Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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000
FXUS65 KSLC 192159
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
359 PM MDT Mon Oct 19 2020

.SYNOPSIS...The stable northwest flow aloft will maintain warm
temperatures and dry conditions through Wednesday. A storm system
moving southeast through western Canada will push a mostly dry
cold front south across Utah Wednesday night through Thursday.
A second colder system could arrive for the upcoming weekend.

&&

.SHORT TERM (Through 12Z Thursday)...Little change in the overall
pattern is expected with the persistent, stable west-northwest
flow aloft maintaining warm and dry conditions across the
forecast area into Wednesday. Mid/high-level moisture embedded in
the flow with bring periodic cloud cover, but no real chance at
precipitation through Wednesday.

Changes will develop toward the end of the current short term
forecast period. Amplification of the eastern Pacific upper ridge
beginning Tuesday will take the shortwave on the downstream side
of the ridge and drive it southeast through western Canada and
then into the Pacific Northwest/northern Rockies by early
Wednesday. This shortwave supported by the upper jet and favorable
thermal support will strengthen as it moves into the western
CONUS.

The associated surface front will reach into northern Utah late
Wednesday afternoon, then strengthen as the trailing low-level
cold advection accelerates the boundary south across the state
Wednesday night through Thursday.

The main impact from this feature will be winds and temperature
changes. Anticipating fairly strong pre-frontal and post-frontal
winds in vicinity to the front across northern and western Utah
late Wednesday afternoon and evening.

Precipitation looks to be quite limited along and behind the front
across far northern Utah. Fairly strong low-level forcing will
lack mid-level support for decent synoptic-scale lift as the core
of the trough remain well removed to north of the state. Can not
entirely rule out precip, though it would likely remain confined
to areas near the Idaho and Wyoming borders with Utah Wednesday
evening.

.LONG TERM (After 12Z Thursday)...As a glancing cold front lifts
E/NE out of northern Utah and southwest Wyoming Thursday morning,
there will be noticeable weather changes. Thursday highs for much of
the northern half of the CWA will be shortly after midnight.
Afternoon temperatures will be around 20 degrees cooler from
Wednesday to Thursday. Not much precipitation will come with the
front, but the highest QPF values will be over far northeast Utah
and southwest Wyoming, generally less than 0.05". Light snow amounts
in the northern Wasatch and in the Uintas is possible.

While Thursday will bring big weather changes, the main focus of the
long term forecast is around increasing confidence of a digging
longwave trough from the north later Saturday. Where uncertainty
starts to build in is with the depth of CAA, precipitation
potential, and wind potential.

Two general setups are looking to be most possible. With both, much
cooler weather is expected. The main difference is with PoPs,
precipitation type, and wind. If the trough is more progressive, or
faster, the area would not cool down as much, less precipitation
would be expected, and winds would be strongest to the north. A more
amplified positively tilted trough would result in colder
temperatures, more precipitation, including mountain and valley snow
stretching into southern Utah, and more wind extending further south.

As guidance comes into better agreement regarding the weekend cool
down, precipitation, and wind potential, more confidence in details
of the forecast will be made. For now, expect cooler temperatures
after the first frontal push Wednesday night/Thursday morning and
likely a bigger surge of cooler conditions with better precipitation
potential later Saturday into Sunday.

&&

.AVIATION...VFR conditions are expected at KSLC through the valid
TAF period. Northwest winds will transition to the southeast around
03Z with less high cloud cover.

&&

.FIRE WEATHER...The well established stable northwest flow aloft
will persist through Wednesday. Moisture embedded within this
northwest flow will bring periodic mid and high level cloud
cover with no real chance for precipitation across northern
Utah through early Wednesday.

Fairly strong winds aloft will generate gusty surface winds
across portions of northern and eastern Utah the next couple of
days. The strongest winds will occur across the higher exposed
terrain, with elevated winds briefly showing up across the far
northern valleys again Tuesday afternoon.

A storm system dropping southeast through western Canada will
settle into the Pacific Northwest/northern Rockies by early
Wednesday. The associated cold front will reach northern Utah
by late Wednesday, then sweep south across the state Wednesday
night through Thursday. Winds both ahead of and behind the cold
front will become strong and gusty by Wednesday afternoon. The
expected winds will fall within critical fire weather criteria,
but humidity values may stay just above the critical threshold
across the north. Down south the already dry air mass along with
the increasing winds could lead the critical fire weather
conditions on Thursday.

&&

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

$$

CONGER/WILSON/VAN CLEAVE

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http://weather.gov/saltlakecity

For frequently asked questions about the Area Forecast Discussion
visit...
http://weather.gov/saltlakecity/general/afd_faqs.php



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