Funnel Cloud in Montana, June 16, 2020

Area Forecast Discussion
National Weather Service Great Falls MT
1130 AM MDT Tue Jun 16 2020

Updated Aviation Section.

.SYNOPSIS...

A large scale, slow moving storm system will produce widespread
rain and mountain snow across the region through Thursday.
Temperatures will range about 10 to 15 degrees below average.
Isolated thunderstorms this afternoon could produce locally heavy
rainfall rates and small hail.

&&

.DISCUSSION...

A large scale closed upper level low will slowly migrate across
the Northwestern U.S. today through Thursday. As such, we will
continue to experience below average temperatures and periods of
widespread rain and mountain snow.

Most of the precipitation today and tonight will spread across
Southwest into central Montana, already seen developing early this
morning on radar. Weak instability beneath a strongly divergent
flow aloft could enable a few embedded thunderstorms this
afternoon, capable of producing locally heavy rainfall rates and
small hail. The 700 mb trough axis crosses the region overnight,
helping to lower snow levels to around 5500 feet across Southwest
Montana. A combination of cold advection and dynamical cooling
associated with the prolonged period of ascent is likely to bring
snow levels below that indicated by model guidance. Thus, forecast
snow accumulation and snow levels are still showing some
variation among the ensemble members, as well as variations in the
location of heaviest precipitation. A winter storm watch remains
in effect for Southwest Montana, but has been raised from 5000
feet to 6000 feet, as expected impacts from accumulating wet snow
now appear at a higher elevation compared to the prior forecast.
Nonetheless, with most backcountry areas open to recreation, snow
accumulations of even a few inches should be considered
significant for this late in the season.

After the mid-level trough axis moves into eastern Montana on
Wednesday, winds shift to northwesterly and a secondary surge of
precipitation develops across Central into Southwest Montana.
Periods of rain and mountain snow continue Wednesday night through
Thursday as the storm system lingers over the region. There is
rather low confidence in the end time for precipitation, as models
vary widely on this outcome. Overall, it is a three day
precipitation event with rainfall amounts of 1 to 2 inches for
central and Southwest Montana, with lesser amounts up to one-half
inch across much of North-central Montana including the HiLine.

The upper low finally drifts far enough away to the east on Friday
to enable partial clearing, but still supporting showers under an
unstable northwest flow aloft. Warmer air returns with fair
weather later Friday through next weekend. Current forecasts bring
temperatures back into the 80s by Sunday, with warm weather
persisting into the following week.

&&

.AVIATION...
Updated 530 AM MDT Mon Jun 15 2020 (16/12Z TAF Period)

VFR or MVFR conditions prevail this morning, with lowering cigs
today and tonight in periods of light rain. A band of light rain
continues to organize and spread from Southwest through central MT.
Precipitation intensity increases during the afternoon along with
possible isolated thunderstorms. Expect widespread MVFR/IFR across
North-central and SW Montana this afternoon, except for KCTB where
downslope northwesterly flow will limit precipitation. There may be
a brief break in rainfall this evening, before the next round of
precipitation arrives overnight through Wednesday. Most mountains
will remain obscured by clouds through the period. An unseasonably
low freezing level with deep clouds and light rain will lead to
icing on ascent/descent.

NOTE: Amendments not scheduled for Ennis (KEKS) due to ongoing
runway closure.

&&

.HYDROLOGY...
The current forecast has about 1 to 3 inches of rain falling over
the higher elevations of Central and Southwest Montana today through
Wednesday. With snow expected at elevations above 5500 feet, where
precipitation amounts will be the highest, some runoff should be
reduced in the short term. Some flooding issues could develop in
burn scar areas or if mountain runoff is more significant. River
flooding is not expected, at this time; however, backcountry streams
and creeks may experience rises and possible flooding in response to
the coming precipitation and snow melt. We will continue to monitor,
and highlights will be issued, as necessary.

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  61  45  59  47 /  60  80  90  60
CTB  64  45  61  47 /  30  60  90  70
HLN  61  43  58  46 /  80  90 100  50
BZN  61  40  55  42 /  90  80 100  50
WYS  54  35  49  31 /  90  90  90  50
DLN  56  39  50  37 / 100 100 100  20
HVR  67  48  66  49 /  40  50  90  70
LWT  58  41  56  44 /  80  60 100  70

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from this evening through Wednesday morning
above 6000 feet for Beaverhead...Gallatin...Jefferson...Madison.

&&

http://www.weather.gov/greatfalls