Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 131248

Area Forecast Discussion
National Weather Service Great Falls MT
548 AM MST Wed Dec 13 2017


Precipitation associated with the approaching shortwave trough
has already reached northern Idaho, moving southeast at about
50kts. Radar reflectivity is outpacing most model guidance, and
only a handful of stations are confirming precipitation reaching
the ground. Observed trends also suggest the trough may move a
bit further than forecast. Therefore, an early update was issued
to advance the timing of precipitation onset across Southwest MT
to around 15Z, exiting before 00Z. This rapid motion and absence
of surface reports have prompted a reduction in forecast snow
totals. Mid-day snow accumulation still warrants caution on area
roadways. However, the event will have pretty much ended by



A fast-moving weather system will spread light snow across
southwest Montana today. Snow accumulations of up to 1 inch are
possible. Skies clear tonight through Thursday. Another system
arrives late Friday with strong winds and chances for rain or


Updated 1140Z.
Clouds are increasing in coverage from the northwest this morning.
Areas of light snow will develop across southwest Montana between
17Z and 21Z, causing mountain obscuration and occasional MVFR
conditions. The snow will exit the area from the northwest after
03Z. VFR conditions prevail over North-central MT. PN


.PREV DISCUSSION.../ISSUED 440 AM MST Wed Dec 13 2017/

Today and Tonight...A shortwave system is seen developing over
Washington early this morning. This system is expected to move
quickly through southwest Montana today and produce areas of
light snow generally south and west of a line from about Helena to
White Sulphur Springs. Model guidance indicates cooling mid-
level temperatures, supporting snow as the expected precipitation
type. Snow is expected to accumulate on roads as valley inversions
have maintained subfreezing temperatures over the last week.
Moisture is limited, but reasonably strong lift through the
dendritic growth zone will raise precipitation efficiency.
However, snowfall may take some time to saturate the lower
atmosphere and reach the surface. This, combined with fast storm
motion is expected to keep snow totals below one inch at lower
elevations, with up to 3 inches possible at isolated mountain
locations in Madison and Gallatin Counties. Snow will develop from
northwest to southeast through mid- to late-morning. Roads could
become slippery by mid afternoon, and icy after dark should any
daytime snowmelt occur. Snow ends for most areas this evening.

Thursday and Friday...Generally clear skies return to end the week
as the ridge strengthens again to our west. Southwest valleys will
remain chilly while downslope westerly winds maintain mild
temperatures for North-central Montana. A period of very strong
winds is possible Thursday night into early Friday along the Rocky
Mountain Front, so will have to monitor for possible high-wind

Saturday through Wednesday...The persistent large scale ridge
finally breaks down on Saturday, allowing a Pacific weather system
to cross the northern Rockies. This will bring rain and mountain
snow through the forecast area. Several inches of snow will be
possible at higher elevations through Saturday, but snow levels
remain uncertain to determine possible accumulations and timing of
snowfall for plains and valley locations. Sunday is trending drier
with ridging aloft. Unsettled northwesterly flow with embedded
shortwave systems will follow Monday through Wednesday next week. PN


GTF  43  27  43  33 /  10   0   0   0
CTB  46  24  43  30 /   0   0   0   0
HLN  33  17  35  19 /  40   0   0   0
BZN  29  14  33  13 /  60  20  10   0
WEY  25   7  29   6 /  60  10   0   0
DLN  35  15  35  17 /  80  10   0   0
HVR  47  24  42  25 /   0   0   0   0
LWT  41  25  40  28 /   0   0   0   0



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