Northeast tornadoes in 2018

By the numbers:

During an average year the United States averages 1,224 tornadoes. The number of reported U.S. tornadoes so far in 2018 is well below average. Looking at the trend and percentile chart shows we're far below the 25th percentile; we're also virtually tied with 2005. 2005 holds the modern record for having the least number of tornadoes.

 

 

 

 

While most of the Contiguous United States is well below average, the Northeast is not. Why is that?

The same pattern that is responsible for all the rain this Summer, is to blame for the increased tornado activity.  We've had a persistent Upper Level  Low (ULL) and a strong Bermuda High in the Atlantic .  The Anticlockwise flow around the ULL and the clockwise flow around the Atlantic high has streamed a lot of heat and humidity northward into the Northeast. This has resulted in a lot of severe weather and flooding caused by torrential rainfall.  

New England has seen more than a dozen tornadoes this year. And while New York State and Pennsylvania are average to slightly below average, the trend has been above average by this time of year.  New Jersey, Maryland and Delaware haven't seen any so far.  Here are a couple of charts I made. One shows the number of tornadoes in the Northeast to date (these are the ones that I know of and can verify). The other shows the annual number of tornadoes per state based on an average from 1950-2010. It's easy to see New England is well above that average.  

 

 

 

Thunderstorms:  

Thunderstorms need 3 basic things to form. The first is moisture in the lower and middle atmosphere.  The second is instability, Warm air at the surface with cooler drier air aloft is the key for instability to develop. The warmer the surface air the greater the instability.  The third thing is lift. Lift is something that causes the warm moist air to rise in the form of an updraft. Differential heating (The Sun heating the ground and air close to the surface) is the most common way for lift to develop; but it can be caused by fronts, sea/lake breezes, outflow boundaries from earlier storms, a dryline can also be a source of lifting air aloft.  The hot moist southern air and the constant influx of frontal systems moving through has produced the perfect conditions for thunderstorms and severe weather.

Severe thunderstorms generally need a 4th ingredient. Winds moving indifferent directions and speeds with height.  This is called wind shear.  Storms can create vertical and horizontal shear, as can other things.  There are many forms of severe weather; but the kind that gets the most attention is tornadoes. The wind shear that causes supercells and tornadoes normally comes from the jet stream.  

The Jet Stream:   

The jet stream is a ribbon of high-speed wind (located at nearly 6 miles above the surface) that is related to the location, of the pole-to-equator temperature contrast. The greater the temperature difference between the air masses the faster the jet stream.

Most of the tornadoes in the CONUS form in the Plains (Tornado Alley) or in the deep south (Dixie Alley).  Normally the position of the Jet Stream is the reason for this. But this year we've see the Jet Stream much farther north. This has made it difficult for storm systems to tap into that moist warm air from the Gulf of Mexico (GOM).  This year we've seen the jet primarily over the Mid west or near the Canadian border; this has allowed the southern heat and moisture to move into the Northeast.  The overlap of the warm moist air and the Jet Stream nearby has provided the most favorable conditions for tornadoes to develop in the Northeast this year.

While increased awareness and platforms like social media can be attributed to some of this increase in reported tornadoes; the vast majority of the increased number of Northeast tornadoes can be blamed on the position of the Jet Stream.