textproduct: Pendleton

KEY MESSAGES

- Widespread shower chances today through early Thursday, with periods of mountain rain showers

- Isolated thunderstorm chances (15-25%) will develop over northeast OR today, then across Cascades and Blues Thursday and Friday

- Below normal temperatures through early next week, with freezing temperatures in mountain valleys Thursday and Friday nights

DISCUSSION

Water vapor satellite imagery tonight shows an upper shortwave trough across the PacNW and northern CA, with the trough beginning to pivot northeast. Per area radar, isolated showers/thunderstorms have been developing along the base of the Cascade east slopes, with this activity mostly in WA.

Today through this weekend, the PacNW will be under the influence of upper level low pressure that will bring a couple rounds of rain showers to the lower elevations, with more persistent chances across the mountains (confidence 70-90%). Temperatures across the region will be nearly 10-15 degrees below average each day, with afternoon high temperatures at their lowest in the mid 60s and low 70s Thursday and Friday (confidence 70-85%). Starting with today, a negatively tilted upper shortwave trough will lift northeast across the region followed by the arrival of an upper closed low offshore the PacNW. These systems will produce the best chances of precipitation east of the Cascades, with the main rain chances spreading northeast across the forecast this afternoon through the evening. Prior to that, the shortwave passage will result in isolated thunderstorm chances across the northern Blues and Wallowa county this afternoon. Through tonight, there is a 50-90% chance of 0.1 inches of precip in the lower elevations, with the highest chances along the Blue mountains foothills and central OR. As for the mountain zones, there is generally a 50-80% chance of 0.5 inches of rain falling in the next 24 hours, with those chances near 100% along the OR Cascade crest.

Thursday through the weekend, there is good consensus amongst ensemble members that the upper closed low will park itself over the PacNW, with the center of the low remaining west of the Cascade crest through this period (confidence 60-85%). During this time, persistent rain shower chances (30-60%) will remain over the Cascades and eastern mountains each day, with shortwave impulses Friday and the low passage Sunday/Monday producing light rain chances (15-40%) in the lower elevations. Isolated thunderstorms will be possible over the mountains Thursday and Friday as weak instability develops in these areas with additional support from the cold core of the low aloft. As for precipitation amounts, the Cascades and Blues will see 0.1 to 0.25 inches of daily precip (confidence 65-90%), with locally higher amounts where thunderstorms develop.

Sunday into Monday, ensemble and deterministic members show the upper closed low beginning to push east, with the low over the forecast area during the day Sunday. While light to moderate rain showers are forecast across the area, the upper low being overhead will also result in slight chances (15-25%) of isolated thunderstorms area-wide.

Monday and Tuesday, ensemble members are in agreement that the PacNW will remain under upper level troughing, however disagree on the placement of the upper trough axis. Members that favor the trough axis to the east of the region keep the forecast area mainly dry, while members depicting a trough axis overhead or west of the Cascade crest results in light precipitation over the area. At this time, confidence is low (20-30%) in forecast details early next week. Lawhorn/82

AVIATION /18Z TAFS THROUGH 18Z THURSDAY/

Regional radar shows rain to the northeast and west of the area resulting from an upper trough moving inland. RDM and BDN should already be seeing -SHRA and VFR to MVFR ceilings Today while thunder risk remains more probable over the higher terrain/mountain areas. -SHRA and MVFR conditions should spread eastward over the valley terminals and into the Columbia Basin PDT/ALW/PSC as well Tonight as the system evolves eastward.

Preliminary Point Temps/PoPs

PDT 74 45 68 42 / 40 90 20 30 ALW 76 49 68 47 / 40 100 30 40 PSC 77 47 72 46 / 20 80 0 10 YKM 74 47 70 48 / 40 40 10 10 HRI 76 47 71 46 / 40 90 0 20 ELN 69 43 66 42 / 50 60 30 20 RDM 63 37 64 35 / 90 80 20 50 LGD 72 46 65 42 / 60 90 70 80 GCD 72 44 63 41 / 60 100 70 80 DLS 71 50 69 50 / 90 80 30 30

PDT WATCHES/WARNINGS/ADVISORIES

WA...None. OR...Air Quality Alert until noon PDT today for ORZ041-505-506- 509>511.