Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 210449

Area Forecast Discussion
National Weather Service Great Falls MT
1050 PM MDT Thu Apr 20 2017

Updated Aviation Discussion.


Pacific cold front extends south-southeastward from east of Cut Bank
to near Big Timber. Farther aloft, negatively-tilted 700 mb trough
axis extends south-southeastward from the MT Rockies to the CO
Rockies. As both of these features continue drifting eastward
tonight, convergence along the 700 mb shortwave trough axis and
sufficient moisture should allow a swath of steadier precipitation
to progress eastward across much of the CWA. Many locations in the
western two-thirds of the CWA should receive about 0.10" to 0.25" of
liquid precip by daybreak Friday. Localized amounts nearing 0.50"
are possible in the Highwoods, Little Belts, Big Belts, and
Bridgers. Precipitation should mainly fall as wet snow for
elevations at and above 4500 feet and primarily rain for lower
elevations. The winter weather advisory for Kings Hill Pass remains
valid and will take effect at 9 PM this evening.



Rest of Today through Friday Night...Latest surface analysis shows a
slow-moving cold front moving into western/southwestern MT at this
time. This front will continue to progress east through this
evening. At the same time, an area of low pressure will develop
along the front in the lee of the Rockies along the MT/AB border.
This low and the associated front will then trek SE through MT on
Friday, exiting to the east Friday night. Moisture, lift, and
convergence will be focused along the slow-moving front tonight and
will likely lead to a band of steadier/heavier precip across parts
of the forecast area. It still appears the I-15 corridor will be the
focus of precip through tonight, with a sharp cutoff in precip
across our far eastern counties (ie. Hill, Blaine, Fergus). Given
the sharp cutoff, I lowered pops in the above-mentioned counties
through tonight. As is often the case with these systems, the axis
of heaviest precip may shift some, but confidence is fairly high
that the I-15 corridor will be the focus through tonight.

On Friday, a deep and moist N/NW flow will develop as the front
moves east and precip will likely become more upslope driven,
favoring north-facing slopes/foothills. For the central MT
mountains, this looks like another much-needed precip event given
the lower than average snow/snowpack in that area over the winter.
As for precip type, confidence in accumulating snow is highest above
4500 feet. Below that, though, a transition to snow cannot be ruled
out at times late tonight or early Friday morning, but warm
ground/road temps should limit accumulations. Of note, though, any
lower elevation area that sees heavier snowfall rates could see some
accumulation. This may especially be the case in the Bozeman area
where the axis of heavier precip may reside tonight. As for snowfall
amounts, I lowered accumulations just a bit in the Little Belts over
Kings Hill Pass, but still think there will be enough snow to keep
the Winter Weather Advisory going there. The most impactful period
there will likely be late tonight into early Friday morning. Even
so, road temps there are in the 40s/50s right now, so much of the
snow will likely melt over the pass initially. 5-10 inches is likely
for the mountains near/south of Bozeman with primarily recreation
impacts. For this reason, we`ll hold off on issuing any advisory for
that area. Precip will quickly move out Friday night from west to
east as an upper level ridge builds in. Martin

Saturday through Wednesday night...model consensus has come into
a little better agreement with the upper level pattern through
portions of the long term. The long term begins with H500 upper
level ridging in place Saturday and through the early morning
hours on Sunday, before a quick moving shortwave moves from the
Pacific Northwest and across the Intermountain West. The
aforementioned shortwave has moved further south in this evening
runs of the GFS and Canadian compared to 24 hours ago, with the
H500 wave axis tracking from west to east across Southern
Montana/Northern Wyoming through 12z Monday. The ECMWF still
maintains a more northerly track with the shortwave, taking the
wave from west to east across Northern Idaho and Northern Montana.
These tracks would induce surface cyclogenesis somewhere over
Northeastern Wyoming/Eastern Montana by Sunday evening, with a
cold front pushing south and east across North Central and
Southwestern Montana through the evening hours on Sunday and into
the morning hours on Monday. Rain, rain changing to snow in the
mountains, and even a few thunderstorms are likely Sunday
afternoon and evening ahead of the advancing cold front and within
the WAA region of the approaching shortwave (especially south of
a Great Falls to Lewistown line at this time). Rain is then
expected to change over to snow at all elevations across North
Central and Southwestern Montana during the morning hours on
Monday as colder air is drawn in behind the departing cold front
and dynamic cooling through the precipitation column continues. A
changeover back to rain at lower elevations is then expected for
the day on Monday, however, temperatures will be well below normal
for this time of the year.

Following the previously discussed system...a messy, progressive,
and generally zonal upper level flow pattern will commence. The
messy nature is especially true, as every long term guidance
(GFS/Canadian/ECMWF) have differences in location and timing of
the embedded waves as they race through the flow. One thing that
can be said with some certainty, is that a wet and active pattern
is in store for much of North Central and Southwestern Montana
through the first half of the upcoming work week. Temperatures
through this period will be at or below normal for the final week
of April. - Moldan


Updated 0450Z.

Conditions will gradually worsen to varying degrees at all terminals
from west to east through the night as precipitation increases in
coverage, with cigs gradually deteriorating to MVFR predominant for
all but north-central plains locations, where cigs may remain VFR.
For now, continued to keep VIS somewhat conservative in the TAFs,
but adjustments are likely as the system evolves and as confidence
increases regarding where the heavier/steadier precip will develop.
Within periods of heavier precipitation, VIS could certainly lower
to IFR at times. Precip should primarily be RA at all terminals, but
there is the potential for a RA/SN mix late tonight or early FRI AM.


GTF  39  48  32  60 /  70  80  20  10
CTB  41  49  31  58 /  60  60  10   0
HLN  39  52  34  62 /  80  60  20   0
BZN  36  48  30  61 /  80  60  40  10
WEY  28  40  22  50 /  80  40  20   0
DLN  32  49  30  59 /  70  40  20   0
HVR  43  53  33  59 /  30  60  20   0
LWT  38  46  31  57 /  50  80  40  10


Winter Weather Advisory from 9 PM this evening to noon MDT
Friday For the Little Belt Mountains in Central Montana,
including Kings Hill Pass in Cascade...Judith Basin...Meagher.


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