Cold + Snow in March = Crabby People. Why is it so cold?

If you are like the many people I've heard from in the past week or so, you are very sick of the cold and snow that the area has seen over the past few weeks.  Many people are longing for the warmer and snow-less days of spring that typically begin around this time of year.  However, we have been stuck in a cold and snowy pattern that doesn't seem to let up.  This has led to many crabby people around the region!  And it has left many wondering why.

Negative and positive phases
of the Arctic Oscillation

It all has to do with the Arctic Oscillation (AO).  The AO is an index of sea-level pressure variations north of 20°N latitude and is either given a positive or negative value.  This index is directly related to the degree to which Arctic air can penetrate into the mid-latitudes, where we live here in the Midwest.  When the AO is positive, this symbolizes low pressure in the Arctic.  This allows the mid-latitude jet stream to blow strongly and consistently from west to east in what is known as a zonal or near-zonal flow.  This keeps the cold Arctic air locked into the polar region.  However, when the AO becomes negative, this means that there is high pressure in the polar region, resulting in larger swings of the jet stream (much less zonal) and allows for greater movement of very cold polar air into the mid-latitudes.  These large swings in the jet stream also result in strong storms sliding along the stream which can produce several inches of snow at a time.

Well, right now, the AO is negative.... very negative.  As the chart below shows, the AO is currently at its lowest level of the winter with an index value near -5.5!  This means that the high pressure over the arctic is strong, and is sending plenty of cold air southward into the eastern half of the nation (as you are probably well aware of)!

The Arctic Oscillation Index during the 2012-2013 winter months. The black line is the observed number, while the red lines are the predicted values for the near future.

Luckily, the AO index is predicted to rise dramatically through April 1st, which, if correct, means that the cold air will be retreating back into the polar regions and more spring-like air will move into the region.  Rejoice spring lovers!  Granted, it does not mean a complete reversal of the pattern, but it will mean that the cold conditions we are seeing across the eastern half of the country will ease up.

The April temperature outlook (left) and precipitation outlook (right) released by the Climate Prediction Center on Thursday, March 21st, 2013

And the April outlook released today from the Climate Prediction Center tends to agree.  They are anticipating slightly above average temperatures for the eastern and southern portions of the United States during the month of April, with below normal temperatures along the west coast.  And if you are wondering about precipitation, it appears the Great Lakes is expected to see above normal amounts of precipitation while the coastal areas may see below normal precipitation.

So it appears that the month of April should be near normal temperatures but wet for the Midwest.  So, yes spring lovers.  Rejoice.  Spring is on the way!

 

Time for a bigger rain gauge!

Wait…. What?  We’re in the middle of a drought and you’re talking about getting a bigger rain gauge?  What’s going on here?


The map shows location of selected rain gauges,
with blue (red) triangles depicting sites with
significant increasing (decreasing) trends,
and white circles showing sites
 with little or no change.
(Credit: Adapted from Villarini et al. (2013))

Even with the current drought situation plaguing much of the Midwest, some of you may actually want to consider getting a bigger rain gauge if you enjoy weather-watching.  Why?  Well, heavy rain events are becoming more common!  A study by the University of Iowa recently published in the Journal of Climate suggests that heavy rains have become more frequent in the Upper Midwest over the past 60 years, holding true even with the current drought situation.  The study also found that temperatures over the center of the nation are rising too, which helps to explain the heavy rain issue.  Warmer air can hold more water vapor, and when more water is available in the atmosphere for precipitation, a greater chance for heavy rains exists.

Lead author Gabriele Villarini stated “We have found that there is a tendency toward increasing trends in heavy rainfall in the northern part of the study region, roughly the Upper Mississippi River basin.  We tried to explain these results in light of changes in temperatures.  We found that the northern part of the study region – including Minnesota, Wisconsin, Iowa, and Illinois – is also the area experiencing large increasing trends in temperature, resulting in an increase in atmospheric water vapor.”

However, the current drought that has affected the Midwest is not part of the study.  It has occurred too recently (over the past year or two) while the study focused on rainfall and temperature data from about 1950 to 2010.  Also, it should be noted that the study was not looking at average annual rainfall, but rather just heavy rainfall events.  So while the area may currently be experiencing a deficit in overall rainfall, the area may be in the normal range when it comes to the number of heavy rainfall events/days.

In addition to the rising temperatures adding to the water vapor in the atmosphere, other studies point to changes in irrigation over the Ogallala Aquifer (which runs from Nebraska to northern Texas) as a source of increased water vapor in the atmosphere.  This suggests that human changes in land use, land cover, and agricultural practice also contribute to the increased frequency of heavy rains.

So in our warming atmosphere, you can expect to see more heavy rainfall events across not only the Upper Midwest but possibly the globe as a whole.  This will also increase the frequency and risk of flash flooding in the area as heavy rains quickly flow into area streams, creeks, and rivers before the ground can absorb the moisture (for those of you in the Coulee Region, think back to August of 2007 when parts of the area received over a foot of rain in about 2 days).  Keep this in mind if you live in or plan to move to a flood prone area in the future so you are aware of the risk.  It also raises the importance of early warnings from the National Weather Service and local media.  I will be going over the meanings of the warnings, watches, advisories, etc. that the National Weather Service issues coming up in April during Wisconsin’s Severe Weather Awareness Week (April 15^th – 19^th), so watch for that.  It never gets old going over these important messages since they save lives!