textproduct: Pocatello

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 /18Z TAFS THROUGH 18Z FRIDAY/

Issued at 1200 PM MDT Thu Aug 20 2026

VFR conditions expected through the period with light winds and mostly sunny skies, becoming partly sunny, as mid and high clouds move in as we push into the day Friday. Only a very isolated precip chance through the period with models showing it limited to areas south of KBYI and KPIH so have excluded any mention from the forecast.

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.