Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 161130
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
430 AM MST Sat Dec 16 2017

...Aviation Section Updated...

.SYNOPSIS...

Light snow will continue over Southwest Montana and the mountains
of Central Montana through the early afternoon today, before
ending from west to east. Skies will begin to clear this evening
and into the morning hours on Sunday, which could allow
temperatures to fall sharply in areas where fresh snow has fallen.
Sunday will be generally quiet, save for the Continental Divide
were upslope flow will lead to snow.

&&

.DISCUSSION...

Today through Monday...main forecast concerns in the short term is
the on-going light snow and what to do with current winter weather
headlines. After very poor model performance with last nights/this
mornings expected snow from the 00z Friday model runs, confidence in
this evenings model solutions is low to say the least.

H500 shortwave trough will continue to cross the Northern Rockies
and push east into the Northern High Plains by the early morning
hours on Sunday. For most of the day today (Saturday), best synoptic
forcing will reside across Southwest Montana (before departing the
region as the shortwave trough slides east around the mid/late
afternoon hours), with continued light orographic lift over the
Little Belts/Central Montana mountains. Moisture across the region
is pretty limited however, with PWATs generally only ranging in the
0.15" to 0.25" range. With this all in mind, expect light to at
times moderate snow to continue across portions of Southwest Montana
and over the Central Montana mountains (especially over the Little
Belts [ie Kings Hill Pass]) through the early afternoon hours. For
this reason, kept Winter Weather Advisories for areas generally
south of a Augusta to Belt line, and dropped the advisories for
areas north of this line (including the City of Great of Falls).

H500 heights will begin to rise this evening and into the afternoon
on Sunday as a somewhat positively tilted ridge overspreads the
region, however, this ridge will be quickly suppressed with nearly
zonal flow by Sunday evening. Much of the day on Sunday will be
quite (save for the Continental Divide where upslope flow will
provide for a chance of snow), with sunny to mostly skies during the
morning and early afternoon hours, becoming cloudy as the westerly
flow pumps moisture into the region. H700-H500 cross barrier flow
begins to strengthen in earnest by 00z Monday, with H700 winds
peaking between 09z-18z Monday at around 50-65kts. With a lack of
ridgetop inversions and limited mixing over this time frame, am not
confident in these strong winds reaching the surface. For this
reason, have held off on any High Wind products at this point. PWATs
over this same time frame will climb to around 0.35" to nearly 0.5"
as Pacific Moisture streams eastward. This combined with upslope
flow along the Continental Divide and mountains of Central/North
Central Montana, should allow for some decent precipitation to
develop, with most of the precipitation being in the form of snow at
higher elevations. This event will need to be closely monitored for
the consideration of future winter weather headlines. - Moldan

Monday night through Saturday...Medium range models continue to
show a moist and unsettled northwest flow aloft over the region
Monday night and Tuesday. This pattern still looks favorable for
precipitation over the western mountains with a chance for light
rain and snow over the plains. Temperatures will remain slightly
above seasonal averages on Tuesday. Tuesday night will see a
vigorous shortwave trough move inland through the Pacific Northwest
with this feature then continuing eastward across Montana on
Wednesday. Models are now in much better agreement with regards to
surface features and indicate that a strong Canadian cold front will
sweep southward through the state with a good likelihood for snow
expected across much of the forecast area Wednesday. Model solutions
for Thursday through Saturday keep a deep upper level trough over
the Great Lakes region with north to northwest flow aloft over
Montana. This pattern is expected to be generally dry and models
suggest that westerly surface winds will prevail over the region
through this period. However, models also indicate that the boundary
between very cold air to the east and somewhat warmer air to the
west could slosh back and forth between the east slopes of the
Rockies and eastern Montana. As a result, confidence in
temperatures, winds and precipitation during the later part of the
week and into the weekend is rather low. For now have kept the
forecast close to a blend between the various models. mpj

&&

.AVIATION...
Updated 1130Z.

Light and at times moderate snow will continue to affect Southwest
and portions of Central Montana through the morning and into the
early afternoon hours today, before coming to an end from west to
east and north to south. MVFR/IFR conditions are expected at the
KEKS and KBZN terminals over this timeframe, with IFR conditions
most likely during the morning hours. MVFR/VFR conditions are
expected at the remaining terminals across Central and North Central
Montana then over this span of time. VFR conditions will return to
all terminals across the region by the evening hours, as transient
ridging builds in behind the departing system. Mountain obscuration
is likely along the Continental Divide, Southwest and portion of
Central Montana into the afternoon hours due to falling snow and low
ceilings. - Moldan

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  35  21  41  35 /  30  10  10  20
CTB  35  21  40  27 /  20  10  10  20
HLN  30  16  37  28 /  40  10  10  30
BZN  29   8  33  24 /  70  30  10  30
WEY  26  -4  20  15 /  60  20  10  40
DLN  32   8  33  25 /  60  20  10  20
HVR  38  20  41  28 /  10   0  10  30
LWT  34  17  39  31 /  40  20   0  20

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 2 PM MST this afternoon
Beaverhead...Broadwater...Gallatin...Jefferson...Madison.

Winter Weather Advisory until 11 AM MST this morning Cascade...
Central and Southern Lewis and Clark...Judith Basin...Meagher.

&&

$$

http://www.weather.gov/greatfalls



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