Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 191011

Area Forecast Discussion
National Weather Service Great Falls MT
411 AM MDT Mon Mar 19 2018


A compact disturbance moving along the US and Canadian borders
today will bring light snow accumulations to the Hi-Line, Bears
Paw Mountains, and Northern Rocky Mountain Front. Additional light
snow is possible tonight across Southwest Montana, with the
highest amounts being confined to the mountains. Temperatures
then warm each day into the middle of the work week, which will
lead to increased snowmelt.



Today through Wednesday...main forecast concerns over the short term
period is precipitation today, followed by warming temperatures and
precipitation chances across the mountains of Southwest Montana.

Compact H500 shortwave moving along the Montana/Alberta/Saskatchewan
borders will bring light snow to generally the Hi-Line and Bears Paw
Mountains today through the evening hours tonight. Over the plains
snowfall accumulations of 1 to 3 inches are possible, with 2 to 5
inches in the mountains. Given the high sun angle for this time of
year and very low snow rates, most of the accumulations should be
confined to grassy surfaces, which will have little impacts to
travel. Along the Rocky Mountain Front, snowfall accumulations of 1
to 4 inches,  with 4 to 8 inches at Marias Pass and localized
amounts up to 10 inches at ridgetops, are expected through this
evening. For this reason, have kept the Winter Weather Advisory in
place through noon today for elevations above 5000 feet, with the
advisory falling off thereafter despite additional light
accumulations due to the fact of the high sun angle/low snowfall
rates once again. In addition to the compact H500 shortwave moving
across the US/Canadian border, a weak wave with weak H700
frontogenesis will also cross Southwestern Montana this evening,
bring the chance for accumulating snow to the mountains of Southwest
Montana and even the valleys.

By Tuesday, H500 ridge over California and the Desert Southwest will
begin to slide east, all the while amplifying north across the
Rockies by Wednesday morning. At the surface, the pressure gradient
along the Continental Divide and mountains of Central Montana will
begin to strengthen, peaking during the afternoon hours on Tuesday.
This will lead to breezy southwest winds for areas south and west of
a Lewistown, to Great Falls, to Cut Bank line generally from late
tonight through Tuesday afternoon. These breezy southwest winds
should help temperatures to warm further during the day on Tuesday,
so have gone above Superblend guidance for these areas. During the
day on Wednesday, the H500 ridge axis slides east across Southwest
and North Central Montana, with the flow becoming increasingly
southwesterly by the evening/overnight hours. 1000-500 mb
thickness values on Wednesday climb to around 435-445dam and warm
air advection increases ahead of the approaching H500 trough over
the Pacific Northwest, which should support high temperatures in
the 40s/50s south and west of a Cut Bank, to Loma, to Lewistown
line, and the 30s north of this line (with the coolest reading
over Blaine and Hill counties where a substantial snowpack still
resides). Precipitation chances over the mountains of Southwest
Montana and along the Continental Divide on Wednesday will be on
the increase as the H700-H500 southwest flow begins to usher in
additional Pacific moisture, but will likely be very showery in
nature. - Moldan

Wednesday night through Sunday...Towards the middle of the week, a
large upper level low will slide south off the west coast of North
America, allowing an upper level ridge over the Rockies to amplify.
This ridge will not stick around long, though, as the upper low
gradually works inland by the end of the week. Locally, it is
expected that this evolution will put an end to the moderating
temperatures and drier conditions expected through mid-week,
transitioning to cooler and more unsettled conditions once again.

As the low moves inland, a broad SW flow aloft develops across the
West, potentially allowing an atmospheric river to develop and move
across the Central Rockies. For now, it looks like the best
moisture, then, will stay south of Montana. That said, a potent
shortwave, forecast to move through the area late Thursday into
Friday, may be able to briefly tap into some better moisture. The
models differ on the handling of this wave, but it bears watching as
it has the potential to produce some modest precipitation amounts,
some in the form of rain, which could have hydrology impacts (please
see the "Hydrology" section for further details). Warm air aloft
ahead of this wave combined with near/below freezing surface temps
may allow a window of mixed precipitation to develop across parts of
Central Montana Thursday/Thursday night. There may also be an
opportunity for stronger winds along the Rocky Mountain Front
Thursday night/Friday. After this initial wave passes, a gradual
cooling trend is expected which should lead to primarily a rain or
snow scenario by the weekend. For now, the forecast over the weekend
reflects a generally unsettled pattern as it is too early to get
specific regarding the timing/impacts of each wave embedded within
the trough. MARTIN


Updated 0525Z.

Southwest Montana : MVFR conditions are possible through the early
morning hours at the KBZN and KEKS terminals due to ceilings and
visibilities from snow showers and mist, however, conditions are
generally expected to be VFR for the remainder of the 1906/2006 TAF
period. Mountain obscuration is likely due to a broken/overcast mid-
level cloud deck for the remainder of tonight and through the day on

North Central Montana : MVFR, IFR, and even LIFR conditions are
expected through much of the 1906/2006 TAF period at the KCTB, KHVR,
KGTF, and KLWT terminals, with a period of MVFR conditions possible
at the KHLN terminal early this evening then improving to VFR
thereafter. LIFR conditions are most likely to occur at the KCTB,
KHVR, and KGTF terminals tonight and early Monday morning due
reduced visibilities from freezing fog/snow and low ceilings as a
upper level disturbance crosses the region. Mountain obscuration
will occur over this timeframe. Snow and fog will gradually come to
an end during the late morning/early afternoon hours today, however,
low ceilings are expected to remain. - Moldan


Updated 19/400 AM.

Temperatures are expected to warm into the 40s and 50s across
most low elevations of North Central and all of Southwest Montana
by Tuesday and continue into the middle of the work week. At mid
and high slope elevations, temperatures are expected to warm into
the 30s generally. These warming temperatures will lead to
increased snowmelt, especially at low elevations where snow
remains. A more gradual snowmelt is anticipated at mid and high
slope elevations, but will be closely monitored.

Towards the end of the week, a more unsettled weather pattern is
expected to develop, with an increasing chance of precipitation,
some likely in the form of rain. At this point, a general 0.10-0.25"
liquid equivalent is expected from Thursday through the weekend. Of
note, though, a potent system moving through late Thursday/Friday
has the potential to produce over a half an inch of liquid
equivalent precipitation across parts of the area and bears watching
as this could certainly lead to additional hydrology concerns. The
one saving grace, even if the higher precip amounts are realized, is
that temperatures will be cooling during this time making the threat
of rain lower with time. Based on all this, then, there does not
appear to be a significant heavy rain on snow potential this
week, but stay tuned for future updates as we continue to get more
clarity with the late-week system. MARTIN


GTF  38  24  45  22 /  10  10   0   0
CTB  33  20  42  21 /  60  10   0   0
HLN  45  26  47  26 /  10   0  10  10
BZN  40  22  44  22 /  10  10  10  20
WEY  32  14  33  20 /  40  30  30  40
DLN  35  17  43  23 /  10  30  10  20
HVR  28  15  36  13 /  80  20   0   0
LWT  37  24  42  22 /  20  10   0   0



Winter Weather Advisory until noon MDT today above 5000 feet for
Northern Rocky Mountain Front.

Flood Advisory continues until further notice for snowmelt in
Jefferson...Broadwater...and Northern Gallatin Counties.


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