Monday, January 6, 2014

An analog context for the cold wave

Much of the country is experiencing some of the coldest weather it has seen in many years this week with temperatures struggling to get above zero Fahrenheit and wind chills in the frigid -50 to -60 degree range in some places.  Here's this morning's surface temperature analysis:
Somewhat complicated plot, but I like it because it nicely divides the above freezing (red) and below freezing (blue) temperatres by the color of the contours, giving an idea of the extent of the cold air.  You can see the strong cold front that has been pushing through the eastern United States today as it trails behind a low pressure center that was over southern Ontario this morning.  Frigidly cold temperatures.  This air has been travelling rapidly down from the arctic over the past several days.  Here's a plot showing the air trajectories over the past three days for parcels of air in the lowest 1km of the atmosphere.  You can see that they have been travelling all the way from the Arctic Ocean down to the midwest.

One notable feature of this air transport is that it has been relatively slow to warm up as it plunged south.  The bottom panel of that plot shows the temperature of this air (in Kelvin) as it has moved along (it's read from right to left with the left side being last night).  Some warming of the air has come because the air has descended a bit.  But in terms of sensible temperature it has only maybe warmed about 10 F (difficult to estimate) over its entire journey.  Part of this is because it has traveled over an area that is now entirely covered by snow, keeping the low-levels of the atmosphere relatively cold.  Here's the latest snow depth estimate map from the National Operational Hydrologic Remote Sensing Center (NOHRSC):

You can rest assured that most of northern Canada has a solid snow pack too.  All of this snow worked like kind of a refrigerator, keeping that cold air cold as it moved south.

We can see this cold air mass nicely outlined by looking at the temperatures above the surface.  Just above the surface at 850mb we see from this morning's NAM analysis that we had -30 to -35 Celsius temperatures over the upper midwest:
When is the last time we had temperatures this cold?  One ever-developing tool in meteorology is the ability to search for analogs--that is, to find example patterns of atmospheric conditions in the past that are very similar to what we are experiencing now or expecting in the forecast.  The cold wave of January 1994 is a good analog for our current cold weather outbreak, though that cold wave was a bit longer in duration.  Here's an example of the 850mb temperatures from the middle of that event:
A similar pool of very cold air over the upper midwest.  That's -36 degrees Celsius over northern Wisconsin and Minnesota.  Wikipedia's brief page about this event has some of the following nuggets of trivia:

  • Chicago got down to -21 Fahrenheit with wind chills down to -55
  • Major snowfalls across the eastern half of the US
  • Reagan National Airport in DC had a record low high temperature (for the 20th Century) of 8 Fahrenheit
  • Pittsburgh got down to a record low -22 Fahrenheit
Do these headlines sound at all familiar?  Maybe it's because we're seeing something similar again:
  • Chicago got down to -17 Fahrenheit this morning with wind chills down to -42 Fahrenheit
  • The 11 inches of snow that has fallen in Chicago is the most in a single event since 2011
The bulk of the coldest air is just moving into the eastern part of the US right now.  Given how similar this event seems to be to the 1994 case, we might therefore by analog expect these kind of chilly temperatures for DC and Pittsburgh later. It doesn't look like we'll get as cold---DC is forecast to have a high of 17 tomorrow and Pittsburgh is only forecast to fall to -10---but still, this gives us an idea of what to expect.

Another side of this kind of analog forecasting is that we can use it to try and describe more abstract effects of the weather on lives and property.  Trying to describe the societal impacts of weather is something that the meteorological community still struggles to do well.  We can give you all the numbers you want for how cold it is going to get or how strong the winds will blow, but what does that mean for peoples' health and safety?  We've developed some systems to try and address this---the Saffir-Simpson Hurricane Scale, for instance, relates the wind speed of a hurricane to the type of damage we might expect from such a storm.  But we don't have these kinds of scales for everything and they are not perfect.

By looking at past analogs, we can get an idea of what we could expect based on what happened last time.  For instance, during the cold of January 1994, over 100 people died due to consequences of the cold weather.  Quoting from that Wikipedia article, United cancelled over half its flights, pipe explosions due to freezing cut off water to thousands of homes, Chicago schools closed, and hundreds of drivers in Chicago could not start their cars due to the cold.

What do we have right now?  O'Hare is backed up for four days to get flights out due to cancellations from the snow and cold, Chicago's schools are closed...we'll have to wait to see how many people suffered from exposure or lost their utilities.  My point is that by using analog forecasting we can get a hint of the kinds of consequences of the weather that our models cannot predict by turning to past experiences.  This is a powerful forecasting tool that is only beginning to be exploited...

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