textproduct: Pendleton

KEY MESSAGES

- Continued mountain rain and snow through Thursday. *Winter Weather Advisory Issued*

- Breezy winds Tuesday and Wednesday.

- Cooling trend Tuesday onward.

DISCUSSION

Current radar and visible satellite imagery shows slight returns across the Cascade and Blue Mountains under mostly cloudy skies. This is in response to an upper level low pressure system that is passing through the area today, ahead of another upper level trough on Tuesday. These quick hitting systems will keep precipitation at higher elevations across our mountain zones, with chances (35-55%) extending across the east slopes of the Cascades, Central Oregon, and the Blue Mountain foothills through the early part of the week. The second system, poised to move through the Pacific Northwest Tuesday into Wednesday, will bring with it a cooler airmass that will drop snow levels from around 5000 feet Tuesday morning to around 3000 feet Wednesday morning. 3-7 inches of snowfall is expected across the Washington Cascades at elevations above 4000 feet through Wednesday afternoon. This has warranted the issuance of a WINTER WEATHER ADVISORY across the upper slopes of the Washington Cascades at elevations of 4000 feet and above from 5 PM Tuesday to 5 PM Wednesday. Travel may be hazardous during these times, with moderate to heavy snowfall possible.

The second system will pass to our south and push into southern Idaho and northern Utah early Wednesday morning, just as a third upper level low pressure system develops off the British Columbia coast and slowly tracks south off the Washington/Oregon coasts. This system stalls offshore through the remainder of the week, keeping light mountain showers in the forecast through Friday afternoon. Due to the origin of this third system being that of the interior of British Columbia and Alberta (more continental), it will bring drier air as observed from the ECMWF and GFS AI ensemble guidance, which suggests precipitable water (PWAT) values of 0.2-0.45" or 70-80% of normal. This dry air is expected to keep any snow or rain accumulations minimal as the NBM advertises a 20-40% chance of 48 hour snowfall totaling 1 inch or greater Thursday through Friday across Snoqualmie and Santiam Passes. White Pass is considerably higher with a 70-90% chance for 1 inch or more snowfall, and 25-35% chance of 3 inches of snow or more Thursday through Friday.

The combination of the passing system Tuesday and the approaching third system off the British Columbia coast will tighten the pressure gradient across the region, leading to breezy winds across the Basin. Southwest winds of 15-25 mph with gusts of up to 40 mph will be possible, peaking overnight Tuesday into Wednesday across the Simcoe Highlands, north-central Oregon, Blue Mountain foothills, and the Columbia Basin. There is a low chance of winds reaching advisory criteria as the NBM highlights a 20-30% of wind gusts reaching 45 mph or higher.

Continued systems dropping into the area from the north will keep flow aloft primarily out of the northwest, advecting cold air into the area through the week. ECMWF and GFS AI ensembles suggest this third system eventually moves inland late in the week. A building upper level ridge just behind this offshore system will erode and weaken the trough ahead of its passage through the Pacific Northwest. This greatly weakens the system and leads to likely drier conditions developing Friday evening as the bulk of the moisture is forced south in response to the incoming upper level ridge. The ridge continues to enhance cooler temperatures, with highs over the weekend only reaching into the low-50s with lows dipping into the upper-20s across lower elevations of the Columbia Basin. Guidance does struggle with the ridge strength, which correlates to how efficient it is at weakening the earlier offshore system and how the ridge's influence lingers before the following system later in the week. Currently, guidance is split with 50% of ensemble members suggesting a stronger incoming ridge and 50% advertising a weaker ridge and more substantial trough passage. Stay tuned. 75

AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/

VFR conditions initial across all sites that will prevail for most of the period. Seeing lower FEW/SCT less than 3000 feet forecasted to roll through most sites with light to moderate shower activity. Most of the precip will be done by 20Z, with some lingering showers near the Foothills/higher elevated TAF sites through the remainder of the period. Can see some MVFR conditions developing (5-15% each TAF site) where the heaviest showers occur. Winds will be generally elevated with all sites seeing prevailing 10-15 knot sustained winds with DLS and BDN seeing gusts up to 20-25 knots.

Preliminary Point Temps/PoPs

PDT 54 39 55 33 / 60 50 40 20 ALW 55 43 54 39 / 60 50 50 50 PSC 58 40 61 37 / 20 0 10 10 YKM 56 37 60 33 / 20 0 0 20 HRI 57 42 58 37 / 40 30 10 0 ELN 52 33 54 28 / 40 10 10 50 RDM 50 34 47 28 / 50 20 40 0 LGD 54 37 50 31 / 80 80 80 50 GCD 52 35 47 29 / 70 50 50 10 DLS 57 43 59 39 / 70 70 20 60

PDT WATCHES/WARNINGS/ADVISORIES

WA...Winter Weather Advisory from 5 PM Tuesday to 5 PM PDT Wednesday for WAZ522. OR...None.