Helena Valley West Central, MT 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

423
FXUS65 KTFX 142320
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
420 PM MST Thu Nov 14 2024

Aviation Section Updated.

.KEY MESSAGES...

 - Breezy to windy conditions continue, especially Saturday night
   into Sunday along the Rocky Mountain Front, adjacent high
   plains, and the Central Montana highway 200 corridor.

 - Snow expected for Central and Southwest Montana Friday evening
   through early Saturday. General winter weather impacts will be
   mostly confined to mountain areas and the Bozeman area.

 - Another round of snow and slightly below average temperatures
   arrive late Sunday through Tuesday, mostly impacting mountain
   areas and the lower elevations of Central and Southwest
   Montana.

&&

.DISCUSSION...
/Issued 211 PM MST Thu Nov 14 2024/

 - Meteorological Overview:

Lighter upper level winds and relaxing surface pressure gradients
will ease surface winds over the next day or two. Mostly dry
conditions are expected through early Friday before a Pacific
Northwest trough ejects eastward and brings a round of
accumulating snow Friday afternoon through early Saturday.
Ensembles continue to highlight a closed low developing over the
Great Basin and then trekking northeast into the eastern portion
of Montana, despite much of the system`s energy diving into the
southwestern CONUS. The meteorological setup is favorable for some
accumulating snow over the southwest and the north facing slopes
of the central MT island ranges along the highway 200 corridor
amid weak northerly flow and H700 temperatures cooling to around
-10C on the backside of the low. Model guidance still struggles
with the northern extent of the snow and precise storm totals.
This is complicated further with the latest hi-res guidance being
a little more aggressive than their longer term counterparts.

This system exits the region later on Saturday, leaving another
period of transient ridging through Sunday afternoon. The
combination of mid- level winds increasing above 50 kts with
tightening surface pressure gradients will bring an onset of windy
conditions and strong wind potential Saturday night into Sunday.
Given the strongest winds aloft looking to occur during the
overnight hours, the concern for the strongest winds (sustained 40
mph, gusts 58 mph) are expected to be mostly confined to the
Rocky Mountain Front, adjacent high plains, and to areas along the
central MT highway 200 corridor.

Ensembles then favor a broad trough moving into the Northern
Rockies late Sunday into early next week followed by a period of
milder ridging for the second half of the week. Although snow
amounts with this early week system don`t look all that
impressive, and will again favor mountain areas, it will have an
ample supply of colder air aloft with -30C temperatures at 500 mb.
This should increase accumulating snow efficiency for mountain
areas and will introduce steep lapse rate for potential heavier
bursts of snow for lower elevations, especially following the cold
frontal passage on Monday. Surface temperatures also look to fall
slightly below seasonal averages for Monday and Tuesday. A return
to mild, breezy, and drier conditions is then expected for the
second half of the week. -RCG

 - Forecast Confidence & Scenarios:

Winds :

Saturday night and Sunday looks to be the windiest period, with
the strongest winds along the Rocky Mountain Front, adjacent high
plains, and the Central MT highway 200 corridor. Since the
strongest winds aloft look to occur during the overnight hours,
there will be a heavy dependence on mountain wave activity to mix
the stronger gusts to the surface and the stronger winds will come
in waves, separated by periods of lighter winds. The current
probabilities for sustained winds over 40 mph and or gusts over 55
mph are running 70 to 90% along the Rocky Mountain Front and
adjacent plains, with a 60 to 80% chance for gusts exceeding 74
mph in the Browning and East Glacier area. Probabilities for
sustained winds over 40 mph and or gusts over 55 mph are still
running in the 50 to 70% range along the Central MT highway 200
corridor. A high wind watch was issued for these respective areas.
Gusty winds are expected again during the day on Sunday for the
aforementioned areas in addition to the remainder of
Central/North-central Montana and the north to south oriented
southwest valleys, though wind speeds generally look to stay in
the 30 to 50 mph range for the areas outside of the high wind
watch.

Snow :

Although confidence remains high with snow moving into Central
and Southwest Montana late Friday into early Saturday, there
continues to be discrepancies on how far north the snow will
extend and whether impacts are felt at lower elevations along the
highway 200 corridor from Great Falls to Lewistown. Confidence is
higher for the north facing foothills of the central island ranges
where there is over a 70% chance for 2 inches of snow or more.
Probabilities for amounts in excess of 4 inches continues to
remain above 70% for most of the Madison/Gallatin mountains.

Winter weather advisories have been added for the central MT and
southwest mountain zones east of I15 given the probabilities
listed above and the increased confidence of the low pressure
system development and associated northerly flow/H700 temps around
-10C. I considered adding the western mountain zones and the
Bozeman area to the winter weather advisory, but decided to hold
off and give model guidance another shift or two to settle down.
The lower elevation areas between Great Falls and Lewistown were
also left out of the advisory with the shorter opportunity for
heavier snow, lower snowfall exceedence probabilities, and border
line temperatures. Given the forecast uncertainty, additional
highlight adjustments remain a possibility over the next 24 hours.

Snowfall early next week looks to favor mountain areas along the
Continental Divide and Southwest Montana where probabilities for 2
inches or more of snow are running around 60 to 80%. However, the
colder nature of this system should make for more efficient
mountain pass accumulations and introduce short periods of heavier
snow showers for lower elevations. - RCG

&&

.AVIATION...
15/00Z TAF Period

VFR conditions will prevail across all terminals during this TAF
period with few to no high-level clouds except for the KEKS and KBZN
terminals. At the KEKS and KBZN terminals snow will affect the
terminals at the end of this TAF period reducing visibility to MVFR
and IFR levels respectively. At the KBZN terminal at the very end of
this TAF period there is a 30% chance for having low MVFR ceilings.
At the end of this TAF period there is a 30% chance for a rain/snow
mix at the KHLN and KLWT terminals. At the KCTB, KHVR, and KGTF
terminals winds will be gusty for the first six hours of this TAF
period before they become light and variable. At the KCTB terminal
there will be isolated instances of low-level wind shear for the
first 6 hours of this TAF period. For the first six hours of this
TAF period there will be mountain wave turbulence. At the very end
of this TAF period in Southwestern Montana there will be mountain
obscuration. -IG


The KWYS TAF will not be issued until airport operations resume
next spring.

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

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  25  45  25  44 /   0  10  60   0
CTB  23  44  19  41 /   0   0   0   0
HLN  24  44  25  41 /   0  40  80  10
BZN  20  41  23  39 /   0  70 100  50
WYS  10  37  15  33 /   0  80 100  70
DLN  20  37  20  37 /   0  70  90  10
HVR  24  49  22  41 /   0   0  10  10
LWT  24  45  23  38 /   0  30  90  60

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
High Wind Watch from Saturday evening through Sunday afternoon
for East Glacier Park Region-Eastern Glacier, Western Toole, and
Central Pondera-Northern High Plains-Southern High Plains-
Southern Rocky Mountain Front.

Winter Weather Advisory from 2 PM Friday to 8 AM MST Saturday
for Big Belt, Bridger and Castle Mountains-Gallatin and Madison
County Mountains and Centennial Mountains-Little Belt and
Highwood Mountains-Ruby Mountains and Southern Beaverhead
Mountains-Snowy and Judith Mountains.

High Wind Watch from Saturday evening through Sunday afternoon
for Cascade County below 5000ft-Judith Basin County and Judith
Gap.

&&

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