Issue Date: Thursday, September 10th, 2020
Issue Time: 10:00AM MDT

— Flooding is NOT expected today

Today the upper-level low pressure system that has been spinning around the Four Corners region (see satellite image below) will begin to work its way northeastward across Colorado, which will continue to generate light, scattered precipitation across the state. Most of this precipitation should fall as snow over the high mountain terrain (over 10,000 ft), but it should stay mainly rain for valley and plains locations. Some of this rain could fall on snow that has accumulated over the last couple days, which could create heavier runoff with added snowmelt. However, rain rates should be light enough and flows low enough that riverine flooding should be avoided.  Temperatures will remain well below average across the state today, but slightly warmer than the last couple days, with high temperatures reaching into the 50Fs for lower elevations. As the Low shifts northeast, precipitation should shift from the southern half of the state this morning and early afternoon to the northern half later this evening and overnight. Most mountain regions should see precipitation today, with some ongoing rainfall over the far eastern plains. Some weak instability may be able to develop over the mountain regions, creating more scattered showers rather than broad stratiform precipitation. However, rain rates with these cold showers should remain low enough that flooding is not expected today.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. 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, Urban Corridor, Palmer Ridge, Raton Ridge, & Southeast Plains:

Light rain will likely continue to fall along eastern portions of the Southeast and Northeast Plains caused by upper-level lift along the eastern edge of the upper-level low. Rain rates with these weak showers should stay below 0.3 in/hr, so flooding is not expected. Total precipitation accumulations should stay below 0.5 inches. Little to no precipitation is expected over the northern Palmer Ridge, western Northeast Plains, and Urban Corridor due to downslope flow. Temperatures should remain above freezing during the day and likely even overnight due to the high humidity and dew points above freezing, so freezing road surfaces should not be much of an issue for all but maybe some higher elevation spots of the Palmer Ridge.

Primetime: ongoing

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

Numerous ongoing showers will continue over the mountains as the upper-level low creates lift and convergence along southwestern slopes. With some limited daytime heating through the clouds, some weak instability could develop to make more popcorn-like showers. However, rain rates should remain below 0.4 in/hr under these weakly convective showers, so flooding is not expected. 24-hour precipitation accumulations up to 1 inch are possible over isolated higher elevation mountain locations. Precipitation should fall as snow above 10,000 ft, where up to a foot of new snow could fall. The snowfall should reduce the threat of rapid runoff along steep terrain at high elevation, and showers will likely move off quickly under the stronger mid-level flow around the low pressure system. However, rain on current snowpack at lower elevations could increase runoff in creeks and low-lying areas, but flooding is not expected. As the Low moves northeastward during the day today, the focus for precipitation should shift from the southern mountains this morning and early afternoon to the northern mountains later this afternoon and overnight. Temperatures will get up into the 50Fs for valley locations, maybe even close to 60F in the Grand Valley and Southwest Slope, so any precipitation should fall as rain over the lower elevations.

Primetime: ongoing

Issue Date: Wednesday, September 9th, 2020
Issue Time: 8:50AM MDT

— Flooding is NOT expected today

It’s not often that you get the first snow of the season in early September. The downside is more stress to the crops and a likely end to the growing season, but the upside is increased wildfire containment and less smoke. It’s a cold start to the day with temperatures shown in red below. Most of the precipitation this morning is over the eastern plains with some lingering snowfall and rain over the southern mountains and near the Grand Valley. The stratiform (low intensity) precipitation over the eastern plains will continue to move slowly to the northeast and should clear the border by mid-afternoon. Slick road conditions are likely with freezing rain being reported.

Today the 500mb high will linger around the four corner regions, which will keep showers and snowfall ongoing over the southwest corner. Flow around the Low will pull some additional moisture northward for some more snowfall over the southern and central mountains from late this afternoon into the overnight hours. This precipitation will likely spill into the southern I-25 corridor early tomorrow morning before spreading northeastward into the Southeast Plains. That means additional, heavy snow is expected over the southern mountains with a rain/snow mix over the lower elevations. Freezing rain may be an issue over the adjacent plains, so be sure to keep track of the icy roads by following CDOT. Freezing temperatures are likely overnight once again, so stay tuned to your local NWS office for the latest on those warnings. As anticipated, flooding is not forecast today with the stratiform precipitation and snowfall.

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:

Southwest Slope, San Juan Mountains, Central Mountains, & San Luis Valley:

Ongoing precipitation is forecast over the southwest corner of the state today. For 24-hour totals, isolated areas of the western San Juan Mountains could see just under 2 inches. This could be another foot to foot and a half of snow over the south facing slopes at the highest elevations. Generally, additional snow totals between 6-12 inches are expected. The Southwest Slope could see totals up to 1.25 inch (south) and 0.75 inches (north), which would be great for the drought-stricken area even if this isn’t a drought buster. Most of this precipitation will fall as rain unless it’s over an elevated region. Further north in the south Central Mountains, additional snowfall between 2-5 inches is possible. As for the SLV, expect totals up to 0.25 inches over the edges of the valley with slightly less precipitation in the central valley. Rain may cross over to snow overnight as temperatures drop. The Freeze and Winter Storm Warnings continue through tomorrow for these zones, and more details can be found from your local NWS office.

Primetime: ongoing

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

Best chance for more snowfall is over the Southeast Mountains and southern Front Range later today and overnight. Further south, totals by morning between 4 and 6 inches will be possible with totals north between 2 and 4 inches possible. The Palmer and Raton Ridge may see another 1 to 2 inches. The big concern is the potential for freezing rain at the lower elevations tonight into tomorrow morning. Please stay tuned to your local NWS office and CDOT for the latest.

The eastern plains are going to get some mixed precipitation this morning through mid-afternoon. Higher totals from the stratiform precipitation are expected further south with 24-hour totals just under 1 inch possible. Further north, near the Kansas/Nebraska border, totals up to 0.30 inches will be possible. Flooding is not expected, but freezing rain may cause some slick roads.

Primetime: ongoing

Grand Valley, Northern Mountains, & Northwest Slope:

An additional dusting of snow can be expected over the Northern Mountains early today with just some flurries for the northwest corner of the state. Over the Grand Valley, pockets of precipitation up to 0.50 inches will be possible (south) with more general totals in the 0.10 to 0.25 inch range. The next wave of precipitation is expected to arrive tomorrow morning and work it way northward from the south. So today, expect more isolated precipitation with the best chances for accumulation over the Grand Valley. Gusty surface winds and sustained winds in the 15 to 25 mph range over the northwest corner will likely cause a bit of a wind chill. There’s a good chance that most of these regions (excluding the Grand Valley) will experience another night of freezing temperatures. Stay tuned to your local NWS office for the latest.

Primetime: ongoing

Issue Date: Tuesday, September 8th, 2020
Issue Time: 10:55AM MDT

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

A strong synoptic low pressure system is strengthening over western Colorado and eastern Utah today, which will pump strong southwesterly flow and create lift as it stalls out near the Colorado/Utah border. This lift can already be seen generating a broad thick cloud cover over western Colorado this morning (blue/green colors on infrared satellite image below). This southwesterly flow will draw in some moisture from Arizona to create some instability along the southern San Juan Mountains, where a few bands of heavy convective rainfall could set up late this morning into early afternoon before the snow starts at the higher elevations. This rainfall should transition to snow after a few hours as the freezing level drops, but a Low flood threat has been issued along and south of the Continental Divide in the southern San Juan Mountains and southeastern portion of the Southwest Slope where the thunderstorms and heaviest rainfall are likely to fall. The cold front is working its way into northern and central parts of the mountain regions, which should keep it mainly snowfall for all but the lower elevation valleys of western Colorado.

Cold air and upslope flow over the plains of eastern Colorado are generating rain and snow this morning, with snow already occurring over the Palmer Ridge and mountains. Mixed precipitation will continue today as temperatures likely won’t get out of the 30Fs over the Urban Corridor, Palmer Ridge and Northeast Plains, and 40Fs over the Southeast Plains. Currently it is mainly rain below 6,000 ft elevation, but this will transition over to snow as bands of heavier precipitation move overhead from strong upper-level lift on the east side of this upper-level low. It should transition to all snow for the Urban Corridor during the evening hours as temperatures drop below freezing. The highest snow accumulations by morning will be over the higher elevations, but the Palmer and Raton Ridge will also pick up slightly higher totals. Some isolated locations along the Front Range and much of the high elevations of the Southeast Mountains could see 18-24 inches of snowfall. Due to the mainly steady rain/snow expected over the plains and adjacent mountains, no flooding is expected over these zones.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. 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, San Juan Mountains, & San Luis Valley:

It will generally be warmer over these southwestern regions with higher snow lines. Strong southwesterly flow will pump in some moisture and cause some bands of convection to develop over the southern portion of the Southwest Slope and the southern San Juan Mountains. Before switching over to snow, these bands could produce an inch of rain and may produce localized flooding issues. A Low flood threat has been issued for this area. Precipitation should become more widespread and stratiform in nature by late afternoon, which should lower the flood threat. Heavy snowfall is possible over the San Juan Mountains overnight due to the strong cross-mountain wind component favoring enhanced lift, with 12-24 inches possible over the high terrain near the Continental Divide. The western San Juan Valley could get 6-12 inches of snow as well, once the snow line drops to valley level after sunset.

Primetime: ongoing

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

Heavy snowfall is expected above 6,000 ft along the Front Range and above 7,000 ft along the Southeast Mountains as upslope flow impinges on these mountain ranges behind the strong cold front. With rain transitioning to snow over the immediate adjacent plains later today and tonight, pockets up to 6 inches are possible for the I-25 corridor. Higher totals (6-10 inches) are expected over the higher elevations of the western Palmer and Raton Ridge as well as near the Wyoming border.

Northeasterly surface winds between 15 and 20 mph are also forecast over the eastern plains, which should produce a bit of a wind chill. Precipitation totals (rain) up to 1.50 inches (south) and 0.75 inches (north) are possible by morning. No flooding is expected due to the more gradual accumulation. A hard freeze is likely over much of the region tonight, so tune into your local NWS office for the latest on the warnings issued.

Primetime: ongoing

Northwest Slope, Northern Mountains, & Central Mountains:

Cold air is infiltrating its way into the Northern & Central Mountains and Northwest Slope as the cold front pushing through this mountainous terrain. Strong winds up to 60 mph are being observed along the Northwest Slope behind this cold front. Snow will fall mainly above 9,000 ft elevation over these zones, with 6-12 inches of total snowfall possible by morning, but the slightly higher freezing level over western Colorado will limit snowfall over the lower valleys to just a couple inches or less by morning. No flooding is expected with more stratiform precipitation expected for these zones.

Primetime: ongoing

Issue Date: Monday, September 7th, 2020
Issue Time: 10:40AM MDT

— Flooding is NOT expected today

Gusty downslope winds will impact the northern Front Range through 1PM today as the jet stream dips south into Colorado today, which will keep fire danger high. Currently, wind gusts have reached over 50 mph at Berthoud Pass, and gusts over 40 mph have been reported in western Larimer County (MesoWest). The increasing pressure gradients with a southward diving trough will further increase winds for much of the central and northern mountains as the day goes on. Unfortunately, these strong winds are also co-located with dry air, as indicated by the orange/red colors on the water vapor satellite image below. With above-average temperatures across the state today, relative humidity will dip into the low teens and single digits again today. This dry air and strong winds will increase fire danger to critical levels along the Northwest Slope, Northern Mountains, and northern Front Range. The Red Flag Warning continues for these regions. The active Cameron Peak wildfire will continue to produce smoke and very poor air quality for the Front Range and Urban Corridor today. Additional smoke from fires across the US West is also infiltrating much of western Colorado this morning, which will likely hang around throughout the day.

Dry air across the state will limit chances for precipitation during the daytime today, but change is on its way. Around sunset, a strong, gusty cold front will make its way south into the plains of eastern Colorado. This cold front will increase moisture and drop temperatures rapidly overnight. Precipitation should develop over the Northern Mountains and northern Front Range by 10PM, with much of this precipitation falling as snow above 6,000 ft elevation. Up to 0.5 inches of precipitation, or around 5 inches of snow, could accumulate above 6,000 ft by 6AM tomorrow over the Northern Mountains and northern Front Range. Showers and a mix of rain and snow could bring some precipitation accumulation up to 0.3 inches to the lower elevation plains regions as well. Expect a transition from rain to snow tomorrow morning over the lower elevations. However, due to the more stratiform/stable nature of the precipitation/atmosphere behind this cold front, rain rates are expected to remain below 0.3 in/hr, so no flooding is expected. The good news is that this precipitation should help suppress and contain the ongoing wildfires.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. 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, Urban Corridor, Front Range, Palmer Ridge, Raton Ridge, Southeast Mountains, & Southeast Plains:

It will remain warm and dry during the day today with temperatures lower than yesterday, but still above average. Downslope winds along the Front Range have produced wind gusts up to 60 mph as well, which will allow the Cameron Peak wildfire to remain active and produce smoke across the northern Urban Corridor and Northeast Plains. A Red Flag Warning remains in place through the day today along the mountains. Details can be found from your local NWS office. A lee low pressure will develop over the Southeast Plains today, which will largely allow dry westerly winds along the western half of the plains regions and southerly to easterly winds along the eastern border. Winds gusts should stay under 30 mph over the plains. This lee low pressure will set up a dry line along the eastern border counties, separating extremely dry air from more moist air. However, a cap should prevent any storms from firing along this dryline today.

Near or after sunset, a strong cold front will begin to work its way into northern Colorado. This cold front will quickly drop temperatures into the 40Fs and 30Fs as it moves south through the eastern half of the state. The front should make its way through all of the plains regions by 6AM, and it could create wind gusts up to 60 mph with its passage overnight. Precipitation should mainly stay over the northern half of the plains regions and northern mountains by early tomorrow morning, where isolated pockets up to 0.5 inches of liquid-equivalent precipitation (up to 5 inches of snow) could fall over the highest peaks of the northern Front Range. Initially warm soils will cause some melting at first. Lower accumulations are expected over the Northeast Plains (up to 0.3 inches) with rain transitioning to snow in the morning hours. Flooding is not expected.

Primetime: 6PM to ongoing

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

Temperatures will be above average again today with plenty of dry air and gusty winds ahead of the approaching system. Fire danger will be critical today for the Northwest Slope and Northern Mountains, where westerly wind gusts up to 40 mph and relative humidity in the single digits are expected.

A strong cold front will work its way into northern and eastern Colorado as a trough moves south over Colorado, but this surface front will have issues getting over the high terrain. The slower moving front will likely work its way into the Northern Mountains and Northwest Slope by early morning. The front will quickly drop temperatures for northern valley locations to near or below freezing, and some showers could bring up to 5 inches of snow to high elevation locations of the Northwest Slope and Northern Mountains by tomorrow morning. Precipitation may start as rain initially for lower elevation locations of the Northwest Slope. No flooding is expected, but the precipitation should start to increase south as the front pushes through.

Primetime: 6PM to ongoing