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508
FXUS65 KSLC 041015
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
415 AM MDT Sat May 4 2024

.SYNOPSIS...
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Saturday will be warm with southerly
winds increasing ahead of a storm arriving on Sunday. This storm
will bring a cold front that will be precluded with strong wind
gusts, particularly across western and southeastern Utah, on
Sunday. Widespread valley rain and mountain snow will accompany
the cold front with much colder temperatures. Conditions remain
unsettled and cold through midweek.
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&&

.SHORT TERM (Through 12Z/6AM Monday)...
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Our next storm is moving
onshore across northern California and the PNW in the form of a
low pressure system that will begin to phase back into the mean
flow and tilt negatively as it strengthens on its way to the Great
Basin. This trough will bring strong southerly winds followed by
colder temperatures and widespread valley rain and mountain snow,
focused on northern Utah/SW Wyoming.

Today, southerly flow and WAA will begin increasing ahead of this
storm. This will result in temperatures in the mid 70s across
most Utah valleys with mid 80s across lower Washington county and
Lake Powell. 700 mb flow aloft increases to near 50 knots across
Utah late Saturday and Sunday as this trough approaches from the
west. These strong winds aloft will mix to the surface with
advisory level magnitudes across western Utah beginning Saturday
afternoon, and approaching warning level criteria for western Juab
and Millard counties. These strong winds will be capable of
producing reduced visibilities from blowing and drifting dust. As
this trough moves further east winds will increase across
southeastern Utah up through Castle Country on Sunday with
widespread winds 35-45 mph gusting 60-65 mph Sunday afternoon.
These winds will weaken as the direction shifts to the northwest
behind a cold front that sweeps through the area from northwest
to southeast during the day on Sunday.

A 100+kt jet will help to direct an atmospheric river onshore the
west coast with a generous portion of this moisture making it into
our CWA. Ensemble mean PWATs on Sunday are forecast to be 0.66"
for KSLC which would place this in the 90th+ percentile for this
time of year. This moisture/jet will pair with a strong
baroclinic zone, potentially undergoing frontogenesis, to produce
widespread valley rain and heavy mountain snow along and behind
it. Ahead of the front, atmospheric profiles are forecast to
remain dry up to about ridgetop levels so only a few scattered
showers and thunderstorms will accompany the pre-frontal
environment. Peak precipitation rates across northern Utah will
occur around midday Sunday as the cold front passes through.
Temperatures will fall dramatically. H7 temperatures ~7C will
quickly plummet to ~-7C in a matter of a few hours. Snow levels
will also be falling to 5500-6000 feet by Sunday evening, and
eventually to ~5000 feet by Monday morning which would result in
precipitation transitioning to snow along the benches of northern
Utah. A moist west/northwest flow will succeed the cold front.
Steep lapse rates and lake induced instability will allow showers
to continue to the east of the Great Salt Lake early Monday
morning.

.LONG TERM (After 12Z/6AM Monday)...High confidence exists for cool
and unsettled conditions to persist through midweek across Utah and
SW Wyoming. On Monday, northern Utah and SW Wyoming will reside
beneath a region of moist cyclonic flow, and this will maintain
persistent showers, particularly over high terrain areas of northern
Utah as well as adjacent valleys. During the morning, snow levels
will be quite low, on the order of 5000-5500 feet elevation,
suggesting the atmosphere will be cold enough to support snow down
to as low as benches. The ingredients will be in place to support
lake effect snow (sufficient lake sfc to 700mb delta Ts, favorable
fetch and sufficient moisture through the DGZ), and in fact local
lake effect snow probabilities suggest a low to medium chance (30-
40%)for lake effect snow during the morning hours. The few available
CAMs that extend out to 60hrs do indeed indicate localized heavier
convective showers streaming off of the lake across the Northern
Wasatch Front between Ogden and Bountiful.

Majority of the model solution space suggests a slight reduction in
shower activity Monday evening and early overnight Monday, although
at least a few showers are likely to continue over the terrain of
northern Utah owing to favorable upslope flow trajectories and
sufficient moisture during this time. A trailing shortwave will act
to enhance shower activity once again across northern Utah and SW
Wyoming beginning early Tuesday morning, with showers persisting
though much of the day. Tuesday morning appears to be another
favorable period for lake effect, with locations from Bountiful to
Salt Lake favored. Highest uncertainty both Monday and Tuesday
mornings is whether temperatures will be cold enough to support snow
in valleys with this activity, as the relatively warm lake may
modify the boundary layer for those areas adjacent the downstream
waters. We are confident that temperatures will be cold enough to
support snow on benches, however with 1 to 2 inches of snow not out
of the question. High enough precipitation rates could support snow
down to valleys, or a combination of snow/graupel in convectively-
driven showers. A high end scenario suggests up to 1" of snow in
valleys for either period. Dry advection begins to take hold late
Tuesday into Wednesday, reducing the coverage and intensity of
shower activity. Should enough boundary layer moisture along with a
sufficiently saturated DGZ (with respect to ice) we cannot rule out
yet a 3rd consecutive morning with lake effect potential, this being
Wednesday morning. Probabilities, while low, are non-zero in the 15-
25% range. With an increased northwest flow, this would favor areas
from Bountiful to SLC. In the 48 hours from around sunrise Monday
morning to around sunrise Wednesday morning, several additional
inches of snow are likely across the northern mountains, with an
additional 6 to 12 inches expected, per the most likely scenario,
with locally higher amounts in the Ogden area mountains and Upper
Cottonwoods, where favorable upslope and lake effect will act
together to enhance amounts. 90th percentile amounts are as high as
16-24 inches.

Temperatures of this magnitude for early May that support snow down
to valley floors in the overnight and early morning hours are
flirting with being outside of climatology for the time of year.
Monday morning lows are expected to range from the upper 20s to low
30s across valley locations of southwest Utah and central Utah, with
low to mid 30s across northern Utah thanks to clouds and
precipitation. Slightly warmer temperatures are in store for Tuesday
morning with even colder temperatures for Wednesday morning, with
widespread upper 20s to low 30s lows currently forecast once again
for southwest Utah, central Utah valleys and most valleys of
northern Utah outside of Urban areas and away from the moderating
effects of the Great Salt Lake and Utah Lake.

Majority of the ensemble solution space suggests negative height and
corresponding negative temperature anomalies will remain in place
through Thursday across Utah and SW Wyoming, with about 1/3 of the
distribution maintaining negative height anomalies by Friday. The
magnitude of the anomalies will be diminishing, however, leading to
a slow, gradual warming trend. After a cold Wednesday with
temperatures 10F to 20F below seasonal normals, temperatures will
rebound to within 10F of normal by Friday, with just over 3/4 of the
model distribution suggesting temperatures will return to normal by
Saturday. The remaining 1/4 of the solution space maintain a warming
trend while keeping temperatures suppressed slightly below normal.
Under this scenario, a slight chance of showers, especially over
the terrain, persists into Saturday. Otherwise,  a few showers will
persist, largely coincident with daytime heating and over the
terrain, through Thursday.
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&&

.AVIATION...
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KSLC...AVIATION...KSLC..VFR conditions will remain in place through the
valid TAF period. Southerly winds are also forecast to be in
place through the period as well, gradually increasing this
afternoon and remaining southerly and gusty through Saturday
night, with gusts in the 30 to 40kt range possible. There is a
low, or 10 to 15 percent chance for nearby showers late this
afternoon and early evening.

.REST OF UTAH AND SOUTHWEST WYOMING...VFR conditions are expected
across the area through the valid TAF period. Southerly winds
will gradually increase this afternoon and remain southerly and
gusty tonight. There is a low to medium (30-40 percent chance) for
LLWS conditions to develop overhead steeper walled valleys
overnight, as winds at the 2kft AGL level will increase into the
30-45kt range after 06Z tonight. Can`t rule out isolated shower
development over northern Utah terrain late this afternoon and
early evening, otherwise widespread shower development will begin
around the end of the period across northwest Utah. Patchy blowing
dust may limit surface visibility over portions of SW Utah late
this afternoon through tonight.
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&&

.SLC WATCHES/WARNINGS/ADVISORIES...
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UT...Wind Advisory from 3 PM this afternoon to noon MDT Sunday for
     UTZ101-122.

     Winter Storm Warning from 6 AM Sunday to noon MDT Tuesday for
     UTZ110>112.

     High Wind Warning from 3 PM this afternoon to noon MDT Sunday
     for UTZ115.

     High Wind Watch from Sunday morning through Sunday evening for
     UTZ120-121-128-130.

WY...None.
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&&

$$

SHORT TERM...Mahan
LONG TERM...A. DeSmet
AVIATION...A. DeSmet

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