textproduct: Pendleton

KEY MESSAGES

- Mountain snow showers continue through this evening Slight chance of isolated thunderstorms in the mountains this afternoon

- Slight chance of isolated thunderstorms in the mountains this afternoon

- Mostly dry conditions will prevail through the workweek, except for isolated mountain showers Sunday through Tuesday

DISCUSSION

Today through Sunday: A trough passage through the PacNW has brought mountain showers across the forecast area tonight, with showers also impacting portions of the Blue mountain foothills. Snow levels are on a steady descent, with area cameras showing light snow developing across portions of the northern Blues and the Cascades at this time.

A cooler airmass will continue to filter into the PacNW today as the upper trough remains over the region. Surface pressure gradients will tighten this afternoon, leading to breezy westerly winds (15-30 mph with gusts up to 40 mph) developing through the Cascade gaps and into the remainder of the lower elevation zones. Snow levels will briefly drop below 4kft across much of the area this morning, then rebound just above 4kft by the afternoon. Areas of the Cascades and eastern mountains above this will see light to moderate snow showers develop throughout today, with moderate to locally heavy snow accumulations along the Cascade east slopes. Winter weather advisories were issued for the WA Cascade upper east slopes and the OR Cascade east slopes through this evening, with 70-85% chance of snow accumulations greater than 6 inches along the WA Cascade east slopes and 5 inches for the OR east slopes. Otherwise, increasing low to mid level lapse rates will increase this afternoon across the mountains, while surface CAPE is expected to increase to 200-300 J/kg. Combined with low to mid level upslope flow into the Cascades and Blues, the increasing instability in the afternoon will lead to isolated thunderstorms capable of small hail and gusty outflow winds.

Sunday, a shortwave diving south will pull the upper trough south and east of the PacNW. This will limit showers to the higher terrain of the Blues, Wallowas, and along the Elkhorn crest through Sunday afternoon. Snow levels will quickly rise to 4.5kft to 5kft, limiting snow showers to above these elevations. Elsewhere, conditions will remain dry with winds becoming light to locally breezy in the lower elevations.

Monday through Friday: There is good agreement through Wednesday that upper level ridging will develop offshore, placing the PacNW under a northwest flow aloft. A series of weak shortwaves diving down the northwest flow Monday and Tuesday will bring a chance (15-30%) of isolated showers to the WA Cascades, northern Blues, and Wallowas(confidence 60-70%). Elsewhere, conditions will remain dry.

Wednesday through Thursday, nearly two-thirds of ensemble cluster members are in agreement of a shortwave trough feature pushing the ridge east, then across the PacNW Thursday with isolated mountain showers. By Friday, these members push the shortwave and precipitation chances east of the region, with upper level ridging once again building offshore.

Other than precip chances, the PacNW will feel the influence of the upper ridge through a gradual warming trend during the week, with a return to high temperatures in the upper 70s to lower 80s in the lower elevations by Thursday and Friday (confidence 55-65%). Lawhorn/82

AVIATION /12Z TAFS THROUGH 12Z SUNDAY/

VFR conditions to prevail through the period. Weather system moving through the region will bring rain showers to sites DLS/PDT/ALW/PSC this afternoon through the evening, except light rain showers will also impact site ALW this morning. Winds will increase to 12-20kts with gusts 25-35kts later this morning at all sites, with light winds developing after 00Z-03Z at sites RDM/BDN/YKM/ALW/PSC. Lawhorn/82

Preliminary Point Temps/PoPs

PDT 59 36 62 39 / 30 30 0 0 ALW 58 40 62 42 / 70 50 10 0 PSC 65 38 70 41 / 20 20 0 0 YKM 64 37 70 41 / 10 0 0 0 HRI 63 37 67 40 / 20 10 0 0 ELN 56 34 63 38 / 10 10 0 0 RDM 54 26 60 28 / 10 20 0 0 LGD 51 34 54 33 / 70 60 10 0 GCD 52 32 54 31 / 30 60 10 0 DLS 60 40 67 43 / 30 20 0 0

PDT WATCHES/WARNINGS/ADVISORIES

WA...Winter Weather Advisory until 11 PM PDT this evening for WAZ522. OR...Winter Weather Advisory until 11 PM PDT this evening for ORZ509.