High Wind in Montana, March 1, 2017

Area Forecast Discussion
National Weather Service Great Falls MT
520 AM MST Wed Mar 1 2017

Aviation Section Updated

.DISCUSSION...

Today through Thursday Night...Strong west winds already impacting
areas from the Rocky Mtn Front east across eastern Glacier and
Pondera counties early this morning with winds sustained at 39kts
gusting to 46 kts at KCTB at 3am. Gusts over 50kts occurring
along the immediate east slopes with severe driving conditions
reported by MDT in areas N/W of Browning. Upper level Jet and mid
level wind maximum along with surface trough will shift east
across N-central MT this morning with potential for widespread
gusts 40-45 kts and localized gusts 50-55kts in areas highlighted
with a High Wind warning. Blowing snow and significant reductions
in visibility will primarily be confined to areas from the
Continental Divide to areas immediately adjacent to the Front
Range this morning, where snow is also falling, primarily near the
continental divide, and a Winter Wx Advisory is in effect.
Localized blowing snow and areas of drifting snow are possible
during periods of higher winds over the remainder of north-central
this morning, but significant visibility reductions are not
anticipated. Surface low pressure tracks east along the MT/AB/SK
border this afternoon with NW to SE axis of snow showers expected
to develop this afternoon across N-Central MT in association with
mid level convergence and a weak frontal boundary that sags to the
S and W this evening. Snow showers will also become numerous this
afternoon over the Mtns of central and SW MT with orographic
precipitation continuing along the Continental Divide. Could see
1-3 inches of snow accumulation late this afternoon and evening
across Judith Basin and Fergus counties, including the Geyser,
Stanford and Lewistown areas as snow shower activity will be
concentrated there with higher amounts in the Little Belts.
Precipitation chances diminish on Thursday with temperatures
warming back to near near or above seasonal averages by Thursday
afternoon. Hoenisch

Friday through Saturday night...main forecast concerns will be the
potential for gusty downslope winds across North Central
Montana/Rocky Mountain Front and snow across the Continental Divide.
00z models are in decent agreement with the progression of the upper
level ridge, which as of 12z on Friday will generally be located
from Central Nevada/Utah, northeast through North Central Montana
and the Canadian Province of Saskatchewan. This upper level ridge
will transition east to over the Central and Northern High Plains by
00z Saturday, as a shortwave moves onto the coast of Washington and
British Columbia. Over this same time, near-zonal flow will be in
place with a H500 jet streak of 60 to 70+ knots over the Rocky
Mountain Front and Lewis Range. Despite the quintessential cross
barrier flow and sharpening lee surface trough, am not as impressed
with the potential for widespread high winds across the CWA due to
weaker upper level winds from previous model runs. With this being
said, far western portions of Teton, Pondera, and Glacier counties
will need to be monitored, as H700 winds approach 50-60kts and 00z
GFS Bufkit soundings depict strong stability above mountain top
levels (which would support the potential for strong downslope
winds).

In addition to the potential for strong winds, the near zonal flow
will usher bountiful Pacific moisture up against the Continental
Divide. This moist upslope flow will support snow along the
Continental Divide throughout the day and evening on Friday (and
then again through the day on Saturday), with several inches of snow
being possible across the Divide through Friday evening. Further to
the east, the downsloping winds will all but eliminate the potential
for any moisture, save for areas along and immediately near the
Rocky Mountain Front. Snow will then spread south and east across
Southwestern Montana Friday night and into Saturday morning, as the
H500 shortwave moves from the Washington/British Columbia coast
inland into South Central British Columbia/North Central Washington.
As the downslope winds relax Saturday morning, snow and even rain
showers will begin to spread east across most of the CWA, with the
exception of North Central Montana where dry conditions are still
expected to prevail. Temperatures over the Friday and Saturday time
frame will be at or above normal values.

Sunday through Wednesday...The better consistency/agreement shown
in Tuesday's model guidance has really dropped off with the
recent 00Z guidance. As was mentioned yesterday, and per WPC, it
appears the culprit for the uncertainty continues to center around
the evolution of the upper level wave pattern from northern
Canada into the eastern/northern Pacific. The ECMWF has remained
the most consistent in recent days, keeping an upper level trough
over the Pacific. Meanwhile, the GFS/GEFS/CMC push the trough
inland in a much more progressive pattern. Given that the EC and,
to some extent the CMC, have been the most consistent of late, the
forecast is a blend of the previous forecast and the ECMWF/CMC.
Of note, though, given the variability, even with the more
consistent models, forecast confidence in the long-term period is
definitely below normal. Note: WPC's Extended Forecast Discussion
is a great resource for a more detailed look at model trends in
the long-term (www.wpc.ncep.noaa.gov).

Based on the above, then, here is the best guess for weather across
the local area. Originally, it looked like a strong Pacific cold
front would sweep through the area on Sunday. The trend, though, is
now more towards a lee-side low developing near/north of West
Yellowstone and quickly deepening as it shifts E/NE into the
Dakotas. This solution suggests a stripe of steadier precip may
develop across the eastern half of our CWA on Sunday as the low
deepens. Ultimately, though, the exact track will make all the
difference in which areas see the higher precip amounts or whether
the heaviest precip stays well east of the forecast area.

In the wake of Sunday's system, the EC/CMC generally agree on a
moist, west/southwest flow continuing which would favor the
western/southwestern mountains for precip and likely limiting precip
for valleys/plains due to downsloping. By the middle of next week,
another SFC low may lead to a more widespread precip event, but
given the model differences, confidence in this is low right now.
Temp-wise, Sunday's system should usher in cooler air after a
moderating trend this week. However, this cooldown may end up being
short-lived as the moist W/SW flow brings milder temperatures back
once again by mid-week. Martin

&&

.AVIATION...
UPDATED 1220Z.

The main aviation concern in the short term will be increasing winds
at all terminals, first across the plains of central Montana this
morning, followed by an increase for valley terminals by late
morning/early afternoon. Wind gusts of 30-45kt will be common, with
a few terminals potentially seeing higher gusts. In addition to the
gusty winds, LLWS will remain possible. Later this afternoon and
evening, a frontal boundary will sag south into central Montana.
Convergence along this boundary may lead to areas of SHSN.
Confidence is highest at KLWT and have introduced a prevailing group
for -SN with lower CIGS/VIS. Elsewhere, lower CIGS/VIS will be
possible, but confidence wasn't high enough to add to the TAFs yet.
That said, any location that sees -SN this afternoon or evening will
likely see MVFR or lower CIGS/VIS. Martin

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  39  26  43  32 /  20  40  10  10
CTB  38  21  39  30 /  50  50  20  10
HLN  38  26  41  26 /  20  20  10  10
BZN  35  24  37  24 /  20  30  20  10
WEY  24  16  28  16 /  50  50  40  20
DLN  36  21  39  23 /  20  20  10  10
HVR  35  19  39  29 /  30  20  10  20
LWT  36  21  40  27 /  30  80  50  20

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
High Wind Warning until 5 PM MST this afternoon Cascade...
Central and Southern Lewis and Clark...Chouteau...Eastern
Teton...Fergus...Judith Basin.

Winter Weather Advisory until 5 PM MST this afternoon Northern
Rocky Mountain Front...Southern Rocky Mountain Front.

High Wind Warning until 5 PM MST this afternoon Eastern
Glacier...Eastern Pondera...Toole.

&&

weather.gov/greatfalls

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GREAT FALLS MT
420 AM MST WED MAR 1 2017

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

1221 AM     NON-TSTM WND GST 8 SE EAST GLACIER PARK  48.36N 113.11W
03/01/2017  M70 MPH          GLACIER            MT   MESONET

            MESONET STATION DEEP CREEK RAWS /DCRM8/.

&&

BH