172
FXUS65 KTFX 011929
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
128 PM MDT Wed May 1 2024

...Aviation Section Update...

.SYNOPSIS...

A wintry spring storm will bring periods of rain and snow to the
area through Friday morning. Temperatures will remain cool to
below normal through the rest of the work week before warming up
this weekend.

&&

.UPDATE...

No significant changes were needed this morning. Daytime heating
will combine with cool temperatures aloft to result in numerous
showers this afternoon, with precipitation type largely being rain
at lower elevations, with snow in the mountains. That said, the
atmospheric profile at lower elevations will support quite a bit of
cooling where precipitation falls, so I wouldn`t be surprised to
hear of a few graupel reports this afternoon and early evening.
Although instability this afternoon looks to be on the low side
(Largely on the order of 200 m2/s2 or less), a few rumbles of
thunder cannot be ruled out.

Precipitation transitions to be more stratiform in nature this
evening as additional moisture and upper level support pivots around
an upper level disturbance in southern SK. Precipitation type
transitions to mostly snow at lower elevations tonight, though
surface temperatures may struggle to cool enough to support any
accumulations on roads there. Confidence remains high for
impactful snow across the Northern Rocky Mountain Front and in the
Little Belts, where a Winter Storm Warning remains in effect. The
chance for intermittent 1" per hour snowfall rates across Kings
Hill Pass late tonight is around 60%. Given northerly to
northwesterly upslope component of wind across the Little Belts,
we will be taking a look at whether additional Winter Weather
Advisories will be needed at lower elevations near the Little
Belts for tonight and tomorrow morning, with primary concern for
impacts being between the Belt and Geyser areas. -AM

&&

.AVIATION...
128 PM MDT Wed May 01 2024 (01/18Z TAF Period)

Note: The NWS Great Falls Weather Forecast Office manages the
following TAFs: KCTB, KHVR, KGTF, KLWT, KHLN, KBZN, KEKS and KWYS.

The next weather system is upon us, causing conditions at our
airfields to degrade, due to snow/rain, blowing snow or mist, and/or
low ceiling.

Throughout this TAF period, KCTB experiences MVFR/IFR conditions,
with gusty northwesterly winds. All other airfields begin this TAF
period in VFR conditions, and begin slipping into MVFR conditions,
from 01/21Z. Southwesterly to northwesterly gusty winds are
forecast, with the strongest winds along The Rocky Mountain Front,
where winds are forecast to gust up to 45KT. Across portions of
central Montana, winds are forecast to gust to 35KT and 40KT. The
strongest winds are forecast to occur during today`s afternoon and
evening hours. Mountain obscuration should be expected.
- Fogleman

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

&&

.PREV DISCUSSION...
/ISSUED 544 AM MDT Wed May 1 2024/

Key Points:

1. The weather system has slowed a little since the last update
but is generally still on track to arrive today with the greatest
impacts expected tonight through Thursday afternoon.

2. Precipitation has already begun in the higher elevations of the
Rockies and will become more widespread across north-central and
central Montana by this afternoon. Higher elevations will have
snow the entire time while lower elevations will oscillate between
daytime rain and nighttime snow.

3. Cool and wet conditions will continue through the remainder of
the week before warming up this weekend.

Impact and Timing By Region:

Glacier National Park and the Rocky Mountain front... The
greatest impacts are now through Thursday. Heavy wet snow and
gusty winds up to 40 mph are possible. Slippery roads and periods
of reduced visibility due to blowing snow are possible which will
make travel hazardous at times, especially along Marias Pass.
People traveling through this area should be prepared for rapidly
changing conditions and ideally avoid travel if at all possible.
By Thursday evening snow is expected to decrease in intensity,
however, light snow showers are expected to continue through
Friday morning.

Hi-Line east of Shelby... Precipitation will start off as rain
and transition to snow heading into late Wednesday night/early
Thursday morning. Along the Canadian border in the Sweet Grass
Hills, higher snow amounts will be possible. However, along
Highway 2, snow amounts are expected to be less than half an inch
with most precipitation falling as rain and most snow melting
quickly upon reaching the ground.

Lower elevations of central Montana (including Great Falls,
Helena, and Lewistown)... Precipitation will pick up this
afternoon, starting off as rain and slowly transitioning to snow
after sunset tonight. The heaviest precipitation is expected
Thursday morning through Thursday afternoon. How much snow
accumulates on the ground is a bit tricky considering how warm the
ground is and depends on how much snow falls in the early morning
hours before transitioning back to rain during the day. The main
concern in this area will be deteriorating road conditions as
slushy roads may make travel difficult at times and below freezing
temperatures overnight, particularly Thursday night into Friday
morning, may allow for some of the slush to freeze which will make
the roads slippery. This back and forth between nighttime snow
and daytime rain will start tapering off by Friday afternoon.

The mountains of central Montana (including the Snowies, Judith, Big
Belts, Little Belts, Highwood Mountains)... Snow will pick up
this afternoon with the greatest impacts expected late tonight
into Thursday morning which may make morning travel in the area very
difficult at times, particularly along Highway 89. Snow showers
will linger through Friday morning and taper off by Friday
afternoon. There is a bit of concern with upsloping potential
along the Little Belts through the duration of the event which
may enhance the expected snow amounts and allow for periods of
heavy snow and reduced visibility in the region. Anyone driving in
the area or in the backcountry will need to be prepared for
rapidly changing conditions.

Southwestern Montana south of a line between Butte and Bozeman...
This region will be the least impacted comparatively. Light rain
showers are expected for lower elevations with mountain snow
generally above 5000 feet. There are some concerns along Bozeman
Pass with a slight upslope potential and slushy conditions
possible but not enough to raise alarm bells at this point in
time. With this weather system starting up north and moving south,
southwestern Montana will not experience significant impacts
until late Thursday morning into Thursday afternoon. Generally
light showers will continue through Friday morning before tapering
off.

Advisory Update:

One major change with this forecast was to expanding the winter
weather advisory to include the Bears Paw Mountains. Looking at
how northwesterly flow sustains itself over that section of the
hi-line for most of the event, the potential for upsloping is high
enough to warrant advisory-level snowfall. It is worth noting that
the most significant impacts will be confined to the Bears Paw
Mountains and the area immediately surrounding the mountains with
otherwise minimal impacts at the lower elevations of far southern
Blaine county. The start and end time for this advisory was
delayed 6 hours from the other advisories in central Montana to
account for the delay in precipitation arriving to this portion
of the region.

Extended forecast (Friday afternoon and beyond)... After this
trough exits the region, ensemble guidance points towards a ridge
building over the intermountain west Friday into Saturday. This
will allow for much warmer conditions to return to Montana for a
couple days with a lurking potential of 70 degree plus
temperatures for portions of the plains. But cool weather lovers
will not need to hold their breath for too long as ensemble
guidance also points towards another large trough moving onshore
sometime late in the weekend into early next week which will bring
the potential for another round of unsettled weather and cooler
temperatures.

-thor

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  51  33  43  31 /  60  80  90  60
CTB  47  31  42  29 /  80  90  90  20
HLN  51  36  50  32 /  50  50  70  40
BZN  47  26  50  28 /  50  40  60  30
WYS  40  17  44  18 /  60  40  50  40
DLN  45  25  50  28 /  40  20  20  30
HVR  54  33  45  31 /  50  70  90  50
LWT  47  28  40  28 /  40  80  90  50

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until noon MDT Thursday for East Glacier
Park Region.

Winter Weather Advisory until noon MDT Thursday for Northern
High Plains-Southern Rocky Mountain Front.

Winter Weather Advisory from 6 PM this evening to 6 AM MDT
Friday for Bears Paw Mountains and Southern Blaine.

Winter Storm Warning until 6 PM MDT Thursday for Little Belt and
Highwood Mountains.

Winter Weather Advisory until 6 PM MDT Thursday for Big Belt,
Bridger and Castle Mountains-Snowy and Judith Mountains.

&&

$$

http://www.weather.gov/greatfalls