Area Forecast Discussion
Issued by NWS Salt Lake City, UT

000
FXUS65 KSLC 261035
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
435 AM MDT Fri Apr 26 2024

.SYNOPSIS...An active pattern will continue into the weekend,
resulting in valley rain and mountain snow along with a cooling
trend.

&&

.SHORT TERM (THROUGH 12Z SUNDAY)...Cooler, wet weather will continue
across the area today and through the bulk of the short term
forecast period as a series of weather disturbances continue to
cross the area. This will bring mountain snow and valley rain along
with scattered thunderstorms to much of the forecast area.

A weak disturbance is currently rippling through northern Utah. This
has resulted in the line of showers and isolated thunderstorms
tracking east across the Wasatch Mountains at this time. Slightly
farther upstream, the next storm system is carving into the Pacific
Northwest states and into the western Great Basin. An associated
upper jet is rounding the base of the mean trough, punching into the
desert southwest. Upper diffluence along the left exit region of
this jet and ahead of the approaching trough has already resulted n
some precipitation across southwest Utah, and this will only
increase from western to eastern Utah through the morning hours and
into the afternoon, when scattered to widespread showers are
expected. A closed circulation is then progged to develop over Utah
tonight, with bands of showers expected to rotate counter-clockwise
through the area. Precipitation will increasingly favor western and
northern Utah heading deeper into the night with moisture wrapping
around the low in association with a TROWAL feature. By tomorrow
afternoon, the low center will be east of Utah, with a north to
northwest flow settling over Utah. This will cause precipitation to
become increasingly confined to the spine of Utah and eastern
valleys through the afternoon and evening hours tomorrow.

The ECMWF-based Extreme Forecast Index is still indicating a signal
for anomalously high QPF today and, to a slightly lesser extent,
tomorrow. However, while showers may be fairly widespread, the
heaviest precipitation rates will be associated with the stronger
showers, which can be hit and miss at times. In addition, latest
HREF ensemble means of SBCAPE values remain under 250J/kg today,
with isolated spots of up to 500J/kg. As a result, have trended with
a bit lower (though still substantial) QPF for today. However, with
cooler air spreading into the area, snow levels will be lower today
and tomorrow, generally in the 7500-8500ft range. This will result
in better snow accumulations. While snow amounts have trended down a
bit this shift, significant accumulations are still expected above
around 8000-8500ft, and have maintained going Winter Weather
Advisories. Impacts will remain limited below this elevation range.
However, will have to monitor how the additional rain in the lower
elevations impact streamflows, which are already a bit high due to
snowmelt.

Otherwise, afternoon max surface temperatures will be significantly
(~10F) cooler today, falling below climatological normals, with
similarly cool temperatures continuing tomorrow.


.LONG TERM (After 12Z/6AM Sunday)...With the main portion of the
late week/ early weekend storm system well east of the area, Sunday
will feature generally warmer and drier conditions. That said,
lingering moisture in the lower levels will combine with a trailing
shortwave disturbance on Sunday which will help to bring another day
of scattered showers and isolated thunderstorms across the northern
Utah mountains and southwest Wyoming. Not expecting much in the way
of rain/ snow accumulations, however, high elevation terrain seeing
multiple showers could see upwards of 1 inch of snowfall (above
~7,500-8,000ft). Temperature-wise, we should see warming of about 5
degrees across the board as warmer air moves in overhead.

Moving into the first half of the week, ensemble guidance continues
to point toward a broad trough remaining in place over the PacNW
region, which will keep an active storm track just to the north of
us. Within this broad trough, expecting a couple rounds of shortwave
troughs progressing through the mean flow which are showing
increasing potential of skirting along the northern half of Utah.
The first of these waves is progged to swing through the area
through the day on Monday, helping to kick off another round of
showers across northern Utah and southwest Wyoming. In addition to
the showers, a surface cold front is also anticipated to drop into
northern Utah/ southwest Wyoming, however, there is some uncertainty
with how far south it progresses. At this point, the resulting
airmass will have a limited lifespan over the area, so not expecting
much in the way of cooling. The shortwave lifts out of the area
through Monday night, however, lingering moisture and weak
instability may still allow for afternoon cloud buildups to develop
into light showers in the northern Utah mountains on Tuesday
afternoon.

A second wave looks to progress to the north of the area in the wake
of the aforementioned on Wednesday, however, there is considerable
uncertainty in how far south this wave will track. In fact, only
about 30% of ensemble members progress this feature far enough south
to generate appreciable precipitation. However, these 30% of
ensemble members supporting a deeper trough would also favor much
cooler temperatures than forecast. For example, the spread between
the 25th and 75th percentile in NBM temperature forecasts is about
16 degrees (ranging from 62 to 78)! Will certainly need to monitor
trends in this trailing wave.

A progressive pattern remains in place heading into the late week as
zonal flow remains steadfast over the northern half of the forecast
area and another trough develops in the PacNW region. Ensembles hold
off on the trough moving into the region until the weekend, which
would also delay the associated precipitation and cooler air. Still
seeing plenty of uncertainty in the models at this time, with a
nearly 50/50 split between above average mid-level heights vs below
average.

&&

.AVIATION...KSLC...Lowering cloud bases and increasing shower
activity will be the name of the game today as a storm system
arrives in Utah. Expecting vicinity showers through a majority of
the day, with the most likely period to see showers impacting the
terminal from 20-01Z. During this time, there is a 30-40% chance of
thunderstorms moving over KSLC. CIGs will remain around 4-5kft AGL
through the day, lowering to around 3kft AGL with passing showers.
Expect variable winds through the morning with light northerlies
prevailing after 15Z.

Rest of Utah and Southwest Wyoming...An active weather day with
widespread showers and isolated to scattered thunderstorms is
expected across Utah and southwest Wyoming during the day today. VFR
conditions will prevail outside of showers with CIGs generally in
the 4-5kft AGL range. Within showers, expect MVFR CIGs. At higher
elevation terminals (e.g. KEVW/ KBCE) expect periods of rain-snow
mix to bring MVFR to IFR due to VIS restrictions.


&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Winter Weather Advisory until 6 AM MDT Sunday for UTZ111-112.

     Winter Weather Advisory until 6 PM MDT Saturday for UTZ113-117-
     125.

WY...None.
&&

$$

Cheng/Webber

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