textproduct: Pendleton

KEY MESSAGES

- Isolated Thunderstorms and scattered showers across the Cascade Crest with breezy conditions in the lower elevations today

- Widespread rain shower chances and isolated thunderstorms across the forecast area Monday into Tuesday

- Breezy lower elevations winds develop Tuesday through Wednesday

DISCUSSION

Today through Sunday: Satellite and radar imagery this afternoon shows an upper low swinging into the PacNW with shower/isolated thunderstorm activity mostly west of the Cascade crest. A few isolated showers have made there way east into portions of the WA Columbia Basin. Otherwise, breezy Cascade gap winds have spread into the lower elevations.

Breezy winds and mostly Cascade crest rain showers will continue through the evening as the upper low tracks across the PacNW. A few isolated thunderstorms will be possible along the Cascade crest and east slopes through this evening thanks to weak CAPE (100-250 J/kg), moderate mid-level lapse rates, and upslope flow. Shower activity will diminish across the Cascade crest tonight as the upper low pushes east of the forecast area with a transient ridge following close behind. However light showers will be possible through the early morning across the Eagle Caps and ridges in eastern Wallowa county. Breezy winds today will become locally breezy tomorrow as a cross-Cascade pressure gradient weakens slightly under the transient ridge.

Mid to high elevation valleys will see near to below freezing temperatures develop Sunday morning. Temperatures below freezing in central Oregon and Wallowa county will be dependent on how quickly a drier airmass on the backside of the low settles into the region tonight, as well as winds becoming light and terrain induced.

Monday through Wednesday: Ensemble cluster guidance starts in great agreement Monday of an upper shortwave trough amplifying over the PacNW as it rounds a closed low in the northeast Pacific. The trough will continue to swing east over the region through Tuesday and push into the northern Rockies by Wednesday morning. Offshore, ensemble guidance is in agreement in an upper level ridge developing sometime Tuesday through Wednesday, but disagreement grows in the positioning and amplitude of the ridge Tuesday night through Wednesday.

Confidence is mod-high (65-80%) in widespread showers developing Monday as the trough moves inland, then shower activity retreating to the mountains Tuesday as the trough axis moves east of the Cascade crest. Isolated to widely scattered weak thunderstorms will also develop in areas underneath and just east of the trough axis Tuesday afternoon into the evening hours. By Wednesday, forecast confidence becomes low-mod(30-50%) in shower activity continuing across the Cascade crest and eastern mountains through Wednesday, owing to the ensemble uncertainty in the evolution of the offshore ridge. Otherwise, there is mod- high confidence (70-80%) in breezy winds (15-25mph with gusts up to 45mph) developing through the Cascade gaps and spreading into the lower elevations Tuesday and Wednesday.

Thursday through Friday: Ensemble guidance remains in disagreement over the evolution of the synoptic pattern through the end of the work week. About 40% of ensemble cluster members favor the upper ridge extending across the PacNW, resulting in drier and warmer conditions developing during this period. Meanwhile, the remaining members favor the PacNW just on the edge of a broad upper level low centered over the Canadian Plains while an upper ridge amplifies poleward offshore. These solutions would all result in light rain showers over the WA Cascade crest and higher terrain over the northern Blues, and weaker warming trend across the forecast area. Confidence in the forecast through this period is low (20-35%). Lawhorn/82

AVIATION /00Z TAFS THROUGH 00Z MONDAY/

VFR conditions prevail with breezy conditions for all sites. Winds will gusts up to 20-30 mph, with mostly dry conditions. No CIG or VIS issues expected.

Preliminary Point Temps/PoPs

PDT 40 70 46 68 / 0 0 20 20 ALW 45 70 50 70 / 0 0 0 10 PSC 42 75 48 73 / 10 0 10 10 YKM 40 74 48 69 / 10 0 10 30 HRI 41 73 47 71 / 0 0 20 20 ELN 36 66 42 64 / 10 0 20 40 RDM 28 69 42 63 / 0 0 20 60 LGD 38 67 42 68 / 10 0 0 30 GCD 36 68 41 67 / 10 0 0 30 DLS 43 72 51 67 / 30 0 20 70

PDT WATCHES/WARNINGS/ADVISORIES

WA...None. OR...Freeze Warning from 11 PM this evening to 8 AM PDT Sunday for ORZ050-511.