As the atmosphere has yet more surprises under its belt, a very powerful line of thunderstorms have developed, and are set to impact the area within the next couple hours.

Quick Synopsis

Areas Affected and Likely Factors

• Philadelphia – 10:15 PM to 10:45 PM
• Trenton – 10:45 PM to 11:15 PM
• New York City – 12:30AM to 1:00 AM

• Damaging Winds
• Heavy Rainfall
• Remote Possibility of a very weak Tornado

Atmospheric Setup 

Thunderstorms are feeding off of the latent heat release that was released by the preceding thunderstorms that impacted areas of the Mid-Atlantic and areas of Pennsylvania, as a secondary line of more intense thunderstorms is developing, and is sustaining itself. The teams expects the thunderstorms to develop, as moisture transport at the 850 Millibar level is very strong, as per the SPC Mesoanalysis page. This provides the moisture needed to support such thunderstorms.

Thermodynamic Indices

Latest RAP mesoanalysis shows that many factors are supporting thunderstorm development and sustainability for the major cities:

In the lowest 300 Millibars (approximately 7,000 feet) from the surface and up, levels of  Most Unstable Layer CAPE (MUCAPE) that are conducive for thunderstorm development are in excess of 500 Joules per Kilogram. This provides energy for the thunderstorms to form, as rising air parcels will interact with colder air parcels and create more instability. Keep in mind that instability is needed to create thunderstorms and convective growth, and CAPE is the energy source of a convective system.

Mid level lapse rates are not very impressive, but do support thunderstorms, and low level wind propagation to the surface. However, what may limit the amount of wind that the surface experiences from the thunderstorms is the low level lapse rates, as thy are low, and are more stable. This will help to limit the amount of wind that descends and mixes down to the surface. However, in a thunderstorm that has intense updrafts and downdrafts, the stable air is overrode to a degree, and how much wind one experiences is very variable based on the specific atmospheric conditions over your location. In short, expect wind gusts of up to 35 MPH in the weaker downpours and thunderstorms, and stronger convective cells will produce winds to 60 MPH, as more winds from above are mixed down to the surface.

Although the possibility is very remote, the team must caution for the possibility of a very weak tornado in a stronger thunderstorm cell. This is due to the amount of low level speed and directional shear that is present over the mentioned areas. If a cell with rotation develops, the added speed and directional shear will tend to air in development of a rotating thunderstorm cloud, or a mesocyclone. Thus, a very weak “spin-up” tornado may develop.

The team sees that in the image below, which is a hodograph. As one ascends, the winds at each level begin to turn in a clockwise motion, or veer. This hodograph provides a visual representation for such phenomenon  This in turn aids in the increased probability for a very weak spin up tornado.

With that said, the amount of “corkscrew” motion in the atmosphere is also very high, and that motion of the air will also aid in enhancing any rotation that develops in a thunderstorm. This is known as helicity. The graphic below shows the latest short term model analysis (RAP) of the values of helicity.

Let it be known that the team is not forecasting a definite tornado to develop, we are just outlining the possibility.

Below is a recent radar image of the line of the thunderstorms.

Into the Night and Early Morning

Once the initial line of string thunderstorms pass, the night will consist of increasing winds with gusts of up to 60 MPH regardless of any thunderstorm or not. Rainfall amounts should range between 1″-2″ for all, and creeks and streams will rise. If you live in a flood prone area, please heed any warnings. As the front passes, the winds will still be very strong, and winds gusts of up to 70 MPH are possible all along New Jersey and especially Eastern Pennsylvania. The graphic below depicts the wind speeds for overnight tonight, and into Thursday Morning.

Conclusion

A strong line of developing thunderstorms are moving towards the major cities, and will likely produce heavy rain and damaging winds in the late evening areas and into the overnight hours. Also, if you get a break from theses storms and do not feel the effects outlined above you most likely have a very stable airmass right above you due to the low level lapse rates being so slow, but remember it will not take much to get those effects down to your location. Regardless, please be careful and heed all warnings. Also, watch out for dense fog while driving.
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