textproduct: Pendleton

KEY MESSAGES

- Patchy Dense Fog Possible overnight.

- Light Mountain snow accumulations return this weekend.

- Heavy Mountains snow accumulations are trending in the models from late Tuesday onward next week.

DISCUSSION

High pressure influencing the area Today as the mesoanalysis shows a flat ridge and 700-500 mph as high thin cirrus over spreads the region. That weak subsidence can keep winds light with additional potential for go overnight. Ran the forecast builder to make fog grids based on lowest dew point depressions and lightest winds under 4 mph which basically mirrors the fog that developed in the lower Columbia Basin last night. Patchy dense fog is also mentioned in central Oregon from The Dalles to Bend. The overspreading cirrus may work to inhibit the fog developments, owing to the 20-30% chances and uncertainty for sub one half mi visibility from the HREF members.

A surface cold frontal system slides across the area Friday night into Saturday bringing low land showers and mountain snows. The atmospheric river portal shows a couple of signals from the control runs of EC and GFS, which fairly well agree on a weak IVT impinging on the WA coast this weekend, and then a longer duration atmospheric river farther south across sw Oregon and northern CA into the latter half of the week. Both of these time frames result in NBM model pops with mountains snows. The snow levels start out quite high for the first episode especially for the eastern Mountains but drop and become more favorable for advisory level snows accums on a 2+ day basis for the entire Cascades eastern slopes as well as the northern Blue Mountains by the Tuesday Nights through Friday period. Confidence for advisory levels is high given some of the NBM spreads. The NBM member 72 hour Tuesday Night through Friday afternoon 25th and 75th percentile snow accums range form 7 to 18 inches at Tollgate, 7 to 21 inches near the Snoqualmie Pass and 12 to 30 inches at the Santiam Pass. High temperatures look to continue to reach the upper 40s across the Valleys through the week but colder air develops across the foothills and mountain zones Tuesday onward.

AVIATION /06Z TAFS THROUGH 06Z SATURDAY/

VFR conditions are expected at most TAF sites through the period. -RA will begin to impact DLS and YKM by late Friday night. The only exception is PSC, which has already begun to have fog and stratus. Guidance continues to not handle this well, but it is evident on satellite imagery along the Columbia River. For now, have kept everything MVFR, but it is very possible PSC ends up IFR or lower. Winds remain light.

Preliminary Point Temps/PoPs

PDT 32 49 37 48 / 0 0 20 40 ALW 35 49 40 49 / 0 10 40 60 PSC 30 48 36 50 / 0 0 20 20 YKM 30 45 32 46 / 0 10 20 20 HRI 30 49 36 49 / 0 0 20 20 ELN 30 42 31 43 / 0 20 40 20 RDM 24 49 29 46 / 0 0 10 40 LGD 28 49 35 48 / 0 0 30 70 GCD 28 50 34 47 / 0 0 10 70 DLS 35 47 38 47 / 0 20 50 50

PDT WATCHES/WARNINGS/ADVISORIES

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