Spring and Severe Weather Season are Here

After a very warm start to spring last year, this year's below average temperatures that plagued much of the nation have left many wondering whether spring will ever arrive.  To the right is an image from NOAA depicting the temperature anomalies across the northern hemisphere during March.  Many parts of the eastern two-thirds of the nation saw temperature anomalies anywhere from one to as much as five degrees Celsius below average.  However, areas in the southwestern United States actually experienced above normal temperatures.

The reason for this can be seen in the two images at left, with the top image showing the Arctic Oscillation (AO) and the bottom image the North Atlantic Oscillation (NAO).  During much of the month of March the AO was negative.  It even reached record negative levels during the later part of the month.  When the AO is negative, the frigid air that is normally confined to the polar regions is blocked and forced to sink into the mid-latitudes.  During this time, if the NAO is also negative then the cold air tends to flow into the eastern half of the United States.  This was also the case for much of the month, especially the mid to late portion.

The red lines in the figures above depict the model forecast for these indexes over the next half month.  As can be seen, after one last cold shot of air this week, the AO is forecasted to spike well into the positive numbers.  During this same time, the NAO is forecasted to continue to trend towards neutral to perhaps even slightly positive.  This is a signal that spring is here!  The latest temperature outlook from NOAA also depicts this, forecasting above normal temperatures across most of the country for the month of April, as seen in the image on the right.  Now that spring has arrived however, severe weather will begin to get rolling with it.  The slow start to severe weather season this year was due entirely to the slow onset of spring and now that spring is here, severe weather will arrive with it.

Over the past four days, there has already been 242 severe hail reports, 119 reports of damaging wind, and 6 tornado reports.  While the preliminary count of tornado reports thus far this year remains very low, it is important to remember that this year really is not that far from average in terms of tornado reports up until this point.  In fact, 2011 had only 15 more tornado reports to date.  Last year the severe weather season got off to a very quick start due to the early spring.  However, the early spring led to an early summer and thus an early end to the severe weather season.  This year our spring has started much later but the risk of the onset of summer leading to an early end of severe weather season exists yet again.  This is because if the AO and NAO go positive and stay positive, cold air will be cut off from the United States.  Additionally, a neutral SOI tends to be correlated with lesser amounts of severe weather across much of the nation.  Lastly, if we take a look at the current sea surface temperature anomalies on the left, we see that sea surface temperatures are below average for most of the Gulf of Mexico and Atlantic seaboard.  Taking all of this into account, while there will definitely be severe weather this spring, especially during the month of April, I do not expect this to be an above normal year for severe weather.  However, it probably will not end up being as quiet as last year was overall.  Therefore, the next couple months will likely become active.

That being said, it appears that a severe weather outbreak will be possible next week though the exact placement of the event is still unknown.  Warm air and moisture from the Gulf will begin to surge northward starting later this week and this will ripen the atmosphere for severe weather.  Areas that should keep a close eye on this involving situation range from the southern and central Plains to the Southeast and Midwest.