textproduct: Pendleton

KEY MESSAGES

- Significant mountain snowfall through Wednesday *Winter Weather Advisories Active*

- Additional snowfall Thursday onward, lowland potential

DISCUSSION

Current radar and visible satellite imagery shows bands of moisture extending southwest to northeast across the area under overcast skies. This is in response to a slow moving cold front passing through our area today, which is associated with an upper level trough that is dropping along the British Columbia coast and becoming a closed low later this afternoon off the Washington/Oregon coasts. This feature will slowly open back up into a trough late Tuesday before sliding through the Pacific Northwest on Wednesday. This slow moving low/trough will advect cold air into the region, allowing high temperatures to reach into the upper-30s to mid-40s across lower elevations of the Columbia Basin and Central Oregon with low temperatures dipping into the upper-teens to mid-20s through Wednesday. The trough will also tap into moisture on its decent along the coast, allowing for persistent mountain snowfall to occur through Wednesday, especially across the Oregon Cascades and the northern Blue Mountains. Thus, Winter Weather Advisories are active across both regions until 11 PM Tuesday for the east slopes of the Oregon Cascades and until 3 PM Wednesday through the Northern Blue Mountains as 6-10 inches of snowfall will be possible. Confidence in these snow amounts is high as the NBM advertises an 85- 95% chance over the Oregon Cascades and an 75-85% chance across the Northern Blue Mountains of receiving 6 inches of snowfall or more through 10 AM Wednesday.

A reinforcing upper level trough follows closely behind the earlier week system, exhibiting a similar path along the British Columbia coast Wednesday before opening up and moving across the Pacific Northwest on Thursday. This will keep cold air over the region and allow snow levels to drop to near the Columbia Basin floor Wednesday and Thursday mornings. All ensemble members suggest up to an inch of snowfall through the Northern Blue Mountain foothills Wednesday and Thursday morning. In addition, 51% of members advertise up to an inch across the Lower Columbia Basin including the Tri-Cities, Hermiston, and Yakima areas. However, amounts near an inch are more optimistic as the NBM highlights a 35-60% chance of measurable snowfall (0.01" or greater) along the Northern Blue Mountain foothills Wednesday and Thursday morning and less than a 10% chance Wednesday and a 10-20% chance for Tri-Cities, Hermiston, and Yakima on Thursday morning. There is also a 40-60% chance of Thursday morning snowfall across Central Oregon (Bend/Redmond/Madras) per the NBM, but LREF ensemble members are more optimistic with a 70-80% chance. These discrepancies across model suites point to low confidence in expected snow amounts across lower elevations late in the week, which is in response to marginal conditions coupled with the overall strength and path of the incoming upper level system being in question.

Unsurprisingly, better chances for measurable snowfall will occur over the Cascade, Blue, and Wallowa Mountains later in the week, with current amounts reaching advisory criteria across the Oregon Cascades (5-8") and near advisory amounts over the Northern Blue Mountains (6-10") Thursday through Friday. This may lead to advisories being issued Wednesday afternoon/evening, but current confidence is low due to the overall spread of 6-8" related to 48- hour snow totals Thursday through Friday. Also of note is that the NBM provides only a 50-60% chance of advisory snow amounts across the Northern Blue Mountains. There is greater confidence via the NBM over the Oregon Cascades of 80-90% of advisory level snowfall, but the 6-8" spread in snow amounts is still present which would barely meet criteria if the 25th percentile snow amount comes to fruition. Stay Tuned. 75

AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/

VFR conditions prevail with light winds overnight. Chances of rain in DLS/PDT/ALW. Precip will end in PDT over the next hour or two, while ALW and DLS will see their chances over the morning hours.

Preliminary Point Temps/PoPs

PDT 27 45 27 41 / 60 30 40 40 ALW 30 46 29 41 / 70 30 60 50 PSC 29 48 28 47 / 20 10 20 20 YKM 23 41 23 41 / 10 20 10 10 HRI 29 47 27 45 / 40 10 30 20 ELN 21 36 22 37 / 20 30 20 20 RDM 22 39 18 37 / 20 30 20 20 LGD 25 41 24 37 / 90 50 70 80 GCD 24 39 22 35 / 80 60 50 60 DLS 29 44 29 44 / 20 50 30 40

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 Tuesday for ORZ509.