019
FXUS65 KPIH 181137
AFDPIH

Area Forecast Discussion
National Weather Service Pocatello ID
537 AM MDT Tue Aug 18 2026

.KEY MESSAGES...

- A Passing Wave to our North brings Convection Today: Building
  high pressure suppresses organized forcing, but lingering
  monsoonal moisture and a passing wave to our north will spark
  mostly isolated showers and storms mainly 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 220 AM MDT Tue Aug 18 2026

Convective activity picks up slightly this morning and continues
through today, especially this afternoon, 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 today before surging above average, especially Thursday
and Friday, as the high pressure 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 /12Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 522 AM MDT Tue Aug 18 2026

Expect VFR conditions during the period with overall light
winds. A passing wave to the north this morning will bring VCSH
showers for BYI and PIH. Models continue the best chance of
thunderstorms this afternoon/early evening is across our
northern areas. Have a PROB30 for thunderstorms this
afternoon/early evening for DIJ and IDA with gusts of around 30
kts possible with any thunderstorms. There is less than a 10
percent chance for thunderstorms for SUN and BYI and less than
a 20 percent chance of thunderstorms for PIH this afternoon.
Starting early this evening expect light winds and clearing
skies as high pressure builds in.

&&

.FIRE WEATHER...
Issued at 220 AM MDT Tue Aug 18 2026

A shortwave feature passing to our north today will flatten the
high pressure ridge keeping convective, mostly isolated,
activity going across higher terrain today, especially along
the Montana Divide. Today, expect temperatures slightly below
normal.

The high pressure ridge will rebound on Wednesday, which will
allow us to warm back to above-normal temperatures 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, model ensembles show a
deep PacNW low shifting the ridge eastward this weekend and
opens the door for the return of monsoon moisture.

&&

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

&&

$$

DISCUSSION...TW
AVIATION...TW
FIRE WEATHER...TW