Severe Weather Threat With Tornadoes Possible Today

6z SPC Forecast, indicating a 10% tornado probability within 25 miles of a point (top) and a 30% severe wind (58mph gust or higher) probability within 25 miles of a point (bottom).

We have one of the most dynamic storm systems for the month of September in quite some time. The Storm Prediction Center has a “slight” risk of severe thunderstorms for the entire Metro area, but even more impressive is the 10% tornado contour and the 30% severe wind contour. There is also a 5% hail contour, but I would honestly be surprised if there were any hail reports.

We have a potent, amplified longwave trough diving down into the entire eastern half of the country with an associated powerful shortwave moving through the Northern Great Lakes and into Canada. This trough is phasing with ample tropical moisture to the south, helping to create a powerful, dynamic storm system. There will be a warm front moving across the area in the early morning hours, and a powerful cold front moving from west to east, crossing the Metro area in the late evening hours. There are two primary threats with this storm system: discrete low-topped supercells that may form some weak tornadoes, and a squall line entering the region in the late evening hours helping to produce very strong winds. Click “Read More” below to read full-length post. 

00z NAM 250mb Jet Forecast, Valid for 00z Wednesday, September 19, or 8pm EDT Tuesday, September 18.

With the event on the 8th, the best lift escaped well to the north and west well before organized convection could make it to the Metro area. Although this might still be a hindrance to an extent, the trough is much more amplified this time around and larger in size, which helps to create a larger area of ascent. This image is valid for 8pm,  and notice how powerful and large the 250mb jet streak is, helping to create a broad right entrance region, favorable for low-level ascent which helps to form and  maintain organized convection.

00z NAM Helicity Forecast, Valid for 21z Tuesday, September 18, or 5pm EDT Tuesday, September 18.

00z NAM Surface-Based CAPE Forecast, Valid for 19z Tuesday, September 18, or 3pm EDT Tuesday, September 18.

Models had initially been lackluster regarding the instability, and without instability, any updrafts would be weak and not able to take advantage of the high low-level shear, and instead would be sheared apart. This would have effectively turned the tornado threat into a heavy rain threat. Although the instability still is not staggering on the models, the recent NAM model trended upward with the surface-based CAPE, which would lead to updrafts robust enough to potentially form supercells. This is because the very strong low-level jet is helping to advect rich, warm moisture from the warm September oceans. Additionally, the NAM shows an area of clearing skies, which would help to heat the surface, generating more lift. If we can get surface-based CAPE values of greater than 1000 J/KG, that would suffice for an isolated tornado threat. The NAM does indeed show this occurring at 3pm, as the image shows.

00z NAM Forecast Sounding for JFK, Valid for 21z Tuesday, September 18, or 5pm EDT Tuesday, September 18.

Now let’s take a look at a NAM forecast JFK sounding, valid for 5pm. It shows SSE surface winds of around 30 knots, and winds just above the surface veering towards the SSW in excess of 50 knots. This creates lots of low-level shear, and combined with the forecast lift from the trough and SB CAPE/low level instability, this can generate tornadoes. Additionally, since the discrete convection in association with a moisture tongue is surface-based in nature, the air parcels can fully utilize the relatively high surface-based CAPE that is depicted, as opposed to utilizing the much weaker mixed-layer CAPE and slow mid-level lapse rates. Also note the moisture in the low-levels, but drying in the mid-levels, which helps to generate some clearing skies, helping to further destabilize the atmosphere. Since the mid-level lapse rates and mixed-layer CAPE is so low, combined with the fact that most of the shear is in the low-levels of the atmosphere, the thunderstorms will be very low-topped in nature, as air parcels will not be able to sufficiently rise vertically past 15,000 feet. This means very little in the way of lightning and hail.

Also note the lack of inversions in this sounding. This would help to mix down the very strong low-level jet winds to the surface, providing any convection can be generated. Thus, there is a very high synoptic wind threat as opposed to just a high severe weather/convective wind threat.

00z NAM 1000mb Relative Vorticity Forecast, Valid for 21z Tuesday, September 18, or 5pm EDT Tuesday, September 18.

As noted before, the potentially low CAPE is potentially the biggest hindrance to the tornado threat. With such a large trough and plume of moisture, it is possible that most of the area stays cloudy and showery, and doesn’t clear much, meaning there is not enough instability for sufficient lift to generate organized, rotating convection. This would relegate the tornado threat to the south and west of the Metro area. However, models have hinted at some enhanced low-level convergence, which would additionally help to generate lift, potentially offsetting the lack of instability. There will potentially be a low-level vorticity maximum running out well ahead of a squall-line, helping to generate localized lift in New Jersey, New York City, and Long Island – the 00z NAM forecast for 21 hours shows this well.

When you combine the potential lift with the exceptional low-level shear, it certainly makes sense that the SPC has a 10% tornado contour. I am a bit skeptical of how large it is, and in particular, how far north it extends considering the CAPE is so low to being with, especially in further north areas. That being said, as far as a Metro area tornado threat is, I could certainly see low-topped supercells producing isolated EF0 and EF1 tornadoes in South Jersey sometime around 2-3PM and extending NNEward towards the NYC/western Long Island areas at around 4-6PM.

00z NAM 900mb Jet Forecast, Valid for 23z Tuesday, September 18, or 7pm EDT Tuesday, September 18.

As the cold front approaches, there is the potential for a squall-line to produce damaging winds, especially considering how strong the low-level jet is to begin with, so not much convection is needed to generate severe winds. Instability will be waning a bit as time goes on and the best lift will be moving out of the area, but there is still definitely more lift this time around than there was on the 8th. The NAM is forecasting a ~70 knot 900mb jet streak for western Long Island at 7pm, which is absolutely staggering for a September non-tropical system. If this can get mixed down to the surface, widespread 50-60mph gusts can be expected, and perhaps some localized gusts of higher than 60mph.