Winter in Portland is a tale of two climates and there are two important facts you should consider when gardening here. The first is that our proximity to the Columbia River Gorge significantly influences our weather and the second, is that winter here may or may not even show up. And statistically that holds true. And climate is nothing if not held hostage by statistics. In the last 20 years we have had a major intrusion of arctic air (lows in the teens or lower with highs in the twenties or lower) once exactly every four years. That is both significant in its symmetry but also how consistent it has been for a surprisingly long time. The truth is we average 7 zone 9 (above 20ºF) winters out of 10 and just three years per decade- for the past three decades that have slipped down to Zone 8 (below 20ºF) Last year was the coldest winter overall (real meteorological winter Dec-Jan-Feb as measured by the NOAA) in 38 years! It was the first winter since 2013 where a real , deep, arctic airmass came and stayed. It was the completely rare case of cold and snow repeating over and over – way out of sync with our normal climate and truly unusual. Time for a real perspective.
Say no to PTSD and garden in the present
It left a kind of gardening PTSD (Portland Traumatic Snow Disorder) that has carried over into this season. Here is a reason that you should get over last years trauma. It was somewhat inevitable statistically but still very rare and likely a one off occurrence. Those same statistics that the majority of our winters are mild upholds this argument well. And this years return to mild weather is a further validation of lasts years rarity. We hadn’t had weather that consistently cold and snowy and icy in four decades and it could easily be that long again before a repeat looms. So, let your trauma fade- status quo is on your side. As a child who grew up here and a complete life long weather hound I can tell you how really difficult it is for snow to fall here. Last year was beyond a fluke, it was a once in a generation occurrence. Snow/ice is by far the hardest thing to predict here and the truth is a very specific succession of events must come together for it to happen at all. This years white christmas was another very rare fluke. Only 6 times in 150 years has snow been observed on the ground on Christmas Day in downtown Portland and it has only been recorded FALLING on Christmas twice. The reality is that in a thirty year period we experience an average only 2.6″ of snow per year- thats spread out over a long time and includes the majority of years where NO snow is observed at all. Last year was a HUGE anomaly and trauma may be complicit in gardeners fearing weather patterns will repeat. No fear, here are the real statistics. Let go of last years garden PTSD and live in the moment. This years refreshingly average moment.
December and January vastly different from last year.
Here’s an example of December’s weather, notice the profound two week warm and two week cold period in the first and second half of the month respectively.
Fourteen days below freezing in December had me second guessing myself around Christmas. Compare that to the similar total of 17 days below freezing in December 2016 and you’d think we were about to repeat last years Cold. However 2018 has begun very differently. In fact, January 2018 has been the second mildest January of all time and the mildest since 2006. Check it out:
A confused year but vigilance
As of now there is a weak La Nina, which forecasters have been very attached to in their forecasts. Each long range forecast has shown a wetter/colder future until that future arrives and models modify the temps and precipitation and winter is held off for another several weeks. For us at Xera it is a constant game of waiting. Our weather tends to operate in two week cycles. Two weeks mild – a ridge (which usually involves fog and cold at the surface- inversion) which makes us tend to not notice them as warm and two weeks cold a trough- stormy, cold, and rainy. How cold we get is completely dependent on the jet stream and its the way it orients the flow of air from the north or northeast. During cold regimes we are more likely to experience an ice/snow event or an arctic intrusion so we watch the weather then very carefully . However, there is a typical end to cold intrusions. The past shows us that real arctic air after February 15 is incredibly rare and has only occurred twice in 100 years of record keeping. The door on cold air slams shut. There’s an old garden adage in Oregon that you wait until after Presidents Day to prune roses in earnest. Apparently wise advice and based on a long standing fact that a hard freeze is then unlikely
Arctic forecasts and evil retrogression
This two week alternating cold/warm pattern is what we watch here at Xera. (Its part of a global pattern that is very complex). A trough of low pressure in the upper atmosphere divides arctic air to the north from warmer high pressure to the south . As the jet stream plows along it produces these ridges and troughs ahead of it. They usually move along with regular alacrity. If a strong trough sets up over the PNW it allows cold air to the south and a naturally occurring dome of high pressure can shift its position in the Gulf of Alaska. If this ridge expands and becomes stable its called a blocking ridge or a Rex Block. This becomes apparent in the forecast as the block of high pressure moves ever so slightly to the west- it retrogrades or moves opposite to the standard flow. This sends the jet winding up and into Alaska/Yukon and then it makes a right hook and spills directly down from the north east over the PNW. Then high pressure east of the Cascades and relatively lower pressure over the ocean sucks cold, dense, dry arctic air through the Columbia River Gorge- a reversal of the more common western, zonal flow. Retrograde highs are the worst as they orient the flow NE to SW and deliver the coldest arctic air directly over us from the the north and northeast. We don’t like the word Retrograde at Xera. Bad word, very bad word. Luckily, with the help of statistics we know that it will happen in a major way about once every 5 years- a long time between events and they luckily are seldom a surprise- with quite a bit of forecast lead time with current models. Lesser intrusions of cold air follow this pattern too but often the cold air is shunted to the east of the Rocky Mountains and delivers us only a glancing blow- a few days of cold air pouring out of the gorge.
What if the jet stream fizzles?
A retrograding high is the set up for a truly arctic air event and while we are in a colder two week scenario there is always the possibility of this arctic door opening. If however, the trough moves through fairly quickly, a warm atmosphere replaces the trough of a cold atmosphere and there is no time for cold air to build. This is our more normal and consistent zonal (west to east) flow. What has been happening in recent years is the prospect of a de-engergized jet stream. That is the typically huge temperature gradient from the poles to the tropics that energizes the jet stream may be failing. The poles are heating up MUCH faster than the equatorial latitudes. This depressed temperature gradient is having a profound implication for our weather. The jet has lost the fuel of a wide pressure gradient and we must prepare for a new pattern- AKA ‘The Lazy Jetstream’. As I mentioned normally ridges and troughs would move through the region quickly as the Jet stream propelled them across the upper latitudes. Arctic air was kept north of this active jetstream corralled as they say and weather also moved along much more consistently.
No denying change is happening
A weaker jet stream blows this all to hell. Weather patterns become static and as ridges of high pressure persist they form huge troughs of cold air downstream. That has been the scenario this year over North America. The Western U.S. has had persistent ridging and the eastern U.S. with a straight north south trough orientation plunging cold air deeper and farther to the south- snow and ice to the Gulf of Mexico not once but three times so far this winter. The actual weather in the Arctic has been far above normal- all the cold air left- drained away with a lack of an atmospheric dam and as nature abhors a vacuum that cold air was replaced by lighter warm air- yet more warm weather in the arctic. The sluggish jet stream is changing the way we experience weather. If you are persistently hot and dry that could continue for an extended time and the opposite is true cold and wet can persist. It seems that once patterns take hold they have been loathe to get on out. Could this have been an ingredient in last years persistent cold? At the moment an example of this is the persistent drought in California. A stubborn, stubborn ridge of high pressure has formed and re-formed over the winter months. Just a handful of storms have made it inland and they are facing a pretty dire return to drought unless things change drastically. A sluggish jet stream will effect everything from flash droughts and heat waves that persist and build to an open gateway to the arctic where cold air simply bleeds far south unimpeded by a healthy intervening rapid jet stream. In essence we could be looking at a sloppy mess. How this is impacted by a more energized, warmer atmosphere that can hold and release more water is just a piece of what is keeping climate scientists awake at night. Its complex. It has great implications from droughts to coastal storms virtually every way the weather is delivered. And I personally have been keeping close attention.
Predictions of an active cold winter evaporate
If I was observing this winter on paper without any other input I would swear that we are having an El Nino winter. Weak, splitty storms and stretches of mild high pressure. Not at all what you would expect for a WEAK LA NINA- (La Nina is the colder and wetter climate cycle) and this appears to have tripped up forecasters this winter. La Nina has been factored into the forecasts but has simply failed to show up. I had a feeling based on this past summer and autumn and tossing in the statistics of arctic intrusion that we would see a repeat of the non-event mild winter of 2015/2016. And as my hunch was correct, that seems to be playing out.
Watching Climate Change in Real Time
In reality we are seeing climate change in real tangible ways. If true it would dovetail with 2017 being the second warmest year ever recorded despite the fact that 2016 was the warmest- inflated by a strong El Nino (warmer Pacific). 2017 was expected to be significantly cooler as the large El Nino had dissolved into a weaker La Nina. The cooler waters were expected to buffer the planets overall fever. Instead, 2017 was shockingly the second warmest year since record keeping began. Could it be that the real news is that the rate of global warming (not just climate change, but warming) has usurped stronger existing patterns and systems. We may have entered scary new territory. I’m sure this will be the subject of quite a bit of research……not by our pathetic denier government. Remember that 17- yes 17 of the warmest years ever recorded have happened since 2000. Yep. Not a good time to deny.
Mildest January since 2006 which was the mildest of all
January has been extraordinarily mild and uneventful. Just TWO days below freezing rather than our average of 12. BTW Portland averages 26 days below freezing per year- this year we are at 16- as I said the majority being in December and not at all arctic. (Our low of 23ºF is right on average for mild years) we typically see a zone 9a winter for four out of five winters. The long range forecasts for February at first showed a hint of cold air but as time has elapsed the models have inevitably trended warmer and warmer. In fact there is a good chance of sunny, dry warm weather for an extended period this month. Weak La Nina aside the most accurate forecast models keep pointing to a warm ridge of high pressure. It will be fun to see what comes to pass. An early spring? A famous False February Spring?
Groundhog Day is passe
So, despite the prediction of the groundhog in two days it appears the climate dice are set for the rest of this winter. Remember that we can still get a freeze up until April 15 and we could even see some wet snow. But real arctic cold is likely a memory. Wahoo.
Here’s to spring!
Happy planting in 2018!