Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 130335

Area Forecast Discussion
National Weather Service Great Falls MT
835 PM MST Tue Dec 12 2017


Previous update mainly focused on the High Wind Highlights and
clouds. Took another look at clouds and Wednesday`s snow event for
this evening`s update. Those looking to view the Geminid Meteor
Shower tonight are still looking for abundant cloud cover across
the south (mainly from Helena south). Satellite trends showing a
brief clearing for central and northern portions early in the
early overnight hours. An upper level disturbance will then bring
increased clouds late tonight through the day Wednesday. This
mentioned disturbance could still bring scattered light snow to
the southern half of the CWA. This evenings models appear snow
start time may delay until after 6 AM Wednesday morning.
Widespread light snow is then possible for these areas through all
of Wednesday. Accumulations still look to be light...with around
an inch at lower levels..and 1 to 4 inches across the mountains.


An upper level ridge of high pressure will be over the region
through Friday, bringing mostly quiet weather to the region. A
weak, fast-moving system will bring light snow to southwest
Montana during the day Wednesday, with snow accumulations up to 1
inch. Another system arrives for Friday and will bring increasing
chances for rain or snow to the region.


Updated 2335Z.

VFR conditions will generally continue across the forecast area over
the next 24 hours, but a disturbance may cause flight conditions to
worsen across southwest Montana after 12Z.

A disturbance exiting the area will most of its mid and high level
cloudiness with it, as well as causing gusty westerly winds over the
plains to decrease. However, another disturbance will bring
increasing clouds again after 09Z. Areas of light snow will also
move southeast across southwest Montana between 12Z and 21Z, causing
mountain obscuration and occasional MVFR conditions. The snow will
exit the area from the northwest between 21Z and 00Z.


/ISSUED 435 PM MST Tue Dec 12 2017/

Tonight through Thursday...

Winds will diminish through the evening as a passing disturbance
responsible for today`s strong winds and fire weather threat moves
away from the area. Expect some clearing especially over northern
and eastern parts of our area, but some clouds will remain along
the Continental Divide and much of southwest Montana. Models seem
to be catching up to the next fast-moving system in northwest flow
that will move across western and southwestern Montana during the
day Wednesday. Decent dynamics combined with good (but not
excellent) moisture in the dendritic snow growth layer suggest up
to a quick inch of snow for most of the mountains and valleys of
Southwest MT, with localized higher amounts to 3 inches on
northwest-facing slopes. This system will move through primarily
during the daytime hours, so there will likely be at least some
travel impacts. Yet another system will come immediately on its
heels, dropping south from Alberta and Saskatchewan over north-
central and northeast Montana. Models suggest very little moisture
to work with despite decent dynamics, so precipitation should be
limited to minimal amounts and primarily over the higher
elevations such as the Bears Paw and possible the Little Belts
Wednesday night. High pressure at the surface and aloft move
overhead Thursday, resulting in continued strong inversions. Fog
will be likely especially over the southwest valleys Thursday
morning, especially if there is fresh snow from Wednesday`s
expected minor snow event. CC

Thursday night through Saturday...A very slow change in the weather
pattern is expected to begin, as the upper-level high pressure ridge
will start to break down. Because of how strong the ridge is, it is
going to take a few disturbances to knock it down. Strong winds are
possible on Friday ahead of the precipitation. Expect snow over much
of the CWA Friday into Saturday, with the potential for some rain to
mix-in at lower elevations. Minor snow accumulations remain possible
as a shortwave trough traverses the region Friday evening into
Saturday morning. Thereafter, quieter weather is expected as another
high pressure ridge aloft builds-in from the west.

Sunday through Tuesday...Both the ECMWF and GFS continue to depict a
transition day on Sunday, with breezy southwesterly winds developing
as the aforementioned ridge is broken-down by another disturbance
that looks to remain north of Montana. These downslope winds will
warm temperatures a bit over North-Central Montana and allow only a
small chance for a passing shower over the plains. For Monday into
Tuesday, the aforementioned forecast models still differ on when the
next storm system will arrive from the Gulf of Alaska region. Thus,
while forecast confidence remains low during this time period,
precipitation probabilities may increase over much of the CWA.


GTF  32  44  23  46 /  10  20  10   0
CTB  29  46  20  45 /   0  10  10   0
HLN  23  33   5  25 /  20  40  10   0
BZN  22  29   0  23 /  20  60  20  10
WEY  13  24  -6  18 /  20  40  20   0
DLN  24  35   3  28 /  20  40  10   0
HVR  30  47  19  45 /   0  10  10   0
LWT  31  42  21  41 /   0  10  20  10



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