textproduct: Pendleton

KEY MESSAGES

- Snow showers today into Wednesday.

- Potential for additional snowfall Thursday onward.

- Drier late week with potential return of winter weather by the weekend.

DISCUSSION

Satellite reveals a broad closed low churning just offshore, with radar showing showers enveloping a good portion of the west side of the Cascades of Oregon and Washington. Some of these showers are spilling over to the lee of the Cascades into central and north-central Oregon. Showers will continue to spread east through the day, driven by the combination of sufficient moisture, steep lapse rates, and synoptic-scale ascent associated with the aforementioned low. Cold air associated with this low will keep snow levels low enough to facilitate low-elevation snowfall, albeit the showery nature means it will be difficult to forecast precise totals; some areas will receive snow and others will not. On that note, some NWP guidance continues to indicate a mesoscale low will develop later this afternoon through the evening, inducing a mix of rain and snow showers for the lower Columbia Basin and Blue Mountain foothills. NBM probabilities of measurable snowfall for these areas range from 20 to 70 percent, highest along the foothills. Winter Weather Advisories remain in effect for the snow showers in the Oregon Cascades and northern Blue Mountains.

By Wednesday, ensemble clusters show the closed low transforming into an open wave as it treks inland, though another shortwave with origins in the Gulf of Alaska is expected (95 percent confidence) to be hot on its heels for Wednesday night and Thursday. Ensemble cluster analysis shows a range of scenarios for this second wave, with the most notable solution showing advisory-level snow potential for central Oregon and other lowland areas. Confidence in the need for advisories is currently low (30 percent or less).

The active weather pattern will likely (80 percent confidence) taper off by Thursday night with mostly dry conditions then forecast into the weekend as an upper-level ridge builds over the Rockies and a large, very deep closed low develops offshore. By later in the weekend and early next week, this low may drive an atmospheric river, heavy snow, and windy conditions. Currently, of the previously mentioned hazards, confidence is highest in breezy to windy southerly winds in wind-prone locations of eastern Oregon and southeast Washington Saturday and Sunday. Specifically, the most likely areas to receive advisory-level winds are the Grande Ronde Valley and slopes/base of the northern Blue Mountains. 86

AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/

A low pressure system impacting the area today will bring scattered snow showers through tonight. Sites impacted by snow showers will see reduction in vsby and CIGs to at least MVFR conditions, with moderate confidence (60-70%) of IFR or lower conditions developing with the snow. Sites RDM/BDN will see a prob30 chance of snow showers through the mid afternoon, while a surface low feature is expected to bring rain, then snow shower impacts to site PDT/ALW later this evening through tonight. A prob30 chance of snow showers have also been included at site PSC for the overnight period, though confidence is overall low (20-30%) in snow showers developing at this site. Otherwise, VFR conditions will prevail through remainder of the period. Winds will be light, less than 12kts, through the period. Lawhorn/82

Preliminary Point Temps/PoPs

PDT 27 40 23 37 / 50 40 20 40 ALW 29 40 25 38 / 70 60 20 40 PSC 29 46 25 42 / 30 20 10 20 YKM 24 42 25 39 / 10 10 20 20 HRI 28 45 25 40 / 40 20 20 20 ELN 22 38 22 35 / 10 20 30 30 RDM 17 36 20 34 / 10 10 30 40 LGD 24 36 20 36 / 80 80 30 60 GCD 22 34 20 36 / 60 60 40 70 DLS 29 44 30 41 / 20 30 50 50

PDT WATCHES/WARNINGS/ADVISORIES

WA...Winter Weather Advisory until 3 PM PST Wednesday for WAZ030. OR...Winter Weather Advisory until 3 PM PST Wednesday for ORZ502. Winter Weather Advisory until 10 PM PST this evening for ORZ509.