textproduct: Pendleton

KEY MESSAGES

- Persistent chances of mountain rain and snow through Thursday, with a Winter Weather Advisory in effect for the Upper East Slopes of the WA Cascades later tonight

- Breezy winds today and Wednesday

- Below normal temperatures through the weekend

DISCUSSION

Water vapor imagery tonight shows the PacNW sitting between a departing upper shortwave trough to our east and a shortwave trough diving down the PacNW coast. A modest upper level jet has developed between the two systems and is facilitating the passage of a shortwave impulse along the WA/OR state border. Per nighttime satellite imagery and radar returns, light to moderate rain showers have developed along the OR/WA border, with showers banking up against the Northern Blues.

Today through Friday: The passage of yesterday's upper low/trough started a prolonged period of the PacNW remaining under the influence of a broad troughing/low pressure pattern that'll persist into the weekend (confidence 85-95%). The approaching shortwave trough offshore the PacNW will bring reinforcing rain/snow chances across most of the eastern mountains into the afternoon. As the shortwave trough pushes into ID late this afternoon, shower chances will diminish across the eastern mountains while rain/snow chances continue across the Cascades and northern Blues.

Later this evening, a second upper closed low will develop over central BC, then remain parked just offshore WA Wednesday afternoon through Friday. The arrival of the upper low to the PacNW will result in a cooler airmass filtering into the region, with snow levels lowering from around 5-5.5kft this morning to 3.5-4.5kft by tomorrow morning. Increased shower activity ahead of the low later this evening and overnight will result in snow accumulations between 3-7 inches above 4kft in the WA Cascades, with accumulations generally 1-3 inches above 5kft in the OR Cascades, Elkhorn Crest, Eagle Caps, and interior northern Blues(confidence 65-85%). A Winter Weather Advisory is currently in effect for the Upper East Slopes of the WA Cascades this evening through tomorrow afternoon.

Light mountain snow/rain showers will continue through Friday as the upper low remains parked offshore. However, any additional accumulations through Friday will be very light, with 48-hr probabilities of snow accumulations exceeding 2 inches only 30-60% along portions of the Cascade crest, with chances closer to 10-20% or near zero over the Blues, Wallowas, and mid slopes of the Cascades.

Otherwise, the parade of upper low pressure systems through the region will result in afternoon temperatures lowering into the 50s by tomorrow, then lower further into the mid 40s to low 50s Thursday/Friday (confidence 65-90%). Breezy winds (15-25mph with gusts to around 35mph) will develop through the Cascade Gaps and lower elevations today with the shortwave passage, then mainly across central OR to the Columbia Basin Wednesday with the approach of the second closed low (confidence 65-80%).

Saturday through Monday: Ensemble guidance is in agreement of the upper closed sliding inland starting Friday night into Saturday, with continued chances of light rain/snow in the mountains through at least Saturday. Overall, there is good agreement amongst ensemble members that by Monday, an upper ridge will slide over the PacNW with widespread dry conditions developing (confidence 45-60%). However, late Saturday through Sunday there is disagreement in how quickly the upper trough moves out of the region, as well as disagreement in the strength of the incoming upper ridge. Confidence in precipitation chances late Saturday into Sunday is low (15-20%), while there is moderate confidence (45-60%) in the forecast late Sunday into early next week. Lawhorn/82

AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/

VFR conditions initial across all sites that will prevail for most of the period. Seeing lower FEW/SCT less than 3000 feet forecasted to roll through most sites with light to moderate shower activity. Can see some MVFR conditions developing (15-30% each TAF site) where the heaviest showers occur. Winds will be generally elevated with all sites seeing prevailing 10-15 knot sustained winds.

Preliminary Point Temps/PoPs

PDT 55 33 52 31 / 40 20 40 20 ALW 55 39 51 34 / 70 40 70 30 PSC 61 37 57 30 / 10 10 30 0 YKM 60 33 55 30 / 0 20 30 0 HRI 59 37 55 29 / 10 0 20 0 ELN 54 28 48 21 / 10 60 60 10 RDM 47 28 47 19 / 40 0 10 0 LGD 50 31 50 27 / 70 50 40 50 GCD 47 29 48 27 / 50 20 0 0 DLS 59 39 55 32 / 30 70 90 40

PDT WATCHES/WARNINGS/ADVISORIES

WA...Winter Weather Advisory from 5 PM this afternoon to 5 PM PDT Wednesday for WAZ522. OR...None.