893
FXUS65 KPIH 182347
AFDPIH

Area Forecast Discussion
National Weather Service Pocatello ID
547 PM 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 1225 PM MDT Tue Aug 18 2026

A shortwave feature passing to our north today will flatten the
ridge of high pressure over Idaho. This will keep isolated showers
and thunderstorms in the forecast today, particularly along the
Montana Divide region and eastern mountains. We have continued to
increase PoPs above National Blend amounts to reflect convection
today depicted in high resolution models. The Storm Prediction
Center has included the northeastern corner of our CWA, including
Island Park and Driggs, in a Marginal Risk of severe weather with a
general risk of thunder extending from the MT Divide to Idaho Falls
southward to Preston. The main impacts will be wind gusts to around
45 mph and small hail.

Dry conditions are in store for mid- to late week as the ridge
rebounds tomorrow, allowing us to warm back into above-normal
territory late this week. The warming trend will continue through
Friday, with widespread 90s across our lower elevations by Thursday
when the ridge axis shifts directly overhead. 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. The ECMWF ensemble indicates precipitation as early as
Thursday afternoon while the GFS ensemble maintains dry conditions
areawide until Friday afternoon. Saturday will be the busiest
weather day for Southeast Idaho, with scattered thunderstorms for
portions of the region, but the active pattern will continue into at
least the first part of the next week as low pressure remains off
the coast with an area of high pressure set up over the
Southwest.

&&

.AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 514 PM MDT Tue Aug 18 2026

Will have some showers through 01Z at IDA with prob30 thunder
ending by 01Z as well. Have vicinity showers at DIJ with tempo
showers until 02Z with clearing after that.

All TAF sites sites will have clear conditions on Tuesday.

&&

.FIRE WEATHER...
Issued at 1225 PM MDT Tue Aug 18 2026

A shortwave trough passing through Idaho today brings isolated
thunderstorm chances to much of the higher elevation areas through
the afternoon and evening hours. The main threat with storms today
will be gusty winds of 50+ mph with a Marginal Risk for severe
weather in part of the upper Snake River Plain near the Montana
Divide and Island Park area. This little bit of moisture around will
keep our temperatures nearly identical to yesterday. Tomorrow, a
ridge of high pressure builds in over the area and means
temperatures begin to warm again with highs in the 80s to low 90s
for most and min RHs generally at or below 20 percent. Winds look to
remain on the lighter side during the "dry" time of Wednesday and
Thursday, so we are expected to stay below Red Flag warning criteria
at this point. Models are hinting at some isolated thunderstorms
returning to the higher elevations as early as Thursday afternoon,
but those look VERY isolated at this point with some increasing, but
still isolated activity returning on Friday as monsoon moisture
returns to the area. This activity picks up as high pressure over
the Four Corners region is pushed just a bit eastward by an area of
low pressure moving into the Pacific Northwest. That low moves
northeastward over the weekend and Saturday looks to be the most
active day for our area, so we will watch it as the next best chance
for needing any Red Flag warnings for scattered thunderstorm
coverage. Activity continues on Sunday, but appears to be a bit more
isolated once again. Next week there is some disagreement on how
systems will behave, but with high pressure remaining over New
Mexico and Texas and troughing off the West Coast, we keep the door
open for more moisture to keep working into the area each at least
through Monday and Tuesday.

&&

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

&&

$$

DISCUSSION...Cropp
AVIATION...GK
FIRE WEATHER...AMM