textproduct: Western and Central Wyoming

KEY MESSAGES

- Weather system exits the area today, but a few more flakes of snow may fly before it does so.

- A parade of weather systems march through the region through early next week. Impressive snowfall totals are likely (80%) for western mountains/valleys.

- Temperatures remain above normal east of the Continental Divide through Sunday, but drop back to normal next week. Temperatures will remain slightly above normal west of the Continental Divide through Sunday and close to normal next week.

UPDATE

Issued at 1245 AM MST Thu Dec 26 2024

A mostly zonal pattern will start today, with several embedded shortwaves bringing rounds of snow to western portions of the forecast area. This will be the beginning of a atmospheric river event (this is reflected by higher than normal PWAT values), as a strong area of high pressure will be in place over the Pacific off the Mexican/California coasts. The current Winter Storm Warnings and Winter Weather Advisories still begin at 12Z this morning. Will keep the previous forecast discussion, as most of the details remain largely unchanged through Friday night. Will update sections as needed.

DISCUSSION

Issued at 113 PM MST Wed Dec 25 2024

There will be a stark contrast in weather conditions west of the Continental Divide compared to east of the Divide for the next 5 days. An onslaught of weather systems will bear down on western portions of the area tomorrow through Monday. For western mountains and valleys, snow will be falling/accumulating consistently from Thursday afternoon until Monday afternoon, with little breaks in between. Temperatures look to warm enough Sunday afternoon (highs upper 30s) for a possible (40%) mix of snow/rain in Jackson Valley/Star Valley. This is likely (70%) to limit snowfall accumulations Sunday afternoon in these valleys.

This weather pattern makes it difficult to message impacts, because of the persistent snowfall expected for a long duration (84+ hours). Will now discuss in detail winter highlights and the complexities at play with this pattern. Let's start off with moisture since this is the slam dunk part of the forecast. Taking a look a PWAT's, it is no surprise snowfall amounts will be high from this long duration winter event. On average, PWAT's will be around 0.3-0.5" through Monday morning (0.3" is in the 90th percentile). Of note is a noticeable decrease to less than 0.2" Thursday night and this coincides with a lull in snowfall rates/accumulation for a 6-12 hour period Thursday night. For the remainder of the period from Friday through Monday, moisture will be plentiful (PWAT's 0.3-0.5") across the area. One may surmise that this would lead to substantial snowfall accumulations across the forecast area, but the issue is each weather system will be progressively moving across the area from west to east, so mountain shadowing/downsloping will be prevalent. This will keep most of the low-level moisture east of the Continental Divide as lee- side downsloping dries/warms the low- levels. The opposite will occur on the windward (west) side of the mountains, with persistent moist upslope flow dumping all the moisture/snow west of the Continental Divide.

For winter highlights decided to go with Winter Storm Warnings for the Tetons/Gros Ventre, Salt River/Wyoming Range, and Star Valley. The two aforementioned mountain zones are slam dunks with snow totals from a foot to 2 feet. Star Valley is marginal, but it's likely (70%) the foothills will receive over 8" and most of the valley at least 6", so could see moderate travel impacts there. Went with Winter Weather Advisories for the Jackson Valley and YNP where snowfall amounts are likely (70%) to be just below Warning criteria (8"). There will be higher amounts around a foot around South Entrance in YNP, but the rest of the Park will only be about 3-6". All of these highlights are in effect from 12Z (5 AM) Thursday to 06Z Saturday (11 PM Friday). There will be new highlights issued for these same zones once the current highlights expire at 11 PM Friday night as snow will continue and become heavier this weekend.

Snowfall ratios will be fairly high as snowfall efficiency will be enhanced due to favorable temperatures aloft. The dendritic growth zone is likely (80%) to be deep and well within the cloud layer for the majority of the period (Thursday-Monday). The exception is on Sunday as WAA increases and brings in warmer air that will drop ratios for a good 12 hour period. The most logical timing for Winter highlights is to keep the timing the same (5AM Thu-11PM Fri) for the first winter event and have a second set of highlights for the second event from Saturday through Monday. For all the ski lovers out there, this will lead to a surplus of fresh powder over this holiday weekend. Wind will be a key factor through this weekend as well, although the strength of the wind has trended slightly weaker with the past couple of model runs. There certainly will be drifting of snow and blowing snow at times from Thursday through Monday. The windiest time period will be Saturday through Monday.

Beyond Monday it will be cooler and mostly dry as the strong weather systems push east. The upper-level pattern should become more zonal and moisture is expected to drop substantially, but some lingering snow showers remain possible (40%) across western WY through the middle part of next week. By the latter half of next week uncertainty in the weather pattern grows considerably, so will have to wait and see what awaits then.

AVIATION /06Z TAFS THROUGH 06Z FRIDAY/

Issued at 1000 PM MST Wed Dec 25 2024

West of the Divide (KBPI, KJAC, KPNA, and KRKS) Terminals:

VFR ceilings expected to start the period around the 4-6kft ahead of some restricting visibility around 09Z at KBPI/KPNA. Snow will move into KJAC by 15Z that will bring MVFR ceilings and visibility with possible IFR conditions construed to a tempo group at this time due to lower confidence IAW MOS/LAMP data. Increasing snow chances at KBPI/KPNA after 18Z with less confidence at KRKS after 19Z. Again, MVFR conditions expected with snow with gusty south of westerly winds up to 25kts at KRKS with slight breezes up to 15-18kts at other locations. Diminished after 00Z ahead of the next round into the next cycle early Friday morning.

East of the Divide (KCOD, KCPR, KLND, KRIW, and KWRL) Terminals:

Some mid level ceilings at around 6-8kft remaining VFR throughout the period. Some low confidence snow across KCOD/KWRL after 18Z during the afternoon hours. KCPR will see the strongest wind gusts out of the southwest with daytime heating and mixing to the surface up to 30kts after 16Z before diminishing after 01Z with radiational cooling. Breezy winds remain at all locations even into the overnight hours under continued cloudy low to mid level ceilings but remaining VFR. Otherwise, the next chance for snow come Friday into the next couple of TAF cycles.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for the latest information on icing and turbulence forecasts.

RIW WATCHES/WARNINGS/ADVISORIES

Winter Weather Advisory from 5 AM early this morning to 11 PM MST Friday for WYZ001-013-014.

Winter Storm Warning from 5 AM early this morning to 11 PM MST Friday for WYZ012-023-024.