Area Forecast Discussion
Issued by NWS Great Falls, MT
FXUS65 KTFX 051119
Area Forecast Discussion
National Weather Service Great Falls MT
420 AM MST Mon Dec 5 2016
...AVIATION SECTION UPDATED...
Today...A shortwave ridge will build-in from the Pac NW while at the
surface, an arctic high pressure ridge will build south-
southeastward from the Alberta plains. However, lift and decreasing
moisture ahead of a shortwave trough dropping southeastward from
Canada will allow snow showers to linger, but decrease in both
coverage and intensity over the CWA. The best potential for
lingering snow showers will be along the Rocky Mountain Front and
northern slopes of the central MT mountains due to upslope flow.
These areas look to receive another 1-3 inches of snow accumulation
by this evening. No more than an additional coating to 1 inch is
expected elsewhere. Thus, decided to cancel the winter storm warning
and most winter weather advisories. However, issued a winter weather
advisory for the Rocky Mountain Front and Cascade, Meagher, and
Judith Basin counties to account for lingering snow showers and some
blowing snow complicating the morning commute. This will especially
be the case along passes such as Kings Hill and Marias Pass. The
advisory is in effect until 5 PM this evening. Daytime highs
today will only reach the teens or lower 20`s in most places.
Tonight through Wednesday...The aforementioned shortwave ridge will
continue eastward over the CWA tonight and then be followed by two
more shortwave troughs dropping southeastward from Canada as these
pinwheel around a mid- to upper-level low centered over SK and
associated with the arctic air mass. The first shortwave trough
passage should occur during the day on Tuesday, while the other
should occur during the day on Wednesday. At the surface, the arctic
high pressure ridge will prevail over our CWA throughout the period.
Scattered snow showers are expected to accompany the shortwave
trough passages. These snow showers should be especially prevalent
over the mountains. During the period, additional snow amounts
should range from no more than 1 inch on the plains and in the
valleys to 1-4 inches in the mountains. Temperatures will be about
20-30 degrees below average, with many locations dipping below zero
for morning lows on Tuesday and Wednesday. However, abundant cloud
cover should prevent lows from nearing record territory.
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.
Cloudy skies and periods of light snow will continue today as cold
air from Canada moves into Central and Southwest Montana. Local MVFR
conditions will continue near snow. MVFR and IFR conditions will
become more common tonight as cold air continues to settle across
the region with areas of snow and fog.
.PRELIMINARY POINT TEMPS/POPS...
GTF 16 -3 6 -11 / 30 20 20 20
CTB 13 -7 -1 -12 / 20 20 20 20
HLN 21 9 16 1 / 60 20 20 10
BZN 21 6 16 -2 / 20 20 20 20
WEY 15 -2 13 -8 / 10 10 30 20
DLN 22 5 16 0 / 10 10 30 20
HVR 17 -3 5 -5 / 40 20 20 20
LWT 18 0 4 -7 / 50 30 20 20
Winter Weather Advisory until 5 PM MST this afternoon Northern
Rocky Mountain Front...Southern Rocky Mountain Front.
Winter Weather Advisory until 5 PM MST this afternoon Cascade...