209
FXUS65 KSLC 201016
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
416 AM MDT Thu Jun 20 2024

.SYNOPSIS...Temperatures will trend warmer across the area through
the weekend west of I-15 before anomalous moisture spreads into
eastern Utah late Thursday through Friday, bringing showers and
thunderstorms with locally heavy rainfall and an associated flash
flood risk along with cooler temperatures. Drier conditions are
expected over the weekend and into early next week with hot
temperatures, although enough low level moisture is expected to
linger across central and southern Utah to support a daily shower
and thunderstorm threat through early next week.

&&

Key Messages:

* Anomalous moisture will work into eastern Utah, mainly along and
  east of I-15 by Thursday night and Friday. This moisture will
  support locally heavy rainfall and an associated flash flood
  risk Friday and Friday evening. In addition, there is a risk for
  a few severe thunderstorms with a microburst threat Thursday,
  along with a microburst and large hail threat on Friday. Areas
  most as risk include recent burn scars, slot canyons, normally
  dry washes and slickrock.

* Heat re-surges across the forecast area, allowing high and low
  temperatures to rise to 10-15 degrees above average by Sunday.
  Hot temperatures combined with warm overnight lows will bring an
  increasing risk of heat related illness through at least Monday,
  especially for the urban areas. There is also increasing heat
  risk across southern Utah by the middle of next week.

.SHORT TERM (Through 12Z/6AM Saturday)...Main focus in the short
term period of the forecast is the heavy rain and severe weather
potential, particularly on Friday. Anomalous moisture for mid
June standards is already in place across adjacent states in
Colorado, New Mexico and northeast Arizona, with surface dew
points having risen into the 40s and 50s across these areas. Model
guidance continues to be in good agreement in showing this
moisture gradually advecting north and west into eastern Utah
over the next 24 hours. On the leading edge of the axis of higher
moisture content air, isolated to scattered showers and
thunderstorms are expected to develop this afternoon across
southeast Utah, with the primary hazards including isolated dry
lightning and gusty and erratic microburst and outflow winds.
Locations with the greatest risk of experiencing these winds late
this afternoon and evening include locations generally east of
I-15 and south of I-70, with probabilities steadily increasing as
one travels south and east.

On Friday, near-record values of total column moisture are
forecast to be in place across eastern Utah, when PWATs are
forecast to surge to 225-250% of normal, or up to 1.2-1.3". In
addition, a broad trough will have moved into the Great Basin,
sending a jet streak across central and northern Utah during the
day. This jet streak will place favorable entrance region dynamics
across southeast Utah. In addition, the jet will support
increasing shear, with 0-6km bulk shear values increasing into the
40-60kt range. Finally, owing to the anomalous moisture in place,
SBCAPE values are forecast to increase into the 500-1500J/kg
range, (highest values south and east earlier in the day).
Therefore, more than sufficient ingredients are in place
(moisture, lift, wind shear and instability) to support organized,
strong convection. An analysis of available CAMs suggest
convection on Friday will initiate over terrain features of
central and southeast Utah and propagate off to the north and
east through the afternoon and evening. Ensemble QPF tracks are in
the 1-2" range along the heaviest convection, with up to a 60%
chance of exceeding 10-yr ARI and a 20% chance of exceeding 100-yr
ARI. Put in simpler terms, the QPF forecast on Friday afternoon
and evening has a 20% chance of being a 1 in 100-year event, with
the highest signal across the Swell, where several slot canyons
reside. EFI QPF values approach the 90th percentile over SE Utah,
with SoT values of 8, suggesting both a high risk for an anomalous
event while suggesting a portion of the distribution is well
above the QPF mean. Given the risk of heavy rainfall, flash
flooding is probable in/near recent burn scars, slot canyons,
normally dry washes and slickrock. We have issued a Flood Watch to
highlight this risk, along with our neighbors in Grand Junction.
Coordination with WPC has placed a slight (level 2 of 4 risk) of
excessive rainfall outlook over these areas as well.

In terms of the severe weather potential, CAMs all show a signal
for a few long-track supercells across SE Utah on Friday
afternoon. EFI values for shear and CAPE are remarkable, with
extreme forecast index values above the 90th percentile generally
east of I-15 and south of I-70, with a pocket outside of
climatology over SE Utah. CAPE profiles on forecast soundings are
impressive. HREF 30% 50kt wind contours are filled in across SE
Utah once again on Friday, highlighting the risk for damaging
thunderstorm wind gusts. Given the CAPE and shear profiles, the
risk of large hail exists as well. Finally, EHI and helicity
parameters suggest a healthy potential for rotating updrafts,
especially near the SE corner of Utah. CAMs suggest activity will
quickly wane late in the evening. After coordination with SPC,
eastern portions of Utah were included in a marginal risk for
severe weather both Thursday and Friday.

Despite the quality of the ingredients in place to support both a
flash flood risk and severe thunderstorm risk, we need to discuss
some of the uncertainties with this event. One uncertainty is how
far west thunderstorm initiation will occur. The ensemble mean
products suggest the bulk of the convection will remain east of
I-15 and south of I-70, with some heavier convection also
extending as far north as the Uintas. If we look at the spectrum
of the distribution, the high end of the distribution does suggest
a potential for heavy rainfall and strong thunderstorms as far
west as Zion NP, Lower Washington County and perhaps even the
Wasatch Front. Additionally, there is some uncertainty as to how
much cloud cover will be in place Friday morning, as higher than
forecast amounts of cloud cover will limit instability/buoyancy
generation. Finally, with deep-layer shear values forecast to be
so high, it`s possible that shear/updraft/cold pool balance could
be difficult to achieve, resulting in thunderstorms that struggle
to sustain themselves. Either more cloud cover than currently
forecast and/or too much shear would act to limit the extent and
severity of both the heavy rainfall and severe thunderstorm risk.
Either way, those with plans to recreate on Friday, especially
along/east of I-15 should consider alternate plans.

.LONG TERM (After 12Z/6AM Saturday)...Saturday looks to have at
least some potential to be another interesting day across portions
of the area. Similar to Friday`s setup, forecast region will be
positioned on the NW periphery of a strong ridge extending
throughout the southern Plains, though the northern stream trough
influence will be lessened as it exits. Still, southern and eastern
Utah in particular will see continued favored moisture content with
elevated PWAT values (around 125-200% of normal) in addition to
enhanced upper level jet flow overhead. While less than that of
Friday, this jet flow should still yield marginal deep layer shear
around 20-30 kts or so. Provided any lingering cloud debris from
Friday has cleared, modest destabilization (MUCAPE ~250-750 J/kg)
should also occur. Given the departure of the northern stream
trough, a little less trigger mechanism is noted, but combination of
terrain and weak PVA should yield some scattered convective
development by the afternoon, albeit likely with lesser of an areal
extent. Given the aforementioned ingredients, the ECMWF Extreme
Forecast Index (EFI) continues to also highlight southern to eastern
Utah in shading depicting anomalous CAPE-Shear values, further
supporting at least some potential for more mature and strong
convection. Additionally, given the moisture profile, any mature
convection should be able to efficiently produce rainfall, so those
recreating in rain sensitive areas (slickrock areas, slot canyons,
dry washes, etc.) should remain weather aware. Elsewhere, dry
conditions are expected, with increasingly hot weather as H7 temps
continue upwards.

Sunday onward into midweek, anomalous moisture lingers especially
across southern and eastern Utah leading to continued daily chances
of isolated to scattered convection, and the nearby strong ridge
continues to yield very warm temperatures. By Sunday, northern Utah
warms sufficiently that the urban corridor in particular will see
heat approaching potentially dangerous levels (upper 90s to low 100s
across much of Wasatch Front). Opting to hold off at least one more
set of guidance given it is still a few days out, but it appears
increasingly likely some sort of heat related headlines may be
needed for a portion of the region. The following several days show
similar levels of heat to maybe ever so slightly cooler, though
spread in guidance also increases a bit. For the southern half of
Utah, conditions will gradually warm day to day moving into the
middle of the week, with increasing potential for heat related risk
accordingly. By Wednesday, current suite of guidance shows
widespread upper 90s on into the 100s, with current NBM giving ~25%
chance to hit or exceed 110 at KSGU. Given the prior mentioned
lingering moisture and limited daily convective potential, these
high marks will have some potential to be modulated by whatever does
manage to develop. In any case, suffice to say the heat is very much
back as we move into astronomical summer.

Looking Thursday onward a little beyond the coverage of this
forecast package, guidance shows increasing consensus on a trough
deepening southward from the PacNW. Less consensus is noted on the
eventual evolution including how quickly it advances inland and how
deep it remains when it does. While some guidance shows any trough
influence could come as early as Thursday, ensemble membership
increases by Friday, with only a limited number of members showing
little to no influence. Still, this looks to be the next window to
see the heat abated a bit, at least across the northern half of the
forecast region or so.

&&

.AVIATION...KSLC...Fairly benign weather expected in near term. Some
mid/high level moisture will result in SCT to BKN cloud cover
accordingly. Weak southerly drainage flow likely becomes more light
and variable prior to flipping back northwest mid to late Thursday
morning. Typical diurnal shift back to southernly drainage flow then
expected late Thursday evening.

.REST OF UTAH AND SOUTHWEST WYOMING...Advection of mid/high level
moisture will result in SCT to BKN mid/high level clouds across
northern terminals, while southern terminals more favored to remain
clear. Otherwise, anticipate winds to follow a fairly typical
diurnal directional pattern at area terminals.

&&

.FIRE WEATHER...Warm and dry conditions will give way to increasing
moisture spreading into southern/eastern Utah by later in the day
Thursday through Friday. Initially, on the fringes of the moisture
surge, midlevel moisture will bring the threat of isolated dry
lightning Thursday afternoon across south-central through southeast
Utah, along with isolated strong microbursts potentially in excess
of 50 mph. Additionally, minimum relative humidities Thursday will
remain on the low side at least through the early part of the
afternoon. This, combined with general southerly winds gusting to
around 30 mph will bring isolated or borderline critical fire
weather conditions to portions of south-central Utah where fuels
have cured.

As the moisture continues to increase Thursday night into Friday,
showers and thunderstorms will increase, with the threat gradually
transitioning from dry lightning and microbursts to locally heavy
rain, still primarily focusing on southern/eastern Utah, once again
mainly along and east of I-15. West of I-15, moisture will increase
just slightly. Thereafter, the airmass will trend a bit drier, but
lingering low-level moisture may maintain at least isolated
thunderstorms through early next week across central and southern Utah.

Temperatures will trend cooler across eastern Utah for Friday in
conjunction with the deeper moisture, then rebound for the weekend
and becoming very hot again Sunday into the early part of next
week.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Flood Watch from Friday afternoon through Friday evening for
     UTZ113-120-121-128>131.

WY...None.

&&

$$

ADeSmet/Warthen

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