Issue Date: Tuesday, August 31st, 2021
Issue Time: 9:10AM MDT

— Flooding is NOT expected today
— An Elevated Flood Threat is expected tomorrow

Colorado awoke to mainly clear skies this morning, but as shown in the visible satellite image, below, monsoonal moisture has begun to creep into the Four Corners area overnight. However, it will take about another 24 hours before this actually translates into rainfall. Grand Junction’s morning Precipitable Water (PW) increased to 0.64 inches, but boundary layer moisture remains very low. Meanwhile, Denver’s morning PW is a meager 0.52 inches, with no rainfall expected east of the Continental Divide until later Wednesday afternoon.

For today, mid and high-level clouds will continue to stream into western CO as steering flow slowly turns from westerly to southwesterly/southerly. No rainfall is expected statewide until the overnight hours when isolated to scattered light showers will enter the higher terrain of the western slope. Flooding is NOT expected today, but an elevated flood threat 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, Southwest Slope, San Luis Valley and San Juan Mountains:

Seasonably warm and dry today with increasing clouds later this afternoon. Isolated-to-widely scattered showers are possible overnight over the higher terrain of the San Juan Mountains. Max 1-hour rainfall 0.25 inches. Flooding is NOT expected today.

Primetime: 1AM to ongoing

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

Very hot and dry today with some increase in clouds especially in the foothills and higher elevations later this afternoon. Flooding is NOT expected today.

Issue Date: Monday, August 30th, 2021
Issue Time: 9:20AM MDT

— Flooding is NOT expected today

As shown in the water vapor image, below, yesterday’s zonal flow across Colorado has shifted northward, leaving our state in between two main features. To the south, a relatively weak monsoon ridge, with some residual boundary layer moisture, will cause isolated storms over the very high terrain along the NM border. However, only short-term moderate intensity rainfall is expected, so no flood threat is needed here. Further north, a jet streak over WY will guide a weak disturbance across mainly northern and eastern CO this afternoon and evening. Moisture will be the limiting factor for today’s rainfall chances. This morning’s Precipitable Water (PW) at Denver was 0.59 inches, down notably from yesterday’s 0.80. Over the western slope, Grand Junction’s morning PW continued to be far below seasonal normal at 0.46 inches. Pockets of much drier air have also infiltrated parts of central Colorado with very shallow moisture profiles noted along most of the Front Range. With a sharp moisture gradient developing over eastern Colorado, we expect a dryline-type convergence zone over the Northeast Plains and Southeast Plains this afternoon and evening. Strong shear moving into the area this afternoon suggests a few storms will have severe potential for large hail and gusty winds. However, westerly steering flow will also increase throughout the day as the incoming disturbance passes through the area. Short-term heavy rainfall will be possible but 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 and Southeast Plains:

Much warmer today with widely scattered showers and storms mainly north of I-70. Max 1-hour rainfall up to 1.5 inches is possible, which could lead to nuisance ponding. However, flooding is NOT expected today. Large hail and damaging winds will be possible with the strongest storms.

Primetime: 4PM through 10PM

San Luis Valley, Raton Ridge, Southeast Mountains, Southwest Slope and San Juan Mountains:

Seasonably warm with isolated to widely scattered showers and weak storms mainly over the higher terrain along the NM border. Max 1-hour rainfall up to 0.6 inches possible. However, flooding is NOT expected today.

Primetime: 1PM to 8PM

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

Hot and mainly dry today with an isolated shower or weak storm possible mainly over the higher terrain along and west of the Continental Divide. Max 1-hour rainfall up to 0.3 inches. Flooding is NOT expected today.

Primetime: 1PM to 7PM

Issue Date: Sunday, August 29th, 2021
Issue Time: 10:15AM MDT

— A LOW flood threat has been issued for the Front Range, Urban Corridor and western Palmer Ridge

It’s a cool and cloudy start to the morning over eastern CO behind yesterday’s cold front. Morning temperatures are running around 8-12F cooler than this time yesterday for most locations. Lower-level moisture has also returned behind the front, and PW at Denver was measured at 0.86 inches. Also noticeable in the sounding was a strong cap right above 700mb, which will help to limit the chance for precipitation north today. The front has had little affect over western CO where PW remains around 0.40 inches. It is likely that PW near the southern border is probably slightly higher, so another round of diurnally driven, scattered storms will be possible over the southern San Juan Mountains. Otherwise, it should remain dry with smoky conditions continuing.

Flow aloft across the state will be zonal behind the trough, and only minimal mid-level energy is forecast to move through this flow. That means the cap will likely not be broken over the northern portion of the Front Range/Urban Corridor. Best chances for post-frontal upslope flow storms to develop will be over the southern Front Range/Palmer Ridge intersect, Southeast Mountains and western Raton Ridge where clearing began earlier in the day. There is a chance for a stronger storm or two to develop over the Palmer Ridge/southern Front Range, if mid-level energy with decent daytime heating can break the cap. Guidance this morning is showing higher confidence in this solution. With the increase in low-level moisture behind the front and slower steering flows aloft, if storms develop, heavy rainfall and localized flooding issues would likely accompany them. So, a LOW flood threat has been issued for this possibility. Elsewhere, rain rates from storms today should be on the lower end, so flooding is not expected.

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.

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

Best chance for post-frontal upslope flow storm development today will be over the western Palmer Ridge, southern Front Range, Southeast Mountains and western Raton Ridge. If a storm can break through the cap over the Palmer Ridge/Front Range intersect, heavy rainfall will be possible with max 1-2 hour rainfall totals up to 1.40 inches. A LOW flood threat has been issued for this reason. Elsewhere, rainfall rates should be more gradual with storm totals just under an inch possible. Additional mountain, scattered storms will likely form over the southern San Juan Mountains where max 1-hour rain rates up to 0.75 inches will be possible. Finally, it will be possible for an isolated storm or two to develop over the far Southeast Plains along a trough axis, but confidence in this occurring is low. If it does, isolated storm totals up to 1.50 inches may occur, so flooding issues would not be likely.
Primetime: 1PM to 10PM

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

It will be another smoke-filled and dry day for much of the western portion of the state. Near surface smoke may mix out a bit over the lower elevations by this afternoon, but it will likely remain thick over the Northern Mountains. Afternoon highs will reach into the 90Fs for the lower elevations and upper 80Fs for the mountain valleys. A couple storms off the high terrains may spill into the San Luis Valley this evening, but more wind than rainfall is likely as storms advance away from the source of lift. Isolated storm totals on the edges of the SLV up to 0.35 inches will be possible. Elsewhere, rainfall is not forecast today.

Issue Date: Saturday, August 28th, 2021
Issue Time: 10:15AM MDT

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

A “cold” front has moved into the northeast corner of the state this morning, but it won’t do much cooling with high temperatures over the area still reaching around 90F. This feature should be stationary throughout the day, and northerly winds behind the boundary will likely pull the plume of smoke, marked below, across the region. Today, the base of the trough (to our north) moves eastward, and flow aloft becomes more zonal (westerly). This drier flow should help to scour out any remaining surface moisture and any new moisture return behind this morning’s front. This set up is expected to keep afternoon rainfall activity on the quieter side with only scattered, high-based storms developing over the mountains and elevated ridges (west), similar to yesterday. The tightening surface gradient will also produce some gusty, westerly surface winds over the Northern Mountains where a Red Flag Warning has been issued.

Tonight, a second surge of cooler air will push through eastern Colorado associated with the trough. This cold front is forecast to not only generate much cooler temperatures tomorrow, but it will likely produce 35 to 45 mph gusts as it slides south. The higher wind gusts will be most likely over the adjacent and far eastern plains during the evening and overnight hours. Moisture is also expected to increase along and behind the front, so it may generate some evening storms and showers. Best chance for a couple of vigorous thunderstorms to develop will be over the eastern plains with more general showers forecast back to the west. The main forecast question for today is the amount of instability that will be over the eastern plains when the lift from the front arrives. Since there is the possibility of a couple stronger storms in a high moisture environment with enhanced convergence along the boundary, a LOW flood threat has been issued. Localized flooding threats include road/low-lying area 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:

Southeast Plains, Northeast Plains & Palmer Ridge:

A couple stronger storms may develop over this area this evening and overnight as lift from the front moves south. With high moisture in place from the surface Low, and enhanced convergence along the boundary, storms that develop may be capable of causing localized flooding issues. Max 1-hour rain rates up to 1.75 inches and storm totals up to 2.50 inches will be possible. If the storms develop, they could cause localized road flooding, local stream/creek bankfull conditions, field ponding and flooding of low-lying areas. A LOW flood threat has been issued.
Primetime: 6PM to 3AM

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

Not much rainfall expected this afternoon though some moderate showers with lightning may develop across the southern high terrains again, both east and west. Other than that, western CO should remain dry. Over the elevated ridges (east) and southern Front Range, showers accumulations are forecast on the lower end with the downscoping flow aloft. Max 1-hour rain rates up to 0.50 inches (west/south) and 0.25 inches (adjacent plains) will be possible. Additional showers may develop over the eastern high terrains and immediate adjacent plains overnight as the cold front drops south. With the dry low-levels and decreasing instability, storms are only expected to produce light to moderate showers. Flooding is NOT forecast today.