Area Forecast Discussion 
FXUS65 KTFX 211157
AFDTFX
Area Forecast Discussion
National Weather Service Great Falls MT
555 AM MDT Fri May 21 2021
...UPDATED AVIATION SECTION...
.SYNOPSIS...
A low pressure system will continue periods of snow, possibly with
some rain, across much of the region today, especially early this
morning and again later this afternoon through this evening. A
minor shift in this system will help decrease precipitation into
Saturday, but it will likely bring another round of rain and snow
for Sunday into Monday, as temperatures remain below normal.
Temperatures will then warm back closer to normal by mid-week, but
with a continued unsettled weather pattern.
&&
.DISCUSSION...
Today through Saturday...A deep low pressure system over the Great
Basin will continue to spread moisture into Montana today,
bringing periods of light to moderate snow this morning and again
late this afternoon into the early evening. Accumulating snow will
likely decrease from the south across southwest Montana early this
morning, but upslope snow will likely persist through the entire
morning over the plains. Although the snow has decreased somewhat
along the Rocky Mountain Front and south through much of Lewis and
Clark County, it should increase again early this morning, so will
leave the Winter Storm Warning going for that area for now. It is
a similar situation with the areas in the Winter Weather Advisory;
snow will likely continue through at least the morning hours, so
will leave that Advisory as is as well. Breezy northerly winds
through favored southwest Montana valleys this morning should also
decrease throughout the day as the surface pressure gradient
there on the leading edge of the Canadian surface high pressure
should weaken. The low pressure system is then forecast to shift
north into the Pacific Northwest later tonight into Saturday,
which should cut off a significant portion of the Pacific moisture
feed and decrease the overall chance for precipitation as the flow
aloft shifts a bit more out of the southeast. Temperatures will
remain well below normal with highs in the 30s today and in the
20s tonight, but temperatures should start to moderate into the
40s to lower 50s for Saturday.
Sunday through Monday...The low pressure system will move a strong
shortwave trough north into the area Sunday into Sunday night,
which should bring another round of substantial precipitation to
the area; forecast models are indicating the potential for
widespread 1 to 2 inches of additional precipitation on an already
saturated ground. This may cause some localized flooding issues,
especially due to heavier showers associated with possible
thunderstorms. Although temperatures may not cool off as much as
with this system (highs mostly in the 40s and 50s with lows in the
30s), the snow level may lower enough to hinder significant runoff
and consequently limit widespread river flooding concerns.
However, this situation will continue to be monitored.
Tuesday through next Friday...After the exit of the weekend
system, the pattern will remain somewhat unsettled, but similarly
strong systems appear unlikely at this time. The pattern becomes
more progressive with the passage of weak shortwave troughs.
Overall, this will bring a good chance for showers and possibly
a few weak thunderstorms, mainly in the afternoon and evening
hours as the airmass destabilizes. Temperatures will also warm
back closer to normal, which will keep most of the precipitation
in the form of rain, but with some high mountain snow. -Coulston
&&
.AVIATION...
555 AM MDT Fri May 21 2021 (21/12Z TAF period)
Low clouds with periods of snow will continue the widespread
mountain-obscuring MVFR/IFR conditions through the next 24 hours.
Local LIFR conditions are possible beneath heavier snow bands. Rain
could mix with the snow at times between 20Z and 02Z. A few
thunderstorms may also move over KWYS between 21Z and 03Z.
-Coulston
Refer to weather.gov/zlc for more detailed regional aviation
weather and hazard information.
&&
.HYDROLOGY...
No flooding is expected through Saturday, as the current storm
system begins to diminish and exit the region.
However, another strong spring storm is forecast to affect the
region from Sunday into Monday. Widespread precipitation amounts
of 1 to 2 inches are possible with this second spring storm, with
isolated higher amounts. There is the potential that some flood
highlights might be needed by early next week. Brusda
&&
.PRELIMINARY POINT TEMPS/POPS...
GTF 36 29 42 34 / 80 90 20 60
CTB 35 29 41 30 / 90 60 20 30
HLN 40 31 48 38 / 70 80 20 60
BZN 43 28 53 37 / 70 80 30 80
WYS 52 29 59 32 / 90 80 50 80
DLN 42 29 52 36 / 70 80 40 60
HVR 38 31 46 34 / 80 60 10 40
LWT 36 27 48 37 / 70 80 20 80
&&
.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 9 AM MDT this morning Cascade...
Eastern Glacier...Eastern Pondera...Eastern Teton...Jefferson...
Judith Basin...Liberty...Toole.
Winter Storm Warning until 9 AM MDT this morning Central and
Southern Lewis and Clark...Northern Rocky Mountain Front...
Southern Rocky Mountain Front.
&&
http://www.weather.gov/greatfalls
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