Tuesday, January 29, 2019

Baby, It's Cold Outside!

I don't know about you but I have pretty much had my fill of the "polar vortex" stuff. Yes, it's real, and meteorologists have known about it for a long time. It seemed to come into popular vogue during the winter of 2013-2014, when a rather sudden onset of cold and snow in late January hammered the eastern U.S. for the next six weeks. Many locations in the northeast experienced the most severe winter on record.

The fact is that these frigid Arctic outbreaks have occurred in many winters, and they were related to changes in the polar vortex. The cold air now overspreading the central and eastern U.S. this week is one of the more stronger cold air outbreaks in recent memory. How bad it is is often a matter of your experience and perspective. I was living in central Illinois during the late 1970s and early 1980s when the state experienced four of its most severe winters on record. I went back and looked at a few of the cold air outbreaks in the past. One thing I soon realized was that many people under the age of 25 or 30 haven't experienced this kind of severe cold.

This graphic from NOAA depicts how the polar vortex affects our weather in the U.S.

Data I looked at was the frequency of low temperatures below zero in Chicago by 10 year periods going back to 1959. I picked Chicago since it seems to be the high population "ground zero"  for this Arctic outbreak (no pun intended). I also looked a runs of days of low temperatures at different thresholds.

As of today, Chicago has experienced only 15 days with low temperatures less than -20°F since 1872. The all-time record low is -27°F on January 20, 1985. There have been 71 days in the record with a maximum temperature 0°F or below. Only six of those have occurred in the past 30 years, the most recent prior to this year on January 6, 2014.

If we look at runs of days with minimums of -10°F, the longest is seven days from January 14-20, 1995. The longest run of days of -15°F or lower in Chicago is four, December 22-25, 1983. In general persistent, severe cold is not that common in Chicago in the past 20 to 25 years.

This chart shows the frequency of different thresholds of low temperatures in Chicago for ten years periods starting with 2018 and back to 1959.

Frequency of minimum temperature below 0°F for different thresholds for Chicago, IL

What this shows is that the frequency of temperatures below zero is 42 percent less in the past 30 years, compared to the 30 years prior. The frequency of low temperatures -10°F or lower is 61 percent less for the same periods.

So, those under 35 years of age in the Midwest likely have experienced far fewer days with the extreme cold that is knifing through the U.S. this week. Record lows temperatures will be set, and no doubt a few all-time record lows will topple as well. The frigid air combined with the strong, gusty winds driving this cold south will produce dangerous wind chill readings that few have experienced. This cold will affect a significant portion of the eastern U.S. population. This map from Ryan Maue of Weather.US shows the extent of the cold through the next five days, and the percentage of the lower 48 states experiencing the cold. He estimates almost 91 million people will experience temperatures 0°F or lower, and 25 million will experience temperatures -20°F or colder.

Finally, this plot shows the trajectory the air is taking into the U.S. from its source in the Arctic.

Stay warm, everyone!

1 comment:

  1. in the late '70's while i was living near Lexington Ne we set a record for days in a row below zero