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FXUS65 KBOU 030651 AFDBOU

Area Forecast Discussion National Weather Service Denver/Boulder CO 1151 PM MST Sat Dec 2 2023

KEY MESSAGES

- Mountain snow and winds will increase tonight.

- Moderate to heavy snow and significant blowing snow are expected to produce very difficult travel conditions in the mountains tonight and Sunday morning.

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

- Late Monday through Thursday will be mild and dry.

- The next system to potentially impact Colorado will arrive late Thursday night through Saturday. Mountain snow and colder temperatures are likely but impacts across the plains remain uncertain at this time.

UPDATE
Issued at 804 PM MST Sat Dec 2 2023

A strong northwesterly flow aloft is over the forecast area, between one system exiting into the Central Plains, and another approaching from the northwest. Currently, we are under a regime of weakening QG descent in the wake of the first system. Latest highway cameras are filling up with snow this evening. That next system will shift eastward overnight as weak to moderate mid/upper level QG ascent develops overnight, coupled with strong orographic ridgetop winds. The nose of a 110 kt upper level jet sits over northwest CO by 12z Sunday, then translates eastward as well. Strong downslope winds still expected to develop this evening, with the best wave amplification until 09z, before it starts to shear out and break down. For Sunday, strong bora wind coupled with the upper level jet, and another dose of mid/upper level QG descent that will continue into Sunday afternoon. Overall no changes to the ongoing highlights, with the heaviest snowfall expected to occur from 06z tonight through 18z on Sunday.

SHORT TERM
(This evening through Sunday)
Issued at 236 PM MST Sat Dec 2 2023

Water vapor satellite imagery showing this morning's trough spinning over western Nebraska at this time. Best subsidence is over eastern Wyoming and northeast Colorado with the mountains seeing more favorable conditions for snow. Looking at a variety of data (satellite, radar, observations, web cameras) appears snow has mostly ended or decreased to flurries in the valleys with light snow lingering over the mountains. Most roads have at least partially thawed in the mountains due to insolation and light snowfall rates. This is expected to quickly change by early evening with the loss of sunshine and snowfall increasing. The next wave which will be stronger is off to the northwest over the Pacific Northwest. Widespread clouds and snow are ahead of it, with radar showing widespread snow over northern Utah and eastern Idaho. Appears we are still on track heavy snow and windy conditions in the mountains late tonight and Sunday morning.
Travel is expected to become very difficult in the mountains by Sunday morning due snow and blowing snow with only a slow improvement through the afternoon. Cross sections show moisture slowly shallows through the afternoon which should cause snow to become light, though they may be another uptick in snow Sunday evening.

For areas east of the mountains, windy conditions are expected tonight and Sunday. The HiRes models show a surge of stronger winds traveling down the foothills this evening, likely associated with a mountain wave, which doesn't last long. Windy conditions, possibly strong, reform (or could continue through the night due to strong winds just off the surface) Sunday morning as the trough pushes through the region. The strong winds are expected to continue in the higher foothills and mountains through Sunday evening. A few snow showers could move off the higher terrain Sunday morning. If they survive, snowfall will be light with only a dusting at most expected.

LONG TERM
(Sunday night through Saturday)
Issued at 236 PM MST Sat Dec 2 2023

A shortwave trough will be exiting Colorado quickly on Sunday night as it heads southeastward. On the backside of this trough there will be a wave of deeper moisture that moves across the northern mountains. This will re-intensify snow showers in the mountains and moderate snowfall will occur in the Park Range and northern Front Range mountains. Winter weather highlights continue through 11pm Sunday night and it is possible the highlights could be extended a few more hours. Warm air advection will be ongoing in the low levels Sunday night. This will strengthen an inversion over the foothills and adjacent plains and will likely allow for a stronger mountain wave to develop and for winds aloft to be pulled towards the surface in the foothills and adjacent plains. A High Wind Warning was issued for the foothills and depending on the strength of the mountain wave, it could be extended to last through much of Sunday night. Winds gusts up to 70-80 mph can be expected.

Strong northwesterly flow aloft will continue on Monday with drier air moving into Colorado. Snow showers will come to an end by the afternoon and winds will begin to relax during the evening hours.
The warm air advection will allow for temperatures to warm to the mid to upper 50s across the plains.

Colorado will be rid of the troughing and northwesterly flow on Tuesday as a board ridge aloft moves overhead. This will lead to mild and dry conditions with much lighter winds than previous days. Highs will reach the 60s in the spots across the plains.

The ridge will continue to impact weather across Colorado on Wednesday and Thursday. This will lead to gorgeous weather for early December as highs across the plains will reach the mid to upper 60s.
The record highs for Denver on Wednesday and Thursday are 73 and 70, respectively. Those will likely be be just out of reach but the Thursday one could be a close call.

Models are in agreement that a trough will move over the Intermountain West on Friday and towards Colorado on Saturday. What models don't agree on is the intensity and exact location of this trough. Ensembles show that there is a good chance the plains will see some snow with this system but they are all over the place with regards to amounts. This system will have to be watched as there is a small chance it brings a widespread, moderate to heavy snow. There are two things that models have higher confidence with for this system. Those are a healthy snow for the mountains, and colder weather for all areas with the single digits to teens expected.

AVIATION
(For the 06Z TAFS through 06Z Sunday night)
Issued at 1144 PM MST Sat Dec 2 2023

VFR through Sunday. West to northwest winds at 10-20 knots are expected to develop at KDEN/KAPA around 13z, transitioning to stronger northwest winds with gusts to 25 knots by 18z. There is a chance of brief snow showers Sunday morning. At KBJC stronger west winds are expected, with occasional gusts in the 30-40 knot range, especially Sunday morning. The winds will decrease in all areas in the 22z-24z time frame.

BOU WATCHES/WARNINGS/ADVISORIES
Winter Weather Advisory until 11 PM MST Sunday for COZ030-032.

Winter Storm Warning until 11 PM MST Sunday for COZ031-033-034.

High Wind Warning until 11 PM MST Sunday for COZ035-036.

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