FXUS65 KTFX 140451
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
951 PM MST Wed Nov 13 2024

Aviation Section Updated.

.KEY MESSAGES...

 - Breezy through much of the upcoming week, especially on the
   plains.

 - Light mountain snow at times through Thursday.

 - Accumulating mountain snow across Southwest Montana Friday
   into Saturday.

&&

.UPDATE...
/Issued 907 PM MST Wed Nov 13 2024/

Main adjustment to the forecast was to decrease winds a bit over
the plains for early this evening to better reflect current
conditions. However, a shortwave trough is still on track to move
northeast over the plains through this evening, which should help
translate the gusty southwest down to the surface soon enough.

Have also added a slight chance of showers to the plains tonight
into Thursday morning, as high-resolution models are hinting
toward a few light showers moving over the area with the passage
of the shortwave. -Coulston

&&

.DISCUSSION...
/Issued 907 PM MST Wed Nov 13 2024/

 - Meteorological Overview:

Brief upper level ridging across the region is shifting eastward
ahead of a broad Pacific trough. A weak wave progressing through
the trough late today into Thursday will result in mainly light
pass- level snow across the Northern Rocky Mountain Front.
Additionally, southerly flow will result in breezy conditions
across the region, most notably in north-south oriented valleys
across Southwest Montana this afternoon and evening. The broad
troughing continues to shift eastward to close out the week, with
an additional embedded wave crossing the region Friday afternoon
into Saturday morning. This will be discussed further below in
the confidence and scenario section, but there is uncertainty
with the north/south placement of this wave, which will plays
large role on where precipitation will end up falling.

After the broad troughing exits late Saturday, zonal flow
develops across the Northern Rockies, with another trough quickly
entering the Pacific Northwest Sunday morning. The main story
with this system will be for breezy to strong winds, with a
slightly lesser concern for light snow heading into early next
week, mainly across Southwest Montana. Guidance then favors an
upper level ridge building in toward the middle of next week,
though with surface high pressure looking to develop to our west
and relatively low surface pressure over the plains, breezy winds
look to continue at times next week.

 - Forecast Confidence & Scenarios:

Winds:

The first concern is the remainder of the afternoon and evening
across southwest Montana, where a modest pressure gradient will
be in place. At this time the chance for a 58 mph gust across
Southwest Montana is only greater than 50% across terrain, with
chances generally 30% or less across lower elevation locations.
The lone exception is around Norris Hill, where the probability
is around 40%. Given the limited probabilistic support, we will
continue to hold off on High Wind Warnings for the remainder of
today and tonight. Areas in north south oriented valleys (Such as
the Madison River Valley) will undoubtedly see sustained winds on
the order of 25-35 mph, with gusts exceeding 50 mph at times.
Isolated instances of gusts in excess of 55 mph will be possible.

Another area of low confidence for strong gusts will be along the
Rocky Mountain Front out to Cut Bank around daybreak Thursday.
Ensemble (GEFS) averaged max gusts tomorrow morning in Cut Bank
is around 45 mph, with around 20% of members having a gust in
excess of 50 mph. No High WInd Warnings are being considered for
this area, but will be an area to watch.

The next period of potentially impactful winds is late Saturday
night through Sunday night across most areas. Probabilistic
guidance gives around a 90% chance for a 58 mph gust in Browning
late Saturday night through Sunday, and a 70% chance in Cut Bank
over the same timeframe. Confidence in this wind event is higher
given ensemble support, but it is too early to warrant a High
Wind Watch.


Snow:

Initial concerns will be for the Northern Rocky Mountain Front
through the day tomorrow. Confidence is high that the greatest
snow amounts will remain above pass level, hence limiting impacts
to commuters. The chance for 3 inches of snow over Marias Pass
from 5 PM this evening to 5 PM Thursday is around 40%, and
decreases to 0% for East Glacier Park over the same timeframe.
Given the limited areal extent of relatively minor impacts, I
have opted to hold off from issuing a Winter Weather Advisory for
this area at this time.


The next round of snow arrives Friday into early Saturday.
Deterministic and ensemble guidance have not been consistent with
the north/south placement of precipitation over the last couple
days and this trend is persisting into today. Confidence is high
that there will be accumulating snow in the Gallatins and
Madisons, with lesser confidence in snow as you move further
north or west. The chance for 3" snow across the aforementioned
ranges Friday into Saturday is greater than 60%. Elsewhere in
terrain the probability is 50% or less. At lower elevations the
location with the best chance for 3" (Around 20%) is the south
end of Bozeman. Minor impacts to passes such as Bozeman Pass,
Raynolds Pass, and Targhee Pass are most likely, with limited
confidence on impacts to other passes.

Another round of troughing will be around early next week, but
does not look to feature a long duration precipitation event,
rather a relatively quick period of snow as the leading edge of
the wave moves through. The chance for 6 inches of snow with this
system is only greater than 50% in the high terrain of the
Northern Rocky Mountain Front and in the Gallatins and Madisons.


Ridging mid to late next week:

Cluster guidance is nearly unanimous (90% chance) that an upper
level ridge will build in late next week, which would favor a
return of above average temperatures and a dry period. Conversely
there is a small subset of guidance (Around 10%) that does keep
upper level troughing around toward the end of next week, leaving
the door open that the region remains unsettled. -AM

&&

.AVIATION...
14/06Z TAF Period

VFR conditions are expected to continue at the terminals across
North Central, Central, and Southwest Montana through at least
15/06Z, but clouds and mountain precipitation may obscure
mountain tops on the Continental Divide for much of the period.
Otherwise, mid- and high-level cloudiness will decrease after
12Z.

Winds will remain the primary concern for the area for much of
the period. A passing disturbance will keep mid-level winds
strong through at least 12Z, causing mountain wave turbulence,
gusty south to southwest surface winds, and widespread low level
wind shear. In the wake of the disturbance, winds should decrease
somewhat, especially after 15Z, lessening the threat for
turbulence and low level wind shear, but surface winds will
remain gusty and more westerly through at least 00Z.
-Coulston

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  39  53  26  45 /  10  10   0  10
CTB  34  48  23  45 /  10   0   0   0
HLN  33  53  25  45 /  10  10   0  40
BZN  28  51  22  42 /  10   0   0  50
WYS  18  36  11  38 /  30  30  10  70
DLN  25  47  21  38 /  10  10   0  50
HVR  35  56  25  49 /  10  20   0   0
LWT  35  52  26  45 /  10   0   0  30

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
None.
&&

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