031
FXUS65 KSLC 062127
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
327 PM MDT Mon May 6 2024

.SYNOPSIS...A cold, unsettled pattern will continue through
midweek across much of the region. A gradual warming trend can be
expected late in the week into the weekend with the potential for
lingering mountain showers and thunderstorms.

&&

.SHORT TERM (Through 12Z Wednesday)...A gradual decline in
coverage of precipitation is noted in the last hour as shortwave
ridging builds into the area in the wake of a cold upper level
low. This isn`t the end of the precipitation however, as the longwave
trough will remain in place across the region through midweek.

Afternoon upper air and satellite analysis indicates that an upper
level low is advancing into the northern High Plains. A shortwave
ridging is crossing the Great Basin. An active, relatively strong
polar jet is noted across the Pacific Northwest into northern
California.

The longwave trough will be slow to depart the West, with the next
shortwave trough pushing a reinforcing cold front through northern
Utah late Wednesday night into Tuesday morning. Lingering showers
will again shift to a period of moderate to occasionally heavy
precipitation across portions of northern Utah Tuesday morning.
With loss of synoptic support and daytime heating, expect showers
to gradually decrease in coverage Tuesday evening into Tuesday
night.

Given 700mb temperatures near -10C or so Tuesday morning into
Tuesday afternoon, rain may mix with or change to snow across the
Wasatch Front. Any accumulations are expected to be limited to
grassy and elevated surface but it is something to monitor.

Given the longer duration of winter storm conditions for the
northern mountains, extended the winter storm warning through 06Z
Wednesday. A wind advisory was also added with the morning package
for the western Uinta Basin, as wind gusts in the 45-50 mph are
fairly widespread across the Basin.

.LONG TERM (After 12Z Wednesday)...Long term forecast features a
lobe of a positive tilted trough breaking off Wednesday into
Thursday, and then retrograding southwestward through Utah before
finally getting picked back up into southern stream jet during the
weekend. Pattern evolution then becomes a bit uncertain late
weekend into early next week, though generally models favor
ridging with somewhat quieter and more mild conditions.

Starting with temperatures, system carries some winter-like
characteristics with H7 temperatures around -7C to -11C overhead
with the initial cold air surge early Wednesday. At lower levels,
this will translate to likely the coldest temperatures of the
forecast period. To that end, a number of lower elevation
locations will see temperatures push near to even below freezing
for a period of time Wednesday morning. Of those zones where more
of the agricultural growing season has started, higher confidence
is noted in Eastern Box Elder County (especially the northern
half), Toole/Rush Valleys (especially the southern half), Utah
Valley (particularly western half), and much of the San Rafael
Swell. A Freeze Watch has been issued for these locations
accordingly, and if guidance trends colder, further expansions to
the watch area may be necessitated. Currently for the Salt Lake
Valley north to around Ogden, for example, NBM probabilities show
around 10-20% chance of subfreezing temperatures at lower
elevations, and around a 30% chance at bench locations.
Temperatures Wednesday afternoon will also be quite cold for this
time of year, with highs areawide running about 10 to 20 degrees
below climatological normal. The good news (if you aren`t a fan of
this colder weather) is that as the low becomes cutoff, H7 temps
will quickly moderate through the work week, resulting in a fairly
quick surface warmup in turn. By the weekend, afternoon highs
likely return near to slightly above seasonal normal.

Next aspect of the system to monitor is in regards to downslope
wind potential in prone areas of northern Utah as the cutoff low
slides further southward. General consensus is that best
juxtaposition of ingredients (such as easterly low level flow and
cold air advection over the terrain) would likely be Friday
morning, and potentially also Thursday morning. As it stands,
ensemble clusters show ~55% of members with at least some
combination of ingredients (albeit weaker signal) coming together
Thursday morning. Friday morning almost all members show a
favorable combination of ingredients, with ~40% carrying a bit
stronger signal. In any case, given the nature of downslope events
and how things need to align just right (including in time), it
currently remains something to keep an eye on in regards to how
things ultimately trend, especially once we get within the high
resolution model window. If it does occur, would anticipate a bit
more of a diurnal trend as stronger May sun angle helps lessen
daytime cold advection, but downslope events tend to be a bit
tricky in nature.

As the low moves overhead, at least something of a mildly
unsettled pattern persists in regards to precipitation chances.
First, with the anomalously cold airmass and strong post frontal
deep north- northwesterly flow Wednesday morning, GFS derived lake
effect probabilities maintain ~20-30% chance of
showers/enhancement downstream of the Great Salt Lake. Given the
cold airmass, snow levels are forecast to be near valley floors
(~4500 ft MSL), and higher precipitation rates could result in
further lowering. Otherwise, most activity looks to be somewhat
diurnally triggered given the less stable environment, and
generally over the higher terrain day to day through the remainder
of the week. Additionally, as the airmass moderates through the
week, will see snow levels gradually increase each day.

Eventually by the weekend guidance favors the cutoff low catching
the stronger southern stream westerlies and shifting out of the
region. That said, a small amount (~25%) of mostly ECMWF ensemble
members (and the ECMWF deterministic run) do show some potential
for the low to hang around, which would result in cooler and more
unsettled conditions than what this forecast includes. Given
ridging is favored however, current forecast indicates a continued
warmup and limited (and mostly confined to high terrain) diurnal
precipitation chances.

&&

.AVIATION...KSLC...Broken to overcast ceilings in VFR range will
prevail through the evening. West winds will prevail, but
intermittent southeast winds could occur through roughly 01Z when
winds transition to southeast. Scattered rain showers will push in
by around 06Z, dropping ceilings into MVFR range. Winds will
transition to northwest around then, but directions could change
from 06-16Z, largely depending on location of showers. Drier
conditions with gusty northwest winds are likely around 16Z. Gusty
winds will continue through the day, with scattered showers from
19Z into the evening.

.REST OF UTAH AND SOUTHWEST WYOMING...Isolated valley rain
showers snow showers in southwest Wyoming will last through the
evening. VFR conditions will prevail, but ceilings and visibility
in MVFR range or lower are likely with snow showers. Scattered
showers will push in by around 06Z and last through much of the
morning. MVFR ceilings are likely for most locations, with MVFR or
lower ceilings and visibility in southwest Wyoming with snow
showers. Drier conditions are likely later in the morning with
gusty west to northwest winds. Winds will stay gusty through the
day with scattered showers by around 19Z. Dry conditions will last
through the day in southern Utah with gusty west to northwest
winds.

&&

.FIRE WEATHER...The cool and unsettled pattern continues across
the region, with a gradual transition to a more showery mode
across the northern Utah mountains. Another cold front will cross
the area Tuesday, reinforcing the cooler than normal temperatures.

By Wednesday morning, the majority of the guidance begins a
gradual warming and drying trend...with a subset keeping a
weakening low across southern Utah into the weekend. The main
impact if this slower transition occurs will be to keep the threat
of afternoon and evening thunderstorms across the higher terrain
of southern Utah and adjacent valleys.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Freeze Watch from late Tuesday night through Wednesday morning
     for UTZ102-103-106-121.

     Winter Storm Warning until midnight MDT Tuesday night for
     UTZ110>112.

     Wind Advisory until 8 PM MDT this evening for UTZ114.

WY...None.
&&

$$

Kruse/Warthen/Wilson

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http://weather.gov/saltlakecity