What a difference a year can make.
Last winter brought record warm and dry weather, which was the start of a drought. This winter has been considerably colder and more snowy. And the winter has dragged into spring, possibly enhancing the flood risk. What happened? Below is a brief explanation as to the reasons why the two winters are so different, and how unusual is this?
Large Scale Climate Signals
The first thing we need to consider is what the upper air patterns were like in the late winter and early spring of 2012 versus 2013.
Below is an image showing the "Jet Stream" winds flowing around the Northern Hemisphere during the January - March 2012 and 2013 periods. During the late winter and early spring of 2012 the Jet Stream was stronger over the Eastern Hemisphere and across the Gulf of Alaska and northern Pacific. For the same period in 2013, the Jet Stream, while it has been weaker, has been a bit farther north allowing the colder air to develop and move farther south. Typically when the Jet Stream in farther north the colder air remains farther north. In this case the northern shift of the Jet Stream more frequently pushed stroms across the northern plains. With more frequent northern storm systems, colder air was readily drawn south from Canada. This is another example of how unusual this winter has been. Perhaps the largest difference is the overall weaker Circumpolar Jet in 2013, associated with the more consistent and more negative Artic Oscillation (AO). While the AO was negative in the 2012 winter and spring, it has been more negative for the same time period in 2013.
Average 2012 January to March departure from normal (left) and 2013 January to March departure from normal (right) wind speed (anomaly) at 250mb for the Northern Hemisphere. Click for a larger version. Image provided by the NOAA/ESRL Physical Sciences Division, Boulder Colorado from their Web site at http://www.esrl.noaa.gov/psd/
Below is an image of the 500mb height anomalies during those two periods. On the left is the average departure from normal heights in the mid levels of the atmosphere for January to March of 2012, and the right image is the same period in 2013. The most significant difference for our area is the fact that last year a large upper ridge was centered over the Ohio Valley with low pressure over the Alaska region. For January to March of 2013, there was high pressure over the Gulf of Alaska and low pressure over the Great Lakes region. During 2012 the upper circulation around the upper ridge consistently brought much warmer air to the much of the United States. By contrast, this year the circulation around the high in the Gulf of Alaska has helped bring colder air into the northern plains. As indicated for both winters, the AO was in a negative phase. However, during the late winter of 2012, the colder air was shunted into the Eastern Hemisphere. This year, the colder air from the polar latitudes has been driven southward into our region. A deeper snow-pack has helped regenerate and keep the cold air in place. .
Average 2012 January to March departure from normal (left) and 2013 January to March departure from normal (right) 500mb height anomaly for the Northern Hemisphere. Click for a larger version. Image provided by the NOAA/ESRL Physical Sciences Division, Boulder Colorado from their Web site at http://www.esrl.noaa.gov/psd/
How large an area is covered by this colder air?
Typically the near surface temperature patterns mimic the upper air patterns in the long term. During the January - March 2012 period the coldest surface air was consistently located over Alaska (under the upper low) and across much of the Eurasian continent., again under the lower atmospheric pressures. In the high polar latitudes, the warmer air was shunted south into the U.S. mainland under the Ohio Valley upper ridge. During the January - March 2013 period the coldest surface air has been farther north of the Siberian region and far northern Canada. The circulation around the upper system has driven the colder air into the U.S., with the axis of coldest air into the Northern Plains. As earlier indicated, the deep snowpack across much of Canada into the northern plains has helped regenerate and sustain the colder than normal airmass.
Average 2012 January to March departure from normal (left) and 2013 January to March departure from normal (right) surface temperatures for the Northern Hemisphere. Click for a larger version. Image provided by the NOAA/ESRL Physical Sciences Division, Boulder Colorado from their Web site at http://www.esrl.noaa.gov/psd/
Average 2012 January to March departure from normal (left) and 2013 January to March departure from normal (right) surface temperatures for the North American Region. Click for a larger version. Image provided by the NOAA/ESRL Physical Sciences Division, Boulder Colorado from their Web site at http://www.esrl.noaa.gov/psd/
Is this really that unusual?
Day to day variances in the temperature, precipitation, clouds, wind speed or direction is what we call "weather." When we string together several years worth of "weather" we call that climate. Everyone generally understands that both the weather and climate are variable. In describing the climate we use statistical terms such as normal, or average in an attempt to describe what one may expect on any given day. However, what the statistics cannot show is what is really extreme, and what is a "normal range" of variations. To that end, it is sometimes difficult to fully quantify "Is this weather extreme?" or "Is this normal?" Without teaching a class in statistics, we can show what an expected range in temperature is as an example.
Based on the 30 year climatology based on the data from 1981 to 2010, the "normal" high temperature on April 10th at the University of North Dakota/NWS climate station is 51 degrees. If we dig deeper at those 30 years, we see that a "normal range" of temperatures is actually anywhere from 40 to 62 degrees! This is based on a technique called the standard deviation, a statistical tool that shows what is the expected spread in a set of data. So, the high temperature on April 10th 2012 was 36 degrees, which is below the normal range. However, 2 days later on April 12th 2012 the high was 68, a 32 degree swing in 2 days. That was then above the expected range and typifies the "weather" during a northern plains spring. Both the extremes, while unusual, are not unexpected or unprecedented.
Another technique is to look at the latest, or earliest day some weather element was observed. The first 1 inch of snow, or the latest 50 degree day. That perhaps better captures how extreme a particular weather pattern is. Let's look at the last 50 degree day in Fargo, as an example. Based on the data available, the temperature did not reach 50 degrees or better until April 17th in 1881; second place is April 16th in 1979. So far the high temperature in Fargo has yet to reach 50 degrees, and based on the forecast (as of April 10th) this record may be broken. If Fargo does not exceed 50F by next Wednesday, that would be unprecedented.
What about the Future?
The most asked question now is when will the late winter weather end, and spring begin? While the overall pattern has remained the same into April, there are signs that it is beginning to change, which may well bring warmer weather to the region. The best way to keep track of the forecast is via our web-site at www.weather.gov/fgf
For more information please contact Mark Ewens, Climate Services Focal Point at 701.795.5198 or email Mark ar Mark.Ewens@noaa.gov