Helena Valley, Montana 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

387
FXUS65 KTFX 140217
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
817 PM MDT Sun Apr 13 2025

.KEY MESSAGES...

 - Gusty winds diminish through the afternoon and early evening
   today.

 - Trending warmer and dry through Tuesday.

 - Transitioning to cooler and wetter for Wednesday evening into
   Thursday, with large uncertainty in precipitation amounts and
   potential impacts.

&&

.UPDATE...

8:15 PM MDT : Skies are clearing quickly, and winds subsiding,
from west to east this evening, with these conditions promoting
ideal conditions for radiational cooling overnight tonight. Have
thus lowered temperatures by 2-4 degrees from the on-going
forecast to account for these ideal conditions. Otherwise the
remainder of the forecast remains on track. Still some concerns
for fog developing in the valleys of Southwest and Central Montana
where recent precipitation has fallen over the past 12-24 hours,
but with BUFKIT soundings still supporting a very dry boundary
layer overnight I have kept any mention of fog out at this time.
Surface observations within these valleys in Southwest and Central
Montana currently have dewpoint depressions of 20 to nearly 40
degrees, which helps to support the aforementioned BUFKIT
soundings. None-the-less, would not be surprised if a few
localized areas (especially near water sources and where recent
precipitation has fallen) see patchy fog through Monday morning.
- Moldan


6:45 PM MDT : Early evening update has been published; which
resulted in the addition of low end (i.e. 15%) PoPs for areas
along and northeast of a line from Judith Gap, to Great Falls, to
Marias Pass where scattered radar returns have been persistent so
far early this evening. - Moldan

&&

.DISCUSSION...
/Issued 643 PM MDT Sun Apr 13 2025/

 - Meteorological Overview:

Upper level troughing over eastern MT and the Dakotas is quickly
exiting eastward this afternoon, allowing for upper level ridging to
begin building in. At the surface, low pressure across ND is also
exiting eastward, which has allowed for precipitation to largely
diminish. Strong mid level northwesterly flow behind this surface
low is quickly diminishing, which was the source for strong wind
gusts in Fergus county and vicinity this morning and early
afternoon. Given this opportunity for strong winds has subsided, the
High Wind Warning has been canceled.

Largely benign conditions will build in tonight as skies clear and
winds further diminish. Recent precipitation combined with light
winds and clear skies may promote some fog formation toward sunrise
tomorrow, but forecast soundings aren`t too enthused with these
prospects. For this reason I have withheld mention of fog at this
time. Other than that, Monday and Tuesday are looking pleasant, with
temperatures near or a touch above average Monday and above average
on Tuesday.

Heading into Wednesday flow aloft begins to become more zonal as
troughing from BC begins sliding southward toward WA/OR/ID. This
troughing slows and becomes a closed upper low at some point
Wednesday evening or night across the aforementioned states. There
is general agreement for this system to continue sliding southward
toward CA/NV and vicinity toward Thursday and Friday. The track of
this system will bring colder air into the region behind a Canadian
cold front Wednesday afternoon or evening, with a period of snow
likely to develop in tandem. Ahead of this front there looks to be
enough instability for showers or perhaps a few thunderstorms,
mostly across Southwest Montana Wednesday afternoon.

There is quite a bit of uncertainty with this system, but at least
some lower elevation snow is appearing likely Wednesday
afternoon/evening through Thursday. Additional concerns and a
discussion on scenarios/uncertainty is included below in the
forecast confidence and uncertainty section.

Heading toward Friday and Saturday, clusters are in relatively good
agreement that brief, weak ridging will attempt to build in across
the region promoting a trend toward drier and warmer conditions
again. By Sunday confidence becomes much lower in the evolution of
the pattern, with low-end opportunities for precipitation creeping
back into the forecast. -AM

 - Forecast Confidence & Scenarios:

The range of scenarios for the Wednesday/Thursday timeframe is quite
large. The more aggressive scenario portrayed by the GFS, holds on to
the upper disturbance in the WA/OR/ID area longer than other
guidance. This would result in a longer duration window for snow at
all elevations, with wind becoming a greater concern. This more
aggressive scenario features a barrier jet along the Rocky
Mountain Front, which would raise concerns for blowing snow in
addition to falling snow for this area.

Guidance that is quicker to send the upper disturbance southward
towards CA/NV results in a more narrow window for snow, and as a
result has much lower precipitation amounts. These quicker scenarios
also feature less of a wind concern at lower elevations.

To put into perspective the spread of potential outcomes, the
difference between 10th percentile snow amounts (Reasonable low-end
amount) and 90th percentile (Reasonable high-end amount) is greater
than 5 inches for Cut Bank, Great Falls, Lewistown, Helena and
Bozeman, among other locations. That spread between low and high-end
amounts increases to around a foot or more over passes such as
Marias Pass, Rogers Pass, and Kings Hill Pass.

The takeaway at this point is for at least a period of cooler
weather and mountain snow Wednesday PM into Thursday, with higher
end scenarios including heavy mountain snow and impactful lower
elevation snow and wind as well. -AM

&&

.AVIATION...
14/00Z TAF Period

Mid level clouds will clear out throughout the evening. Breezy
winds will also move out of KHVR and KLWT this evening.
Confidence remains low on any fog developing overnight. Monday
will bring a few clouds in the sky with calm winds. -Wilson

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

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  26  60  37  71 /  10  10   0   0
CTB  25  58  32  67 /  20  10   0   0
HLN  27  61  36  72 /  10   0   0   0
BZN  20  57  30  70 /  10   0   0   0
WYS   9  54  19  62 /  10   0   0   0
DLN  23  60  33  70 /  10   0   0   0
HVR  25  58  30  74 /  20  20   0   0
LWT  23  52  33  68 /  10  10   0   0

&&

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

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