textproduct: Pendleton

KEY MESSAGES

- Patchy dense freezing fog this morning, mainly for the Blue Mountains region and fog-prone valleys within the Columbia Basin.

- Near-normal temperatures and mostly dry conditions until a weak weather system arrives late Wednesday.

DISCUSSION

GOES-18 water vapor imagery shows an amplified upper-level ridge extending from California northeast through the Canadian Rockies. Upstream in the Pacific, a low-pressure system is developing in the vicinity of 39N, 142W. Nighttime microphysics imagery reveals mostly clear skies across the forecast area, albeit with low stratus and/or fog in portions of the eastern Columbia River Gorge, lower Columbia Basin, Blue Mountains, and cold mountain basins. At the surface, most valley/basin locations are nearly saturated at around 90-100% RH, and additional fog formation remains a concern for early morning. Have held off on any headlines due to the patchy nature, but will continue to monitor the situation.

The aforementioned area of low pressure is expected (90% confidence) to develop into a closed low by Tuesday, eventually moving inland over southern Oregon or northern California by late Tuesday. Ensemble guidance suggests the low will be too far south to produce significant precipitation for our forecast area, and mostly dry conditions (PoPs of 20% or less) are forecast to continue until late Wednesday into Thursday when ensemble guidance highlights the arrival of a shortwave trough. Clusters of ensemble solutions continue to indicate significant spread in how much precipitation will accompany the shortwave; roughly one-third of members place noteworthy precipitation across the forecast area, while the remainder keep precipitation confined to the Cascade crest.

By Friday, ensemble clusters are showing medium potential (~65%) of wet weather for the Washington Cascades, with significantly lower chances (~30%) for wet weather east of the Cascades. Ensemble guidance is trending towards mostly dry solutions (75-85%) by Saturday with an upper-level ridge overhead and precipitation displaced to the northwest of our forecast area.

AVIATION /12Z TAFS THROUGH 12Z TUESDAY/

Predominantly VFR conditions are forecast at TAF sites over the next 24 hours, with the exception of some sub-VFR conditions this morning. Highest confidence (40-80%) in sub-VFR conditions will revolve around ongoing or forecast low stratus and fog this morning at DLS/PSC. A ribbon of stratus has been developing in the eastern Columbia River Gorge early this morning, and while DLS has yet to report a ceiling associated with it, confidence was high enough in temporary MVFR CIGs to include a TEMPO group. At PSC, anticipating some temporary VSBY reductions as patchy fog has been developing in the Columbia Basin. Otherwise, winds of 12 kts or less are forecast, and mid/high cloud cover will increase through the period as a weather system approaches from the Pacific. While light precipitation cannot be entirely ruled out later this evening into tonight, chances are too low (15% or less) to include a mention in the 12Z TAFs.

Preliminary Point Temps/PoPs

PDT 58 36 51 31 / 0 0 10 0 ALW 57 39 52 35 / 0 0 10 0 PSC 58 38 54 31 / 0 0 0 0 YKM 55 39 57 30 / 0 0 10 0 HRI 58 38 53 30 / 0 0 10 0 ELN 53 34 51 27 / 0 10 10 0 RDM 59 31 51 23 / 0 0 0 0 LGD 53 37 57 31 / 0 0 0 0 GCD 57 36 57 31 / 0 0 10 0 DLS 59 41 57 35 / 0 10 10 0

PDT WATCHES/WARNINGS/ADVISORIES

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