Issue Date: Saturday, August 7th, 2021
Issue Time: 9:05AM MDT

— Flooding is NOT expected today

The trough will be overhead today and associated westerly flow aloft has started to push a thick smoke plume into the state (dashed oval below). Smoky conditions have caused a statewide Air Quality Alert to be issued, and smoke is expected to be thickest in the valley locations. A cool front has dropped through the state, which paired with the incoming smoke will bring slightly cooler conditions to the northern portion of eastern Colorado this afternoon. Finally, there’s some morning cloud cover along the northeast border associated with some mid-level lift out in front of the trough.

As far as rainfall chances today, it should stay dry across the majority of the state. PW at Grand Junction has fallen to 0.40 inches, and PW at Denver (0.72 inches) will continue to drop with the dry, westerly flow ramping up. There’s still a chance for an isolated storm or two over the northern Front Range and Southeast/San Juan Mountain intersect, but only light rainfall and cloud cover is anticipated. Best chance for seeing measurable rainfall this afternoon will be over the Palmer Ridge and over the far northeast and southeast corners of the state. With only moderate max 1-hour rain rates forecast, 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:

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

A few isolated storms may form over the elevated ridges this afternoon, but quick E/SE movement of the storms, small storm cores and high cloud bases will limit the flood threat. The main storm threats will be lightning and gusty outflow winds up to 60 mph. As the storms move eastward, outflow boundaries may help initiate a few more storms over the Southeast Plains. Additional storms may track into the northeast corner of the state this evening. Max 1-hour rain rates up to 1 inch are possible, so flooding is NOT expected.
Primetime: 12PM to 9PM

Southeast Mountains, San Juan Mountains, Front Range & Urban Corridor:

A few isolated, weak storms may form along and near the Continental Divide (northern Front Range) or south near the Southeast/San Juan Mountain intersect. Storms will likely only produce a few sprinkles and maybe a little lightning, but more just cloud cover. Any accumulations should remain under 0.10 inches. Storms will likely dissipate as they move off the mountain, and a weak storm or two could develop over northern Weld County. Flooding is NOT forecast.
Primetime: 12PM to 5PM

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

It will be quite smoky, but it should stay dry over these forecast zones today. An isolated storm may pop up near the Continental Divide, but cloud cover and some virga are the most likely outcome. The thick smoke will likely keep high temperatures a couple degrees cooler. Flooding is NOT forecast.

Issue Date: Friday, August 6th, 2021
Issue Time: 9AM MDT

— Flooding is NOT expected today

There’s a slight pattern change today as a quick-moving trough aids in shifting the ridge axis eastward. Not a ton of moisture is associated with this feature, but PW at Grand Junction has bounced up from 0.49 inches yesterday to 0.71 inches this morning. Moisture will likely rise a little more throughout the day as the feature moves east, so plenty for scattered storms to develop over the mountains with the diurnal flow and enhanced lift. However, with most of the moisture located in the mid-level of the atmosphere, the main threat from storms today will be strong outflow winds with the inverted-V profiles. High cloud bases and quick eastward movement of storms will also help to limit the rainfall threat. As the storms spill into the adjacent plains by early afternoon, with additional development of some storms over the Palmer Ridge, many will produce more virga and cloud cover than wetting rainfall. As anticipated, flooding is NOT expected today.

Accompanying the trough and more westerly flow will be some heavy smoke from the wildfires over the western US. The visible satellite imagery below shows a very thick smoke plume circulating around the Low. Higher near surface smoke looks to already have reached Utah, so the forecast for it to reach the western border by this evening looks on track. It is expected to begin to advect into eastern Colorado by tomorrow morning.

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, Southeast Mountains, Central Mountains, San Juan Mountains & Front Range:

There will be enough moisture for scattered storm development over the mountains this afternoon. Increased lift will help to expand the coverage of the scattered storms as well. Max 1-hour rain rates up to 0.25 inches will be possible for a few of the stronger storms that form along and near the Continental Divide. Training storms may allow for isolated totals up to 0.50 inches. The main threat from storms will be strong outflow winds, which could gust up to 50 mph. This will be most likely to happen over the central and northern high terrains this afternoon and evening. A second round of storms may develop over the Central Mountains tonight, which could help to increase local accumulations a bit. Flooding is NOT forecast.

Primetime: 1PM to 11PM

Northeast Plains, Urban Corridor, Raton Ridge, Palmer Ridge & Southeast Plains:

The quick easterly movement of storms will spill the mountain storm activity into the adjacent plains by early afternoon. There may be some additional storm development over the Palmer Ridge and areas south are expected to stay dry. Max 1-hour rain rates up to 0.25 inches will be possible with most storms producing only cloud cover and virga. If, and it’s a big if, storms can make it into the far Northeast Plains before falling apart, isolated totals up to 0.60 inches will be possible. Flooding is NOT forecast. A Red Flag Warning has been issued for Yuma County today from 1PM to 7PM.

Primetime: 2:30PM to 10:30PM

Grand Valley, San Luis Valley, Northwest Slope & Southwest Slope:

It should stay dry over the lower elevations of these regions though scattered storm development is still forecast over the elevated plateaus. More cloud cover and virga are expected than measurable rainfall today, so flooding is NOT forecast. A handful of storms could produce some brief, but strong outflow winds. The tightening pressure gradient is expected to increase the southwest surface winds over the Northwest Slope and Grand Valley with gusts up to 40 mph possible. Paired with low relative humidity values, a Red Flag Warning has been issued.

Issue Date: Thursday, August 5th, 2021
Issue Time: 8:30AM MDT

— Flooding is NOT expected today

The prolonged plume of very moist monsoonal air has abruptly moved east out of Colorado over the past 24 hours. Precipitation Water has collapsed in its wake, from 1.15 to 0.61 inches at Denver, and from 0.77 to 0.49 inches at Grand Junction. This is the lowest PW at Grand Junction since July 10th, a remarkably long stretch! And similarly, after 20 consecutive days with a flood threat, flooding is NOT expected today.

Taking a look at the visible satellite image, below, the upper-level ridge is located over western Arizona, with very hot conditions over western Colorado and areas further south and west. A disturbance is currently entering the northern California coast. With a tightening pressure gradient to its east, this disturbance is supporting quite a push of concentrated smoke from the CA wildfires to the northeast. Additional wildfires are ongoing over southwest Canada, combining with the CA smoke plume and resulting in some of the most concentrated smoke particle transport into CO thus far this season. Fortunately, the morning smoky skies should improve a bit later this afternoon as winds aloft turn more westerly.

In terms of rainfall potential today, with limited PW, almost the entire state is expected to stay dry. There are two exceptions. First, in the San Juan Mountains, plenty of morning sunshine will cause some weak instability this afternoon, fueling scattered showers and weak storms. Second, over the northern Front Range and Urban Corridor, isolated showers and weak storms are possible as a weak disturbances races across the area. However, rain rates are expected to stay well below flood thresholds in both places.

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, San Luis Valley and Southwest Slope:

Widely scattered showers and weak thunderstorms possible this afternoon, mainly over the higher terrain above 9,000 feet. Max 1-hour rainfall up to 0.5 inches. Flooding is NOT expected today.

Primetime: 2PM through 9PM

Northwest Slope, Grand Valley, Central Mountains, Northern Mountains, Palmer Ridge, Raton Ridge, Southeast Plains, Front Range, Urban Corridor and Northeast Plains:

Smoky, mainly dry and seasonably warm today. An isolated shower or weak thunderstorm cannot be ruled out over the northern Front Range and Urban Corridor. However, the main impact from these storms will be gusty winds, with only up to 0.3 inches of rainfall expected.

Primetime: 4PM through 8PM

Issue Date: Wednesday, August 4th, 2021
Issue Time: 10AM MDT

— A LOW flood threat has been issued for the Southeast Mountains, Raton Ridge, Southeast Plains and portions of the Urban Corridor, Northeast Plains and Palmer Ridge
— A PM Update is possible this afternoon

There are two quite distinct air masses shown in the water vapor imagery below. To the west of the green dashed line, drier air (yellow shades) has begun to fill in behind the trough with increasing northwesterly flow. PW at Grand Junction has dropped to 0.77 inches, and it is expected to continue to drop at least another tenth throughout the day. Whereas to the east of the green dashed line, the monsoon moisture plume is keeping dew points and PW well above average. PW at Denver this morning was measured at 1.15 inches, and with extra lift from the vorticity maximum, moderate rainfall is already occurring over the central plains.

So as far as rainfall chances, to the west of the Continental Divide, storms may again be possible this afternoon and evening with the residual moisture in the area (indicated by morning fog in the mountain valleys). However, storms that develop with the diurnal flow should be high-based and isolated in nature, which will reduce the flood threat and produce more cloud cover than measurable rainfall. To the east, the slow-moving vorticity maximum will continue to enhance mid-level lift in a moisture-rich environment. So as the day goes on, and a bit more heating occurs, expect storm coverage and intensity to expand across the plains. Longer duration rainfall (2 to 6 hours) may allow for some higher end rainfall accumulations that could cause flooding issues. Moreover, some stronger thunderstorms may develop over the Southeast Plains (south) where a bit more instability could build. While nearly fully-grown crops may help to mitigate the flooding issues, isolated totals just over 3 inches are possible and could still cause some road flooding, field ponding and local creek/arroyo flooding. For these reasons, a LOW flood threat has been issued.

Lastly, an additional set of more vigorous thunderstorms may be possible over the Southeast Mountains and elevated ridges where clearing has already begun. With steering flows from the northwest, some of these storms may spill into the immediate adjacent plains. Slightly faster steering flows should somewhat mitigate the flood threat, but the high moisture content will allow for heavy rainfall in the storm cores, and training storms could increase local accumulations. The LOW flood threat has been extended back to the west for this reason. Should the flood threat become more elevated over this region 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:

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

A quieter day is ahead over the northern portions of these regions behind the trough. Ongoing storms will expand in coverage and strength south and east. Max 1-hour rain rates up to 1.75 inches (east) & 1.50 inches (west) with totals up to 3 inches (east) and 2 inches (west) will be possible. This may cause local flooding issues with flood threats including debris flows (mountains), local creek/arroyo flooding, field ponding and road flooding.
Primetime: Ongoing to 10PM

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

A much drier day is ahead with the drier air working in from the northwest. Isolated storms will still be possible with residual moisture, but high storm bases should limit the flood threat. Storms may produce some brief outflow winds and cloud cover is more likely than measurable rainfall. Isolated max 1-hour rain rates up to 0.50 inches will be possible near and along the divide (south) with isolated accumulations under 0.10 inch elsewhere. Flooding is NOT forecast.
Primetime: 1PM to 8PM