textproduct: Pendleton

KEY MESSAGES

- Active weather pattern will return this evening with mostly rain across the region, and light high mountain snow developing by Sunday

- Breezy to locally gusty southerly winds across portions of central and northeast OR will continue through the weekend

- A train of upper low pressure systems will bring progressively cooler conditions across the forecast area, with light to locally moderate mountain snow accumulations, and potentially a dusting in portions of central OR

DISCUSSION

Water vapor satellite imagery this afternoon shows an upper level low and surface cold/occluded front approaching the PacNW from the northeast Pacific. Meanwhile, there is increasing cloud cover along the Cascade crest into central OR, with partly cloudy conditions across the remainder of the forecast area. Area-wide radar imagery shows very light showery activity staying west of the Cascade crest at this time.

Today: A transient upper level ridge is sliding across the region this afternoon, and is responsible for the dry conditions with mostly partly cloudy skies across the forecast area. By tonight, the transient ridge will push to the east as the upper low and attendant fronts approach closer to the PacNW and BC. Surface pressure gradients across the Blue mountains are expected to strengthen in response, producing breezy winds along the Base of the Blue mountains and through the Grande Ronde valley. In fact, southerly winds between 25-35 mph with gusts 40-50mph are expected to develop later this evening and persist overnight in the Grande Ronde valley when the pressure gradient is at it's peak.

Sunday through Thursday: Tomorrow, the upper low in the northeast Pacific will swing the cold/occluded front across the region, with a broad upper level trough following Monday through Tuesday. The incoming frontal boundary will push a band of precipitation across the region throughout Sunday, with the frontal boundary stalling across far eastern OR/WA Sunday night into Monday morning. Monday, the upper trough will drop into the PacNW, pushing whats left of the frontal boundary into the northern Rockies, with precipitation mainly limited to the Cascades, Blues, Elkhorn crest, and the high terrain across Wallowa County. During the peak precipitation on Sunday, snow levels will mainly be above 6kft in the mountains, resulting in any snow accumulations being limited to the Cascade peaks, with a rain/snow mix down to around 5kft. Behind the frontal boundary, snow levels will drop to around 4kft, resulting in light snow developing along the Cascades Sunday evening through Monday morning, with rain transitioning into snow along the eastern mountains late Monday morning into the late afternoon. In addition to lowering snow levels, temperatures will cool region-wide through Monday, with mod-high confidence(65-80%) in afternoon high temperatures in the mid 40s to low 50s.

Tuesday through Thursday, ensemble cluster members are in good agreement that a transient ridge will slide into the PacNW Tuesday with dry conditions. Tuesday night through Thursday, ensemble guidance now has an upper low swinging into and across southern OR and northern CA, with disagreement amongst ensemble solutions in the strength of the incoming low.

For Tuesday, while the ridge is overhead conditions will remain dry, however temperatures will remain cool as a cold airmass remains over the region. By late Tuesday night, the next round of precipitation will arrive to the Cascades, then lift northeast across the region as the upper low pushes inland. There is moderate confidence (50-60%) that snow levels will lower down to around 2.5-3.5kft Tuesday night and Wednesday morning, then increasing to around 3-4kft by Wednesday afternoon and evening. This will result in a light rain/snow mix in some lower elevation locations (confidence 35-50%) Wednesday morning, while there is moderate confidence (35-55%) in 2 to 3 inches of snow accumulations across the Cascades and eastern mountains through Wednesday night. Late Wednesday into Thursday, forecast confidence is low (15-20%) owing to positioning disagreements of the upper low as it continues to move inland as well as 24 hour run to run differences amongst ensemble solutions. From the most recent ensemble run, ensemble cluster solutions range from light mountain snow continuing through Thursday, light mountain snow and lower elevation rain continuing through Thursday, or light rain and mountain snow ending Thursday morning. Lawhorn/82

AVIATION /06Z TAFS THROUGH 06Z MONDAY/

Predominantly VFR conditions for all TAF sites until 06z on the 16th. Low level wind shear present for KPDT(6z - 12z), and KALW(6z- 12z) at 2000ft with southerly winds at 40kt. KBDN(6z-17z) at 2000ft with south southwesterly winds at 40kt. Light rain and rain showers for TAF sites as frontal precipitation band move eastward. Periods of MVFR conditions related to low cloud bases present, with highest confidence at KPDT and KALW closer to the end of the TAF period.

Preliminary Point Temps/PoPs

PDT 42 55 37 48 / 30 80 90 10 ALW 45 52 41 47 / 10 80 90 30 PSC 40 57 36 52 / 50 50 50 0 YKM 37 54 31 51 / 70 20 0 0 HRI 41 56 36 50 / 50 70 60 0 ELN 33 48 26 45 / 70 20 10 0 RDM 35 51 25 47 / 20 30 10 0 LGD 37 47 35 46 / 0 90 90 70 GCD 36 50 35 47 / 0 90 90 50 DLS 42 52 37 49 / 70 60 30 20

PDT WATCHES/WARNINGS/ADVISORIES

WA...None. OR...Wind Advisory until 7 AM PST Sunday for ORZ049-507.