The Science Behind The Kent Island Tornado

I wrote this for the American Geophysical Union’s AGU Blogosphere:

 

A tornado hit Kent Island Maryland early Monday. The storm crossed the Chesapeake Bay from Annapolis and came ashore just south of the Chesapeake Bay Bridge. My photo from the WBOC Chopper. Damage pics in the video below.

We rarely see tornadoes on the Eastern Shore, but a strong EF 2 (winds near 125 mph) hit Kent Island just SE of the Bay Bridge at 1:30 AM Monday morning. Tornadoes are rare in Maryland and the last tornado in Queen Anne’s County was in 2012.

The Dover, Delaware NEXRAD Doppler showed intense rotation at the time of the tornado.

Some of my thoughts about it:

1. Usually, the Chesapeake Bay weakens storms as they cross over it, but when the Bay is really warm I’ve seen them get stronger, and this can happen in July/August during hot summers when the Bay gets very warm. It’s VERY warm with water near Annapolis around 30º C! This is at least 3ºC above the hottest we would usually see all summer.

2. The land vs water temp. difference causes a local difference in pressure which causes winds to be different along the shoreline and a boundary forms with a difference in wind/air density and temp. across it. In short, the warm water of the Chesapeake Bay heats the air and it expands with the pressure lower than over the land.

3. The air along this boundary on the shore can rotate horizontally due to density differences, but if a strong cell comes over it at the right angle, it can tilt this slowly rotating air vertically, stretching the air column, and forming a tornado. This horizontal rotation of air is called helicity, but the angle relative to a storm cell is crucial to possible tornado development.

Storm relative helicity is an important variable that we watch in possible severe weather situations, but this was over too small of an area to measure. We have models that can forecast the synoptic scale helicities, but not really the smaller mesoscale helicity (on the order of tens of kilometers). It’s there though…

4. A similar case happened at a camp ground in Accomack County on the Eastern Shore of Virginia on 24 July in 2014, with two deaths. The campground was right near the Bay, and this event in Queen Anne’s County was 3 years later, to the day.
Below are images of the reflectivity and velocity data at about the time the tornado hit. It weakened quickly after it moved inland and this is no surprise.

 

You can read more about it from my friend Matthew Cappucci who is an atmospheric science student at Harvard. He wrote an in-depth piece for the Washington Post, and it’s well worth a read.

My live shot yesterday from Chopper 16 is below: