textproduct: Pendleton

DISCUSSION

Key Messages:

1. Freezing Fog and light freezing drizzle early today. *Special Weather Statements Issued*

2. Low clouds and chilly temperatures linger through weekend.

3. Mountain snow possible Monday, better chances Wednesday.

Current radar and infrared satellite imagery shows dry conditions as the stubborn stratus layer continues to drape across the Columbia Basin and adjacent valleys. Pockets of freezing fog are also being observed via webcams across portions of North-Central and Central Oregon, primarily along the US-97 and 197 corridors. This is in response to the upper level ridge that has been parked offshore over the last several days. An upper level shortwave is dropping along the front side of the ridge, allowing for some high level clouds to ride over the lower stratus deck that is attributing to a feeder-seeder situation. This has resulted in light freezing drizzle across the Blue Mountain foothills and lower Columbia Basin of Oregon, and light snowfall over Central Oregon overnight into the early morning hours. As a result, a thin glaze of ice has accumulated on roadways and surfaces. Of particular concern are bridges, overpasses, sidewalks, and lightly used corridors. A special Weather Statement has been issued through the morning across Central/North-Central Oregon and the Blue Mountain foothills addressing these travel concerns, so use caution if traveling through these areas this morning.

As previously mentioned, an upper level shortwave is passing over the area today, ushering in a cooler airmass as flow aloft becomes more from the north. This synoptic feature is fairly weak and is not coupled with a frontal boundary that would effectively scour out the Columbia Basin and bring about and end to the stratus deck. There should be slight improvement across adjacent valleys and lifting of the deck over the Basin as a slight uptick in winds will promote low level mixing, but full removal is not expected as a more substantial system will be needed. The lingering cloud cover and infusion of cooler air will lead to a slight cooling trend over the weekend with highs in the upper-20s to low 30s and lows in the upper teens to low 20s across the Columbia Basin, Blue Mountain foothills, Central Oregon, Columbia Gorge, Wallowa County, and the Yakima/Kittitas/Grande Ronde Valleys.

The upper level ridge currently located offshore will begin to push onshore Saturday into Sunday as a stronger shortwave erodes the backside of the ridge and flattens it as it progresses onshore into Monday. This will lead to upper level flow becoming more from the west as the shortwave approaches and passes on Monday. Even through this shortwave is expected to be stronger than the one today, it still does not possess a significant frontal boundary to fully scour out the stratus deck across the Columbia Basin. However, there is a slight chance (15-20%) that it will bring enough moisture to allow for light snow showers across the Cascade and Blue Mountains during the morning and afternoon hours.

The upper level ridge rebuilds behind the departing shortwave late Monday into Tuesday, before shifting east as another shortwave slides onshore. This one does look to have a frontal boundary associated with it, albeit weak. This secondary system may bring with it widespread rainfall to lower elevations of the Basin, as indicated by 56% of ensemble members. There is significant uncertainty with both system early next week, as ensembles struggle with shortwave strength and timing as only 19% of ensemble members highlight greater than a 40% chance of measurable precipitation over the Cascades and greater than a 30% chance of measurable precipitation over the northern Blue Mountains on Monday. 79% of ensemble members suggest measurable precipitation across our mountain zones with the system later in the week. At this time, the second system looks more promising for mountain snowfall, but the effectiveness of lifting the stratus deck over the Basin is still uncertain. 75

AVIATION...00Z TAFs

Low stratus with several TAF terminals reporting <4000 feet of CIGs through the night. Low ceilings will continue to impact the region through the night as the stratus deck continues to dominate the pattern. A few sites (PDT/RDM/BDN) are seeing visibility issues coupled with the low CIGs, but not expecting quarter mile of fog as of yet, but ceilings will remain in LIFR territory through the late morning hours today. Otherwise, winds remain light and calm for the most part with general <10 mph winds dominating.

PRELIMINARY POINT TEMPS/POPS

PDT 28 22 28 23 / 0 0 0 0 ALW 30 23 30 23 / 0 0 0 0 PSC 34 25 33 25 / 0 0 0 0 YKM 31 26 30 25 / 0 0 0 0 HRI 32 25 32 25 / 0 0 0 0 ELN 28 23 27 23 / 0 0 0 0 RDM 27 20 29 20 / 0 0 0 0 LGD 30 23 30 23 / 0 0 0 0 GCD 27 20 28 21 / 0 0 0 0 DLS 34 26 34 27 / 0 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None.