157
FXUS65 KSLC 122107
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
307 PM MDT Sun May 12 2024

.SYNOPSIS...As a low exits into Colorado, high pressure will build
over the Great Basin today, bringing a continued warming trend and
drier conditions than the past few days. A weak storm system will
graze northern Utah late Monday afternoon into the evening before
another system moves through the area Tuesday into Wednesday,
bringing less warm and more unsettled conditions. High pressure
will return late in the week with a renewed warming and drying
trend.

&&

.SHORT TERM (Through 12Z/6AM Tuesday)...Early afternoon analysis
products show a departing low pressure system located over eastern
Colorado and a building ridge over Nevada, with modest lingering
cyclonic flow hanging back over Utah and SW Wyoming within broad,
weak northerly flow. Dry advection has been ongoing today with
PWATs dropping to 0.4". Just enough moisture lingers to support
isolated showers and perhaps a thunderstorm from SW Wyoming SW
through the Book Cliffs, where thunderstorm probabilities hover
just above 15%. Any lingering convection will quickly wane with
the loss of daytime heating this evening. Above-mentioned ridge
over Nevada will build eastward across our area this evening while
simultaneously flattening due to an approaching Northern Rockies
shortwave. This shortwave will track through Montana and Idaho
Monday afternoon/evening while clipping extreme northern Utah in
the process. As a result, scattered showers and thunderstorms will
break out across northern Utah and SW Wyoming Monday afternoon
and evening. Besides lightning, main hazard appears to be gusty
outflow winds, with HREF probabilities for 40-50kt winds topping
out around 70% across northern Utah late Monday afternoon and
evening. The combination of SBCAPE values up to 750J/kg and deep-
layer shear values of up to 35kt by 6PM supports the notion of
some organized convection. With modest boundary layer moisture
creeping into southern Utah by late Monday afternoon/evening,
can`t rule out isolated convection developing here as well,
confined primarily to high terrain areas.

.LONG TERM (After 12Z/6AM Tuesday)...Long term period starts with a
fairly weak northern stream grazing shortwave and associated frontal
boundary beginning to bring light impacts to the region. The
shortwave will dig southeast from the northern Rockies on into the
central/northern Plains Tuesday into Wednesday, with the weak and
somewhat diffuse frontal boundary sagging southward into the local
forecast region accordingly. This boundary will likely stall
somewhere across southern Utah before ultimately washing out. In
regards to temperatures, this system will result in slightly cooler
afternoon highs both Tuesday and Wednesday, though temperatures will
still largely be near to slightly above climatological normal for
mid May.

Combination of the system moving through overhead, cold frontal
boundary sagging southward, and diurnal heating will trigger some
isolated to scattered convection both Monday and Tuesday, with
slightly more coverage currently anticipated Tuesday. On both days
convection will generally be favored over the high terrain and
adjacent downwind valley locations, as well as some extra focus
along and likely south of where the cold frontal boundary ends up.
Limited CAM guidance so far, but what is available suggests this may
be somewhere near the I-70 corridor Tuesday afternoon at least.
Given moderately strong daytime heating south of the boundary as
well as somewhat limited low level moisture, could see dry subcloud
layers help contribute to a few stronger wind gusts. While initial
glance shows maybe a bit less coverage north, what does develop will
have a bit more deep layer shear to work with given better jet
support nearer to the trough, and with mid level lapse rates around
7 C/km or so, wouldn`t completely discount a stronger storm or two.
The ECMWF Extreme Forecast Index also shows some slight shading to
CAPE-Shear across portions of northern Utah, giving a bit more
support to this idea. Convection will largely wane with loss of
diurnal heating late Tuesday evening, with increasing redevelopment
by early Wednesday afternoon. Remnant boundary will likely be losing
more definition by Wednesday, so anticipate coverage a bit more high
terrain and adjacent valley focused.

Guidance has come into a bit better agreement in ridging nosing in
from the west at least for Thursday. This will help kick off a
warming trend, as well as introduce enough stability to largely
preclude area convection.

Friday on into the weekend continues to see some inconsistency
between guidance sources in just how exactly the pattern evolves as
a more marked upper jet begins to push into the PacNW. For Friday at
least, the majority of ensemble members lean in favor of continued
ridging and associated warm/dry conditions, with only about 25%
bringing some sort of impacts from northern troughing
(cooler/slightly unsettled). Clusters of plausible solutions then
become even more spread for the weekend, ranging from continued
ridging and warm/dry conditions, or varying degrees of cooler/wetter
trough scenarios. Deterministic model runs suggest some potential
for something similar to the early/midweek system with more grazing
style impulses, but even then spread is quite high. For temperatures
for instance, NBM at KSLC has 11F of spread between 25th/75th
percentile values for Saturday afternoon highs, increasing further
to 13F Sunday. As expected, similar spread is noted in H7
temperatures across global ensemble members for the northern portion
of the forecast region. With all that, will just have to keep an eye
on day to day trends to ultimately see things pan out.

&&

.AVIATION...KSLC...Northwesterly winds will continue until a
transition to southeasterly around 03-04z. VFR conditions will
prevail.

.REST OF UTAH AND SOUTHWEST WYOMING...Scattered showers will
continue roughly east of the central spine of Utah mountains through
02-03z. These showers may be capable of producing gusty and erratic
outflow winds up to 25kts. Otherwise, expect VFR conditions and
largely diurnally-driven winds.

&&

.FIRE WEATHER...The area of low pressure that had been in place
over the past few days will exit Colorado this evening, taking
moisture with it. A few, lingering isolated thunderstorms will be
in place early this evening, mainly over high terrain areas and
adjacent valleys. On Monday, an area of low pressure will swing
through the Northern Rockies, dragging a weak cold front into
northern Utah on Monday afternoon and evening. This will bring
isolated to scattered showers and thunderstorms to northern Utah
during this time, with a threat for gusty, erratic outflow winds.
Another disturbance will force the cold front southward, and as
such the threat for isolated to scattered showers and storms will
shift into central and southern Utah for Tuesday, primarily in the
afternoon and evening. A few, isolated showers and storms may
linger across southern Utah on Wednesday as the now weakening
front will linger across this area. A modest cooling trend will
build in behind the front with an equally modest increase in
humidity. Wetting rains should be isolated during this period. A
ridge will build across the area for Thursday and Friday with a
warming and drying trend. This will cause increasingly poor
overnight recovery across southern Utah. A grazing trough sliding
eastward near the Idaho/Utah border next weekend will bring an
increase in winds, a slight cooldown and a low (15% or less)
chance for showers as of today`s forecast.

&&

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

$$

ADeSmet/Warthen/Cunningham

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