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FXUS65 KBOU 020309 AFDBOU

Area Forecast Discussion National Weather Service Denver/Boulder CO 809 PM MST Fri Dec 1 2023

KEY MESSAGES

- Periods of snow and blowing snow will continue this evening through Saturday.

- Snow and winds increase Saturday night with areas of moderate to heavy snow and significant blowing snow creating very difficult travel Saturday night and Sunday morning.

- Strong winds are expected to develop across the mountains and higher foothills Saturday afternoon into Sunday, with a few gusts greater than 75 mph possible.

UPDATE
Issued at 802 PM MST Fri Dec 1 2023

Scattered snow showers have been floating across the Palmer Divide late this afternoon, and are moving east into the plains as they dissipate. A few spots in Elbert and Lincoln Counties may see a dusting to an inch of accumulation where showers are most persistent, and have thus increased PoPs and QPF accordingly.
Otherwise, minimal changes made, with the below forecast on track.
We remain in store for an overall highly beneficial mountain snow event this weekend, helping boost snowpack numbers, which have been somewhat slow to climb so far this season.

SHORT TERM
(This evening through Saturday)
Issued at 223 PM MST Fri Dec 1 2023

A cool but quiet start to the weekend is expected across the plains and Denver metro...with a different story for the high country.

Most of the plains will be cool but dry on Saturday as the flow aloft increases. Temperatures should warm up a bit compared to today with highs in the low to mid 40s. We should also see gusty winds develop through the day as we mix into the strengthening mid-level flow.

Across the high country, only a few light snow showers remain this afternoon, in a lull between shortwaves. There should be an increase in snow coverage through this evening as moisture increases ahead of our next shortwave trough traversing the area, though the heaviest snow will hold off until after midnight.
Orographic snow showers will continue through the day Saturday with additional light snowfall accumulations likely. Have opted for a Winter Weather Advisory tonight through Saturday evening due to the combination of snow and blowing snow across some of the busier travel corridors in the mountains, with a Winter Storm Warning over the Park Range where heavier snow is expected.

Winds will also increase across the higher elevations Saturday afternoon and evening behind the passage of the shortwave. Winds will generally remain in the high country during the early afternoon, as gusts will largely be mechanically mixed. As subsidence increases in the wake of the shortwave, we should see some fairly impressive mountain wave amplification late Saturday into Sunday. Guidance differs on the timing of this, but model cross-sections and Sangster data largely support the potential for wind gusts >75 mph. A High Wind Watch goes into effect Saturday afternoon... which will continue into Sunday.

Both the snow and wind will continue through the weekend...
continue reading the Long Term discussion below for additional details.

LONG TERM
(Saturday night through Friday)
Issued at 223 PM MST Fri Dec 1 2023

A second and stronger short wave trough embedded in the the northwest flow aloft will dive southeast across the Northern and Central Rockies Saturday night and Sunday. The 12Z model runs are stronger with this system than previous runs, and thus have higher snowfall amounts. Ridge top winds reach 45 to 60 knots Saturday night and then slowly decrease Sunday afternoon through Sunday night. This will lead to good orographic lift in addition to the lift from the short wave. Expect a period of moderate to heavy snow late Saturday night and Sunday morning to accompany the wave as it passes overhead. It will also become very windy, leading significant blowing and drifting snow over the mountain passes.
Travel in the mountains late Saturday night through Sunday morning is expected to become very difficult. Best to avoid traveling in the mountains during this time. We decided to issue a Winter Storm Warning for all of our mountains due to the blowing snow and increased snowfall amounts. For the mountain valleys, a winter weather advisory was issued where travel conditions will still be hazardous, but not as bas as over the mountains. Snowfall amounts for this storm look to be 1 to 2 feet for the mountains near Rabbit Ears Pass, with 6 to 14 inches for the rest of the mountains of north central Colorado. The mountains valley are expected to see 4 to 10 inches with the higher amounts expected to be found from Frasier to Grand Lake.

East of the mountains, high winds to 80 mph will be possible in the foothills. A High Wind Warning was issued beginning Saturday mid afternoon. Appears a mountain wave sets up Saturday afternoon and evening. The threat for high winds will continue through Sunday morning. Even though the mountain wave breaks down Saturday night, Strong subsidence once the short wave passes will help transport stronger winds aloft to the foothills. Even with strong downslope winds, there will be a chance a little snow survives onto the foothills and near by plains. Any accumulation is expected to be light and less than an inch.

For next week, ridging spreads across the intermountain west Monday. This will cause northwest flow aloft over Colorado to slowly decrease through the day. Light snow should linger in the northern mountains and end late in the day. Elsewhere, dry and mild conditions will prevail. Highs look to be in the upper 50s to around 60. Warming trend continues into Tuesday and Wednesday with ridging now over Colorado. Highs will push well into the 60s by Wednesday. The ridge flattens Thursday with west to southwest flow aloft over the Central Rockies. This will keep mild temperatures in place, though a weak cold front could knock temperatures down a notch Thursday and Friday. No rain or snow is expected for the upcoming week through Friday once the mountain snow ends on Monday.

AVIATION
(For the 00Z TAFS through 00Z Saturday evening)
Issued at 413 PM MST Fri Dec 1 2023

Main concerns will be briefly lowered ceilings (somewhere between 6000-8000ft AGL) late this afternoon and evening, and stronger winds late Saturday morning into Saturday afternoon. Current easterly winds will become more southerly this evening, with a slight increase in speeds however largely remaining at or below 10 kts.

VFR conditions are expected tonight and Saturday. Guidance is in good agreement in mixing stronger west-northwesterly flow to the terminals near 18z Saturday, continuing into the afternoon/evening.

BOU WATCHES/WARNINGS/ADVISORIES
Winter Weather Advisory from 8 PM Saturday to 11 PM MST Sunday for COZ030-032.

Winter Storm Warning until 11 PM MST Sunday for COZ031.

Winter Weather Advisory until 8 PM MST Saturday for COZ033-034.

Winter Storm Warning from 8 PM Saturday to 11 PM MST Sunday for COZ033-034.

High Wind Watch from Saturday afternoon through Sunday morning for COZ035-036.

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