Helena Valley Northeast, Montana 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

720
FXUS65 KTFX 251130
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
530 AM MDT Fri Apr 25 2025

Aviation Section Updated.

.KEY MESSAGES...

 - Increasing opportunity for afternoon showers and thunderstorms
   across Central and Southwest Montana heading into the weekend.

 - Impactful rain and snow Sunday into Monday across portions of
   Central and Southwest Montana.

 - Another opportunity for precipitation Tuesday into Wednesday
   of next week.

&&

.DISCUSSION...
/Issued 327 AM MDT Fri Apr 25 2025/

 - Meteorological Overview:

Today through Tonight : Upper level ridging over the Northern
Rockies this morning will transition east and over the High Plains
through the evening and overnight hours tonight as longwave
troughing over the Eastern Pacific digs to over Southern California
through Saturday morning. Strengthening low level jet over the
plains of Central and North Central Montana will lead to increasing
south to southeast surface winds from the late morning through early
evening hours before gradually decreasing overnight tonight.
Increasing southerly flow aloft in response to the digging trough
will help to transport moisture northward and over Southwest Montana
through the afternoon hours today, with isolated to scattered
showers and thunderstorms developing initially over the higher
terrain of Idaho/Montana before moving north over areas along and
south of the I-90 corridor in Southwest Montana through the
overnight hours tonight.

Saturday through Monday : Longwave trough and attendant closed low
over Southern California will lift northeast and over the Desert
Southwest and Great Basin through the day on Sunday before opening
and continuing to lift northeast over the Central Rockies and
Northern High Plains on Monday. Strong warm air advection ahead of
this lifting longwave trough will help to push high temperatures
into the mid-60s to low 70s on Saturday, with temperatures on Sunday
cooling slightly before dropping to near normal on Monday in wake a
southward surging cold front from Sunday evening thru Sunday night.
Shower and thunderstorm chances will expand further north across
Central and into portions of North Central Montana from Saturday
morning and through the evening hours as a subtle wave within the
southerly flow aloft lifts north and over the Northern Rockies, with
a brief break from precipitation expected during the overnight hours
on Saturday. By the mid-morning hours on Sunday precipitation
chances will begin to increase once again as the core of the
longwave trough and attendant closed begins to approach the Northern
Rockies from the south, with initially showery/convective
precipitation on Sunday morning/afternoon transitioning to more of a
stratiform event from Sunday night through Monday morning,
especially across Southwest Montana where the heaviest precipitation
will fall with this event. Initially high snow levels of 7000ft
across North Central Montana to 8500ft across Southwest Montana
Sunday afternoon will fall through the overnight hours to around
3500ft to 6000ft by Monday morning across North Central and
Southwest Montana respectively. How fast these snow levels fall,
especially across Southwest an into Central Montana along and
southeast of a Dillon, to Three Forks, to White Sulphur Springs
line, will be dependent on precipitation rates Sunday evening as the
event transitions from convective to stratiform.

Tuesday through next Friday...ensemble clusters largely favor
northwest flow aloft over the Northern Rockies on Tuesday and
Wednesday before transient ridging over the Pacific Northwest slides
east from Thursday through Friday, with the leading mode of
uncertainty to end the work week being the position and/or timing
of the ridge amongst the ensemble members. - Moldan

 - Forecast Confidence & Scenarios:

Confidence remains very high that measurable precipitation will
occur across Southwest and into portions of Central Montana from
Sunday morning through Monday morning, with NBM10thPct QPF values of
0.01" or greater occurring along and south of the Montana Hwy 200
corridor over this timeframe. While these values are nothing to
write home about they do show continued long term consistency within
the ensemble spread, especially along and south of the I-90
corridor, since this past Monday and indicate a high degree of
certainty in measurable precipitation occurring. Where confidence
begins to degrade is in how much precipitation falls throughout the
event, and then with how far north does the precipitation extends as
there will likely be a sharp northern cutoff in the stratiform
precipitation from Sunday afternoon/evening through Sunday morning.

With respect to how much precipitation could fall across Southwest
Montana, especially along and south of the I-90 corridor, climate
anomaly indicators like the ECMWF EFIs continue to support values of
0.8 to 0.95 along and southeast of a Dillon to Bozeman line from
Sunday evening through Monday morning; with these values by
themselves signifying a climatologically unusual or extreme event.
This is impressive given the fact that the months of April, May, and
June are the Northern Rockies climatological wet season. Furthermore
a shift of tails (SOT) of 2 over these same areas supports that at
least a few of the ensemble members within the ECMWF signify a
very extreme precipitation event. Given this potential for an
unusual to very extreme precipitation a baseline of the NBM75thPct
for precipitation seems appropriate, with these values ranging from
0.5" to 1" across all elevations southeast of a Dillon to Bozeman
line and in portions of the Bridgers, Gallatin, Madison, and Tobacco
Root Mountains near 1.5". For a potential worst case scenario the
NBM90thPct supports values of 1" to 1.25" across lower elevations
while the aforementioned mountains approach 2".

How far north the stratiform precipitation extends from Sunday night
through Monday morning remains highly uncertain, with a huge spread
existing between the NBM25thPct and NBM75thPct along the MT Hwy 200
corridor for areas like Great Falls and Lewistown where 25thPct
values range from 0.05" to 0.15" while the 75thPct ranges from 0.4"
to 0.65". This spread between the 25th and 75th percentile become
less dramatic further north along the Hi-Line from Cut Bank to
Havre, which increases confidence that rainfall amounts are likely
to remain minor and below 0.05" to 0.10" over this same timeframe.

Finally confidence in snowfall amounts, mainly across Southwest
Montana, remains low (especially at and below pass level) given
marginal snow levels throughout the event. However, it should be
noted that the same ECMWF EFIs do support the potential for a
climatologically extreme snowfall event for areas along and
southeast of a Dillon to Bozeman line from Sunday night through
Monday morning. Spring storms with heavy precipitation rates during
the overnight hours have been notorious at producing snowfall 1000ft
to 2000ft lower than what model guidance supports given the strong
dynamical cooling that occurs, which certainly raises the potential
for snow occurring at least at pass level Sunday night through Monday
morning. - Moldan


&&

.AVIATION...
25/12Z TAF Period

Weak high pressure aloft will continue VFR conditions for much of
North Central, Central, and Southwest Montana with mostly clear
to partly cloudy skies through at least 26/12Z.

However, dense fog and low clouds in the KWYS valley area will
likely continue terrain-obscuring LIFR/VLIFR conditions there
through around 16Z. Similarly, low clouds along the ridge tops
east of KBZN and south of KLWT may move into those terminal areas,
but confidence is low in that happening. Otherwise, a disturbance
with weak instability will then bring increasing VFR cloudiness
to Southwest Montana after 18Z and continuing until around 04Z,
possibly with a few showers and maybe a thunderstorm to the KWYS
area. Thunderstorm coverage should be isolated at most, so have
not included mention of thunder in the KWYS TAF.

A mid-level southeasterly wind flow will also increase through
the period, likely causing some mountain wave turbulence. After
15Z, the gusty southeast winds should mix down to the surface,
then decouple somewhat after 03Z. -Coulston

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

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  64  34  70  43 /   0   0  30  20
CTB  61  32  67  40 /   0   0  10  10
HLN  65  37  70  46 /  10  10  40  20
BZN  59  35  69  38 /  10  10  20  10
WYS  58  29  61  27 /  20  30  10  10
DLN  61  38  66  37 /  10  20  20  10
HVR  64  33  71  40 /   0   0  10  10
LWT  54  33  68  40 /  10   0  20  30

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$
http://www.weather.gov/greatfalls