Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 181046
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
346 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.

&&

.DISCUSSION...

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 CWA 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 CWA 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 CWA. 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

&&

.AVIATION...
Updated 0540Z.

Cloudy skies and windy conditions will persist overnight with
widespread VFR conditions expected through about mid-morning Monday.
Strong mid- to upper-level winds will also cause a substantial risk
for moderate to severe turbulence aloft. Surface conditons rapidly
deteriorate across North Central Montana as low-level clouds and
precipitation increase along a frontal boundary expected to set up
from around KCTB to KHVR by mid-day Monday. This boundary is
expected to sag southward by Monday evening, possible reaching as
far south as Rogers Pass-KGTF-KLWT. Locations south of this boundary
should see only scattered snow showers and areas of mountain
obscurations. mpj

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  45  27  35  27 /  10  60  60  40
CTB  39  23  27  17 /  80  80  90  60
HLN  42  29  38  26 /  10  30  30  30
BZN  37  27  37  26 /  20  40  30  20
WEY  23  16  28  19 /  40  50  40  40
DLN  37  25  38  26 /  20  30  20  20
HVR  41  23  31  22 /  90  30  80  70
LWT  44  25  36  27 /  30  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.

&&

$$

http://www.weather.gov/greatfalls



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