397
FXUS65 KTFX 171150
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
550 AM MDT Mon Jun 17 2024

Aviation Section Updated

.SYNOPSIS...

Well below normal temperatures are expected across the Northern
Rockies through the middle of the work week as an anomalously
cold June storm moves overhead. This storm will also bring
beneficial rain and mountain snow to Southwest through North
Central Montana.

&&

.UPDATE...

Precipitation coverage and intensity will increase across Central
and North Central Montana through 8 AM MDT as strong upper level
divergence associated with the exit region of a potent upper level
jet overspreads the Northern Rockies. This burst of heavier
precipitation, primarily west of a Wolf Creek, to Great Falls, to
Loma, to Conrad line may allow rain to mix with or change over to
snow as snow levels fall to as low as 4000-4500 feet. Snow
accumulations below 5500 to 4000 feet will generally be confined
to grassy surfaces and less than 1 inch. - Moldan

&&

.AVIATION...
17/12Z TAF Period

Widespread rain/snow snow showers continue to spread across
terminals this morning and will last through the TAF period.
Precipitation will transition VFR ceilings to MVFR ceilings this
morning (afternoon for KBZN/KEKS). During the evening, ceilings will
drop to IFR ceilings (except KGTF dropping to IFR morning). KLWT has
a 70% of dropping to LIFR ceilings around midnight Tuesday. There`s
a 10-30% for thunderstorms this afternoon, which can drop
visibilities to MVFR/IFR categories. Mountain obscuration will be
persistent through the TAF period from low ceilings and mountain
snow showers. -Wilson

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

&&

.PREV DISCUSSION...
/ISSUED 347 AM MDT Mon Jun 17 2024/

Today through Wednesday...Positive tilted longwave trough over the
Western CONUS and Canada, with an attendant closed low over the
Pacific Northwest, will slowly lift northeast and over the High
Plains through Tuesday afternoon. This track combined with the mid-
level (H700) low tracking from over Northern Idaho (this morning) to
Central Montana (this afternoon/evening) and eventually Northeast
Montana/Western NoDak (Tuesday morning/afternoon) will place much of
Southwest through North Central Montana beneath favorable dynamics
for widespread lower elevation rain and mountain snow. This
precipitation will be very beneficial for the entire CWA, with most
ASOS climate sites running a June precipitation deficit of between
0.75" and 1.50".

Anomalously, by 2-3 standard deviations, cold air associated with
this disturbance combined with strong dynamics have made for a
tricky snow level forecast, with the operational NBM showing snow
levels ranging from 6000-8000 feet throughout the event. Recent June
snowstorms (i.e. June 7-8th, 2020 and June 13-14, 2022) with equally
anomalous cold air and strong dynamics have brought impressive
snowfall to areas along the Continental Divide, with actual snow
levels being much lower than the operational NBM guidance. Given the
higher snow level bias of the operational NBM during these types of
events, decided to lean into and below the NBM10th percentile for
this event, which brought snow levels down as far as 4500-6500 feet
across Southwest through North Central Montana. Winter Weather
Advisories remain in effect for most mountainous zones; however, a
few zones (i.e. Southern Rocky Mountain Front, Little Belt and
Highwood, and Gallatin and Madison County Mountains and Centennial
Mountains) across Central and Southwest Montana may require an
upgrade to a Winter Storm Warning should snowfall amounts trend any
higher. Held off on upgrading these areas given that half of the
snow will be falling during the daylight hours today, with a high
June sun angle working against snow accumulations. None-the-less,
cold and raw conditions will befall anyone out in the backcountry,
with the risk for hypothermia being a significant risk for those who
are unprepared. Additionally, the heavy, wet nature of the snow will
likely lead to tree damage at higher elevations and the potential
for power outages. On the liquid side of the storm, NBM
probabilities for greater than 0.5" range from 20-40% across the Hi-
Line and far Southwest Montana to 80%+ across Central Montana, with
the probability for 1" of QPF being 30-60% north of the US Hwy 12
corridor and southeast of a Havre to Choteau line.

High temperatures over the next several days will be well below
normal, with today being the coldest as highs struggle to rise out
of the 40s to low 50s. Overnight temperatures will fall into the 20s
to 30s tonight and Tuesday night, with wind chill values falling to
as low as the teens in the mountains over this same timeframe.

Thursday through Friday...ensemble clusters favor southwesterly flow
aloft over the Northern Rockies as shortwave troughing moves from
over the Eastern Pacific/Western Seaboard and towards the Northern
and Central Rockies. This southwesterly flow regime will help to
maintain scattered to numerous showers and storms, particularly over
Central and North Central Montana, each day. Additionally,
temperatures will moderate to near normal.

Saturday through next Monday...quasi southwest to zonal flow is
favored within the multi-model ensemble mean during the period,
which is expected to translate to well above normal temperatures and
isolated showers/storms each day across the Northern Rockies. -
Moldan

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  45  37  53  36 / 100  90  90  30
CTB  48  35  51  32 /  90  70  80  20
HLN  49  40  58  40 / 100  90  90  40
BZN  52  33  54  32 / 100 100  90  30
WYS  50  27  49  24 /  90  90  80  20
DLN  51  31  55  33 /  90  90  70  20
HVR  52  39  58  38 /  90  80  70  30
LWT  48  34  51  33 / 100 100  90  30

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 6 PM MDT Tuesday for Big Belt,
Bridger and Castle Mountains-East Glacier Park Region-Elkhorn
and Boulder Mountains-Gallatin and Madison County Mountains and
Centennial Mountains-Little Belt and Highwood Mountains-
Northwest Beaverhead County-Snowy and Judith Mountains-Southern
Rocky Mountain Front.

&&

$$

http://www.weather.gov/greatfalls