Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 210036

Area Forecast Discussion
National Weather Service Great Falls MT
530 PM MST Mon Nov 20 2017



A cold front continues to move through southwest Montana
overnight, then begins to dissipate. Light rain and snow can be
expected along and north of this front through the overnight
tonight, with some patchy freezing drizzle also possible in a few
locations. The cooler airmass begins to retreat back north on
Tuesday followed by stronger warming and increasing winds
Wednesday and Thursday.



Tonight through Wednesday...Cold front has reached the mountains
as of mid-afternoon and will continue to move south through the
afternoon and evening, ending up over far southwestern MT early
Tuesday morning. Lower levels moisten rather quickly the remainder
of this afternoon and evening behind the front for the
development of low clouds and light upslope precipitation across
north-central MT the next several hours, while increasing mid
level moisture and incoming upper level jet energy from the west
is expected to produce an area of more widespread precipitation as
the energy passes through the region this evening. Cooling of the
airmass in low levels will allow snow levels to lower this
afternoon and evening with precipitation type as mostly snow in
all areas by late this evening. However, the moist layer becomes
fairly shallow this evening across north-central MT, allowing for
some potential for patchy freezing drizzle. Overall snow
accumulation later today and tonight look to be less than an inch
for most lower elevation locations, but will need to watch the
potential for some mesoscale precipitation banding over central MT
this evening, which could support locally higher amounts. As of
right now, this looks like more of a nuisance event with little to
no impact on roads (but perhaps some impacts on elevated surfaces
such as car windshields), but this will be closely watched in case
a short-fused advisory is needed for some areas. Precipitation
ends quickly Tuesday morning as drier air moves into the region
with a building upper level ridge. Weak energy ahead of stronger
warming aloft will bring increasing clouds and perhaps some
precipitation again Tuesday night with more widespread
precipitation focused along the continental divide with snow
levels rising to well above mtn passes. Strong warming by
Wednesday will push afternoon temperatures to around 20 degrees
above seasonal averages with many lower elevation locations likely
to reach or exceed 60 degrees. CC/Hoenisch

Wednesday night through Monday...An unusually-strong high pressure
ridge aloft Wednesday night and Thanksgiving morning will be broken-
down by a Pacific weather disturbance later Thursday into early
Friday morning. A cold front accompanying this disturbance will
sweep eastward through the area late Thanksgiving evening into early
Friday morning. Another high pressure ridge aloft should then become
established over our CWA later Friday through the upcoming weekend.
However, several Pacific weather disturbances may traverse this
ridge. Forecast confidence then decreases for Monday of next week.
The latest GFS depicts ridging aloft persisting over our region,
while the latest ECMWF progs another Pacific disturbance breaking-
down the ridge. Next week Monday`s forecast reflects a compromise
between the two model solutions. Atmospheric profiles will remain
favorable for periods of strong and gusty downslope winds along the
Rocky Mountain Front, especially Wednesday night into Thanksgiving.
This period will continue to be monitored for potential high wind

The above pattern will result in periods of upslope precipitation
along the Continental Divide and SW MT mountains. Elsewhere,
Thanksgiving evening into Friday looks to offer the best potential
for precipitation due to the aforementioned cold frontal passage. In
addition, some rain and/or snow showers are possible at lower
elevations on Monday of next week. Thanksgiving will be very mild,
with morning lows and afternoon highs expected to be about 15-25
degrees above-normal. In fact, some locations are forecast to have
near-record highs that day. Thanksgiving night will mark the
beginning of an overall cooling trend. However, temperatures are
still expected to be above-normal during the remainder of the long
term period.


Updated 0030Z.

Moisture ahead of an approaching disturbance will override a
northwest to southeast oriented cold front to spread rain and snow
with mountain obscuration across much of the forecast area through
10Z. Terminals along the Hi-Line (KHVR) and in southwest Montana
(KHLN KBZN KEKS) will mostly remain VFR with occasional MVFR
conditions. The terminals along north- and east-facing slopes (KCTB
KGTF KLWT) will have mainly MVFR to occasionally IFR conditions in
mostly snow due to enhanced upslope effect. The heaviest
precipitation will likely occur through 06Z. However, with drier air
aloft moving over the area between 03Z and 09Z, patchy freezing
drizzle may mix with the snow at times along those north- and east-
facing slope areas. Am only expecting a light mix, but it may be
enough to make runways extra slick. The disturbance will exit the
area after 10Z, leaving some MVFR/IFR conditions in low clouds and
fog until 15Z. High pressure will then build into the area after 15Z
with decreasing low cloudiness and increasing high cloudiness
through 00Z.


GTF  23  47  45  62 /  90  10  40  20
CTB  15  38  37  57 /  70  10  20  30
HLN  25  43  35  56 /  70  10  20  20
BZN  23  44  35  55 /  80  10  40  10
WEY  23  39  32  42 /  80  30  50  40
DLN  28  47  37  57 /  40  10  20  10
HVR  12  32  31  56 /  40   0  30  10
LWT  20  43  40  60 / 100  10  10  10



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