FTB 09-22-2020: Widespread Scattered Storms Forecast for the Mountains

Issue Date: Tuesday, September 22nd, 2020
Issue Time: 9:15AM MDT

— Flooding is NOT expected today

A well-defined area of lift can be seen over Utah in the mid-level water vapor imagery below (orange “X”). As this feature moves eastward throughout the morning, it will help trigger more widespread scattered storms over the mountains this afternoon. Precipitation is expected kick off a little sooner today with plenty of moisture over the area, and storms should begin to pop over the mountains by midday. Steering flow remains light, but is slightly stronger than yesterday, so storms will slowly move to the east this afternoon. This will cause rainfall to spill into the adjacent eastern plains favoring the elevated ridges for accumulations once again.

PW has increased from yesterday and was measured at 0.60 inches in Grand Junction and 0.65 inches in Denver this morning. A slight increase in mid-level moisture is also forecast as the upper level wave arrives. With the majority of the moisture in the mid-levels again, high cloud bases will limit the amount of rainfall reaching the surface. However, totals are still expected to slightly rise from yesterday, but still remain well below flood threat criteria. Therefore, flooding is not forecast. Best chance for a rumble of thunder will be over the southern San Juan Mountains and Palmer/Raton Ridges where more daytime heating can build instability. The main threat from the weak thunderstorms that develop will be gusting outflow winds and lightning.

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

The majority of the rainfall this afternoon should occur south of I-70 once again, but an isolated storm or two may develop over the northern Front Range. Higher accumulations are expected over the Southeast Mountains and elevated ridges where isolated totals up to 0.50 inches (west) and 0.75 inches (east) are possible. Elsewhere, rainfall totals from the larger storms will generally be between 0.20 and 0.30 inches. Storms aren’t expected make it too far off the mountains, so the eastern plains should remain dry. The main threat from stronger storms that occur today will be gusting outflow winds, especially where virga is present, and lightning. Gusts up to 45 mph may be possible. Flooding is not forecast, and near surface smoke should clear out a bit this afternoon over the northern Urban Corridor.

Primetime: 12PM to 11PM

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

Best chance for measurable rainfall today will be over the high terrains south of I-70, and the most accumulation is again expected further south. Over the Central Mountains, rain totals up to 0.35 inches will be possible, and over the southern San Juan Mountains, isolated totals up to 0.65 inches will be possible. Best chance for a thunderstorm or two will be over the San Juan Mountains, and the main threats will be strong outflow winds and lightning. Light rainfall may fall over the valleys as storms move east, but plenty of virga is forecast as well. Over the western high terrains, totals should be between 0.10 and 0.15 inches. A second wave of energy may keep storms lingering over the mountains this evening, but the majority of activity should end just after midnight. Flooding is not forecast, and near surface smoke should improve throughout the day across the northern portion of these zones.

Primetime: 11:30PM to 1AM