High Wind in Montana, May 18, 2021

Area Forecast Discussion
National Weather Service Great Falls MT
535 AM MDT Tue May 18 2021

.SYNOPSIS...

A cold frontal passage this morning will bring cooler temperatures
to the region today, though still above normal for many areas. A
major spring storm impacts the region with widespread rain and
snow beginning Wednesday and lasting through much of Friday.

&&

.DISCUSSION...

Today through Tuesday Night...

The weather starts a deteriorating trend as an upper level low
moves into the PacNW. As this upper level low moves in, a weak
low pressure system will move across Southern British Columbia and
Alberta, which will cause a cold front to swing through our area,
sweeping away the recent warm air mass that we've had the past
few days and replacing it with noticeably cooler air. While winds
behind this front will be breezy, it looks like this front will
not have enough moisture to produce much, if any, in the way of
rainfall as it passes. The best chance for any shower or
thunderstorm activity this afternoon into the evening will be over
far eastern parts of north-central and southwest Montana, and
extending east through northeast and south-central/southeast
Montana.

Wednesday through Friday Night...

Current model guidance continues to be in great agreement in
depicting an upper level low slowly moving south from the PacNW
coastline to central Nevada from Wednesday through Friday. Such a
scenario places Montana, particularly north-central and west-
central Montana, in strongly veering flow from the surface to the
mid-levels promoting significant moisture and lift across the
region. Most prominently situated to receive significant moisture
is western parts of north-central Montana, where low-level
northeasterly upslope flow and moisture advection combines with
significant moisture advection and overrunning in the mid-levels
with southwesterly and then southerly flow from the strong upper-
level low. Details are becoming clearer in that significant rain
and snow amounts are expected, with potentially a foot or more of
snow across the Continental Divide and parts of the Rocky Mountain
Front, with lowering accumulations extending east out onto the
north-central plains. Out on the High Plains, the likely dividing
line between more significant snow accumulations and mostly just
rain appears to be along and west of a Great Falls-to-Rudyard
line. East of this line, snow accumulations will be highly
elevation-dependent, with lower elevations seeing just a trace to
perhaps one-half inch of snow, while mountain locations are
likely to see closer to 3 to 6 inches. Models do continue to trend
cooler with each set of runs, so it is possible more significant
forecast snow accumulations may need to be extended east.

The Winter Storm Watch was kept as-is for now. An upgrade to a
Winter Weather Advisory or Winter Storm Warning is likely within
the next 24 hours. The main concerns are the tourist and
recreational traffic to the area in the last few weeks, and
especially those recreating in the backcountry. Also of note is
the expected very wet nature of the snow, which when combined with
breezy conditions seems to cause the vast majority of power
infrastructure issues in this region. With leaves on many trees
now, another issue will be the heavy wet snow on trees with
leaves and the possibility of resulting tree damage.

Saturday through Monday...

Unsettled weather continues over the region with the area
remaining under cyclonic flow from whatever is left of the slow-
moving upper-level low, with models generally in agreement that
this feature will weaken but be very slow to exit the region.
Daily chances of showers and/or thunderstorms are expected, and
possibly even some light accumulating snow in the mountains
through Saturday night, but with temperatures trending gradually
warmer back closer to normal for most areas by early next week.

&&

.AVIATION...
535 PM MDT Mon May 17 2021 (18/12Z TAF period)

Mostly VFR conditions are expected over the upcoming TAF period,
with the exception of a few showers and thunderstorms, mainly at
KWYS and KLWT. A cold front will push through the area today,
brining gusty winds to all terminals this afternoon. Any showers and
thunderstorms that do develop will be limited in coverage and
dissipate after sunset. Areas along the Rocky Mountain Front will
need to watch for LLWS and mountain wave turbulence as westerly
winds cross the mountains. Ludwig

Refer to weather.gov/zlc for more detailed regional aviation
weather and hazard information.

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  70  37  56  34 /  10   0  40  90
CTB  60  34  55  31 /   0   0  20  80
HLN  72  41  58  39 /   0   0  60  90
BZN  78  38  66  38 /  20  20  60  80
WYS  70  33  66  34 /  20  20  50  80
DLN  74  37  63  36 /   0  20  60  80
HVR  75  40  62  36 /  10  20  10  70
LWT  75  38  62  36 /  20  20  20  80

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from Wednesday evening through Friday
afternoon Central and Southern Lewis and Clark...Northern Rocky
Mountain Front...Southern Rocky Mountain Front.

&&

http://www.weather.gov/greatfalls

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GREAT FALLS MT
646 AM MDT TUE MAY 18 2021

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

0639 AM     NON-TSTM WND GST 3 SW CUT BANK           48.60N 112.37W
05/18/2021  M56 MPH          GLACIER            MT   ASOS

            ASOS STATION KCTB CUT BANK AIRPORT.

&&

BRUSDA