Friday, November 03, 2023

Winter 2022-23 Recap - Part III - The Drivers

Part I and II recapped the temperatures and snowfall for last winter. Why did these temperatures and snowfall occur?  What were the individual drivers of each period last winter?  I am sure I will miss some features in this long post so let me know.  We start in October with our first snowfall of the season.

Strong cold front with a ton of vorticity spun up over the Great Lakes.  Temperatures at 1000 feet off the ground (850 mB level) dropped to between -4 and -6 the night of October 18th.


850 mB temperatures

500 mB height anomalies were extremely low and deep

500 mB height anomalies

Cold temperatures were widespread across the central US and Ohio Valley

This resulted in light accumulations across the eastern 1/2 of northern Ohio

By November, the pattern became more variable.  First 10 days of the month featured well above normal temperatures followed by a sharp cold spell then another milder stretch around Thanksgiving.

The pattern across the country was about as polar opposite west to east as it can get during the first half of November. HUGE ridge and warmth across the east, trough and cold west

The pattern flipped the second half of the month. 

The first week of November was incredibly warm and snow free.  By the middle of the month, temperatures averaged in the 30s for 10 straight days with frequent snowfall.

The last 7 days of November temperatures moderated.  Snowfall was sporatic

Around Thanksgiving, there were indications that the middle/third week of December was trending MUCH colder than normal and the cold would be fairly widespread. A precursor to the extreme cold which invaded the central US the week of Christmas.  My tweet on November 22 indicated this. The duration and day-to-day intensity of the cold was not known at that time. 

December 12 projections started picking up on the Christmas cold.

By December 14, I drew up long range model comparisons to the December 25th cold in 2000.

By December 19, the cold signal for Christmas week was very strong stretching across the continent.

Our forecast 5 days before the cold!

Cold extended into central Florida

Progression of the cold across the central US

Snowfall forecast for northern Ohio

Wave height on Lake Erie

December temperatures were overall below normal with a quick rise by New Years Day.

The ridge in the Pacific shifted north of the Arctic Circle. Trough developed across the Great Lakes and then the COLD. Strong Greenland ridge developed.

The Northern Atlantic and Arctic pressure pattern were strongly negative as the Greenland ridge strengthened. (Blue bars below)

Arctic Oscillation Phases

North Atlantic Oscillation Phases

The Greenland block historically correlates to colder temperatures across the eastern US. This matched up with late December.

By January, a HUGE northern Pacific ridge developed locking in a milder pattern across the eastern US

The polar vortex rapidly strengthened in January.  By early January, the pattern was showing signs of what we experienced in November. Fast moving systems with frequent changes in the temperature with little extreme cold. 

Greenland Block (ridge circled in black) weakened for most of January and February.  NAO shot up.

Temperatures across much of eastern North America and US went well above normal.  Here is the progression from late December through February.

Overall temperature ranks over the two month period were top 5 across much of the eastern US.

January temperatures vs normal for Cleveland.  Notice most days were in the 40s and 50s.

February temperatures vs normal for Cleveland.  More 50s and a few 60s.

December through February were some of the cloudiest winter periods on record for parts of the Great Lakes and New England. H/T Brian Brettschneider

However, by March the Greenland Block resurfaced.

The Madden Julian Oscillation (MJO) quickly migrated into phases 7, 8 and 1 with off-the-charts magnitude. The forecast was indicating this in late February.

Actual MJO verified just as strong.  These tropical changes strongly hinted at a colder March eastern US with an active storm track.

MJO phase and magnitude in March

In summary, the colder March pattern shift was the result of many factors? First since mid/late February, the temperatures in the upper atmosphere had been warming over the North pole. The sudden warming in the stratosphere over the pole translates to high pressure over the north pole at the surface with colder temperatures and often a weaker polar vortex. (

Great graphic below showing the warming in the upper atmosphere from November through March. As the polar vortex weakened, the displacement propagated down into the lower atmosphere first in late December. It rapidly strengthened in January. Then a late February warming with a downward propagation in March and again in late April

Polar Vortex Behavior from Nov to April (red indicates weakening--colder)

These changes high in the atmosphere over the pole propagate to the lower levels and further south into the tropical regions. These changes intensified the MJO (Madden Julian Oscillation) as it transitions out of warmer phases (2-3-4 much of the winter) to colder phases (8-7-1). (see MJO images above)  The jet stream responded with more variability off of eastern Asia east over the northern Pacific and into North America.  This has resulted in high pressure developing near Alaska and low pressure undercutting north of Hawaii Greenland high pressure which reinforced the undercutting storm track strengthening the jet stream and the trough as it moved east into the Ohio Valley along with colder air.  

Overall March temperatures vs normal.  Notice little relative warmth.

All of this occurring in a slowly decaying La Nina along the equatorial Pacific. Blue colors (cooler water)

If this late season pattern shift would have occurred 6 weeks earlier, our winter would have been MUCH different:  COLD and more SNOW in January and February.