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FXUS65 KBOU 081649 AFDBOU

Area Forecast Discussion National Weather Service Denver/Boulder CO 949 AM MST Fri Dec 8 2023

KEY MESSAGES

- Snow and travel impacts increase toward evening

- Keeping a close eye on how far east heavier snow progresses, with Denver metro area on edge of this storm.

- Heaviest period of snowfall this evening.

UPDATE
Issued at 948 AM MST Fri Dec 8 2023

The banded snow developed on the Palmer Divide early this morning, but broke up a little since. There will still be banded precip working toward the southeast across the Limon area and Lincoln county into early afternoon, but the rest of the area is in a rather pronounced dry pocket. Road temperatures are warm enough to limit any travel impacts.

The main concerns are still for snow and travel impacts late this afternoon and evening. Models have come into much better consensus and the convective allowing models have come back to reality with regard to location. Earlier runs back to yesterday insisted we would downslope locally near the Front Range, but appears most synoptic scale models were much better focusing closer to Denver metro, the Palmer Divide, and Southern Front Range Foothills as persistent mid level QG forcing dominates the eventual evolution of this snow event.

As a result, our confidence has certainly increased for north/northeast upslope derived snowfall, as shown in the updated PoP forecasts. We'll have mostly dry conditions through mid afternoon, but then snow increases in the mountains first and then starts to spread eastward into the Foothills and I-25 Corridor late this afternoon and moreso this evening. We think most of the commute in Denver (at least the first half through 5 pm) will be fine, but then snow intensity and coverage is likely to increase 6-7 pm as we start to bend winds into the foothills from the north/northeast. Meanwhile, the foothills will be an hour or two earlier. The airmass is quite unstable with initial lapse rates of 7-9 C/km! Thus, locally heavy snowfall is expected and road conditions will be changing rapidly.

The main question is how far east this makes it, and that eastward progression will likely be driven by amount of drying in the low levels. It's not very moist, that's for sure, with most upstream dewpoints only in the mid teens. In many cases this will result in a sharp west-east gradient in snowfall amounts and this case should be no different. Thus, the western suburbs of Denver are shaping up for a healthy snow while northeast side of Denver toward Fort Collins could get only a light amount, and Greeley may very well see nothing. We may need a Winter Weather Advisory expansion into parts of Denver. But with the forecast literally coming down to just a few miles, we'll look at all of the latest 12Z data and ensembles before making that decision toward mid afternoon. Stay tuned.

SHORT TERM
(Today and tonight)
Issued at 430 AM MST Fri Dec 8 2023

GOES mid level water vapor displays the first batch of moisture along the western slopes. A few METARs present light to moderate snow occurring throughout Clear Creek, Grand, and Summit counties.
We're getting a buy one, get one free for deal for snowfall today. A few mesoscale/synoptic features are causing uncertainty snowfall amounts and may lead to higher snowfall totals across the southern foothills and Palmer Divide.

Firstly, surface temperatures today remain slightly warm across the plains with a few areas reaching the lower 40s this afternoon. Skies will likely remain mostly cloudy across the region.

An upper level jet with the a vort max near the entrance region arrives this morning over our CWA between 12Z-15Z. This incoming trough axis does begin to drop too far south but will provide westerly flow to increase orographic lift across the mountains. Once the first wave pushes east of the mountains this morning, snow becomes spotty across the plains with little accumulations between trace to a half an inch on grassy surfaces. Next, a few model solutions indicated weak frontal system this morning pushing south from the Wyoming border to the Palmer Divide. Given the increase in frontogenesis, 200-300 J/kg of CAPE and cooler temperatures across the Palmer and southern foothills, this may lead to banded snowfall beginning as early as 8-9AM.

Light to moderate snowfall will continue in the mountains throughout the afternoon. By late afternoon, temperatures will be cooler and our entire region should be cool enough for snow. South of Denver, such as Elbert, Douglas, and parks of Lincoln counties remain favorable for jet induced snow bands. As the trough axis deepens, northerly flow will provide favorable upslope for the Palmer Divide and a few cities south of Denver to receive periods of moderate snowfall. Forecasted snowfall totals for the Palmer Divide and parts of the southern foothills range from 3-6 inches but if snow bands persists through Saturday morning, totals could reach 7-8. Thus, the Winter Weather Advisory remains in effect through 5 AM Saturday.
Elsewhere across the lower elevations, expect a slick commute this evening given that will be the heaviest period of snowfall. Light accumulations of 1-3 inches, with the largest totals south of Denver, are possible for the plains through Friday night.

LONG TERM
(Saturday through Thursday)
Issued at 430 AM MST Fri Dec 8 2023

North-northwest flow aloft will prevail behind the exiting system for Saturday. May see light snow linger over the Palmer Divide until around sunrise. Cross sections show moisture remains somewhat trapped over the mountains, with cloudy skies and pockets of light snow. Ridge top winds won't be very strong Saturday, this should keep any orographic produced snowfall light. Over the eastern plains, gusty north winds up to 45 mph and temperatures in the mid to upper 30s will make for a chilly day.

For Saturday night and Sunday, northwest flow aloft ramps up while a ridge builds over California and Nevada. Temperatures will warm into the mid to upper 40s across northeast Colorado. It is looking like a windy day for the foothills and mountains. High winds will be possible Sunday morning when a mountain wave is expected to form. Model run-to-run consistency has been poor the last few days, including for Sunday, so will hold off from issuing a High Wind Watch for now. For the mountains, more light snow is expected. The stronger ridge top winds will lead to better orographic lift, but lapse rates and moisture are not that great.
Snowfall again will be light where it occurs with only up to an inch or two possible.

The ridging over the western states doesn't last long and flattens by Monday as a trough moves onto the Pacific coast. Mild and mainly dry conditions are expected for Monday, though can't ruled out a little light snow for the mountains. The trough intensifies into a closed low somewhere over the west by late Tuesday. For Wednesday and Thursday, models are showing something different regarding the closed low each run, so confidence is low in the forecast Tuesday through Thursday. Temperatures are expected to cool back to near normal. As far as precipitation goes, will keep PoPs fairly low for now with only a chance/slight chance for snow in the mountains.

AVIATION
(For the 06Z TAFS through 06Z Friday night)
Issued at 430 AM MST Fri Dec 8 2023

There will be two separate waves of snowfall during the TAF period. Visibilities will likely remain at VFR conditions through 21Z Friday afternoon. As southeasterly winds continue at KDEN and KAPA, cloud decks of SCT/BKN could lower between 5-8k ft. This morning, the first wave of snow showers will occur for all terminals with little to no accumulation for KDEN and KBJC. KAPA could experience a trace to a half an inch this morning through mid afternoon.

Given the nature of this system, there is higher confidence that VFR conditions will continue but there could be a greater chance of lower cigs near 2-5k. By Friday evening 00-02Z the next batch of moisture arrives. There is higher confidence that this round will produce snowfall accumulations on surfaces and lower visibilities near MVFR conditions through 06-07Z. Expecting visibilities to bounce back to VFR due to our forcing exits the region and winds remain elevated between 10-14kts keeping fog development away.

BOU WATCHES/WARNINGS/ADVISORIES
Winter Weather Advisory until 5 AM MST Saturday for COZ031-033- 034.

Winter Weather Advisory from 8 PM this evening to 5 AM MST Saturday for COZ036-041.

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