textproduct: Salt Lake City

KEY MESSAGES

- An atmospheric river will bring valley rain and high-elevation dense mountain snow to northern Utah through Wednesday morning. Warm conditions will limit most of the accumulation to above 8,500', reducing road impacts. - Windy conditions are in store through tomorrow for ridgetops and parts of southwest Wyoming.

- Unusually warm conditions arrive by Friday and persist for the next 7-10 days. The chance of significant precipitation for that same period is low.

DISCUSSION

Water vapor satellite imagery this morning tells the story: a ribbon of moisture transport is currently streaming from just east of Hawaii northeastward to Northern California and inland across the Northern Rockies and northern Great Basin (and thus northern Utah). The 250 kg/m*s contour of Integrated Water Vapor Transport - a typical minimum threshold to define an atmospheric river (AR) - is shown on global model initializations stretching into northern Utah, though decaying on its way. As is typical for these events, very warm air is in place - e.g. Alta currently near 40F - and strong ridgetop winds are moving in - e.g. Logan Peak currently gusting to 55MPH. Lastly, light precipitation is already ongoing at high elevations per area radars, especially from Ben Lomond Peak northward.

Per high-res models, the bulk of the precipitation should arrive this evening and continue overnight. The latest model runs have trended towards a quicker end to the widespread precipitation with drying behind a subtle shortwave trough tomorrow morning, followed by showery conditions tomorrow. Throughout the event, terrain favored by westerly to northwesterly flow will see the highest totals.

For valleys, expect a general tenth to third of an inch of rain from about Nephi northward, with the I-15 corridor from Salt Lake to Ogden seeing a quarter to half inch. Bench areas of Davis/Weber County have a 75% chance of more than a half inch and 20% chance of over an inch. For the mountains, expecting very high snow levels of 8 to 9 thousand feet and unusually dense snow for our area with snow-to-liquid ratios of around 5-to-1 tonight to 8-to-1 tomorrow. Snow Water Equivalent totals for the favored areas - Bear River Mountains, central and northern Wasatch, and western Uintas - will be near to over 2 inches. With the high snow levels and low snow-to-liquid ratios this only translates to around 6-16", which is still in line with the going Winter Weather Advisory. The high snow levels will limit transportation impacts to mostly the highest parts of the Cottonwood Canyons and Wolf Creek Pass. Lower snow levels near the Idaho border suggest snow at Logan Pass by tomorrow, though amounts will not be as significant (30% chance of more than 6"). The heavy, dense nature of the snow falling on top of lighter snow will also increase the avalanche danger for the mountains of northern Utah, which has already been considerable. Ridgetop winds of 80+ mph will also result in snow loading. See the Utah Avalanche Center for more specifics of that hazard.

Speaking of winds, strong 700mb winds combined with a stout surface pressure gradient across southwest Wyoming will result in gusty west winds today and tomorrow, locally nearing but not quick reaching warning criteria (50% chance of gusts over 50MPH in the Bridger Valley area).

The majority of precipitation will taper off by Wednesday afternoon as high pressure makes its way back over the area, bringing a return to dry conditions with temperatures staying rather mild. Confidence is still relatively high on dry conditions continuing for the first part of the weekend. From the weekend through next week, mean ridging will be the dominant feature over the Western US, though a pesky Pacific closed low may bring unsettled conditions into the Great Basin Sunday into early next week. There remains quite a bit of spread with regard to the evolution and potential timing with that closed low (as is common for those features). That said, the chance of significant precipitation is low given the unfavorable moisture track and weaker dynamics. Interestingly, even with the potential low in vicinity, temperatures will almost certainly remain above normal, e.g. 90% chance of highs into the 50s for SLC Monday in comparison to the normal of 50. Looking farther ahead into next week, there is high confidence of significantly-warmer-than-normal conditions, with a 75% chance of SLC highs in the 60s by the end of the workweek.

REST OF UTAH AND SOUTHWEST WYOMING

Increased west-southwest winds prevail for most regional terminals today, with strong gusts exceeding 30 kts expected this afternoon across far NE UT & SW WY. Showery precipitation will gradually fill in across the northern portion of the airspace this afternoon, with a heavier band moving in later this evening that will result in further CIG/VIS reductions down to MVFR or perhaps brief IFR conditions from KPVU north. Across southern Utah, VFR conditions prevail through the period.

SLC WATCHES/WARNINGS/ADVISORIES

UT...Winter Weather Advisory until 5 PM MST Wednesday for UTZ110>112.

WY...None.