textproduct: Pendleton

KEY MESSAGES

- Low weather concerns with periodic light rain chances that are mainly confined to the mountains, highest over WA Cascades, into tonight, Friday, and Sunday-Monday.

- Increased potential (60-80%) for a stagnant weather pattern Tuesday-Thursday that promotes air quality and fog concerns as confidence grows for light winds in tandem with weak mixing under a strong upper-level ridge.

- Slightly above average temperatures through the weekend with the greatest departures above normal in central OR and the mountains.

UPDATED AVIATION...06Z TAFs

All sites except RDM/BDN are seeing MVFR or lower due to VIS of 5SM or lower, with YKM seeing CIGs below 2500 ft. Light rain and BR has been impacting sites DLS/PSC/ALW/YKM again this evening and rain will slowly trickle off through the late evening before becoming dry overnight and the remainder of the TAF period. YKM/PSC/DLS may see times of low CIGs (300 ft or less) after 06Z for YKM and 11Z for PSC and 15Z for DLS. These 3 sites have 30% chances of VIS as low as 1/4SM and CIGs as low as 100 FT. Otherwise, sites RDM/BDN are expected to remain dry. Winds will be light, 6kts or less, through the period.

DISCUSSION

Tonight through Thursday. Latest water vapor imagery shows plume of moisture lifting northeastward across the region as a weakening warm front lifts north across WA. A low-level jet stream is lifting north in tandem with the best moisture west of the Cascades, of which latest precipitable waters are around 1.5 per MIMIC-TPW v2 on the coast with limited penetration inland. Latest radar display shows some weak returns moving east across the Basin with scant amounts seen over the last hour over north central OR. With this current activity, limited confidence in precipitations chances outside the crest of the WA and northern OR Cascades owing to limited spill over given the largely meridional trajectory. Limited impacts expected owing to high snow levels, 5-7 kft, with Snoqualmie pass most notably expected to see at least a quarter inch of rain (>60% confidence) mainly in the window of now through 4 am PST. Meantime, the next 6-12 hrs will see mid-level ridging building overhead with expectations of precipitation ending during the early morning hours.

The next weak disturbance moves east into southern BC/WA by late Friday on a northerly track as it rotates around the periphery of the upper ridge centered over CA-PacNW. This acts to dampen the ridge slightly and provides another window for precipitation tied primarily to the crest and upper eastern slopes of the WA Cascades. Ensemble IVT forecasts show weak moisture tap, short duration surge of Pacific moisture, and a northerly trajectory that will work to limit sensible weather impacts and penetration east of the Cascades. Snow levels remained elevated again, generally 6 kft and higher, which could be attributable to these warm flavor southerly setups and weak frontal systems that are staying largely offshore/coast. A larger upper trough is then expected to flatten the upper ridge as it evolves and moves into the area late Sunday and Monday that is characterized by another moisture with a southerly fetch and strong meridional component limiting best precipitation chances to the crest, eastern slopes, and eastern mountains. While some showers may spill over into the lower elevations, low confidence in the occurrence of wetting rain. Highest chances (80% or higher) confined to the crest where 24-hr amounts over 0.25 inches or more is highly likely.

After the end of this unsettled pattern, confidence in high pressure dominating the region Tuesday into early Friday is growing (60-80%). Strong, persistent upper-level high pressure is forecast with raw ensemble guidance showing heights greater than the 99th percentile. Concerns turn to a potential stagnant weather pattern owing to anticipated limited daytime mixing, lower transport winds and light surface winds, and a stable environment in lower elevations/Lower Basin and adjacent valleys. This increases concerns for conditions that could limit dispersion leading to air quality impacts. Meantime, this poses a increased potential for stratus/fog in this window mid-early next week and will be monitored.

Preliminary Point Temps/PoPs

PDT 38 52 37 51 / 20 10 10 0 ALW 40 52 39 52 / 30 20 20 10 PSC 35 47 34 48 / 30 20 10 0 YKM 35 45 33 47 / 50 30 10 10 HRI 35 48 35 48 / 30 10 10 0 ELN 34 42 33 42 / 70 40 20 10 RDM 35 55 33 57 / 20 10 0 0 LGD 36 48 34 51 / 20 10 10 0 GCD 35 53 35 54 / 20 10 0 0 DLS 38 49 40 51 / 50 40 20 10

PDT WATCHES/WARNINGS/ADVISORIES

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