Severe thunderstorms possible this afternoon and evening

12:30pm: This morning, we initially had a strong area of showers and rumbles of thunder in Pennsylvania that was moving toward our area. if it hit us, the atmosphere would have stabilized and the severe threat for thunderstorms would have significantly gone down.

However, the EML we discussed yesterday was able to work its way into the region and provide a capping area that completely disintegrated this area of showers. This means that clouds will quickly be disintegrating, and we will quickly be able to destabilize the atmosphere, making it very ripe for severe weather.

In fact, surface-based CAPE values are already in excess of 3,000 J/KG in many areas, which is actually ahead of schedule. When this is combined with the strong mid-level lapse rates and strong wind shear, severe weather is expected to hit a decent chunk of our area later this afternoon. The primary threat will be damaging wind gusts and vivid lightning, but large hail is also a threat, and a tornado threat is non-zero.

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An energetic mid level disturbance, impressive wind shear, and moderate to significant atmospheric instability will combine to create an enhanced threat for organized severe thunderstorms in the Northeast US on Tuesday. While some uncertainties still exist in regards to exactly how the event will unfold, confidence is rising in the heightened threat for severe thunderstorms — even in the NJ and NYC Metro Area. While not everyone will experience a severe storm, the threat for organized thunderstorms is much higher than normal today.

Thunderstorms are forecast to develop over Northern Pennsylvania and New York State, and begin progressing southeastward throughout the day. Meanwhile, atmospheric destabilization will occur over much of New England, the NYC Metro, New Jersey, and the Mid Atlantic. Instability parameters are expected to be quite high, especially by Northeast US standards. With more than adequate wind shear for thunderstorm organization, these storms are expected to spread southeast toward the coast during the late afternoon and evening.

This post will serve as a Live Blog throughout the day for updates and information on the latest analysis as well as a relay point for Watches and Warnings from the Storm Prediction Center and Local NWS offices. A de-brief on the threat is below:

Here’s what we know about the setup:

The aforementioned energetic trough will be running into quite an unstable airmass. Models forecast surface based CAPE (juice for thunderstorms, in geek terms) to be much higher than it typically is in this part of the country. Values over 2500-3000 j/kg are not common around here — in fact, they’re more common well to our west over the Ohio Valley and into the Midwest.

Meanwhile, plenty of wind shear in the mid and upper levels will be present in the atmosphere. This means that thunderstorms which form — as a result of forcing from the incoming disturbance — will be able to organize themselves, and won’t just form and then die. The shear will help the storms to remain organized, and will also help them progress eastward as a cold front slides through the Northeast.

This will all aid in the development of severe thunderstorms, which will slide toward the coast into unstable air right over our heads. The result, while it won’t be the same for everyone, will be a heightened threat for severe storms. Some of these storms could produce dangerous lightning, strong winds, hail, and even an isolated tornado.

What might change or go wrong?

Everything. We’re kidding. But, in reality, a lot could change in regards to this setup as the day goes on. There are a few potential mitigating factors that could limit the potential for severe weather in our area.

1) Prolonged clouds or morning showers: This is a biggie. If clouds stick around in the morning for longer than most forecast models suggest they will, the atmosphere won’t get nearly as unstable as those same models suggest later in the afternoon. The severe weather threat would then take on an entirely different life — and we probably would not see storms of the same intensity.

2) Better forcing to our north: Forecast models keep our area right on the fringes of the best forcing for convection (or thunderstorms). If this shifts slightly northward throughout the day, or if the forcing is more focused on New England, the front will push through our area without any widespread thunderstorm development. This is something we will have to watch as the day goes on.

What’s the bottom line with this thing?

The bottom line is that we’re expecting a heightened potential for strong to severe thunderstorms Tuesday afternoon and evening. While not everyone will experience a severe thunderstorm (these things are hit and miss by nature) more areas will than usual. These storms will be capable of producing dangerous lightning, flooding heavy rains, strong wind gusts, hail, and isolated tornadoes.

The storms are expected to develop in the early to mid afternoon, and push through the area right around rush hour. They’ll be off the coast and out of the picture by the mid to late evening hours.

Stay tuned throughout the day today for further information on the storms, including the latest watches warnings from SPC and the local NWS. The top of this post will be updated as a Live Blog, so make sure to jump up there for the latest.