College Place, WA 99324 Weather

Key Messages

- Dry conditions will persist into the weekend as a ridge of high pressure moves overhead.

- Near- to sub-freezing low temperatures tonight into Saturday morning. Freeze Warnings in effect.

- High confidence (70-90%) in cooler conditions and periods of showery weather early next week, followed by medium confidence (40-60%) in periods of active weather middle to late week.

Discussion

Water vapor imagery shows broad troughing over the western CONUS, moving out of our forecast area. Meanwhile, precipitation has tapered off across the Blue Mountains as a very dry northerly to northwesterly flow aloft replaces the previous air mass and a low-amplitude upper-level ridge moves overhead. Morning soundings (12Z or 18Z) at SLE, OTX, MFR, and UIL all show low PWATs of 0.25-0.35" and 12Z HREF model guidance suggests values ranging from 0.15" to 0.45" across the forecast area today into tonight. Lingering breezy conditions will taper off this evening into tonight. Moreover, the combination of clear to mostly clear skies and the dry air mass has set the stage for near- to sub-freezing temperatures tonight into Saturday morning. While most public forecast zones have observed a season-ending freeze, a widespread freeze has not yet occurred in the lowest elevations of our forecast area. Confidence is medium-high (50-95%) that a season-ending freeze will be observed in the lower Columbia Basin, eastern Columbia River Gorge, Yakima Valley, and the foothills of the northern Blue Mountains. Lowest confidence in a widespread freeze is for the eastern Columbia River Gorge; highest confidence in a freeze is for cold-prone locations along the Blue Mountain foothills and lower Columbia Basin. That said, opted to upgrade all Freeze Watches to Freeze Warnings.

Uncertainty in the 500-hPa height field burgeons Sunday through the remainder of the forecast period due to continued ensemble spread in solutions regarding the evolution of a closed low that is currently developing in the Gulf of Alaska. While ensemble solutions have begun to cluster around a closed low entering, then tracking inland, across the Pacific Northwest region later Sunday through Tuesday of next week, significant uncertainty remains with regard to the track of the low, the depth of the low, and how quickly it will move inland. Based on 12Z ensemble guidance, the best chances of precipitation will be Monday and Tuesday. Even then, chances of a wetting rain (0.10" or greater) are low (40% or less) for the lower elevations, Washington Cascades, and Ochoco-John Day Highlands. Higher chances (40-70%) of a wetting rain are forecast for the Oregon Cascades and northeast Blue Mountains.

Ensemble solutions are roughly split (50/50) Wednesday through the remainder of the week on whether another trough/closed low or ridge will be present over the Pacific Northwest. Continued cool, showery weather would accompany the closed low or troughing scenario, while dry conditions would result from the ridge.

Aviation

High pressure will create light and variable winds overnight (under 5 knots). More consistency in wind direction will develop on Saturday (under 10 knots). Clear sky or cirrus and VFR conditions at all terminals.

Pdt Watches

WA...Freeze Warning until 10 AM PDT Saturday for WAZ024-027>029. OR...Freeze Warning until 10 AM PDT Saturday for ORZ041-044-507.