textproduct: Boise

UPDATE

Updated aviation discussion.

KEY MESSAGES

- High temperatures 5 to 15 degrees above normal through Wednesday.

- Shower and thunderstorm chances return starting this afternoon, and increasing Wednesday through Friday.

- Cooler temperatures by the end of the week and into the weekend.

SHORT TERM /Through Thursday Night/

Issued 401 AM MDT TUE OCT 20 2026 The upper-level low pressure system has moved further south along the west coast over the course of this morning. It is expected to remain largely stagnant off the central California coast through Thursday morning before moving onshore and NE through the Great Basin Thursday night. The short term period will be defined by southerly flow aloft and increasing shower and thunderstorm chances day over day.

Today will see temperatures remain 10-15 degrees above normal, or low- mid 70s in the valleys and upper 60s in the mtns. PWAT/moisture values will steadily increase through the day, culminating above the 90th percentile by the end of the day. This moisture will be fuel for showers and possible thunderstorm development starting late this afternoon, with chances continuing all the way into Thursday night. Daytime heating along with elevated instability will support afternoon/evening development over the w-central ID mtns and along the northern NV border late today, although chances will be low (10-15%). An embedded wave lifting through the area tonight into Wednesday will extend another low (10-15%) chance of showers (and possibly a rumble of thunder) across much of the area. A low chance of showers and thunderstorms returns to the w-central ID mtns and areas along the NV border Wednesday afternoon/evening. Early Thursday morning will see precipitation chances increase from south to north as the closed low pushes into the Great Basin. Widespread precipitation chances of 60-90% are forecast Thursday, with 20-30% chances of embedded thunderstorms. Precipitation chances gradually decrease west-to-east late Thursday night as a cold front moves in from the northwest.

Any thunderstorms will result in localized heavier rainfall rates/totals underneath the core, especially Wednesday and Thursday when higher PWATs are widespread. Through the period, southerly flow aloft will keep snow levels on the higher side (9-10kft MSL), giving rain its opportunity to shine as the dominate precipitation type. Over the course of this event (Wednesday through late Thursday), precipitation totals in lower elevations range from 0.15-0.45 inches (lowest in eastern Oregon), mountain valleys ranging from 0.50-0.80 inches, and highest elevations up to 0.90 inches; with the bulk of the precipitation falling late Thursday into early Friday. While this will be more stratiform and long duration in nature, steep terrain/burn scars may experience increased runoff and land movement.

LONG TERM /Friday through Tuesday/

Issued 401 AM MDT TUE OCT 20 2026 The low pressure system will be on the way out Friday as it passes overhead then NE and flow aloft shifts out of the northwest. The cold front will be through the area Friday morning, bringing a cool down to around 5 degrees below normal. Precipitation chances also clear out when it moves through. Of particular concern for this front is the gusty west-northwest winds. Friday afternoon, winds across ridge tops and open terrain will gust to 20- 35 mph. The strongest winds, however, will blow in the Snake River Plain east of Mountain Home where gusts will reach 35-45 mph. Lowering snow levels on Friday behind the front will allow for a transition to snow/wintry mix for elevations above 6.5 kft MSL. This switch will be at the tail end of the precipitation, however, so only a dusting of snow is expected along high elevation passes such as Banner Summit.

Pattern uncertainty increases behind this system, however Saturday at the very least should be dry with temperatures near-normal again. Near-normal for this time of year does mean that valley floors Saturday morning could see freezing to below freezing temperatures. Cluster analysis for the past few days was confident on upper-level troughing over the Pacific NW late this weekend, however some model solutions are now introducing high amplitude upper-level ridging over the area. This stark difference in synoptic pattern creates a large range of scenarios. At the time of this forecast, it does seem models are now converging on keeping the trough offshore and upper- level ridging building in.

AVIATION /12Z Tuesday through Wednesday/

Issued 516 AM MDT TUE OCT 20 2026

VFR and high clouds. Locally reduced visibility due to smoke from prescribed fires. Virga increasing Tuesday, with 15% chance of showers and thunderstorms over central ID in the afternoon/eve. Surface winds: variable less than 10 kt, except NE to SE 10-20 kt over Boise Mountains and s-central ID. Winds aloft at 10kft MSL: S to SE 10-20 kt.

KBOI...VFR and high clouds. Surface winds: SE 4-8 kt, then variable or light northwest late this afternoon.

BOI WATCHES/WARNINGS/ADVISORIES

ID...None.

OR...None.