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FXUS65 KBOU 100625 AFDBOU

Area Forecast Discussion National Weather Service Denver/Boulder CO 1125 PM MST Sat Dec 9 2023

KEY MESSAGES

- High winds up to 80 mph in the higher mountains and Front Range Foothills on Sunday.

- Areas of blowing snow along Highway 93, between Golden and Boulder, a concern late tonight and Sunday

- Light snow mountains through Monday, possibly spilling onto northern plains Monday night

- Still uncertainty around cutoff low through the Southern Rockies Wednesday, but most impacts should stay south

UPDATE
Issued at 723 PM MST Sat Dec 9 2023

Not much change to the forecast at this hour. Minor adjustments for slower clearing of the clouds upstream of the Front Range and a pocket of drier air from the central mountains out to around Limon. It still looks like foothills winds will really kick up around 12z, though there will be some increase before then.

SHORT TERM
(This evening through Sunday)
Issued at 258 PM MST Sat Dec 9 2023

Scattered radar returns over northern Weld and Morgan counties this afternoon, but likely more virga vs showers. This will taper off by late afternoon. For tonight, some lingering snow showers over the higher northwest facing slopes but dry otherwise. For Sunday, we have opted to upgrade the High Wind Watch to a High Wind Warning for the Front Range mountains and foothills. NAM12 spatial cross-sections still show a favorable profile for an amplified mountain wave, with the strongest downward transport from 15z to 21z. the models do show an increase in the depth of the moisture layer west of the Continental Divide after 21z, which may help decrease the magnitude of the gusts late in the day. The Sangster/Pressure Gradient still showing warning criteria winds at 12z and 18z, so will keep the timing of the highlight as is, 5 am to 5 pm on Sunday. Increasing moisture west of the Continental Divide will translate to an increase in orographic snowfall along the higher west and northwest facing slopes in the afternoon, but primarily light snow. At the base of the foothills, strong winds will produce areas of blowing snow along Highway 93, between Golden and Boulder. This will result in poor visibility and slick/slushy spots along the windier sections of the highway.
Gusty northwest winds across the plains as well, with gusts to 35 mph possible along the northern tier zones. High temperatures on Sunday will climb back to near 50 in the windier areas, a little cooler along the South Platte River Valley.

LONG TERM
(Sunday night through Saturday)
Issued at 258 PM MST Sat Dec 9 2023

Strong westerly flow aloft and the mountain wave is expected to weaken Sunday evening. This would occur as the low level gradients relax and mountain top stable layer weakens. Light snow in the mountains will also aid mountain wave breakup. We expect a dusting to 3 inches in the high country, given fairly shallow moisture profile.

On Monday, we'll see some flat ridging and thus most of the light snow in the mountains should be existing or pushing to the border area. By Monday night, there is good agreement that a backdoor cold front slips across the plains with shallow upslope flow.
Overall, there is limited moisture, and depending on the amount of low level moisture could set the stage for a bit of light snow or even some light freezing drizzle on the plains given the thermal structure. We've cooled off temperatures a few more degrees for Tuesday considering the shallow cold airmass which will likely be locked in across the plains.

By Wednesday and Thursday, we'll focus on the cutoff low which is expected to move into the Desert Southwest and across the Southern Rockies. Overall the track keeps most snow to the south of our forecast area, but there are just a few outliers that would be farther north. We didn't have access to the WPC Cluster analysis which would have been beneficial to diagnose this closer, but it appears most solutions would keep the threat of any snowfall over the mountains and southeast/east central Colorado plains, and any snow would be light. We'll keep an eye on that should anything change.

Beyond that into next weekend, a couple weak waves in west/northwest flow would be possible but overall confidence in timing and location of any disturbances is low. Forecast will be close to climatology in this situation.

AVIATION
(For the 06Z TAFS through 06Z Sunday night)
Issued at 1125 PM MST Sat Dec 9 2023

VFR conditions to prevail through Sunday. Highs clouds to increase overnight and persist through Sunday. After the southerly drainage winds at DEN and APA, west-northwest winds will develop around 18Z. Gusts to 30 knots will be possible, through late afternoon 22-23Z. At BJC, west winds are expected to be stronger with gusts to 40 knots possible. HiRes models show these stronger winds may not be widespread with some eddies producing weak easterly winds.
To sum it up, wind forecast confidence is low with the gusty west-northwest winds the most likely scenario, but far from certain.

BOU WATCHES/WARNINGS/ADVISORIES
High Wind Warning from 5 AM to 5 PM MST Sunday for COZ033>036.

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