FTB 09-23-2019: Cool Morning to Start Fall with Mostly Dry Conditions Forecast

Issue Date: Monday, September 23rd, 2019
Issue Time: 9:10AM MDT

— Flooding is NOT expected today

Fall officially began early this morning, and cooler morning temperatures certainly made it feel more seasonable. Lows dipped into the 30°Fs over the northwest corner (lower elevations) and San Luis Valley again with Denver bottoming out at 48°F. Highs should rebound quite a bit today with a building ridge and drier air mass overhead. There’s some morning cloud cover over the state associated with some energy moving through the southwest flow aloft. Expect this cloud cover to continue on and off today with only clear skies forecast for the far southeast corner. The upper low over Nevada will dig south throughout the day and be cut off from the main flow by early tomorrow morning. This feature is forecast to sit south and west of the state, which should keep a drier air mass in place for the majority of the state. Thus, rainfall is not anticipated for the northern 2/3 of the state.

Over the southern 1/3 of the state, moisture associated with the mid-level energy below (orange “X”) is forecast to move in, so some isolated showers and weak thunderstorms will be possible over the southern Southwest Slope and San Juan Mountains this afternoon. This vorticity max will move eastward this evening, so storms are expected to spread into the San Luis Valley and maybe even the Southeast Mountains by morning. With instability dwindling after sundown, rain rates will really start to drop off, so the accumulating rainfall might not make it all the way east. A light dusting of snow may also be possible for the highest elevations of the eastern San Juan Mountains.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. For Zone-Specific forecasts, scroll below the map.

Zone-Specific Forecasts:

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

Best chance for rain this afternoon and evening will be over southern tiers of these regions. Highest totals are forecast for the eastern San Juan Mountains and eastern Southwest Slope due to a bit more instability and moisture available during the afternoon. Max 1-hour rain rates up to 0.2 inches will be possible with most totals below this threshold. Some light showers may also occur over the San Luis Valley as this feature moves east, but totals should remain under 0.1 inches with more mist and cloud cover than measurable rainfall. Can’t rule out a light dusting of snow over the highest elevations of the San Juan Mountains later tonight, but large accumulations are not expected. Low temperatures should also be warmer, so freezing temperatures should be avoided in the San Luis Valley.

Primetime: 2PM to 4AM

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

Highs this afternoon will rebound into the 80°Fs with some upper 80°Fs over the Southeast Plains where skies remain clear. The drier air mass overhead and inversions in the upper atmosphere will prevent accumulating rainfall, though on and off cloud cover can be expected with the shortwave. The 50°F dew point line in the visible satellite imagery above is to the east of the area, which is usually a good marker for rainfall chances. Should be a beautiful day with calm winds except over the far Southeast Plains where some breezy southerly surface winds are expected (15 to 20 mph).

Central Mountains, Northern Mountains, Northwest Slope, Grand Valley:

These areas should remain rain-free as well with dry air in place and a building ridge. Highs will barely reach 80°F over Grand Junction with 70°Fs forecast for the other valleys. Still a little warm for this time of year, but appreciated after the late hot streak. Lows should be about 5°F warmer than last night, so not thinking any Freeze Watches will need to be issued for the lower elevations.