Too damn hot.
Meteorological summer is defined as June-July-August according to the National Oceanic and Atmospheric Administration. This past summer will go down in history for two main reasons. The June heatwave was a historical event that exceeded EVERYTHING we’ve recorded here in 150 years. The colossal heatwave of June 25-29 was an event that most meteorologists would have thought impossible. In fact it was so extreme that world climatologists have had to re-think how, where, and how severe heatwaves can become. More importantly it gives us a scary view of the future.
Heat domes have been occurring more frequently around the globe. They are cut off high pressure systems from the jet stream that sit and essentially bake. This is most likely from a jet stream of decreased intensity brought about by higher global temperatures. The jet stream which is powered by the temperature gradient between the equator (hot) and the Arctic (cold) and has begun to show this lack of propulsion. Especially during the warmest months. The weather in essence stalls. Our heat dome was wildly unusual for a lot of reasons, the most conspicuous was for its intensity. An intensity of high pressure that could only be possible with human induced global warming and it caused the mercury to soar. On June 26 we hit 108ºF- that eclipsed 150 years worth of local records. The 27th we warmed to an astonishing 112ºF. and on the 28th we peaked at PDX at an almost unfathomable 116ºF. To put that in perspective its not only 9 degrees above the all time record and the all time record was broken three consecutive days. Local high temperature were beyond extreme. This led to astonished climatologists.
High Temperatures in the Portland Metro Area. on June 28, 2021
PDX 116ºF Troutdale 116ºF
Salem 117ºF Beaverton 115ºF
Hillsboro 115º Lake Oswego 114ºF
116ºF (put this in more perspective) exceeds the all time temperature for some notable hot cities and comes close to the hottest of some famous heat centers. Sacramento, Tucson, Dallas, El Paso and Atlanta are just five hot cities that have not exceeded 115ºF. And the all time highest temperature in Las Vegas is just 117ºF- one degree shy of Portland’s record and a straight tie with Salem’s all new all time peak. This is astonishing.
Recently, heat domes have been more conspicuous in Europe and Asia and less obvious in North America. That may be about to change. So far global warming has shown an increase in temperatures on the west and east coasts of North America. The central part of the country has seen status quo or slightly lower temperatures. This does not refute warming. In fact, the increasing temperatures of the Gulf of Mexico are pulling moist and therefore, ocean cooled air north from that source. As low pressures dive over a semi-constant high pressure ridge in the west they slide down towards the gulf. This draws moisture north to the interior of the continent. Therefore the central U.S. is not seeing our precipitous temperature spikes and it is seeing an increase in precipitation.
This situation highlights the most dangerous part of this heat regime. Drought. Drought which has been increasing across the entire west has lasted more than 20 years- a mega drought with little end in sight and the worst consequence of drought increasing mega fires (100,000+ acres). Droughts can become self reinforcing. The air is further heated (much like a metal frying pan) as the ground loses moisture. In fact our mega drought has reached a new and scary point. There is so little moisture in the southwest and west now that the suns heat which usually evaporates water from the soil, and somewhat slows daily heating, has gotten so severe that that process is over. The suns energy is going directly into heating the dry ground and that makes it possible to attain higher temperatures during the day and at night. It also becomes a self reinforcing loop. The drier the ground, the hotter it can get and this reinforces the semi-permanent high pressure ridge, initially responsible for the drought. It will take a huge change in climate regime to bring drought relief to the west. A change more than just several wet winters will cure.
Local flora hated the heat dome
For me the most surprising thing about this extreme heat event was its affect on our native flora. All over the region Douglas Firs, Western Hemlocks and other predominate conifers of our forests had a severe reaction. As an example there is a 85′ mature douglas fir behind my garage. On the fourth day of the heat wave I noticed severe sun scald on the western side of the tree. This was repeated all through out our region. From western Canada to southern Oregon conifers fried and if there was the reflected heat of asphalt or a roof that process was even more severe. Obviously, this increases fire danger but it has a more profound effect on our forests. Douglas firs are native over a huge area of North America from south central Mexican highlands to Central British Columbia so this was surprising. Upon further investigation I noted that Douglas Fir are one of the last native trees to put on new growth. This process of unfurling and climatizing their new growth was in mid throw when our heatwave hit- it was tender and fried. The trees were vulnerable. It also showed the upper temperature limit of many forest trees. Western Hemlock which is predominantly native in areas that exceed 60″ of rain and are cooler suffered horribly. All of this is a very bad indicator for a future which looks extremely hot. We can honestly say that Tsuga heterophylla (Western Hemlock) will see its natural range shrink probably in our lifetime.
Native trees that did do well were not as surprising. Ponderosa Pine, one of the most widespread species in North America sailed through relatively well. Oregon White Oaks which can stand quite a bit of heat were also unscathed. Based on freeway plantings of natives around the city it should be obvious to us which trees should be planted and which are best left to more coddled sites. Some other winners were Pacific Madrone ( Arbutus menziesii) Incense Cedar (Calocedrus decurrens) and Big Leaf Maple (Acer macrophyllum).
On the other hand some plants loved the heat. Crape myrtles and Gardenias have never bloomed so well.
How to garden in heat
Water BEFORE the heat wave. Unlike winter weather heat waves are relatively easy to forecast and with certainty up to a week out. This gives time to prepare. If extreme heat is forecast ( I define that for Portland as two or more days above 97ºF), I begin watering immediately. Plants should enter a heatwave thoroughly hydrated. Do not wait to water flagging plants when the heat has arrived. By all means water but don’t expect the plants to respond as well. In the case of this extreme heat (above 105ºF) I water EVERYTHING in the garden at least once. That includes drought adapted plants such as Manzanita and Grevilleas. One deep soak will not endanger them and it will give them resources to draw from as the heat and dry intensifies. And if you’d like to know just how much to water use your hands and ears. Containers should be watered until they are heavy. If they are really large water until you hear and see bubbles coming up to the surface. In the ground I use my ears too. Water until you can actually hear the ground absorbing water. It will make subtle draining noises that makes sense to the gardener.
LA NINA LA NINA
Probably the best news we could have at this time is the formation of LA NINA! La Nina is the cold/wet phase of El Nino/Southern Oscillation and it has already begun forming in the tropical pacific. La Nina winters are characterized by cooler, wetter conditions in the PNW. They also, increase snowpack and the possibility of an arctic event. So, we’re patiently waiting for fall rains to begin. Give it a week or two.
Xera Plants loves fall planting. Its an ideal time to insert any native or drought adapted plant. We’ll have tons in stock through Halloween.
Happy Fall Planting,