textproduct: Pendleton

KEY MESSAGES

- Mountain snow today into Thursday

- Breezy to windy through Thursday

- Near- to sub-freezing morning lows Thursday and Friday for the lowlands

DISCUSSION

Ensemble and deterministic NWP guidance are in excellent agreement that the next weather system to affect the region, an upper-level closed low currently centered over southeast Alaska, will continue to track along the coast of British Columbia and eventually into the Pacific Northwest through Thursday. Its attendant cold front is expected (99 percent confidence) to slowly traverse the forecast area overnight (Washington Cascades) through Wednesday afternoon (far northeast Oregon), lowering snow levels below mountain pass elevations.

Winter weather headlines remain in effect, and confidence is high (80 percent) in 4-8 inches of snowfall for the Washington Cascades, 6-12 inches for the Oregon Cascades, and 5-10 inches for the northern Blue Mountains with locally higher totals along the crests. The bulk of the snowfall will likely (80 percent confidence) end by Wednesday morning (Washington Cascades) to Wednesday afternoon (Oregon Cascades and Blues) as the front sags southeast, but snow showers will persist through Wednesday night or Thursday morning as the aforementioned upper low moves overhead.

Precipitation associated with the cold front in non-mountain areas will be mostly rain below 1500-2000 feet, though uncertainty still exists in snowfall between 2000 and 4000 feet. Much of this uncertainty is driven by how long the frontal band lingers over the lowlands (i.e. how much precipitation falls and how much rate-driven cooling will occur within the band), and also in the location/magnitude of post-frontal snow showers.

Widespread breezy to windy southwesterly to westerly winds are expected this afternoon and tonight in advance of the cold frontal passage. Confidence was too low (50 percent) in widespread advisory-level sustained winds or wind gusts to issue any wind headlines. By Wednesday afternoon, winds will become westerly to northwesterly in the wake of the front and remain diurnally breezy to windy through Thursday. NBM probabilities of exceeding advisory levels vary widely (30-80 percent) across the lower elevations each day, highest for wind-prone locations of the foothills of the Blue Mountains and north-central Oregon as well as the Kittitas Valley.

The cold air mass associated with the upper low Thursday morning has prompted concerns for near- to below-freezing morning temperatures. NBM calibrated probabilities suggest medium-high chances (60-90 percent) of freezing temperatures for the Yakima and Kittitas valleys, medium probabilities (30-70 percent) along the foothills of the Blue Mountains, and low- medium-medium (20-50 percent) for the lowest elevations including the eastern Columbia River Gorge and lower Columbia Basin. Have opted to issue a Freeze Watch where chances of freezing are highest (Kittitas and Yakima valleys and the foothills of the northern Blue Mountains of Oregon), but will note there are still a lot of moving parts to the forecast with uncertainty in how much cloud cover will persist and how windy conditions will remain overnight into the morning hours. Also worthy mentioning, with the notable exception of several CAMs (and perhaps a few other inputs not examined), raw inputs to the NBM are largely not supportive of sub-freezing temperatures while bias-corrected inputs are. This is not uncommon, but when conditions are not calm, raw inputs are more likely to have their warmer temperatures verify than when radiational cooling is efficient and guidance struggles to resolve cold pools.

An upper-level ridge will likely (80 percent confidence) build into the Pacific Northwest Friday into the weekend, yielding lighter winds and mostly dry conditions. The main headline- worthy consideration for Friday will be near- to sub-freezing temperatures for the lowlands. This set-up looks more favorable for mostly clear to clear skies and lighter winds relative to Thursday morning, so while the coldest air mass will have exited the region to the east, radiational cooling should be more efficient.

By Sunday through Monday, uncertainty in the 500-hPa pattern grows, but some flavor of a 500-hPa closed low (~85 percent of members) or trough (~15 percent of members) is advertised by all ensemble clusters to be in the vicinity of, or over, the Pacific Northwest. Will note the trend in 12Z ensembles has been towards the offshore cut-off low, a solution that anecdotally materializes more often relative to the more progressive solutions.

AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/

VFR conditions currently prevail with concerns revolving around winds the next 24- hrs and deteriorating flight categories 12Z on tomorrow associated with a cold front. This active 24-hr window will see persistent breezy winds with elevated winds continuing overnight. This will be followed by a modest wind shift between 12Z-18Z tomorrow at terminals tomorrow with peak gusts expected around 25-35 kts. While there are low chance ahead of it this afternoon (23Z-04Z) at KALW and KPSC terminals, more robust chances (60% and higher) will be seen after 9Z tomorrow morning at terminals from west to east and north to south.

Preliminary Point Temps/PoPs

PDT 41 52 31 53 / 50 90 40 20 ALW 42 52 34 53 / 80 90 60 30 PSC 44 58 34 61 / 30 70 30 0 YKM 38 55 30 59 / 60 40 10 0 HRI 42 56 32 59 / 30 80 20 0 ELN 32 48 29 52 / 60 30 10 0 RDM 34 48 23 49 / 40 80 30 0 LGD 39 48 29 46 / 90 100 80 60 GCD 38 49 26 45 / 70 100 70 60 DLS 41 53 35 57 / 80 80 20 0

PDT WATCHES/WARNINGS/ADVISORIES

WA...Winter Weather Advisory from 5 PM this afternoon to 11 PM PDT Wednesday for WAZ030. Freeze Watch from Wednesday evening through Thursday morning for WAZ026-027. Winter Weather Advisory until 11 PM PDT Wednesday for WAZ522. OR...Winter Weather Advisory from 5 PM this afternoon to 11 PM PDT Wednesday for ORZ502. Freeze Watch from Wednesday evening through Thursday morning for ORZ507. Winter Storm Warning from 5 PM this afternoon to 5 AM PDT Thursday for ORZ509.