Issue Date: Thursday, August 19th, 2021
Issue Time: 10:40AM MDT

— A HIGH flood threat has been issued for the Grand Valley, Southwest Slope & Northeast Plains
— A MODERATE flood threat has been issued for the Northwest Slope
— A LOW flood threat has been issued for the San Juan Mountains, Central Mountains, Northern Mountains, Front Range, Urban Corridor and Southeast Plains

PM Update: Very heavy and widespread rainfall is becoming increasingly likely over the far Northeast Plains this evening/overnight due to strong moisture convergence in the area and training storms aided by the jet. A HIGH flood threat has now been issued for Sedgwick, Phillips and Yuma Counties, and the MODERATE flood threat has been shifted slightly further south. Max 3-hour rain rates up to 5 inches will be possible and local accumulations could slightly exceed that amount by morning. These totals have the potential to cause local creek/stream flooding, heavy field ponding and severe road flooding. Please stay tuned to your local NWS office for the latest on real-time warnings that are issued.

It’s a busy weather day ahead for most of the state, so we’ll just go ahead and jump right into the discussion. Over western Colorado, above average moisture (PW just over an inch) is already combining with upper-level lift from the jet, which is producing moderate showers this morning. Snow was being reported at Wolf Creek Pass, so it’s possible that there is a dusting of snow on some of the high peaks. This first round of rainfall is forecast to continue to move northeast throughout the morning before the next set of storms to arrives with the cold front early this afternoon. Within this second wave of storms, rain rates should pick up in intensity and coverage, and a good soaking is anticipated for most of the region. A couple severe storms might be possible south where there is a better chance for a break in cloud cover, which would allow some instability to build. Moderate rainfall accumulations over a longer duration should lead way to an increase in runoff as soils become saturated, which could cause local streams and creeks to run at bankfull or minor flooding conditions. There is also an uptick in the risk for mud flows and debris slides with embedded convection as the soils become more unstable. A HIGH/MODERATE flood threat has been issued for this area, and the flood threat is over a longer duration rather than the typical 1 to 2 hour threat. Storms are expected continue into the overnight hours, but should decrease in intensity by the early morning hours.

As the closed Low system, marked below, slowly meanders southeast throughout the day, the area of lift currently over western Colorado will move east with it. This mid-level energy arriving at peak heating, diffluent flow aloft and a convergence boundary should help to spark some showers and thunderstorms over the Front Range and adjacent Urban Corridor this afternoon and evening. Additional severe thunderstorms may be possible over the Northeast Plains later this afternoon and overnight, which would be capable of producing strong outflow winds, large hail and a possible tornado or two. Throughout the day, surface moisture is expected to be reinforced over the area with southeasterly winds on the east side of the surface Low, which combined with the increased dynamics in the area will make storms capable of producing very heavy rainfall. Thus, a MODERATE/LOW flood threat has been issued. Drier air on the south and east side of the surface Low along with downsloping winds should limit the chances for precipitation over the southern half of eastern Colorado.

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:

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

Another round of more vigorous storms is on its way this afternoon. Max 1-hour rain rates up to 1 inch with storm totals up 3.50 in the HIGH threat area are possible. This longer duration threat with more widespread rainfall could cause excessive runoff and flooding of local streams, creeks and roads. There is also an increased threat for mud flows and debris slides with embedded convection over saturated soils. Thus, a HIGH flood threat has been issued. Storms will likely continue over the mountains into the overnight hours with the heavier rainfall ending around midnight. Moderate and lighter rainfall should end around 4AM. Stay tuned to your local NWS office for the latest on real-time warnings that will likely be issued today.
Primetime: Ongoing to 4AM

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

Increasing chance for rainfall across the northern portion of eastern Colorado today. Max 1-hour rain rates up to 1.25 inches (west) and 2 inches (east) will be possible. Severe thunderstorms will also be possible over the Northeast Plains with the main threats being very large hail, strong outflow winds and perhaps a tornado or two this afternoon and overnight. On top of these threats, very heavy rainfall could accompany the storms with isolated totals up 4.50 inches possible. The potential for this very heavy rainfall is most likely along the border. A MODERATE threat has been issued for this reason with the main threats being road flooding, heavy field ponding and flooding of local streams/creeks. There is an outside heavy rainfall across the far Southeast Plains if the moisture doesn’t scour out too far east on the southeast side of the surface Low. If storms can pop or back-build into the area, max 1-hour rain rates up to 1.75 inches will be possible. A LOW flood threat has been extended south for this reason. Storms will likely continue into the overnight hours making this an overnight flood threat, and some showers are possible over the Urban Corridor tomorrow morning.
Primetime: 12PM to 10PM (west); 3PM to 4AM (east)

Southeast Mountains, San Luis Valley, Raton Ridge & Palmer Ridge:

Downsloping winds and lower moisture over these regions from the surface Low should keep the areas mostly rain-free today. There is still a chance for a couple showers to develop over the Southeast Mountains where rain rates up to 0.15 inches will be possible. As anticipated, flooding is NOT forecast.

Issue Date: Wednesday, August 18th, 2021
Issue Time: 9:45AM MDT

— A MODERATE flood threat has been issued for the Grand Valley, Southwest Slope and southern Northwest Slope
— A LOW flood threat has been issued for the San Juan Mountains, Central Mountains and Northern Mountains
Note this is an overnight threat with a second round of rainfall arriving at the western border by early tomorrow morning

The combination of increasing southwesterly flow aloft and an incoming cold front associated with the troughing pattern will generate the next moisture surge into western Colorado today. An uptick in dynamics with the approaching cold front and jet stream is also anticipated, which means more widespread storms and a couple severe thunderstorms will be possible out west. With PW rising to close to an inch across the western border (0.92 inches this morning), storms that develop today are expected to pose a heavy rainfall threat. Slightly faster steering flows with the incoming jet will likely aid in limiting the threat over any one area, and ongoing showers this morning may help to limit the areal coverage of instability that can develop this afternoon and evening. Nonetheless, mid-level lift and moisture will be plentiful for widespread coverage of stronger storms, and the increasing southwesterly flow will help to pull additional disturbances to our southwest into Southwest Slope and San Juan Mountains.

Initially, the flood threat today will be on the 1 to 2 hour timescale due to the possibility of training storms, but by tonight and into tomorrow morning, the flood threat turns to more of a 3 to 6 hour threat with multiple waves of rainfall forecast over the next 48-hours. Higher-end local accumulations from longer duration rainfall, as well as the quick downpours from storms this afternoon and evening, are forecast to cause an increasing threat for mud flows/debris slides, road flooding and local creek/stream flooding. A MODERATE flood threat has been issued for portions of western Colorado, and the flood threat will continue overnight and into tomorrow morning.

Over eastern Colorado, especially the Southeast Plains, dry air and southwesterly steering flow should keep the plains and Southeast Mountains dry this afternoon. There’s a better chance for some spillover storms along the Front Range and Urban Corridor, but faster storm movement and slightly lower moisture should create more of a wind than heavy rainfall threat. Nuisance road and low-lying area ponding may be possible under the stronger storms that develop (north), but flooding is not forecast for this area.

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:

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

Widespread rainfall is forecast with the incoming trough along with cooler temperatures today. Storm movement towards the northeast will be relatively fast, but training storms may help to increase local totals. Storms initially will be capable of producing 1-hour rain rates up to 1.50 inches, and a couple severe storms may be possible over the border counties. The main threats will be strong outflow winds and large hail, but a brief weak tornado is also possible. By this evening and into tomorrow morning, the rainfall threat becomes longer duration (3 to 6 hour) with the upper-level jet aiding in lift for overnight rainfall. Totals by tomorrow morning up to 2.50 inches over the Grand Valley and Southwest Slope forecast regions will be possible. Flood threats over the next 24-hours include local stream/creek flooding, mud flows and debris slides (steeper terrains) and road flooding. A MODERATE flood threat has been issued. Note that a second round of rainfall will likely arrive to western Colorado early tomorrow morning before the next flood threat bulletin is released.
Primetime: 1PM to Ongoing

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

Southwest steering flows should keep dry air in place over the plains, Raton Ridge, San Luis Valley and Southeast Mountains limiting the chances for precipitation. A couple high-based storms may drop light rainfall over the Southeast Mountains, so flooding is NOT anticipated. Better chance for storm spillover and a couple thunderstorms will be across the Front Range, Palmer Ridge and Urban Corridor. Max 1-hour rain rates up to 0.40 inches will be possible. This could cause some nuisance ponding in low-lying areas/intersections over impervious areas, but flooding is NOT forecast.
Primetime: 2PM to 11:30PM

Issue Date: Tuesday, August 17th, 2021
Issue Time: 8:40AM MDT

— Flooding is NOT expected today
— An Elevated Flood Threat is expected Wednesday and Thursday mainly over western Colorado

Major changes are underway for Colorado’s weather over the next few days, but today will act as a calm transition period with limited rainfall. As shown in the visible satellite image, below, a weakening upper-level ridge will remain in control of our weather for one more day. Moisture remains lacking with Grand Junction’s morning Precipitable Water (PW) coming in at 0.71 inches, and Denver’s at a paltry 0.53 inches. Just like yesterday, boundary layer moisture remains very limited and has fallen a bit even along the NM border.

With upper-level heights beginning to fall today in advance of the incoming strong trough, signaling a destabilizing atmosphere, isolated to scattered showers and weak storms are expected mainly over the higher terrain of northern and central Colorado. Some isolated showers could also begin to pop up overnight over northwest Colorado. But due to the aforementioned dry boundary layer, flooding is NOT expected today.

The northern half of the state will continue to experience smoky skies throughout the day until steering winds aloft turn more westerly and southerly. Much better air quality is expected beginning tomorrow.

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:

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

Isolated to widely scattered showers and weak storms this afternoon, with the highest coverage over the northern and central higher terrain. Max 1-hour rainfall up to 0.4 inches. Gusty winds will also be possible with the strongest storm. Flooding is NOT expected today.

Primetime:
12PM through 9PM

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

Mainly dry and continued hot today with an isolated shower or weak storm possible in the foothills. Max 1-hour rainfall up to 0.3 inches, along with possibly gusty winds. Flooding is NOT expected today.

Primetime: 2PM through 9PM

Issue Date: Monday, August 16th, 2021
Issue Time: 9:35AM MDT

— A LOW flood threat has been issued for the Southwest Slope and San Juan Mountains

Visible satellite imagery below shows the 500mb High further west this morning, and plenty of smoke out West from the ongoing fires (gray hue). Colorado remains under northerly flow over the western border, which is moving a disturbance south this morning (orange “X”). There is another disturbance over the far Southeast Plains, and both disturbances are creating cloud cover with some embedded rainfall. Today, an incoming, west coast trough will start to push the High first south then west. That will place Colorado under both northerly and variable upper-level flow. Mid-level lift is forecast to remain over the western portion of the state, which will help generate more widespread showers and thunderstorms over the high terrains, especially south.

Drier and stable conditions east of the Continental Divide are expected keep the area mostly rain-free today, although storms will be likely once again along and near the Divide and perhaps south over the southern Southeast Mountains. PW at Grand Junction remains around the same as yesterday (0.79 inches), but with slower steering flows and some training storms forecast, local accumulations should get a boost. Morning cloud cover may limit the coverage of instability, but some pockets of greater instability should still be able to form with strong August heating. That should allow for some stronger thunderstorms to develop over the Southwest Slope and San Juan Mountains that will be capable of producing heavy rainfall in their storm cores. Increased runoff may also be an issue if storms track over the areas that have received rainfall the last couple of days. A LOW flood threat has been 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:

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

Max 1-hour rain rates up to 1.25 inches with storm totals up to 1.75 inches will be possible over the San Juan Mountains and Southwest Slope this afternoon where stronger instability can develop. Cloud cover is already starting to break-up, which indicates more instability may be able to build. This could lead to isolated road flooding, mud flows/debris slides and local stream flooding, especially if storms track over the same areas as the last couple of days. Additional threats include strong outflow winds and perhaps some hail. A LOW flood threat has been issued. Storms to the north should be anchored to the high terrains with the weak northerly steering flow. Max 1-hour rain rates up to 0.50 inches will be possible with more storms producing totals around 0.25 inches. Lower elevations should stay dry.
Primetime: 12:30PM to 10PM

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

More stable conditions are forecast across the lower elevations this afternoon, which should keep rainfall confined to areas along and near the Continental Divide. There could also be a couple storms that form over the southern Southeast Mountains. Isolated max 1-hour rain rates up to 0.75 inches will be possible (central/south) with storms that develop over the Northern Front Range producing totals closer to 0.30 inches. Over the far Southeast Plains, if storms can overcome the cap along a dryline, 1-hour rain rates up to 1.25 inches will be possible. First, we’ll need the cloud cover to clear out and mid-level energy to move through the area during peak heating. We’ve got the first ingredient already occurring. Flooding is NOT forecast for these regions today.
Primetime: 1PM to 8PM