250
FXUS65 KTFX 071140
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
540 AM MDT Tue May 7 2024

UPDATED AVIATION SECTION

.SYNOPSIS...

An impactful spring weather system will bring widespread rain,
mountain snow, and strong gusty winds though early Thursday. The
most impactful conditions will occur over the Central ranges and
their neighboring northerly foothills, where the combination of
strong winds and heavy snow will cause blizzard conditions at
times. High pressure moves in towards the weekend for moderating
temperatures and drier conditions.

&&

.DISCUSSION...

A deepening mid - level low will remain over the plains of
Eastern Montana and Western N/S Dakota through Thursday, placing
North- central and Southwest Montana in a favorable position for
extended periods of lower elevation rains, mountain snow, and
strong gusty north to northwesterly winds. Rain over the
eastern/northern portions of the plains will continue to slowly
retrograde to the southwest today, reaching Southwest Montana by
this afternoon/evening. Newer model guidance has trended to more
of a northerly flow aloft for Tuesday night into Wednesday for an
overall westward shift/expansion of the heaviest core of rainfall
(1 to 3 inches) deeper into Central/North-central, but areas along
and east/southeast of a Havre to Great Falls line are favored to
see the higher 2+ inch totals according to current trends and NBM
probabilistic forecasts. This widespread rainfall will be
beneficial, but there will be at least some hydrologic concerns to
deal with. Please see the hydrology section for more information.

As previously advertised, colder air aloft, along with this
system`s dynamics and orographic north to northwesterly upslope
flow will result in periods of moderate to heavy mountain snow.
Nearly all mountain ranges in the forecast area are now covered
under winter highlights, with the exception of Western Beaverhead
County. Areas over and near the central ranges, namely the Little
Belt and Highwood Mountains are still progged to see the highest
snowfall amounts and overall greatest impacts. Probabilistic
guidance still supports snowfall amounts in the 1 to 2 feet range
with wind gusts routinely above 40 mph. The Rocky Mountain Front,
Big Belts/Bridgers, and the Madison/Gallatin Ranges will also see
higher end snow totals and potential impacts. As a mater of fact,
the Madison/Gallatin zone 330 was upgraded to a Winter Storm
Warning to address the increased synoptic favorability for heavier
snow.

There are a few lower elevation areas that may see at least a
period of impacts from wet snow, most notably over the northerly
upslope areas along the MT highway 200 corridor in between Great
Falls and Lewistown. The current Winter Weather Advisory here
covers the concerns for this area as do the advisories in effect
for the remainder of the mountain areas not under warnings, but I
did delay the start time for these advisories to this evening,
closer to when the impacts are expected. I also considered
upgrading the Bear`s Paw Mountain zone to a Winter Storm Warning
with snow totals nearing 8 inches or so over the peaks, but
decided against it with warmer air actually moving in from the
north tonight into Wednesday.

Winds were initially a little slower to develop than anticipated,
but have started to increase during the overnight hours and that
trend will continue into Tuesday with a tightening pressure
gradient and H700 flow increasing and peaking in the 50 to 60 kt
range this afternoon. The current High Wind Warnings handle the
situation reasonably well and more or less match NBM probabilistic
guidance, so no changes were made to these products.

The greatest impacts from rains, mountain snow, and winds will
occur between Tuesday afternoon and Wednesday morning before
things slowly tapper off and becoming more showery in nature
Wednesday afternoon through early Thursday. Ensemble then favor
the low pressure system moving towards the south and weakening,
with high pressure building into the Northern Rockies for drier
conditions and temperatures warming above average this weekend.
- RCG

&&

.AVIATION...
540 AM MDT Tue May 7 2024 (07/12Z TAF Period)

A strong northwesterly flow aloft will keep widespread low level
wind shear, mountain wave turbulence, and strong northwest surface
winds across North Central (KCTB KHVR), Central (KLWT KGTF KHLN),
and Southwest (KBZN KEKS KWYS) Montana though at least 08/12Z. High
Wind Warnings are in effect for much of the area.

The system responsible for these winds will also continue to spread
rain and mainly mountain snow and blowing snow southwest over the
terminals through 21Z, lowering mostly low VFR conditions to at
least widespread mountain-obscuring MVFR/IFR by then. An increase in
precipitation intensity will further worsen conditions further to
mostly LIFR by around 03Z over much of North Central and Central
Montana. Winter Weather Warnings/Advisories are in effect for
mountain areas and areas adjacent to them. -Coulston

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

&&

.HYDROLOGY...

A moisture laden spring weather system will bring prolonged
periods of lower elevation rain and mountain snow through early
Thursday. NBM probabilistic guidance supports widespread rainfall
amounts in the 1 to 3 inch range over much of Central/North-
central Montana, with the highest amounts occurring along and
southeast of a Havre to Great Falls line.

While impactful 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.
Additionally, low lying areas may see ponding of water and or
ground saturation. Anyone near waterways should remain alert for
changing conditions and be prepared to seek higher ground should
water rises occur. - RCG

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  46  34  43  37 / 100 100 100  80
CTB  45  33  44  34 / 100 100  90  20
HLN  49  37  46  39 /  70 100 100  60
BZN  48  31  44  32 /  60 100 100  80
WYS  42  24  44  27 /  70  90  80  40
DLN  46  31  47  35 /  20  80  90  40
HVR  48  38  57  40 / 100 100  90  50
LWT  39  29  40  33 / 100 100 100  90

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
High Wind Warning until 6 AM MDT Wednesday 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 Warning from noon today to midnight MDT Wednesday
night for Big Belt, Bridger and Castle Mountains-East Glacier
Park Region-Snowy and Judith Mountains-Southern Rocky Mountain
Front.

Winter Weather Advisory from 6 PM this evening to 6 AM MDT
Wednesday for Bears Paw Mountains and Southern Blaine-Eastern
Toole and Liberty.

High Wind Warning until midnight MDT tonight for Big Belt,
Bridger and Castle Mountains-Canyon Ferry Area-Elkhorn and
Boulder Mountains-Gates of the Mountains-Helena Valley-Meagher
County Valleys-Upper Blackfoot and MacDonald Pass.

Blizzard Warning from noon today to midnight MDT Wednesday night
for Little Belt and Highwood Mountains.

Winter Weather Advisory from 6 PM this evening to noon MDT
Wednesday for Cascade County below 5000ft-Fergus County below
4500ft-Judith Basin County and Judith Gap.

Winter Weather Advisory from 6 PM this evening to midnight MDT
Wednesday night for Elkhorn and Boulder Mountains-Upper
Blackfoot and MacDonald Pass.

High Wind Warning until 6 PM MDT this evening for East Glacier
Park Region-Eastern Glacier, Western Toole, and Central Pondera-
Northern High Plains-Southern High Plains-Southern Rocky
Mountain Front.

High Wind Warning until noon MDT today for Little Belt and
Highwood Mountains.

Winter Storm Warning from 6 PM this evening to midnight MDT
Wednesday night for Gallatin and Madison County Mountains and
Centennial Mountains.

&&

$$

http://www.weather.gov/greatfalls