Potential for Widespread Severe Storms Today

There is strong potential for widespread severe weather today, primarily from a squall line, with damaging winds, a few embedded tornadoes, and some discrete supercells out ahead of the squall line that may also produce a few tornadoes. The SPC continues its moderate risk for severe weather, with 45% hatched for winds, 10% for tornadoes, and 15% for hail.

Not much has changed since the event overview post from yesterday morning. However, one thing to note is that at 500mb, the shortwave energy and area of vorticity appears to escape to the northeast a bit quicker than previous runs. Instead of the strongest area of vorticity being in Pennsylvania at 00z, Sunday, it is now located in NW Upstate NY. This will result in some of the best shear escaping to the northeast a bit faster, and not extending as far to the south into New Jersey.

00z NAM 500mb Forecast, Valid 00z Sunday, September 9th, or 8pm EDT Saturday, September 8th

That being said, it is still a very impressive synoptic setup with lots of large-scale ascent from a potent trough swinging to the east, creating height falls and very fast mid and upper level winds with strong 250mb divergence. Most of the northeast will be in the right entrance region of the jet, also promoting large-scale ascent. Additionally, the large trough and storm system will help to promote strong southerly flow at the surface, helping to advect rich low-level moisture, with dewpoints in the low 70s. Some areas will see a SE flow, but considering how warm the ocean is in the early fall, the ocean provides a good moisture source without providing much in the way of stabilization.

00z NAM 0-1km Helicity Forecast, Valid for 00z Sunday, September 9th, or 8pm EDT Saturday, September 8th.

With the vorticity escaping to the northeast a bit faster, this also means that the best low-level helicity will also be confined to areas north of the immediate Metro region, but still not too far away. The values of 200 meters squared per second squared are confined to the southern Hudson Valley and adjacent areas, as opposed to extending southward into New Jersey. That being said, there is still enough low-level helicity for isolated spin-ups in any part of the Metro region, but the best threat for that appears to be to the north of New York City.

ooz NAM CAPE Forecast, Valid for 21z Saturday, September 8th, or 5pm EDT Saturday, September 8th

As far as instability is concerned, it is not off the charts, but it is certainly sufficient enough given the dynamics and shear at play. What is important to note is that it is not all about the environment that a storm is currently in, but also about the environment that is ahead of the storm — since a storm, especially a fast-moving squall-line — will tap into that environment. With the July 26th event, the CAPE out ahead of the squall-line was very weak, and the line gradually weakened. The forecast CAPE valid for 21z tomorrow is certainly a sufficient environment for the survival of a squall-line.

I did not post the helicity valid for 21z, but it showed the strongest values in the southwest and southern Hudson Valley (essentially shift the 00z values a bit to the west). What’s interesting to note is that a great area of juxtaposition between the best 0-1km helicity and strong instability appears to be in the southern Hudson Valley. It is this region that I think especially has to watch out for tornadoes, not just ahead of the squall-line but also in the squall-line itself.

00z NAM Forecast Sounding for KSWF, Valid for 21z Saturday, September 8th, or 5pm EDT Saturday, September 8th.

A sounding that really stood out was the 21z sounding at KSWF, which is Stewart International Airport, located in the southern Hudson Valley, near Newburgh, NY. As one heads to the valleys, south and southeast winds are often “trapped”, leading to increased low-level helicity and thus a higher tornado threat. The winds increase from 15 knots out of the SE at the surface to around 45 knots out of the SW at 850mb, as a strong, closed 850mb low in SE Canada really helps to get the low-level jet cranking. Additionally, the CAPE is 2085J/KG. It would honestly surprise me if there were no tornado reports in the Hudson Valley tomorrow based on these soundings. There is also very good 0-6km shear, though the directional shear is a bit lackluster once you go above 850mb. The extremely fast low-level jet and very fast mid and upper level winds will help provide strong winds in the mixed layer that can easily be mixed down with any form of convection, and a fast-moving squall line which will be able to stay organized with the sufficient shear and instability.  The forward speed of the squall-line will only add to the very strong winds that are already possible. Additionally, as I mentioned in yesterday morning’s discussion, the dry punch of air at the mid-levels helps to support strong downdrafts and downbursts because of the evaporational cooling that would take place when it rains. This leads to the surrounding environment becoming colder than the temperature of an air parcel, leading it to sink rapidly.

The reason why the convective mode eventually becomes a linear squall line is because of a rapidly advancing cold front. For modes where the cells are discrete for the entire day, you generally want the frontal boundaries to be slowly moving or stall. This is the trickiest part of the forecast – how much in the way of discrete, organized convection can form ahead of the line before its arrival? Based on this 21z sounding, if anything discrete forms, it could certainly become tornadic, and this is why the SPC has issued a 10% tornado outlook for parts of the area. In the July 26th event, there was not a whole lot of lift and the instability was weak out ahead of the line, so anything discrete that tried to form was mainly from the outflow of the squall. Thus, it would either get absorbed by the squall line (the tornado in eastern PA that day is an example) or they quickly died.

In this case, however, there is lots of large-scale ascent from the powerful trough, meaning that an area of ascent can race well out ahead of the squall line, when instability should still be strong as well as helicity and shear. When you combine that with a strong 250mb divergent flow, it is certainly possible for discrete cells to form, much more-so than what was seen on 7/26. This is another reason why everyone in the Metro area needs to be wary of tornadoes, though as previously mentioned, the strongest risk for tornadoes should be in the southern Hudson Valley. I’d also put NE PA in a relatively strong risk area for tornadoes as well, and this zone should extend to the north and east until about eastern upstate NY near the Massachusetts border.

Even if the discrete convection does not work out, there will still be enough shear and helicity in association with the squall line that there may be several tornadoes embedded. The difference between a 5% tornado zone and a 10% tornado zone essentially comes from the discrete supercells that could form before the squall line hits. So this could be a two-part deal, which is why I strongly urge everyone remain wary of the situation.

00z NAM Forecast Sounding for KEWR, Valid for 00z Sunday, September 9th, or 8pm EDT Saturday, September 8th.

A quick look at the KEWR sounding from 00z suggests a favorable shear profile for organized convection and strong winds. However, notice how the low-level winds are west of south and the sounding, while unstable, is not off the charts with instability. There is still enough veering to be wary of a tornado threat, but not to the same degree as areas a bit further north.

The KSWF sounding shows pretty mediocre mid-level lapse rates. This is partially compensated for by the dry air aloft and strong ascent, but these lapse rates might prevent this outbreak from being even worse than it already will be and also helps to make the hail threat tertiary compared to the wind and tornado threat. I cannot rule out severe hail, but it is just not as much of a concern as severe winds and tornadoes are.

The EWR sounding seems to have better mid-level lapse rates, but that is because rain has already begun to cool the mid-level temperatures towards their wet-bulbs. Either way, mid-level lapse rates is one potential caveat to this event. Another caveat explained in yesterday morning’s discussion is the potential for convective debris and cloud cover out ahead of the line. But data has grown in agreement that there should still be enough CAPE for the most part to suffice given the strong shear profiles. That said, with the shortwave escaping to the northeast a bit quicker, that might mean a tad less lift as well, which could make the mid-level lapse rates and smaller CAPE a bigger hindrance. The instability does weaken a bit as you head eastward, so I am still not sure about the extent of a tornado threat east of New York City, but a severe weather threat certainly remains. The marine layer also might provide slight moderation given how strong the southerly and southeasterly flow might be, also leading to more clouds and convective debris. But one thing to note – lots of people talk about the “bust” of June 10th, 2010 regarding convective debris, but remember the date of that event…early June. The ocean was a lot colder, so the strong large-scale ascent and colder waters made the marine layer a bigger issue. But since the ocean temperatures are so warm in early September, the marine layer will only provide a small amount of moderation, if any at all, which is why the SPC does indeed extend their moderate risk to the east of New York City.

Stay safe tomorrow, everyone, and we will keep you posted all day tomorrow on Facebook and Twitter!