Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 172043

Area Forecast Discussion
National Weather Service Great Falls MT
243 PM MDT Mon Oct 17 2016

Today through Wednesday...Light rain and mountain snow showers will
continue to shift east this afternoon as an upper-level trough of
low pressure moves over the Northern Rockies. Additional
precipitation amounts will generally be less than one-tenth of an
inch. Another wave will bring some cooler air into the region and
snow levels will fall to around 6000 feet. Any snow accumulation
will stay confined to above pass levels with 1 to 3 inches possible.
A mix of light rain and snow is possible for mountain passes along
the Continental Divide. Showers will linger in the mountains and
southwest Montana valleys on Tuesday while downslope winds will keep
the plains across north-central Montana dry. By Wednesday, a weak
ridge of high pressure will move across the region with drier
conditions for most areas, with the exception of the mountains along
the Continental Divide where an isolated shower is possible.
Temperatures through the forecast period will generally be near to
slightly below seasonal averages. MLS

Wednesday Night through Monday...Medium range models are in good
agreement for this period up through Friday night, then some
differences for Saturday onward cause some uncertainty in the
forecast. High pressure aloft off the California/Baja coast
Wednesday night is forecast to move east into the Southwestern
U.S./Northern Mexico border area through Friday night, strengthening
the upper level ridge over the Western/Central U.S. As a result,
Montana will be under the influence of a somewhat moist but
anticyclonic westerly flow aloft. This will keep breezy westerly
downslope winds over much of the TFX forecast area, while weak
disturbances keep a good chance of showers along and west of the
Continental Divide. Models hint at a few showers possibly moving off
the mountains across North Central Montana Thursday into Thursday
night with the passage of a relatively stronger disturbance.
However, mid level (700 mb) winds are forecast to potentially reach
the 45 to 60 kt range across this area, with the strongest winds
along the Rocky Mountain Front. These potentially strong westerly
downsloping winds will likely keep any rain showers over the plains
on the light side, if they reach the ground at all. The greater
threat will likely be the strong wind itself. This set-up could
bring 25 to 40 mph sustained winds with gusts of 45 to 70 mph to the
east slopes of the Rockies. As mentioned above, the stronger winds
would be closer to the Rocky Mountain Front, while the lower end
winds would be out over the adjacent plains. Will keep an eye on
this, as high wind highlights may be needed.

For Saturday into Saturday night, the outlying Canadian model moves
a strong disturbance through the state, while the GFS/ECMWF continue
the westerly flow. Have therefore followed the consistency of the
GFS/ECMWF continued with just scattered mountain showers. Sunday
into Monday, the ECMWF digs a deep upper level trough just off the
Pacific coast, amplifying the ridge over Montana. The GFS, on the
other hand, has a weaker shortwave that moves into the Pacific
Northwest and keeps the area under more of a southwest flow. At this
point, solution differences are minimal, as these models both
forecast temperatures to warm up a bit and bring an increased chance
of mountain showers. Overall, temperatures hover around seasonal
averages with maybe a 5 degree swing either way with disturbance
passages.  Coulston


Updated 1800Z.
Light rain and mountain snow showers will move across central and
southwest Montana today as an upper-level disturbance slides over
the Northern Rockies. Ceilings may briefly lower to MVFR and
mountains will be obscured through the period. Westerly winds will
become breezy this afternoon but winds gusts should be less than 20
to 25 kts. Showers will diminish late this afternoon and evening
from west to east though they may linger over the mountains, which
will keep cigs lowered and mountains obscured. MLS


GTF  40  53  37  53 /  20  20  10  20
CTB  35  51  35  52 /  20  10  10  10
HLN  35  52  35  52 /  20  20  20  20
BZN  34  50  33  49 /  30  30  30  20
WEY  29  38  24  41 /  50  60  40  20
DLN  33  46  30  47 /  20  40  30  10
HVR  39  50  32  53 /  30  20  20  20
LWT  37  51  34  49 /  40  20  10  20




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