162
FXUS65 KTFX 220247
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
847 PM MDT Tue May 21 2024

.SYNOPSIS...

After a few showers and thunderstorms this afternoon and evening,
our next impactful spring snowfall is expected to arrive on
Thursday. Expect heavy, wet snow across the higher terrain, with
snow possibly even reaching the valleys of Southwestern Montana.
Heavy wet snow that falls may result in some damage to trees and
power lines, and will be difficult to move. This snow will begin
to taper off Thursday night, though wet weather will continue
through the weekend.

&&

.Update...

Evening update has been published, with mainly minor adjustments
made to PoPs and low temperatures. Bumped PoPs up slightly through
the remainder of the evening hours, especially over the plains of
Central Montana, to account for recent radar trends which
continued to show scattered showers drifting east. These and other
showers will become more isolated in nature with the loss of
diurnal heating; however, a few elevated showers may persist into
the early morning hours on Wednesday over the mountains and
eastern zones. Rain and mountain snow will begin to increase in
coverage beyond 3AM MDT west of the I-15 corridor with the arrival
of the next upper level disturbance, especially along Continental
Divide north of the Montana Hwy 200 corridor. Low temperatures
were also nudged up by several degrees across most locations, with
the anticipation that increasing cloud cover will limit
radiational cooling throughout the nighttime hours. Finally,
patchy fog was introduced across eastern portions of the Hi-Line
where skies will remain partly clear through the overnight hours
and where recent rain has fallen from today`s showers. -  Moldan

&&

.AVIATION...
22/00Z TAF Period

VFR conditions are expected to continue across North Central (KCTB,
KHVR), Central (KLWT, KGTF, KHLN), and Southwest (KBZN, KEKS, KWYS)
Montana through at least 22/15Z.

Through 22/15Z: Weak high pressure aloft will cause lingering
clouds, showers, and breezy westerly winds to decrease across the
region through 03Z. However, a low pressure area will move into the
Pacific Northwest after 09Z, bringing increasing mid-level
cloudiness. Increasing southwest winds on the leading edge of this
low will also cause a period of mountain wave turbulence and low
level wind shear over far Southwest Montana, mainly south of a KEKS-
KBZN line.

After 22/15Z: Increasing moisture and instability the southwesterly
flow aloft will spread mountain obscuring low VFR/MVFR conditions in
showers over the region. Southerly winds will also become quite
gusty south of Interstate 90, while scattered thunderstorms could
cause erratic wind gusts up to 45 kt at times across much of
Southwest Montana. -Coulston

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

&&

.HYDROLOGY...

A Flood Watch remains in effect for the Little Belts Wednesday night
into Friday. The chances for an inch and two inches of precipitation
have increased slightly (Largely around 70% and 30% respectively)
for the Little Belts over this period. There is some uncertainty as
to how low snow levels fall with this system. Nonetheless, if most
precipitation across the Little Belts Falls as rain, there are
concerns for flooding on creeks and streams in the area given recent
precipitation/saturated soils. -AM

&&

.PREV DISCUSSION...
/ISSUED 625 PM MDT Tue May 21 2024/

Key Points:

-A few gusty showers and thunderstorms are expected across the
plains today

-Another heavy wet snowfall is expected in the higher terrain
Wednesday and Thursday that may lead to travel delays and damage
to trees and power lines

-Wet weather continues through the weekend, possibly resulting
in stream and river rises

Today and Tonight Widely scattered showers and thunderstorms
have begun to pop up across North Central and Southwestern Montana
this afternoon as moisture pushes in from the North thanks to a
low pressure system in Southern Alberta and Saskatchewan. This
moisture is interacting with a surface trough that is currently
positioned across the area, which is creating just enough
atmospheric lift to help these showers get going. For the most
part, these showers will not be much of an issue today and just
bring beneficial rainfall, though a few rumbles of thunder and
isolated wind gusts will be possible, especially across the plains
where the sun has been able to help create just a bit of
instability. Across the Southwest, expect clouds to be a bit
thicker, helping to limit surface heating and thus limiting how
intense the showers are able to become. Ludwig

Wednesday through Thursday night... The next round of impactful
precipitation will begin to move into the area tomorrow. A closed
upper level low will dive southward from BC into the Pacific NW
tonight into Tuesday, slowly turning eastward Wednesday evening,
moving across southern Idaho and vicinity before exiting eastward
Thursday night. Ensembles continue to favor Southwest through
Central Montana as the areas with the best chance for an inch of
liquid precipitation, though a bit of uncertainty is creeping in.

The first complicating factor to these chances is that the regime
appears to be convective in nature through the day Wednesday, before
transitioning to a more stratiform event Wednesday night. This would
lead to decreased confidence in impacts at any one location through
the day Wednesday. Additionally, there is a subset of guidance that
forms a H7 low much further north than the H5 low that will be
largely over southern ID. This would favor higher precipitation
amounts further north, further adding to a complicated forecast.
Regardless, mountain snow will be around in the afternoon Wednesday,
with snow levels falling through the overnight into Thursday
morning. Current guidance suggests that most major mountain
ranges south of a line from Great Falls to Lewistown have at least
a 50% chance of seeing at least 6 of snow, with the potential
for some of the higher passes to see over a foot of heavy, wet
snowfall. There is some concern that this heavy wet snow will
result in some tree and power line damage, especially now that
trees have begun to come out of dormancy and some trees may have
experienced some stress from the heavy snowfall event earlier this
month. This concern will extend into the far Southwestern
Valleys, where snow levels will likely fall to the valley floors
early Thursday morning, resulting in a coating of heavy wet snow
at locations like Bozeman, Dillon, Ennis, and West Yellowstone.
For now, I have included Bozeman, Dillon, and Ennis in a Winter
Weather Advisory with a forecast of 1-3 inches of snow, though
there is some concern that a period of heavier precipitation could
result in snow lasting longer than currently forecast, which
could result in notably higher snowfall totals in the lower
elevations. (Note that West Yellowstone was not mentioned in the
Advisory as they are currently under a Winter Storm Warning.) With
all of this said, this system should not be nearly as impactful
as our storm earlier in the month, though that is not to say that
this will be a minor event. This system will likely result in
scattered power outages, along with very difficult travel thanks
to the heavy and wet nature of the snow, as heavy wet snow is very
energy intensive to move. AM/Ludwig

Friday through Sunday... The region looks to get a brief break
Friday, as one system departs and another begins its approach.
Showers will still be around given cool air aloft, but not expecting
many impacts from these showers. Another system looks to work in
quite similarly to the Wednesday and Thursday system for this
weekend. An upper level disturbance will drop southward from BC into
the Pacific NW and then turn eastward. There is a bit more
uncertainty with respect to track and timing with this system
however, which will play a role in impacts. The chance for a half an
inch of precipitation with this system is largely lower than 50%,
owing to the aforementioned uncertainty.

Sunday night into early next week... Ensembles favor upper level
ridging building in behind the departing system Sunday night into
early next week, which will allow temperatures to trend back
closer to normal Monday and then above normal for Tuesday. This
period looks largely dry, but there will be a non-zero chance for
a shower or thunderstorm. -AM

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  45  57  37  48 /  20  80 100  80
CTB  42  53  34  51 /  20  90  80  40
HLN  47  61  37  48 /  20  90 100  90
BZN  38  60  32  45 /  10  80 100 100
WYS  30  48  25  41 /  20  90  90  90
DLN  42  57  30  44 /  20  90  90  90
HVR  40  57  37  53 /  40  70  70  40
LWT  37  56  34  43 /  30  70  90  90

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning from 6 PM Wednesday to noon MDT Thursday
for Elkhorn and Boulder Mountains-Northwest Beaverhead County-
Ruby Mountains and Southern Beaverhead Mountains-Southern Rocky
Mountain Front-Upper Blackfoot and MacDonald Pass.

Winter Weather Advisory from midnight Wednesday night to 6 PM
MDT Thursday for Gates of the Mountains.

Flood Watch from Wednesday evening through Friday morning for
Little Belt and Highwood Mountains.

Winter Storm Warning from 6 PM Wednesday to 6 PM MDT Thursday
for Big Belt, Bridger and Castle Mountains-Gallatin and Madison
County Mountains and Centennial Mountains-Little Belt and
Highwood Mountains.

Winter Weather Advisory from 6 PM Wednesday to 6 PM MDT Thursday
for Beaverhead and Western Madison below 6000ft-Gallatin Valley-
Madison River Valley-Missouri Headwaters.

&&

$$

http://www.weather.gov/greatfalls