textproduct: Salt Lake City

KEY MESSAGES

- Hazardous travel is expected due to freezing rain for areas across northern and eastern Utah, with the greatest confidence expected along I-80 from the Salt Flats to the Nevada border. Elsewhere, scattered to isolated freezing rain is expected.

- Light to moderate high elevation snowfall will bring travel impacts to elevations above 8,000ft from early Thursday morning through Friday evening.

- An active weather pattern will follow this initial storm as we head into early next week, bringing continued threat for high elevation snowfall and travel impacts.

DISCUSSION

Persistent valley inversions are evident once again across much of Utah as very little warming has been observed for much of the area as compared to yesterday. Adding insult to injury, the valley haze is notably more dense today and has restricted visibility at KSLC to as low as 5 miles. The good news is that we are introducing our next weather system into the environment through the remainder of the evening which will help to provide at least some ventilation to help clear valley inversions. The bad news... a variety of impacts are expected across the region as warm overrunning air pairs with strong valley cold pools. The result... areas of freezing rain.

Our approaching storm system is already noted on KICX radar as weak returns are observed in northern AZ/ southern NV. This area of precipitation will gradually push into southern Utah through the remainder of the evening hours, spreading into central and northern Utah through the early morning hours on New Year's Day. With the warm antecedent temperatures, and relatively warm temperatures confined within the weak low, snow levels will be quite high through much of the event (8,000-8,500ft). As such, impacts related to snowfall will only be expected in the highest elevation areas across the forecast area (e.g. upper Cottonwoods, Tushar mountains, Brian Head area, and Escalante area mountains). For the valley areas, the dominant precipitation type becomes a bit complicated, however, liquid precipitation is expected.

That said... models have maintained consistency with favoring freezing rain across many of the Utah valleys late tonight/ early Thursday morning, with the most consistency pointing toward the West Desert region from the Salt Flats westward to the Nevada border. There is a medium to high probability (60-75% chance) of freezing rain in this area from the time of precipitation onset early Thursday morning (around 3AM) through as late as noon Thursday. An additional area consistent amongst hi-res guidance is the I-80 corridor through the Tooele Valley. Given the consistency and relatively high confidence, have issued a Winter Weather Advisory for these areas for the threat of freezing rain. Elsewhere, residual cold air will bring at least a short-lived period of freezing rain for many valley areas across northern Utah including areas closest to the Great Salt Lake north of Salt Lake City (low lying terrain west of the I-15 corridor), Cache Valley, through the mouths of steep canyons (Cottonwoods, Ogden, and Weber), and for various eastern valley areas (highest confidence SR-10 corridor and areas south of I-70). Have not issued a Winter Weather Advisory given the lack of confidence. That said, it does not take much freezing rain to cause significant impacts. Make alternate plans if you are planning to drive between 3AM to 7AM, especially along I-80 west of Salt Lake City.

This initial wave of precipitation will progress through the northern area through Thursday afternoon, with rain/ snow becoming more showery in the wake of the main precipitation shield. A more robust shortwave trough is expected to progress through the region from late Thursday through Friday morning, bringing a secondary wave of precipitation and a weak cold frontal boundary. This period is anticipated to bring the greatest impact to the mountainous terrain as snow levels will gradually drop to around 7,000ft by Friday morning, with the additional forcing helping to drive snow rates up. In particular, anticipate winter driving conditions in the Cottonwoods, Parleys, Sardine Summit, and the higher elevations of Logan Canyon by early Friday morning. Models have made a significant jump in the high end potential for this second wave which may warrant the issuance of headlines for the mountainous terrain.

Brief ridging is expected to follow in the wake of the aforementioned shortwave through the day on Saturday before an active storm track resumes. An upstream, quasi-stationary longwave trough off the U.S. west coast will likely eject a series of shortwave troughs over the forecast area, each providing enough moisture and lift to spread precipitation across much of Utah and southwest Wyoming. That said, there is some uncertainty with how quickly this occurs and how robust the energy becomes. Currently, ensemble guidance supports the whole longwave feature translating inland around the Tuesday/ Wednesday timeframe. There is some uncertainty with how this evolves which will ultimately determine the placement of the best precipitation and the coldest air (thus influencing snow levels).

Big picture... the storm track will remain active heading into next week but the details of individual precipitation events are fairly uncertain at this time.

REST OF UTAH AND SOUTHWEST WYOMING

Unsettled pattern will begin overnight into the new year with precipitation spreading across the area from south to north. Snow levels will start around 5000-7000 ft, but increase quickly. That said, valley areas with low level thermal inversions and other well shaded/sheltered areas will see a low end chance (~5-15%) of periods of freezing rain/drizzle. Overall, most terminals outside of highest elevations will see rain, with periods of MVFR CIGS.

SLC WATCHES/WARNINGS/ADVISORIES

UT...Winter Weather Advisory from 1 AM to 11 AM MST Thursday for UTZ101-102.

WY...None.