-- Discussion containing changed information from previous version are highlighted. --
735
FXUS65 KTFX 061735
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1135 AM MDT Mon May 6 2024

.SYNOPSIS...

A Pacific weather system is on track to initially bring strong
northwest winds to much of North Central, Central, and Southwest
Montana with some areas of precipitation. While the strong winds
will linger through Wednesday, the low elevation rain and mountain
snow will increase and become heavier, especially over North
Central and Central Montana. The system will then exit the area
Wednesday night into Thursday, for decreasing winds and
precipitation, and a period of drying and warming conditions into
the weekend.

&&

.UPDATE...

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Forecast appears to be mostly on track so far, so no major changes
are planned until the afternoon forecast package aside from
expiring the Winter Weather Advisory for Southwestern Montana.
Ludwig
-- End Changed Discussion --

&&

.AVIATION...
1105 AM MDT Mon May 6 2024 (06/18Z TAF Period)

Light to moderate rain has overspread areas along and northeast of a
KLWT, to KGTF, to KCTB line late this morning, with this area of
precipitation expected to slowly lift north and east of a KLWT to
KHVR line through 00-06z Tuesday. MVFR/low-VFR conditions will
predominately prevail beneath and outside of this area precipitation
through much of the TAF period, with CIGS falling to IFR/MVFR at the
KCTB and KHVR terminals beyond 12z Tuesday as precipitation pivots
back to the southwest from southern Canada. Mountains will be
obscured over the next 24 hours.

In addition to the lower ceilings throughout much of the TAF period,
strong and gusty west to northwest winds will occur along and north
of the I-90 corridor, particularly at the KHLN, KLWT, KHVR, and KCTB
terminals where wind gusts in excess of 35kts are expected. Mountain
wave turbulence and instances of low level wind shear will be a
threat for the entire 0618/0718 TAF period. - Moldan

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

&&

.HYDROLOGY...

A slow moving Pacific weather system will bring an extended
period of lower elevation rain and mountain snow this week, with
the heaviest precipitation falling Tuesday through early
Thursday. NBM probabilistic guidance highlights the eastern
portions of Central/North-central MT with the heaviest rainfall
amounts for this period, largely supporting 1 to 3 inches of
rainfall/snow melt liquid equivalent over locations east of a
Great Falls to Shelby line Tuesday through Thursday.

While flooding is generally not expected, some rivers, creeks,
and streams along the Canadian border in Hill and Blaine Counties
are expected to approach or reach action stage Wednesday and
Thursday, mainly impacting unpopulated rural locations. Anyone
near waterways should remain alert for changing conditions and be
prepared to seek higher ground should water rises occur. - RCG

&&

.PREV DISCUSSION...
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/ISSUED 1105 AM MDT Mon May 6 2024/

Key Points:

- A significant spring storm remains on track to impact the
  Northern Rockies through the middle of this week; bringing
  strong winds, widespread low elevation rain, and heavy mountain
  snow.

- Strong northwest winds of 30 to 45 mph gusting to 65 mph will
  develop today over much of North Central and Central Montana and
  continue at times through Wednesday.

- The heaviest forecast rainfall amounts have spread west to
  include the plains for much of North Central and Central
  Montana. Amounts are mostly in the 1 to 2 inch range for this
  afternoon through Thursday, with up to 4 inches of rain/snow
  water equivalent possible in the mountains.

- The heaviest mountain snowfall amounts have remained
  consistently in the 1 to 2 feet range for most areas Tuesday
  through Wednesday, but up to 3 feet of snow is possible in the
  Little Belt Mountains.

Today through Tonight... A low pressure area aloft that is over
the Utah/Idaho/Wyoming borders early this morning is forecast to
be moved northeast across Wyoming today and into the Dakotas
tonight. Moisture wrapping around this system will continue light
to at times moderate snow with gusty southerly winds in the
mountains of Southwest Montana, so the Winter Weather Advisory
will remain in effect there until noon today. Precipitation will
gradually shift northeast on the back side of the low pressure
area, focusing over the area along and east of a Lewistown to
Havre line this afternoon. The moisture axis is then forecast to
move further to the northeast overnight as the low center lingers
over North Dakota. However, strong (45 to 60 kt) low- to mid-
level winds on the backside of the low will also combine with a
strong surface pressure gradient to bring a 70 to 85 percent
probability of strong northwest winds of 30 to 45 mph with gusts
to 65 mph to much of North Central and Central Montana beginning
this afternoon. Have therefore upgraded the High Wind Watch for
much of North Central Montana to a Warning and expanded it to
include much of Central Montana. This potential seems to be driven
partially by areas of downsloping off the Sweetgrass Hills, Bears
Paw Mountains, and the island mountains of Central Montana, as
well as by the northwest winds blowing through northwest to
southeast oriented valleys in Central Montana. This system will
also begin the downward trend in temperatures, as they should fall
closer to seasonal averages.

Tuesday through Thursday... The low pressure center is then
forecast to retrograde west into Eastern Montana Tuesday into
Tuesday night as it merges with a large low pressure trough moving
east across the Pacific Northwest. This shift west of the
precipitation-laden system will spread moderate rainfall over much
of North Central and Central Montana, which will persist into
Wednesday night. Probabilistic guidance continues to give much of
the area an 80+ percent chance for at least 1 inch of
precipitation, with a 50 to 70 percent probability of 2+ inches
east of Interstate 15. Temperatures cooling up to 15 degrees below
normal will help lower snow levels to between 4000 and 5000 feet,
resulting in at least a 50 percent chance of 12+ inches of snow
for most mountain areas with a 70+ percent chance of at least 24
inches of snow in the Little Belt Mountains. Have therefore
continued the Winter Storm Warning for the Little Belts and
upgraded the Winter Storm Watches to Warnings for the East Glacier
Park Region and for the remaining mountain areas of Central
Montana. The approach of the low pressure center and its
associated strong northwest flow mentioned earlier will continue
the strong surface winds through Tuesday into Tuesday night, which
will cause reduced visibility in the mountains due to wet blowing
snow, and a few power outages are possible due to the winds
blowing wet accumulated snow on some power lines. Winds should
decrease through the day on Wednesday and the snow should taper
off Wednesday night into Thursday as ensemble guidance members are
in good agreement with moving a high pressure ridge southeast into
Montana, forcing the weather system into the Central Rockies and
High Plains.

Friday through next Monday... Most of the ensemble guidance keeps
at least a weak high pressure ridge over Montana through this
period, which should keep the area mostly dry as temperatures warm
to around 15 degrees above normal by Monday. However, the NBM
brings some weak disturbances through the area Sunday and Monday,
so there is a chance of mainly mountain precipitation for those
days. -Coulston
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&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  56  39  47  36 /  40  20  60  90
CTB  54  37  45  35 /  20  30  70  90
HLN  55  38  49  39 /  30  10  50  80
BZN  53  31  48  32 /  70  20  50  80
WYS  44  26  40  26 /  60  50  80  80
DLN  50  29  45  32 /  40  40  40  60
HVR  56  38  48  37 /  80  80  90 100
LWT  47  34  42  31 /  80  40  70  90

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
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High Wind Warning until 6 AM MDT Wednesday for Bears Paw
Mountains and Southern Blaine-Eastern Toole and Liberty-Fergus
County below 4500ft-Hill County-Judith Basin County and Judith
Gap-Little Belt and Highwood Mountains-Northern Blaine County-
Snowy and Judith Mountains-Western and Central Chouteau County.

Winter Storm Warning from noon Tuesday to midnight MDT Wednesday
night for Big Belt, Bridger and Castle Mountains-East Glacier
Park Region-Little Belt and Highwood Mountains-Snowy and Judith
Mountains.

High Wind Warning until midnight MDT Tuesday night for Big Belt,
Bridger and Castle Mountains-Canyon Ferry Area-Elkhorn and
Boulder Mountains-Gates of the Mountains-Helena Valley-Meagher
County Valleys-Upper Blackfoot and MacDonald Pass.

High Wind Warning until 6 PM MDT Tuesday for East Glacier Park
Region-Eastern Glacier, Western Toole, and Central Pondera-
Northern High Plains-Southern High Plains-Southern Rocky
Mountain Front.
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&&

$$

http://www.weather.gov/greatfalls