textproduct: Pendleton

DISCUSSION

Radar shows light showers across the Washington Basin and across the Washington Cascade crest, that will continue to develop through the evening hours today. Otherwise, mid to upper level clouds roam the region, while temperatures continue in the low to mid 60s through the lower populated regions. Expecting things to continue to be dry for much of the region as a ridge moves in the area, limiting much of the moisture from the current PacNW atmospheric river. Hydro concerns continue for parts of the Yakima and Naches rivers, as rains from the system brought elevated river levels across the Yakima/Kittitas Valley through the Washington Basin. Most rivers have already crested and will improve going through the weekend. The lone exception of this will be the Yakima River at Kiona, which is expected to reach 'Moderate' flooding Friday through Saturday, then drop after that. There is small concern (20 to 50 percent chance) that we will see a "second increase" in river levels due to an oncoming system next week (will get to later in the discussion), but some hydro forecasts show some rivers in Naches will remain 'Bank Full' going through next week, while Yakima shows levels going below action this weekend, but rise close to 'Action' stage sometime mid next week. Great certainty can't be placed on this forecast due to some uncertainties with next week system, but additional monitoring may be needed for a possible second flooding event next week.

As said earlier, our next system will enter the region sometime next week, bringing in a colder air mass compared to the system we've seen this week. As the ridge that sets over us this weekend begins to propagate, upper level winds will begin to shift towards a southwest/south-southwest motion, allowing for some moisture build-up and bring in our next rain/snow chances through next week. Agreement between the Euro and GFS ensembles show a trough moving over the PacNW region Monday into Tuesday, developing shortwaves and instability across the area. A secondary atmospheric river is likely into next week with the ECMWF EPS showing 90+% chances of IVT >250 kg/m/s and 50 to 70% chances of >500 kg/m/s Monday and Tuesday. Snow levels will drop from the current level of 7500 to 8500 feet, down to 3500 to 4500 feet by Tuesday afternoon (50 to 70 percent chance). This will periodically bring in moderate to heavy mountain snow (50 to 70 percent chance). NBM advertises six-hour intervals of 3 to 5 inches of snow for Snoqualmie Pass.

There's disagreement with clusters going into Tuesday onwards, with models struggling to capture the upcoming troughs strength, which will be a factor of how much snow will develop. About 45% of members suggest a solution of a stronger trough with heavier snow potential (especially in the Washington Cascades and Eastern Slopes) with more widespread coverage through Central Oregon all the way through the Eastern Mountains. Regardless, it is looking like we will be in a cooler, wetter pattern going into next work week with multiple days of light to moderate valley rain and mountain snow (>90 percent chance).

AVIATION...06Z TAFS...VFR conditions are forecast through the period, with very low (10 percent or less) chances of rain this evening as an upper-level ridge gradually pushes into the region through Friday. With mostly light winds under the ridge, there is a low (30 percent) chance of some fog/mist and low stratus developing overnight into Friday morning at DLS/YKM/PSC, and again Friday evening. Chances of fog and stratus increase for all sites heading into the weekend, but the most prone will be the Basin sites.

PRELIMINARY POINT TEMPS/POPS

PDT 42 59 37 57 / 10 0 0 0 ALW 48 59 42 56 / 20 0 0 0 PSC 45 59 38 52 / 10 0 0 0 YKM 42 58 37 55 / 20 0 0 0 HRI 43 59 37 53 / 10 0 0 0 ELN 41 54 36 51 / 40 10 0 0 RDM 30 61 29 61 / 0 0 0 0 LGD 40 56 36 56 / 20 0 0 0 GCD 35 57 35 59 / 0 0 0 0 DLS 46 59 40 56 / 20 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES

OR...None. WA...Flood Watch until 7 AM PST Friday for WAZ026-027-521>523.