642 
FXUS65 KTFX 142127
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
227 PM MST Mon Jan 14 2019

.SYNOPSIS...

Mostly clear skies and light winds persist through Tuesday. 
Patchy fog is possible again in Southwest Montana valleys tonight 
where cold air remains trapped. Otherwise, temperatures remain 
above average but begin to gradually cool Tuesday and Wednesday. 
A return to more winter-like conditions is expected Thursday and 
Friday as colder air moves into the region from Canada while 
moisture from the Pacific moves overhead bringing a period of 
accumulating snowfall Thursday and Friday. 

&&

.DISCUSSION...

Rest of this afternoon through Wednesday...another chilly night is 
in store for the cold prone basins of Southwest and Central Montana, 
with most locations falling into the single digits below zero to 
single digits above zero. As has been the case for numerous nights 
now, patchy to areas of fog are expected to develop in the Big Hole, 
Centennial, Hebgen, Boulder, and Helena Valleys during the overnight 
hours tonight. Winds will also become breezy through the Madison 
Valley tonight and into Tuesday morning as southerly low level flow 
and an increasing pressure gradient develop. Tuesday will be a 
cooler day across the region as cold air advects into the region, 
with the one exception being the valleys of Southwest Montana where 
this cold air advection will help temperatures warm (thanks to it 
eroding the stubborn temperature inversion which has locked these 
valleys in with cold air compared to the surrounding mountains). 
Clouds return and light snow develops by Tuesday afternoon/evening 
along the MT/ID border and around West Yellowstone, as moisture 
moves into the area. Snowfall accumulations from Tuesday night 
through Wednesday of around 1 to possibly 2 inches is possible for 
areas like West Yellowstone, Raynolds, and Targhee Passes. Given how 
dry the air has been as of late in the mid- and upper levels, it may 
take some time for the increasing moisture Tuesday afternoon/evening 
to completely saturate the atmospheric column. If this is the case, 
even lighter snowfall amounts would be observed from Tuesday night 
through Wednesday. By Wednesday a cold front will begin to push 
south from Canada, bringing cooler temperatures for the days on 
Thursday and Friday. - Moldan

Wednesday Night through Monday...Models continue to advertise an 
amplifying upper level weather pattern late this week as the upper 
level ridge currently over the region stretches and migrates to the 
NW while a large scale trough deepens across central and eastern 
Canada. The fist in a series of cold surface high pressure systems 
moving south around the Canadian trough will spread cooler air into 
N-central MT Wednesday night, spreading south across the remainder 
of central and SW MT Thursday and Friday. Meanwhile, the next 
Pacific trough opens up and moves inland spreading moisture 
across the deepening cold airmass, setting up a period of 
accumulating snowfall Thursday and Friday. Models are in somewhat 
better agreement now with timing and placement of precipitation 
Thurs/Friday leading to greater confidence in the overall forecast
for this period. Light snow looks to increase initially over SW 
MT Thursday morning then spread northeast with the most persistent
focus for overrunning precipitation generation occurring over N- 
central MT late Thursday into Friday morning before precipitation 
diminishes Friday afternoon. Overall snow forecast has changed 
little with 2-4 inches of snowfall over much of north-central MT 
with slightly lower amounts in the SW MT valleys and slightly 
higher amounts across the Mtns. low level NE surface winds will 
keep the cold air in place through Friday though winds are not 
expected to reach values that would create any impactful blowing 
or drifting of snow during this period. Surface high pressure 
sliding by to the east of the region will allow for the 
development of a lee-side trough of low pressure late Saturday 
and Saturday night. This will bring some warming to areas along 
the east slopes of the Rockies by late Saturday along with an 
increase in SW surface winds in the same areas, which may create 
some blowing/drifting of snow. Additional warming and more 
widespread windy conditions appear likely Sunday as flow aloft 
from the west increases and surface low pressure moves east across
southern Alberta. Hoenisch

&&

.AVIATION...
Updated 1705Z.

Fog in the Big Hole, Centennial, Hebgen, Boulder, and Helena Valleys 
will continue to dissipate through 18z. Otherwise, VFR conditions 
are expected for the duration of the 1418/1518 TAF period. 

For planning purposes, am expecting increasing mid-level clouds 
Tuesday afternoon as moisture begins to be transported north within 
southerly flow. This will lead to increasing precipitation chances 
Tuesday afternoon along the Idaho border, in addition to mountain 
obscuration across all of Southwest Montana. - Moldan

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  20  38  21  37 /   0   0   0   0 
CTB  18  34  15  32 /   0   0   0   0 
HLN  10  35  17  39 /   0   0   0  10 
BZN   4  36  13  39 /   0  10  10  10 
WEY -13  31  10  30 /   0  20  40  70 
DLN   8  35  16  37 /   0  10  10  10 
HVR  16  35  14  31 /   0   0   0   0 
LWT  20  38  20  39 /   0   0  10  10 

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

http://www.weather.gov/greatfalls