Winter Storm in Montana, February 8 to 9, 2018

Area Forecast Discussion
National Weather Service Great Falls MT
439 AM MST Thu Feb 8 2018

Aviation Section Updated

.SYNOPSIS...

Another in a series of winter storms will impact a large portion of
Central and Southwest Montana through Friday. The greatest impacts
are expected over Central Montana, with lesser impacts further
south. The valleys of Beaverhead and Madison Counties may not see
much precipitation at all. Some sleet and freezing rain will be
possible across southern sections of Central Montana and parts of
Southwest Montana. Otherwise, mostly snow is expected. Very cold
temperatures follow this system.

&&

.DISCUSSION...

The forecast regarding the incoming winter storm remains
complicated, with continuing issues regarding precip-type, snowfall
amounts, and what, if any, impacts will occur with any
sleet/freezing rain.

Water vapor this morning shows a well-defined shortwave moving
through central/southern BC at this time. Ahead of this wave, warm
air advection/overrunning induced precip continues from Havre to
Lewistown, while upslope snow continues along the Divide. In between
these two areas, the 12z TFX sounding shows a small, but well-
defined dry layer just above the surface where downsloping is
occurring. This has led to a lack of precip from Cut Bank through
Great Falls for most of last night/early this morning. It is this
area that precip- type and snowfall amounts remain the most
uncertain. Due to the downsloping, precip may not develop until
this afternoon when the low-level flow becomes more E/NE,
effectively ending the downsloping. Until that time, any shallow
low-level moisture/lift beneath the dry layer could allow pockets
of very light freezing rain to develop. The lack of better low-
level lift should preclude significant icing, but this will be
closely monitored. Based on this, though, I have continued the
trend of lower snowfall amounts in that downslope area, especially
the I-15 corridor south of Conrad. If low/mid level moistening
were to occur quicker than current hi-res models suggest, though,
snowfall amounts would end up a bit higher.

By this afternoon, the cold air should work all the way to Helena,
then reach the I-90 corridor this evening. North of the front,
precip should be all snow. There remains the potential for a few
bands of moderate to heavy snow with rates approaching 1"/hr along
the Hi-line and this is where the greatest impacts from snow are
expected. Further south, the snow may trend more convective in
nature, meaning more widely-separated, but brief, periods of
heavier snow, but a wider variance with snowfall amounts. Any one
band could produce localized higher amounts. So, given all these
factors, we have decided to keep the Winter Storm Warning as is.
In that Conrad to Great Falls area, we expect lower snowfall
amounts, but the combination of freezing rain and snow could lead
to greater impacts. A downgrade may be necessary if models trend
towards less ice potential. For Helena to Bozeman, we upgraded the
Winter Storm Watch to a Winter Weather Advisory as impacts look
to be less significant than further north, but still worth
highlighting. Beyond this system, it turns very cold Friday night
with widespread -10 to -25 degree temps over the plains. Some
moderation is possible afterwards, but more cold air comes in over
the weekend or early next week, possibly accompanied by some snow
and breezy conditions. MARTIN

&&

.AVIATION...
Updated 1139Z.

MVFR/IFR/LIFR conditions, LLWS, and possibly some FZRA/PL to impact
the forecast area over the next 24-36 hours.

As of 11z, -SN continues to primarily impact areas from CTB to HVR
and south to LWT. VIS has been reduced to a 1SM at times, but thus
far the snow hasn't been too heavy. With time, though, increasing
lift should lead to a greater threat of VIS falling below 1SM at
times in moderate to heavy snow, especially HVR. Further south, dry
air continues to limit precip outside of some very light snow,
sprinkles, and drizzle. Where temps are below freezing, some
FZDZ/FZRA may occur, mainly GTF. This potential will be closely
monitored, but confidence is not high enough to include any
FZRA/FZDZ mention at any site for now. Eventually, SN should spread
south reaching GTF later this morning and SW MT late this
afternoon/evening (possibly starting as a brief wintry mix before
changing over). For now, the threat of significant icing looks low,
but not completely out of the question. The 12z TFX sounding shows
strong winds just above the surface and LLWS is likely this morning,
especially GTF/LWT/EKS. MARTIN

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  21  -2   1 -14 /  80  90  40  10
CTB  11  -8   2 -13 / 100  90  30  10
HLN  46   5  12  -3 /  60  90  60  30
BZN  45  10  21  -2 /  40  90  70  50
WEY  35  17  27 -10 /  70  60  70  40
DLN  48  15  28   8 /  10  40  50  50
HVR  10 -11  -2 -21 / 100  70  10   0
LWT  27  -3   5 -16 / 100  90  40  10

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 4 PM this afternoon to 5 PM MST
Friday Central and Southern Lewis and Clark...Jefferson.

Winter Storm Warning until 6 AM MST Friday Blaine...Chouteau...
Eastern Glacier...Fergus...Hill...Judith Basin...Liberty...Toole.

Winter Weather Advisory from 6 PM this evening to 5 PM MST
Friday Broadwater...Gallatin...Meagher.

Winter Storm Warning from noon today to 11 AM MST Friday
Cascade...Eastern Pondera...Eastern Teton.

Winter Storm Warning until 5 PM MST Friday Northern Rocky
Mountain Front...Southern Rocky Mountain Front.

&&

http://www.weather.gov/greatfalls

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GREAT FALLS MT
640 AM MST THU FEB 8 2018

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0630 AM     SNOW             3 W HAVRE               48.54N 109.74W
02/08/2018  E5.0 INCH        HILL               MT   TRAINED SPOTTER

            RECEIVED 4-5 INCHES OVERNIGHT. SCHOOLS ARE
            CLOSED, BUSES NOT RUNNING. US HWY 2 IS A
            MESS.

&&

MPJ