Helena Valley West Central, MT 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

727
FXUS65 KTFX 220430
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
930 PM MST Sat Dec 21 2024

Aviation Section Updated.

.KEY MESSAGES...

 - Light mountain snow and lower elevation rain arrives late
   tonight and into Sunday morning.

 - Breezy to gusty winds are expected along the plains next week,
   particularly Sunday and Monday.

 - There`s a slight chance for light freezing rain north of the
   Havre to Harlem line early Sunday morning.

 - Daily chances for light mountain snow next week, with lower
   elevation mixed precipitation Christmas Eve into Christmas
   morning.

&&

.UPDATE...
/Issued 812 PM MST Sat Dec 21 2024/

The main change with the evening update was to bump up the start
time of the Winter Weather Advisory for Hill and northern Blaine
counties. Light radar returns are making their way northeastward
from Central Montana. Although little precipitation has been
observed so far, I didn`t see a reason to risk not having the
Advisory in effect given cold surfaces. Overall, a light glaze
looks most likely, with any ice resulting in very slick surfaces
tonight into early Sunday.

The other concern this evening is for a period of strong winds
tonight into Sunday morning along the Rocky Mountain Front and
adjacent plains. Latest forecast soundings are showing ~50kts
within the stable layer around Cut Bank. It seems reasonable that
a mountain wave would be able to direct a strong gust or two
toward the surface tonight. Although this is the case, the
probability for a 50 kt gust is only around 50-60% in Cut Bank.
Hence, I have elected to issue a Special Weather Statement to
cover the forecast conditions, noting that an isolated gust of
50kts will be possible in and around Cut Bank tonight.

No other changes were made at this time. -AM

&&

.DISCUSSION...
/Issued 812 PM MST Sat Dec 21 2024/

 - Meteorological Overview:

The rest of the afternoon remains dry across the region as the
upper- level ridge slowly exits to the east. An upper-level trough
moves in this evening, bringing light mountain snow and lower
elevation rain through Sunday morning. Along the Hi-line, where
shallow colder air at the surface Sunday morning, can allow a
brief few hour window for light freezing rain. This will be mainly
north of U.S Route 2 from the Havre to Harlem corridor. Strong
warm air advection will transition freezing rain to rain shortly
after. With this ridge breakdown comes stronger winds along the
Rocky Mountain Front and breezy winds across the plains.

Next week looks to be a more active pattern effecting mostly the
higher elevations. Multiple disturbances move through the region,
bringing light, mountain snow/mixed precip at lower elevations,
and gusty winds to the plains. Better chances for widespread
precipitation arrives on Christmas Eve to early Christmas morning
and late Friday night to Saturday morning. Daily minor
accumulations of snow along mountain passes will have a minor
impact to holiday travel. Temperatures also remain well above
average through the week.

 - Forecast Confidence & Scenarios:

In terms of snow today, Marias Pass has a 20-30% chance for 3" of
snow. With this higher end amount only having minor impacts to
travel, I have omitted from issuing a winter weather advisory. In
other mountain passes, minor snowfall amounts that range up to
2" likely. Throughout the week, there`s a 30-40% chance for 2" of
snow at pass level daily, leaving to minor accumulations/ travel
impacts. Late Friday to early Saturday morning, there`s a 30%
chance for areas above 7,000ft to getting 4" of snow.

For the winds Sunday and Monday, vertical profile soundings show
35-45kt winds mixing down at the Cut Bank area. Along the
immediate Rocky Mountain Front area, there`s a 40% for 70mph winds
in isolated areas. Strong winds will likely border high wind
criteria. With less than 80% confidence in these winds exceeding
criteria, I have also omitted high wind products at this time.
Gusty winds continue throughout the week, but all look to stay
below high wind criteria.

I have medium confidence in the light freezing rain chances for
areas north of the Havre to Harlem line Sunday morning. Higher-
res guidance gives a 20-40% chance for 0.01" of ice. This window
will be brief, since temperatures will quickly warm above
freezing in the morning, transitioning to rain and leaving
accumulations light (up to a few one hundreths).-Wilson

&&

.AVIATION...
22/06Z TAF Period

The main concern this TAF period will be for gusty winds to develop
across most areas. Low-level wind shear will be possible at times
before gusty winds make it to the surface overnight into Sunday, but
the duration looks to be relatively brief. Hence no LLWS is included
in TAFs at this time. The other concern will be from precipitation.
Light freezing rain looks likely in the KHVR area overnight.
Additionally, mountain snow will obscure mountains, mostly along the
Rocky Mountain Front. -AM

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

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  36  48  33  51 /  30  10   0   0
CTB  35  43  27  46 /  30  10   0   0
HLN  31  45  29  47 /  50   0   0  20
BZN  27  44  26  43 /  40  10   0  50
WYS  18  32  20  34 /  40  10  40  90
DLN  25  41  26  42 /  10   0  10  30
HVR  26  42  24  44 /  70  10   0   0
LWT  33  45  27  48 /  50  10   0  10

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 8 AM MST Sunday for Hill County-
Northern Blaine County.

&&

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