textproduct: Pendleton

DISCUSSION

Satellite imagery tonight shows an upper low to the south pushing mid to high level cloud decks north into central and northeast OR. Meanwhile, low stratus decks have also developed from La Pine to Madras in central OR.

Today and Thursday: An upper low setup over Northern CA will spin a weak plume of moisture and shortwave impulses into southern OR today, resulting in a chance (15-30%) of a dusting of snow across the higher terrain of southern Deschutes/Crook/Grant counties. Upper level ridging is then expected to build back over the forecast area late this evening through Thursday with dry conditions prevailing.

Friday through Tuesday: There is growing confidence that Friday will mark the beginning of a period of active weather across the the forecast area that will persist into early next week. By Friday afternoon, ensemble cluster solutions show a trough dropping out of the Gulf of Alaska with the main low circulation setting up offshore the PacNW. This trough will continue it's journey south and setup offshore central CA by late Sunday afternoon, with the trough becoming a closed low by late Sunday afternoon (confidence 45-70%). There is growing agreement during this period that with the trough remaining offshore and weak moisture support, mountain and lower elevation precipitation remain fairly light. To provide an idea of how much snow the mountains may see, the 48-hr probability of 6 inches or more is 25-45% across the Cascade crest, Blues, Eagle Caps, and Elkhorn Crest through Sunday afternoon. Meanwhile, the lower elevations will see mostly rain through Saturday, by cold air advection behind into the region Saturday night into Sunday will bring snow levels closer to the surface over central OR and the Yakima/Kittitas valleys, allowing for a rain/snow mix to develop in these areas (confidence 40-55%).

Sunday night through Tuesday, ensemble guidance is in good agreement of a second upper trough diving south out of the Gulf of Alaska and setting up between 40N and 50N. However, disagreement arises between the ensemble members on the position of the upper trough and the amount of precipitation the region will see. While the general trend of cooling temperatures and at least light snow in the lowlands is consistent amongst the differing cluster solutions, about half of the ensemble members favor moderate snow accumulations in the northern Blues and the OR Cascades, while the other half favor lighter snow accumulations in the mountains. Meanwhile, 48-hr probabilities of snow accumulations greater than 6 inches from the NBM show a 35-50% chance in the Oregon Cascades and northern Blues, but chances drop to 15-25% in the WA Cascades. Confidence is moderate (35-45%) in widespread light snow or rain/snow mix developing Monday night across the lower elevations. Lawhorn/82

AVIATION /06Z TAFS THROUGH 06Z THURSDAY/

VFR conditions are expected at most TAF sites through the period. The exceptions will be BDN, RDM and ALW, where there could be some IFR CIGS in the morning. There is also a 30-40 percent chance at DLS, but at this point not forecasting it outright in the TAF, just have some sct clouds. Winds will be light.

Preliminary Point Temps/PoPs

PDT 29 48 31 49 / 0 0 0 0 ALW 32 48 34 49 / 0 0 0 0 PSC 30 49 32 49 / 0 0 0 0 YKM 29 47 29 44 / 0 0 0 0 HRI 29 49 30 49 / 0 0 0 0 ELN 28 45 30 42 / 0 0 0 10 RDM 24 51 24 49 / 0 0 0 0 LGD 28 51 27 49 / 0 0 0 0 GCD 28 49 28 50 / 0 0 0 0 DLS 33 51 35 49 / 0 0 0 10

PDT WATCHES/WARNINGS/ADVISORIES

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