textproduct: Pendleton

DISCUSSION

The first in a series of upper shortwaves have arrived to the region tonight, with satellite showing mid to upper level cloud cover spreading across WA and norther OR, while radar shows light showers along the WA Cascade crest, with light spill over along the east slopes and adjacent valleys. Otherwise, surface observations show persistent breezy winds along the Blue mountain foothills and through the southern Grande valley, with patchy dense freezing fog impacting the Kittitas valley.

Today through Monday: The PacNW will be situated under a west southwest flow aloft through Monday as a series of shortwave passages to the north flatten upper level ridging over the western CONUS. The passing shortwaves will continue to clip the WA Cascades with light showers, however, snow levels rising to well above 6kft before this afternoon will greatly limit any snow concerns for the passes. Otherwise, the remainder of the forecast area will continue to see no precipitation.

Breezy winds through the Grande Ronde valley and along the Blue mountain foothills will continue throughout today and Monday as a strong pressure gradient persists between the Columbia Basin and the Lower Treasure valley. Strongest winds with gusts 35-50mph will continue to be possible, especially at the exits of the Ladd and Pyles Canyons and between 2.5kft to 3.5kft along the Blue mountain foothills. Lastly, a strengthened temperature inversion has resulted in persistent freezing fog impacting the Kittitas valley tonight, and with a lack of strong winds developing, this area of fog will only persist throughout the remainder of this morning, with slight lifting into the afternoon. By late this evening, would expect the freezing fog to descend and expand across the Kittitas valley and persist through at least Monday morning.

Tuesday through Saturday: The upper level ridge over the western CONUS will amplify poleward Monday night through Tuesday, bringing a return to area-wide dry and stagnant conditions. The ridge will briefly flatten again Wednesday as a shortwave slides into BC to the north, but precipitation will remain well north of the PacNW. Ensemble guidance is in agreement that the ridge axis will setup offshore and remain there through the weekend, placing the eastern half of WA/OR under a northerly flow aloft. A continental airmass will advect into the region under the northerly flow aloft, resulting in a cooling trend across the northern two-thirds of the forecast area. In addition, low level inversions across the Columbia Basin and adjacent valleys will strengthen under the stable conditions and dry air aloft, promoting the development of widespread low level stratus and dense fog/freezing fog into next weekend (confidence 45-65%). Lawhorn/82

AVIATION...12Z TAFS

VFR conditions currently at all sites, which will stay the course through the period. There is a slight chance (15-25%) that KPSC may experience reduced visibilities due to fog this morning and KYKM Sunday evening, but confidence is currently too low to include in tafs. Otherwise, winds will stay light and below 8 kts with ceilings dropping from 25kft to 10kft toward the end of the period. 75

PRELIMINARY POINT TEMPS/POPS

PDT 51 34 56 34 / 0 10 0 0 ALW 49 38 55 38 / 0 10 0 0 PSC 47 35 53 33 / 10 0 0 0 YKM 45 34 52 35 / 10 10 10 0 HRI 47 34 53 33 / 0 0 0 0 ELN 41 33 48 36 / 20 30 30 0 RDM 51 28 54 28 / 0 0 0 0 LGD 43 31 48 33 / 0 0 0 0 GCD 48 29 52 32 / 0 0 0 0 DLS 49 37 51 36 / 0 10 0 0

PDT WATCHES/WARNINGS/ADVISORIES

OR...None. WA...Freezing Fog Advisory until noon PST today for WAZ026.