The Storm Prediction Center (SPC) issued a Slight risk for a significant part of the Mid-Atlantic, and even an Enhanced risk in a more concentrated area, including New Jersey, Pennsylvania, Delaware, and Maryland.
"Multiple supercells have developed within a line south of the D.C. area, with notable rotation on radar. The moist and warming air mass has lead to 2000-2500 J/kg SBCAPE, including a favorable amount of low-level CAPE as 700 mb temperatures remain tempered. The 16Z special sounding from IAD shows an impressive low-level hodograph structure, with observed storm motions resulting in 280 m2/s2 0-1 SRH. As such, it appears RAP-based objective analysis is underestimating the shear. The environment east/northeast of the ongoing cells remains favorable for a supercell tornado threat, bounded on the north by the stationary front."
Levels of instability that day exceeded amounts of 2000-2500 J/kg of CAPE, as we can see by the orange contours.
These 2 observed soundings from the NWS Sterling Virginia office show a significant change in a 3-hour period out ahead of overall convective initiation. Not much is going on in the 12Z (8am EST) sounding, minimal cape, fairly linear flow aloft. In just three hours' time, (15Z 11am EST sounding) significant veering winds out ahead of the storms along with a substantial increase in CAPE, SRH, and Lapse rates all helped further the instability for the day.
These 2 soundings are from Pivotal Weather, and these were saved from September 1st, 2021 from the 00Z run. The first sounding modeled for 21 Z (5pm EST), was for the Philadephia area showing significant veering winds at the time of the storms along with a substantial CAPE, SRH, and Lapse rates. Looking at the second sounding, it is clearly indicative of higher tornado potential with increased parameters since the previous sounding. One thing to note between the two soundings is the increase in both Cape and a well-defined hodograph.
This image of the Significant Tornado Parameter for 22Z (6 pm EST) was initialized for the 12Z sounding. Indicating a high probability of tornadoes for southeastern PA, eastern NJ, and Southern New Jersey. This would play a major role in the decision for us to go out and chase on September 1st 2021. The Significant Tornado parameter is a model output derived from the 0-6km shear parameter and can be a critical model output for predicting tornadoes in a region based on the probability. We rarely see numbers this high across such a widespread region of PA, MD, and NJ. This is especially rare as this all occurred on September 1st 2021, and not within the climatological Severe Season of March through June.