I can’t even tell you who came up with the phrase “Fake Cold”, but the last 4 days have been the perfect example!
Of course FAKE COLD is not a real meteorological phrase but a term we use here in our area, in the lower elevations of the Pacific Northwest.
What is it? Probably better to define a “regular cold” airmass or modified arctic airmass. That’s when cold (usually dry too) air comes down from the north or northeast (out of Canada). It settles down into the Pacific Northwest, we forecast highs in the 30s, or colder, under sunny skies west of the Cascades. Of course it would typically be 5-10 degrees colder east of the Cascades in the valleys. For example, last New Year’s we had a cold near-arctic airmass descend south over us…with 850mb temps down around -8 to -10 over our area…that’s cold! We ended up with high temperatures similar to what we’ve seen the past two days; mid-upper 30s.
FAKE COLD is when a cold airmass didn’t mainly come from Canada but is generated by air sitting over the valleys/basins of our region in winter. Generally it’s the Columbia Basin in Eastern OR/WA. Less often it can be in the Willamette Valley. It works best with snow-covered ground, but you see the same weaker effect without snow cover. Usually the airmass cools over several days; caused by long winter nights when more heat leaving the lower atmosphere than is taken in during the daytime. In this case there is no mild wind to bring in warmer air so the lowest part of the atmosphere gradually cools. Take a look at the 5pm temps at PDX for the past 4 days…all of them windy and well-mixed.
About 8 degrees cooling in the past 72 hours.
In the Gorge at 3,500′ (3 Corner Rock)
About 10 degrees cooling there in 3 days.
Same thing at The Dalles, 9-10 degrees cooling.
Interesting that all the well-mixed locations have cooled the same 8-10 degrees don’t you think? Meanwhile the 850mb temperature over Salem at the beginning of this episode didn’t drop below -4. That would typically give us high temps in the mid 40s this time of year, which is what we started out with on that first east wind day back on Wednesday. Today we barely hit 36! Troutdale only PEAKED at 33.
So that is FAKE COLD…an airmass that started just a little on the cool side and has now become similar to a weak arctic blast. Dewpoints have also been steadily dropping.
Uh-oh, now we’re all primed for snow & ice…if/when the moisture shows up…
More on that in the next hour.
Chief Meteorologist Mark Nelsen