Medium Range Analysis: Severe Weather Chances this Weekend

Good evening! We hope you all had a great Monday. For the most part, the area had a beautiful, summer day, but some western suburbs of NYC had some strong thunderstorms and flash flooding. These storms quickly weakened as they headed towards NYC, as the pattern right now is generally not favorable for severe weather in our area. But that may change come this weekend, when the general synoptic pattern may start to favor strong to severe thunderstorms.

It’s pretty far out in time, so plenty can change — but at this range we like to look at general features of the pattern and what they signal, since there is inherently less noise in broader signals. One thing we like to look at is a pattern that can favor advection of an EML (Elevated Mixed Layer) into the region. The EML is a layer of very hot and dry air from around 700mb (around 10,000 feet) and up that originates in the desert southwest. This hot and dry air air is not contaminated with moisture and thus it can cool quickly with height, creating a lot of mid-level instability. These EMLs often combine with rich, tropical Gulf of Mexico moisture in the low-levels of the atmosphere in tornado alley and lead to some of the biggest severe weather outbreaks.

Of course, tornado alley/the Plains is located much closer to the desert southwest than our area, so these EMLs can advect into these regions readily without contamination or modification. This is one of the biggest reasons why severe weather in the Northeast is not nearly as impactful as it is in the Plains. When the EML tries to make it towards our area, there are so many weather disturbances and remnant thunderstorm complexes with moisture that can contaminate the EML on its way. It also usually turns out that a lot of patterns that advect the EMLs into our area more efficiently do not contain the low-level moisture and instability necessary to complete the puzzle for big thunderstorm outbreaks. However, somewhat contaminated EMLs — but with still some of its original profile remaining — along with good wind shear and low-level moisture and instability can still trigger some pretty sizeable thunderstorm outbreaks in the Northeast, and this is what may be occurring this weekend.

When we take a look at EML patterns into the Northeast, but ones that can also contain the necessary instability beneath the EMLs for severe weather, we often start at 700mb. This is because the bottom of the EML layer is usually a very warm area of 700mb temperatures that advects over the top of an already established warm and moist airmass — but slightly cooler than the EML above. This allows the EML to initially be a cap in the atmosphere — preventing any weak updrafts from turning into thunderstorms and thus weeding them out. Only the strongest updrafts can turn into thunderstorms, and considering the environment they enter with the EML, they can then become severe.

Monday’s 12z GFS illustrates this potential pattern well. The above is an animation of the 700mb temperatures, winds, and heights from Friday morning through Sunday evening. There is a very warm plume of 700mb temperatures in the Rockies that moves northeastward into the Northern Plains, then quickly gets scooted east by a fast area of westerlies into the Midwest and the Northeast. This evolution makes perfect sense due to the strong 700mb height gradient from a large ridge in the Tennessee Valley, SE US, and SW Atlantic and a big vortex/trough in SE Canada — the plume rotates clockwise over the top of this ridge.

This ridge also helps provide a very warm and unstable low-level airmass into the area, and the vortex in SE Canada can help provide plenty of dynamics and lift in the atmosphere. Our area — as modeled — is juxtaposed very well between these features to provide a good combination of lift and instability, and the strong height gradient also helps to provide strong westerly winds aloft, which is great for providing wind shear in the atmosphere.

The 700mb temperatures up to 10C and the way they are being advected into the region does resemble some EML cases in the Northeast, and the above pattern is one that favors this advection. Of course, the true EML gets very contaminated as it will run into plenty of thunderstorm complexes in the Midwest during its trek into our area — but the idea is there.

Here is a quick look at how all of this comes together in a forecast sounding for Northern New Jersey on Sunday:

Notice in the above sounding how in between 850mb and 700mb, the temperature profile briefly juts out to the right. This is the capping inversion between the warm, moist low-levels and the remnant EML overhead. Also notice how from that capping layer and above, the dewpoints also quickly jut to the left, which shows much drier, mixed air. And then the temperature profile above the inversion juts back to the left — and thus the temperatures go back to decreasing with height again, and we strongly juxtapose mid-level instability and that dry air. This is an indicator that an EML — and why it stands for Elevated Mixed Layer — is overhead. In the strongest EML cases, the lapse rates are much higher, which shows that this EML is certainly contaminated and not a classic one. But the remnant EML is still enough to provide additional mid-level instability and dry air aloft, and the pattern that led to its advection has a very strong bout of westerly winds aloft, leading to good deep-layered wind shear for severe weather.

The prevalence of the dry air aloft and instability also leads to many severe hail analogs being listed, and the downdraft CAPE values being over 1,000 J/KG would also lead to severe wind gusts being transported to the surface. In a lot of our other severe weather setups, they did not produce severe weather over a widespread area because the mid-level instability was generally lacking — the CAPE profile was short and “skinny”, and only produced high surface-based values because of low-level moisture and low LCL heights. But a profile like the one above is sufficient for instability in all levels of the atmosphere and no manufacted/inflated CAPE numbers from low-level moisture. Thus, the above setup is much more likely to produce widespread severe weather than the “typical” northeast severe weather case.

Taking a step back a bit, this sounding is still almost six days away, and a lot can change between now and then. It could turn out that the EML becomes even more contaminated, and the amount of instability aloft decreases and we return to a “typical” run-of-the-mill northeast thunderstorm threat. Or, the EML could provide too much of a cap near 700mb and no updrafts — not even the strongest ones — break through the cap, and only further north in New England closer to the vortex in SE Canada where stronger westerlies and lift are present is where the thunderstorms would actually develop. But the general evolution of the 700mb winds, heights, and temperatures over the next several days does pique our interest for this weekend. It could be Saturday, or Sunday, or even both days, depending on how exactly it evolves.