textproduct: Pendleton

KEY MESSAGES

- Dry conditions through Sunday morning

- Active weather returns late Sunday into Monday

- More widespread precipitation chances Wednesday through Thursday

DISCUSSION

Satellite this afternoon reveals clear skies over the Pacific Northwest aside from scant, wispy cirrus. At the surface, an anomalously warm, dry air mass is in place across central and eastern Oregon. Breezy southeast winds are also present, especially noticeable through the Grande Ronde Valley. Meanwhile, temperatures in the Columbia Basin and vicinity are closer to seasonal averages.

Another chilly night is on tap for cold-prone valleys and basin areas. That said, increasing mid- and high-level clouds associated with a strengthening jet aloft and a modest increase in PWATs, coupled with the aforementioned breezy winds, should hinder radiational cooling more than last night -- at least in spots. Have added a mention of frost to the gridded forecast for locations within the Columbia Basin where temperatures and moisture are supportive.

A return to more active weather is slated for later Sunday into Monday as a shortwave trough and attendant cold front sweep through the Pacific Northwest. Snow levels will drop low enough by later Sunday afternoon through Sunday night to facilitate some light snowfall across the mountain passes, but meager moisture should preclude the need for any winter weather headlines (80 percent confidence).

Current deterministic NWP guidance is advertising an 8- to 12-mb difference in MSLP between PDX and GEG late Sunday, certainly supportive of widespread breezy to windy westerly winds. Will note, however, that the lack of a strong jet aloft (most guidance shows 40 kts or less at 850 mb) should mitigate the wind threat. Delving into chances of reaching advisory-level winds, the NBM shows a 25 percent chance or less of reaching gusts of 45 mph or greater across the wind-prone locations of the Columbia Basin.

While ensemble clusters depict some spread in model solutions, drying conditions as well as chilly overnight lows (sub- freezing in cold-prone locations of the Columbia Basin) are then likely (75 percent confidence) Monday night through Tuesday in the wake of the trough as another very dry air mass moves over the region.

Looking ahead, all ensemble clusters show a return to troughing, with a more robust weather system present in over half of the members Wednesday through Thursday. Chances of precipitation are medium-high (40-70 percent) for the lower elevations, while chances of mountain precipitation are very high (80 percent or higher) on Wednesday. To touch on some uncertainty, ensemble QPF clusters show roughly 43 percent of spread can be explained by differences in QPF magnitude across the Pacific Northwest, while an additional 15 percent are due to differences in the north- south axis of QPF. Keep in mind, these are raw, global ensembles, and individual model bias is present in solutions.

Current probabilities of exceedance from the NBM place medium- high (50-90 percent) chances of 6 inches or more of snowfall in the 48-hr period ending 5 AM Friday for the Oregon Cascades, while the Washington Cascades have medium (40-60 percent) chances, and the northern Blues have a low (10-40 percent) chance. Will note that low-elevation snowfall is possible for central and eastern Oregon, but am hesitant to even place a number on confidence in occurrence at this lead time, so will just say confidence is low in amounts. 86

AVIATION /00Z TAFS THROUGH 00Z SUNDAY/

VFR conditions will prevail for the next 24-hours. Winds will be light and variable at all sites. Some slightly stronger 10-15 knot winds developing at around 18/19z at most of the sites. Wind threat remains well below advisory level. No CIG or VIS issues expected.

Preliminary Point Temps/PoPs

PDT 60 32 64 40 / 0 0 0 0 ALW 60 34 63 42 / 0 0 0 0 PSC 62 35 67 41 / 0 0 0 0 YKM 59 35 64 36 / 0 0 0 0 HRI 62 35 67 41 / 0 0 0 0 ELN 55 28 56 34 / 0 0 0 0 RDM 68 26 66 35 / 0 0 0 0 LGD 62 30 65 37 / 0 0 0 0 GCD 67 33 66 38 / 0 0 0 0 DLS 62 38 64 40 / 0 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES

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