textproduct: Salt Lake City

KEY MESSAGES

- Near record to record temperatures will continue through Thursday.

- There is a 35% chance wind gusts in excess of 45 mph will occur across western Utah near the Nevada border Wednesday afternoon and evening.

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

- An area of moderate to heavy rain will occur across southern Utah below 8000 feet Wednesday into Thursday. Expect enhanced runoff across this area, with all streams, slot canyons, and other waterways running high.

- The unsettled pattern will continue with gradually falling snow levels, reaching as low as 6000 feet by Friday evening and continuing to fall toward valley floors by Saturday morning. Another 6 to 12 inches of snow will occur across most Utah mountains Friday into Saturday.

- There is a 30% chance of measurable snow for the valley floors of northern Utah Friday night into Saturday morning.

DISCUSSION

It is another extremely warm overnight period (for the cool season) for many locations across northern and western Utah. The Salt Lake City Airport broke the all time high minimum temperature for December and tied the all time high minimum for the entire winter season (December through February) on December 22. So far, the low after midnight has fallen only to 58F. It is significantly less windy than it was at this time last night, so there is still some hope the northern and western valleys will decouple through 9 AM. If this doesn't happen, will again need to update the forecast and push the high temperatures at or above the 95th percentile.

Early morning upper air and satellite analysis indicates a strong, active jet persists across the southwestern United States. Semi-tropical moisture associated with the eastern Pacific ITCZ is being entrained into this jet circulation. An upper level trough is approaching the Pacific Coast. Looking at the current mesoanalysis across the Beehive State, precipitable water values are quite low, around 0.20 inches or so for most of southern Utah.

As a 140kt+ jet max rounds the base of the trough this evening into tonight, a shortwave ejecting ahead of the main circulation will shift across the area early to mid-Wednesday morning. This is loosely associated with the edge of the deeper moisture from the ITCZ, around 99.5th percentile for the NAEFS climatology. Deep moisture advection will continue, exceeding the max in the NAEFS climatology by Wednesday evening.

Meanwhile, the upper level trough will continue to shift eastward, with preferential jet dynamics shifting toward southwestern Utah Wednesday afternoon. The period of heaviest precipitation is expected from roughly 18Z Wednesday through 12Z Thursday. The best forcing for precipitation will shift east of the area Thursday afternoon into Thursday evening.

While 700mb winds approach 50 kts across western Utah Wednesday afternoon and evening, deep moisture combined with extensive cloud cover will likely preclude widespread areas of wind advisory criteria across western Utah. Current forecast is for an approximate 35% chance of wind gusts in excess of 45 mph for several hours along the Utah/Nevada border. Given this low probability, no wind headlines will be issued with this package.

Early runs of the CAMS suggest hourly rates, particularly in areas prone to southerly upslope, will range from 0.10-0.40" (25th to 75th percentile) with a 10% chance of hourly rates in excess of 0.50" in embedded convection. A portion of the CAMS are struggling to saturate the lowest levels across the St. George metro, however, think with the depth of the moisture with this system, this will be overcome by Wednesday evening at the latest.

The snow level will start around 9500 feet Wednesday afternoon and fall to near 8500 feet through Thursday morning across the southern Utah mountains. Snow levels will fall as low as 7500 feet in heavy precipitation. The southern mountains above 8500 feet will see 6 to 12 inches (25th to 75th percentile) through Thursday night. Issued a winter weather advisory for these locations with this package.

Storm total precipitation (largely rain below 8000-8500 feet, snow water equivalent above 8500 feet) for southwestern Utah will range from 0.40-1.00" (25th to 75th percentile) for lower Washington County, 0.75-1.75" (25th to 75th percentile) for much of the remainder of southwestern Utah by Thursday evening. There is a 10 percent chance storm totals with exceed 2.50" for the Pine Valleys eastward to Zion National Park and other areas with preferential upslope in southwesterly flow.

All area streams, normally dry washes, slot canyons, and other waterways will be running Wednesday through at least Friday. The current forecast from the Colorado Basin River Forecast Center for the North Fork of the Virgin River, for example, shows a peak flow of around 570 cfs (from a small base flow around 40 cfs) by early Thursday morning. Anyone with outdoor plans in these areas should consider alternate plans outside these waterways. The Weather Prediction Center continues to outlook southwestern Utah in a marginal risk from Wednesday morning through Thursday evening, meaning there is a greater than 5% chance rainfall will exceed flash flood guidance with 25 miles of a point. While the threat for flash flooding is very low, CAMS do indicate the potential for embedded convection, so will need to monitor this threat for the bulk of the storm.

While the southern part of the state will see the highest precipitation totals with this landfalling atmospheric river, snow levels will be quite high across northern and central Utah, leading to rain in many areas below 9000-9500 feet or so through at least Thursday morning. Expect a general 0.15-0.40" (25th to 75th percentile) of rain for the northern and central valleys, except 0.40-0.75" (25th to 75th percentile) for the northern Wasatch Front and Cache Valley. For the Wasatch Range, expect around 0.70-1.50" of rain below 9000-9500 feet, snow water equivalent for those highest peaks through Thursday night. For the high Uintas above 9000 feet, expect 6 to 12 inches of snow (25th to 75th percentile) through Thursday night. Issued winter weather advisory for the western Uinta Mountains above 9000 feet with this package.

Attention then turns to a colder upper level low and longwave trough making landfall near the northern California coast by Thursday evening which will bring an end to the record temperatures across the region. As the trough shifts eastward Friday, cold air advection associated with this system will shift into northern Utah. Snow levels near 8000 feet Thursday evening will fall to near 6500 feet by Friday morning and to near valley floors by Saturday morning as 700mb temperatures fall from near -1C Thursday evening to -8C by Saturday morning.

The most focused period of precipitation associated with this second system is associated with a strong jet max crossing the region Friday evening into early Saturday morning. Showers will linger into the early afternoon.

The mountains of Utah are a relatively easy forecast for this second system, with the bulk of the higher terrain seeing 6 to 12 inches of snow (25th to 75th percentile). No real snow level issues with this part of the storm. The bigger question (and one that I know is on a lot of peoples' minds) is the potential for measurable snow for the valley floors of the Wasatch Front. There is a period of precipitation that overlaps with lower snow levels in a subset of the guidance Saturday morning. This is the crux of the issue...whether the forcing for precipitation moves east of the valleys prior to the coldest air arriving. NBM probabilities for measurable snow for the valley floor continue to hover around 30% for the lowest portions of the Wasatch Front, increasing with elevation to greater than 70% above about 4800 feet. Forecasting the overlap between the cold temperatures and cessation of precipitation is difficult this far out, but something to keep an eye on.

No headlines for this second system at this time, though the current forecast would lean toward winter weather advisories for most mountain areas of Utah Friday into Saturday.

REST OF UTAH AND SOUTHWEST WYOMING

Gusty southerly winds will pick up after ~18z across most areas, with largely gusts to 25-35kts expected. BKN-OVC mid- to high-clouds will persist across the area, with some rainfall moving into southwestern UT after 23z. These showers may bring lower CIGs around 3000ft to KCDC and KSGU, and may briefly enhance southerly wind gusts (10% chance of exceeding 40kts).

SLC WATCHES/WARNINGS/ADVISORIES

UT...Winter Weather Advisory from 11 AM Wednesday to 11 PM MST Thursday for UTZ112.

Winter Weather Advisory from 5 AM Wednesday to 11 PM MST Thursday for UTZ125.

WY...None.