275
FXUS65 KSLC 072140
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
340 PM MDT Tue May 7 2024

.SYNOPSIS...A slow moving upper level trough will gradually cut-
off from the mean flow across southern Utah through the week. A
gradual warming trend is expected...though diurnal showers and
thunderstorms will remain possible across the higher terrain and
adjacent valleys into the weekend, especially across central and
southern Utah.

&&

.SHORT TERM (Through 06Z Thursday)...Mountain snow showers and
valley rain showers continue across northern Utah this afternoon.
At times, very gusty winds have been reported with these showers,
likely associated with higher momentum mix down and a relatively
dry sub-cloud layer in the valleys. Afternoon upper air and
satellite analysis indicates a broad, longwave trough remains
across the central and western United State. The main forcing with
this trough is shifting into the High Plains...however the
backedge of the trough will gradually shift south and eventually
cut-off from the mean flow by early in the long term forecast
period.

Expect showers to gradually end near to after sunset with the
exception of a potential period of lake effect south and east of
the Great Salt Lake. CAMS are nearly universal in developing a
band of transitory precipitation, extending to near Parleys
initially, then gradually shift to the Cottonwoods and then
western Salt Lake County and eventually diminishing near to after
10-12Z. If the residence time in any one location were to last a
few hours and precipitation changes to snow, slushy accumulations
would be possible on area roads. The most likely location for this
to occur would be the western side of the Salt Lake Valley
between 08-12Z. This is something to monitor moving through the
overnight period.

Next up, the Freeze Warnings. Upgraded the watches to warnings for
all zones and added the western Canyonlands zone for a >80% chance
of freezing temperatures in these locations. Even in areas not
currently expected to reach freezing such as Weber, Davis and Salt
Lake Counties, patchy frost will be possible. Those who have
vulnerable plants should either cover or bring them indoors.

Finally, with modest cold air advection and 700mb flow impacting
the gap area in Washington County, expect wind gusts to 50 mph
near canyons including towns such as the aptly named Hurricane and
Toquerville. Added a winter weather advisory for this location as
well.

Wednesday will feature occasional snow showers across the northern
mountains, however, accumulations will be light. Do not
anticipate the need to extend the winter storm warning.

.LONG TERM (After 06Z Thursday)...Early Thursday a broad positive
tilt trough will extend into the region, with a lobe of the
trough gradually cutting off over Utah and slowly moving
southwestward thereafter. This feature will then more or less
churn somewhere in the UT/AZ/NV/CA border region into the weekend.
In comparison to yesterday, guidance now is leaning in favor of
this feature lingering a bit deeper into the weekend or even early
next week before ejecting eastward out of the region, though
uncertainty is still noted in how exactly the flow evolves.

One of the impacts of this system will be the potential for
downslope winds across prone areas in far northern Utah. By
Thursday morning, most guidance moves the center of the
trough/cutoff low far enough south to begin imparting easterly
flow across northern Utah, and around 85% of ensemble members
carry some sort of light cold air advection over the terrain (two
necessary ingredients). High resolution guidance is just starting
to come into view, and suggest downslope winds begin to pick up
after 3am or so and peak sometime near or just after sunrise.
Still early, but HRRR does show potential for gusts generally in
the 35-50 mph range, primarily in prone areas of eastern
Weber/Davis Counties. Thursday night into Friday morning the low
becomes more favorably positioned, and ~80% of ensemble members
(especially ECM Ens members) show an enhanced cold air advection
component over the terrain. As a result, still thinking early
Friday on into mid morning Friday or so show the greatest
potential for stronger downslope winds. Currently thinking around
~60% chance to see gusts in excess of 45 mph in those typical
downslope wind prone areas, and ~15% chance of gusts in excess of
60 mph. Will begin to get a better idea as high resolution
guidance comes into view. Depending on the exact position of the
low, it looks likely some sort of easterly gradient is maintained
Friday night into Saturday morning, but almost all guidance ceases
any notable cold advection. Higher uncertainty here, but initial
thinking is maybe a few gusts, but probably much more marginal
overall.

As the low shifts around overhead or in relatively close vicinity
to the forecast region, it will also limit stability and help
maintain at least something of an unsettled day to day pattern. In
general, expecting isolated to scattered showers (and maybe a
thunderstorm or two), largely diurnally driven in nature, and with
the highest coverage over the high terrain and adjacent valleys.
Given guidance trending towards the low lingering longer into the
weekend, if not early next week, this will likely be the day to
day pattern for much of the long term period.

Once the low splits off from the parent trough, initially very
cold H7 temperatures will begin to moderate day over day. This
will impart a warming trend over the forecast region, with
Thursday afternoon highs around 5-10 degrees below climatological
normal increasing to near normal by Saturday, and likely slightly
above normal by Sunday on into early next week.

&&

.AVIATION...KSLC...Scattered rain showers will continue through
roughly 01z, with gusty and erratic winds and lowered CIGs to 4000
ft AGL possible near these showers. There is also a 25% chance
for thunder over KSLC, as well as graupel. Gusty west-
northwesterly winds as high as 40kts will continue until roughly
01-02z. Overnight (~04-12z), a band of lake-effect showers is
possible, mainly to the west of KSLC. However, there is still a
30% chance that this band directly impacts the terminal, bringing
a transition to snow and low (15%) chance for IFR conditions.

.REST OF UTAH AND SOUTHWEST WYOMING...Scattered showers will
continue mainly north of KDTA through roughly 03z. These showers
will be capable of producing gusty and erratic outflow winds and
MVFR-IFR conditions (especially at sites above ~6000ft ASL that
will receive snow). Lake-effect showers are possible over the
western Salt Lake Valley overnight, potentially impacting KU42.
Gusty northwesterly winds will likely decrease after sunset across
all but far southern UT (south of KMLF). A weak front pushing
through southern UT later this evening will produce a transition
to gusty northerly to northeasterly winds overnight. KSGU may
waver between gusty northeasterly winds and light westerly winds
overnight (40% chance of the lighter winds). There is also a 30%
chance that KENV could see gusts exceeding 25kts overnight.

&&

.FIRE WEATHER...While the mountain snow showers and valley rain
showers will gradually end tonight, the system responsible for
this precipitation is going to slowly shift southward and largely
remain across the region through the weekend. Sensible weather-
wise, this will mean a gradual warming trend as this area of cut-
off low pressure weakens, with showers and thunderstorms possible
along the higher terrain and adjacent valleys each afternoon and
evening into the weekend.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Freeze Warning from midnight tonight to noon MDT Wednesday for
     UTZ102-103-106-121-130.

     Winter Storm Warning until midnight MDT tonight for UTZ110>112.

     Wind Advisory from 9 PM this evening to 8 AM MDT Wednesday for
     UTZ123.

WY...None.
&&

$$

Kruse/Warthen/Cunningham

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