textproduct: Salt Lake City

SYNOPSIS

A significant winter storm will impact northern and central Utah late Thursday into Sunday morning.

DISCUSSION

One of the bigger winter storms of the fall season looks to be loading up for the northern mountains Friday into the weekend. Early afternoon upper air and satellite analysis indicates an upper level low is shifting across the 4 Corners area. A strong eastern Pacific ridge remains displaced offshore. A strong jet max is rotating around the top of this ridge.

Aside from some light snow showers across mainly southern Utah and patchy fog across northern Utah, the next 24 to 36 hours will be relatively quiet, cold and dry.

A strong jet max will round the northeast side of the ridge and shift into the Interior West Thursday. This, combined with modest warm air advection will support the development of precipitation across northern Utah after midnight Thursday night. With a cold airmass in place, initial precipitation will be all snow to valley floors. As warm air advection continues, snow levels will gradually rise to around 5500 feet by Friday night, 6000 feet by Saturday morning. At least one round of moderate to heavy precipitation is likely, from Friday afternoon through Saturday morning.

There is a small subset of ensemble members that support a more north and east jet position, which would result in a less prolific storm across much of northern Utah. This subset continues to shrink however and didn't represent enough of the ensemble members to cluster around this solution. While very unlikely, this scenario will need to be monitored as storm totals would be significantly less.

Periods of northern mountain snow and northern valley rain/snow will continue through Saturday, gradually ending later Saturday into Sunday. There is a subset of ensemble members that end precipitation by Saturday evening. While, this would also decrease storm totals, though at this time, amounts would likely remain in the warning range for the mountains.

Looking at potential storm totals, the 25th to 75th percentile storm totals for the Wasatch Mountains are 1 to 2 feet, with locally up to 30 inches for the upper Cottonwoods, Ogden area mountains and Bear River Range (all areas that do well in this type of moist northwest to west flow). For the western Uinta Mountains, storm totals (25th to 75th percentile) are lower but still sizable, 8 to 18 inches.

As far as valley snow, while most valley locations across northern Utah and Uinta County, WY will start as snow, valley floors north of Ogden will shift to rain sometime after 2 PM Friday. This will limit accumulations to around 2 inches or less. For the benches, expect 2 to 5 inches. The heaviest valley snow is likely in the Cache Valley, where a combination of cold antecedent temperatures and colder mid-level temperatures will hold snow into the evening and potentially overnight hours, especially for the benches.

For the Wasatch Back, which has a wider range of elevations, expect 6 to 12 inches of snow for higher elevation areas such as Park City and the Eden/Liberty area, with less in lower elevation areas such as Heber and Huntsville.

Given the confidence in winter storm warning amounts of the northern mountains and Wasatch Back is greater than 70%, issued a winter storm watch for these areas from 09Z Friday to 12Z Sunday. Advisories are likely going to be needed for the Cache Valley, Uinta County, WY and the Bear River Valley/Bear Lake area...but will see the trends in the snow levels (and how quickly a changeover to rain will occur for the Cache). Expect winter driving conditions on all area mountain routes across northern and into central Utah late Thursday night into Sunday...and monitor the forecast for potential impacts to valley routes.

In the further extended portions of the forecast, guidance continues to vary on whether northwest flow remains well entrenched across the region...bringing several weak disturbances through northern Utah next week...or a significant block ridge builds into the Interior West.

REST OF UTAH AND SOUTHWEST WYOMING

Cigs have lifted above MVFR at northern Utah and southwest Wyoming terminals but may return through the night ahead of the next round of midlevel clouds around 12z tomorrow. That said, there exists a 30% percent chance of fog developing at EVW and LGU after 12z. For southern Utah terminals, CDC vicinity snow showers are expected to decrease this evening, though residual cigs just above MVFR may persist into late morning, while SGU should remain clear. Northerly winds will persist for most regional terminals into late evening, delaying any typical diurnal shifts.

SLC WATCHES/WARNINGS/ADVISORIES

UT...Winter Storm Watch from late Thursday night through late Saturday night for UTZ108-110>112.

WY...None.