309
FXUS65 KSLC 041018
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
418 AM MDT Thu Jul 4 2024

.SYNOPSIS...High pressure will continue to build into the West. A
long duration heat wave will impact southwestern Utah
Sunday...reaching into northern Utah by Tuesday. This heat wave
will continue through the remainder of the work week...and
potentially beyond.

&&

.SHORT TERM (THROUGH 12Z SATURDAY)...A dry northwesterly flow aloft
remains in place over Utah this morning in between a mean trough to
the east and high pressure building over the West Coast. This
pattern will maintain near to slightly below seasonal temperatures
across the area through Friday. Besides some breezy conditions today
across the western Uinta Basin and Castle Country/San Rafael Swell
leading to locally critical fire weather conditions through this
evening, generally benign weather can be expected through the short
term forecast period.

.LONG TERM (After 12Z/6AM Saturday)...Main theme of the long term
portion of the forecast will be very hot and dry conditions. Global
deterministic and ensemble forecast systems are in excellent
agreement in their relative depictions of the presence of an
unseasonably strong high pressure ridge into the region. While there
remain some minor strength and positioning differences among the
model solution space, this doesn`t take away from the fact that
temperatures may approach record values in the Sunday through
Tuesday timeframe across Utah. By Sunday, EFI values and anomaly
plots suggest the most likely location for unseasonably hot
temperatures reside across SW Utah, with this extreme heat then
expanding into northern valleys by Tuesday and persisting through
the end of the workweek, coincident with the center of the ridge and
associated heat dome migrating overhead. Existing Excessive Heat
Warnings will likely need to be expanded in both geographical extent
and temporal extent as we get closer to the event. There is some
uncertainty as to how far out to extend these at this juncture, as
temps may ease ever so slightly across southwest Utah by mid-week,
but still remaining above normal.

So, how hot are temperatures likely to get? In St. George,
temperatures will reach or exceed 110F each day from Saturday
through at least Thursday (with lows in the upper 70s to around
80F), with a 10% chance of tying the State`s all-time record high of
117F on Sunday, per latest NBM probabilities. Thus, forecast
temperatures in terms of magnitude through the period have some
potential to rival the extreme heat wave of early to mid July 2021.
Excessively hot temperatures will also overspread Zion NP and the
Glen Canyon area where warnings are in effect. Farther north, the
hottest days are most likely to begin on Tuesday and persist through
the workweek, where high temperatures are forecast to meet or exceed
100F. Here, latest NBM advertises nearly a 30% chance of tying SLC`s
all-time record high of 107F by Thursday or Friday of next week. Day-
to-day variations in temperature trend as well as the magnitude of
the temperatures can be explained by the subtle differences in the
strength and positioning of the ridge through this time.

The bottom line is that the cumulative effects of this excessive
heat will place significant stress on people and pets, particularly
those without adequate cooling and those that spend significant time
outdoors, which adds to the vulnerability. Avoid strenuous outdoor
activities during the hottest time of the day, drink plenty of water
and limit exposure to the sun.

Towards the end of the workweek, an increasing portion of the model
solution space advertises an increase in PWAT across the region,
with this moisture originating from the western Mexican coastal
region and the Gulf of California. Latest consensus advects this
moisture to the north and into the Great Basin in the Thursday-
Sunday timeframe next week. The trajectory of this moisture plume
will ultimately be dictated by the position of the ridge by this
time, which ensemble means generally suggesting the vicinity of the
Four Corners region. It`s too early to conclude whether this
moisture will be limited to the mid-levels (tending to favor dry
lighting assuming sufficient instability) or will be manifest in an
increase in sensible low-level moisture all the way through the
precipitation-bearing layer, which would be more supportive of more
widespread rains. Either way, moisture does appear sufficient to
begin to support at least low-end probability for precipitation
across high-terrain areas of southern Utah as early as next
Thursday/Friday, with the potential gradually increasing with time.
Uncertainty does exist surrounding the timing and extent of any
moisture, which will dictate where and when precipitation chances
will appear in the forecast as we approach this period of time.

&&

.AVIATION...KSLC...Northwesterly flow will prevail over the KSLC
terminal for the next several hours, switching to a southerly flow
late. A period of light and variable flow is expected between
07-08Z before southerly drainage flow establishes.

.REST OF UTAH AND SOUTHWEST WYOMING...Predominantly northerly
flow is expected through the remainder of the daylight hours.
After sun down, light terrain driven flows are expected across
all terminals through the overnight hours under clear skies.

&&

.FIRE WEATHER...A moderate northwesterly flow aloft will continue
today as a trough grazes northeast Utah. Winds will be enhanced in
downslope regions, particularly the eastern Utah valleys. These
winds will combine with low humidities to maintain locally critical
fire weather conditions for the western Uinta Basin and San Rafael
Swell where fuels have cured. Northwesterly flow aloft will be
maintained through the end of the week across the area as strong
high pressure builds over the West Coast. Another grazing
disturbance over the weekend will bring another round of critical
fire weather conditions due to gusty winds and low humidities,
especially on Sunday. Otherwise, the airmass will remain dry for an
extended period. As the upstream high pressure gradually builds
inland, expect a warming trend across the area with hot temperatures
developing this weekend and continuing through much of next week.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Red Flag Warning until 9 PM MDT this evening for UTZ482-489.

     Excessive Heat Warning from 9 AM Sunday to 9 PM MDT Monday for
     UTZ123-124-131.

WY...None.
&&

$$

Cheng/ADeSmet

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