324
FXUS65 KBOI 152320
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
520 PM MDT Tue Sep 15 2026

.UPDATE...Updated aviation discussion.

&&

.KEY MESSAGES...
- Slight warming and drying trend Wednesday, with light winds.

- Cold temperatures tonight will bring patchy fog and frost to
  the mountain valleys.

- A low pressure system will bring light precipitation and
  isolated afternoon thunderstorms across the area Thursday and
  Friday. A few storms could be strong on Thursday producing
  heavy rain and small hail.

&&

.SHORT TERM /Through Thursday Night/...
Issued 228 PM MDT TUE SEP 15 2026
An upper level low continues to track slowly southward along
the Pacific Northwest coastline today, strengthening as it drops
toward northern California through Wednesday. As the low moves
south, amplified southerly flow aloft will overspread southeast
Oregon and southwest Idaho. This pattern shift will drive a
steady warming trend across the region, boosting temperatures
back to near seasonal norms through Wednesday afternoon. Dry
conditions will generally prevail today and early Wednesday
under the building ridge ahead of the main trough axis, though
mid and high level cloudiness will increase from west to east.

By Thursday, the upper level low will begin to shift eastward
and move inland from the Oregon coast. As it pushes inland, an
associated upper level jet streak will position itself across
southwest Idaho. The resulting upper level divergence and strong
synoptic forcing will provide favorable dynamics for broad
scale ascent. Expect increasing instability and moisture
transport, leading to expanding shower coverage and a risk for
embedded thunderstorms from Thursday afternoon into Thursday
night. Thunderstorm parameters featuring 200-400 J/kg surface
based CAPE and 0-6 km bulk shear of 45-65 kts suggest a marginal
threat for organized thunderstorms capable of producing heavy
rain, gusty winds, and small hail. The primary caveat remains
the exact placement of the dry slot. Recent high-resolution
model runs position the mid-level dry slot directly over
southwest Idaho Thursday afternoon, which would focus the
highest potential for robust thunderstorm development across
southeast Oregon and eastern Idaho. Any shifts in system timing
or track will alter this dry slot orientation. Temperatures on
Thursday will remain mild, though cloud cover and shower
activity will moderate afternoon highs slightly compared to
Wednesday.

&&

.LONG TERM /Friday through Tuesday/...
Issued 228 PM MDT TUE SEP 15 2026
The upper-level low pressure system will continue to move through
the area Friday into early Saturday. Precipitation / thunderstorm
chances and overall coverage will be lower compared to Thursday,
with the primary threat area being the west-central Idaho mountains
and Baker County. Chances will range from at least a 30-70% chance
of showers and a 10-25% chance of thunderstorms through Friday
night. Over the weekend starting Saturday afternoon, upper-level
ridging builds in causing warming and drying conditions.
Temperatures will start slightly below normal and trend slightly
above normal (3-5 degrees) by Monday. An upper-level troughing
pattern looks to return by early-to-mid next week, bringing slightly
cooler temperatures and a slight chance of mountain rain
showers.

&&

.AVIATION /00Z Wednesday through Thursday/...
Issued 520 PM MDT TUE SEP 15 2026

VFR. Increasing high clouds, mainly north and west of KTWF/KJER.
Skies clearing late Wed/AM. Surface winds: Variable up to 10 kt
this evening. Becoming SW-NW 6-12 kt overnight with gusts to
around 20 kt in SE Oregon. Winds aloft at 10kft MSL: SW 5-15 kt.

KBOI...VFR with increasing high clouds this evening. Surface
winds: Variable (leaning W-SW) 3-6 kt overnight. Becoming
northwest 5-10 kt Wed/PM.

&&

.BOI WATCHES/WARNINGS/ADVISORIES...
ID...None.

OR...None.

&&

$$

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AVIATION.....NF
SHORT TERM...JDS
LONG TERM....CH