textproduct: Boise

DISCUSSION

No change to current forecast of cloudy, dry, breezy, mild wx today, then rain spreading north to south tonight, then becoming moderate to heavy Tuesday with snow level continuing to slowly rise. Heaviest pcpn targets the Boise Mountains where totals of 1.50 to 2.25 inches are forecast late tonight through Wednesday morning. That much rain may bring mud and rocks down steep slopes and we advise avoiding roads below steep slopes, such as the Banks- Lowman Road, until pcpn ends on Wednesday. With the main event now in the short term, hi-res guidance allows more precise timing of the onset of pcpn. For Ontario/OR and Boise: around 2 AM MST Tuesday, Mountain Home: 4-5 AM MST, Jerome and Twin Falls: 6 AM MST. Latest models show Wednesday becoming windy in eastern ID but generally sparing western ID and OR. The western Magic Valley should be the windiest area in our CWA, currently estimating westerly 20-30 mph with gusts 40 mph.

AVIATION

VFR today. Increasing and lowering ceilings this evening across SE Oregon and mtns of SW Idaho. Rain and snow developing after 00Z this evening and continuing through Tuesday with low-VFR and local MVFR in valleys. IFR and snow in mtns will obscure terrain. Snow levels of 5-7kft overnight will rise to 6500-8500 ft on Tuesday. Surface winds: SW-SE 5-15 kt. LLWS across portions of SE Oregon through 18Z. Winds aloft at 10kft MSL: SW 25-35 kt.

KBOI...VFR with steady SE winds. Low-VFR ceilings and rain developing after 06Z this evening.

PREV DISCUSSION

SHORT TERM...Today through Wednesday night...Mild southwest flow will persist today ahead of a trough off the coast. It will be cloudy but mostly dry today, with light precipitation developing across the northern mountains this afternoon. Snow levels today will be 5-6kft MSL. Highs today will be a couple degrees cooler than yesterday due to increased cloud cover, but will still be 5-10 degrees above normal. Winds out of the south will be breezy ahead of the system, especially across higher terrain where gusts 20-35 mph will occur.

A much wetter pattern is still on track starting late tonight and continuing through late Tuesday night or Wednesday morning. Widespread rain and snow will develop across the area as a deep subtropical plume of moisture, currently located off the coast of California, is advected into our area under strong westerly flow aloft. To put it in context, precipitable water values are forecast to increase to the 95th to 99th percentile for this time of year, or near record highs. Precipitation amounts continue to trend higher, with 0.25-0.75" rain across most lower elevation sites (locally up to 1" near the foothills where westerly flow will enhance precipitation amounts). Most mountain locations will see 1-2" of rain or liquid equivalent, aided by upslope flow. Warmer air aloft will accompany this moisture, resulting in rising snow levels. Snow levels tonight of 5-6kft north and 6-7kft south will rise through the day Tuesday to 6500-7500 ft north and 7500-8500 feet by Tuesday evening. This will limit heavy snow to elevations above 6-7kft, where 8-16 inches will be common and locally up to 2 feet. Higher mountain valleys, such as McCall, may see light snow accumulations Tuesday morning before turning to rain. Widespread precipitation will cool high temperatures by around 5 degrees from today, and winds will remain gusty in the higher terrain.

Precipitation will decrease late Tuesday night into Wednesday as the moisture plume sags south and a weak cold front moves in from the north. Snow levels will fall to 3500-4500 feet as precipitation ends, with a dusting of snow in mountain valleys possible. Temperatures will be similar to or a couple degrees cooler than Tuesday behind the front. However, the biggest story for Wednesday will be the gusty winds. The strongest winds are expected across south-central Idaho, including the Magic Valley where gusts of 35-45 mph will develop.

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 with precipitable water values reaching the 90th percentile by Saturday night. Though moist, not seeing much dynamic energy or flow aloft to capitalize on the increased moisture. The ensemble mean (NBM) continues to carry a general 20-30% chance of precipitation for Sat/Sun, with deterministic solutions and the WPC QPF forecast focusing development of light precipitation across our southern zones along the NV border. The NBM trends 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.