399
FXUS65 KSLC 302308
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
408 PM MST Thu Jan 30 2025

.SYNOPSIS...High pressure will bring strengthening valley
inversions to northern Utah through Friday. An atmospheric river
will bring the potential for very heavy snow, especially for the
northern mountains this weekend into early next week.

&&

.SHORT TERM (Through 00Z Tuesday)...
KEY MESSAGES...

- Significant and long duration atmospheric river event to impact
  the northern Utah mountains later Friday into Monday morning
  bringing heavy accumulations of atypically dense snow.

- Snow levels will initially start near 5000 feet or so later
  Friday but will rise to 7000 feet for the Bear River Range,
  7500 feet for the Ogden area mountains, 8000 to 8500 feet for
  the Cottonwoods and Provo area mountains by Sunday afternoon.
  This will introduce the potential for rain on snow, with
  localized rises near creeks and streams.

- Winter driving conditions are expected on many mountain routes
  across northern Utah, most impacted routes include Logan Summit
  (US-89), Sardine Summit, and the Cottonwood Canyons later Friday
  through Monday morning. Traction laws may be enacted.

- Model spread remains wider than average on how far south this
  atmospheric river event will shift. Ranges for
  precipitation/snow totals remain larger than normal due to the
  range of solutions.

- Winter Storm Warnings are now in effect for the Wasatch
  Mountains and western Uinta Mountains. Winter Weather Advisories
  are in effect for the Book Cliffs and Wasatch Plateau.

Early afternoon upper air and satellite analysis indicates upper
level ridging remains in place across the Great Basin. An upper
level trough is approaching the Pacific Northwest, with the
beginning of landfalling atmospheric river event taking shape.

For Utah, northern valley inversions remain quite stout in many
locations, with orange/unhealthy for sensitive groups conditions
forecast through Friday for Salt Lake and Utah County, through
Saturday for Duchesne County. All other monitored northern Utah
valleys are forecast to continue to see yellow/moderate air
quality through Saturday. The strongest inversions are in the
western Uinta Basin and Bear River Valley of Utah, where
temperatures continue to be quite suppressed. For the Wasatch
Front, noticeable haze continues.

Southwest flow ahead of the landfalling atmospheric river will
spread across Utah Friday, with mountains seeing warming
temperatures and many northern valleys holding temperatures down
with strong inversions.

Expect the initial shortwave trough and attendant speed max to
build precipitation into northern Utah as early as Friday
afternoon, particularly across the Bear River Range. By Saturday,
IVT anomalies will be near the max for the time frame in the NAEFS
CFSR climatology for all time hours...showing how profoundly
abnormal this IVT is.

By Saturday evening, the speed max will be in a preferential
location to steadily enhance precipitation rates across the
northern Utah mountains, particularly in areas that favor west to
west-southwesterly flow. This will also continue a period of
modest to strong warm air advection, that will gradually increase
snow levels to near 7000 feet for the Bear River Range, 7500 feet
for the Ogden area mountains, 8000 to 8500 feet for the Cottonwoods
and Provo area mountains by Sunday afternoon.

Northern Utah will sit in an area of preferential lift through
Sunday, with moderate to heavy precipitation continuing for much
of northern Utah (especially, again, those areas that do well in
west to west-southwest flow). As the jet shifts north early
Monday, precipitation will gradually shift north and eventually
diminish by Monday evening along the Utah/Idaho border.

While higher elevation snow will initially favor typical Utah
powder, consistent modest to strong warm air advection will
gradually shift toward dense to very dense snow more reminiscent
of the Sierras or southern New England than Utah by Sunday
afternoon. This will increase the risk of weight loading in areas
that receive the heaviest precipitation, especially mountain
areas north of I-80 and bring significant travel impacts to routes
through Sardine Summit, Logan Summit and up to the Cottonwood
Canyon resorts Friday afternoon through Monday morning. Those with
travel plans in these areas should be prepared for severe winter
conditions.

What could go wrong with this forecast? A few questions remain...
1. If the atmospheric river position is further north, the
heaviest amounts will shift northward over the current forecast.
2. If the atmospheric river position shifts further south, heavier
accumulations can be expected to shift southward.
3. If the western Uinta Basin and Bear River Valley of Utah
remains heavily inverted, a period of light to moderate snowfall
will be more likely Saturday afternoon through Sunday. This will
need to be monitored.
4. If the snow levels/precipitation rates remain favorable, the
Ogden Valley, particularly near Eden and Liberty will see heavier
snow accumulations than forecast and may need to be added to the
winter weather advisory.
5. If the snow levels increase as fast as forecast, areas with
low/mid level snow cover may see a period of rain on snow, and
related rises on area creeks and streams as well as the potential
for some overland flooding. The highest threat of this is in the
northern Ogden Valley.
6. Another area of concern is western Uinta County, WY. Depending
on snow levels and eastward extent of precipitation, there is a
threat of heavier snow for western Uinta County.

For now, issued Winter Storm Warnings for the Wasatch Mountains
and western Uinta Mountains as well as a Winter Weather Advisory
for the Book Cliffs and Wasatch Plateau...based on these areas
having the highest confidence of significant impacts.

.LONG TERM (After 00Z Tuesday)...Southwest Wyoming and Utah will
get strong warm air advection through Tuesday, with atmospheric
river (AR) moisture lifting north and diminishing. Precipitation
will become more isolated to scattered throughout southwest
Wyoming and northern Utah. With the warm air advection, snow
levels will be around 7000-8000 feet. For valleys, there will be
unseasonably warm conditions, with highs ranging from the 50s to
60s for most locations, while lower Washington County gets to
around 70F. The trough that brought the AR event will dig
southward early in the week, which will push the most enhanced
moisture north, bring warmer conditions locally, and also gusty
winds. Winds will peak in southwest Utah, but be enhanced
throughout southwest Wyoming and Utah through Tuesday and into
Wednesday for portions of Utah.

The longwave trough will dig southward through the week, with a
baroclinic zone tracking in from the northwest. There is
uncertainty with models and ensembles on timing, most likely
Wednesday, and how far that system will dig. Those uncertainties
give lower confidence on details like how cold conditions will
get, snow levels, and how far south precipitation reaches.

Compared to the AR event through the weekend, this event would
likely come with lower snow levels and higher snow ratios. There
would likely also be more precipitation into southern Utah, but
less total precipitation throughout Utah.

&&

.AVIATION...KSLC...Light, northwesterly winds will transition to
southeasterly around 02-03z, with increasing southerly winds after
17-18z Friday. Light rain and lowered CIGs resulting in mountain
obscuration are likely to arrive around 21-00z Friday.

.REST OF UTAH AND SOUTHWEST WYOMING...Winds will remain fairly
light and terrain-driven through the overnight hours, increasing
out of the south to southwest after 17-18z Friday with gusts to
15-25kts in most areas. Precipitation is likely to enter northern
Utah from the west around 18-20z.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Winter Storm Warning from 5 PM Friday to 8 PM MST Monday for
     UTZ110.

     Winter Storm Warning from 11 PM Friday to 5 PM MST Monday for
     UTZ111-112.

     Winter Weather Advisory from 5 AM Saturday to 8 PM MST Sunday
     for UTZ113.

WY...None.
&&

$$

Kruse/Wilson/Cunningham

For more information from NOAA`s National Weather Service visit...
http://weather.gov/saltlakecity