Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 060520

Area Forecast Discussion
National Weather Service Great Falls MT
1020 PM MST Mon Dec 5 2016

...Update to Aviation


Water vapor satellite imagery reveals a subtle shortwave feature
rotating through central Montana this evening. Surface pressure
continues to rise while IR satellite shows at least BKN mid-level
clouds drifting north to south with the deepening Arctic airmass.
Radar suggests snow flurries may be associated with the deepening
upslope flow and weak disturbance aloft. The forecast has been
updated to add a bit more widespread coverage of light snow
flurries. Any new snow accumulations will range from essentially
none to perhaps 0.1 inches along the Rocky Mountain Front and
other northwesterly aspects of mountain ranges across central and
southwest MT. Given the persistent low cloud cover, have opted to
remove freezing fog overnight, even though it can`t be completely
ruled out for some areas. Temperatures remain on track for a cold
night. Wind chill advisories will go into effect tonight as
scheduled. PN


.AVIATION...Updated 0520Z.

Cold upper level trough will remain over the region for moist
northwest flow aloft. Occasional flurries and areas of light snow
will cause MVFR/IFR conditions at times. Generally light west to
northwest winds expected to prevail through the forecast period. mpj


/ISSUED 456 PM MST Mon Dec 5 2016/
Tonight through Wednesday...The transition to winter conditions
has started. Light northerly winds continue to reside over North
Central MT this afternoon, resulting in light snow
showers/flurries across Central MT. That trend should continue
into the evening hours before slightly drier air moves into the
region diminishing the snow activity. Any new snow accumulations
should be less than a quarter of an inch tonight. With some
partial clearing, the main concern will be overnight lows. Most
areas will go into the single digits below zero, but if several
hours of clearing occur in areas with a snow cover, temperatures
could fall into the teens below zero. Temperatures will remain
cold for both Tue/Wed afternoon as a trof of low pressure resides
over the region. Weak upper level disturbances will be moving
southward in this trof, producing a few light snow showers over
Southwest MT, mainly on Tue. Any snow accumulations should be less
than a half inch, and mostly in the mountains. Mostly dry
conditions are expected Tue night/Wednesday...along with colder
temperatures. A wind chill advisory has been issued for North
Central MT for tonight/Tue...which could likely be expanded in
area and length of time for Wednesday into Thursday. Brusda

Wednesday night through Sunday...Cold, arctic air will have spread
across the zones by Wednesday night and high pressure will be
building aloft. This will keep the air mass dry and allow the
beginning of a slow warming trend. A high pressure ridge will have
built over the Northern Rockies Thursday while, farther west, a low
pressure trof will be moving onto the West Coast. Moisture ahead of
this trof will be moving into the ridge and will spread across the
Rockies when the ridge moves east Friday. The trof to the west will
weaken as it moves through the Pacific Northwest and the chance of
widespread precipitation will increase but accumulations will remain
on the low side. West winds aloft will strengthen Friday and Friday
night. The cold air at the surface will keep these winds from mixing
down and only higher elevations should have windy surface
conditions. This westerly, unsettled flow will continue through
Sunday. Chances of precipitation will diminish somewhat Saturday
night but another disturbance in the flow aloft will increase
chances again by Sunday night. Timing of these disturbances,
however, is still of very low confidence. The cold air will have
moved away from the Rockies by Sunday and, with winds aloft still
strong, surface winds will become windy and gusty Sunday. These
winds will also inhibit precipitation along the lee slopes of the
Rockies and adjacent plains. Temperatures will begin the period well
below seasonal normals. The air mass will trend warmer but
temperatures will still below normals Sunday.


GTF  -3   5 -15   3 /  20  10  10  10
CTB  -6   0 -15   2 /  20  20  20  10
HLN   2  11  -8   7 /  30  20  20  10
BZN   3  14 -12   7 /  20  20  20  10
WEY  -6  11 -18   7 /  20  20  20  10
DLN   2  14  -8   8 /  20  20  20  10
HVR  -2   6  -3   4 /  10  10  10  10
LWT  -2   5 -10   2 /  20  20  20  10


Wind Chill Advisory from midnight tonight to 5 PM MST Tuesday
Eastern Glacier...Eastern Pondera...Liberty...Northern Rocky
Mountain Front...Toole.

Wind Chill Advisory from midnight tonight to noon MST Tuesday
Blaine...Cascade...Central and Southern Lewis and Clark...
Chouteau...Eastern Teton...Fergus...Hill...Judith Basin...
Meagher...Southern Rocky Mountain Front.


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