I’ve spent a great deal of my life monitoring the weather,in Oregon its a great distraction and there is always a ton to learn. I tend towards catastrophism which means that I always compare any slight catastrophe to the worst case scenario. Its my way of putting things in perspective. As we all know peoples perspective of the weather can vary greatly. We have notoriously short weather memories in the PNW. Especially gardeners. So far this winter has been very average as far as I’m concerned especially in terms of ultimate low temperature. ( hint: we matched our average annual lowest temperature of 20.8ºF) The real anomalies are that average temperatures in November and December were both about -3ºF below normal- this is perfectly in line with La Nina conditions. However, unlike most La Ninas it has been surprisingly dry.
Pre-Xmas Polar offshoot and ice storm
As far as freezing low temperatures we are right on track with nights below 32ºF , 20 so far this season on our way to the average of 33. per year. Our lowest temperature occurred on Dec.21 and 22nd when an arctic front plowed through with 1.5″ of sleet in the Portland area and dangerous freezing rain as far south as Cottage Grove, in the southern Willamette Valley, both days the low at PDX was 20ºF. What made this ice storm different was the depth and breadth of the spread of arctic air. It pushed over and through the coast range and freezing rain was reported from Florence north on the beaches. This cold air was propelled through the Columbia Gorge as arctic high pressure built in the Columbia Basin and was drawn through the gap. This resulted in very strong off shore ( E ) winds that lashed all of northwest Oregon continuously for 72 hours. This was just a small off-shoot of the HUGE ARCTIC FRONT that mowed through the rest of North America with enormous temperature drops, frigid lows and powerful winds. This Polar Plunge caused complete havoc on almost all broadleaved evergreens and many other plants over vast swaths of the mid-south to the deep south. Photographs of entire landscapes as far as you can see with dead Schipka laurels and Japanese privet Sad. Most should recover it will just take time and patience.
This was the coldest airmass to affect our area since January 2017.
Temperatures in NW Oregon dropped to the upper teens to near 20ºF in conjunction with strong continuous east winds of 25-40mph with gusts to 60 mph that ferried arctic air directly into the region. This caused a shocking amount of damage to plants in the Portland Metro. Arctic air is not only cold it is very dry, this forces plants to transpire water at a higher rate at the same time the ground is frozen and the plant cannot take up water to reenforce desiccated cells. The plant loses water but cannot replace it. Along with actual subfreezing temperatures this can cause damage that seems far worse than the low temperature would suggest. That low of 20ºF was achieved on December21 and 22nd.-during that 72 hour stretch the wind gusted continuously over 35 mph and the temperature climbed no higher than 25ºF. So, not only were plants being freeze dried by the wind they had no warmer daytime recovery. This causes a lot more stress.
On top of that .18″ of ice coated 1.5″ of sleet. Many rosette forming slightly tender plants showed damage from this combination of factors which was much more severe than an ultimate low of 20ºF would suggest. In my years of observation temperatures below 28ºF with winds of 20 or more miles per hour is when you begin to lose hardiness at a greater rate than the actual temperature. Just like the windchill chart for humans, the effects of lower temperatures plus higher winds leads to exponentially more plant damage and the loss of hardiness zones. The damage in my garden bears that out. For plants from drier climates such as Agaves fast drainage was essential and the addition of very low temperatures and a continual layer of freezing rain proved fatal for many as their thaw was followed by rot.
Canary in the coal mine
One plant that is particularly adept at showing the effects of subfreezing wind is native to New Zealand. Cordyline australis is common on the coast and now inland, In Eugene which has virtually no subfreezing wind I regularly observed Cordylines undamaged down to about 12ºF in a very calm environment. Below that temperature they would begin to droop and list flop over several weeks after the freeze when real damage is apparent. In my neighborhood in North Portland all of the Cordyline australis have frozen and gone limp where the meristem turns green, they are flopped over showing signs of freeze damage. . Many of these were very large specimens often multitrunked and above the first story of homes. This was the product of an unprecedented string of warm winters in W. Oregon. Most of these were actually planted as annuals, but have persisted and thrived during this warm stretch. They are so large and established now that only tip damage has occurred and they are all likely to recover in the next growing season.
Strong winds erase zones
The coldest temperature I recorded at home and in my neighborhood was 19ºF for several consecutive hours and below freezing for 74 continuous hours. These local Cordylines are showing the same amount of damage I observed at 12ºF in wind free Eugene. The wind had removed a little over one half zone of hardiness. So, even if you did not record temperatures below 20ºF (Zone 9a) plant damage is manifesting as a half zone or more colder. Thats how you get a zone 9 freeze and zone 8 damage. Subfreezing wind. Its a huge factor in plant hardiness.
La Nina continues to roar
We are experiencing one of only several triple La Ninas, since 1950- that is three La Nina’s in a row, Usually La Nina is observed every 3-5 years but this one has persisted for three surprising years. For more information Google Triple La Nina. La Ninas are associated with colder, wetter, snowier winters in the Pacific Northwest. So far the temperatures have born out the colder conditions as November/December and now February appear on track to be below normal. Uncharacteristically, rainfall in the lowlands is slightly below normal. The NOAA Climate Prediction Center follows the La Nina code, they continue with below normal temperatures into early April. February through April 2023 predicted temperature outlook:
Precipitation in the next three months of ending in May is about as average as this map can portray for the PNW. Very little out of the average. Over the entire country below average in the SW is on track but that is also the driest time of year there. There is also a bulleted zone over the upper midwest with a strong prediction of wetter than normal.
Almost every climate model is showing the demise of La Nina by April. if you like sunny warm springs thats good news. The national climate prediction center suggest normal to above normal temperatures for the second half of spring with average precipitation: So, if you are sick of the cool doldrums of winter chill and wet and darkness there is more than a strong indicator that warmer conditions are in the cards. Check out the current prediction for next summer. Whew! Remember that La Nina actually causes the earths climate to cool slightly – though global warming is having an affect on this mitigating it somewhat. In a No Nino we lose the cooling effects of La Nina and we’re exposed to the full aspect of climate change. June, July, August 2023 Seasonal temperature outlook is above average for the whole country:
The remains of winter
This fact cannot be repeated enough the possibility of arctic air literally plunges by mid February. It can still freeze, and it can still snow but the chances of truly arctic air (below 20ºF) plummets. This is one reason gardeners are advised to wait until mid-february (Presidents Day) to prune many plants most importantly roses. It doesn’t appear that the remainder of this month is going to vary much. Long range forecasts are hinting at more snow and sub freezing temperatures next weekend. This weekend will be pleasant. By March we should all be crossing our fingers for the demise of La Nina. She’s clearly overstayed her welcome.