572
FXUS65 KPIH 210322
AFDPIH

Area Forecast Discussion
National Weather Service Pocatello ID
922 PM MDT Thu Aug 20 2026

.KEY MESSAGES...

- Dangerous Heat & Expanding Advisories: Strong high-pressure
  ridging will push valley highs into the mid-to-upper 90s
  today, prompting active Heat Advisories. Temperatures will
  surge an additional 1 to 3 degrees on Friday as 500mb heights
  peak, expanding Heat Advisory coverage.

- Surging Moisture & Saturday Storm Peak: A low-end (15%)
  isolated storm risk remains over southern zones this evening.
  Flow turns southwesterly Friday ahead of an approaching
  Pacific Northwest trough, drawing deeper moisture north and
  making Saturday the peak day for widespread showers and
  thunderstorms.

- Warm & Unsettled Into Next Week: As the main trough exits
  toward the Canadian Prairies Sunday into Monday, residual
  moisture and a rebuilding Four Corners ridge will maintain
  isolated afternoon storm risks alongside valley highs near
  90F.

&&

.DISCUSSION...
Issued at 147 PM MDT Thu Aug 20 2026

Hot, dry, and unseasonably warm conditions dominate across
eastern Idaho as strong upper-level ridging amplifies over the
Intermountain West. Valley temperatures have already scaled into
the lower 90s early this afternoon and remain on track to top
out in the mid-to-upper 90s during peak diurnal heating.
Consequently, Heat Advisories remain active for lower-elevation
zones through this evening. H5 heights will continue to rise
into Friday, nudging afternoon maximum temperatures up by an
additional 1 to 3 degrees over today`s readings. An expanded
Heat Advisory will take effect Friday to account for this
secondary surge in thermal stress. While satellite imagery
currently reveals mostly sunny skies, high-resolution
convective-allowing models (CAMs) continue to show a low-end
(~15%) chance for an isolated shower or thunderstorm across
southern zones late this afternoon and evening.

A synoptic transition begins tonight into Friday as an upper-
level low-pressure system moves inland over the Pacific
Northwest. This feature will back regional flow to the
southwest, opening a mid-to-upper level moisture channel into
Idaho. Cloud cover will steadily increase overnight, with
isolated to scattered shower and thunderstorm chances expanding
across the forecast area through Friday afternoon.

Saturday continues to stand out as the height of convective
coverage across the region. The combination of advancing
shortwave dynamics, upper-level divergence, and elevated
precipitable water (PWAT) values will support widespread showers
and embedded thunderstorms. As the primary upper low tracks
northeastward toward the Canadian Prairies on Sunday, sufficient
mid-level lift and moisture will linger across the northern and
eastern highlands to round out the weekend with continued
scattered precipitation chances.

Long-range guidance indicates that the broad Four Corners high-
pressure ridge will attempt to re-amplify across the Great Basin
heading into the new workweek. However, the ridge axis does not
appear strong enough to completely cap the atmosphere. Lingering
boundary-layer moisture will combine with daily diurnal heating
to maintain isolated shower and thunderstorm chances through at
least the first half of next week. Afternoon temperatures will
moderate slightly behind the weekend trough but will hold near
90 degrees across valley floors  remaining a few degrees above
seasonal climatological norms.

&&

.AVIATION /00Z TAFS THROUGH 00Z SATURDAY/...
Issued at 922 PM MDT Thu Aug 20 2026

Winds will be light tonight into tomorrow morning. Expect
shower development with isolated thunderstorms late Friday
afternoon, continuing through the night.

&&

.FIRE WEATHER...
Issued at 1200 PM MDT Thu Aug 20 2026

A ridge of high pressure remains over eastern Idaho today, but we
see the return of monsoon moisture as early as this afternoon.
However, it will very limited with isolated storms only expected in
parts of the South Hills and Southeast Highlands (generally forming
along the terrain) or Fire Weather Zones 413 and 427. This
afternoon, min RHs will range from 10 to 20 percent across the area
with wind gusts around 20-25 mph outside of thunderstorm activity.
So while fire weather conditions are elevated at times, mainly out
across the desert, we`re not quite at any Red Flag Warning criteria.
Friday will be a very similar day, though afternoon thunderstorms
will increase a bit in coverage. They will still remain isolated,
but expand farther north into parts of the Central Mountains and
Snake River Plain. These storms Friday afternoon could produce gusty
winds around 50 mph. The bigger concern starts to move into the
area overnight Friday into Saturday with a more organized push of
monsoon moisture and as an area of low pressure moves through
southwestern Canada to flatten our ridge a bit. There will be the
potential for stronger wind gusts early Saturday, though we will
have to see how those move north out of Utah into our area. These
will likely be a mix of wet and dry storms as PWATs range from 0.60
to 0.75" in the Central Mountains on Saturday to 0.80 to 1.00"
everywhere else. Storms look to be more scattered on Saturday, but
some questions about how much lightning a few areas will actually
see with these has given us pause for issuing a Fire Weather Watch
today for Saturday. It seems prudent to wait for the hi-res models
to provide a bit clearer of a picture tomorrow to decide on Red Flag
Warning issuance or not. A few of our zones are "approaching
critical" instead of "critical" now, so that gives another point to
consider. Thunderstorm activity will continue on and off for the
second half of the weekend and through the first half of next week
and will be a bit of a mix of isolated and scattered storms. Outside
of storm activity, winds will be breezy on Sunday with gusts of 25
to 30 mph, but min RHs will have improved to 20 to 40 percent area
wide.

&&

.PIH WATCHES/WARNINGS/ADVISORIES...
Heat Advisory until 10 PM MDT this evening for IDZ051-054>057-
075.
Heat Advisory from noon to 10 PM MDT Friday for IDZ051>059-075.

&&

$$

DISCUSSION...McKaughan
AVIATION...13
FIRE WEATHER...AMM