Sunday's setup and November tornadoes

November tornadoes ripped through towns in downstate Illinois as well as south of Chicago, Indiana and Michigan Sunday morning and afternoon.  Severe storms of this nature are not common during the month of November, but they can happen.  Remember the EF3 Caledonia tornado that struck on the afternoon of November 22nd, 2010?  How about the EF3 tornado that ripped through the town of Poplar Grove in January 2008?  Looking back at history, as well as to Sunday, shows tornadoes can happen any time of the year given the right atmospheric setup.

You may have noticed Sunday was an unusually warm and somewhat humid day.  Temperatures were in the middle 60's around sunrise with dew points in the upper 50's/low 60's.  The high temperature reached 69°!  That type of moisture, this far north, isn't typically felt in the middle of November.  So, what caused the rare tornado outbreak across Illinois and the southern Great Lakes?

A very strong, fall storm system brought together the three key ingredients for storms to form: warmth & moisture creating instability, strong wind shear (turning winds with height) and a lifting force, in this case the cold front.  The winds within the jet stream as they came across the Midwest through Iowa and Central Illinois were on the order of 150+ mph!  Usually during the month of November the atmospheric winds tend to be stronger than what they are in late spring and summer.  This is due to the temperatures changing from north to south across the United States.  During the fall and spring there is typically a larger temperature contrast than there is in the summer when temperatures from Canada to the Gulf are a little more uniform.  The sharp contrast is what strengthens the jet stream, therefore creating those stronger winds above.  However, during the late fall and winter months we just don't have the sufficient moisture in place to generate thunderstorms.  This, unfortunately, wasn't the case Sunday morning.  As the strong jet stream winds came into Western and Central Illinois Sunday morning, they interacted with the strong, southerly winds at the surface.  The change in wind direction as well as wind speed with height increases the wind shear within the atmosphere.  This wind shear is what's needed to cause thunderstorms to rotate.  With the wind shear in place along with the instability, thanks to the incredibly high dew points, all we needed was a forcing mechanism to ignite rotating thunderstorms.  Storms were quick to form mid to late morning and continue into Eastern Illinois through the afternoon and into Indiana and Ohio during the evening.

Many have asked if tornadoes can occur in November and the answer to that is YES!  The Chicago National Weather Service looked at data for their County Wide Area (CWA) dating back to 1950 and found that 12 tornadoes across Northern and Northeast Illinois occurred during the month of November.  Most of them were closer to Chicago, but just 3 years ago we had the EF2 Caledonia tornado move through Boone County.  An interesting statistic from College of DuPage Meteorology Professor Victor Gensini indicates that, including yesterday, there have been 194 tornado warnings issued in the month of November in Illinois dating back to 1986.  Of those 194 warnings, 101 (or 52%) of them were issued during Sunday's tornado outbreak!  That's amazing!  Victor Gensini also stated that when looking back to 1950, the strongest November tornado in Illinois was an F3 tornado that occurred east of St. Louis back on November 15th, 1988.  A preliminary survey conducted from the Lincoln National Weather Service office of the Washington, IL tornado has rated that one an EF4.  If that holds true, then Sunday's tornado outbreak will have produced the strongest Illinois tornado in November dating back to 1950!

This is why it's so important not to let your guard down even as the temperatures fall.  It's our job as the First Warn Weather Team to provide you with the most accurate, up to date information so you can make informed decisions when severe weather strikes.  It's also our responsibility to help you and your family stay safe, no matter what type of weather we have!