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fxus65 ktfx 181752 
afdtfx

Area forecast discussion
National Weather Service Great Falls Montana
1052 am MST Mon Dec 18 2017

Synopsis...

An active weather pattern is developing over the region with
temperatures turning colder throughout the upcoming week. Strong
winds continue through this morning along the Rocky Mountain
front and over portions of central Montana. Snow and blowing snow
continues along the Rocky Mountain front and in central Lewis And
Clark County. Areas of rain and snow will develop across north
central Montana during the day on Monday with widespread snow
expected tonight through Wednesday.

&&

Am update...

A strong 140kt jet continues to drop south, with the jet Max located
right along the US/Canadian border. Simultaneously, this continues
to push a Canadian cold front south. As of 10am, surface obs show the
front has just crossed the border and stretches from Cut Bank to
near Havre. This is close to or a bit slower than the previous
forecast indicated and I have adjusted the forecast some to reflect
this trend. For the hi-line, this means falling temps through the
day.

As the front continues to drop south, lift associated with the jet
overhead will allow a band of snow to develop along the hi-line this
afternoon (cut bank just started snowing recently). With time, jet
dynamics will weaken for a short time, which should allow the band
of snow to weaken/diminish by this evening, but not before putting
down a quick inch or so from Cut Bank to Havre. Lift will increase
markedly late tonight into Tuesday, so the decrease in lift this
evening will be temporary. Ahead of the front, steep low level lapse
rates will be increasing, although winds aloft will be decreasing.
Given the steep lapse rates, gusty winds will continue from Great
Falls to Lewistown, but the weakening winds aloft should limit the
high wind potential and we'll plan to allow the current High Wind
Warning to expire at 11am. For now, no additional changes to the
winter weather products, but upgrades to advisories or warnings are
likely this afternoon. Martin

&&

Aviation...
updated 1748z.

Moderate to significant impacts to aviation are expected across
central and north-central Mt, first kctb to khvr this afternoon,
then spreading south towards kgtf/khvr late tonight/Tue.

A Canadian cold front will push south across central/north-central
Montana this afternoon/evening, with a band of snow bringing a period of
MVFR/IFR conditions to kctb and possibly khvr. A brief lull in
precip may occur overnight, followed by widespread snow developing
across central/north-central Montana Tuesday with IFR or lower conditions.
For now, VFR conditions are expected across SW Montana. Mtn
obscurations are likely across the entire forecast area. Martin

&&

Previous discussion...
/issued 434 am MST Mon Dec 18 2017/

Today through Wednesday...it is one week before Christmas and
just a couple of days before the winter solstice, and much of my
forecast area can expect to see a cold, snowy and very active
winter weather pattern develop over the next few days. A strong
upper level jet and strong downslope pressure gradients will
continue to fuel strong and gusty winds along the Rocky Mountain
front and over portions of central Montana this morning. Models
are in good agreement showing winds diminishing below high wind
criteria by late morning as a boundary moves southward from Canada
and turns winds to a northerly direction this afternoon. This
boundary is also expected to be the focusing mechanism for areas
of rain and snow today before lifting back northward overnight
with a good likelihood for accumulating snows expected over the
north central Montana plains late tonight and Tuesday. Conditions
become even more active Tuesday night and Wednesday as a strong
upper level shortwave trough sweeps inland through the Pacific
northwest and then across the northern Rocky Mountains. A surface
low pressure system is expected to develop along the
Montana/Wyoming border by early Wednesday morning, allowing an
associated Canadian cold front to sweep southward through the
Treasure state with widespread accumulating snows and much colder
temperatures.

Current highlights for this series of weather systems include a
High Wind Warning which will remain in effect for the Rocky
Mountain front and portions of central Montana through late this
morning. The existing Winter Storm Warning for the northern Rocky
Mountain front will be allowed to continue through Wednesday
morning. I have added a Winter Weather Advisory for the southern
Rocky Mountain front and portions of central Lewis And Clark
County for this morning through Wednesday morning for snow and
areas of blowing snow, including Rogers Pass. However, it should
be noted that this advisory does not include Helena or MacDonald
Pass. Finally, I have left the Winter Storm Watch in place for the
remainder of central and north central Montana so the day shift
can have at least one more model run to determine the proper
highlights needed for this area. Mpj

Wednesday night through Monday...main forecast concerns in the
long term is another decent shot for snow County Warning Area wide (thursday
night through Friday night), followed by well below normal
temperatures.

H500 trough will be overhead Wednesday evening, with most lingering
snow/snow showers ending prior to or around the midnight hour, as
large scale subsidence associated with the downstream side of an
eastward advancing ridge overspreads the region. At the surface,
high pressure (~1025 to 1035mb) over southern Alberta/Saskatchewan
will slide south and east across north central Montana during early
morning hours on Thursday. The combination of clearing skies, light
winds, cold air advection, and ideal timing for radiational cooling,
will allow temperatures across the County Warning Area to fall into the single
digits above or below zero. Areas that have seen recent fresh
snowfall will be most at risk of bottoming out, with some areas
possibly observing double digit below zero readings. Thursday will
be generally quiet for most locations as a transitory and positively
tiled h500 ridge moves overhead. The one exception to these quiet
conditions will be the northern Rocky Mountain front, where the h700-
h500 flow will generally be northwesterly, which will aide in some
degree of orographic lift and snow showers. Temperatures on Thursday
will be well below normal, with most locations struggling to reach
out of the teens and into the low 20s.

By Thursday evening, the next h500 shortwave trough will begin to
move southeast from the coast of British Colombia and into the
intermountain west/northern rockies. 12hr h500 height falls of ~80m
to 150m are expected to occur throughout the overnight hours on
Thursday/Friday morning. At the surface, an area of low pressure
moving east from central Alberta into Saskatchewan will drag a cold
front from northwest to southeast across the region. The combination
of increasing synoptic ascent from the upper level trough, surface
front, and northwest orientated flow, will allow for an area of
light to at time moderate snow to develop from Thursday night
persisting into the afternoon/evening hours on Friday across most of
the County Warning Area. At this time, snowfall accumulations of 2-4" appear
possible south and west of a kctb to kgtf line, with generally 1-2"
northeast of this line. Mountains across central and southwest
Montana could see totals approach 4-6". Model differences then begin
to arise by 00z Saturday with the evolution of the h500 pattern and
subsequent surface highs/lows. Despite these differences, north
central and southwest Montana will be to some degree under the
influence of northwest flow aloft and strong high pressure at the
surface. This would lead to temperatures well...well below normal
for highs, with some locations falling into the 20s to 30s below
zero for overnight lows (cold basin in southwest montana).
- Moldan

&&

Preliminary point temps/pops...
gtf 47 27 35 27 / 20 60 60 40
ctb 37 23 27 17 / 70 80 90 60
hln 43 29 38 26 / 20 30 30 30
bzn 45 27 37 26 / 20 40 30 20
wey 25 16 28 19 / 50 50 40 40
dln 38 25 38 26 / 10 30 20 20
hvr 39 23 31 22 / 70 30 80 70
lwt 43 25 36 27 / 40 80 60 40

&&

Tfx watches/warnings/advisories...
Winter Storm Watch from late tonight through Wednesday morning
eastern Glacier...eastern Pondera...Liberty...Toole.

Winter Storm Watch from Tuesday morning through Wednesday
morning Blaine...Cascade...Chouteau...eastern Teton...Fergus...
hill...Judith Basin...Meagher.

High Wind Warning until 11 am MST this morning Cascade...central
and southern Lewis And Clark...eastern Glacier...eastern Teton...
Judith Basin...northern Rocky Mountain front...southern Rocky
Mountain front.

Winter Weather Advisory until 11 am MST Wednesday central and
southern Lewis And Clark...southern Rocky Mountain front.

Winter Storm Warning until 11 am MST Wednesday northern Rocky
Mountain front.

&&

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