One year later, a look back at the Pacific Northwest’s 2021 Heat Dome

 The following article was written by Michael Fagin. Fagin is an operational meteorologist providing weather forecasts to clients in the Pacific Northwest and providing custom forecast for groups climbing Mt. Everest and other major peaks. Michael is also a travel writer with a focus on weather and wine.

Anyone who lives in the Pacific Northwest remembers the scorching heat of late June 2021. Poor Seattle, where an estimated 44% of homes have air conditioners, had three consecutive days of temperatures 100+ degree Fahrenheit from June 26th to June 28th. June 28 had a new record of 108 degrees, which is an astonishing 34 degrees above normal.

Of course Puget Sound is not in Washington’s main wine growing region. So how hot was it in the Columbia Basin? Hanford, near the center of the basin, officially broke the record for the hottest day ever in Washington at 120 degrees on June 29.

How about Oregon? Portland also had three consecutive days of 100+ degrees with an all-time record of 116 on June 28. Salem, in the middle of the state's main growing region, had a record of 117. Oregon tied an all-time high of 119 at Pelton Dam in eastern Oregon.

Why was it so hot?

What caused the extreme temperatures? There were several things going on.

First, there was an exceptional, and perhaps record-breaking, strong ridge of high pressure in the upper atmosphere (see map, photo credit). Under these conditions air sinks and compresses and, according to physics, the air mass warms as this happens. Furthermore, there were southeast winds at times that brought in dry, warm air. The bottom line, this all contributed to the record breaking heat.

Another reason for the record heat? Willamette Valley and Puget Sound usually get westerly winds that bring cool marine air at times and a cooling fog. Great for the Pinots!

However, during the record heat there were easterly winds at the surface and aloft. As this air from the Cascade crest is forced toward Willamette Valley and Puget Sound, physics once again dictates that when air sinks it warms up. Thus, there was the perfect combination of strong upper-level ridge (aka heat dome) and the east winds leading to record temperatures.

Right after this heat wave, some growers gave a sigh of relief. They had a wet spring which helped to provide much needed water for the vines during the heat wave. Also the heat was early in the summer, so they did not expect much damage since the berries were small and green. Also, the grapes benefitted from the canopy, which was full of green growth to protect the berries to some extent.

What also took the edge off the record heat in many areas in Washington and Oregon was that there was a cool fall. Looking at Growing Degree Days, although 2021 was much higher than normal, it was not as high as the last warm season of 2015 (Growing Degree Days are average temperatures over 50 from April 1st to October 31st). The graph shown here is for the Red Mountain appellation, which is representative of this trend found in some other Washington regions in 2021.

Were yields impacted? Yes, according to the Washington State Wine Commission. They indicated that the record breaking heat in 2021 resulted in lower yields.

Is this the new normal?

Is June 2021 going to be a regular pattern? There is little doubt that our summers are getting warmer, as the graph indicates (photo credit NOAA). However, there is debate whether this event was caused by global warming.

According to extensive research from meteorologist Cliff Mass, his conclusion and title says “Was Global Warming the Cause of the Great Northwest Heatwave? Science Says No.” Dr Mass makes several key points that brought this extreme event.

First of all, the ridge of high pressure at 18,000 feet “was the most intense ever observed in the region.” This high pressure brings strong sinking of the air mass along with the strong warming associated with this.

Secondly, there was this “supercharger” in action. The supercharger was the positioning of high pressure and low pressure. There was the aforementioned upper level ridge of high pressure over the northwest and a trough of low pressure off the Northern California coast.

This unique position and combination brought strong southeasterly winds that pulled air from the desert Southwest. Then this air mass subsequently descended the west slopes of the Cascades where the air was further compressed and warmed.

“Everything had to come together just right to give us this extreme event,” Mass said.

In concluding, Dr. Mass thought this heat from last June was a “Black Swan” heatwave, given the exceptional atmospheric conditions that occurred. Dr. Mass went on to say “global warming marginally increas[ed] the peak temperatures by perhaps a few degrees.”

Meanwhile the international organization WorldWeatherAttribution.org (WWA) took a different view and issued a statement saying “In summary, an event such as the Pacific Northwest 2021 heatwave is still rare or extremely rare in today’s climate yet would be virtually impossible without human-caused climate change. As warming continues, it will become a lot less rare.”

Will this record-breaking heat dome be a yearly occurrence? Probably not. How often will events like this occur? It is not known.

What is known is that events like this could be perilous for the wine industry. Had this event occurred later in the growing season when grapes were more fully developed, the results could have been devastating. Warmer summers also have growers concerned with which varieties will thrive in this type of environment.

Heat events like this aren't the only concern. With summer predictions of above normal likelihood of significant wildland fires there will be concern once again of the risk of potential smoke impact should these fires occur. 

Although much of Washington and Oregon has had a cold and wet spring in 2022, there is concern that we get dry and warm conditions in July and August. After all, this has been the default pattern recently. In conclusion, the wine industry in Washington and Oregon will continue to be concerned with these summer weather conditions in the years to come.

Sean P. Sullivan

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