textproduct: Pendleton

KEY MESSAGES

- Significant mountain snow and lowland potential overnight *Winter Storm Warnings and Advisories Active*

- Additional mountain and lowland snow Thursday

- Warmer weekend temperatures with mountain snow persisting

DISCUSSION

Current radar and visible satellite imagery shows light to moderate returns along the Cascade and Northern Blue Mountains under partly cloudy to mostly cloudy skies. This is in response to an upper level low pressure system dropping along the Washington/Oregon coasts as an associated cold front slowly moves across the region. A surface low pressure has developed along the frontal boundary over Central Oregon, which is expected to slowly meander along the Northern Blue Mountains through Wednesday morning. Periods of heavy snow will be possible as terrain influences the synoptic forcing associated with the present surface low to allow for substantial snow amounts of up to 15 inches across the Northern Blue Mountains through Wednesday afternoon. Snowfall is expected to peak before 7 AM, but will linger into the afternoon. Thus, the Winter Weather Advisory has been upgraded to a Winter Storm Warning across the Northern Blue Mountains until 3 PM as 10-15 inches of snowfall is possible. These amounts align more with the 75th percentile (1 in 4 chances) than the 50th (1 in 2 chances), but of note is that the overall mean is significantly higher than the 50th. This relates to many ensemble members hinting at much higher snow totals as evident in the 90th percentile (1 in 10 chances) of receiving 25-30 inches over the Northern Blue Mountains. These parameters were taken more into account as a number of snow squalls occurred last night across Central Oregon, including the Bend/Redmond area and John Day and training snowfall has already set up across the Northern Blue Mountains.

Due to the nature and progression of the surface low pressure along the terrain, and the presence of cold air aloft mixing lower to the surface as evident with the earlier snowfall across portions of the Northern Blue Mountain foothills, Winter Weather Advisories have been issued across the Southern Blue Mountains, Wallowa County, Grande Ronde Valley, John Day Basin, and the Northern Blue Mountain foothills through 3 PM Wednesday. 3 to 6 inches of snowfall is expected across the Grande Ronde Valley, Wallowa County, and the Southern Blue Mountains, with 2 to 4 inches of snow along the Northern Blue Mountain foothills and the John Day Basin. Much like the Northern Blue Mountains, overall probabilities are low but the mean and 75th percentiles advertise advisory snow amounts. The Winter Weather Advisory across the east slopes of the Oregon Cascades continues to stay active until 11 PM tonight as an additional 2 to 5 inches will be possible. Confidence is high in these snow amounts as the NBM suggests a 70-80% chance of 2 inches or more snowfall and a 55-65% chance of 4 inches or more snowfall across Santiam Pass this evening.

The upper level low currently located along the coast will open into a trough and move through the Pacific Northwest into Wednesday morning ahead of a reinforcing trough that will drop along the Washington/Oregon coasts on Thursday. This will again bring northwest flow aloft to the region and another round of mountain snowfall with a chance for light accumulations across lower elevations of Central Oregon and the Blue Mountain foothills. The timing will coincide with the overnight period into Thursday morning, with mountain snow and lower elevations precipitation tapering off through the evening. Currently, the best chance (40-60%) for reaching Winter Weather Advisory snow amounts resides along the east slopes of the Oregon Cascades. Snow amounts expected across Santiam Pass Wednesday evening through Thursday are between 8 and 9 inches - which would be a higher-end Winter Weather Advisory. However, confidence is still lacking as the overall spread (25th vs. 75th percentiles) is 4 inches, which could lead to sub-advisory snow amounts. In addition, LREF ensemble clusters suggest 3-4 inches of snowfall, which align more with a sub-or low-end advisory levels. Further analysis will be necessary to determine if an additional winter weather product will be needed later in the week across the east slopes of the Oregon Cascades.

An upper level ridge begins to build across the Pacific Northwest Friday in the wake of the departing trough, effectively tapping into additional moisture and warmer temperatures Friday through the weekend. In response to the transient ridge building, only light mountain snow is likely Friday morning. Another upper level low pressure drops along the British Columbia coast over the weekend and slowly dissipates through the early part of the week. This will bring a return to mountain snow Saturday through Monday. 75

AVIATION /06Z TAFS THROUGH 06Z THURSDAY/

Areas of snow showers will impact sites through the night. Mainly PDT and ALW is currently experiencing a heavy winter precip band passing through currently. Conditions are expect to last for the next few hours (especially more-so overnight for ALW). Given that, we'll see MVFR/IFR conditions during moderate to high snow bands passing through, while improving to VFR as the snow showers move out. Otherwise, winds generally less than 15 mph with mostly mid to high level clouds prevailing through the second half of the period.

Preliminary Point Temps/PoPs

PDT 28 39 24 38 / 60 30 10 20 ALW 30 39 26 38 / 70 50 10 20 PSC 30 47 26 42 / 30 10 10 10 YKM 25 43 26 39 / 10 20 20 20 HRI 30 45 26 41 / 40 20 10 10 ELN 24 38 23 34 / 10 20 30 30 RDM 19 37 21 36 / 20 10 20 30 LGD 24 36 18 37 / 80 70 10 40 GCD 22 34 19 36 / 70 50 20 50 DLS 30 44 31 42 / 20 30 40 40

PDT WATCHES/WARNINGS/ADVISORIES

WA...Winter Weather Advisory until 3 PM PST Wednesday for WAZ029. Winter Storm Warning until 3 PM PST Wednesday for WAZ030. OR...Winter Weather Advisory until 3 PM PST Wednesday for ORZ049- 050-503-505-507. Winter Storm Warning until 3 PM PST Wednesday for ORZ502. Freezing Fog Advisory until 10 AM PST Wednesday for ORZ502.