It has certainly been a cold May to this point and indications are that the second half of the month will continue to see more colder than average days than above average days. With the average currently near 70 degrees in many parts of North Dakota now, even a 65 degree day, pleasant yes, would still be below average. Our weather memories tend to be short, although, most of you reading this would likely remember how cold and wet Spring 2013 was in North Dakota. Below is the average statewide May temperature since the late 1800s.
You will quickly notice that many recent Mays have been below average. To emphasize the past decade, below is the average May temperature in North Dakota since the year 2000.
The green line is the smoothing the yearly changes which helps show that this century, the trend has clearly been for colder than average temperatures in the month of May. Many years have been cooler than average, some, like 2002 and 2004 have been significantly below the 1981-2010 average. Even the years with above number temperatures have not been much above normal. Not only has the month of May been below average in recent years, so has spring (March through May) as a whole. Below is the average March-May temperature in North Dakota since 2000.
Because some March and Aprils have been warmer than the average, a few more springs have finished near the “average” line than just May alone, but even springs have generally been running colder in the past decade. Although there are likely many reasons for this, one factor has been the recent shift of the Pacific Decadal Oscillation to the negative phase.
Historically, a -PDO has lead to colder than average springs in the state and this past decade has also leaned in that direction.