textproduct: Salt Lake City

KEY MESSAGES

- Mild temperatures with highs around 10 to 15 degrees above normal will persist through Friday.

- A broad system approaches the area late Friday into Saturday, resulting in increasingly gusty conditions Saturday. There is a 40-50% chance of wind gusts in excess of 45 mph across portions of Utah's West Desert Saturday afternoon.

- Unsettled conditions return Saturday, initially bringing periods of rain and thundershowers as well as high elevation snow. A colder system will reinforce widespread precipitation chances, lower snow levels, and bring a period of more substantial mountain snow alongside continued breezy winds.

DISCUSSION

Conditions across the region remain relatively quiet this evening as marginally cooler temperatures are being observed across the northern area thanks to a subtle baroclinic zone that dropped into northern Utah and southwest Wyoming. Low amounts of moisture pairing with weak low- to mid-level convergence across west-central Utah has lead to the development of isolated high- based showers in Utah's West Desert. No precipitation has been noted at surface-based observation sites as the near surface (and remainder of the low-level environment below 10,000ft) remains very dry. These isolated high-based showers are expected to continue through the overnight hours and into the afternoon hours on Thursday. Though low levels of moisture will remain in place over the northern area tomorrow, it is unlikely (less than 15-20% chance) that appreciable precipitation (0.05 inches or greater) will be observed in the valley areas. That said, anticipate light rain showers in the mountainous terrain of northern Utah, with light snow at the highest elevations (9,800ft+).

The upper-level pattern will undergo a significant change from Thursday afternoon/ evening forward as an offshore low begins to pivot into the Great Basin through the upcoming weekend. As the pattern shifts and we enter a period of unsettled weather, the main concerns for weather will be focused on the potential for wind, moderate valley rain, mountain snow, and colder temperatures. During this pattern change, there will be two distinct active periods; the first expected late Friday through Saturday (warmer system, limited impacts) and the second expected Sunday morning through at least late Monday (colder system, greater potential for impacts).

The initial period of unsettled weather will come in the form of an ejecting shortwave trough from the offshore low, effectively breaking down the ridge in place and opening the door for the low to move inland. Southwesterly winds will begin to increase during the day on Friday, however, peak wind gusts across the forecast area will remain at, or below, 30-35 mph. One final surge of warm air will allow for temperatures to increase by around 2-4 degrees for much of the area before the warmth is shunted northeastward. Cooler temperatures and moisture will begin pushing into the region Friday evening through Saturday morning as the shortwave trough tracks overhead. Much of the precipitation resulting from the moisture increase and broad lift will be focused across the northern half of the area, with snowfall confined to the high elevation (9,000-9,500ft and above) due to warm antecedent temperatures. Southerly winds will continue to increase on Saturday as the surface pressure gradient tightens and flow enhances aloft, resulting in gusty winds at the surface. Current guidance shows the best potential for stronger winds (40-45+ mph) across the west-central and east-central portions of Utah on Saturday afternoon, where there is upwards of a 40-50% chance of wind gusts exceeding 45 mph. This will be the main concern for Saturday.

The meat and potatoes of the pattern change arrive by Sunday as the aforementioned low begins to shift inland, bringing continued enhanced winds, a greater increase in moisture, and a strong cold front. Uncertainty still exists in the precise timing and location of which areas will see the greatest precipitation amounts, however, model trends have seemingly shifted farther south for the best energy and a skosh later on the timing of the frontal boundary. That said, current model consensus points toward a period from Sunday night into Monday morning as the best potential to see heavier precipitation across the region as the cold front progresses eastward, with post-frontal instability showers continuing through Monday and a portion of Tuesday. As it stands, the greatest amount of uncertainty exists across northern Utah, namely due to uncertainties in the track of the best mid-level forcing and productivity of post-frontal showers.

Current model spread (25th to 75th percentile) in 48-hour precipitation amounts through Tuesday morning range from around 0.1-0.3 inches for southern Utah valleys, 0.05-0.15 inches for eastern Utah valleys, and 0.25-0.75 inches for northern Utah valleys... all of which is expected to be rain below 6,000ft elevations. For the mountainous terrain, the possibilities for snow and snow-water equivalent ranges from 5-10 inches of snow (0.3-0.75in SWE) across the southern/central mountains and 6-16 inches of snow (0.75-1.5in SWE) across the northern mountains based on current guidance. The higher end of these forecast ranges are reflective of ranges/ portions of ranges that are often favored during storms (e.g. upper Cottonwoods, western Uintas, Tushar Mountains, and Brian Head area).

As mentioned, there are some uncertainties to work through before we get into this event, but the big-picture story remains the same; a significant pattern change will occur this weekend and will bring colder temperatures and a shot of widespread precipitation across Utah which will be favored in the mountains.

REST OF UTAH AND SOUTHWEST WYOMING

The airspace will see VFR conditions through the night with some high clouds over northern Utah. Winds will be generally light and terrain driven.

SLC WATCHES/WARNINGS/ADVISORIES

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