Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 212253
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
450 PM MDT SAT MAY 21 2016

.UPDATE...

448PM...Canceled Severe Thunderstorm Watch for Fergus County.

.DISCUSSION...

Tonight through Monday...A very cool and moist area of high
pressure remains entrenched over North-central MT. Further east,
an inverted surface trough stretches from east of Billings to
near Glasgow. This creates a warmer and more unstable convective
environment over eastern Montana, but it does stretch westward
just enough to create potential for severe thunderstorms over far
eastern Fergus County. While temperatures are presently in the
lower 50s, there is some concern that a band of thunderstorms just
now moving into southern Fergus county could produce marginally
severe elevated storms, mainly along and east of MT 191. Any
thunderstorms rolling off the higher terrain south of Lewistown
could ride over a layer of enhanced low level shear in this area.
The severe storm threat is expected to end as this presently
active band of storms moves through the county.

As precipitation tapers for most areas tonight, attention turns to
a heavy snow potential over Glacier NP. As the center of the upper
low cuts through northwest MT, winds will shift to the west and
produce an extended period of upslope winter precipitation over
GNP. Snow levels drop to around 6000 feet through Monday. Westerly
wind gusts could reach near 50 mph and snow will cross to the
eastern side of the Continental Divide. Those working / recreating
in GNP this weekend through Monday will encounter cold exposure
hazards due to wet snow and wind. Snow will reduce visibility at
times to near zero, with a rapid deterioration in conditions above
5500 feet. A winter weather advisory is posted above 6000 feet to
address these concerns.

Even as the upper low moves into Alberta Sunday, periods of rain
are expected through Monday. Isolated thunderstorms embedded
within areas of rain are possible as well, mainly during afternoon
hours. Temperatures remain well below seasonal average. PN

MONDAY NIGHT THROUGH SATURDAY...This period will continue to be
unsettled, but most of the activity will likely occur during the
diurnally unstable afternoon and evening hours. A lingering low
pressure trough will continue a good chance of showers over the area
through Tuesday, with a slight chance of thunderstorms over
Southwest Montana under the base of the trough. Temperatures will
remain 5 to 10 degrees below normal during this time. A broader
trough will then develop over the western and central United States
for Wednesday and into the Memorial Day weekend, as a series of
disturbances from the Gulf of Alaska reinforce the trough.
Temperatures will warm back closer to normal from mid week through
the end of the week, which will increase the instability during the
afternoon and early evening hours and make these periods the most
likely for scattered showers with a few thunderstorms. There are
some differences among the forecast models as to just how strong
these disturbances will be and how much precipitation will be
produced. Have therefore gone with near to slightly above
climatological POPs. This uncertainty carries through the holiday
weekend, but it appears at this time that the period will continue
to be unsettled with occasional showers and maybe a few
thunderstorms.  Coulston

&&

.AVIATION...
UPDATED 1810Z.
Moist and unstable southerly flow aloft will continue as will
widespread rain and isolated thunderstorms. Strong thunderstorms are
possible along and east of a KHVR to KLWT line. Thunderstorms should
end by 04z but areas of light rain will continue overnight. MVFR/IFR
conditions prevail due to low ceilings, fog, and precipitation.
Gusty erratic winds are possible near thunderstorms. Mountains
obscured.

&&

.HYDROLOGY...
Periods of showers and thunderstorms will continue to affect the
region over the next few days. Some of the stronger storms could
produce rainfall amounts of 0.50 to 0.75 inches. With breaks in
the precipitation, major rivers/creeks are not expected to rise
above flood stage at this time.

However, some minor low land flooding or agricultural field
flooding can not be ruled out for small streams that flow out of
the mountains, as snowmelt from higher elevations will add to the
runoff rises. For this reason, a Flood Watch has been issued for
Clear Creek with the potential it will rise near or slightly above
flood stage Sunday morning. Brusda/Martin

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  41  56  43  54 /  50  40  20  40
CTB  39  50  40  51 /  60  40  40  50
HLN  41  56  43  57 /  30  40  20  30
BZN  36  58  39  59 /  20  40  30  30
WEY  31  44  31  48 /  40  50  50  50
DLN  36  53  36  56 /  20  40  20  30
HVR  42  60  44  57 /  60  40  40  40
LWT  39  61  41  58 /  50  40  20  30

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from midnight tonight to noon MDT Monday
for MTZ009-048.

Flood Watch for Clear Creek in Blaine County through Sunday
night.

&&

$$

weather.gov/greatfalls


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