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 /12Z TAFS THROUGH 12Z THURSDAY/

VFR conditions to prevail through the period. Isolated showers/thunderstorms have already developed in the area ahead of an approaching upper weather system, with prob30 chance that showers will impacts sites PDT/ALW/PSC through this morning, with periodic showers at site YKM. Later this afternoon and evening, widespread rain impacts will develop at all sites, with rain ending around 2Z-3Z at sites YKM/RDM/BDN/DLS, around 6Z for site PSC, and late overnight for sites PDT/ALW. Confidence in MVFR or lower CIGs/vsby developing with precip is low (25-30%). Breezy winds (12-17kts with gusts to around 25kts) will develop at most sites this afternoon and early evening, but will otherwise be light in the morning and overnight. Light winds will prevail at site DLS through the period. Lawhorn/82

Preliminary Point Temps/PoPs

PDT 74 45 68 43 / 50 90 30 30 ALW 76 50 69 49 / 60 90 40 40 PSC 77 47 73 44 / 30 70 10 10 YKM 74 48 69 46 / 30 60 20 10 HRI 76 47 71 44 / 40 80 10 20 ELN 69 43 65 41 / 40 60 30 20 RDM 63 36 63 32 / 90 80 20 50 LGD 72 46 65 43 / 80 100 70 70 GCD 72 45 64 42 / 60 100 70 70 DLS 71 49 67 48 / 90 80 40 40

PDT WATCHES/WARNINGS/ADVISORIES

WA...Air Quality Alert until 10 AM PDT this morning for WAZ026-522- 523. OR...Air Quality Alert until noon PDT today for ORZ041-505-506- 509>511.