Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 201813

Area Forecast Discussion
National Weather Service Great Falls MT
1213 PM MDT Thu Apr 20 2017

Aviation Section Updated


Latest surface analysis and satellite imagery shows a frontal
boundary pushing into Idaho at this time. Moisture continues to
increase ahead of the front across SW MT, but the low levels still
appear to be fairly dry and it will likely take a little more time
before low level moisture increases enough for precip to develop at
lower elevations (ie. Dillon/Ennis areas). The going forecast
handles this well, so no changes were needed for SW MT. Across
central Montana, temps should warm into the mid 60s this afternoon
ahead of the front which should allow just enough instability for
isolated shower development prior to the front`s arrival. For this
reason, I introduced low-end pops from Great Falls to Havre late
this afternoon. An isolated thunderstorm cannot be ruled out
anywhere this afternoon, but given the very weak instability,
confidence is too low to include thunder in the forecast for now.

12Z model update: Taking a quick look at the latest incoming
guidance, there doesn`t appear to be any significant changes to the
system for tonight/FRI and I don`t anticipate significant changes to
the forecast. However, we`ll continue to assess the latest guidance
to see if any changes are needed. Martin


Updated 1813Z.

VFR conditions prevail initially, but conditions will gradually
worsen at all terminals from west to east, especially tonight
through Friday. A cold front moving in from the west will lead to
increasing clouds and lowering CIGS with mtn obscurations
developing. Precipitation will increase in coverage through tonight
as well with MVFR or lower CIGS/VIS likely. A band of heavier precip
is expected to develop across parts of central/SW MT tonight/FRI AM.
Within this band, more significant reductions to VIS will be
possible. For now, I kept 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 this band, 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 or a switch to all SN late tonight or early FRI AM.


/ISSUED 535 AM MDT Thu Apr 20 2017/

Today through Saturday...Short term models continue to show that
a negative tilt trough currently along the Oregon coast will move
inland today. This feature will push short wave ridging over the
forecast area eastward with an increasingly moist southwest flow
aloft developing through the day with a chance for showers over
my western zones by afternoon. This trough will settle over
central Montana for Thursday night and Friday with widespread
rain and mountain snows anticipated for much of the forecast area
along with much cooler temperatures. Forecast precipitation
totals are still expected to remain generally below one half inch
at lower elevations with the highest mountain peaks in Southwest
and Central Montana seeing as much as an inch of liquid equivalent
in the form of 6 to 12 inches of snow, especially in the
mountains near Big Sky. The heaviest snow amounts are expected to
be well above pass levels for most areas but have issued a Winter
Weather Advisory for the Little Belt Mountains with Kings Hill
Pass likely seeing 4 to 8 inches tonight through about mid-day on
Friday. Bulk of energy associated with the upper level trough is
expected to remain well south of Montana and will evolve into a
closed low over the central and southern Great Plains for Friday
night and Saturday. This will put Montana beneath a building short
wave ridge although scattered showers are expected to linger over
the forecast area through Saturday morning. However, by Saturday
afternoon am expecting mostly clear skies with temperatures
rebounding back toward seasonal averages. mpj

Saturday night 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 evening 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


GTF  64  39  48  32 /  20  70  80  20
CTB  61  41  49  31 /  10  60  60  10
HLN  59  39  52  34 /  50  80  60  20
BZN  58  36  48  30 /  30  80  60  40
WEY  45  28  40  22 /  70  80  40  20
DLN  51  32  49  30 /  50  70  40  20
HVR  69  43  53  33 /  20  30  60  20
LWT  63  38  46  31 /  10  50  80  40


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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