Issue Date: Sunday, July 18th, 2021
Issue Time: 9:20AM MDT

— A LOW flood threat has been posted for parts of the San Juan Mountains

As shown in the water vapor image, below, an upper-level ridge has strengthened significantly over the past 24 hours and is now positioned over western Colorado. Plenty of moisture is still “stuck” underneath the ridge with morning radiosonde PW coming in at 0.91 inches at Grand Junction and 0.82 inches at Denver. However, a notable area of subsidence and upper-level drying was noted along the WY/NE border today so PW should slightly decrease through the day over the eastern half of Colorado. On the other hand, PW at Albuquerque came in at an impressive 1.21 inches, with a moist adiabatic type sounding profile indicating very efficient rain rates are possible. A simple interpolation between Albuquerque and Grand Junction implies the southwest part of Colorado currently has PW at or around 1.0 inch, which is significantly above normal for this time of year. Despite the warm mid-level temperatures we expect scattered to numerous showers and thunderstorms again for the southern high terrain. However, today, the LOW flood threat will be limited to just the climatologically favored San Juan Mountains where E/ENE steering flows of 10-15mph along with instability up to 800 J/kg will support heavy rainfall cores. Elsewhere, lower rainfall coverage is expected. However, isolated strong-to-severe thunderstorms will once again be possible over the Northeast and Southeast Plains, though steering flow of 25mph+ will limit point rainfall to below flood threat thresholds.

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, San Luis Valley, Southeast Mountains, and Raton Ridge:

Scattered to numerous showers and thunderstorms especially over the higher terrain of the San Juan Mountains where max 30-min rainfall up to 0.7 inches and max 1-hour rainfall up to 1.2 inches is expected. A LOW flood threat has been posted for the San Juans for isolated flash flooding, debris slides and mud flows. Elsewhere, isolated to widely scattered storm coverage is expected with max 1-hour rainfall up to 0.8 inches.

Primetime: 12PM through 10PM

Northeast Plains, Palmer Ridge and Southeast Plains:

Hot with isolated to widely scattered showers and thunderstorms possible this afternoon and evening. Max 1-hour rainfall up to 0.9 inches (west) and 1.5 inches (east), which is just below flood threat thresholds. However, nuisance street/field ponding is possible under the strongest cells. Severe weather, in the form of large hail and damaging winds, will also be possible especially in eastern areas along the KS and NE borders.

Primetime: 2PM through 11PM

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

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

Primetime: 1PM through 9PM

Issue Date: Saturday, July 17th, 2021
Issue Time: 9:40AM MDT

— A LOW flood threat has been posted for parts of the Northeast Plains, Palmer Ridge, Southeast Mountains, Raton Ridge, Southeast Plains and San Juan Mountains
— Over the Northeast Plains, the threat persists into the early overnight hours

Mostly clear skies greeted most of Colorado this morning as a ridge of high pressure aloft has begun to build along the NM border. A couple of weak disturbances were noted on satellite and water vapor imagery, one over the Northwest Slope and a second along the southern KS border in association with a dissipating complex of overnight storms. However, most of today’s rainfall activity will be generated by high elevation heating, a staple of Colorado summer. In terms of changes over the past 24-hours, the ridge has strengthened slightly and will continue to do so throughout the day. This will bump up high temperatures by 3-5F over most of the state. Ironically, the strengthening ridge will likely increase chances of heavy rainfall over the higher terrain as subsidence will tend to dissipate weaker updrafts, leading to fewer cells overall but with higher potential rain rates over cells that do stay intact. Near-surface and total column moisture has increased statewide with radiosonde PW at 0.91 inches in Grand Junction and 0.80 inches at Denver; morning model initializations showed up to 1.4 inches along the OK border. Dewpoint temperatures are generally 1-3F higher compared to yesterday morning. Some drying was noted along and west of the UT border, which will make it into the northern and central parts of the Western Slope, reducing storm coverage a bit. Weak, generally westerly steering flow of 15mph is expected over most of the state, transitioning to more northerly over eastern Colorado especially this evening as the ridge extends northward.

In terms of sensible weather, we expect scattered to numerous thunderstorms over the climatologically preferred higher terrain of the Palmer Ridge, Southeast Mountains and San Juan Mountains this afternoon. A Low flood threat is warranted for these regions as max instability in the 700-1,100 J/kg range along with light steering flow will support the chance of 30-60 minute periods of heavy rainfall mainly this afternoon and early evening. Meanwhile, as storms move off the Cheyenne Ridge along the WY border, guidance suggests they will encounter upslope moist flow, aided by a nocturnal low level jet, which will support a ~3 hour of period of heavy rainfall late this afternoon and into the early overnight hours. Enough shear will be present to also sustain the threat of severe weather of the large hail and damaging wind variety. Outflow boundaries from this possible complex could trigger some more isolated storm activity along the Palmer Ridge into the overnight hours.

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:

Northeast Plains:

Warmer with scattered showers and thunderstorms possible, beginning late afternoon and last into the early overnight hours. Max 1-hour rainfall up to 2.5 inches and max 3-hour rainfall up to 3.7 inches warrants a Low flood threat for isolated flash flooding. Large hail and damaging winds could accompany the strongest cells.

Primetime: 4PM through 2AM

Raton Ridge, Palmer Ridge, Southeast Plains, Southeast Mountains, Central Mountains, Northern Mountains, Front Range and San Juan Mountains:

Warmer with scattered to numerous showers and thunderstorms this afternoon and evening, especially over the higher terrain. Max 1-hour rainfall up to 1.5 inches (east) and 1.1 inches (west) warrants a Low flood threat for isolated flash flooding, debris slides and mud flows. A couple of storms could persist into the late evening over the Palmer Ridge.

Primetime: 12PM through 10PM

Grand Valley, Northwest Slope, Southwest Slope, San Luis Valley and Urban Corridor:

Warmer with isolated to widely scattered thunderstorms possible this afternoon and evening. Max 1-hour rainfall up to 0.8 inches (east) and 0.5 inches (west). Flooding is NOT expected today.

Primetime: 12PM to 9PM

Issue Date: Friday, July 16th, 2021
Issue Time: 10:10AM MDT

— A LOW flood threat has been issued for the Northeast Plains and Southeast Plains

Over the eastern plains this morning dew points are 60F+, which is helping to produce fog with the cool morning temperatures. This should begin to burn off with a little more heating. As for the large-scale pattern today, the ridge will continue to build over the state, which should increase afternoon high temperatures another few degrees. A shortwave is forecast to move around the High (orange “X”), and it is already generating some cloud cover with a plume of moisture out in front of it. PW at Grand Junction was measured at 0.75 inches with a large majority of the moisture aloft. Higher PW values and surface moisture are located across the southern border again. This should set the stage for another round of storms and showers over the western high terrains favoring the San Juan Mountains for better storm coverage and moderate rainfall accumulation.

As this area of lift moves east, it will also help to generate some high-based showers over the eastern mountains. Steering flows from the WNW are expected to push the scattered storms into the adjacent plains by early afternoon. Initially, the storms are expected to be high-based and produce only moderate rainfall, but as they move into deeper moisture over the far eastern plains, the storms should begin to intensify. A couple severe storms capable of producing hail and strong outflow winds will be possible. With slightly weaker northerly steering flows forecast by this evening, storms over this region may be capable of producing local flooding issues. A Low flood threat has been issued with the flood threats including road/local creek flooding and field ponding.

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:

Northeast Plains & Southeast Plains:

Storms are expected to arrive over the eastern plains early this evening and may continue overnight with enhanced lift from the low-level jet. Isolated severe storms may develop that could produce large hail and strong outflow winds. Southeasterly surface winds throughout the day will help to keep higher surface moisture over the region, which in turn, will help to intensify the rainfall rates. Winds aloft turns more northerly this evening, which will likely steer some stronger across the northern border and into the Northeast Plains. At this time, storms are forecast to move slower and may do some back-building, which will increase the flood threat. Max 1-hour rainfall rates up to 2 inches and localized storm totals up to 3 inches will be possible. A Low flood threat has been issued for possible road flooding, field ponding and local creek flooding.

Primetime: 5PM to 2AM 

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

Scattered storms are forecast to develop over the mountains and Palmer Ridge with the next moisture plume arriving to the area. Better storm coverage is forecast south where there’s slightly higher moisture. Max 1-hour rain rates up to 0.40 inches (north) and 0.60 inches (south) are possible over the mountains. Over the adjacent plains, storms are more likely to occur over the Palmer Ridge and south where there’s better southeast surface winds. Max 1-hour rain rates up to 0.90 inches will be possible. Flooding is NOT forecast.

Primetime: 1PM to 11:30PM

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

Storms should pop over the high terrains and elevated plateaus again by early this afternoon. Further north, storms should be more isolated in nature and may produce gusty outflow winds. Max 1-hour rain rates up to 0.30 inches will be possible. Better coverage of storms is anticipated over the San Juan Mountains where there’s slightly higher moisture. Moderate rainfall is forecast with max 1-hour rain rates up to 0.75 inches possible. Places that see training storms over the San Juan Mountains may see storm totals up to 0.90 inches. Flooding is NOT forecast.

Primetime: 1PM to 10:30PM

Issue Date: Thursday, July 15th, 2021
Issue Time: 9:35AM MDT

— Flooding is NOT expected today

The upper-level ridge has started to build northward again, and the more northerly flow has helped to pull in a drier air mass over the state (north). PW at Grand Junction and Denver has dropped to around 0.80 inches, and there’s a noticeable dry layer in the mid-levels within both soundings. As the ridge moves eastward throughout the day, the rising heights will produce warmer temperatures along with an increase in stability. Between the increase in stability and decrease in available moisture, a downtick in afternoon storm activity and rainfall totals are forecast.

There’s still some lingering moisture that can be seen in the water vapor imager below (blue shades), which is associated with the departing trough. This residual moisture will allow for some high-based, scattered storms to develop with the diurnal cycle over the mountains and elevated ridges. Storm coverage is expected to be greatest over the Central, San Juan Mountains and Front Range. As storms move to the southeast with the northwest steering flow, they are expected to strengthen some over the adjacent plains. The stronger storms that develop may produce some small hail and strong outflow winds. Faster storm movement and less available moisture should only allow for moderate rainfall accumulations, so flooding is not expected 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:

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

Scattered storms will fire over the high terrains this afternoon favoring the Front Range and Palmer Ridge for better coverage. High storms bases and quicker storm movement should allow only moderate rainfall accumulations and plenty of virga. Max 1-hour rain rates up to 0.50 inches will be possible along with brief, gusty outflow winds. As storms move into the adjacent plains, max 1-hour rain rates up to 0.40 inches (west) and 1.40 inches (east) will be possible. Threats for the stronger storms that develop (east) will be small hail and damaging outflow winds. Flooding is NOT forecast.
Primetime: 1:30PM to 11PM

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

Scattered, high-based storms will develop over the mountains by early afternoon. Best storm coverage will be over the Central and San Juan Mountains and elevated ridges of the southern Northwest Slope. Max 1-hour rain rates up to 0.50 inches (west/south) and 0.25 inches (central) will be possible. Storms may produce some brief, gusty outflow winds as well. Flooding is NOT forecast.
Primetime: 1PM to 9PM