Issue Date: Sunday, August 15th, 2021
Issue Time: 9:20AM MDT

— A LOW flood threat has been posted for parts of the San Juan Mountains, Central Mountains and Southeast Mountains

It is quite unusual for any part of Colorado to wake up to thunderstorm activity especially this late in the summer, but such is the case this Sunday morning. A narrow line of elevated convection is fueling some very short pulses of heavy rainfall over the Northeast Plains. This is a good indication that today’s atmosphere is ready to generate rainfall, although the vast majority of it will fortunately not pose a flood threat.

Looking at the visible satellite image, below, we continue to see a very messy picture over Colorado. Our state lies on the eastern periphery of a seasonably strong upper-level ridge that is centered over CA. With a fairly strong pressure gradient between the ridge and a trough over the Central Plains, southeasterly steering winds continue to remain strong, above 25mph for most of Colorado. This will limit point rainfall for most areas. Switching to the moisture analysis, Denver’s Precipitable Water (PW) is 0.79 inches, up slightly from yesterday. On the other side of the Continental Divide, Grand Junction’s PW has also increased a bit to 0.8 inches. However, at both soundings, moisture was “top-heavy”, which is not a good sign for heavy rainfall potential. Further south, however, the moisture profile looks better, with PW up to 0.9 inches along the NM border and higher near-surface moisture. Mid-level temperatures are colder today than yesterday and there is more substantial synoptic scale lift today, both of which will result in an uptick in rainfall coverage. The key to the heavy rainfall potential will be to find the juxtaposition of instability and relatively slow steering flow, with the southern high terrain appearing to meet both criteria.

Translating everything into sensible weather, we expect scattered to numerous showers and thunderstorms over basically the entire Continental Divide spine of higher terrain. Over northern and central areas, only very short-term heavy rainfall is expected. Further south, over the San Juans and Southeast Mountains, 60-90 minutes of heavy rainfall will be possible, which warrants a LOW flood threat for isolated flash flooding, debris slides and mud flows.

Isolated severe storms are expected in the foothills and plains east of the Continental Divide. The main impact looks to be damaging wind gusts over 60mph, though hail up to 1.25 inches is also possible.

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:

Central Mountains, San Juan Mountains, Southwest Slope, San Luis Valley and Southeast Mountains:

Numerous showers and thunderstorms likely this afternoon and evening with max 30-minute rainfall up to 0.8 inches and max 1-hour rainfall up to 1.2 inches. The highest coverage will be at elevations above 7,500 feet. A LOW flood threat has been posted for isolated flash flooding, mud flows and debris slides especially over steeper terrain. A few storms with gusty winds will be possible late into the evening.

Primetime:
1PM through 10PM

Southeast Plains and Northeast Plains:

Isolated to widely scattered thunderstorms this afternoon with ax 1-hour rainfall up to 1.5 inches possible. Some nuisance field and road ponding is possible today but flooding is NOT expected today. Damaging winds and large hail up to 1.25 inches will be possible with the stronger storms today.

Primetime: 12PM through 10PM

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

Scattered showers and thunderstorms expected this afternoon and evening, mainly over the higher terrain above 7,500 feet but a few storms will make it into the foothills and plains east of the Continental Divide. Max 30-minute rainfall up to 0.6 inches, with max 1-hour rainfall up to 0.9 inches. Gusty winds will be possible with the stronger storms especially later this afternoon.

Primetime: 12PM through 11PM

Issue Date: Saturday, August 14th, 2021
Issue Time: 9:25AM MDT

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

Colorado awoke to mainly clear skies this morning. However, as shown in the water vapor image, below, the atmosphere today remains in a rather confused state with pockets of higher moisture intertwined with drier air. Precipitable Water (PW) at Denver this morning was 0.74 inches, down from yesterday’s 0.92 inches. However, PW at North Platte (NE) and Dodge City (KS) were over 1.30 inches, signifying a strong moisture gradient over eastern Colorado. Grand Junction’s morning PW was 0.67 inches, up slightly from yesterday, but with a moisture profile not conducive for much, if any, rainfall. The PW in Albuquerque was an impressive 1.15 inches, and up to 1 inch of PW is estimated over far southwest Colorado. This appears to be the area to watch for flooding potential today.

In the mid-levels, a strengthening upper-level ridge, currently centered over Utah will support a generally clockwise flow of air around its eastern fringe. Little to no forcing features are easily discernable, making for a challenging forecast. An ongoing storm complex over western Nebraska was being fueled by elevated convection and is expected to dissipate later this morning. However, it could send an outflow boundary south along the KS border and spark an isolated to widely scattered severe storm. Flooding looks unlikely with this situation as steering winds will be strengthening through the day and exceed 25 mph by afternoon. With adequate instability and strong wind shear, however, large hail and damaging winds will be possible with any storms that form.

Over most of the higher terrain, moisture is not overly impressive this morning, but scattered showers and thunderstorms are still expected this afternoon. The vast majority of the activity should be benign with only short-term moderate intensity rainfall possible. Over the San Juans, however, a bit more moisture with slower steering flow does support storms capable of 30-60 minute duration heavy rainfall. A LOW flood threat has been posted for this limited area for 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:

San Juan Mountains and Southwest Slope:

Scattered showers and thunderstorms likely this afternoon and evening with max 30-minute rainfall up to 0.7 inches and max 1-hour rainfall up to 1.1 inches. A LOW flood threat has been posted for isolated flash flooding, mud flows and debris slides especially over steeper terrain. A few storms with gusty winds will be possible late into the evening.

Primetime:
1PM through 10PM

Southeast Plains and Northeast Plains:

Hot and mainly dry today though isolated to widely scattered thunderstorms are possible mainly along the KS border this afternoon and evening. Max 1-hour rainfall up to 1.5 inches possible, which could cause some nuisance field and road ponding. However, flooding is NOT expected today. Severe weather wise, large hail and damaging winds will be possible with the stronger storms.

Primetime: 2PM through 10PM

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

Scattered showers and thunderstorms expected this afternoon and evening, mainly over the higher terrain above 8,000 feet. Max 30-minute rainfall up to 0.5 inches, with max 1-hour rainfall up to 0.8 inches. Gusty winds will be possible with the stronger storms especially later this afternoon. A few storms could make it eastward off the higher terrain by late evening, but only short-term moderate intensity rainfall is expected, along with possibly gusty winds.

Primetime: 12PM through 11PM

Issue Date: Friday, August 13th, 2021
Issue Time: 10:20AM MDT

— A MODERATE flood threat has been issued for the Southwest Slope, San Juan Mountains, Central Mountains, Front Range, Urban Corridor and Palmer Ridge
— A LOW flood threat has been issued for the Southeast Mountains, Northern Mountains, Northeast Plains, Raton Ridge and Southeast Plains

Today the 500mb High will continue to strengthen, and the majority of the state will be under the influence of the clockwise flow. Over the far southeast corner of the state, there’s still some westerly flow with yesterday’s front draped over the central eastern plains. Monsoon moisture has started to fill in statewide as shown by dew points > 50F over eastern Colorado and at a few stations over the southwest corner. PW has risen to 0.60 inches over Grand Junction and 0.92 inches at Denver. Dew points should remain high throughout the day as the surface flow will be favorable for moisture advection in a post-frontal, upslope flow environment. With slow steering flows under the building ridge and some mid-level energy in the area, widespread, heavy rainfall producing storms are forecast today. Additionally, a couple severe thunderstorms will be possible over the eastern plains.

Storms over the mountains will pop by early afternoon and are forecast to be widespread in nature. It is likely that outflow boundaries will help to produce additional storm coverage in the moisture-rich environment. A couple stronger thunderstorms possible with the main threats being hail and brief outflow winds. Best chance for these storms will be over the Southwest Slope high terrains and southern Front Range. A MODERATE flood threat has been issued for these areas due to a higher likelihood for stronger and training storms producing heavy rainfall.

Rainfall might not be as easy to come by over the adjacent plains today as Denver’s sounding this morning showed a decent cap, which could inhibit convection this afternoon. Morning model guidance has also quieted over the far eastern plains. Should an outflow boundary from a storm to the north or enhanced convergence from a developing Denver Cyclone be able to trigger convection with the eroding cap, a couple severe thunderstorms may be possible over this forecast region later this afternoon and evening. This would also hold true for any storms that track into the state off the Cheyenne Ridge. On top of severe weather, these storms will also be capable of producing very heavy downpours, so a LOW flood threat has been issued.

Finally, further south over the far Southeast Plains, it is possible that a convergence boundary may move northward from New Mexico and Oklahoma late this afternoon. It may be able to help break the cap over the area, if it does so, a couple severe thunderstorms may be possible. In addition to hail and strong outflow winds, heavy rainfall would be likely due to dew points in the low to mid-60s. Less confidence that the boundary will make it northward into the state, so only 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:

Central Mountains, Southeast Mountains, Urban Corridor, San Luis Valley, & Front Range:

More copious storms are is forecast today with some mid-level energy and a post-frontal upslope flow environment. In addition to heavy rainfall, storms may produce some small hail and strong outflow winds. Max 1-hour rain rates up to 1.75 inches and storm totals up to 2.75 inches will be possible if multiple storms track over the same area. Flood threats include mud flows and debris slides along with road flooding and local stream flooding. A MODERATE/LOW flood threat have been issued.

Primetime: 1PM to 10PM

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

It should stay more dry north and across the western border (north), but some healthy moisture has moved into the Southwest corner of the state. This will help produce numerous storms over the Southwest Slope and San Juan Mountains this afternoon. Max 1-hour rain rates up to 1.75 inches will be possible and a couple rounds of storms could bump totals near 3 inches. A MODERATE flood threat has been issued for road flooding, mud flows/debris slides and local stream/creek flooding. Stronger thunderstorms could produce hail and brief outflow winds as well.

Primetime: 1PM to 11PM

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

There’s a chance for some severe thunderstorms to develop late this afternoon/evening out east both north and south. This would only occur if storms can break through the cap. Max 1-hour rain rates up to 2.50 inches with storm totals up to 3.50 inches would be possible. Further north, there’s a bit more low-level wind shear, so a brief, weak tornado may be possible. The main threats from the severe storms would be large hail and strong outflow winds. Additional storms could develop over the plains from other outflow boundaries, so a large area of a LOW flood threat has been issued. Flood threats for storms that can develop include road flooding, field ponding and local stream flash flooding.

Primetime: 3:30PM to Midnight 

Issue Date: Thursday, August 12th, 2021
Issue Time: 9:50AM MDT

— A LOW flood threat has been issued for the Southeast Plains, Palmer Ridge, Raton Ridge and Northeast Plains
— Possible PM update may be issued 

The departing trough to our north will allow a ridge to begin to build today, which will pull the moisture plume, that was sitting to our south over the last few days, northward. There is already a sharp increase in moisture across the southern border as shown in the water vapor imagery below. While PW at Grand Junction was measured only at 0.33 inches, just to our south over Albuquerque, PW was over 1 inch. PW has also begun to rise over Denver with a morning measurement of 0.63 inches with PW over an inch at Amarillo and Dodge City. So, it’s a strong moisture gradient. This will translate to an uptick in rainfall today, particularly over the southern mountains and Southeast Plains.

Initially, the dry surface layer will cause storms that develop over the mountains to produce less efficient rain rates and promote strong outflow winds. Furthermore, an increase in some mid-level energy will allow storms to be more widespread than just the typical diurnally driven thunderstorms that we’ve seen the last couple of days. Over the plains, slightly better surface moisture will translate into some higher rain rates. Enhanced convergence along a couple boundaries and some mid-level energy from the south will be the main forcing mechanisms for storms. While the severe threat will be limited with the pulse-like storms, some hail and strong outflow winds could still occur. Very slow steering flows and enhanced convergence may allow for some local heavy downpours within the high moisture environment, so a LOW flood threat has been issued.

Finally, an axis of instability may help generate some storms over the foothills, Urban Corridor, and perhaps the Northeast Plains overnight. Unsure about the amount of moisture that will develop over the area behind the front, and if the lift will be enough to overcome the cap. For now, we’ll go with no flood threat for this region and issue a PM update if the threat for heavy rainfall increases.

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:

Best chance for heavy rainfall this afternoon will be over the Palmer Ridge and Southeast Plains. Enhanced convergence along boundaries, lee troughing and mid-level energy could help anchor storms and promote some heavy downpours. Max 1-hour rain rates up to 1.90 inches will be possible with isolated storm totals up to 2.75 inches. Field ponding, arroyo flooding and road flooding will be the main threats from the stronger storms that can develop. A LOW flood threat has been issued. Additional overnight storms may be possible over the Northeast Plains along an axis of instability that develops behind the front. If storms can break the cap with some lift, max 1-hour rain rates up to 1.75 inches will be possible.

Primetime: 1PM to 3:30AM

Central Mountains, Southeast Mountains, San Juan Mountains, San Luis Valley, Front Range & Urban Corridor:

Storms should pick up in coverage over the mountains this afternoon and evening. Further north storms will be located along and near the Continental Divide, and further south, storms should have more widespread coverage. Isolated max 1-hour rain rates up to 0.50 inches will be possible, but most storms will produce lower accumulations and more brief windy conditions and dangerous lightning. Flooding is NOT anticipated from these storms.

Overnight, some additional rainfall over the Front Range and Urban Corridor could be possible. For this to occur, first, moisture will have to increase, and second, lift will have to be sufficient to break the cap. If both of these occur, heavy rainfall would be possible with the storms that develop. However, low confidence in this outcome, so going with no flood threat for now. If the overnight, heavy rainfall threat increases, a PM update will be issued.

Primetime: 1PM to 3:30AM

Grand Valley, Northwest Slope, Northern Mountains, & Southwest Slope:

Storms should be confined to the mountains this afternoon, though an increase in cloud cover will likely occur over the Southwest Slope. Hot temperatures return, especially over the Grand Valley and near surface smoke will continue to promote poor air quality.