617
FXUS65 KTFX 030547
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1145 PM MDT Sun Jun 2 2024

Updated Aviation Discussion

.SYNOPSIS...

Clouds and areas of mostly mountain rain showers increase tonight
into early Monday before additional scattered showers and isolated
thunderstorms sweep through the region with a cold front later on
Monday. Strong westerly winds develop behind the cold front Monday
night with periods of stronger winds affecting much of the area
through Tuesday night. Above normal temperatures and drier
conditions return for later next week.

&&

.UPDATE...

No major updates were needed to the forecast. Widespread
precipitation will continue to slide in from western MT this evening
and overnight into the Continental Divide. Light precipitation is
also expected to develop in portions of Central and North Central MT
this evening and overnight.

&&

.AVIATION...

03/06Z TAF Period

Pacific moisture continues to stream east and increase across the
region tonight with precipitation and areas of mountain obscuration
increasing along the continental divide while cloud bases lower but
remain VFR at most TAF sites with only some very light showers
through Monday morning. South to southwest winds increase later
tonight through Monday morning ahead of an approaching Pacific cold
front with the front bringing a shift to stronger west winds as it
passes through the area Monday afternoon. Showers will move east of
the continental divide with the cold front Monday afternoon with
potential for tome thunderstorms as well across eastern terminals.
Mountain wave turbulence will become increasingly widespread Monday
afternoon and increase through Monday night. Hoenisch

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

&&

.PREV DISCUSSION...
/ISSUED 520 PM MDT Sun Jun 2 2024/

Remainder of this afternoon and tonight...Modest instability
remains sufficient for isolated shower and thunderstorm
development this afternoon/evening, mostly over the southwest and
areas of higher terrain. As mentioned in the earlier update,
gusty winds may accompany some of the stronger cells, but this
occurrence should be rather sparse. Upper level moisture streams
in from the west tonight leading to additional light rain shower
development during the overnight hours. This activity will mostly
be confined to areas along Continental Divide, but locations to
the east may also see some sprinkles or light showers, especially
over the eastern portions of Central/North- central MT.

A Pacific cold front moves across the region Monday afternoon
with any lingering light stratiform precipitation transitioning to
scattered showers and isolated thunderstorms moving eastward
along the front. The risk for thunderstorms is greatest across the
eastern portions of central and southwest MT with gusty erratic
winds being the primary concern with any storms. Colder air behind
the front may result in rainfall over the higher terrain along the
Rocky Mountain Front briefly changing to snow in the evening
before precipitation diminishes Monday night.

Westerly flow aloft increases markedly Monday evening following
the cold frontal passage, ushering the first of a couple period of
strong surface winds that will impact the Rocky Mountain Front
and Central/North-central MT through Wednesday. The initial period
of stronger winds will primarily be driven by mountain wave
activity and more localized to locations along the Rocky Mountain
Front, Central/North-central ranges, and their immediate adjacent
lee side plains. The second surge of westerly flow on Tuesday will
be stronger and more widespread. The potential for surface wind
gusts in excess of 55 mph becomes more expansive Tuesday afternoon
and evening given the wide swath of H700/H500 winds aoa 60 kts.
Ensemble statistical guidance tools such as North American
Ensemble Forecast System (NAEFS) and EC Extreme Forecast Index
(EFI) highlight forecast H700 winds to be between 3 and 4 standard
deviations above climatology during the peak of the event.

The primary limiting factor for this event to reach its full
potential looks to be the peak of the strongest upper level flow
occurring overnight Tuesday. This does limit the amount of winds
expected to mix to the surface Tuesday night, but there will be
another opportunity for mountain wave activity with the strongest
winds likely occurring on the lee sides of terrain. Wind
exceedance probabilities haven`t changed a whole lot since
yesterday, still generally running around above 90% along the
Rocky Mountain Front and adjacent high plains, 50-80% along and
east of the Central/North- central ranges and in the MacDonald and
Bozeman Pass areas. Other areas including the remainder of the
plains and the southwest valleys currently have probabilities less
than 50%.

Given the said meteorological factors and the fact that this late
season event will be more impactful than normal with higher
summer traffic and fully leafed trees, the High Wind Watches were
upgraded to High Wind Warnings. Additionally, The MacDonald Pass
zone and the remainder of the plains were added to the warning and
the start time for the MT highway 200 corridor and Bear`s Paw
Mountain areas were moved forward to Monday evening to cover
mountain wave winds Monday night. Lastly, a High Wind Watch was
added for the Gates of the Mountains, Big Belts/Bozeman Pass, and
the Helena Valley. Despite lower wind exceedance probabilities fro
some of these areas, the pattern supports at least a period of
strong winds, most likely Tuesday afternoon and evening. - RCG

The upper level jet lifts north Wednesday with winds aloft and at
the surface gradually relaxing by Wednesday afternoon and evening
as upper level ridging begins to build across the western US.
longer range model ensembles continue to support amplification the
the ridge late this week into the weekend with temperatures
trending above seasonal averages. There continues to be some
divergence however with the precise location of the ridge axis
leaving some uncertainty still at this time in exactly how much
above average temperatures will be and the location/origin of
moisture moving into the area around/over the ridge by next
weekend. - Hoenisch

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  55  69  44  69 /  30  40  10   0
CTB  50  64  42  62 /  30  20  10   0
HLN  55  69  44  70 /  20  80  30   0
BZN  50  67  39  69 /  20  90  70   0
WYS  45  54  37  62 /  20  90  80   0
DLN  52  65  39  69 /  10  80  30   0
HVR  54  74  47  72 /  30  40  30  10
LWT  50  67  40  67 /  20  80  60   0

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
High Wind Warning from 6 PM Monday to noon MDT Wednesday for
Bears Paw Mountains and Southern Blaine-Cascade County below
5000ft-East Glacier Park Region-Eastern Glacier, Western Toole,
and Central Pondera-Fergus County below 4500ft-Judith Basin
County and Judith Gap-Northern High Plains-Southern High Plains-
Southern Rocky Mountain Front-Upper Blackfoot and MacDonald Pass.

High Wind Watch from Tuesday morning through Wednesday morning
for Big Belt, Bridger and Castle Mountains-Gates of the
Mountains-Helena Valley.

High Wind Warning from 9 AM Tuesday to noon MDT Wednesday for
Eastern Pondera and Eastern Teton-Eastern Toole and Liberty-Hill
County-Northern Blaine County-Western and Central Chouteau
County.

&&

$$

http://www.weather.gov/greatfalls