FTB 09-28-2021: Widespread Precipitation Forecast for the Mountains and Western Colorado

Issue Date: Tuesday, September 28th, 2021
Issue Time: 10:25AM MDT

— Flooding is NOT expected today

Currently, there’s a hearty plume of late season moisture over the state associated with the cutoff Low (blue and white shades in the water vapor imagery below). This moisture and lift from the Low are helping to produce some morning, scattered cloud cover as well as some isolated, weak showers over the elevated regions of western Colorado. This cutoff Low is expected to track NNE throughout the day, so moisture and lift are expected to remain over the state. The Low may also generate some weak showers over the plains this afternoon, but only light rainfall accumulation is expected. With the plume overhead, PW at Grand Junction has risen to whopping 0.83 inches, which is close to the 95th percentile for this date. Over Denver, PW has risen more modestly to 0.68 inches, but that’s still in the 90th percentile for this time of year. That means there is more than enough moisture for widespread precipitation to develop over the mountains and western Colorado this afternoon and evening. An incoming trough from our west (orange dashed line) will also help increase lift and perhaps add a little more moisture later this evening over western Colorado. So, expect a second round of precipitation to arrive to the western border later this evening and continue through tomorrow morning as the trough axis moves east.


There are a few things that should limit the flood threat today. The first is that both soundings still show a relatively dry surface layer. That means that the initial storms that develop this afternoon will likely produce some brief windy conditions, and higher cloud bases should help temper rainfall rates. The second is that CAPE values are expected to be on the lower end, which will limit the convective/downpour potential of storms. More stationary storms, as well as the possibility for multiple storm cores tracking over the same area, are expected to increase local totals; however, dry soils should be able to absorb most of the precipitation and limit the runoff. With instability dropping off quite a bit after sundown, overnight precipitation is expected to only produce moderate hourly rainfall rates. Additionally, colder overnight temperatures are expected, which should help turn rainfall to snow over the highest elevations. For these reasons, flooding is NOT expected today. This event should be a much needed, widespread soaking for the mountains and western 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:

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

Widespread showers should develop by early afternoon. Minimal CAPE and more pop like storms should limit the flood threat, although some decent, but isolated, storm totals will be possible by morning. The reason for the higher storm totals is that a second round of overnight precipitation is forecast as the trough axis moves overhead. Some snow will likely fall at the top of the highest peaks. Max 1-hour rain rates up to 0.65 inches will be possible this afternoon, and isolated 2-3 hour totals up to 1.75 inches are forecast by morning (Central/San Juan Mountains). Flooding is NOT expected today.
Primetime: 11:30AM to Ongoing

Grand Valley, Northwest Slope & Southwest Slope:

Best chance for some afternoon storms will be over the elevated plateaus/mesas and Southwest Slope. A second round of precipitation is expected overnight, which will likely help bring measurable precipitation to the lower elevations of the Grand Valley and Northwest Slope. Max 1-hour rain rates up to 0.50 inches and isolated 2-3 hour totals up to 1 inch will be possible. Flooding is NOT expected.
Primetime: 11:30AM to Ongoing

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

It will be a bit drier over the Southeast Mountains/Raton Ridge and San Luis Valley today; however, isolated storms are still likely to develop. Max 1-hour rain rates up to 0.25 inches will be possible over the Southeast Mountains/Raton Ridge and storm totals up to 0.15 inches can be expected over the SLV. Light showers may also be generated over the eastern plains as the Low moves overhead. Rainfall accumulation over the plains should be light, with totals remain at or under 0.15 inches.
Primetime: 1PM to 9PM