All AgWeatherNet stations reached below freezing Wednesday, Feb 10. How often does this happen?

Mark Ingalls, AgWeatherNet Intern

Wednesday morning (February 10, 2021) was cold across much of North America, and Washington State was no exception. While the state did not see the extreme cold that the northern Plains and much of Canada saw, morning lows were well below average. Most of Western Washington was in the 20s with a few teens around Bellingham (thanks to cold winds coming down the Fraser River). East of the Cascades, most AgWeatherNet stations recorded lows below 20 with the exception being along the Columbia River downstream of the Tri-Cities. Two stations in the Methow Valley even got below zero.

For the purposes of this, AgWeatherNet's Tier 1 mesonet and Tier 2 all-in-one ag focused stations are considered.

It turns out, AgWeatherNet typically observes a few nights a year where the entire Tier 1 and Tier 2 networks reach below freezing. February 10 wasn't even the first time this was observed in 2021, an honor which is held by January 23. Usually for this to happen, offshore flow is required in Western Washington to limit the moderating effects of the Pacific Ocean. Offshore flow was in place Wednesday morning thanks to extreme cold and associated high pressure in interior British Columbia and Alberta.

From late-December to mid-February, there are many nights where all but a couple AgWeatherNet sites reach below freezing. On January 22, for example, Wallula SW was the only station with a low above freezing. Other frequent holdouts are Seattle, Grayland, and Long Beach. A summary for each station is provided in the following paragraphs.

Due to the comparatively warm Pacific Ocean, AgWeatherNet's two coastal sites (Long Beach and Grayland) don't drop below freezing often. So far for the winter of 2020-2021, Grayland has dipped below that magic number about twenty times. The Seattle station, located on the University of Washington campus, has to contend with comparatively warm water in the Puget Sound as well as Lake Washington. On top of that, it's location in Seattle places it within the city's urban heat island.

Finally, there's Wallula.SW. Despite being along the Columbia River, it is perched a couple hundred feet above it but still within Wallula Gap. On winter nights with strong temperature inversions on either side of the gap it acts as a funnel for air to pass through rather than having to go over the Horse Heaven Hills. This produces frequent gusty winds that help to keep the Wallula.SW station warmer than nearby locations like Kennewick, Touchet, and Hermiston. Seattle and Wallula.SW also get below freezing several times per winter but getting all four (along with the rest of the network) below freezing on the same night can be hard to do.

Sean Hill, AgWeatherNet's Application Systems Analyst, helped gather data for this report.

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