Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 220224
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
824 PM MDT Sat May 21 2022

.SYNOPSIS...

A low pressure system moving out of the Pacific Northwest will keep
a chance of rain and snow showers, with mainly afternoon and evening
thunderstorms, across the region this weekend with better chances
across Southwest Montana. Unseasonably cold temperatures are
expected again tonight with near record low temperatures in some
areas. High pressure moving into the region for Monday will help
keep the area mostly dry, as temperatures warm back closer to normal.

&&

.UPDATE...

The evening update has been published with minimal amendments.
Model guidance continues to show development of showers across
Southwest Montana in association with an embedded mid-level
disturbance, specifically over Beaverhead County. Probabilistic
guidance from the NBM v4.1 suggests a high (70%) confidence that
at least a trace of snowfall accumulation can be expected along
western portions of Southwest Montana, steadily dropping to 10-15%
of 1" or greater accumulating (with exception to Pioneer Mountain
around 45%). Nevertheless, presence of lightning was noted near
West Yellowstone early today and its suspected that a chance of
showers may still occur tonight. Otherwise, the current forecast
remains on track, describing the current situation well. - Pierce

&&

.AVIATION...
521 PM MDT Sat May 21 2022 (22/00Z TAF period)

VFR conditions prevail across all terminals under mid- to high-level
clouds; however, scattered showers are expected across Southwest
Montana near KWYS and KEKS, briefly reducing visibility and cloud
ceilings to MVFR conditions. Winds along the surface remain light
and veer northeast to southeast during the overnight hours across
most terminals. - Pierce

Refer to weather.gov/zlc for more detailed regional aviation
weather and hazard information.

&&

.PREV DISCUSSION...
/ISSUED 522 PM MDT Sat May 21 2022/

An unseasonably deep and broad upper level trough encompasses much
of western North America, including the Northern Rockies and MT,
while at the surface a cold airmass and unseasonably strong surface
high pressure area extends from the Canadian Rockies southeast
across MT and the Northern US Plains. Within the upper level trough
there is a deformation axis draped from southern Alberta through
north-central MT and mid level circulation center over WA with an
additional embedded shortwave disturbance over ID.

Deeper instability across SW MT and the approaching mid-level
system/disturbance from WA/ID will focus the best chance for showers
and possibly a weak thunderstorm or two across areas mainly south of
I90 through this evening. Further north, cumulus cloud development
will result in mainly light showers, limited by the more shallow
instability. Showers end for most areas this evening, but may
continue through tonight across Beaverhead county as the disturbance
continues into ID. Another widespread frost/freeze is likely tonight
as surface high pressure slides east from MT to the Dakotas. Coldest
minimum temperatures, in the mid 20s, are expected across portions
of north-central MT where lesser cloud-cover is anticipated and a
few more daily/late season records may be broken.

Another cool day is expected Sunday with showers focused again
across SW MT as the upper level disturbance continues onward through
SE ID into WY by Sunday night. A shortwave ridge within the broader
upper trough moves across the area Monday, for mainly dry conditions
with temperatures returning to seasonal averages. One more upper
level disturbance dives SE across the Northern Rockies Monday night
and Tuesday as the large scale trough moves east into the central
US. SW MT again appears to have the best chance for showers Monday
night into Tuesday with a more widespread coverage of showers and
thunderstorms possible across the area with daytime heating and
instability Tuesday afternoon/evening.

Upper level ridging is still expected to move across the region
Wednesday through Thursday for mainly dry conditions with
temperatures pushing above seasonal averages. By Thursday afternoon
temperatures in the 70s will be widespread with a few locations
pushing into the lower 80s.

Longer range models and their ensembles continue to support the
overall idea of larger scale troughing moving back into the region
late this week into the holiday weekend, tilting odds in favor of
cooler and unsettled conditions with increased chances for
precipitation. There is still however a fair amount of spread in the
details, including the arrival of moisture and eventual track of
upper level features, which will dictate weather precipitation is
more widespread or showery through the period. Hoenisch

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  25  60  34  67 /   0   0   0   0
CTB  25  60  34  65 /   0   0   0   0
HLN  32  62  36  67 /  10  20   0   0
BZN  29  57  31  62 /  10  40  10   0
WYS  22  50  21  55 /  20  70  10  10
DLN  30  54  29  60 /  40  50   0  10
HVR  27  62  37  70 /   0   0   0   0
LWT  25  57  33  62 /   0  20  10  10

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

http://www.weather.gov/greatfalls


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