Low topped squall line overnight Wednesday into Thursday

[TalcottWx]

Yeah as sbos was saying about elevated instability, it's the dynamic driven events with elevated instability that have the ability to peak after dark because they are not dependent on diabatic heating. With a near inversion/isothermal layer like we had last night below 850mb, usually it's difficult to get winds down to the surface because as the cool air sinks it eventually hits air as cool as it and the homogeneity prevents the air from sinking farther. A steepl apse rates goes both ways in terms of rising air or sinking air. I'm looking at a sounding for the Taunton area around 9z just before storms hit and there's a brief but steep adiabatic lapse rate between 750-700mb and a constant 13-14C 850 and below. At 12z the lapse rate is a bit steeper, relative to what it was hours earlier, and was probably enough to help that llvl jet (about 65kts even as low as 925mb in places) reach the surface. I'm assuming this was the case in most areas in the warm sector. 

 

A lot of forced events don't have a lot of lightning like we saw with especially storms between RI and Boston. Figuring out which storms will be prolific lightning producers and which ones will not has always eluded me, but I think the steeper the lapse rate between -10C and -30C the better. Lapse rates were relatively steep in that area for New England, around 7C/KM I think, and the dynamics and amount of kinetic energy in the storms allowed for plenty of graupel interaction to allow charge separation. Flashes were "hot" and flickering with a handful of return strokes in our storms though, which was surprising because most of the cold season storms I've witnessed the flashes are "cold" and no there are no return strokes. 

Although we didn't have a blatant inversion, most storms that are elevated produce higher amounts of IC than CG. It's almost like the top of the inversion is the ground relative to the storm in a way. Once that isothermal layer diminished as storms moved through, especially SE MA and RI, we saw storms with higher amounts of CG strikes.

If an inversion is in place, however,  the thunder will sound distant and overhead from IC flashes because the sound bounces off the inversion and stays mostly above the inversion. If there's a CG strike the sound can be deflected off the inversion and that sort of creates an effect where the thunder is louder/quieter in areas you wouldn't think it'd be. 

I think the key to this one was the steep lapse rates and the strong LLJ. Without both we wouldn't have been able to mix down such substantial winds and we really maximized the MUCape because of the combination resulting in a sh1t ton of lightning.

[TalcottWx]

I just got back from a drive, saw a few branches down on 125 in Andover/North Andover, and a tree down on Andover st in North Andover, half mile from Merrimack College. 

How was my alma mater looking? They need to do some work to upgrade the appearance of the outer edges of campus. 

[OceanStWx]

For the novice weenies last night is usually how we get our severe events in Eastern MA. Elevated instability is our best and usually only hope. When we have significant severe and shear it usually results in a south, southeast wind direction in Southern New England which floods areas east of ORH with a marine and stabilized airmass that absolutely destroys surfaced based convection at our latitude.

 

Last night had strong southerly winds but the advection of warm air, humid airmass aloft, theta e, etc creates the elevated instability aloft that negates any problems with relying on surface based instability. One of the great examples is the day of the Springfield/Brimfield significant tornados that were originally discrete. Later on that night we had a wonderful lightning show and gusty winds as a result of the elevated instability that remained into the night time hours and we did well with the linear storm mode that developed. MUCape is the best cape as we saw last night. Great lightning.

 

There are even cases of significant severe weather events in the plains with zero surface based instability. They even produce numerous tornadoes on occasion. It is more rare though compared to the surface based events.

 

That's definitely a much smaller subset than significant severe in a stable environment. It's relatively easy to produce very large hail in a stable sounding, harder to get wind (straight line or rotational) involved. Usually this is when a very shallow nocturnal inversion has formed or a location is just on the cool side of the warm front.

 

Greensburg, KS is a good example. This is from an SPC publication on significant nocturnal tornado events (LLJ enhanced). You can see the purple sounding is the 00z from nearby Pratt, and the red/green is near the tornado time at 04z. You can see how the boundary layer has stablized, but the increasing dewpoints have also increased CAPE available in the sounding (your MUCAPE argument for eastern MA).

 

[OceanStWx]

A lot of forced events don't have a lot of lightning like we saw with especially storms between RI and Boston. Figuring out which storms will be prolific lightning producers and which ones will not has always eluded me, but I think the steeper the lapse rate between -10C and -30C the better. Lapse rates were relatively steep in that area for New England, around 7C/KM I think, and the dynamics and amount of kinetic energy in the storms allowed for plenty of graupel interaction to allow charge separation. Flashes were "hot" and flickering with a handful of return strokes in our storms though, which was surprising because most of the cold season storms I've witnessed the flashes are "cold" and no there are no return strokes. 

Although we didn't have a blatant inversion, most storms that are elevated produce higher amounts of IC than CG. It's almost like the top of the inversion is the ground relative to the storm in a way. Once that isothermal layer diminished as storms moved through, especially SE MA and RI, we saw storms with higher amounts of CG strikes.

If an inversion is in place, however,  the thunder will sound distant and overhead from IC flashes because the sound bounces off the inversion and stays mostly above the inversion. If there's a CG strike the sound can be deflected off the inversion and that sort of creates an effect where the thunder is louder/quieter in areas you wouldn't think it'd be. 

 

That may be more of a symptom of our area than elevated storms overall. West of the Appalachians elevated storm complexes can be quite the lightning barrage at the surface. 

[OceanStWx]

"Interestingly, the 59 knot gust in Hartford occurred about 10 minutes after the highest reflectivity (which was coincident with the highest outbound velocities) moved through. I’m not quite sure why that would be the case. In fact at 0602 UTC there was only light rain over KHFD per radar with the heaviest echoes well north and east."

 

I noticed this effect too. I assumed it was because the storms were moving so quickly there was a slight lag in the precip. Also the winds were higher a little above the surface so higher echoes observed were an artifact of llvl jet pushing the rain farther ahead of the rain closer to the ground. Any thoughts on this?

 

 

I have noticed many times this lag between winds on radar and ground wind. I always thought exactly what you described above. It was most noticeable in the Feb 13 Blizzard but again in March 13 retro. A similar situation occurred with Irene.

 

It would depend on whether the line was tilted or not. If it was a vertical line, even though the radar would "see" precip as being tilted forward, in actually it's just a time delay and the surface precip should match the location of the 0.5 degree scan. I think it's probably unlikely we had a highly sloped squall line in this case, as those tend to be less favorable for wind.

 

It's possible there was some element of stability involved here. That the initial convective line and mixing of the low levels allowed the LLJ to more easily reach the surface behind the precip. It's also not uncommon for the RIJ of bowing segments to continue a good deal behind the leading edge. Another possible answer is local pressure perturbations. Sometimes you get a local low along the line, and a local high behind it, creating a gradient and thus increasing wind flow between the two. That seems another less likely answer though, because wind direction should have been different (usually more out of the east in this type of orientation). 

[CoastalWx]

It would depend on whether the line was tilted or not. If it was a vertical line, even though the radar would "see" precip as being tilted forward, in actually it's just a time delay and the surface precip should match the location of the 0.5 degree scan. I think it's probably unlikely we had a highly sloped squall line in this case, as those tend to be less favorable for wind.

 

It's possible there was some element of stability involved here. That the initial convective line and mixing of the low levels allowed the LLJ to more easily reach the surface behind the precip. It's also not uncommon for the RIJ of bowing segments to continue a good deal behind the leading edge. Another possible answer is local pressure perturbations. Sometimes you get a local low along the line, and a local high behind it, creating a gradient and thus increasing wind flow between the two. That seems another less likely answer though, because wind direction should have been different (usually more out of the east in this type of orientation). 

 

That's what I was wondering. Maybe some sort of subsidence or meso high behind helping with the LLJ?

[ryanjn]

It would depend on whether the line was tilted or not. If it was a vertical line, even though the radar would "see" precip as being tilted forward, in actually it's just a time delay and the surface precip should match the location of the 0.5 degree scan. I think it's probably unlikely we had a highly sloped squall line in this case, as those tend to be less favorable for wind.

 

It's possible there was some element of stability involved here. That the initial convective line and mixing of the low levels allowed the LLJ to more easily reach the surface behind the precip. It's also not uncommon for the RIJ of bowing segments to continue a good deal behind the leading edge. Another possible answer is local pressure perturbations. Sometimes you get a local low along the line, and a local high behind it, creating a gradient and thus increasing wind flow between the two. That seems another less likely answer though, because wind direction should have been different (usually more out of the east in this type of orientation). 

Yeah those explanations make sense. About the RIJ being behind the higher reflectivity, I wonder in some cases if the cells are producing the wind or the cells fire up because of the RIJ forcing them at the nose of a node so to speak. The reflectivity in CT when precip began had sort of a wavy nature to it, undulations, and I wonder if the the bands of precip were caused by this or they were caused by the increased mixing like you said. Kind of a chicken vs egg kind of deal where perhaps they both have an effect on each other though. In terms of there being a lag because of the radar not seeing the sfc, the tilt1 on RadarScope for Upton (not sure if it's 0.5 or not) Hartford is 4.5k ft ASL. So not sure if that's enough height to cause this potential lag or not but I still think it's a possibility because it'd take at least several minutes for the precip to reach the ground from that altitude. 

 

 

SBOS; Campus looks the same as it always does to me. We'll see in the spring if they do a nice landscaping job or put some new mulch down. Didn't see any damage on campus driving by briefly though. 

[CT Rain]

That's what I was wondering. Maybe some sort of subsidence or meso high behind helping with the LLJ?

 

Yeah - could be for sure. What was interesting is that on velocity and reflectivity the maxima were co-located at the leading edge... definitely didn't have the look on velocity of a big RIJ.

 

I think Legro's explanation of the convection serving to temporarily mix the boundary layer quite well is the most likely cause. 

[CT Rain]

I can't recall another winter event like this where we had legit CAPE (albeit elevated) of nearly 1,000 j/kg and a fairly weak inversion. It would seem that the steep mid level lapse rates (6.5C/km) plus the abnormally high T/Td at 925mb was the perfect combination for some fun.

[OceanStWx]

Yeah those explanations make sense. About the RIJ being behind the higher reflectivity, I wonder in some cases if the cells are producing the wind or the cells fire up because of the RIJ forcing them at the nose of a node so to speak. The reflectivity in CT when precip began had sort of a wavy nature to it, undulations, and I wonder if the the bands of precip were caused by this or they were caused by the increased mixing like you said. Kind of a chicken vs egg kind of deal where perhaps they both have an effect on each other though. In terms of there being a lag because of the radar not seeing the sfc, the tilt1 on RadarScope for Upton (not sure if it's 0.5 or not) Hartford is 4.5k ft ASL. So not sure if that's enough height to cause this potential lag or not but I still think it's a possibility because it'd take at least several minutes for the precip to reach the ground from that altitude. 

 

It's usually all interconnected. The pressure perturbations produced by the strongly forced squall line are really what help winds get to the surface. You have lifting into the squall line (high reflectivity), vacating an area of local low, air flows rearward aloft, creates a local high at the surface, wind accelerates frontward back towards the leading edge of the squall. If you can maintain that equilibrium you get a long lived event, if the cold pool becomes too strong the whole circulation falls apart.

 

In this example the precip lag would be occurring downstream. So in that image Ryan used, that reflectivity at 4.5 kft or so would rain out somewhere NE of HFD (it won't fall straight down to HFD). So the echoes that actually reached the ground at HFD would have been some time before Ryan's image. That's why I think something besides the high reflectivity caused that wind gust. 

[CoastalWx]

I can't recall another winter event like this where we had legit CAPE (albeit elevated) of nearly 1,000 j/kg and a fairly weak inversion. It would seem that the steep mid level lapse rates (6.5C/km) plus the abnormally high T/Td at 925mb was the perfect combination for some fun.

 

I think in 2011 we had that event near rt 2 that had a virtual EML. That was in early feb iirc? That had pretty good hail. But, this was clearly a widespread event in Feb that rivaled nothing I have ever seen.

[CT Rain]

I think in 2011 we had that event near rt 2 that had a virtual EML. That was in early feb iirc? That had pretty good hail. But, this was clearly a widespread event in Feb that rivaled nothing I have ever seen.

 

Hmmm I don't remember that one - I'll have to go back and look. I actually thought we would get more hail in this event. 

 

A really bizarre event for sure... pretty much all tied into boundary layer stability. 

[OceanStWx]

I think in 2011 we had that event near rt 2 that had a virtual EML. That was in early feb iirc? That had pretty good hail. But, this was clearly a widespread event in Feb that rivaled nothing I have ever seen.

 

 

Moisten that warm nose up a bit and not hard to get hail out of that look. That's legit EML lapse rates.

 

But this last event, what a good case study it would be. I mean meso models nearly whiffed by 1000 J/kg, let alone actually attaining 1000 J/kg during the event.

 

I do think Ryan is right about the very shallow mixed layer being a red flag. Instead of having to fight the inversion all the way to the surface, it had a little mixed layer to work with once it broke through the weak inversion aloft.

[weatherwiz]

Some awesome re-analysis posts in here!

you know what else is interesting...there seems to be a pretty solid correlation to intense periods of WAA (sharp increase in temp/dewpoint in a very short amount of time). Obviously this leads to a vast increase in instability so I wonder if it's possible that the sudden surge in instability acts to further accelerate upward motion and also enhance mixing.

[CT Rain]

2011021900.74494.skewt.parc.gif

 

Moisten that warm nose up a bit and not hard to get hail out of that look. That's legit EML lapse rates.

 

But this last event, what a good case study it would be. I mean meso models nearly whiffed by 1000 J/kg, let alone actually attaining 1000 J/kg during the event.

 

I do think Ryan is right about the very shallow mixed layer being a red flag. Instead of having to fight the inversion all the way to the surface, it had a little mixed layer to work with once it broke through the weak inversion aloft.

 

The NAM and HRRR did show 500+ j/kg leading up to the event - so at least it was not impossible to forecast. It got more impressive each run leading up to the event.

[weatherwiz]

2011021900.74494.skewt.parc.gif

Moisten that warm nose up a bit and not hard to get hail out of that look. That's legit EML lapse rates.

But this last event, what a good case study it would be. I mean meso models nearly whiffed by 1000 J/kg, let alone actually attaining 1000 J/kg during the event.

I do think Ryan is right about the very shallow mixed layer being a red flag. Instead of having to fight the inversion all the way to the surface, it had a little mixed layer to work with once it broke through the weak inversion aloft.

I think I could see how the models were off with the cape. When you have lapse rates about as stew as we had and a juicy low-level airmass, the modes being off by even 1C in either dewpoint or temp could mean the difference of several hundred J of cape. And in he case of a setup like this the difference between 250-500 and 500-1000 is substantial.

Not to mention I also think that being in the prime region of the ULJ streak and having the activity right along the nose of the 500mb jet provided us with sufficient mid to upper support (70-80 knots of 0-6km shear was fantastic too).

But gettin cape in the 0-2km later really helped to piss the inversion away or at least weaken it substantially.

[weatherwiz]

The NAM and HRRR did show 500+ j/kg leading up to the event - so at least it was not impossible to forecast. It got more impressive each run leading up to the event.

That 12z NAM run showing 700+ of cape was a pretty decent eye opener.

[Ginx snewx]

2011021900.74494.skewt.parc.gif

Moisten that warm nose up a bit and not hard to get hail out of that look. That's legit EML lapse rates.

But this last event, what a good case study it would be. I mean meso models nearly whiffed by 1000 J/kg, let alone actually attaining 1000 J/kg during the event.

I do think Ryan is right about the very shallow mixed layer being a red flag. Instead of having to fight the inversion all the way to the surface, it had a little mixed layer to work with once it broke through the weak inversion aloft.

had severe on 2 feet of snow with that one.

[Ginx snewx]

2011021900.74494.skewt.parc.gif

Moisten that warm nose up a bit and not hard to get hail out of that look. That's legit EML lapse rates.

But this last event, what a good case study it would be. I mean meso models nearly whiffed by 1000 J/kg, let alone actually attaining 1000 J/kg during the event.

I do think Ryan is right about the very shallow mixed layer being a red flag. Instead of having to fight the inversion all the way to the surface, it had a little mixed layer to work with once it broke through the weak inversion aloft.

nothing like deluge rain,hail, constant CG and winds near 50 on top of 2 feet of snow

[ryanjn]

Anyone find any good vids of the storms? I can't find any on youtube. There's gotta be a couple out there somewhere.

[CT Rain]

I went back and pulled a lot of the radar data from the 2 PA tornadoes. That Lancaster County storm was a beast. Rotational velocity of about 60 knots!! 

 

http://www.ryanhanrahan.com/2016/02/28/pennsylvania-tornadoes/

[OceanStWx]

I went back and pulled a lot of the radar data from the 2 PA tornadoes. That Lancaster County storm was a beast. Rotational velocity of about 60 knots!! 

 

http://www.ryanhanrahan.com/2016/02/28/pennsylvania-tornadoes/

 

We've altered our color curves for NROT, but what are those values for the Lancaster tornado?

[CT Rain]

We've altered our color curves for NROT, but what are those values for the Lancaster tornado?

 

at peak intensity it was 0.89 nROT.

 

Seems like since the circulation was so small (spatially) being so far from the radar the nROT values were way lower than they would have been closer to the radar - especially given a delta V of nearly 120 knots!

[CT Rain]

Here's the grab at peak mesocyclone intensity about 10 minutes prior to touchdown.

 

No TDS here either - there was a Lancaster Co storm in 2014 that was unwarned that had a pretty good TDS I believe. 

[CoastalWx]

Is nROT a valuable tool vs looking at gate to gate stuff? Can it be used to enhance confidence or decrease confidence in a possibly tornado or is it just another tool to use?

[CoastalWx]

I know Normal,ized CAPE can help determine skinnt crap CAPE vs fat CAPE..wondering if nROT is similar?

[OceanStWx]

at peak intensity it was 0.89 nROT.

 

Seems like since the circulation was so small (spatially) being so far from the radar the nROT values were way lower than they would have been closer to the radar - especially given a delta V of nearly 120 knots!

 

Not bad considering elevation/range. It's normalized after all, and that type of rotation would be far more common at 7000 ft than 700.

 

Is nROT a valuable tool vs looking at gate to gate stuff? Can it be used to enhance confidence or decrease confidence in a possibly tornado or is it just another tool to use?

 

Starting to look that way. Ekster has been doing work on this across New England (to build a big enough database), but nearly all reported tornadoes feature an NROT value of around 0.80 or greater. So while we're looking at traditional velocity scans first, NROT is becoming a sanity check for issuing TORs.

[OceanStWx]

Basically, if you're seeing values of 1 (true rotational signatures, not those funky shear induced NROT signals) or greater you should probably have a TOR out.

[CT Rain]

Basically, if you're seeing values of 1 (true rotational signatures, not those funky shear induced NROT signals) or greater you should probably have a TOR out.

I went back and looked at OKX the night of the storms and nROT was sky high on that line in CT thanks to the shear induced signal you described. Thankfully cooler heads prevailed on Long Island and they didn't issue a bogus warning.

[CT Rain]

Basically, if you're seeing values of 1 (true rotational signatures, not those funky shear induced NROT signals) or greater you should probably have a TOR out.

Yeah when you're getting a 1 nROT that's a pretty strong mesocyclone.