textproduct: Pendleton

KEY MESSAGES

- Rain chances diminishing over the WA Cascades while ramping up across the Oregon Cascades and Blue Mountains Tonight and Saturday.

- Mountain headwater rivers to run high or flood through the weekend

DISCUSSION

A weak to moderate atmospheric river continues impinging on the PacNW coastline as well as inland across Washington. Over the next 24 hours this effective cold frontal zone will shift southward and lose the more robust IVT altogether. The very high POPs (90% This Afternoon) across the WA Cascades drop off dramatically (15 to 25 percent by This evening) while the Oregon Cascades begin to get a marked increase in rain probabilities through the evening and overnight. Dew points have become more stable across the central Oregon counties/zone westerly and and probably due to surface winds being lighter and trending westerly and northwesterly allowing some boundary layer moisture through the Cascade gaps. As for the temperatures, this weekends anomalously warm regime is about to come to an end as a cold front associated with the decaying AR influences the entire region through late Saturday morning. Deterministic NBM highs temps for Saturday are running about 20 degrees cooler across all areas of the CWA. Still warmer areas like the Lower Columbia Basin and Eastern River Gorge, as well as Central Oregon and the John Day Basin can see temps warm up through the low 60s. A slow moderation in temperatures shows up in the ensembles through Monday but is only notable across the John Day Basin and Hells Canyon reaching low 70s. Meanwhile the decaying atmospheric river may help drop up to a quarter inch of precipitation (rain) across the eastern Mountains on Saturday.

A second atmospheric river is on the horizon which could have sensible weather implications by Tuesday (day 5) for the Washington Cascades. That moisture flux is located around 155W and 25N right now, and modeled by both GFS and EC to hit the PacNW coast, although the GFS appears much stronger than the EC at this time. Russell/71

AVIATION /18Z TAFS THROUGH 18Z SATURDAY/

VFR conditions are forecast today as a ridge of high pressure remains over the region. CIGs will be mostly 15-25 kft through afternoon, though mountain waves may induce periodic BKN060 to BKN080 decks at DLS/YKM.

This evening through Saturday morning, a cold front will sag southeastward across the region, bringing low (10-30 percent) chances of light rain for DLS/YKM/RDM/BDN/PSC and medium-high (30-70 percent) chances at PDT/ALW. Forecast soundings from the 12Z HREF suggest at least some potential for MVFR CIGs, highest chances at PDT/ALW, but have not advertised in the 18Z TAFs yet due to lack of confidence.

Winds of 10-15 kts with gusts of 15-25 kts are forecast for all sites. 86

FIRE WEATHER

A third day with seemingly very similar fire weather risk across central Oregon, with some caveats. Overall light downsloping winds in the single digits to around 15 mph will be common for the balance of the afternoon, however gusts will commonly be in the 20 to as much as 30 mph range. The only caveat is the wind direction and subtle boundary layer moisture that may be coming through the Cascade Gaps. That said the fire weather risk will be Limited this afternoon. Russell/71

HYDROLOGY

The Cascade Slope basins have received the bulk of the precipitation that will be impacting river forecasts over the weekend. All tributaries with mountain headwaters will continue to run high. Forecast points along the Naches and Yakima Rivers will be the most impacted by the surge of mountain runoff as several of these points enter into flood by Saturday, if not already in flood like the Naches at Naches. The Yakima River at Easton is forecast to reach just over the 52.1 ft level, (about 1 ft over moderate flood) but has a 30% chance of reaching 52ft. At Kiona, the river is forecast to reach the 13.9 ft level, but has a 10% chance of reaching 14 ft. Russell/71

Preliminary Point Temps/PoPs

PDT 45 57 30 57 / 50 60 0 0 ALW 47 57 34 56 / 70 70 0 0 PSC 47 62 33 60 / 40 20 0 0 YKM 39 58 29 56 / 10 0 0 0 HRI 48 61 31 59 / 40 30 0 0 ELN 37 49 28 52 / 20 0 0 0 RDM 41 58 23 61 / 20 10 0 0 LGD 45 55 28 58 / 40 80 0 0 GCD 45 57 28 60 / 10 60 0 0 DLS 45 59 33 61 / 30 10 0 0

PDT WATCHES/WARNINGS/ADVISORIES

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