Locally Strong Thunderstorms Possible on Wednesday

Good evening! We hope you enjoyed this summery Tuesday. This article is going to just focus on the potential for thunderstorms on Wednesday afternoon — some of which could briefly pulse to strong to even severe status.

It all starts with a shortwave trough that will be moving into the area on Wednesday, bringing plenty of vorticity with it and thus lift. This is a true lifting mechanism that we have not had over the past couple of days, which should allow coverage of thunderstorms to be higher on Wednesday than on Monday and Tuesday.

Tuesday afternoon's NAM model valid for Tuesday afternoon shows a shortwave trough with embedded vorticity helping to trigger some showers and thunderstorms in New Jersey.

Tuesday afternoon’s NAM model valid for Tuesday afternoon shows a shortwave trough with embedded vorticity helping to trigger some showers and thunderstorms in New Jersey.

In the midst of this warm and humid airmass, surface-based instability should be pretty high already, but the fact that this shortwave trough will be lowering heights a bit but also not contaminating the airmass with too much moisture will help increase lapse rates somewhat significantly. This allows for updrafts to potentially be more robust and for more of the cumulonimbus cloud to be in dense, cold air aloft — both factors allow for more hail formation, the potential for downburst gusty winds, and somewhat vivid lightning.

Tuesday afternoon's 18z NAM valid for Wednesday afternoon shows deep CAPE with strong lapse rates and a dry pocket aloft for hail and strong wind gusts, but next to zero wind shear (NEXLAB).

Tuesday afternoon’s 18z NAM valid for Wednesday afternoon shows deep CAPE with strong lapse rates and a dry pocket aloft for hail and strong wind gusts, but next to zero wind shear (NEXLAB).

One of the main mitigating factors is that even despite this shortwave, the wind fields are extremely weak. This leads to very low wind shear values, which tends to limit storm organization. A thunderstorm can still pulse strong to briefly severe in its initial growth, but without wind shear to separate the updrafts and downdrafts, a thunderstorm will be updrafting rain-cooled, stable air near it and thus choke off and weaken. Additionally, the lack of wind shear prevents storms from rotating, which means these storms most likely will not reach truly severe ceilings, and also not be able to drop any tornadoes.

With weak wind shear preventing the storms from having a long shelf life, as well as the slow wind fields preventing the storms from moving much at all, it becomes prudent to figure out where these storms will initiate, as storms may not move much from their initiation points.

Tuesday afternoon's NAM model valid for Wednesday afternoon shows a few subtle surface convergence boundaries in NJ, favoring multiple storms forming.

Tuesday afternoon’s NAM model valid for Wednesday afternoon shows a few subtle surface convergence boundaries in NJ, favoring multiple storms forming.

Some low-level southeast flow along the coast may converge with some southwest flow further west, placing the initial main convergence boundary in NE PA and NW NJ. However, as the afternoon goes on, this convergence boundary may shift towards NYC, thus putting a decent chunk of the NE PA, NJ, and NYC corridor at risk for storms. But east of NYC looks to be remain east of this surface convergence boundary, so storms may struggle to reach those areas. NYC itself may even remain east of the main action. The best way to view these boundaries are to focus on the bottom left panel in the above image and look for where the streamline arrows are converging. The fact that there is more than one of these boundaries means that multiple cells may form, though the convergence itself with these boundaries is not particularly strong, so not every storm that forms will have the lift necessary to become a strong thunderstorm.

But with the potential for multiple thunderstorms to form in NJ with slow wind fields and thus not a lot of movement, flash flooding is a concern.

Tuesday afternoon's NAM model's total precipitation through 2:00am Thursday morning shows several 1-2"+ of rain pockets in NJ, which is a signal for flash flooding with the convective nature to this precipitation (Tropical Tidbits).

Tuesday afternoon’s NAM model’s total precipitation through 2:00am Thursday morning shows several 1-2″+ of rain pockets in NJ, which is a signal for flash flooding with the convective nature to this precipitation (Tropical Tidbits).

These thunderstorms will not be able to move much with the very slow wind fields, so they may sit and dump heavy rain over some local areas for a relatively long period of time, even if they are weakening. And even when the storms weaken, more storms may be able to trigger in near-by areas to where they initially formed, further reinforcing heavy rain. Thus, the above precipitation map shows several clusters of 1-2″ or even locally more of rain in a good chunk of E PA and NJ. Not surprisingly, as you head towards NYC and east — where you get east of the convergence boundaries — no rain falls at all, and this general idea has credence.

We will be sure to keep you updated on these potential storms in updates the rest of this evening and tomorrow. Have a great night!