, so the LOW flood threat continues overnight.

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:

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

Faster moving storms should help limit the flood threat today back to the west today. However, training storms may be possible in a pool of moisture over the Palmer Ridge with enhanced convergence, so a LOW flood threat has been issued. Max 1-hour rain rates up to 1.25 inches with local storm totals up to 2 inches may be possible. As storms move east, they will likely form a line and be capable of producing large hail and damaging wind gusts along with lightning and heavy rainfall. An isolated tornado may be possible (south). With max 1-hour rain rates forecast up to 2.25 inches, a LOW flood threat has been issued. It’s possible for another round of overnight storms across the Southeast Plains, and training storms may cause 3-hour rain rates up to 3.50 inches, so the LOW flood threat continues into the overnight hours for this area. Flood threats today include road, local stream and arroyo flooding as well as heavy field ponding.
Primetime: 3PM to 6AM

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

The scattered storms that develop over western Colorado mountains and elevated plateaus are expected to favor the northern/central regions with more isolated pops further south. Max 1-hour rain rates up to 0.25 inches are forecast with isolated storm totals up to 0.50 inches possible. Flooding is NOT forecast today, and activity should come to an end by the evening hours.
Primetime: 1PM to 9PM

Issue Date: Thursday, September 2nd, 2021
Issue Time: 9:50AM MDT

— A LOW flood threat has been issued for the southern Front Range, southern Urban Corridor, Palmer Ridge, northern Southeast Plains and southern Northeast Plains
— Possible PM update may be issued

Southwesterly flow aloft continues across the state this morning with the main axis of precipitation to our east. There are still some ongoing, scattered showers over the western portion of the state and Northern Mountains, which indicates a little mid-level lift in the area in an otherwise subsidence promoting environment. Flow aloft by midday will become more westerly as another trough drops in from the Pacific Northwest (orange “X”). This will begin to pull in a drier air mass across western Colorado, which will start to drop PW values and decrease the chances for rainfall over the next several days. However, there should still be enough residual moisture and mid-level lift for a round of scattered, mountain storms this afternoon and evening favoring areas south of I-70. So, expect the current showers to expand a little bit in coverage and intensity as they meander west throughout the day. A couple evening storms may also be possible over the Grand Valley and Northwest Slope, but with instability dropping off rapidly with the sun setting, storms should dissipate rather quickly. With the smaller storm cores and lower-end rainfall rates today, flooding is not expected over this area.

PW at Denver has risen to just under 1 inch over the last 24-hours, and this moisture is forecast to hang on through tomorrow night. With mid-level energy approaching from the west, scattered storms are forecast to return to the area, and more so than yesterday, they are expected to spill into the southern Urban Corridor. Best coverage of showers and storms will be over the southern Front Range and Palmer Ridge where there will be a bit better convergence and mid-level lift. Slower steering flows and training storms may cause some localized flooding issues with the above-average moisture over the Palmer Ridge, if a couple stronger thunderstorms can develop. There is also the possibility of some overnight storms in this area with an uptick in lift forecast. A nice theta-e gradient may work its way into the eastern plains, and if storms generate along this feature, heavy rainfall should be expected. For these reasons, a LOW flood threat has been issued. Should this threat for overnight storms drop off by this afternoon, a PM update will be issued.

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: 

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

Afternoon and evening storms are expected over the central, eastern Colorado as the mid-level lift works its way over from the west. Subsidence elsewhere should keep storm chances limited over the northern/southern mountains. In addition to training and slow-moving storms this afternoon/evening, some storms will be possible over the Palmer Ridge and eastern plains overnight. Rainfall that develops along a theta-e axis (front) overnight may also be capable of producing heavy rainfall. Max 1-hour rain rates up to 1.75 inches and storm totals up to 3 inches will be possible. Therefore, a LOW flood threat has been issued for possible road flooding, bankfull conditions of local streams/creeks, mud flows/debris slides (west) and field ponding under the more robust storm cores. A couple thunderstorms may also be capable of producing small hail, strong outflow winds and dangerous lightning. Note that this is an overnight threat.
Primetime: 12:30PM to 6AM

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

More scattered storms are forecast today over the higher terrains mostly south of I-70. Without much instability being able to build, rainfall rates should be on the lower end and accumulations should be gradual enough, and over a small enough area, to avoid flooding issues. A quick 0.50 inches of rain may be able to fall over a 30-minute period if a couple stronger storms can develop. Isolated morning totals up to 1 inch will be possible over the higher terrains of the Northwest Slope (south), Grand Valley and San Juan Mountains, but flooding is NOT forecast today.
Primetime: Ongoing to 10PM

Issue Date: Wednesday, September 1st, 2021
Issue Time: 10:20AM MDT

— A MODERATE flood threat has been issued for the Southwest Slope, San Juan Mountains, Northwest Slope and Grand Valley
— A LOW flood threat has been issued for the Central Mountains, Northern Mountains, San Luis Valley and Southeast Mountains

Southeastward progression of the trough, to our north, has helped pull the Low, off the coast of California, inland. As the day continues, the Low is forecast to be absorbed into the stronger southwest flow, which will bring enhanced mid and upper-level lift across the state. This will help create two distinct areas of precipitation over the state, one over western Colorado and the Southeast Mountains and the other over the eastern plains along a surface trough. Downsloping flow from the lee trough should help to keep precipitation chances fairly low across the Front Range and northern Urban Corridor.

In the meantime, the location of the California Low has created southerly flow aloft, which has advected the plume of subtropical moisture more into western Colorado. PW at Grand Junction has risen to 1.13 inches, which is just a couple hundredths shy of the daily maximum value. The first batch of showers has already arrived to the northwest corner of the state, and throughout the day, showers and storms are expected to increase over all of western Colorado. With better moisture available within the surface layer today, rain rates and accumulations are expected to be higher than yesterday. One limiting factor to the flood threat will be the lack of daytime heating, which should decrease the chance for widespread thunderstorm development. Another limiting factor will be faster southwest steering flows, which should limit the flood threat from any individual storm. However, continuous southwesterly flow and extra lifting dynamics are expected to help initiate multiple rounds of rainfall, training storms and embedded convection. All of which will increase the flood threat, but over a longer duration than the typical 1 to 2 hour threat. A MODERATE flood threat has been issued, which continues into tonight.

Back to the east, PW at Denver was measured at 0.73 inches, which is expected to increase throughout the day as the moisture plume moves east. A drier surface layer was noted in the sounding, which paired with the faster southwest steering flows, should help thunderstorm accumulations from reaching flood threat criteria over the eastern plains. With an increasing moisture gradient towards the southern border, it is possible that storms that develop over the Southeast Mountains, on southwest facing slopes, may produce some heavier rainfall, which could cause some local flooding issues. Thus, the Low flood threat has been extended into this region.

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, Southeast Mountains, Southwest Slope, Central Mountains, San Luis Valley, Raton Ridge, Grand Valley, Northwest Slope & Northern Mountains:

Longer duration rainfall (3 to 6 hour), in a high moisture environment and with extra dynamics overhead may cause some localized flooding issues over the next 24-hours. Expect this mornings storms to expand in coverage and intensity around noon. By morning, isolated totals up to 3.50 inches may be possible with 3-hour rainfall rates reaching around 2 inches in the MODERATE flood threat area this afternoon/evening. This could cause mud flows, debris slides and excessive runoff over steeper terrains, especially as soils become saturated in areas where training storms set up. Other flood threats include flooding of roads and low-lying areas. A couple stronger storms may develop south with the main threats being lightning and embedded convection. Back over the Southeast Mountains, accumulations will be slightly lower with isolated storm totals up to 3 inches possible by morning. With the dry boundary layer, the initial storms that develop may produce some strong outflow winds. A LOW flood threat has been issued for this area. Storms are expected to continue into the evening hours, but should decrease in intensity shortly after sundown.
Primetime: Ongoing to Ongoing (limited to central, western CO tomorrow morning)

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

Downsloping winds with the lee surface trough will limit precipitation chances further north. Best chance for storms today will be along a surface trough over the eastern plains. Max 1-hour rain rates up to 1.25 inches will be possible. In addition to this rainfall, small hail and strong outflow winds are likely. With dry low-levels and fast steering flows, storms are not anticipated to cause any flooding issues. A second wave of rainfall may arrive to the southern portion of eastern Colorado tonight, which may produce localized totals up to 0.50 inches.
Primetime: 2PM to 4AM