013
FXUS65 KMSO 211913
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
113 PM MDT Fri Aug 21 2026

.DISCUSSION...

KEY MESSAGES:

-  Marginal risk (5% chance for any given location) for severe
   thunderstorms capable of severe wind gusts (60 mph) on Saturday
   areawide.

-  Fire Weather Watches in effect Saturday into early Sunday
   morning across northwest Montana for dry lightning capable of
   new fire starts.

-  Fire Weather Watches in effect Sunday across southwest Montana
   (Fire Zones 110-111) for dry and windy conditions.

A ridge of high pressure continues to bring hot and dry conditions
to the area, with high temperature running 10-15F above normal
today. Water vapor satellite imagery shows monsoon moisture
streaming northward across the Great Basin into the Northern
Rockies, with mid-high level cloud cover beginning to increase.
High-resolution models and forecast soundings suggest atmospheric
instability will begin to support high-based shower and isolated
thunderstorm activity by this evening across central Idaho.
Convection will move northward overnight, reaching western Montana
after midnight. Little to any precipitation is expected as warm
and dry air is present in the low levels. Isolated lightning
activity (5-10% probability) can not be ruled out across north-
central Idaho and southwest Montana this evening through early
Saturday morning as this wave of showers and isolated convection
moves northward.

The risk for strong to isolated (5% probability) severe
thunderstorms enters the picture Saturday afternoon and evening,
with strong to damaging winds being the primary threat. An upper-
level trough is still on track to lift out of the Pacific
Northwest and phase with increasing monsoonal moisture providing
plenty of moisture, lift, and instability for thunderstorm
development. Widespread shower and thunderstorm activity will
increase through the afternoon and evening hours across the
region. A fire weather watch has been issued across the West Lolo
and Kootenai National Forests and the Flathead Reservation as
thunderstorms will bring an increased risk for dry lightning
capable of starting new wildfires.

PWATS are forecast to near and exceed 1.0 inches in southwest
Montana and Lemhi County on Saturday, leading to a marginal
threat for excessive rainfall, due to intense precipitation rates.
Normally vulnerable spots such as burn scars, and areas of steep,
rocky terrain will be monitored closely for excessive rainfall
leading to debris flows and/or rock slides.

A cold front crosses through, from the west, Saturday evening
into Sunday lowering daytime high temperatures by 10-15 degrees;
closer to seasonal normals. However, the post-frontal environment
will also bring deep mixing, strong westerly winds, and a very dry
airmass creating elevated to near-critical fire weather
conditions especially considering the preceding risk of new
lightning starts and holdovers from Friday and Saturday`s
convection. A fire weather watch has been issued for Sunday across
southwest Montana (Fire Weather Zones 110-111), where confidence
is increasing for windy and dry conditions to bring critical fire
weather.

Ensemble model cluster analysis shows modest-high confidence (>80%
agreement) for the Northern Rockies to be under the influence of
southwesterly flow Monday through Wednesday next week. Model
guidance suggests a ridge of high pressure centered over the
Rockies, with a trough of low pressure near the Pacific Northwest
coast. This pattern suggests mainly dry and warm conditions, with
chances for showers and thunderstorms focused across Lemhi County,
ID into southwest Montana.

&&

.AVIATION...High pressure continues to bring hot conditions and
elevated density altitudes across the regional airspace this
afternoon. GOES Water Vapor imagery shows monsoon moisture moving
into the mid-upper levels of the atmosphere south to north today.
Elevated convection developing around and after 22/0000Z near
KSMN and KBTM will spread northward into the overnight bringing
the risks of localized erratic outflow wind gusts of 20-30 knots,
inducing low-level turbulence, and LLWS near these high-based
storm cores. Widespread shower and thunderstorm activity on
Saturday poses threats for strong to severe wind gusts, hail, and
temporary MVFR/IFR weather conditions across the region
particularly during the afternoon hours.


&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Fire Weather Watch from Saturday afternoon through Sunday
     morning for Flathead Reservation...Kootenai...West Lolo.

     Fire Weather Watch from Sunday afternoon through Sunday evening
     for Deerlodge/West Beaverhead...East Beaverhead.

ID...Heat Advisory until 8 PM PDT this evening for Lower Hells
     Canyon/Salmon River Region...Orofino/Grangeville Region.

&&

$$