387
FXUS65 KPIH 211928
AFDPIH

Area Forecast Discussion
National Weather Service Pocatello ID
128 PM MDT Wed Oct 21 2026

.KEY MESSAGES...

- Flash Flood Watch for Local Burn Scars: A Flash Flood Watch
  has been issued for the Wapiti, Ohio, and Big Rock burn scars
  from late Thursday through Friday, where incoming
  precipitation could trigger debris flows and rapid runoff.

- Main Rain Event Late Thursday & Friday: After a dry daylight
  lull on Thursday, widespread moderate-to-heavy rain will
  overspread the region late Thursday evening and peak on Friday
  as a closed Pacific low moves across the Great Basin. While
  models have nudged amounts upward around Sun Valley and
  slightly downward across the Snake Plain, the broader forecast
  remains unchanged: widespread soaking rain across all of
  eastern Idaho.

- Sharp Friday Temperature Drop: Daytime highs will tumble
  significantly on Friday, struggling to reach the lower 50s,
  before sunshine returns Saturday afternoon and Sunday with
  temperatures holding near or just below seasonal norms.


&&

.DISCUSSION...
Issued at 124 PM MDT Wed Oct 21 2026

Afternoon satellite imagery reveals a mix of sun and clouds
across eastern Idaho, while regional radar tracks light returns
across parts of the Central Mountains up toward the Montana
Divide. Areas farther south across the Snake Plain and Magic
Valley remain mostly clear. Temperatures continue to run on the
warm side of seasonal climatology, with lower elevations peaking
in the upper 60s to lower 70s.

A pattern change is already underway to our southwest as an
upper-level low moves onshore along the central California
coast. Between now and the main event, most locations will stay
dry. However, persistent southerly flow will allow high-
resolution convective-allowing models (CAMs) to spark isolated
showers and a few embedded thunderstorms through the early
overnight hours tonight. A noticeable lull in precipitation will
then take hold for much of the daytime hours on Thursday,
providing a dry window ahead of the primary system.

The California upper low will track across the Great Basin over
the next 24 to 48 hours, spreading deep moisture and strong
synoptic lift directly into eastern Idaho. Widespread rainfall
will begin moving into southern and western zones late Thursday
evening, expanding areawide through the day on Friday.

While model guidance continues to exhibit minor spread on
localized placementnudging rainfall totals upward in upslope-
favored areas around Sun Valley and slightly downward across the
Snake Plainthe big picture remains unchanged: widespread,
soaking precipitation is imminent. Due to the total forecast
precipitation, a Flash Flood Watch has been issued for the
Wapiti, Ohio, and Big Rock burn scars given the elevated threat
for flash flooding and debris flows over hydrophobic soils. More
widespread areal flooding remains a low-end possibility if
precipitation overperforms current consensus forecasts, but
broad areal flood statements are not warranted at this time.

Strong cold advection and extensive cloud cover will result in
a significantly colder Friday. Daytime maximum temperatures will
struggle to climb into the 50s across lower elevations, though
a few valley spots may briefly reach that threshold during brief
breaks. Rain will steadily taper off from west to east late
Friday evening and into the early morning hours on Saturday as
the low center pulls eastward.

A much more pleasant weekend is in store once morning clouds
exit on Saturday. While Saturday will remain on the cooler side,
sunshine will break out during the second half of the day and
continue into Sunday. Daytime high temperatures will hold at or
slightly below seasonal averages in the 50s and low 60s. No
widespread organized storm systems are projected behind this
departing low, though isolated showers will remain possible
across higher-terrain zones Monday and Tuesday under a cool,
seasonable regime.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1141 AM MDT Wed Oct 21 2026

East Idaho on lee side of weak feature that brought SHRA to the
region this morning. A few showers still in place on
satellite/radar, but generally past terminals already so believe
threat has been diminished for the remainder of today. High-res
models show a weak feature working through East Idaho for a
brief time this evening/overnight, which may be enough for a few
SHRA at SUN, but should be no impact at the other terminals at
this time. IDA and DIJ may be close, but not enough confidence
to add to terminal forecast. Better confidence for increased
coverage for SHRA and isolated TSRA at all terminals after 18Z
Thursday. CIGS remain VFR for now, with light winds.

&&

.FIRE WEATHER...
Issued at 111 AM MDT Wed Oct 21 2026

A few spotty showers are expected today with little to no
measurable rain. Wetting rains will likely hold off until late
Thursday afternoon and Thursday night.

&&

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

&&

$$

DISCUSSION...McKaughan
AVIATION...DMH
FIRE WEATHER...13