September 8th/9th Severe Thread

[bluewave]

Friday's 12z 4km NAM did very well with the timing and placement of the features.

It had the morning storms, but the sim radar signal was not as robust as verified.

It also took tracked the evening meso right across NNJ, though the southern

end of the line verified weaker.

[Weathergun]

DC had better instability and better ML Lapse rates. Our instability wasn't terrible and our surface based CAPE was actually pretty good. However, it wasn't enough to compensate for our crappy ML Lapse rates once we lost our lift from the shortwave escaping.

DC's instability, however, was.

There wasn't much difference between mid-level lapse rates, on the18z soundings yesterday at OKX and IAD.But Sfc-3km lapse rates at 7.3 C/km at OKX and 8.5 C/km at IAD. I think the earlier convective debris contributed to lower lapse rates in our region. So there was better instability overall for t-storms to the farther south and better forcing t-storms to farther north. With an area of increasing subsidence in between.

[forkyfork]

we were in between two sections of the line that were more perpendicular to the shear vector. bad luck happens.

[nycsnow]

In my neighborhood by the belt parkway exit 11n you can see a perfect line of from when the tornado came through of trees signs etc. then more in my actual neighborhood there's just blocks with trees all over. Breezy point is bad from what I heard from a friend

[IsentropicLift]

Although I had no damage on my property, we lost power for about a half hour with the squall line and a large tree came down around the block from me. One section of town didn't get power back until the next day. It was cool to see so many tornado warnings in the area throughout the day and actually have some verify. One of the few moderate risk/tornado watches for our area that have actually verified.

[earthlight]

we were in between two sections of the line that were more perpendicular to the shear vector. bad luck happens.

The terrible lapse rates also were an issue once the convection outran the best forcing, which shot up to the north and west of our area with the main shortwave over Southeast Canada moving north.

[blizzardof09]

The terrible lapse rates also were an issue once the convection outran the best forcing, which shot up to the north and west of our area with the main shortwave over Southeast Canada moving north.

overall, SPC's forecast was very well done. to me in verified nicely, but just like the june derecho, the eastern zones due to horrible lapse rates and missing the best forcing really killed these storms. takes a very nearly perfect setup to get a truly widespread severe outbreak close to the coast and yesterday just didnt have it in the afternoon. also the storms in the morning played a roll later in the afternoon for sure

[Dsnowx53]

There wasn't much difference between mid-level lapse rates, on the18z soundings yesterday at OKX and IAD.But Sfc-3km lapse rates at 7.3 C/km at OKX and 8.5 C/km at IAD. I think the earlier convective debris contributed to lower lapse rates in our region. So there was better instability overall for t-storms to the farther south and better forcing t-storms to farther north. With an area of increasing subsidence in between.

Good post. I agree.

If we had the lift that earlier model runs were showing, I think that would have compensated for the bad mid level lapse rates.

[earthlight]

^ The shortwave ending up father north added to the problem. Without the increased forcing and lift, the mid level lapse rates being poor was only magnified.

When we started to see the storms resort to outflow boundaries to trigger new weak updrafts, the writing was on the wall.

[IsentropicLift]

^ The shortwave ending up father north added to the problem. Without the increased forcing and lift, the mid level lapse rates being poor was only magnified.

When we started to see the storms resort to outflow boundaries to trigger new weak updrafts, the writing was on the wall.

We had two tornados touch down in the area, multiple warnings all day and plenty of severe weather reports to go around. Just because you may not have had severe weather in your backyard doesn't mean that the area didn't experience severe weather. I think some people make the mistake of thinking that just because they are in a moderate risk that they are guranteed to see severe. Even a 45% hatched means that you have less than a 50/50 shot.

[earthlight]

We had two tornados touch down in the area, multiple warnings all day and plenty of severe weather reports to go around. Just because you may not have had severe weather in your backyard doesn't mean that the area didn't experience severe weather.

Whwre did I even hint at the thoughg that nobody experienced severe weather?

[Dsnowx53]

We had two tornados touch down in the area, multiple warnings all day and plenty of severe weather reports to go around. Just because you may not have had severe weather in your backyard doesn't mean that the area didn't experience severe weather. I think some people make the mistake of thinking that just because they are in a moderate risk that they are guranteed to see severe. Even a 45% hatched means that you have less than a 50/50 shot.

The tornado warnings were mostly all in reference to the discrete convection well ahead of the line, which made sense given the low-level moisture and instability allowed the storms to be surface based, thus tapping the low-level shear, leading to tornadoes and rotating supercells. Also, in the late morning hours, the forcing was still at our latitude.

But we were talking about the reasons behind the squall-line weakening.

[bluewave]

The TDWR did a great job of showing the low level rotation.

[billgwx]

The TDWR did a great job of showing the low level rotation.

Without TDWR, I doubt I'd have issued tornado warnings before hearing of damage. Rotational signatures were present but too weak on the OKX/DIX 88D's, though storm geometry on the 88D's might still have clued me in.

[Dsnowx53]

Without TDWR, I doubt I'd have issued tornado warnings before hearing of damage. Rotational signatures were present but too weak on the OKX/DIX 88D's, though storm geometry on the 88D's might still have clued me in.

Yeah, the SRVs weren't nearly as impressive on KOKX or KDIX. This was the best couplet I could find from either site, and it was already several minutes after the Breezy Point tornado touched down. And it still wasn't even that impressive.

Kudos to you for being on top of the TDWR radar sites and issuing the timely warnings!

[billgwx]

Yeah, the SRVs weren't nearly as impressive on KOKX or KDIX. This was the best couplet I could find from either site, and it was already several minutes after the Breezy Point tornado touched down. And it still wasn't even that impressive.

Kudos to you for being on top of the TDWR radar sites and issuing the timely warnings!

Different story 88D-wise from the August 2007 Brooklyn tornado, when KDIX 88D spun up fiercely with a TVS. That event also became the poster child for using TDWR to watch for tornadogenesis.

Thanks for the kudos! In all honesty warning ops were a team effort...our aviation forecaster first noticed the rotating cells over lower NY Harbor and I watched 'em from that point on.

[bluewave]

Without TDWR, I doubt I'd have issued tornado warnings before hearing of damage. Rotational signatures were present but too weak on the OKX/DIX 88D's, though storm geometry on the 88D's might still have clued me in.

It was great teamwork from your crew. Would it be possible to post the ACARS sounding from JFK at

the time of the tornado? I was wondering if the rules would permit you posting the full study that

you helped write on our local tornado climatology? It looks like a really interesting read from the

abstract. I am also wondering if this tornado along with the August one were the first documented

cases of a tornado circulation developing in the coastal waters south of NYC/LI and coming ashore?

I can remember the warned cell back in October 2007 showing a good couplet south of LI but

weakening as it approached the coast. I know in both cases this year the dewpoints and

the ocean temperatures were right around 75 degrees.

http://journals.amet...ournalCode=wefo

This paper describes the climatology of tornadoes around New York, NY (NYC) and Long Island (LI), and the structural evolution of two tornadic events that affected NYC on 8 August 2007 and 16 September 2010. Nearly half (18 of 34 events from 1950-2010) of NYC-LI tornadoes developed between 5 AM to 1 PM EDT, and August is the peak tornado month as compared to July for most of the Northeast U.S. A spatial composite highlights the approaching mid-level trough, moderate MUCAPE, and frontogenesis along a low-level baroclinic zone.

Shortly before the early morning tornadoes on 8 August 2007, a mesoscale convective system intensified in the lee of the Appalachians in a region of low-level frontogenesis and moderate MUCAPE (~1500 J kg⁻¹). Warm advection at low-levels and evaporative cooling within an elevated mixed layer (EML) ahead of the MCS helped steepen the low-level lapse rates. Meanwhile, a surface mesolow along a quasi-stationary frontal zone enhanced the warm advection and low-level shear. The late afternoon event on 16 September 2010 was characterized by a quasi-linear convective system (QLCS) that also featured an EML aloft, a surface mesolow just west of NYC, low-level frontogenesis, and a southerly low-level jet ahead of an approaching mid-level trough. The QLCS intensified approaching NYC and generated meso-vortices as the QLCS bowed outward. These cases illustrate the benefit of high density surface observations, terminal Doppler radars, and sounding profiles from commercial aircraft for nowcasting these events.

[Weathergun]

I do think the high dewpoints and above normal SSTs, have contributed to more severe/torandic activity, along the coastm, than what usually expect with winds off the water. Instability, shear, moisture convergence, is all enhanced.

[bluewave]

I do think the high dewpoints and above normal SSTs, have contributed to more severe/torandic activity, along the coastm, than what usually expect with winds off the water. Instability, shear, moisture convergence, is all enhanced.

CIMSS recently added a post with some great satellite imagery.

http://cimss.ssec.wisc.edu/goes/blog/archives/11291

[Dsnowx53]

I posted a reanalysis of this event, in case anyone wants to read:

http://www.nymetroweather.com/2012/09/11/a-reanalysis-of-the-saturday-september-8-severe-weather-event-what-caused-the-brooklyn-tornadoes-and-why-did-the-squall-line-weaken/

[Hailstorm]

So what was the maximum gate-to-gate velocity shear in knots at 500-1000 feet depicted on the scans during Breezy Point and Canarsie tornadoes? I know that such values can get as high as 240 knots in the intense tornado outbreaks in the Plains, such as during the 2011 super outbreak. My guess is that in this case on September 8, the value was probably in range of 100-120 knots, since 110 MPH winds were estimated in the Canarsie tornado. Can anyone validate my guess?