Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 220539
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1040 PM MST Tue Feb 21 2017

Updated Aviation Discussion

.UPDATE...

While some parts of the forecast are becoming more clear over the
next 24hrs, the forecast remains a tricky one. The models are doing
fairly well with the large scale features, but this pattern and the
sensible weather at the SFC is very much tied to mesoscale features
in the low/mid levels that the models continue to struggle with
resolving. Latest SFC analysis shows a weak SFC low between Great
Falls and Lewistown with a frontal boundary extending east into NW
ND where a stronger SFC low resides. Decent CAA behind this low will
allow colder air to push south through our far northeastern counties
through Wednesday. Meanwhile, the low back near Great Falls and an
attendant frontal/convergence boundary (that stretches northwest
into NW Montana) will likely stall and remain draped across central
and western sections of central MT.

It is along and north of this stalled frontal boundary where the
greatest coverage of snow showers is expected. North of this
boundary, it should remain cold enough for all snow through
tomorrow, but right near the front itself, temperatures may climb
just enough above freezing for rain to mix in at times. If this were
to happen, it could certainly lower snowfall amounts. At the same
time, though, convergence along/north of the front could enhance
snowfall rates at times. Additionally, a weak low moving along the
MT/Canada border may locally enhance snowfall rates/amounts as well,
especially across the Hi-line and perhaps favored upslope areas near
the Sweetgrass Hills. In the wake of this low, drier downsloping air
may actually limit snow right along the Rocky Mtn Front.

Again, the takeaway here is that confidence remains somewhat lower
given the smaller scale details that will have to be ironed out
almost in real-time. For now, thinking the highest snowfall amounts
through Wednesday will be from Cut Bank to Havre. For the update, I
made adjustment to pops through Wednesday to try to account for the
latest model guidance and trends. I also added in areas of fog
overnight as low level moisture remains high in the wake of earlier
shower activity. The fog will reduce visibility to less than a mile
at times. For now, though, thinking it won`t be widespread enough to
warrant any Fog Advisories. The ongoing Winter Weather Advisories
were left as is. Even in areas that do not see as much snow, the
snow comes after a milder stretch and the Advisory highlights the
change in conditions compared to recent days and as well as likely
travel impacts. Martin

&&

.AVIATION...
Updated 0540z.

Area remains under a moist WSW flow aloft tonight through tomorrow
with several emebedded disturbances rippling across the region.
Precipitation will mainly be focused along a corridor across north
central MT where a weak frontal boundary exists and will fluctuate
north and south slightly as disturbances move through the flow
aloft. Low clouds, fog and and IFR conditions in precipitation will
become more widespread at n-central MT terminals overnight with a
band of moderate snow likely developing over the region late tonight
through early Wednesday morning. SW MT terminals remain mostly dry
through the rest of tonight with increasing  chances for
precipitation and MVFR conditions Wednesday afternoon as the front
sags south. Hoenisch

&&

.PREV DISCUSSION...
/ISSUED 500 PM MST Tue Feb 21 2017/

Tonight through Thursday...Active weather looks to continue
through at least Wednesday in the short term period. This mornings
frontal boundary has made it about as far north as it will get for
the day. A few showers and isolated thunderstorms under weak
instability are developing along or south of this front. These
mainly rain showers could linger into the evening hours for
central and northern portions. A shortwave will then take this
stalled front and push it back south in a form of a cold
front...bringing widespread areas of snow tonight through
Wednesday morning. Challenge of the day was working out model
differences. GFS kept with its timing and location of the snow
mainly in northern and eastern portions...while most remaining
models shifted precip more into the north and central. Tough to go
against the model consensus...so shifted snow and pops slightly
to hit central regions. The biggest difference this caused was
adding accumulating snow into the Great Falls area. Confidence is
medium to high that most of the north and central will see snow
through Wednesday morning. There may still be just enough
lingering warm air during the afternoon that snow may then mix in
with rain. The caveat is if heavier precip continues to fall...as
some models suggest...more snow than rain is possible. With recent
warm weather...the warm ground may initially inhibit
accumulations...however...by the end of Wednesday widespread 2 to
4 inches is possible...with some areas receiving up to 6 inches.
There is the potential for more snow with any banding along the
front. Winter Weather Advisories have been issued to areas with
at least moderate to high confidence of accumulating snow.
Confidence trails off later in the short term...hence ending the
Advisories midnight Thursday. A cut off low then develops in WY
for Thursday...shifting our flow to an upslope easterly
direction...with some unstable nw flow also possible. This will
continue chances for accumulating snow for most the
area...however...exact timing and locating of the heaviest snow
remains uncertain. This will have to be continued to monitored for
any further highlights. Temperatures will be on a cooling trend
through the short term...with highs in the 20s and 30s by
Thursday. Windy conditions are possible across the south and
central through this evening...so will continue the High Wind
Warning. Anglin

Thursday Night through Tuesday...Areas of light snow will linger
throughout much of north-central and southwestern MT Thursday
night into Friday as the region remains under the influence of a
broad upper level trough just to the west. The upper flow becomes
more northwesterly Friday into Saturday, with just isolated to
scattered snow showers mainly over the SW MT mountains. Transitory
upper ridging will influence the region Saturday night into
Sunday, but southwest flow increases moisture push into the area
Sunday night through at least early Tuesday. This will allow for
the potential for widespread light snow accumulations across the
region during that time period. Temperatures look to remain
seasonably cool through the extended period, with temperatures
remaining below freezing generally in the single digits to near 30
above zero across much of north- central and southwestern MT.
Cassell

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  29  37  21  30 /  70  70  40  50
CTB  23  33  17  26 /  80  50  30  40
HLN  31  38  16  27 /  30  30  30  40
BZN  29  37  12  27 /  30  30  40  40
WEY  19  27   7  23 /  60  50  30  40
DLN  25  35  14  24 /  20  20  20  30
HVR  28  36  23  32 /  70  50  40  30
LWT  29  33  19  28 /  60  70  40  30

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until midnight MST Wednesday night
Blaine...Cascade...Chouteau...Eastern Pondera...Eastern Teton...
Fergus...Hill...Judith Basin...Liberty...Meagher...Toole.

Winter Weather Advisory until 6 PM MST Wednesday Eastern
Glacier...Northern Rocky Mountain Front...Southern Rocky
Mountain Front.

&&

$$

weather.gov/greatfalls



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