091
FXUS65 KSLC 212106
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
306 PM MDT Sat Sep 21 2024

.SYNOPSIS...An area of low pressure will move through the Four
Corners region through the weekend, bringing some threat for modest
canyon winds Saturday night across prone areas of northern Utah. A
mostly dry front and grazing system then move through Monday,
followed by high pressure and mostly quiet conditions thereafter.

&&

.SHORT TERM (Through 12Z/6AM Monday)...Afternoon GOES-W water
vapor and visible satellite loop both show clear evidence of upper
level low centered south of the UT/AZ border. This feature will
continue to shift northeastward through the Four Corners region
today and gradually eject through the Central Plains Sunday.

Through the remainder of Saturday, the forecast region will remain
placed more on back side of the cyclonic flow associated with the
upper low. While moisture is far from prolific (PWAT values around
0.5 to 0.7 inches), in combination with modest diurnal
destabilization and energy associated with the low it will be
sufficient to help trigger continued redevelopment of isolated to
scattered convection. Available CAM guidance suggests highest
coverage of such convection will generally be along and east of
Utah`s spine up into the Uintas or so, with activity beginning to
wane fairly quickly through the late evening hours. Given the drier
characteristics to the airmass, biggest threat from any convection
appears to still be some gusty outflow winds. That said, while
instances of more moderate to heavy rain will be limited to a
handful of the strongest cells, can`t entirely rule out seeing
something slip through a more rain sensitive and quick responding
basin. While the airmass will continue to dry moving into Sunday,
there may be enough lingering moisture to result in seeing a few
widely isolated showers develop, primarily along the higher
terrain.

Aside from the precipitation associated with the upper low, its
movement also looks to place it in a favorable area to bring a brief
period of stronger gradient driven canyon winds tonight to some of
the typically prone areas across northern Utah. Given the lack of
other components for a more typical downslope style setup (little to
no cold air advection or critical layer noted), anticipate the winds
to be fairly modest. Mean HREF wind gusts generally fall in the 25
to 35 mph range, with ensemble maximum values noted in the 40 to 45
mph range. The latter max range especially seems like a reasonable
number for some of the locally gustier canyon mouths. High
resolution guidance suggests the most favorable time window for
these winds is just prior to midnight on through around 4am or so,
but likely will see some modest gusts hang on through at least
sunrise. Given the continued departure of the low, a repeat of
strong canyon winds Sunday night looks quite unlikely.

.LONG TERM (After 12Z/6AM Monday), Issued 358 AM MDT...
Deterministic global and ensemble system agreement has improved
through midweek, with consensus favoring broad northerly flow
through Wednesday, with Utah and SW Wyoming residing on the
eastern periphery of a building PacNW ridge. Enough lingering
moisture and cyclonic flow remains to support an isolated shower
across southern Utah Monday and Tuesday afternoon/early evening,
but this activity will be few and far between. Such a pattern also
allows for largely seasonable to slightly above normal
temperatures into midweek. Heading into the second half of the
workweek, upstream PacNW ridge is shown cresting across the Great
Basin, ensuring dry, stable and outstanding early Autumn weather
with high temperatures ranging 5F to 15F above normal. As we head
into the later portion of the week and into next weekend, there is
some uncertainty introduced regarding the strength and timing of
shortwaves progressing through the southwestern Canadian provinces
and International Border region. Solutions range from keeping the
ridge firmly in place to those with stronger grazing troughs that
may offer a slight cooldown along with locally breezy conditions.
Even members with the strongest troughs offer little in the way
of precipitation as we head into next weekend, with only ~10% of
the solution space offering light precipitation amounts by next
weekend.

&&

.AVIATION...KSLC...VFR conditions will persist for the KSLC terminal
through the period. Cumulus buildups are expected into the early
evening over nearby terrain, especially southeast of the terminal.
There is a very low, or less than a 15% chance for a late
afternoon/early evening shower approaching the terminal vicinity
from the east. WNW winds, occasionally gusty up to 20kts, will
continue through 1-3Z before diminishing and turning southeast.
There is a 30% chance of volatility in winds after 03Z Sunday due to
the presence of easterly canyon winds near the terrain east of the
terminal.

.REST OF UTAH AND SOUTHWEST WYOMING...An area of low pressure will
move through southern Utah into Saturday evening, providing showers
and thunderstorms, especially over the higher terrain east of I-15
and south of I-80, with coverage increasing as one approaches the CO
and AZ borders. Largely VFR conditions expected, however can`t rule
out brief mountain obscuration in briefly heavier convection, along
with gusty and erratic winds. Canyon winds expected to develop along
the lee of the Wasatch between KSLC and KBMC after 06Z Sunday, with
local gusts near 30kts through mid-morning Sunday.

&&

.FIRE WEATHER...A trough of low pressure currently centered near the UT/AZ
border will shift northeastward through the weekend. Ongoing
activity Saturday will diminish through the evening. While the
atmosphere will begin to dry Sunday, lingering moisture will be
sufficient for a widely scattered afternoon shower or two
primarily across high terrain, but additional wetting rain chances
remain minimal. On Monday a mostly dry cold frontal boundary will
push through northern and central UT, and while some modest
increase to winds and gusts are expected with this frontal
passage, critical fire weather conditions are not anticipated.
Thereafter on through the upcoming week high pressure is favored
to settle into the region. This will promote a drying trend with
daily minimum relative humidity values and overnight recovery
trending downwards accordingly.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...None.
WY...None.
&&

$$

SHORT TERM...Warthen
LONG TERM...Cunningham
AVIATION...Van Cleave
FIRE WEATHER...Warthen

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