textproduct: Salt Lake City

KEY MESSAGES

- A cold front will pass through Utah and southwest Wyoming through the remainder of today and early Tuesday, bringing a period of light to moderate valley rain and high elevation snowfall. Embedded thunderstorms may bring periods of moderate to heavy rainfall for valleys in northern Utah.

- A colder system impacts the region early Thursday through Friday, bringing better chances for more substantial mountain snow and potential for light valley snow.

DISCUSSION

A broad region of precipitation is currently observed from northeast Utah through western Beaver county as our approaching cold frontal boundary helps to focus an area of lift. This boundary, and the associated precipitation, will continue to shift southeast through the next 6 to 12 hours and allow for valley rain and high elevation (7,000-7,500ft+) snowfall to spread across Utah and southwest Wyoming. Ahead of the boundary, radiosonde observations reveal a substantial amount of elevated instability (300+ J/kg CAPE) for this time of year. We have already seen a number of isolated thunderstorms that likely contain small hail, and this will continue to be a localized threat through the remainder of the day (mainly for areas which have not yet seen the boundary pass). The exception for wetter conditions through the next 12 hours will be the eastern Utah valleys, which are anticipated to remain dry as the boundary progresses due to a significant amount of moisture absorbed by the central mountain spine.

There are a non-neglible amount of hi-res models that are suggesting a cold enough environment to produce snow on the bench areas of the northern Wasatch Front and eastern Cache valley during the overnight hours, however, with the anomalously warm antecedent conditions the thought is that it will be quite difficult to see any significant accumulations of snow (especially on paved surfaces). That said, there is still around a 25% chance that we see around 1 inch of snow accumulation by Tuesday morning in eastern Salt Lake, Davis, Weber, and Cache counties. A majority, if not all, of the impacts tied to snowfall from this current storm are to be expected for elevations above 7,000ft. Initially, snow levels will be upwards of 9,000ft by the time of precipitation onset and will gradually lower to around 6,500ft by early Tuesday morning. Fortunately, a majority of the moisture and broad lift will already be exiting the region by the time temperatures are cold enough to support lower elevation snowfall, so anticipate minimal impacts outside of mountain travel routes.

A brief lull in active weather is expected through the remainder of Tuesday and Wednesday as we prep for our next colder storm system. Northerly gap winds will spread across Washington county on Tuesday morning, with peak gusts approaching the 35 mph mark. While temperatures will be cooler (near seasonal normals) on Tuesday, a stout warming trend is expected into Wednesday as temperatures rebound back to above normal levels alongside dry surface conditions. An uptick in southerly winds can also be expected during the late afternoon on Wednesday as lower level flow enhances ahead of the next frontal boundary.

Models remain in fair agreement of another trough pushing into the eastern Great Basin from late Wednesday/ early Thursday through late Friday, with the base of the feature trending farther and farther south. As such, big picture trends will be an overall cooler end to the week and wetter conditions areawide. Given the much colder airmass associated with this system, thinking that mountainous terrain will see much greater chances for impacts as snow levels will start around 7,000ft overnight Wednesday into Thursday and drop to as low as 4,500ft by Friday morning.

Current model guidance shows the highest amount of uncertainty across the northern half of Utah, with the highest QPF spread existing in the mountains. Total water amounts (25th to 75th percentile) from early Thursday through early Saturday range from around 0.1 inches to 0.5 inches for northern valleys and around 0.25 inches to upwards of 1.5 inches in the high terrain! For the southern area, similar elevations may see 0.01-0.25 inches in the valleys and 0.25-0.9 inches in the mountains. Depending on snow ratios and actualized SWE amounts, mountainous terrain may see 2-12 inches in the northern Utah mountains and 3-7 inches in the southern mountains. The highest amount of uncertainty, however, appears to be focused around the Tushar Mountains where higher end (75th percentile) output is pushing 20+ inches. For valleys, snowfall is not off the table as 75th percentile output is as high as 1 to 3 inches (particularly focused on the east benches).

It should go without saying that there is still a solid amount of uncertainty to work through with this storm, but the big picture trends remain the same. Beyond the scope of this next storm, the pattern looks to stabilize, however, we'll remain between two main weather drivers... a longwave trough to the east and a ridge to the west. Small changes in the amplitude and phase of these two features will have influence on our forecast... so maintaining low end PoPs through the remainder of the forecast period.

REST OF UTAH AND SOUTHWEST WYOMING

A band of heavy precipitation accompanying a cold front passage will continue to impact N UT/SW WY terminals through this evening, with MVFR to IFR CIGS/VIS persisting through this period. Precipitation tapers off by early Tuesday morning for most locations other than some lingering showers along and east of the Wasatch. Westerly winds across N UT/SW WY switch northwesterly behind the frontal passage early this afternoon. Across southwest Utah, expect a period of enhanced wind gusts in excess of 30 kts at times before switching northwesterly this evening.

SLC WATCHES/WARNINGS/ADVISORIES

UT...None. WY...None.