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FXUS65 KBOU 090558 AFDBOU

Area Forecast Discussion National Weather Service Denver/Boulder CO 1058 PM MST Fri Dec 8 2023

KEY MESSAGES

- Snow and travel impacts increase toward evening as heavier snow becomes more widespread in/near mountains and foothills.

- Tight west-east gradient in snowfall expected.

- Strong winds in the mountains and foothills Sunday into Sunday morning.

- Scattered alpine snow showers most of next week, dry on the plains.

UPDATE
Issued at 722 PM MST Fri Dec 8 2023

Our winter system is progressing as expected so far. The southern foothills have been seeing steady snowfall for a few hours with estimated rates around 1"/hr. Elsewhere it has been a slower start with dry low levels, as dewpoints remain in the low to mid 20's, and even lower farther into the plains. That said, snow is filling in quickly from Loveland into Denver and it won't take long for slick conditions to develop as snow intensifies, with road temperatures having already fallen from the mid 40's to the mid 30's over the course of ~2 hours. Snowfall rates could approach 2"/hr in the heaviest bands late this evening, most likely south and west of Denver where northeasterly upslope flow will be maximized. The current forecast looks on track and headlines appear appropriate, so no changes are planned at this time.

SHORT TERM
(This evening through Saturday)
Issued at 246 PM MST Fri Dec 8 2023

Satellite shows convective showers developing over the mountains where a little sunshine was noted today. This is a sign of the amount of instability present in the atmosphere, and also gives us a hint of the potential snow rates for this evening. At the same time, current surface analysis also shows us how dry the low levels are on the plains, with dewpoints in the lower teens advecting in from the northeast behind an initial surge. That is where the forecast problem lies tonight with regard to how much of an impact the dry low levels will have on snowfall and it's eastward extent of the more significant accumulations.

What is agreed upon (as spoken to in the earlier discussion) is that enough upslope will be developing to focus the heavier snowfall in/near the Front Range Foothills and Palmer Divide. Even the vast majority of the mesoscale models now support that, as they've moved away from the localized downslope pushing the snow farther south and east on the plains. We'll have good QG lift through the evening, as evidenced by the upstream satellite showing a growing area of cooling cloud tops over much of Central Colorado, rotating slowly this way. Given the amount of instability, we think heavy snows are a good bet where upslope is focused and dry low levels are minimized, essentially the foothills from around Boulder southward to the Palmer Divide. We wouldn't be surprised that peak rates reach 1-2" per hour.
Therefore, we increased snowfall amounts in those areas based on the latest model output, upslope component, and ingredients including high lapse rates and expected good dendritic growth zone (DGZ). Meanwhile, the adjacent foothills (i.e. Boulder - western Denver suburbs) will also likely pick up appreciable snowfall of several inches in this setup, so added them to the Winter Weather Advisory. Based on experience and reinforced by high resolution model output, we do expect a sharp west-east gradient given the dry low levels and focus of upslope in/near the foothills. Thus, we think the eastern side of Denver won't see that much (maybe an inch or two), and even downtown could be limited in accumulations depending on exactly how sharp the west-east gradient is. A few miles could make all the difference. Meanwhile, farther north and east like Greeley, Fort Morgan, Sterling, and Akron may very well see nothing.

With regard to highlights...Since the impacts are expected after most of the commute for Denver and amounts are under traditional criteria, we'll keep the immediate Denver area out of the Advisory for now. The above could all change, yet, depending again on exactly how much dry low level air impedes the eastward progression. We could even end up with Warnings in the foothills/Palmer Divide if all comes together just right, so stay tuned for updates.

Snow will end from north to south across the plains and foothills late tonight, with partly cloudy to mostly sunny skies expected Saturday. It will be cold, with highs struggling into the 30s with cold advection and areas of fresh snow cover. Mountain areas will see scattered light snow showers linger through the day, with only shallow moisture present. It will also be windy mainly over the eastern plains, with gusts around 40 mph late morning through the afternoon.

LONG TERM
(Saturday night through Friday)
Issued at 145 PM MST Fri Dec 8 2023

Very strong north-northwesterly flow aloft behind the exiting upper trough moves eastward across the CWA Saturday night. Fairly strong northwesterly flow aloft is in place Sunday into Sunday night. It then becomes zonal Monday and Monday night. The QG Omega fields point to downward vertical velocity for the CWA Saturday night into Sunday. After that, weak synoptic scale energy is over the forecast area through Monday night.

There is some moisture embedded in the flow, especially to affect the mountains, Sunday into Monday morning. Along with the mountain top west-northwesterly winds, there should be scattered orographic snow showers. Concerning winds, they will get pretty strong later Saturday night into Monday morning with a mountain wave set-up indicated on the cross section. Areas from the divide eastward into the foothills could see high winds by later Sunday morning into Sunday evening. For temperature, Sunday's highs warm up 2-5 C from Saturday's readings. Monday's highs are similar to Sunday's which are near seasonal normals.

For the later days, Tuesday through Friday, models have the zonal flow decreasing on Tuesday, then becoming more southwesterly Tuesday nigh into Wednesday. From later Wednesday through Friday, the various models show a weak upper closed low somewhere over the western United States embedded in mean upper ridging. Model agreement is still poor with this feature. The amount of moisture varies between the models as well, with the overall picture showing mainly mid and upper level cloudiness through the period.
The lower levels are mostly pretty dry. With orographic help, scattered snow showers can be expected in the high mountains much of the week. The plains should be dry. Temperatures look to stay

AVIATION
(For the 06Z TAFS through 06Z Saturday night)
Issued at 1058 PM MST Fri Dec 8 2023

Snow will continue the next 2-3 hours at KDEN and KBJC. At KAPA, snow will linger a little longer ending around 11Z. VFR is expected to prevail 11-13Z, though there is a slight chance for sct-bkn025 or through 16Z. DEN and APA will be on the edge of the gusty northerly for late Saturday morning and afternoon. Gusts to 25 knots will be possible with stronger winds not far off to the east.

Snowfall amounts still look on track overnight with the most likely totals being;

KDEN: 1-3" KBJC: 2-5" KAPA: 3-5"

BOU WATCHES/WARNINGS/ADVISORIES
Winter Weather Advisory until 5 AM MST Saturday for COZ031-033- 034-036-039-041.

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