307
FXUS65 KTFX 051801
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1159 AM MDT Sun May 5 2024

.SYNOPSIS...

A slow moving Pacific weather system begins to impact the
Northern Rockies today in the form of northward expansion of
shower and isolated thunderstorm activity. Snow also develops over
the higher terrain of Southwest Montana tonight into Monday
morning. Much cooler and wetter weather is expected for much of
the up coming workweek, along with breezy to windy conditions and
localized heavy mountain snow.

&&

.Update...

Morning update has been published, with minimal changes made to
the on-going forecast. Main change this morning was to increase
PoPs slightly through the early morning hours to account for
latest radar trends, especially along the Idaho border and over
the plains of Central and North Central Montana. While most of the
radar returns over the plains of Central and North Central
Montana are only resulting in virga due to dry air in the boundary
layer and mid-levels, I did bump PoPs up into the "slight" range
over a larger area to account for the heavier bursts of
precipitation. Even with this increase I only expect a trace to
potentially a hundreths of an inch with the heavier bursts
through the morning hours over the plains. Further south across
Southwest Montana webcams along the Idaho border show that
precipitation is reaching the ground, which is now supported with
PoPs in the 90-100% range. Otherwise the remainder of the forecast
remains on track, with the aforementioned dry boundary layer and
mid-levels over the plains of Central and North Central Montana
eventually saturating through afternoon hours and supporting rain
and even a few embedded thunderstorms. - Moldan

&&

.AVIATION...
1159 AM MDT Sun May 5 2024 (05/18Z TAF Period)

VFR and MVFR conditions are expected to be the main flight
categories across North Central and Southwestern Montana today,
though periods of IFR conditions can`t be ruled out, especially at
EKS and WYS. Incoming weather system will continue to bring rain and
virga showers to much of the area, with an increasing number of
these reaching the ground as we progress through the day. A few of
these showers may be able to mature into some isolated
thunderstorms, and while there is still a good deal of uncertainty
with regards to these storms, I did go ahead and add them to the
TAFs at HLN and GTF where models seem to be converging on
timing/location. Watch for increasing winds and lowering cigs
overnight tonight and tomorrow. Ludwig


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

&&

.HYDROLOGY...

A slow moving Pacific weather system will bring an extending
period of lower elevation rain and mountain snow next 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...
/ISSUED 545 AM MDT Sun May 5 2024/

Southerly flow aloft and abundant mid- and higher level clouds
have overspread much of North-central and Southwest MT this
morning as a closed Pacific low centered over N CA/NW NV slowly
moves eastward toward S ID. Scattered shower and isolated
thunderstorm activity will overspread the region today in a south
to north fashion. Given forecast MU CAPE levels generally in the
100 to 300 J/kg range, not expecting anything beyond general
thunderstorm hazards like gusty winds, brief downpours, and
isolated lightning strikes. The best chance for convective
activity will be this afternoon over the southwest and the eastern
portions of the plains. Colder air aloft moves in tonight and
introduces falling snow levels and mountain snow, most notably
over the southwestern mountains. NBM probabilities support a quick
1 to 5 inches of snow, with the highest amounts expected over NW
Beaverhead County, including Chief Joseph and Big Hole Passes.
Peak snowfall rates look to occur between 3 and 9 am Monday
morning, so a Winter Weather Advisory was posted for the three
mountain zones south of I90 to address the expected impacts during
the coldest part of the day.

A majority of ensembles continue to highlight the closed Pacific
low lifting northeastward into eastern MT while merging with
another shortwave streaming eastward out of the Pacific NW. There
will be a bit a lull in activity on Monday as steadier stratiform
rains remain to our east/northeast and mountain snows temporarily
diminish. H700 northwesterly flow begins to increase by Monday
afternoon, ushering cold air advection and increasing surface
winds along the Rocky Mountain Front and much of Central/North-
central MT.

By Tuesday, periods of moderate, to at times heavy, rainfall
begin to settle in and more or less remain in place through
Thursday. NBM probabilistic guidance still highlights locations
east of a Great Falls to Shelby line with a 50 to 80% chance for
rainfall amounts in excess of 1 inch and a 30% to 50% chance for 2
inches of rainfall or greater for the 72 hour period ending
Thursday 6 am MDT. Keep in mind that there will be some areas
situated southeast of higher terrain that see reduced rainfall
amounts due to downsloping winds. This widespread rainfall will
generally be beneficial for most, but some minor hydrologic
impacts are expected for those who see the heaviest storm totals,
please view the hydrology section for more information.

Strong and cold northwesterly flow aloft (H700 winds around 40 to
60 kts, temperatures near -10C) will be occurring during this
timeframe as well, resulting in windy conditions for all areas and
heavy wet mountain snow over the central ranges. Monday night
through Tuesday looks to be the overall windiest period, with most
locations in both North-central and Southwest MT seeing 60 to 90%
chances for wind gusts in excess of 45 mph, while the 55 mph
exceedance higher probabilities (70% +) remain confined along the
Continental Divide, the Central MT highway 200 corridor, and the
eastern portions of the north-central plains. After collaboration
with the other MT WFOs, I decided to hold off on issuing a High
wind Watch for now given continued uncertainties with positioning
of the strongest winds and mismatches with timing and NBM
probabilistic guidance.

As mentioned earlier, H700 temperatures will be cold enough to
support mountain snow, heaviest over the central ranges where a
Winter Storm Watch remains in effect Tuesday through Thursday and
the probability for snowfall amounts in excess of 2 feet continues
to run 70% and higher, mostly for the mid- slopes and higher
peaks of the Little Belt Mountains. The combination of strong
winds and heavy wet snow is expected to cause dangerous conditions
for outback recreation over higher terrain and there is also the
expectations for at least some isolated impacts to the power
infrastructure in areas that receive the heaviest snow and
stronger winds. Additional winter weather highlights may be needed
over the higher terrain of the southwest, the Rocky Mountain
Front, and possibly for the Bear`s Paw Mountains. Lower elevations
will mostly see cold, wet, and raw conditions for outdoor
recreation and agriculture activities with daytime temperatures
mostly in the 40s Tuesday and Wednesday.

Rain and mountain snow diminish on Thursday as the responsible
low pressure system weakens and generally moves away from the
region. Ridging aloft becomes more dominant heading into next
weekend for drier conditions with temperatures moderating to near
or slightly above average. - RCG

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  64  43  54  40 /  70  80  60  20
CTB  60  41  53  38 /  40  70  50  20
HLN  67  43  54  39 /  80  90  50  10
BZN  66  36  52  33 /  80  90  90  10
WYS  50  29  42  26 /  80  90 100  70
DLN  58  35  49  30 /  60 100  40  30
HVR  69  44  57  40 /  30  60  80  60
LWT  65  39  49  35 /  40  60  70  30

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from Tuesday morning through Thursday morning
for Big Belt, Bridger and Castle Mountains-Little Belt and
Highwood Mountains-Snowy and Judith Mountains.

Winter Weather Advisory from 9 PM this evening to noon MDT
Monday for Gallatin and Madison County Mountains and Centennial
Mountains-Northwest Beaverhead County-Ruby Mountains and
Southern Beaverhead Mountains.

&&

$$

http://www.weather.gov/greatfalls