textproduct: Pendleton

KEY MESSAGES

- Rain chances return Friday afternoon and late Saturday.

- Breezy winds late Saturday through Sunday morning.

- Mountain precipitation persist through early part of week.

DISCUSSION

Current radar and visible satellite imagery shows mostly clear skies under dry conditions. This is in response to an upper level ridge building in from the southwest on the heels of the departing shortwave that brought light rain to the area early this morning. The axis of the ridge will continue to shift east and elongate, shifting flow aloft from the northwest to more from the southwest into Friday. As this slowly shifts, a shortwave passes through Central British Columbia to allow light showers to develop across the Washington Cascades and Columbia Basin. Rain amounts are expected to stay light across the Columbia Basin, likely 0.02" or less. The NBM advertises a 10-40% chance of measurable rainfall (0.01" or less) across the Columbia Basin, with 10% being over the Tri-Cities and 40% over the Yakima and Kittitas Valley. Showers will primarily be focused across the Washington Cascades, with up to 0.25" across higher terrain. Due to the more southwesterly flow, snow levels will hover around 8000 feet.

A more substantial upper level system will approach the southern British Columbia coast on Saturday, before moving onshore overnight into Sunday. This will bring additional showers across the Cascades late Saturday before extending across the Blue Mountains Sunday morning. Rain amounts of 0.20-0.40" are expected across the east slopes of the Cascades late Saturday, with closer to a wetting rain (0.10") above 4000 feet. The incoming shortwave will be more oriented north-to-south, which will incur more of a rain shadow effect across the Columbia Basin, likely keeping most areas dry. This also aligns with the NBM which shows a 5-15% chance of measurable rainfall expected across lower elevations of the Columbia Basin, increasing to a 40-60% along the Blue Mountain foothills.

The weekend system will also attribute to breezy conditions across central and eastern Oregon Saturday afternoon through Sunday morning, with highest winds expected across the Grande Ronde Valley. The GFS highlights a pressure gradient of 3-4mb between Baker and Meacham, which may lead to advisory-level winds. Sustained south winds of 25-35 mph with gusts of around 45 mph are expected. The NBM suggests a 60-80% chance of sustained winds of 30 mph or greater, with a 50-70% chance of gusts reaching 45 mph or greater. This provides rather high confidence in a Wind Advisory being necessary across the Grande Ronde Valley Saturday evening through Sunday morning.

A weak ridge of high pressure moves in Sunday afternoon, drying conditions briefly before the next shortwave approaches the coast on Monday morning. This again brings more mountain showers along the Cascade and Blue Mountains Monday, ahead of additional showers through the beginning part of the week due to a strengthening upper level trough offshore. The trough will enhance southwest flow aloft, keeping light showers in the forecast across the Mountains through Wednesday. Guidance suggests the best chance for widespread showers that extends across the Columbia Basin will be Tuesday, as advertised by 69% of ensemble members. 75

AVIATION /06Z TAFS THROUGH 06Z SATURDAY/

Mostly VFR conditions to prevail through the period. There is moderate-high confidence (60-80%) that fog will develop around the Tri-Cities area tonight, bringing lowered vsby at site PSC. For tonight's TAF, have included vsby down 3SM between 7Z and 16Z at site PSC, though there is a 25-45% chance that vsby will lower below 3SM. Otherwise, winds will be light, 12kts or less, at all sites through the period. Lawhorn/82

Preliminary Point Temps/PoPs

PDT 37 59 38 66 / 0 10 10 0 ALW 42 57 44 67 / 0 10 10 0 PSC 36 57 37 60 / 0 10 20 0 YKM 36 56 37 59 / 0 20 20 0 HRI 35 57 37 61 / 0 10 20 0 ELN 33 51 34 55 / 0 40 40 0 RDM 32 62 32 69 / 0 10 0 0 LGD 33 53 37 57 / 0 10 0 0 GCD 32 61 32 67 / 0 10 0 0 DLS 39 59 42 62 / 0 10 20 0

PDT WATCHES/WARNINGS/ADVISORIES

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