Strong thunderstorms with isolated tornadoes tonight

A very interesting meteorological setup for severe weather looks to unfold tonight. A large blocking pattern in the Arctic has forced a large trough to dig into the Midwest, and this will eventually swing through to our area, yielding a potent storm system to our northwest. Although it is often much easier to get thunderstorms during the summer when airmasses are warmer, storm systems are usually stronger during the Autumn, as there is often a larger difference in temperature — or a battleground — for a storm to form. Considering this, plus the fact that in early October the Atlantic Ocean is still quite warm, it becomes much easier for severe weather to be supported near the coast. The strong storm system will serve to bring warm, moist air from the Atlantic Ocean, yet also provide colder temperatures aloft, generating plenty of instability. Additionally, instead of the rising air for showers and storms coming from sunshine heating the ground, we are able to generate forcing for lift from that strong storm system. This gives us the basic foundation for severe weather tonight into Wednesday morning. But this situation remains quite unique.

Arguably the most impressive facet of this potential threat is the strong winds just above the ground. When winds change direction (clockwise) from the ground and up, as well as greatly increase in speed, that is when strong wind shear is created, which supports rotating thunderstorms, capable of producing strong wind gusts and isolated tornadoes. Given that surface winds will be out of the southeast, and winds just above the ground will be in excess of 50 knots out of the southwest, plenty of wind shear is generated. When this is combined with instability, those rotating updrafts can be lifted into thunderstorm clouds, and severe weather can be realized. Without the instability to lift rotating updrafts into thunderstorm clouds, thunderstorms greatly diminish in coverage. Lots of recent model data, however, shows a good combination of potentially rotating updrafts and instability, which piques our interest.

Today’s 12z NAM Model valid for 2:00am tonight shows a decent juxtaposition of instability (CAPE) and rotating updrafts (Helicity) for much of the NYC Metro region.

One thing important to note is that the strongest helicity values are located away from the strongest instability, which will save the area from a widespread tornado threat. That being said, there are still pockets of impressive helicity values within the CAPE values of 1000 +. As the overnight progresses, the instability and helicity increase to the northeast, putting Long Island and particularly the Hudson Valley and Southern New England at risk.

While these factors are impressive, there are still a few factors which may prevent this outbreak from reaching its potential:

1) For one thing, tornadoes are more likely to form in discrete thunderstorms. But given the potential for plenty of lifting from the strong storm system, it may be more likely that a large cluster of rain forms, or potentially even a squall-line, with potentially a couple of embedded tornadoes, as opposed to a widespread tornado risk.

2) Another factor is that the winds above 850mb do not turn as much in the clockwise direction (less wind shear above 850mb), which would also support a line of thunderstorms with strong straight-line winds as opposed to tornadic thunderstorms.

That being said, considering how strong the winds just above the ground are, any thunderstorm could still bring wind gusts in excess of 40-50mph. This means that even if the tornado threat does not pan out, the risk for severe weather could still exist.

As far as the potential timing for the worst weather, it appears that Southern and Central NJ have the highest risk for severe weather at around 2:00am, and that threat will shift to the northeast during the overnight. Long Island could see its maximized severe weather threat around 5:00am, with New York City around a similar time. All possibilities are still there: from merely heavy rain, to a few rumbles of thunder and a moderately strong wind gust, to strong and severe wind gusts, or even an isolated few tornadoes. Time will tell what ultimately materializes, but it does appear that as of now, the further east areas (coastal NJ’s Longitude and east) will have a higher threat, due to their closer proximity to the Ocean and its moisture. The exact placement of features could even shift that zone eastward, only placing the tornado threat to eastern Long Island and Southern New England, as well.

We encourage you to stay tuned over the next several hours. We’ll be starting a live thread later tonight with updates from our meteorologists.