Arco, ID Weather

Key Messages

- Warm with Isolated Storms Through Thursday: Above-normal temperatures continue across lower elevations with highs in the upper 60s to lower 70s under southwesterly flow, accompanied by isolated afternoon/evening shower and thunderstorm chances.

- Widespread Heavy Rain Late Week: An upper low tracking from the central California coast overhead will bring another widespread heavy rain event, with the primary precipitation window lasting from late Thursday evening through early Saturday morning. Forecast rainfall amounts call for around 1.0 inch across much of the Snake Plain, 0.50 inch in the Magic Valley, 0.50 to 1.0 inch near Sun Valley with upslope enhancement, and 1.0 to 1.5 inches across the Eastern Highlands.

- Sharp Friday Cooldown: Daytime highs struggle to get out of the 40s areawide on Friday. Temperatures will remain below normal through the weekend before slowly moderating near or slightly below seasonal averages early next week.

Discussion

Issued at 116 PM MDT Tue Oct 20 2026

Above-normal temperatures will persist across eastern Idaho for the next few days as the region remains positioned beneath a broad southwesterly upper-level flow regime. High-resolution convective-allowing models (CAMs) continue to show low-end probabilities for isolated showers and embedded thunderstorms developing during peak heating, though most areas will remain dry. Afternoon highs in lower-elevation valleys will hold in the upper 60s to lower 70s through Thursday.

The driver for our next major pattern shift will move into the central California coast on Wednesday before shifting northeastward toward the northern Great Basin, eventually tracking directly overhead by Friday. Significant 500 mb height falls and strong dynamic lift associated with this incoming low will bring widespread precipitation and a sharp airmass transition to eastern Idaho.

The most favorable window for widespread precipitation runs from late Thursday evening into the early morning hours on Saturday. QPF guidance has remained consistent with recent runs: expect around 1.0 inch across much of the Snake Plain, 0.50 inch in the Magic Valley, 0.50 to 1.0 inch around Sun Valley due to favorable upslope enhancement, and 1.0 to 1.5 inches across the Eastern Highlands although some locally higher rainfall totals are certainly possible and these forecast amounts could still change over the next couple of forecasts. This setup will produce another widespread soaking rainfall event across the region. In addition to the precipitation, strong cold advection and persistent cloud cover will limit Friday daytime highs strictly to the 40s across the entire forecast area.

Precipitation will taper off from west to east during the early hours of Saturday morning. Temperatures will remain well below seasonal norms through the weekend, holding daytime highs in the 50s. A slow moderating trend will take place heading into early next week, though afternoon maximums will likely stay at or just below seasonal averages, keeping conditions noticeably cooler than the unseasonable warmth observed early this week.

Aviation

Issued at 527 PM MDT Tue Oct 20 2026

VFR conditions forecast with increasing mid-level and high clouds with generally light and variable winds. Isolated showers expected to continue across the region for the next few hours with VCSH only anticipated at KDIJ through 04z. The next batch of moisture will lift northward beginning overnight around 06z at KSUN and 09z elsewhere. Thus, have included VCSH from then through the day Wednesday.

Fire Weather

Issued at 129 AM MDT Tue Oct 20 2026

Isolated showers and thunderstorms are expected today and Wednesday. Rainfall amounts should be very light. Widespread wetting rains will arrive Thursday afternoon and Thursday night.