FTB 09-12-2021: Scattered Storms Return & A Couple Severe Storms Possible East

Issue Date: Sunday, September 12th, 2021
Issue Time: 9:50AM MDT

— Flooding is NOT expected today

With the High suppressed to our south and a few shortwaves moving through the flow, the 500mb pattern has become more zonal across the state this morning. This more westerly flow aloft will allow another shortwave, marked below, to move across the state and increase mid-level lift for scattered storm development. In the wake of yesterday’s trough, a weak cold front has pushed through the eastern plains. With a slight increase in overall moisture, upslope flow, and uptick in lift from the shortwave, scattered storms are expected to develop over the northern and central mountains by midday. As these storms are steered into the adjacent eastern plains, they should encounter a little deeper surface moisture along with some marginal shear. This should allow the storms to expand in both coverage and intensity over the border counties, and one or two severe storms may be possible. The main threats from these severe storms would be large hail and strong outflow winds.

Steering flows do look to be a little slower than yesterday, which should help encourage higher local accumulations from the storms today. However, less surface moisture back to the west means that storms will likely produce more of a wind threat than heavy rainfall over the mountains and immediate adjacent plains. Although, dew points over the far eastern plains should remain in the low to mid-50Fs, steering flow should be quick enough to keep rainfall rates just below flood threat criteria. Therefore, 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:

Front Range, Central Mountains, Northern Mountains & Northwest Slope:

Cooler afternoon temperatures are forecast today with storms kicking of just after noon favoring the Front Range and Northern Mountains for development. Similar to yesterday, storms should pulse in intensity as they develop and fairly quickly dissipate or move out of the area. Max 1-hour rain rates up to 0.25 inches will be possible. In addition to rainfall, storms will likely produce gusty outflow winds, especially over the Front Range. Flooding is NOT expected today.
Primetime: 12:30PM to 9PM 

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

As storms move east and off the mountains, they will begin to encounter slightly higher surface moisture. This should allow storms to increase in intensity, especially east of I-25. Over the adjacent plains (Urban Corridor), best coverage of the stronger storms will be north, especially along the Palmer Ridge. Max 1-hour rain rates up to 0.50 inches will be possible. South, max 1-hour rain rates up to 0.20 inches will be possible.

A couple marginal severe storms may be possible this evening as storms make their way into the far eastern plains with the main threats being strong outflow winds (50 to 55 mph) and large hail (1.25 inches). Best chance for this to happen will be north of southern Kiowa County border. Max 1-hour rain rates up to 1.50 inches will be possible with the stronger storms that develop, and storm intensity should begin to drop off around 9PM. However, some showers may linger overnight. Flooding is NOT expected today.
Primetime: 2PM to 1:30AM

Grand Valley, San Juan Mountains, Southeast Mountains, San Luis Valley & Southwest Slope:

It’s expected to remain dry over the lower elevations of these forecast zones today with isolated storm development over the San Juan and Southeast Mountains this afternoon. Best chance for measurable rainfall will be along and near the Continental Divide where isolated storm totals up to 0.20 inches will be possible. High temperatures will drop into the upper 80Fs (lower elevations) with 70Fs for the mountain valleys. Flooding is NOT expected.