167
FXUS65 KTFX 060232
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
832 PM MDT Sun May 5 2024

.SYNOPSIS...

Below normal temperatures, widespread precipitation, and strong
and gusty winds are expected through the first half of the
upcoming work week as a slow moving Pacific storm moves over the
Northern Rockies and High Plains. Significant impacts to travel
are likely over the Central Montana mountains and along the
Continental Divide as heavy snow falls, especially from Tuesday
afternoon through Wednesday night.

&&

.UPDATE...

An upper level disturbance pivoting through the intermountain
west is carving out a surface low across NE WY and E MT this
evening. This surface low will drift northward and perhaps a bit
northwestward this evening, allowing for a pressure gradient to
slowly develop across the region overnight into Monday. Westerly
winds look to develop late tonight and slowly increase in
magnitude through the day Monday.

In the meantime, southerly flow ahead of this system has resulted
in showers and a few thunderstorms across the region this
afternoon and evening. Given waning surface heating, showers and
any isolated thunderstorms are forecast to diminish in coverage
over the next few hours. That does not mean precipitation chances
are over for the evening though, as a more stratiform
precipitation increases in coverage from southwest to northeast
tonight across the region. Greatest impacts from this
precipitation overnight look to be in the mountains of Southwest
Montana, where a Winter Weather Advisory remains in effect. This
area of precipitation will slowly move toward eastern portions of
North-central and Central Montana tomorrow.

No significant changes were needed with the forecast this evening.
Small tweaks were made in the near term using latest guidance to
account for trends. -AM

&&

.AVIATION...
541 PM MDT Sun May 5 2024 (06/00Z TAF Period)

The primary concerns this TAF period will be for showers across the
region this evening and tonight, increasing west winds at the
surface, and additional rain tomorrow across eastern portions of the
plains. Showers across the region this evening have largely just
resulted in virga, but a few instances of light rain have been
observed. Not expecting too many instances of sub-VFR visibility in
rain this evening, but pockets of MVFR ceilings will be around.
Gusty west winds develop late overnight and early tomorrow morning,
peaking in magnitude tomorrow afternoon. -AM

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

&&

.HYDROLOGY...

A slow moving Pacific weather system will bring an extended
period of lower elevation rain and mountain snow next week, with
the heaviest precipitation falling Tuesday through early Thursday.
NBM probabilistic guidance highlights the eastern portions of
Central/North-central MT with the heaviest rainfall amounts for
this period, largely supporting 1 to 3 inches of rainfall/snow
melt liquid equivalent over locations east of a Great Falls to
Shelby line Tuesday through Thursday.

While flooding is generally not expected, some rivers, creeks,
and streams along the Canadian border in Hill and Blaine Counties
are expected to approach or reach action stage Wednesday and
Thursday, mainly impacting unpopulated rural locations. Anyone
near waterways should remain alert for changing conditions and be
prepared to seek higher ground should water rises occur. - RCG

&&

.PREV DISCUSSION...
/ISSUED 541 PM MDT Sun May 5 2024/

Key Points:

- Significant spring storm will impact the Northern Rockies through
the middle of the upcoming work week; bringing widespread rain,
heavy mountain snow, and strong winds.

- Heaviest rainfall amounts continue to be favored over the plains
of Central and North Central Montana, with a 50% or greater chance
than amounts exceed 1" northeast of a Judith Gap, to Great Falls, to
Chester line from Monday morning through Thursday morning.

- Heaviest mountain snow continues to be favored across the Central
Montana mountains, particularly in the Little Belt and Highwood
Mountains, and across the Glacier National Park Area. The Little
Belt Mountains have an 80% or greater chance of seeing a foot or
more of snow from Tuesday through Wednesday, with a 30-70% chance of
exceeding two feet of snow.

- Strongest winds are expected to occur along and north of the
Montana Hwy 200 corridor, with NBM probabilities supporting a 70% or
greater chance that sustained wind speeds reach or exceed 40 mph at
times from Monday afternoon/evening through Wednesday.

A strong and slow moving spring storm will bring widespread impacts
to the Northern Rockies through the middle of the work, with the
highest impacts to travel, agriculture, and outdoor recreation
expected along and north of the I-90 corridor across portions of
Southwest, Central, and North Central Montana. High temperatures
through the middle of the upcoming work week will be below to well
below normal, with Tuesday and Wednesday being the "coldest" days
thanks to widespread cloud cover and periods of moderate to heavy
precipitation. A gradually moderation in temperatures is then
expected for the second half of the work week as the aforementioned
storm pulls away from the region, with temperatures rising above
normal for the upcoming weekend.

A majority of ensembles continue to highlight a closed Pacific low
lifting northeastward into eastern MT while merging with another
shortwave streaming eastward out of the Pacific NW. There will be a
bit a lull in activity on Monday as steadier stratiform rains remain
to our east/northeast and mountain snows temporarily diminish. H700
northwesterly flow begins to increase by Monday afternoon, ushering
cold air advection and increasing surface winds along the Rocky
Mountain Front and much of Central/North-central MT.

By Tuesday, periods of moderate, to at times heavy, rainfall begin
to settle in and more or less remain in place through Thursday. Keep
in mind that there will be some areas situated southeast of
higher terrain that see reduced rainfall amounts due to
downsloping winds. This widespread rainfall will generally be
beneficial for most, but some minor hydrologic impacts are
expected for those who see the heaviest storm totals, please view
the hydrology section for more information.

Strong and cold northwesterly flow aloft (H700 winds around 40 to 60
kts, temperatures near -10C) will be occurring during this timeframe
as well, resulting in windy conditions for all areas and heavy wet
mountain snow over the central ranges. Monday night through Tuesday
looks to be the overall windiest period, with most locations in both
North-central and Southwest MT seeing 60 to 90% chances for wind
gusts in excess of 45 mph, while the 55 mph exceedance higher
probabilities (70% +) remain confined along the Continental Divide,
the Central MT highway 200 corridor, and the eastern portions of the
north-central plains.

As mentioned earlier, H700 temperatures will be cold enough to
support mountain snow, heaviest over the central ranges where a
Winter Storm Watch remains in effect Tuesday through Thursday and
the probability for snowfall amounts in excess of 2 feet continues
to run 70% and higher, mostly for the mid- slopes and higher peaks
of the Little Belt Mountains. The combination of strong winds and
heavy wet snow is expected to cause dangerous conditions for outback
recreation over higher terrain and there is also the expectations
for at least some isolated impacts to the power infrastructure in
areas that receive the heaviest snow and stronger winds.  Additional
winter weather highlights may be needed over the higher terrain of
the southwest, the Rocky Mountain Front, and possibly for the Bear`s
Paw Mountains. Lower elevations will mostly see cold, wet, and raw
conditions for outdoor recreation and agriculture activities with
daytime temperatures mostly in the 40s Tuesday and Wednesday. -
RCG/Moldan

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  44  56  40  47 /  30  30  20  70
CTB  43  54  38  46 /  20  10  30  80
HLN  43  56  38  49 /  50  20  10  40
BZN  39  53  30  47 /  60  60  20  50
WYS  30  45  25  39 /  70  80  70  80
DLN  38  51  28  43 /  60  40  30  40
HVR  46  58  41  48 /  20  70  70  90
LWT  42  51  36  42 /  50  80  30  80

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
High Wind Warning from noon Monday to 6 PM MDT Tuesday for East
Glacier Park Region-Eastern Glacier, Western Toole, and Central
Pondera-Northern High Plains-Southern High Plains-Southern Rocky
Mountain Front.

High Wind Watch from Monday evening through late Tuesday night
for Bears Paw Mountains and Southern Blaine-Eastern Toole and
Liberty-Fergus County below 4500ft-Hill County-Judith Basin
County and Judith Gap-Northern Blaine County-Snowy and Judith
Mountains-Western and Central Chouteau County.

Winter Storm Watch from Tuesday afternoon through Wednesday
evening for Big Belt, Bridger and Castle Mountains-East Glacier
Park Region-Snowy and Judith Mountains.

Winter Storm Warning from noon Tuesday to midnight MDT Wednesday
night for Little Belt and Highwood Mountains.

Winter Weather Advisory until noon MDT Monday for Gallatin and
Madison County Mountains and Centennial Mountains-Northwest
Beaverhead County-Ruby Mountains and Southern Beaverhead
Mountains.

&&

$$

http://www.weather.gov/greatfalls