textproduct: Pendleton

KEY MESSAGES

- Patchy to areas of 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 /18Z TAFS THROUGH 18Z TUESDAY/

KDLS is buried in stratus with 700 ft ceilings this morning and the LAMP guidance in addition to REFS sounding profiles suggests these ceilings breaking out by 19 or 20z. Elsewhere, where localized brief fog occurred, the visibilities have improved to VFR. Light winds at all terminals through the TAF.

Preliminary Point Temps/PoPs

PDT 58 36 50 30 / 0 0 10 0 ALW 57 39 50 34 / 0 0 0 0 PSC 58 38 54 31 / 0 0 10 0 YKM 55 39 57 31 / 0 0 10 0 HRI 58 38 52 30 / 0 0 10 0 ELN 53 34 52 28 / 0 0 10 0 RDM 59 31 51 23 / 0 0 0 0 LGD 53 37 56 31 / 0 0 0 0 GCD 57 36 58 31 / 0 0 0 0 DLS 59 41 58 35 / 0 10 10 0

PDT WATCHES/WARNINGS/ADVISORIES

WA...None. OR...Dense Fog Advisory until noon PST today for ORZ050.