Area Forecast Discussion
Issued by NWS Great Falls, MT
FXUS65 KTFX 290523
Area Forecast Discussion
National Weather Service Great Falls MT
1022 PM MST Mon Nov 28 2016
...AVIATION SECTION UPDATED...
Cancelled the winter weather advisory for portions of southwest
Montana since snow has tapered off in many places and no
significant impacts are expected tonight. Adjusted Tuesday
morning forecast lows downward by several degrees in the high
valleys of southwest MT, including West Yellowstone, per trends in
surface observations and the latest available consensus of model
guidance. A compact shortwave trough over southeastern BC and
southwestern SK this evening is expected to slide south-
southeastward and reach the western half of north- central MT by
daybreak Tuesday. Thus, scattered light snow showers are possible
overnight tonight, especially over the western half of north-
central MT. However, fresh snow accumulations should be no more
than a dusting. The exception will be along the northern and
northwestern slopes of the Big/Little Belt Mountains, Big Snowy
Mountains, Elkhorn Mountains, and the Madison and Gallatin Ranges.
Here, an additional 1-3 inches of fresh snow accumulation cannot
be ruled out overnight tonight. The latest run of the NAM12 has
been depicting the upslope snow showers in these areas quite
nicely over the past several hours and suggests north-
northwesterly upslope flow will result in weak ascent within the
dendritic snow growth zone (DSGZ) tonight. This ascent, combined
with conditionally-unstable lapse rates and abundant moisture
within the DSGZ should allow these upslope snow showers to persist
through at least daybreak Tuesday. The rest of tonight`s forecast
appeared to need no adjustments.
This Afternoon through Wednesday...Occasional snow showers in a
slightly unstable northerly flow aloft will continue over the
mountains of Western and Southwest Montana into this evening.
Will therefore continue the Winter Weather Advisory for mountain
passes across Southwest Montana until 11 pm and re-evaluate the
situation this evening. An additional inch or so is possible on
the passes, while 1 to 3 inches are possible in the mountains.
Snow showers will also bring light measurable snow to the
mountains of Central Montana, but any accumulation will be less
than 2 inches and it should taper off by mid-evening. A push of
northerly surface winds behind a weak Canadian cold front will
spread low clouds, patchy fog, and areas of light snow or
flurries over North Central Montana tonight. Any snow
accumulation there will be minor (likely less than a half inch).
A weak high pressure ridge will then build into the Pacific
Northwest from the west Tuesday into Tuesday night, for
diminishing clouds, winds, and snow showers. The next upper level
trough to impact the area will break down the ridge and move into
the Pacific Northwest on Wednesday, bringing an increasing chance
of showers to the mountains of Western and Southwest Montana.
Will monitor this system mountain snowfall potential.
Temperatures will remain near to slightly below normal through
Wednesday night through Monday...Fairly quiet weather conditions
will start the extended period over the region, before colder air
and a better chance for snow could arrive by next Monday. An
upper level trof will be over the region on Thursday, with a few
light snow showers possible over the Rockies and across Southwest
MT. This system is quick moving as weak shortwave ridging
develops for most of Friday and Saturday. There will still be
some weather disturbances moving through the ridge, resulting in
some passing snow showers, but mostly on Saturday. I did keep
temperatures a touch below guidance for Saturday, as I do not
think it will warm quite as fast as the models have progged,
given the low 1000-500mb thickness yet. For Saturday
night/Sunday, a quick shot of slightly above normal temperatures
will move into the region, along with windy west winds. However,
by Monday, both the GFS/EC are progging an upper level trof to
move through the region, along with a stronger surface cold
front. As a result there is a better chance for snow on Monday,
but kept pops low for now because of the timing. Additionally,
both the EC/GFS prog a period of colder air to move into the
region starting next Monday evening and continuing through much
of next week, with widespread temperatures possibly going below
zero for areas north of Great Falls next week.
The region remains under the influence of low pressure aloft and
Southwest Montana will continue under a chance of light snow tonight.
Low level moisture will keep a threat of low clouds and fog across
most areas in Central and Southwest Montana and local IFR condition
will persist through 15z Tuesday. Mountain obscuration across
Southwest Montana is also expected. Gusty surface winds will
diminish but eastern areas, including KHVR and KLWT will see breezy
conditions through Tuesday.
.PRELIMINARY POINT TEMPS/POPS...
GTF 37 24 36 20 / 20 30 20 0
CTB 33 22 33 17 / 20 20 10 0
HLN 41 26 36 19 / 50 50 30 0
BZN 37 23 34 19 / 70 60 30 10
WEY 30 8 26 7 / 80 50 20 10
DLN 35 18 32 16 / 70 50 20 10
HVR 32 23 34 19 / 20 20 10 0
LWT 35 23 31 20 / 40 30 20 0