textproduct: Salt Lake City

KEY MESSAGES

- Record warm temperatures will continue through Thursday.

- Wind gusts in excess of 45 mph will continue across western Utah through Friday afternoon. Areas of blowing dust may reduce visibilities at times below 5 miles.

- 6 to 12 inches of snow is expected across the southern mountains above 8500 feet and western Uinta Mountains above 9000 feet Wednesday into late Thursday.

- Moderate to heavy rain will occur across southern Utah below 8500 feet Wednesday into Thursday. Expect enhanced runoff across this area, with all streams, slot canyons, and other waterways running high.

- There is a less than 10 percent chance of measurable snow for valley floors of northern Utah Saturday morning.

- 6 to 12 inches of snow will occur across the higher terrain of Utah Friday into Saturday afternoon.

DISCUSSION

Deep moisture advection continues across the region early this morning, with mesoanalysis indicating PWs have risen from 0.20" to 0.60" over the last 24 hours across southern Utah. PWs over 1.00" are starting to creep into southern Nevada. With gusty southerly winds in place, many northern and western valleys remain exceptionally warm for December. It will be another day of recording breaking high temperatures...and potentially record high minimum temperatures across the Beehive State.

As a jet max rounds the base of the trough to the west, deep forcing will shift eastward into the region this morning. Note that there is light ongoing precipitation across the southern mountains early this morning, likely associated with an ejecting shortwave trough ahead of the main circulation as well as the beginning of the strongest moisture advection reaching southern Utah.

By later this afternoon, preferential jet dynamics will combine with near max to max PW values compared to the NAEFS climatology to bring moderate to heavy precipitation to much of southwestern Utah. The period of peak precipitation intensity will stretch from around 21Z Wednesday to 12Z Thursday across southern Utah. By Thursday evening, expect the following 25th to 75th percentile precipitation ranges:

-Lower Washington County: 0.20-0.70" -Zion National Park: 0.40-1.40" lower elevations, 0.70-2.00" upper elevations -Pine Valley Mountains, New Harmony, Kanarraville: 2.00-4.00" (10% chance of 4.50"+) -Cedar City: 0.60-1.25"

The Weather Prediction Center has outlooked portions of southern Utah in a marginal risk for excessive rainfall, meaning a greater than 5% chance of rainfall exceeding flash flood guidance with 25 miles of a point. CAMS suggest that embedded convection will be ongoing from this evening into early Thursday morning, which will bring the potential for higher hourly rainfall rates. Current expectation is hourly rainfall rates will average around 0.10-0.40" per hour (heaviest in preferential upslope areas), with a 10% chance hourly rates will exceed 0.50" per hour particularly across the Pine Valleys/Zion National Park area between 22Z Wednesday and 06Z Thursday. While flash flooding is a very low threat (around 5-10% chance within 25 miles of a point), all area rivers, streams, normally dry washes and slot canyons will be running and should be avoided through at least Friday.

Snow levels overnight have been near about 8000 feet...and will gradually rise with time as the landfalling atmospheric river moves overhead. Expect snow levels to rise to 9000-9500 feet by this afternoon and then fall back toward 8500-9000 feet by Thursday morning across southern Utah. A winter weather advisory remains in effect for elevations above 8500 feet across the southern mountains, with 6 to 12 inches the most probable range.

Further north, deep southwesterly flow will bring moderate snow accumulations to the western Uinta Mountains above 9000 feet, with 6 to 12 inches of snow the most probable range by Thursday evening. Winter weather advisories also continue for this area.

With 50kts + of southwesterly flow at 700mb, gusty southerly winds will continue through at least Thursday for the western valleys. There is some question to the degree of momentum transfer with ongoing precipitation and significant cloud cover, particularly after Wednesday afternoon. HREF suggests the best chance for multiple hours of wind gusts in excess of 45 mph will be this afternoon and evening across the western Utah valleys (>60%, particularly on ridgelines). By Thursday afternoon, probability of reaching this threshold drops consistently for most locations, except those exposed ridgelines. Will keep going wind advisory for now, but based on the newest guidance, the threat of meeting criteria looks to decrease after Thursday evening. Blowing dust will also be a concern, particularly this afternoon and evening.

As the best forcing shifts eastward Thursday afternoon into Thursday evening, attention will then turn to the colder portion of the period. Guidance has shifted a bit...toward a splitting system, with the main trough moving across the northern Intermountain Region Friday evening, and a now cut-off upper level low retrograding southwestward.

The associated cold front will shift into northern Utah Friday evening, bringing quickly lowering snow levels in association with a frontal band of precipitation. The heaviest period of precipitation across northern Utah associated with this front is expected between 00-15Z Saturday. This still provides a window with ongoing precipitation and snow levels near valley floors Saturday morning, however, guidance has suggested the probability of seeing measurable snow for Wasatch Front valley floors has decreased to around 10%. Benches will see a higher chance of measurable snow.

For the mountains of Utah, another 6 to 12 inches of snow will be widespread, with locally up to 15 inches for the upper Cottonwoods by Saturday afternoon. Still a bit too early to expand the winter weather advisories, but will likely need to consider adding headlines with future forecast packages.

After record to near record temperatures across the region through Thursday, with the potential for many locations to see the warmest Christmas Day on record, more typical December temperatures are expected by Saturday. A period of upper level ridging is increasing in likelihood for early next week, with generally warming temperatures.

REST OF UTAH AND SOUTHWEST WYOMING

Gusty southerly winds can be found in locations downwind of higher terrain this morning, and will become more widespread in most areas after 18z. The highest wind gusts can be expected across western valleys, with many areas south of US-6 and west of I-15 expected to see gusts in excess of 40kts through the afternoon. Showers will develop across southwestern UT early this morning, though may have a hard time reaching the 5SM radius around KCDC given strong downsloping winds. These showers will develop across northern Utah by mid- afternoon after roughly ~19- 20z, resulting in mountain obscuration and a low chance for periodic MVFR VIS.

SLC WATCHES/WARNINGS/ADVISORIES

UT...Wind Advisory from 11 AM this morning to 5 PM MST Friday for UTZ101-102-115-122.

Winter Weather Advisory from 11 AM this morning to 11 PM MST Thursday for UTZ112.

Winter Weather Advisory until 11 PM MST Thursday for UTZ125.

WY...None.