Helena Valley Northwest, MT 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

864
FXUS65 KTFX 302137
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
237 PM MST Thu Jan 30 2025

.KEY MESSAGES...

 - Strong winds are possible across portions of Southwest through
   North Central Montana from Friday through Saturday.

 - An Arctic front will surge south Saturday afternoon and
   evening, bringing much colder temperatures to the plains of
   Central and North Central Montana through next week.

 - A prolonged period of accumulating snow, beginning as early as
   Friday along the Continental Divide, is expected in wake of the
   Arctic front and through the upcoming work week.

&&

.DISCUSSION...

 - Meteorological Overview:

After at least a week of dry conditions and above normal
temperatures, a cold and moist westerly flow aloft is expected to
send a series of shortwaves through the Northern Rockies while an
Arctic boundary surges southward and plunges temperatures down to
near and below zero. Snow begins along the Rocky Mountain Front
(RMF) as early as Friday January 31st and continues periodically
into the following week, with the most impactful winds and heaviest
burst of snow for the RMF expected Saturday morning and early
afternoon.  The remainder of the forecast area may see periods of
locally intense snow showers and gusty winds with the passage of a
Pacific front Saturday afternoon before the Arctic boundary surges
southward Saturday night into Sunday.

The Arctic boundary will become stationary or even retreat to the
north some on Sunday, potentially setting up somewhere along or just
to the north of the I90 corridor.  Local knowledge suggests that a
more concentrated area of snow will set up along the boundary Sunday
into Monday, while other areas see periods of lighter end snows.

A stronger shortwave approaches later Monday into Tuesday and
initially brings warm air advection aloft for widespread overrunning
snow, mostly for plains locations.  The final round of snow comes
with the passage of the main shortwave Tuesday night into Wednesday
and may be heavy at times. - Greely

 - Forecast Confidence & Scenarios:

High Winds from Friday through Saturday :

Madison River Valley - The pressure gradient orientated
perpendicular to the north-south valleys of Southwest Montana will
begin to strengthen this evening as surface high pressure over
Eastern Idaho builds and an area of weak low pressure remains in
place over the plains of Central and North Central Montana. This
strengthening pressure gradient, with values of 0.05 to 0.10 mb/km,
will lead to increasing southerly surface winds within the
constricted Madison River Valley of Madison County starting tonight.
Winds will then become strong and gusty by Friday afternoon as the
low to mid-level southwesterly flow increases to between 35-45kts.
While the aforementioned pressure gradient will gradually relax into
the day on Saturday across Southwest Montana, persistent
southwesterly flow of 35-45kts just above the surface will help to
maintain strong and gusty winds through Saturday afternoon. Latest
NBM probabilities for wind gusts in excess of 48kts from Friday
afternoon through Saturday afternoon range from 50-80%, with the
probability for sustained wind speeds in excess of 34kts also ranging
from 50-80%. Given the surface setup and supportive wind
probabilities a High Wind Watch has been issued from noon Friday
through Saturday afternoon.

Lower Elevations of Cascade and Judith Basin County - Increasing
westerly flow ahead of a potent shortwave and associated Pacific
front on Saturday has the potential to produce high winds along
portions of the Montana Hwy 200/US Hwy 89 corridors from Great Falls
to Eddies Corner, along with in/near the city of Cascade. While NBM
probabilities for wind gusts across these areas generally range from
a 50-80% chance the probability for sustained wind speeds in excess
of 34kts are nearly the same if not 5-10% higher. Given the strong
support for high sustained speeds combined with the favorable timing
of the Pacific front (early to mid-afternoon on Saturday) a High
Wind Watch has been issued here from late Friday evening through
Saturday afternoon.

Rocky Mountain Front and adjacent high plains - BUFKIT soundings
marginally support the potential for mountain wave activity from
Friday night through Saturday morning; however, thermal profiles
near ridgetops are less than ideal. Of more concern, and similar to
portions of Central Montana will be the increasing cross barrier
from Saturday morning through the afternoon, which will coincide
with the passage of the Pacific front and shortwave aloft. Should
the strongest winds remain solely with the passage of the Pacific
front then this scenario would likely only result in a 3-6 hour
window of high winds vs the current High Wind Watch timeframe. NBM
probabilities for wind gusts in excess of 48kts for the Cut Bank
area on Saturday are 75%, with identical probabilities for sustained
speeds of 34kts or greater. For the Browning area and points south
along the US Hwy 89 corridor to the MT Hwy 44 Jct the probability
for wind gusts in excess of 64kts are 40-70% and for sustained
speeds of 44kts or greater is 30-60%. One reason for not upgrading
the High Wind Watch with this forecast package was concerns that the
Arctic front could influence/intrude on the aforementioned areas
prior to the Pacific front arriving, which would all but inhibit the
strong winds from mixing down.

Accumulating Snow :

A prolonged period of accumulating snow, with a duration of blowing
snow from Friday through Saturday along the Continental Divide
(especially from Marias Pass to near Browning), remains on track
across the Northern Rockies from the end of the work week through at
least the middle of the upcoming work week. With respect to the
orographic upslope/overrunning from along the Continental Divide
from Friday afternoon through Monday afternoon the latest NBM 25th
Percentile ranges from 8-14" with the NBM 75th percentile ranging
from 14-24". With EFI values ranging from 0.5 to 0.7 across the
Divide it is not out of the realm of possibilities for the NBM 75th
Percentile values to verify , which combined with strong and gusty
winds will produce accumulating impacts. Beyond Monday afternoon
ECMWF EFIs continue to hint at the potential for a climatologically
extreme event with respect to snowfall, especially when the mid-
level warm front and shortwave lift north over the Arctic airmass
from Monday night into Wednesday. - Moldan

&&

.AVIATION...
30/18Z TAF Period

VFR conditions are expected to persist over the region through at
least 31/18Z. The main concern will be gusty winds along portions
of the Rocky Mountain Front as well as the Madison River Valley.

North-Central Montana (KCTB, KHVR): Gusty winds will start off the
TAF period for KCTB with winds slowly tapering off and becoming
light by 31/10Z. There will be some LLWS concerns through the
first 6 hours of the period for KCTB which should largely diminish
by 31/00Z. Otherwise, scattered to overcast high level clouds will
move through during the period.

Central Montana (KGTF, KHLN, KLWT): Generally light winds will
persist at KHLN and KLWT through the period. There will be gusty
winds at KGTF through much of the period with the strongest winds
expected now through 31/06Z.

Southwest Montana (KBZN, KEKS): For KBZN, light and variable winds
will persist through much of the period with some increasing winds
possible after 31/22Z. For KEKS light winds will start off the
TAF period with gradually increasing winds through the day with 30
to 45 kt gusts possible towards the end of the period. Sustained
winds are expected to pick up above 20 kts by 31/06Z with the
strongest winds expected to arrive just outside the 30/18Z TAF
window and continue through the remainder of Friday and Saturday
with gusts up to 50 kts possible. -thor

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

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  36  44  38  43 /  10  10  10  20
CTB  30  41  32  38 /  10  10  10  10
HLN  14  32  27  41 /   0  10  50  70
BZN   7  39  21  40 /   0  20  20  70
WYS   0  29  21  31 /   0  80 100  90
DLN  16  37  23  39 /   0  10  30  50
HVR  27  36  30  42 /  10  10  30  10
LWT  29  43  32  42 /   0  10  30  20

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning from 8 AM Friday to 5 AM MST Sunday for
East Glacier Park Region-Southern Rocky Mountain Front.

High Wind Watch from Friday evening through Saturday evening for
Cascade County below 5000ft-Eastern Glacier, Western Toole, and
Central Pondera-Judith Basin County and Judith Gap-Northern High
Plains.

Winter Storm Watch from late Saturday night through Monday
afternoon for Big Belt, Bridger and Castle Mountains-East
Glacier Park Region-Elkhorn and Boulder Mountains-Gallatin and
Madison County Mountains and Centennial Mountains-Little Belt
and Highwood Mountains-Northwest Beaverhead County-Snowy and
Judith Mountains-Southern Rocky Mountain Front-Upper Blackfoot
and MacDonald Pass.

Winter Weather Advisory from 5 PM Friday to 5 AM MST Sunday for
Gallatin and Madison County Mountains and Centennial Mountains.

High Wind Watch from Friday afternoon through Saturday afternoon
for Madison River Valley.

Winter Storm Warning from 8 PM Friday to 5 AM MST Sunday for
Northwest Beaverhead County.

&&

$$
http://www.weather.gov/greatfalls