Monday, August 15, 2016

Strong to Severe Thunderstorms Possible Tuesday, August 16th, 2016

After a one-day break from beyond oppressive heat and humidity a warm front will be approaching Tuesday morning bringing a return to the hot and sticky airmass.  However, its stay will be rather brief as a cold front is expected to push through in the afternoon.  The heat and humidity will not be the big weather story tomorrow, however, as we will be dealing with the risk for thunderstorms and several of these will have the potential to become strong to severe.  In fact, a few computer forecast models are indicating the threat for a few tornadoes as well.

Overnight and into tomorrow morning an area of surface low pressure will be tracking to the northeast well to the west of southern New England.  As the surface low pushes through western PA into western NY and into Quebec, Canada a warm front will surge northward through southern New England where several computer forecast models indicate the warm front could stall in the vicinity of the MA Pike.  South of the warm front the airmass will become very humid.  Surface dewpoints are expected to surge back into the mid 70's with precipitable water values (PWATS) increasing to over 2.3''...indicative of a very moist airmass:


As the warm front approaches we will have to deal with the potential for showers and perhaps even some thunderstorms and we will have to watch this potential rather closely.  If the morning warmup happens to be associated with any thunderstorm activity there will be favorable parameters in place not only for some of these thunderstorms to become strong but perhaps severe and carry the risk for a tornado.  Below is a bufkit profile for New Haven, CT for 9:00 AM tomorrow morning from the 18z 4km NAM computer forecast model.  We see strong moisture advection, good solid cape in the lowest 6km of the atmosphere and very good low-level directional shear and speed shear indicated by nearly 30 m/s of shear.  We also see pretty low lifted condensation levels (LCL's) which mean cloud bases will be rather close to the ground:


The potential for morning thunderstorms would likely be in the 6 AM to 10 AM timeframe.  This is not a given but is something that needs to be watched given the parameters that will be in place.  In addition to the potential for a tornado, storms could contain some gusty winds along with torrential downpours and frequent lightning.  After we wrap up with any morning activity our attention will quickly shift towards the afternoon and evening hours where the potential will exist for a much more potent and perhaps bit more widespread severe weather threat.

With the warm front getting hung up somewhere across central MA, this will enhance the likelihood that surface winds across much of CT, RI, and SE MA will remain "backed" and be more out of the south to even southeast.  This is significant because as you go up a few thousand feet in the atmosphere the winds will begin to transition to a more southwesterly direction with winds then transitioning to a more westerly direction even higher up.  When you have these change of wind directions with height this is what is called directional shear and creates "atmospheric spin" which can allow for a thunderstorms updraft to not only become tilted but begin rotating if the updraft is able to utilize this.  Winds aloft are also expected to increase throughout the afternoon hours as the area of surface low pressure begins to strengthen.  In fact, some computer forecast models indicate that winds about 5000ft ASL will increase to as much as 30-35 knots with winds even higher up increasing to 40-50 knots.  This increase of winds with height is called speed shear.  One way to measure direction and speed shear is through helicity values.  Looking at 0-1km and 0-3km helicity values for tomorrow they are forecasted to be very impressive.  Typically when looking at the possibility for a tornado you want to start seeing 0-1km helicity values approach 125 m2s2 and 0-3km values approach 150 m2s2.  These values, especially 0-3km helicity certainly approach or even exceed those thresholds:


While we know the wind shear will be there the next question is with regards to how unstable will the atmosphere become and how much surface heating we see.  With such a moist low-level airmass in place it will not take much sun for the atmosphere to rapidly destabilize, even with poor mid-level lapse rates (though this will be a bit of a negative for getting even more potent updrafts to occur).  While clouds could be an issue early on, especially if we see lots of morning activity, computer forecast models indicate we will break out into some sun and strong surface heating will commence with temperatures surging through the 80's.  Given the degree of wind shear aloft, if the atmosphere can become quite unstable the threat for several strong to severe thunderstorms will exist and posing the threat for a tornado, strong to damaging winds, and even some hail.

Below is a bufkit profile sounding for Windsor Locks, CT from the 18z 4km NAM computer forecast model for 6:00 PM tomorrow evening and what we see is a very favorable thermodynamic/dynamic environment for any thunderstorm to become strong to severe and pose a risk for a tornado, strong to damaging winds, and hail.  In fact, this sounding would indicate the potential for a strong tornado would be on the table:



As typical with thunderstorms and severe thunderstorm potential across southern New England, we really won't know what will happen or unfold until we are able to see the details iron out during the morning hours.  These are always very complex setups and in a region where severe weather (especially high end severe weather and tornadoes) aren't as common as further west there are typically always red flags.  These setups and how they unfold and produce all depend on the timing of all the main features...does the strongest instability coincide with the strongest wind shear and strongest forcing from the approaching cold front?  These are some of the questions we will have to iron out tomorrow.

Given how these setups don't produce all that often around here its very tough to say for certain as to how much in the way of severe thunderstorms we'll see or even if we'll see any tornadoes, however, right now that possibility is on the table and we will have to watch how the conditions unfold in the morning.

Thursday, August 11, 2016

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/