Flash Flooding and Excessive Heat in Utah, July 21, 2024

Area Forecast Discussion
National Weather Service Salt Lake City UT
407 AM MDT Sun Jul 21 2024

.SYNOPSIS...High pressure will remain in place through midweek.
Lingering moisture will allow for the development of daily showers
and thunderstorms over the higher terrain, primarily focused over
southern Utah. There are some indications that convection could
become more widespread by late in the upcoming week.

&&

.SHORT TERM (Through 12Z Tuesday)...High pressure remains centered
near the Utah/Nevada border this morning as a Pacific trough moves
northward over the ridge into the Pacific Northwest. This has put
a north to northeast flow over Utah and southwest Wyoming, with a
slight increase in the flow due to the passage of the trough. Have
also seen a bit of an easterly surface gradient develop over
eastern Utah and southwest Wyoming with the passage of the system,
that has allowed a minor downslope wind event to develop. Some
gusts in excess of 40 mph have been observed at the most prone
areas in Farmington, with some gusts in the 30s observed near
Parleys Canyon and in the Cache Valley. The gradient will tend to
weaken during the morning as the trough continues northward.

In the northerly flow, have seen some drying over northern Utah,
with satellite derived PWs in the 0.5 to 0.6 range. The deeper
moisture remains pooled over southern Utah, with PWs of 0.8 and
0.9 observed. Thus, will see good coverage of afternoon showers
and thunderstorms over the higher terrain of southern Utah this
afternoon, with more limited coverage over the mountains of
northern Utah. Steering flow will be a bit greater than the past
couple of days, but still not terribly impressive. Temperatures
over the area will remain on the warm side, with maxes up to 5F
above seasonal normals.

Another trough is expected to approach the Pacific Northwest coast
during the day Monday, acting to flatten the ridge just a bit.
Overall, flow aloft over the area will be quite light, so little
change in the moisture profile and thus little change in the
expected coverage of convection is anticipated for Monday.

.LONG TERM (After 12Z Tuesday)...With a deeper well established
north to northwesterly flow in place over the forecast area,
northern areas in Utah and southwest Wyoming will remain largely dry
while the southern and central portions of Utah continue to see
diurnally driven showers and thunderstorms. With a deeper north to
northwest flow in place, valley areas to the south and southeast of
larger terrain features will face higher chances of seeing
precipitation during the afternoon hours. Similarly, areas prone to
flash flooding that are located in a similar position to more
prominent terrain features will see a higher risk of flash
flooding... though chances are generally lower given a disconnect
from a deeper moisture source.

Similar day after day conditions are anticipated through at least
Wednesday as an area of high pressure will remain positioned over
the central Great Basin. Slow, but gradual, warming is anticipated
through Wednesday with temperatures really only warming by about 1-2
degrees per day. That said, the slow warming may be enough to tip
the scales of where prolonged exposure to heat may become a concern,
especially by Wednesday for the urbanized areas.

Late Wednesday into Thursday, the area of high pressure will begin
shifting eastward, advecting greater amounts of moisture into Utah
and southwest Wyoming. As such, will see increasing coverage of
afternoon showers and thunderstorms across the area on Thursday and
Friday , most notably across northern Utah and southwest Wyoming.
In addition, will likely see more efficient rain as more anomalous
atmospheric moisture will be in place (around 125-150% of normal for
Thursday, and around 140-175% of normal for Friday). Average storm
motion will also change directions to more of a west to east track
as the overhead pattern transitions. This could bring higher
potential for flash flooding for many of Utah's slot canyons,
National and State Parks, and recent burn scars. Will continue to
monitor the trends going forward, especially as high-resolution
guidance comes into play.

Late Friday into Saturday, model guidance has come more into
alignment on a broad trough settling over the PacNW region with a
much weaker area of high pressure positioned to the east of the
forecast area. For us, this increasingly confident scenario will
bring a fairly rapid drying trend as well as bring increasing winds
out of a southwesterly direction. As such, concerns for a transient
fire weather pattern are beginning to increase as the probability
for winds gusting in excess of 25 mph alongside minimum afternoon
humidity below 15% are approaching 20% for portions of central Utah.
At the very least, elevated fire weather conditions will arise on
Saturday and Sunday as winds ramp up out of the southwest alongside
a drying environment.

&&

.AVIATION...KSLC...VFR conditions are expected to prevail over the
KSLC terminal through the valid TAF period. Diurnally driven winds
will persist over the airfield, with southerly drainage winds
reverting to a northerly upvalley flow around 18-19Z. Any smoke from
the nearby fire is likely to disperse to higher altitudes and is not
expected to bring a reduction in surface visibility.

.REST OF UTAH AND SOUTHWEST WYOMING...VFR conditions are expected to
prevail across the area through the valid TAF period. Isolated to
scattered afternoon showers and thunderstorms are expected over the
higher terrain features, and adjacent valley areas, from about Nephi
southward. Gusty and erratic outflow winds in excess of 30kts will
be expected around showers/ storms. Temporary reduction to MVFR
category may be possible from heavier rain and/ or blowing dust.
Outside of storm dominated flows, winds will be largely terrain
driven with the exception of far northeast Utah/ southwest Wyoming
where a more persistent northeast wind is expected.

&&

.FIRE WEATHER...High pressure will remain parked near the
Utah/Nevada border through midweek, with moisture trapped under
this ridge over Utah. This will result in a daily threat of
showers and thunderstorms over the higher terrain, primarily over
central and southern Utah, with some drift into the adjacent
valleys. Any storms that develop will be capable of wetting rains
as well as strong and gusty microburst winds. By later in the
week, the influence of a weak trough will bring more widespread
showers and thunderstorms to the state.

&&

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

SHORT TERM/FIRE WEATHER...Traphagan
LONG TERM/AVIATION...Webber

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http://weather.gov/saltlakecity