FXUS65 KBOU 111746 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 1046 AM MST Mon Dec 11 2023 .KEY MESSAGES... - Areas of light snow and freezing drizzle developing over the northern plains (mainly north of Denver) this evening and overnight - Limited travel impacts expected, except northern border area - Still some uncertainty around cutoff low through the Southern Rockies Wednesday - Wednesday night, but most impacts should stay south of the area. && .UPDATE... Issued at 1004 AM MST Mon Dec 11 2023 Forecast is moving along pretty much as expected, although there is more sunshine initially this morning. That may also allow us to warm a couple more degrees so have nudged the sky and temperature forecast accordingly for today. We're still keeping a close eye on the threat of precipitation and type tonight. There is an organized short wave noted in southeast Idaho, with fairly widespread snow, which is slowly tracking east/southeast. This appears to be along an area of weak frontogenesis and QG lift which is forecast to move across the Wyoming border area this evening. That should bring an dusting to two inches of snow to the far northern mountains, foothills, and Wyoming border area of the lower elevations. Farther south, moisture is far too limited. However, the precipitation in the north this evening combined with a backdoor cold front and shallow upslope could be just enough to moisten the low levels sufficiently for light freezing drizzle. With generally east or southeast flow, the highest odds/coverage of that should stay right next to the foothills roughly from Boulder northward to Fort Collins and the Wyoming border, as already indicated in the forecast. Air temperatures only slip just a few degrees below freezing overnight, and with quite warm pavement temperatures we think any impacts would be minimized with the exception being the northern Front Range Foothills and I-25 Corridor north of Fort Collins. Bridges and overpasses farther south to about Boulder could still be impacted however. That light freezing drizzle could last into Tuesday morning but any upslope forcing is really light. Points from Denver to the east and south would likely stay dry with only low clouds. && .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 18Z TAFS through 18Z Tuesday afternoon) Issued at 1046 AM MST Mon Dec 11 2023 VFR conditions will persist through 04Z-06Z with only increasing middle and high level cloudiness. We do think ceilings will lower to around 5000 feet AGL toward 03Z-04Z, and then drop to MVFR ceilings closer to 06Z-08Z, a little sooner at KBJC. With regard to precipitation, the airmass overall is quite dry in the Denver area, but enough weak upslope to support just a slight chance of flurries or light freezing drizzle once the lower stratus deck develops. KBJC would see a little higher chance and enough to warrant a VCSH there. With regard to winds, we're starting this period with southeast flow, eventually pushing more easterly through this afternoon (about a 30% chance we go more northeast). Then easterly should hold most of this evening until the backdoor cold front moves through and Denver cyclone matures 03Z-08Z. Winds will turn more north or northwesterly at the TAF sites through that time frame, and should hold there through much of Tuesday as long as the cyclone is displaced to the southeast as expected. && .BOU WATCHES/WARNINGS/ADVISORIES... None. && $$ UPDATE...Barjenbruch SHORT TERM...Entrekin LONG TERM...RPK AVIATION...Barjenbruch