Day 81: Abnormally Quiet Tornado Year (So Far)

Posted by pmarsh on 22 Mar 2010 | 2 comments

If you have thought that this year has been unusually devoid of severe thunderstorms and tornadoes, you’d be correct.  This has been an incredibly quiet year.  In fact, based on the SPC Preliminary Storm Report page, up through today, 10 days did not have a single report of severe (convective) weather.  (Please note that these are preliminary reports.  Official numbers will be released in the publication Storm Data a couple months after the event.  Verification takes time…) The no severe weather report days are,

• 2 March 2010 to 6 March 2010 (5 days);
• 14 March 2010 to 18 March 2010 (5 days).

Also, there have been only 14 tornadoes reported in March, compared to an average of around 150.  March tornadoes were reported on

• 8 March 2010 (2 tornadoes);
• 10 March 2010 (7 tornadoes);
• 11 March 2010 (4 tornadoes);
• 12 March 2010 (1 tornado).

The image above was created by the Warning Coordination Meteorologist, Greg Carbin, of the Storm Prediction Center (SPC).  It is the inflation adjusted annual tornado running total and percentile rank.  Please click here to understand the inflation adjusted process.

• The red line was created by taking the maximum number of tornadoes reported on any given day.  In this regard, it can be thought of as the worst case scenario (if you don’t want tornadoes).
• The orange line was created by taking the 75th percentile number of tornadoes reported on any given day.  In this regard, it can be thought of as the upper-bound on “average”.
• The green line was created by taking the 50th percentile number of tornadoes reported on any given day.  In this regard, it can be thought of as the average.
• The blue line was created by taking the 25th percentile number of tornadoes reported on any given day.  In this regard, it can be thought of as the lower-bound on “average”.
• The magenta line was created by taking the minimum number of tornado reports on any given day.  In this regard, it is the best case scenario (if you don’t want tornadoes).
• The black line is the running total for the current year.

The current year, while below average, is still within the envelope of possibilities; although, edging very close the the minimum values.  In any event, the data shown in the graphs suggest that as we head into April, the number of tornado reports should begin to increase rather quickly.

So, what do the last days of March have in store?

The image above (and annotated below) is quickly becoming a staple on this blog.  It depicts the amount of moisture in the atmosphere, as derived by satellites, at 2300 UTC on 22 March 2010.  In the wake of this past weekend’s significant southern plains winter storm, northwest winds have swept dry, continental air out over most of the Gulf of Mexico.  Immediately ahead of this storm, southerly winds are drawing up moist air into the storm, but this moist air is continually being advected away from the Gulf of Mexico as the storm continues to move east.  Also of importance is the fact that numerous cold, dry air intrusions into the Atlantic Ocean has resulted in deep moisture being suppressed to areas south of the 10 degree North latitude.  In fact, the tropical easterlies across much of the Atlantic is transporting dry air, instead of the moist air typically contained in the tropical easterlies.  This is readily apparent when one compares the moisture values of the Atlantic to those in the Pacific at a given latitude.

Why do I pay so much attention to moisture?  Because it is one of the necessary ingredients for deep convection (thunderstorms), and deep convection is a necessary ingredient for (true) severe thunderstorms.  It also plays a crucial role in the development and evolution of surface pressure features, which help dictate our weather.  Check out my Day 79 post to get an idea of just how important rich moisture is to developing cyclones.

With repeated intrusions of cold, dry, continental air into the Gulf of Mexico and Caribbean, the atmosphere is unable to generate sustained areas of deep, rich moisture.  This is detrimental to the formation of long-lived thunderstorms which often go on to be the kind of thunderstorms to generate severe weather.  Unless the pattern changes sufficiently to limit the number of cold, dry air intrusions into the Gulf of Mexico, severe weather should remain quite hard to come by.