textproduct: Pendleton

KEY MESSAGES

- Light rain this evening through Thursday morning.

- Mountain showers return Friday and Sunday morning.

- Breezy weekend winds, primarily late Saturday into Sunday.

DISCUSSION

Current radar and visible satellite imagery shows dry conditions as clouds begin to develop along the Cascade and Blue Mountains. This is in response to the high pressure ahead of an incoming system that is already bringing light showers along the coastal range. Showers will begin to impact the Cascades after 6PM, Central Oregon around 9PM, and beginning to spill into the Columbia Basin around 11PM. Showers reach the Blue Mountains and foothills around 3AM and linger into the early morning hours. Outside of the Cascade and Blue Mountain zones, showers will be intermittent as rain amounts of up to 0.04" are expected across lower elevations of the Columbia Basin - most areas will receive 0.02" or less. The NBM aligns with these amounts as it highlights a 25-45% chance of measurable rainfall (0.01" or greater) across lower elevations of the Columbia Basin. Rain amounts of 0.08-0.15" are expected across the Cascade and Blue Mountains, with higher terrain across the Washington Cascades trending on the higher bound of the range and the Oregon Cascades and Blue Mountains more toward the lower bounds of the range. Snow levels will be hovering around 6000 feet, so the only measurable snowfall will occur across the highest terrain of the Cascade and Blue Mountains.

An upper level ridge follows closely behind the passing system Thursday morning, bringing drier conditions and mostly clear skies for the majority of the day. As a result, high temperatures will bump up about 5 degrees from Wednesday to near 60 degrees across portions of the Columbia Basin. These temperatures are about 5 degrees above average for this time of year, and are expected to stay above normal into next week. The next system, which is rather weak and well to the north, will only clip the Washington Cascades. It does have significantly more moisture than tonight's system, but its passing through Central British Columbia keeps measurable rainfall confined to the Washington Cascades and east slopes. Snow levels will be around 8000 feet, so rain is expected. Cle Elum is currently along the gradient between a wetting rain (0.10" or greater), with higher rain amounts of 0.2-0.3" possible west and at higher elevations.

The parent system of this shortwave is forecast to pass through Washington Saturday night through Sunday morning. The proximity and orientation of this trough passage is expected to extend rain chances across both the Oregon and Washington Cascades, as well as the Blue Mountains. 54% of ensemble clusters suggest little, if any, precipitation over lower elevations of the Columbia Basin. Timing is trending slightly slower as well, with 69% of members keeping precipitation across the Cascades until early Sunday morning versus moving in Saturday night. These differences are directly related to the strength of the incoming system and the ridge that builds ahead of it Friday afternoon/evening. The stronger trough relates to a wetter and earlier scenario, with a stronger ridge more efficient at weakening and slowing the incoming system.

A secondary concern with the weekend's system will be related to breezy conditions across Central and Eastern Oregon, primarily focused on the Grande Ronde Valley. An incoming frontal system, coupled with high pressure to the southeast and the potential for cold-pooling in the Grande Ronde Valley will enhance winds through this area. South winds of 20-30 mph with gusts of 40-50 mph will be possible as the GFS advertises a 4-5mb pressure gradient between Baker and Meacham. Currently, the NBM suggests a 35-65% chance of wind gusts reaching advisory-level criteria of 45 mph or greater. This broad range leads to low confidence in advisory-level winds occurring, at least related to current guidance. In addition to the large range of NBM probabilities, only 46% of ensemble members are suggesting a trend towards higher winds for the Grande Ronde. This does seem quite low, but it is likely the enhancement of cold pooling and a localized pressure gradient is not being realized by these longer range ensemble members. The arrival of shorter term guidance in the next 24-48 hours will greatly improve confidence in this event. 75

AVIATION /00Z TAFS THROUGH 00Z FRIDAY/

VFR conditions currently across all sites, which will stay the course for most sites through the period. The only exception will be reduced ceilings of 1500 feet at KDLS overnight. An incoming system will drop ceilings to near MVFR for KPDT/KALW as periodic light rain impacts all sites overnight. Winds will stay light and below 10 kts for all terminals through the period. Ceilings will improve and lift through the morning and early afternoon. 75

Preliminary Point Temps/PoPs

PDT 38 56 32 56 / 50 20 0 0 ALW 42 54 38 55 / 40 50 0 0 PSC 38 59 32 55 / 20 10 0 0 YKM 36 59 33 54 / 30 0 0 20 HRI 38 59 32 54 / 40 10 0 0 ELN 34 54 28 49 / 40 0 0 30 RDM 33 53 26 59 / 20 0 0 0 LGD 37 52 31 54 / 20 40 0 0 GCD 38 52 29 59 / 10 20 0 0 DLS 42 60 37 57 / 40 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES

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