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Area forecast discussion
National Weather Service Great Falls Montana
1030 am MST Sat Jan 21 2017

..update to aviation...

Update...

Moisture continues to advect north into the area as evidenced by
increasing clouds on recent satellite imagery. Of note, recent
water vapor and visible imagery actually shows a small low/mid level
circulation over western Montana associated with a weak S/W lifting
north. Radar imagery and surface obs show an area of light snow
associated with this feature. While it appears most of the snow
with the S/W will stay west of The Divide, there is some
potential for light snow across far western parts of central
Montana as it lifts NE through the day. I adjusted pops along The
Divide based on latest trends and short term guidance. However,
we'll have to keep an eye on whether the snow can extend any
further east than currently forecast as the S/W moves through.
Across SW Mt, a moist, upslope flow should allow widespread snow
showers to develop along the Montana/Idaho border and I increased pops in
this area through this evening.

The only other issue of note this morning is fog. Both Helena and
Havre airports have dropped down to less than a mile at times this
morning. Based on webcams, though, the fog appears to be more
patchy in nature, so we'll hold off on any fog advisories for now.
But, I did add fog in those areas through noon. Martin

&&

Aviation...
updated 1730z.

A moist southwest flow aloft will prevail over Montana through late
Sunday morning. Several weak disturbances in the flow aloft will
bring a chance for light snow to the western mountains and to much
of southwest Montana through the forecast period. Expect mountain
obscurations with occasional IFR/MVFR conditions in precipitation
impacting lower elevations, including the kbzn, keks and khln
terminals. Mostly VFR conditions will prevail across north central
Montana today but a weak surface cold front will bring a surge of
low level moisture for periods IFR/MVFR conditons this evening
through Sunday morning. Occasional fog will also continue to affect
khln and khvr, mainly during the late night through early afternoon
hours. Mpj

&&

Previous discussion...
/issued 446 am MST Sat Jan 21 2017/

Today through Monday...current water vapor imagery showing nicely
our weather setup for today. With a weak cut off low in the
Dakotas...and a large upper level low spinning just off the coast
of the Pacific northwest...we find ourselves stuck in the middle of
weather systems. The overall result will be light winds and near
normal temperatures for today. A ridge of high pressure to our
south slowly builds northward throughout the day. As that
happens...moist southwest flow enters the state from southwest to
northeast. This could bring a few scattered light snow showers
across the southwest during the day...with snow chances moving
northeast tonight into Sunday morning. By Sunday afternoon the
mentioned high pressure ridge enters into the state...and
diminishes chances for snow...except across the southwest where
moist and breezy southwest flow continues through the day. The
mentioned low off the coast of the western US then sends a
disturbance with surges of moisture to the state Sunday night
into Monday. Then a weak push of cold Canadian air could be
experienced later in the day Monday. These disturbances will
return widespread chances for light to scattered moderate snow
during this time period. At least light snow accumulations are
possible by Monday evening...mainly across northern...western...and
southern portions of the County Warning Area. No highlights issued yet with this
storm...as models still trying to pinpoint the track of the upper
level disturbances...and intensity. And although most areas will
have at least a slight chance of snow...right now looks like
southern areas will have the better chances. Stay tuned for
updates. Winds remain light for today. The southwest flow could
bring some increase winds...especially across the southwest...on
Sunday. With no major airmass changes...near normal temps are
expected today into Sunday. Colder air from Canada may bring
slightly below normal temps for Monday. Anglin

Monday night through Saturday...a broad upper level trough with axis
from central Canada SW into the western US will bring cool and
unsettled conditions early next week, followed by dry northerly flow
aloft as the trough progresses east into the central US with a ridge
amplifying along the West Coast and pushing inland by the end of
next week. Shortwave energy tracking east from southern Idaho into Wyoming
Monday night will bring widespread light snow to southern portions
of the forecast area while a northerly flow with embedded energy
also spreads light snow showers across areas to the north Monday
night through Tuesday night. Amplifying ridge to the west will bring
dry conditions from late Wednesday into next weekend with
temperatures moderating from somewhat below seasonal averages early
next week to slightly above average by next weekend. Surface high
pressure will also strengthen over the interior western US late next
week and will likely lead to the redevelopment of temperature
inversions in the SW Montana valleys, likely leading to lingering cold
temperatures and potential air quality issues in some SW Montana valleys.
Hoenisch

&&

Preliminary point temps/pops...
gtf 31 19 34 19 / 10 0 10 20
ctb 31 17 32 17 / 0 10 10 50
hln 26 8 26 11 / 10 10 10 30
bzn 26 6 28 9 / 20 30 10 30
wey 25 7 22 12 / 50 60 40 50
dln 30 11 32 15 / 20 30 20 30
hvr 28 18 30 18 / 0 10 20 20
lwt 33 19 35 18 / 0 0 10 20

&&

Tfx watches/warnings/advisories...
none.

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