Uncommon Summer Freeze Alert Issued for Two Counties in Oregon

A Freeze Warning is in effect overnight for the Wallowa Valley in northeastern Oregon, with temperatures expected to drop as low as 30 degrees early Sunday.

Wallowa County: Sub-Freezing Temperatures Overnight

The NWS Pendleton office has the warning in effect from 1 AM to 9 AM PDT Sunday for Wallowa County, including Joseph, Wallowa and Enterprise.

Forecasters expect sub-freezing temperatures as low as 30 degrees to settle into the valley overnight, a sharp late-summer cold snap for the region.

What It Means for Gardens and Outdoor Plumbing

Frost and freeze conditions could kill crops and other sensitive vegetation still in the ground this late in the growing season. Unprotected outdoor plumbing is also at risk of damage.

Why a Freeze Warning in Late Summer Isn’t as Strange as It Sounds

A freeze warning in the last week of August looks jarring next to the calendar, but it’s the result of a sharp, region-wide pattern change rather than a fluke. Meteorologist Cliff Mass, who tracks Pacific Northwest weather, has called the shift “the Great Cooling” — a substantial drop in temperatures across the entire region that started around August 26 and is expected to mark the effective end of the region’s hot weather for the season.

Part of the driver is simple astronomy: solar radiation declines quickly this time of year as days shorten and the sun angle drops, and the weeks surrounding the equinoxes are when the Pacific Northwest’s seasons turn over fastest. Layered on top of that seasonal decline is sharply colder air riding in behind a strong cold front and low-pressure system dropping south out of the northern Rockies — the same pattern that’s also bringing the first mountain snow of the season to parts of Montana.

High-elevation valleys like the Wallowa — Joseph and Enterprise both sit above 4,000 feet — are especially prone to hard freezes once cold air moves in, because clear skies overnight let the ground radiate heat away quickly and cold, dense air drains down from the surrounding mountains and pools in the valley floor….