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FXUS65 KBOU 080550 AFDBOU

Area Forecast Discussion National Weather Service Denver/Boulder CO 1050 PM MST Thu Dec 7 2023

KEY MESSAGES

- High winds in the Front Range, and elevated fire weather conditions on the plains come to end early this evening.

- Light snow showers tonight in the mountains, then more widespread snow, colder temperatures, and travel impacts Friday and especially Friday night.

- Considerable uncertainty to snow amounts. Higher totals favoring the mountains, southern foothills, and Palmer Divide.

UPDATE
Issued at 808 PM MST Thu Dec 7 2023

Let's be frank...he or she who says they are confident with regards to the snowfall forecast for the upcoming system is likely not being entirely honest. Over the past few hours, model guidance has spit out about as many different solutions as there are items on a Cheesecake Factory menu, particularly in regards to our second wave of snow Friday evening.

That being said, timing of potential impacts is the one aspect where confidence has managed to increase. For Friday morning, low-level flow will, for the most part, remain westerly. This drying downslope component is simply not favorable for impactful snow accumulations for the Denver metro, regardless of most other factors. This appears well reflected in the latest CAM output, suggesting minimal to nonexistent snow accumulations for the metro in the morning. The Palmer Divide may still squeeze out some minor accumulations, but really the focus for more widespread snow has shifted to Friday evening and night.

Mid-level lapse rates do look robust, between 7-8.5 C/Km, and generally collocated with the DGZ. Most guidance depicts heavier snow bands developing after ~6-7PM Friday *somewhere* in the plains or southern urban corridor, but the location varies wildly.
Our forecast continues to hedge towards our southern foothills and Palmer Divide being favored for the higher accumulations (2-7"), and with the possible contribution of shallow upslope flow in those locations, we opted to hoist a Winter Weather Advisory for these areas beginning Friday evening. That said, essentially nowhere is out of the woods quite yet as far as receiving a heavier band of snow.

SHORT TERM
(This evening through Friday)
Issued at 300 PM MST Thu Dec 7 2023

Most of the Front Range and plains were seeing gusty west winds this afternoon as the airmass became well mixed. There were still a few pockets south and east of Denver that were protected. The winds will be decreasing across the plains through early evening as we decouple and the low level gradient decreases. The mountain wave is still breaking in the foothills as evidenced by radar, but the signature has weakened a little this afternoon as we start to see some snow develop in the mountains. From our observations over the years, mountain snowfall typically disrupts the mountain wave, and thus we think the brunt of this event is over. We'll keep the High Wind Warning until 5 pm. So unless there's significant drying in the mountains behind the afternoon showers we should be able to let the High Wind Warning expire on time.

On to snow...One shallow wave of moisture and light snow showers as discussed above is moving through the mountains and will be mostly over toward dark. Some impacts have been noted in the Rabbit Ears Pass area this afternoon with snow covered roads/slick travel, but that will likely improve again this evening as we dry out again. Then, the next wave as seen in satellite imagery moving into Nevada. This will arrive late tonight and Friday morning. The lift will come in a couple notable features, the right rear entrance region of an upper level jet and rather strong frontogenesis as noted in the 700-500 mb layer. The latter can be a "sneaky" hazard here in the lower elevations of northeast Colorado, so will have to keep a close eye on that and a band of heavier snowfall possibly developing for the morning commute. It was interesting to see the mesoscale models/CAMs pointing that to our south over the Palmer Divide, while the conceptual model and synoptic scale models would show that farther north. Thus, we'll carry high PoPs up to about Denver and at least some light snow accumulations in the morning. Regarding impacts, as long as snow rates don't increase too much we'll likely be able to melt any snowfall (outside of bridges and overpasses) due to sufficiently warm road/pavement temps. The Palmer Divide, on the other hand would be more prone to travel impacts should a heavier band move across. We may need an Advisory to account for this (and the Friday night snow - more on that below), but uncertainty is rather high now so latter shifts will hopefully be able to narrow this threat down.

Finally, given the mountains will likely see an uptick in snowfall later tonight and Friday and impacts to the I-70 mountain corridor, we've opted to expand the Winter Weather Advisory to include the Front Range Mountains and Summit County as well.

LONG TERM
(Friday night through Thursday)
Issued at 211 PM MST Thu Dec 7 2023

Snow is expected to increase across The Palmer Divide, Foothills and adjacent plains Friday evening as the upper trough moves across the region and another surge sweeps across the plains. At this time, there is quite a bit of uncertainty with this system due to run to run inconsistencies, model discrepancies and mesoscale influences.
The biggest uncertainty is how the winds set up behind the frontal passage which will affect the amount of moisture and upslope flow across the Front Range Urban Corridor and nearby foothills. Some of the models show a downslope flow along the foothills into the Front Range Urban Corridor which would result in very little snowfall.
This scenario would focus the heavier snowfall to the east and southeast of Denver with little to no accumulation across the Front Range Urban Corridor. On the other hand, some models such as the ECMWF Hi-Res, RRFS A and UKMET models are showing deeper upslope flow which would result in higher snowfall amounts further west across the foothills and Denver metro. When looking at the SREF Plumes, the general consensus of the models is that the snow across the Front Range Urban Corridor and Palmer Divide will be somewhat light ranging between 1 and 2 inches. However, if deeper upslope flow develops, there is the potential for heavier snow amounts due to the presence of quite a bit of instability and QG lift. SREF Plumes are hinting at this with total amounts as high as 4.5 inches at DIA and 9 inches in the southern/western suburbs. These amounts seem somewhat overdone, however with this type of setup we can't completely rule it out. We've tweaked the forecasts to go middle of the road for now, and also increased the 90% snow probs to account for the potential for higher snowfall amounts. The snow is expected to decrease after midnight with some additional light accumulation possible over the Southern Foothills and Palmer Divide.

Across the mountains, snow and breezy conditions are expected to continue into Saturday morning. Therefore, the Winter Weather Advisory which was just issued by the short forecaster is expected to continue until daybreak.

As the upper trough pushes east into the Central Plains States, gusty northwesterly winds up to 45 mph are expected across the plains making Saturday a cold and blustery day.

Dry and warmer weather is expected Sunday and Monday as upper level high pressure briefly transitions across the Rocky Mountain Region.
On Sunday, windy conditons are expected across the higher mountains and foothills due to a strong northwesterly flow aloft and mountain wave.

For Tuesday through Thursday, models are showing another upper level storm system taking shape across the the Great Basin and Southern Rockies. At this time there great uncertainty on how this system will develop as there is no agreement between the models. For now, this system appears to be of little impact due to a lack of moisture, however time will tell.

AVIATION
(For the 06Z TAFS through 06Z Friday night)
Issued at 1034 PM MST Thu Dec 7 2023

There is still uncertainty in onset timing of snowfall.
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. Friday morning, the first wave of snow showers will occur near all terminals but moisture remain shallow. 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 "snow forcing" exiting 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 Friday to 5 AM MST Saturday for COZ036-041.

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