Weather Service Report


941 
FXUS65 KSLC 230345
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
945 PM MDT Wed Sep 22 2021

.UPDATE...The weakening storm system system with a low center
currently over the OR/ID border remains on track to push a weak
cold front into Utah late tonight into tomorrow. Updated the
forecast to make minor changes including decreasing sky cover for
the rest of the night based on latest observational trends and to
remove any mentionable PoPs prior to midnight. 

&&

.SYNOPSIS...
A weak area of low pressure moves into the region this evening, 
bringing a slight cool down to northern Utah but remaining 
largely dry. Warm and dry conditions prevail through the weekend,
with increasing chances for precipitation through the first half 
of next week.

&&

.SHORT TERM (Through 12Z/6AM Saturday)...
Sensible weather impacts in the short term remain benign with 
generally seasonable and dry conditions prevail through the end of
the work week.

A weakening mid-level low seen moving across the Pacific Northwest
late this afternoon will push a mainly dry "cold" front across 
the region tonight through Thursday morning. Smoke ahead of this
front can be seen on satellite imagery, but remains aloft per
surface PM2.5 concentrations. This matches expectations from the
HRRR smoke forecast, and thus smoke across northern Utah is
expected to remain largely aloft, however behind the weak front
passage, some near-surface smoke is expected to be pulled into
southwestern Utah on Thursday. 

Otherwise, temperatures will fall about 5 degrees into Thursday 
across northern Utah, with little to no change across southern 
Utah. A few showers are possible over northern Utah, closer to the
mid-level support and moisture, but moisture remains overall 
lacking with this storm system, thus any precipitation making it 
to the ground will be largely isolated and non-wetting in nature. 
A few cumulus builds are possible off the southern Utah mountains 
Thursday afternoon in the vicinity of the frontal boundary, and a 
stray shower or two cannot be ruled out, but largely expect 
conditions to remain dry.

By Friday, the mid-level trough will drop to our south with dry,
subsident northwesterly flow in place as upstream ridging builds
back into the area. This will allow temperatures to recover
slightly, but remains near seasonal values with otherwise dry
conditions in place.

.LONG TERM (After 12Z/6AM Saturday)...
A low amplitude ridge will allow for warmer than average temperatures
and dry conditions through the weekend. High temperatures are 
expected to be some 5F- 10F warmer than average. As a longwave 
trough digs inland from the PacNW, that ridge will move downstream
Monday. Weather conditions will remain largely unchanged from 
this weekend.

There are large differences in deterministic guidance with the 
trough. Ranges go from the 12 GFS with a grazing trough to the north 
Wednesday to the 12Z ECMWF with a closed low moving through Utah 
Thursday. Ensemble members give better confidence in how it moves 
through.

Ensemble guidance has good consistency with trough amplitude. Most 
members increase southwest flow Monday, but more so Tuesday after 
the trough axis moves inland. The majority of ensemble guidance have 
the trough amplified enough to move through southwest Wyoming and 
Utah. EOF's at 500 mb show largest variance away from the trough 
axis Tuesday and Wednesday, which is discrepancy in when it moves 
through.

The NBM illustrates this nicely with temperatures, particularly in 
the northern part of the area. Temperatures for Tuesday at Salt Lake 
City have a wide range of possible outcomes. The 25th-75th 
percentile ranges from 71F-85F Tuesday, and from 59F-76F Wednesday. 
Confidence is high in a cool down, but precisely when is still of 
low confidence.

Precipitation chances are expected to increase Tuesday as better 
dynamics move in. The best chances will likely be Wednesday, when 
most ensemble members bring the trough through. Latest forecast 
includes slight chance/chance PoPs for southwest Wyoming and much of 
Utah Tuesday and Wednesday, with that timeframe likely to be 
narrowed down some in later forecasts.


&&

.AVIATION...
KSLC...VFR conditions are expected for the remainder of the valid
TAF period. Southeast flow will prevail through the night. A 
trough will transit the region Thursday morning, shifting the 
winds out of the northwest around 15Z, then ramping up behind the
boundary with gusty northwest winds expected through the 
afternoon. Little moisture is involved with this system and while 
mid and high cloud coverage will increase, no showers or 
thunderstorms are expected for the terminal.

REST OF UTAH AND SOUTHWEST WYOMING...Generally light winds will 
prevail with VFR conditions expected across all terminals for the 
duration of the valid TAF period. A trough approaching from the 
northwest will move through the region early Thursday, driving 
increased northwest winds across the state. Some showers are 
possible along the high terrain in the northeast Utah and
southwest Wyoming Thursday afternoon but with little associated 
moisture, weather impacts should be limited to no more than 
enhanced cloud cover and gusty winds.

&&

.FIRE WEATHER...
A transient ridge will continue to cross the area late this 
afternoon. As we move into the evening, increasing flow aloft 
ahead of the approaching weak trough will bring an uptick in wind 
speeds (mainly through the Bear River mountains). RH values in 
this area will likely drop to near 15% alongside wind gusts of up 
to 25 to 30 mph, creating a period of localized critical fire 
weather conditions. That being said, with fuel moistures/ ERCs in 
this area not reaching critical levels, Red Flag Warnings will not
be issued given this borderline wind and RH forecast.

The weak trough mentioned above will continue to push into the 
area through the overnight hours into Thursday, bringing a 
potential area of light showers over the terrain of far 
northwestern Utah. These showers have little to no potential to 
generate lightning activity given the lack of moisture and 
instability. An uptick in northerly winds is anticipated Thursday 
afternoon across west-central Utah, however, speeds and rising 
afternoon RH values are not likely to create critical fire weather
conditions. Temperatures and RH will start to rebound some on 
Friday, but winds will remain light.

&&

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

$$

Cheng/Church/Wilson/Wessler

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