Lee Cyclogenesis

#2618 "Flurry of Saturday Afternoon Activity"
10x10 inches depicting the weather of
an Alberta Clipper, February 2022

I no longer look at the weather as closely as I did. I spend more time painting now. But these Blog exercises allow me to revisit the meteorology and maybe relearn it better. Being retired, I can take that time to really savour the science and the beauty of how nature works. My goal is to spread that joy of nature to others. Appreciation of the natural world is the first and most important step in its preservation. 

Knowing the weather requires knowing the wind. We have been aiming at understanding the wind for the past couple of months. 

To quickly recap, we explained how the pressure gradient, Coriolis and centrifugal forces create wind in the free atmosphere on the spinning Earth. We added in the force of friction to better understand how winds move near the surface. Finally we spent a couple of week’s figure skating and conserving spin while moving that wind over mountains. We arrived at a better appreciation of why there is a ridge of high pressure over and upstream from those mountains and a trough of low pressure downstream. And that leads us to the lee cyclogenesis of storms and some very important weather. 

Lee cyclogenesis is a very reliable forecast when strong winds cross the Rockies… nearly perpendicular. The storms that result are determined by where the jet stream crosses those mountains and are typically named by the location of the subsequent lee trough. You have most certainly heard of Alberta Clippers, Colorado, Texas and Gulf Storms. The meteorology behind every storm is unique so that these averages are just my generalizations. If you put five meteorologists in a room, you are likely to get six opinions. Mark Twain might have said that “all generalizations are false, including this one” but generalizations can be useful so let’s continue. 

You might be surprised to appreciate that our weather is shaped by what happens over the Pacific - the El Niño-Southern Oscillation (ENSO). The typical jet stream locations in La Niña years are the black line generalizations in these graphics. The preferred La Niña locations of the jet stream flows are in dark blue. 

In El Niño years, the mid latitude polar jet stream can be diverted by a large, warm and dry ridge of high pressure. This large ridge can spread mild winters temperatures all the way eastward to Ontario. The southern, subtropical or Pacific jet stream is directed more across the extreme southern US in the El Niño phase. The science behind ENSO is extremely interesting and important story too but best left for another day. 

El Niño-Southern Oscillation (ENSO) and
Jet Stream Location Generalizations

The average path of the jet stream is revealed 
from the temperature anomaly for the month.
 Remember to place
your left hand in the cold blue
and your right hand in the warm red
and you are looking
in the direction of the wind.  The 6th warmest 
February on record was actually cold 
over eastern North America. 

This past winter was characterized by the La Niña phase of the El Niño-Southern Oscillation (ENSO). The jet stream is variable in location but is typically centred over the mid latitudes from southern British Columbia and Alberta to Colorado. This should be no surprise given how many Alberta Clippers raced across the Prairies and the Great Lakes Basin this past winter. From my vantage in eastern Ontario, it was a refreshingly typical cold season full of cold, snow, ice and wind... and it is not over yet. 

You might also be surprised that this cold pattern over eastern North America is the direct result of Global warming. I have explained this process many times but not today. The climate is changing dramatically as a result of human activity. In the short term, eastern Ontario will be spared from the baking inferno to the south and west and also receive precipitation. This cold and snow of winter can be a good thing. 

The equatorial sea surface temperatures were below average across the eastern Pacific Ocean (La Niña) this past winter and the odds are 53% that this pattern will continue into the Northern Hemisphere summer ... but that is another story. Through my research in performance measurement, I discovered a pattern that supercellular convection (the kind that is responsible for almost all severe thunderstorm events) was favoured over pulse type thunderstorms during the La Niña phase of ENSO. There are very good meteorological reasons for this but I needed more time and data to be certain of those facts and that is another story as well. So many stories... so little time. 

Next week we can bring an end to winter with a revisit to Alberta Clippers and why they are so very important. 

Warmest regards and keep your paddle in the water, be safe,

Phil the Forecaster Chadwick