textproduct: Boise

DISCUSSION

Water vapor imagery shows a deep subtropical moisture plume currently advecting into southeast Oregon and southwest Idaho under strong westerly flow aloft. This atmospheric river will be the primary weather driver through Tuesday night. Currently, radar shows increasing showers across the area, though a dry boundary layer remains a limiting factor for precipitation reaching the ground so far. However, precipitation will become moderate to heavy beginning late tonight with the greatest potential Tuesday morning as the core of the moisture plume arrives.

Heaviest totals are still targeted for the Boise Mountains, where 1.00 to 2.00 inches of liquid equivalent are forecast through Wednesday morning. This significant rainfall on existing snowpack increases the risk of rock and mud slides on steep terrain. Residents and travelers should avoid roads below steep slopes, such as the Banks-Lowman Road, until the event concludes.

Snow levels currently sitting between 5000 and 7000 feet will rise further on Tuesday, peaking between 6500 and 8500 feet. This will keep the bulk of the precipitation as rain for most mountain valleys, though a brief period of snow or a rain/snow mix is possible in the McCall area early Tuesday morning. Significant snow accumulations will be restricted to the highest peaks above 7000 feet. A cold front will move through late Tuesday night into Wednesday, bringing a shift to gusty northwest winds and lowering snow levels to 3500-4500 feet as the moisture exits. No updates needed at this time.

AVIATION

Precipitation continuing to increase from NW-SE with lowering ceilings through the night. Precipitation will persist area- wide through Tuesday. Low VFR-MVFR in rain, IFR/LIFR in snow. mountains obscured. Snow levels: 5.5-7.5 kft MSL, rising to 6-8.5 kft MSL Tue/PM. Surface winds: Variable up to 12kt, increasing to SW 15-25 kt across SE-Oregon & SW-Idaho highlands by Tue/AM. Winds aloft at 10kft MSL: SW-W 30-40 kt, increasing to 40-55 kt overnight. Strongest in E-Oregon.

KBOI...Rain after Tue/09z with low-VFR conditions. Periods of MVFR ceilngs possible (30% chance) after Tue/09z through Tuesday morning. Surface winds SE 5-10 kt.

PREV DISCUSSION

SHORT TERM...Tonight through Wednesday night...Slightly lowered total QPF tonight through Wednesday morning based on latest model consensus, except in the Boise Mountains and West Central Idaho Mountains where amounts actually increased slightly, now 1.00 to 2.25 inch, mostly rain. The greater QPF also increases the danger of rock and mud slides down steep terrain. Persons would be wise to avoid roads like the Banks-Lowman Road and other below steep terrain in Idaho until rain ends Wednesday evening. Most new models also have higher snow levels than our previous forecast, so we lowered total forecast snowfall below 7000 feet through Wednesday morning. Finally, latest models are also windier than before on Wednesday and we increased gusts in south-central ID to near Wind Advisory criteria Wednesday afternoon. Won't issue an advisory with this package but if next model cycle continues the trend we will issue at that time.

LONG TERM...Thursday through Monday...A dry northwest flow will be in place through Friday with the storm track positioned along the US/Canada border. By Saturday winds will weaken and shift to the W-SW as a closed low approaches the CA coast. This wind- shift will inject moisture into the region over the weekend. Though moist, not seeing much dynamic energy or flow aloft to capitalize on the increased moisture. The NBM continues to carry a general 20-40% chance of precipitation for Sat/Sun trending toward drier conditions Monday, though uncertainty remains in the progression of the CA closed low. Temperatures will run at or slightly above normal through the weekend.

BOI WATCHES/WARNINGS/ADVISORIES

ID...None. OR...None.