textproduct: Pendleton

DISCUSSION

Global models remain in agreement that the PacNW will find itself locked into a persistent, strong WNW flow pattern aloft as a result of blocked high pressure centered off the California coast. This pattern will channel a potent atmospheric river right into our region, with the WA Cascades in particular bearing the brunt of this oncoming moisture. The latest QPF forecast from WPC suggests that portions of the central WA Cascade crests may see upwards to 10 inches of liquid precip by the end of the week. While this pattern more or less spares the mountain passes of any significant snow threat, as well as the lowlands of heavy precip as 1) the main band of IVT falls along the northern periphery of our CWA, save for Tuesday when it briefly sags down to the Columbia Basin, and 2) the mid-level winds remain westerly enough to support persistent rain shadowing across the Columbia Basin and central Oregon, all that rain expected to fall across the mountains will run down into the valleys, spurring a flooding threat across primarily the east slopes of the WA Cascades.

Multiple river gauges along the Klickitat, Naches, and Yakima Rivers are forecasted to go into minor flood stage sometime around the midweek, with Wednesday into Thursday being the most likely period. Even some gauges along the foothills of the northern Blues may inch up toward minor flood stage as the Blues get hammered with rain around the same time. The RFC's forecast has wobbled over the past few days, as uncertainty in QPF amounts stems primarily from the positioning of the strongest IVT axis over the PacNW, but both the GFS and ECMWF are in good agreement in depicting IVTs in the 600-800 kg/m/s range slamming the WA Cascades. The one saving grace from this otherwise strong AR event is that it's still relatively early in the winter, so mountain snow packs are on the lower end (SWE below 50% across all basins), precluding any rain on snow excessive runoff threat. Still, given the threat, opted to issue a Flood Watch for our zones at and along the WA Cascades, in order to better advertise the flooding threat over the Hydrologic Outlook that had been in place over the last couple of days.

With the strong upper-level winds accompanying this AR, expect to see surface winds ramping up later today, especially during the late afternoon / early evening hours. Wind Advisories are currently in effect for our Columbia Basin zones, as well as the foothills of the Blue Mountains, Columbia River Gorge, and North Central Oregon starting at 1 PM at lasting until sunrise Tuesday. Didn't see too much across guidance to suggest the need to deviate from ongoing messaging, so the forecast continues to call for wind gusts in the 35-50 mph range across the advised areas, with the ridgetops along the Simcoe Highlands seeing the biggest threat for gusts to exceed 60 mph. HREF suggests probabilities are too low (>5%) to warrant an upgrade to a High Wind Warning, and will opt to keep the Advisory in place for the Simcoe Highlands given that the populated areas within the zone are not anticipated to receive Warning-level winds. Winds abate during the day Tuesday, before picking back up again in the late evening hours, but as of now, the forecasted wind gusts look to remain below Advisory thresholds, save for along exposed ridgetops.

The precip threat finally starts to relent by Thursday as the atmospheric river pattern shifts to our north and east in favor of oncoming high pressure. This high pressure ridge pattern will be 'dirty' enough in the sense that models continue to suggest low-end PoPs across the WA Cascades, but the bulk of the rain threat is expected to end by early Thursday at the latest. Still, expect elevated river levels to persist even into the weekend, especially if the weather pattern continues to support even light shower activity over the Cascades. 74

AVIATION...12Z TAFs

Primary concern through the forecast period will be the winds. All TAF sites will be VFR through the period with VSBY at P6SM and CIGs over 5kft. PDT/ALW/PSC are already seeing winds 10 kts and above while the remaining sites are below 6kts. Timing of the winds will prove to be the challenge through the period. However, model guidance shows the winds to begin to increase between 14-19Z with DLS coming in a bit later after 00Z or later. With that said, winds are expected to be 15-25 kts sustained with gusts to 35 kts with some locally higher winds across PDT/ALW/PSC with sustained wind reaching 30 kts (70-90%). 90

PRELIMINARY POINT TEMPS/POPS

PDT 60 47 56 47 / 70 70 90 90 ALW 59 49 55 47 / 90 80 90 100 PSC 60 49 56 45 / 60 40 80 80 YKM 54 41 51 40 / 80 80 80 90 HRI 62 49 58 47 / 60 50 90 90 ELN 49 37 45 37 / 90 90 70 90 RDM 57 44 58 46 / 50 50 70 50 LGD 49 44 51 45 / 90 90 100 100 GCD 52 44 52 46 / 50 50 90 70 DLS 60 51 57 50 / 100 90 100 100

PDT WATCHES/WARNINGS/ADVISORIES

OR...Wind Advisory from 1 PM this afternoon to 7 AM PST Tuesday for ORZ041-510.

Wind Advisory from 4 PM this afternoon to 7 AM PST Tuesday for ORZ044-507-508.

WA...Wind Advisory from 1 PM this afternoon to 7 AM PST Tuesday for WAZ024-521.

Flood Watch from 7 AM PST this morning through Friday morning for WAZ026-027-521>523.

Wind Advisory from 7 PM this evening to 7 AM PST Tuesday for WAZ027>029.