Review of the 8/10/2016 North Haven, CT tornado

On average, the state of Connecticut gets about 1-2 tornadoes per year and on Wednesday, August 10th, 2016 Connecticut recorded its first tornado since July, 27th, 2014 when an EF-0 tornado also coincidentally occurred in New Haven county.  While the setup yesterday afternoon was not a screaming tornado setup, the setup and ingredients in place are known to have produce tornadoes in the past.  These setups are very challenging to forecast and its difficult to really relay the potential to the public but they don't always produce and given how we usually see a few of these setups per summer, the last thing you want to do is hype up each event as potentially producing a tornado.  However. yesterday afternoon the ingredients in place all came together to produce a short-lived tornado in a small section of North Haven, CT which damaged trees and some structures (mainly from flying debris and downed trees).  We will take a look at the setup and ingredients in place and look at some of the clues that were on hand just prior to tornadogenesis.

Setup/Ingredients 

On Monday, computer forecast models indicated the potential for some strong to even marginally severe thunderstorms, including the risk for an isolated brief tornado across CT for Wednesday.  Computer forecast models were advertising sufficient instability values to go along with ample low-level moisture (dewpoints increasing into the 70's), with directional wind shear (winds changing direction as you increase with height through the troposphere), and a source of lift (nearby warm front).  However, as resulting computer forecast models runs sort of began backing off this potential until late Wednesday morning/early afternoon rolled around.

Below is a bufkit model sounding from the RAP (Rapid Refresh) computer forecast model for New Haven, CT for the hours of 1:00 PM EDT (just prior to tornadogenesis and the tornado touchdown) and 2:00 PM EDT (just after tornadogenesis and the tornado touchdown).  By analyzing the forecast sounding we see several parameters which were suggestive (but not overwhelming) of the potential for a brief tornado:

While none of the parameters here scream tornado, the values and parameters are borderline enough to indicate that the possibility of a tornado is certainly in the cards.  You had a very moist low-level airmass in place with surface dewpoints well into the lower 70's.  While there wasn't a tremendous amount of sunshine (although some peaks), the presence of this rich moisture was enough to allow for a few hundred joules of cape (measure of atmospheric stability) to develop.  With the warm front pretty much overhead of this area, this allowed for the advection of the strong moisture and also allowed for winds in the lowest 5,000ft of the atmosphere to become "backed"...indicating good directional shear.  You had winds at the surface in a southerly to perhaps even southeasterly then slowly turning to a more south-south westerly to southwesterly all within the short vertical distance, thus indicating a solid deal of atmospheric spin and you also had strong shear (>20-30 knots of wind).  All you needed was some sort of strong enough updraft to realize the weak instability that was in place and then utilize the wind shear aloft and boom...the recipe for a brief/short-lived tornado was in place.

Also, thanks to the presence of a very moist low-level airmass and the surface temperatures and dewpoints very similar this yielded very low lifted-condensation levels (LCL's).  Having low LCL's is very critical (especially in New England) for tornadogenesis to occur.  When LCL's are low this indicates the cloud bases are not very high up from  the ground and its likely possible that when a storm's updraft utilizes the instability/shear, the rotating updraft located not very high up has an easier likelihood of reaching the ground.  Anyways, LCL's were between about 500-750 meters!!!  That is about as low as you will ever seen them.  You can see this in the attached image below.

As mentioned the degree of directional shear and strong winds aloft (indicating good speed shear) lead to those curved/somewhat elongated hodographs.  Helicity values (which measure directional/speed shear) were more than sufficient for the possibility of an isolated tornado.  For 0-1km helicity values once you get above 100 m2s2 and when 0-3km values approaches and exceed 150 m2s2, that is a sign there is strong enough directional and speed shear in place to warrant the possibility of a tornado.  As you can see in the attached image, both 0-1km and 0-3km values certainly met these thresholds.  Effective storm relative helicity values of 100 m2s2 were also certainly supportive.

Mixed-layer cape values were well over 250 J/KG and in reality were likely closer to 500-750 J/KG which when combined with the degree of wind shear that was present was more than enough to support a strong enough updraft to realize the wind shear and beginning rotating.

The supercell composite parameter was a 4 across southern CT (pretty high for around here!) which indicated that if a discrete thunderstorm developed, the chances of it becoming supercell-like were pretty high.

Too conclude, while the setup wasn't a screaming tornado-producing setup, several ingredients were in place and suggestive that this potential existed...all we needed was a discrete thunderstorm with an updraft strong enough to utilize these ingredients and that's exactly what happened.

There is also more too this!  For a more detailed and scientific look at the radar presentation check-out the blog post done by Emmy-Award winning meteorologist, Ryan Hanrahan of NBC Connecticut.  He did a fantastic write-up on the radar presentation and how dual-pol radar picked up on this very well.  The link to his blog post is below:

http://www.ryanhanrahan.com/2016/08/10/north-haven-tornado/