Tornado Potential WEST of CT on Tuesday
By Quincy Vagell on August 13, 2012, 1:22pm
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Severe weather is a strong possibility west of Connecticut on Tuesday and the area of target for potential tornadoes may be central and eastern Pennsylvania...
After seeing the Storm Prediction Center (SPC) place portions of the mid-Atlantic area under a "slight risk" of severe thunderstorms, I decided to take a closer look.
The first thing that really jumps out at me is the Skew-T data.
Above is a NAM forecast sounding for north-central PA at 5 p.m. tomorrow.
Notice some nice veering of the wind barbs on the lower right part of the image.
From the surface up to 650mb, there is about 60 degrees of shearing.
For tornadoes, you need some "twisting and turning" of wind in the atmosphere and this forecast is showing the potential for that.
To dive deeper into the potential, it's important to look at other data too.
Here is a forecast for the 0-1km helicity Tuesday evening.
Helicity is a measure of the potential for cyclonic updraft rotation in right-moving supercells. Although values here are only expected to be in the 50 to 100 m^2/s^2 range, it is still a good sign that the potential is there.
Here's another look at a forecast including helicity and the SWEAT Index:
The greatest target for severe weather and potentially tornadic storms is once again centered over central and eastern PA.
Lapse rates look decent, but the highest values are clearly east of Pennsylvania. With that said, yellow still indicates lapse rates of 6C/km or greater, which does support severe weather. I've seen plenty of situations this year where severe weather and even tornadoes formed in the Northeast when lapse rates were around 5 or 6C/km.
Significant Tornado Parameter:
This index is another helpful tool for predicting where tornadoes may form. The parameter factors in numerous variables, including wind shear, storm-relative helicity, sbCAPE, sbCIN and the surface parcel LCL height.
Values here in green show marginal probabilities of tornadoes. As is the case for most of the other parameters, I am focused over central and eastern PA.
It's also important to look at surface maps...
Low pressure is forecast to move towards the PA/NY border Tuesday evening. The 8 p.m. Tuesday forecast from the NAM is shown below. An occluded front is lifting into north-central PA. I've outlined an area in a circle where I believe the best lift is in place for isolated tornadoes.
As I've seen first hand this year, we do live in the Northeast...forecasting severe weather and especially tornadoes can be tricky. Sometimes there is a great set-up and things don't come together. One worry about Tuesday is that clouds and low-level moisture may limit daytime heating and also result in a slightly lower threat of severe weather. At the same time, it's not all about heat. If wind shear remains the way it is, there is still the risk of damaging winds and isolated tornadoes. The hail threat is there, but I don't think it's quite as high as severe wind.
On the other hand, sometimes the set-up looks marginal and things over-perform. That can be the case where a mesoscale feature, such as a meso-low or even local orographic lift can spin up a tornado rather quickly.
A detail not mentioned about is 500mb vorticity.
It looks like one area of vorticity is expected to move through north-central PA early in the afternoon, but the second area comes with the main lower later in the day. The timing isn't perfect, but a bit of vorticity can only help matters.
One final note is that there is almost another mini "tornado alley" out in this area. NY, just north of the PA border, has already seen two tornadoes in the past two months. central and northern PA is also favored for tornadoes, likely given it's location geographically and the fact that local spin-ups can be aided by hilly terrain.
Stephen Barabas and I will most likely be heading out west on I-84 Tuesday morning. It's surprisingly only about 2.5 hours from SW CT to Scranton, PA. From there, we can decide if we need to go north, south or perhaps further west.
Stay tuned for all the details and follow me on Twitter @danburyweather, I'll be sure to keep everyone up to date with my plans, the progress and any severe weather that I encounter!
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