textproduct: Boise

DISCUSSION

Clear skies and lighter winds tonight will allow many lower valley sites to reach freezing, and the Freeze Warning remains in effect for the Treasure and Magic Valleys. However, light southeast winds will develop overnight, which may prevent some areas that tend to see the most effect from these winds, such as Boise, from seeing a widespread freeze. That said, sheltered areas will still see temperatures dip to near freezing. Forecast remains on track and no updates this evening.

AVIATION

VFR. Patchy fog Tue morning. Surface winds: variable less than 10 kt. Winds aloft at 10kft MSL: NW to W 10-20 kt.

KBOI...VFR and clear. Surface winds: SE up to 10 kt overnight, then variable 3-6 kt by Tue afternoon.

PREV DISCUSSION

SHORT TERM...Tonight through Wednesday night...A quiet and dry weather pattern in the short term as we remain under a weak amplitude upper ridge. Enough areas see freezing temperatures in the Snake Plain tonight to warrant upgrading the Freeze Watch to a Warning. That said there will be areas where a southeast wind is enough to hold temperatures above freezing. From there temperatures will moderate Tuesday and Wednesday with the stable air mass favoring more notable warming at higher elevations through Wednesday.

LONG TERM...Thursday through Monday...A mix of zonal flow/weak ridging and above normal temperatures (5-10 degrees above normal) will be ongoing Thursday though midday Friday. This period of quiet weather will be abruptly broken by a potent atmospheric river event beginning late Friday and lasting until mid-Monday. An intense and expansive upper-level trough off the Gulf of Alaska will be drawing in tropical moisture into the Pacific Northwest. This could bring the first significant snowfall for the mountains this fall.

Strong southwesterly flow aloft will quickly overspread the region beginning for SE OR 18z/Fri and SW ID 00z/Sat. This fast flow aloft will help carry precipitable water values to above 0.5", which according to SPC climatology, is above the 75th percentile. The intensity of the pressure gradient (and associated cold front) will create strong wind gusts at the surface, with values of 30+ mph being seen along our southern CWA border. With good moisture values and lifting mechanisms (cold front and favorable flow aloft against topography), confidence in widespread precipitation is high (60% chance and higher) beginning late Friday and lasting through the weekend. QPF totals from 00z/Sat to 12z/Mon have greater than 0.5" for the lower elevation valleys and greater than 1" for the higher elevation mountain valleys.

Snow levels begin this event near 8000 ft MSL, dropping to 5500-7000 ft MSL after the cold fronts initial passage, then hovering around 4000 ft MSL after continued northwesterly flow aloft as the upper-level trough moves east. Highest chances for mountain snow will come late Saturday into Sunday when the snow levels reach more favorable heights, along with another surge of moisture following an upper-level shortwave embedded in the flow. Accumulating snow will be possible down to 5000 feet with up to a foot of snow in total above 7000 feet MSL. Temperatures following this cold frontal passage will be 5 to 10 degrees below normal.

Midday Monday to late Monday, the ECMWF/Canadian and the GFS disagree on another batch of moisture following behind the upper-level trough moving east out of the area. This is leading to lower, but still mentionable, widespread precipitation chances (30%+). What is agreed upon is northwesterly flow aloft, which will keep us cool to end Monday.

BOI WATCHES/WARNINGS/ADVISORIES

ID...Freeze Warning from 5 AM to 10 AM MDT Tuesday IDZ012-014-016. OR...Freeze Warning from 5 AM MDT /4 AM PDT/ to 10 AM MDT /9 AM PDT/ Tuesday ORZ064.