Helena Valley, Montana 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

431
FXUS65 KTFX 210501
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1001 PM MST Wed Nov 20 2024

Aviation Section Updated.

.KEY MESSAGES...

 - Periods of light to moderate mountain snow along the
   Continental Divide through Saturday night, with the highest
   snowfall totals falling north of the MT Hwy 200 corridor to the
   Canadian border
 - Light snow spills over the Continental Divide and over the
   plains of Central and North Central Montana Thursday night
   through Friday night.
 - Minor ice accumulations are possible Friday over most plains
   locations of Central and North Central Montana, in addition to
   some valleys in Central Montana.
 - A strong cold front surges south on Saturday, brining much
   colder temperatures and widespread snow through the remainder
   of the weekend.

&&

.UPDATE...

Only some minor updates this evening, mainly to add the potential
for patchy fog in and around Great Falls. Fog will likely be brief
as precipitation arrives, but still could briefly reduce
visibility to a mile or so. Ludwig

&&

.DISCUSSION...
/Issued 501 PM MST Wed Nov 20 2024/

 - Meteorological Overview:

Rest of today through Friday...Longwave H500 troughing will remain
in place over the eastern Pacific through the period, with overall
rising heights through Thursday afternoon before the next shortwave
rotating around the trough axis begins to eject towards the
northeast and over the western seaboard. This shortwave will
continue to lift east and northeast towards the Northern Rockies
through Friday night, with height falls approaching 100 meters
overspreading Western Montana between 06-12z Saturday. Prior to the
arrival of the aforementioned shortwave, broad ridging will remain
in place just to the east of the Northern Rockies with moist,
southwest flow aloft over Southwest through North Central Montana.
This flow regime, in addition to an abundance of Pacific moisture
(PWATs of 1 to nearly 3 standard deviations above normal), will lead
to continued cloudy to mostly cloudy skies across the CWA through
the end of the work week, with periods of light to moderate snow
falling along the Continental Divide given favorable upslope
conditions. As the H500 shortwave and attendant closed low begins to
approach the Pacific Northwest, strong mid-level WAA associated with
a northeastward lifting warm front will move over the surface to
~2000ft agl cold air banked up against the mountains across the
plains of Central and North Central Montana. This overrunning
scenario combined with mid-level frontogenesis associated with the mid-
level warm front will allow for precipitation to spill east of the
Continental Divide and over all of Central and North Central
Montana. Snow and/or light freezing drizzle will be possible across
all of the plains of Central and North Central Montana from late
Thursday night through at least early Friday afternoon, with ongoing
precipitation through Friday night along the Hi-Line in North
Central Montana. - Moldan

On Saturday, a strong Canadian cold front moves southward
through the CWA. Overall, I am about 90 percent confident in this
front moving southward and causing impacts. Typical impacts from
fronts like this are a few hours of moderate to heavy snowfall and
temperatures falling quickly. The front is likely to be moving
southward through North Central MT on Saturday morning and Southwest
MT late morning into the early afternoon hours. There is a
reasonable chance the front could move through quicker than
currently forecasted, so some adjustments to temperatures/snow
amounts might be need on Saturday.

Confidence is high for cold temperatures Sunday and Monday.
Additionally, once the cold air moves southward into the CWA, it
takes a fairly strong southerly wind to move it back northward this
time of year. Overall, I don`t see a strong southerly wind
developing early next week, so there is the potential that the cold
air could linger a bit longer than currently forecast, especially in
the Havre/Harlem areas. Brusda

 - Forecast Confidence & Scenarios:

Snow Accumulations - Southwest to westerly upslope precipitation
across the Continental Divide, especially north of the MT Hwy 200
corridor, over the next 48 hours (through 06z Friday) will produce
periods of light to at times moderate snow. Latest NBM probabilities
suggest a 50% chance for 6" or more of snow over Marias Pass, with a
15% chance for 10" or more snow here. Higher elevations of Glacier
National Park have a 90+% chance for 6" or more of snow over this
same timeframe, with 40-70% chance for 12" or more snow. By late
Thursday night/early Friday morning precipitation is expected to
expand further east as a mid-level warm front lifts over cooler air
near the surface. Uncertainty remains with how much warm air will
lift north, with the potential for precipitation to stay in the form
of all snow or become a wintry mix of freezing rain/drizzle and
snow. 24hr snowfall probabilities from 06z Friday through 06z
Saturday of 1" or more of snow are in excess of 40% along and
northwest of a Rogers Pass to Great Falls, to Harlem line, with a 20-
40% chance for 2" or more of snow across these same areas. By
Saturday colder air advancing south behind a strong cold front will
change precipitation to all snow, with 50+% chance for additional
accumulations to reach or exceed 2" over the plains of Central and
North Central Montana through early Sunday morning. Light snow will
advance south into Southwest Montana over this same timeframe, but
accumulations are largely expected to remain near 1".

Ice Accumulations due to Freezing Rain/Drizzle - Latest NBM
probabilities suggest between a 20-40% chance for ice accumulations
in excess of 0.01" across most valley and plains locations along and
north of the US Hwy 12 corridor in Central through North Central
Montana from late Thursday night/early Friday morning through Friday
night, with several pockets of between a 10-15% chance for 0.10" or
more of ice along the MT Hwy 200 corridor from near Rogers Pass to
Great Falls. Over this same timeframe there is less than a 10%, but
a non-zero, chance for 0.01" of ice accumulations south of the
aforementioned line across the remainder of Central and Southwest
Montana. Latest BUFKIT soundings suggest that this event may have a
brief 1-2hr window for freezing rain, but with mid-level dry air
near the warm nose and ice nuclei present above said warm nose a
freezing drizzle scenario is more favored. None-the-less, any ice
accumulations would have the potential to create disruptions, if not
hazardous conditions, to travel on Friday (especially during the
Friday morning commute). Overall this is remains a low confidence
forecast with respect to ice accumulations, but a high risk to
anyone wishing to travel from late Thursday night through Friday
night.

Cold temperatures -  Fresh snow, clearing skies, light winds, and a
strong surface high positioned over Southern Alberta and Northern
Montana Sunday night/Monday morning has the potential to allow
temperatures to dip well below freezing, especially over the plains
of Central and North Central Montana. Latest NBM probabilities
support between a 20-60% chance for low temperatures by Monday
morning to fall below 10 degrees below zero across the Golden
Triangle (i.e. between Cut Bank, Havre and Great Falls). - Moldan

&&

.AVIATION...
21/06Z TAF Period

VFR conditions are expected to be the prevailing conditions during
this TAF period, though lowering cigs are possible Thursday
afternoon and evening as moisture pushes into the area. Attention
will first turn to increasing chances for light rain or snow,
first across SW MT (KEKS, KBZN, KHLN) before spreading further
north and east. This rain and snow will cause increased chances
for reduced cigs and vis, along with increased icing concerns.
Ludwig

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  15  33  20  27 /  20  30  60  80
CTB  12  26  10  20 /  30  50  80  90
HLN  23  44  28  41 /  20  40  60  70
BZN  15  43  28  46 /  20  30  40  40
WYS  11  36  22  40 /  50  80  70  30
DLN  17  42  27  44 /  20  30  30  30
HVR  13  33  15  28 /  20  30  50  90
LWT  20  44  20  34 /  10  20  50  70

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 11 AM MST Thursday for East
Glacier Park Region.

&&

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