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FXUS65 KBOU 111209 AFDBOU

Area Forecast Discussion National Weather Service Denver/Boulder CO 509 AM MST Mon Dec 11 2023

KEY MESSAGES

- Light snow in the mountains through this evening, possibly spilling onto northern plains tonight with a little freezing drizzle as well (mainly north of Denver).

- Still some uncertainty around cutoff low through the Southern Rockies Wednesday - Wednesday night, but most impacts should stay south of the area.

SHORT TERM
(Today and tonight)
Issued at 215 AM MST Mon Dec 11 2023

Water vapor imagery showing a persistent and moderate northwest flow pattern across Colorado with a steady stream of mid/high level moisture. There is some orographic impact for the mountains with enough low level moisture for occasional light snow over the mountain passes. Observations and web cams showing a general 1-3 inches of new snow has fallen since this afternoon. Given the moisture and upstream disturbance, far northern mountains could see up to 3 inches of additional snowfall.

Water vapor imagery also showing next weak wave now moving across far northern California and northwest Nevada at this time. This feature is progged to move across Northern Colorado later this afternoon and this evening. Ahead of the wave, enough mid/high level moisture to keep at least partly cloudy skies ongoing today across lower elevations. Temperatures today will be similar to Sunday's readings, except a bit cooler over far northern sections.

There is some weak QG ascent tonight, especially over the far northern border areas with Wyoming and Nebraska. There is a weak associated cold front which will backdoor into the Front Range overnight tonight with some shallow upslope flow developing.
Airmass saturates in the low levels, then becomes more shallow overnight and into Tuesday morning. This means a mixed bag of precipitation types with possible rain/snow in the evening, leading to some freezing drizzle later in the night as moisture depth becomes more shallow. Forcing and moisture will favor precip chances over Larimer and Northern Weld counties, mainly staying north of Denver. In addition, the southeast winds will favor additional lift over Larimer and Weld counties with the better chances for precipitation. Mountain moisture decreases tonight as the wave moves east of the mountains with drying expected west of the Continental Divide.

LONG TERM
(Tuesday through Sunday)
Issued at 215 AM MST Mon Dec 11 2023

An upper level trough will move into the Great Basin on Tue with SW flow aloft across the area. At the sfc, cold high pres will be over the Central Plains. Soundings show some potential for light freezing drizzle thru mid morning on Tue from Denver north to the WY border and in the nrn foothills. Otherwise cross-sections show mid and higher moisture embedded in the flow. Could see a few snow showers over the higher mtn peaks but the rest of the area should be dry in the aftn. Highs will be in the mid 30s to lower 40s across the plains.

For Tue night into Wed, the upper level trough will move SSE from the Great Basin into the swrn US. This will allow for a more southerly component to the mid level flow. Cross-sections show abundant moisture from 700 mb to 400 mb, however, orographics are unfavorable over the higher terrain so any snow shower activity that does occur will be very light. Across the plains, the low level flow will be southeast. Can't rule out a slight chc of rain or snow showers over the far sern portions of the CWA by aftn, however the rest of the plains will be dry. Highs over nern CO will be in the upper 30s to lower 40s.

Looking ahead to Wed night and Thu, the GFS tracks an upper level low from sern AZ across srn NM with most of the precip staying south of the area. The ECMWF has a slightly further northward track across central NM which allows some influx of precip across sern areas of the CWA mainly Wed night. Still not sure about the exact track of the low so will leave in 30%-50% pops over sern areas.
Highs on Thu will be near seasonal normals.

For Fri into Sat an upper level ridge will begin to build into the area. This will lead to a dry pattern with warmer temperatures.

AVIATION
(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 503 AM MST Mon Dec 11 2023

VFR through early this evening with increasing middle and high level cloudiness. Light winds this morning with a light east to northeast directions in the afternoon. Weak cold front will bring in a lowering cloud deck late Monday evening and overnight with MVFR ceilings developing. There is a slight chance of a flurry or patch freezing drizzle at KDEN/BJC late tonight but confidence is low on this developing.

BOU WATCHES/WARNINGS/ADVISORIES
None.

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