textproduct: Pendleton

KEY MESSAGES

- Above normal temperatures and dry conditions through Wednesday

- Return of showers and isolated thunderstorms to the eastern mountains Thursday, with breezy conditions in the lower elevations Thursday and Friday

- Moderate Confidence (60-70%) in freezing morning temperatures Saturday

DISCUSSION

Satellite imagery this afternoon shows an upper closed low spinning away off the coast of CA while a band of mid to upper level cloud cover reaches up into central and northeast OR.

Today through Wednesday: While an upper low continues to sit off the CA coast today, and upper level ridge will slide across northern WA and BC. The forecast area will be under the influence of this upper ridge, resulting in dry conditions, above normal temperatures, and light winds the rest of today. By tomorrow, the upper ridge will be east of the region with south to southwest flow over the forecast area. This will result in continued dry conditions and light winds. To the south, the upper low will begin to swing into central CA, with precipitation ahead of the low reaching southern OR by Wednesday night.

Thursday and Friday: Later this work week, the PacNW will be impacted by two systems: the upper closed low passing to the south and east and upper trough with attendant cold front sliding across the PacNW.

The upper closed low passage will be the main precipitation maker through this period. Showers will begin to develop across the high terrain south and east of John Day Wednesday night, then through Thursday will spread to the remainder of the eastern mountains, central OR, and the eastern half of the Columbia Basin by Thursday afternoon. Shower chances will dissipate from west to east Thursday evening into Friday morning as the upper low continues east of the region. Thursday, 24-hr chances between 75-95% of 0.25 inches of rain are focused around the northern Blue Mountains, Elkhorn Crest, and Wallowa county; along the Blue Mountain foothills and the remainder of the eastern mountains, chances of 0.1 inches are 45-65%. In the remainder of the areas, there is a 30-50% chance of at least 0.01 inches.

The upper trough and cold front passage late Thursday through Friday will bring wind impacts throughout the forecast area. Winds will begin to increase through the Cascade gaps Thursday afternoon, and continue to strengthen overnight as the cold front drags across the intermountain PacNW. Winds are currently expected (confidence 55-70%) to peak overnight Thursday into Friday morning between 25-35 mph with gusts to around 45 mph through the eastern Gorge and Kittitas valley, while winds will be slightly weaker (20-30mph with gusts up to 40 mph) in portions of the Columbia Basin, Central OR, Cascade east slopes, and eastern mountains (including Grande Ronde valley). Breezy winds will carry on into the afternoon Friday, but become light by the nighttime hours.

Lastly, the upper closed low swinging inland will become an open trough with a negative tilt by Thursday afternoon. Ahead of the trough's axis, SBCAPE between 200-250 J/kg will develop across the eastern mountains. Increased CAPE and the negative tilt will result in chances (15-25%) of isolated thunderstorms developing Thursday afternoon.

Saturday through Monday: After Friday, ensemble guidance is nearly split 50/50 in the evolution of the synoptic pattern. The more impactful of the two solutions involves an upper closed low swinging out of the northeast Pacific and across the PacNW late Saturday through Monday. This would result in a return of lower elevations rain showers while snow levels will be low enough (~5kft) for light mountain snow showers to develop. The other solution would be for upper level ridging developing over the PacNW and for any upper trough/low to be blocked from swinging into the region. Under this scenario, dry conditions would redevelop and persist across the forecast area. Forecast Confidence Saturday afternoon through Monday is low (20-25%).

Otherwise, there is good agreement that the earlier trough/cold front passage will allow for a colder/drier airmass to settle into the area Friday night through Saturday morning. The dry conditions and potentially clear skies will at least help with lowering temperatures to or below freezing in lower elevation locations Saturday morning (confidence 60-70%). Will continue to monitor for need of freeze warnings. Lawhorn/82

AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/

VFR conditions with predominantly light, diurnally and terrain-driven winds are forecast through the period for all sites (95% confidence or higher).

Preliminary Point Temps/PoPs

PDT 32 68 38 62 / 0 0 0 50 ALW 39 68 44 61 / 0 0 0 60 PSC 37 69 37 65 / 0 0 0 30 YKM 39 70 40 69 / 0 0 0 0 HRI 34 68 37 64 / 0 0 0 40 ELN 33 67 38 66 / 0 0 0 0 RDM 28 72 31 63 / 0 0 0 30 LGD 39 74 43 64 / 0 10 10 90 GCD 39 76 43 63 / 0 0 20 90 DLS 40 73 44 70 / 0 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES

WA...None. OR...None.