For winter weather nerds in the Pacific Northwest, mid-February is traditionally a tough time to reckon with.  The sun angle is rapidly inceasing and our normal daily temperatures in the lowlands are beginning to creep upward.  The last decade aside, February is historically not as good as December or January from major snowfalls and cold weather episodes.  Often if there is no cold air showing up on the maps by February 15 or 16, it’s appropriate to “stick a fork” in the chance for any more serious winter cold for the year.

Just one week ago, in the throes of a historically warm “false spring” event, I too had consigned myself to the likelihood that we wouldn’t see any more significant cold weather this season.  However, at just about the last possible opportunity, weather models decided to make a Hail Mary play:  A late-season arctic airmass showed up on the maps for the last full week of February.  And now, as we enter a three-day holiday weekend, we’re just 48 hours away from the onset of a late-season cold snap.

SUMMARY

1.  We have a cold front dropping in from the north this weekend.  Showers of valley rain (and mountain snow) should begin tonight and continue through some point on Monday.
2. By Tuesday an arctic airmass is in the Columbia Basin, and will drive a cold east wind through the Columbia River Gorge.  That likely means high temps in the mid/upper 30s in Portland and Vancouver for a couple days, which is a far cry from the 50s and 60s we’ve become accustomed to the last couple weeks.
3. But it doesn’t last long.  By Thursday the airmass is already moderating.  Powerful late February sunshine will prevent a low-level cold pool from lingering east of the Cascades for several days, like it might do if it was midwinter.  Next weekend we should be back to near-normal temperatures.

THE SYNOPSIS

The West Coast has been dominated by an upper-level ridge the past 2 weeks.  When it’s directly over us in mid/late February, the sun angle is often enough to break the inversion and cause warm temperatures in the lowlands.  Generally this means the low 60s, though last Friday we had a perfect setup for unusually warm conditions in the metro area (68 at Vancouver airport, 67 at PDX, and 65 in Cannon Beach along the north coast!).  But now the ridge is going to get pushed back to the west, allowing cold northern systems to slide down from the BC coast.  This morning’s IR satellite shows the front and its clouds approaching:

This means that precipitation should pick up during the afternoon over the Puget Sound and much of Western Washington, with pretty significant snowfall in the mountains.  Shower activity arrives in NW Oregon/SW Washington this evening and continues through the day on Sunday, and at least part of Monday.  Here are the precip totals by 4am Tuesday, when most showers are gone and the cold air is on our doorstep:

The snow map looks like you might expect in the mountains:  healthy totals of 10-16″ for most of the higher slopes.  The lowlands are showing far less snow accumulation, with almost none in the PDX/VAN area.  However the map DOES show little bands of sticking snow across parts of the Willamette Valley.  This is a sign that lowland locations might see a little snow on Monday, if they are lucky enough to get underneath a convective shower or one of the “ocean effect” bands coming in from the northwest.

It’s also possible that we see an uptick in shower activity Monday evening, as the arctic east wind arrives.  Arctic fronts can drop an inch or three of snow as they pass through, though many of them are very dry.  We will have to see with this one.  For now, there is a chance of light snow accumulation during Monday or Monday evening in the metro area, though no guarantee.  Temperatures in the lowlands will be quite chilly on Monday, likely not getting out of the low 40s even during afternoon sunbreaks.

By Tuesday the frigid arctic air is entrenched east of the mountains:

The arctic air & east wind continues for two days – Tuesday & Wednesday.  That means very cold temps for the metro area, at least by late February standards (mid/upper 30s for highs, as opposed to the usual low 50s).  I don’t have the image here, but the ECMWF model is showing low temps around 23-24 at Portland Airport Wednesday morning.  Due to the very low humidity (dewpoints in single digits F), that probably means temps outside the heat island will be in the 15-20 range.  By Thursday, the sun is killing the cold pool on the east side and the wind is weakening:

So…this cold snap will be short-lived, with likely only a couple really cold days (Tuesday & Wednesday) and two cold nights (Wed & Thu morning).  Nonetheless, it will come as quite a shock after last week’s record warmth.

Some of you are probably thinking:  “This cold snap is going to wreak havoc in our gardens!”  Good news….I wouldn’t worry about it too much.  About the only things blooming at the moment are crocus and a few early daffodils, and these flowers are VERY hardy to cold.  To put it into perspective, the month of January was very close to normal temps, and the first 2 1/2 weeks of February have been running only about 2 degrees F above normal.  That’s not enough to create a big shift in early spring phenology!  Now, if the first 6-7 weeks of 2022 had been as warm as they were in 2015, I’d be a lot more worried about a 20-degree freeze in late February causing significant damage.  I recall photos of February plum and magnolia blossoms in Portland that year, and even The Dalles saw a surprising amount of spring plant activity by month’s end.

But if you got fooled by the recent warm weather and started planting stuff that normally shouldn’t go out until late March or April?  Then the joke is on you I guess…

So to review:  We have showers tonight through Monday, with gradually lowering snow levels.  Some of the Monday showers MIGHT produce lowland snow if there’s moisture around as the colder air arrives.  Tuesday and Wednesday are going to be about as cold as late February can get, with east wind and nighttime temps possibly dropping to 20 or lower away from the milder urban microclimate. -Karl