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000
FXUS65 KTFX 170547
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1147 PM MDT Mon May 16 2022

...Aviation Section Updated...

.UPDATE...

A few strong thunderstorms this afternoon and early this evening
across Central and Southwest Montana had reports of heavy rain,
lots of small hail, and wind gusts to 50 mph. However, the threat
for additional strong thunderstorms has mostly diminished this
evening. A few showers and weak thunderstorms may linger through
around 3 am over Southwest Montana along the trailing edge of a
cold front. Otherwise, the mid- and high-level cloudiness will
decrease from the northwest overnight, which may allow patchy fog
to form later tonight in some southwest valleys where storms moved
through.
-Coulston

&&

.SYNOPSIS...

A few showers and thunderstorms may linger into the overnight
hours over Southwest Montana, with patchy valley fog possible
towards morning; however, the plains should remain dry with
decreasing cloudiness. Dry and seasonally mild conditions follow
Tuesday and Wednesday, with increasing west winds on Wednesday.
Temperatures turn much cooler on Thursday and Friday, as a large
scale storm system produces periods of rain and snow.

&&

.AVIATION...
1147 PM MDT Mon May 16 2022 (17/06Z TAF period)

Clouds are decreasing from NW to SE tonight as showers decrease in
coverage. A few showers across Southwest Montana may persist until
around 3Z. Patchy fog will be possible through 15Z across Southwest
Montana, but confidence is low. I have opted to handle fog in TAFs
with VCFG given low confidence. Breezy westerly surface winds then
develop across the Hi-Line (KCTB and KHVR) beginning mid-morning.
Overall, VFR conditions prevail. -AM

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

&&

.PREV DISCUSSION...
/ISSUED 515 PM MDT Mon May 16 2022/

Remainder of this afternoon and tonight...Scattered shower and
thunderstorm development is now underway over the western portions
of Central and Southwest Montana this afternoon. Bulk shear in
the 30 to 40 kt range is overlaid with lapse of 7 to 8.5 C/km.
Primary concerns with these storms will continue to be strong
gusty outflow winds, hail up to 1 inch in diameter, brief heavy
downpours and of course lightning. This activity will move east
and northeastward through this evening, mostly impacting locations
south of a Helena to Lewistown line. The strong/severe threat
ends by 8 or 9 this evening, but lighter showers may continue into
the overnight hours over the southwest.

Tuesday through next Monday...Drier westerly flow aloft settles
in Tuesday and Wednesday, but temperatures trend a bit cooler as
an upper level low moving through Southern Alberta trails a weak
cold front across North- central Montana. Winds and shower
activity increase on Wednesday as a cold upper level Pacific
system approaches the Northern Rockies. Wind gusts in excess of 40
mph can be expected over the Rocky Mountain Front and the plains,
but probability of wind gusts in excess of 55 mph are running
well below 50 percent for most locations.

This system will usher in a period of colder weather with a
potential for widespread rain/snow Thursday through Friday
evening. The latest ensemble guidance support a general tenth to
half inch of liquid precipitation for low elevations with amounts
closer to an inch for the higher terrain. Given the colder nature
of the storm, accumulating mountain snow is likely and lower
elevations could see at least trace amounts of snow. Most
ensembles are now settling on a more northerly path for this
system, which will bring the most significant snowfall to the
central ranges, along the northern Divide, and the northerly
upslope areas of Central/North-central MT. Gusty north to
northwesterly surface winds are looking to be pretty strong
Thursday through early Friday, with probabilities of 55 mph gust
exceedence in the 50 to 80 percent range for much North-central
and portions Southwestern MT. Considerations for winter
weather/wind highlights will be reviewed over the next couple of
days.

High temperatures cool to as much as 10 to 20 degrees below
average with widespread 40s and 50s expected. Overnight
frost/freezes remain possible. Saturday morning still appears to
have the coldest low temperatures, though probabilities for lower
elevation temperatures falling at or below 28 degrees have fallen
off a bit from previous days. Drier northwesterly flow develops
for the second half of the weekend as temperatures inch back
closer to the seasonal average. - RCG

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  41  70  40  71 /   0   0   0   0
CTB  40  64  36  63 /   0   0   0   0
HLN  44  71  42  72 /  10   0   0   0
BZN  42  69  37  70 /  40   0   0   0
WYS  30  64  29  62 /  40  10   0   0
DLN  41  69  37  69 /  10   0   0   0
HVR  46  72  41  70 /   0   0   0   0
LWT  43  69  38  67 /  10   0   0   0

&&

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

$$

http://www.weather.gov/greatfalls