287
FXUS65 KPIH 172248
AFDPIH

Area Forecast Discussion
National Weather Service Pocatello ID
448 PM MDT Mon Aug 17 2026

.KEY MESSAGES...

- Terrain Convection Today: Building high pressure suppresses
  organized forcing, but lingering monsoonal moisture will spark
  isolated to scattered afternoon showers and storms over the
  mountains, with a low-end chance of an isolated cell spilling
  into the Snake Plain. Main threats are gusty outflow winds and
  brief moderate downpours.

- Mid-to-Late Week Heat Surge: Temperatures will steadily rise,
  reaching the low-to-mid 90s across lower valleys Thursday and
  Friday.

- Weekend Pattern Uncertainty: Ensemble guidance hints at a
  deep Pacific Northwest low shifting the ridge east over the
  weekend, potentially re-opening the monsoonal moisture tap.
  Given high model variance regarding ridge placement, a
  consensus middle-of-the-road forecast is maintained for now.

&&

.DISCUSSION...
Issued at 300 PM MDT Mon Aug 17 2026

An upper-level ridge of high pressure will continue building
across southeastern Idaho today. While this building ridge will
eliminate large-scale upper-level support for organized
convection, deep monsoonal moisture remains firmly entrenched
across the region. Driven by increasing surface temperatures
under the ridge, diurnal heating is leading to another afternoon
of isolated to scattered higher-terrain showers and
thunderstorms. High-resolution convective-allowing models (CAMs)
strongly support this terrain-driven convective placement for
this afternoon and evening, though at least an isolated chance
remains for activity or gusty outflows to push into the Snake
Plain. The primary hazards with any developing cells will be
localized gusty outflow winds and brief, moderate downpours.
Convective activity will taper off quickly this evening with the
loss of solar heating.

Convective potential picks up slightly tonight into Tuesday as
a shortwave trough traverses the northern perimeter of the ridge
across Montana. Associated height falls and glancing upper-
level energy will enhance shower and thunderstorm coverage
across eastern Idaho during this timeframe, particularly over
the Central Mountains, Continental Divide, and Eastern
Highlands. NBM guidance appeared too low given the incoming
dynamics, so PoPs were manually adjusted upward to better
reflect expected coverage.

Behind the departing shortwave, the ridge amplifies and this
will shut off the monsoonal moisture plume pushing a drier air
mass into the Intermountain West and significantly reducing
daily thunderstorm chances for the second half of the week.
Daytime temperatures will steadily trend upward Wednesday into
Friday. Expect afternoon highs to remain slightly below seasonal
normals through Tuesday before surging above average,
especially Thursday and Friday, as the ridge axis locks over the
region with low-to-mid 90s looking increasingly likely across
valley locations.

Though there is some uncertainty, by next weekend model
ensembles show a deep Pacific Northwest low shifting the ridge
eastward. This would open the door for the return of monsoonal
moisture, bringing returning precipitation chances and cooler
temperatures. However, significant model differences remain
regarding the exact placement and strength of the Great Basin
ridge, so a middle-of-the-road approach remains the forecast for
now until better consistency is seen in model guidance.

&&

.AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 446 PM MDT Mon Aug 17 2026

Not expecting thunderstorms at TAF sites as through 23Z nothing
has developed so removed. Mainly VFR with light winds overnight
with an increase in winds Tuesday after 18Z with around 10
knots at the sites with gusts up to 20 or so at PIH and IDA.
Based on progression today kept thunderstorms out again Tuesday
afternoon.

&&

.FIRE WEATHER...
Issued at 1248 PM MDT Mon Aug 17 2026

Monsoon moisture advected northward in southwest flow aloft will
bring us another round of isolated storms today mostly across the
mid to high elevations. High resolution models show activity
remaining north and south of the Snake River Plain this afternoon
with isolated showers and storms working across the Snake River
Plain during the early evening. Thunderstorms will be capable of
heavy rain and wind gusts up to around 45 mph.

A shortwave feature passing to our north tonight and Tuesday will
flatten the ridge, keeping activity going along the Montana Divide
overnight and again Tuesday afternoon while keeping temperatures
just below normal. The ridge will rebound on Wednesday, which will
allow us to warm back into above-normal territory later week. As the
ridge of high pressure shifts directly overhead, many low elevation
areas will warm into the 90s on Thursday and Friday. While the
details are still in flux, ensembles show a deep PacNW low shifting
the ridge eastward this weekend and opening the door for the return
of monsoon moisture.

&&

.PIH WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...McKaughan
AVIATION...GK
FIRE WEATHER...Cropp