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Severe weather


tombo82685

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I have 2 questions. In reading the SPC's statement yesterday for their reasoning for issuing the severe thunderstorm watch, they referred to buoyancy. What is bouyancy? Does it relate to some degree how moist the atmosphere is?

 

Also, when looking at shear, which do you look at for thunderstorm development? On the spc's meso analysis site, they have shear effective, sfc to 1km, 6km, and 8km. Which is the best?

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I don't know of any model pages that really forecast shear, besides twisterdata. That site breaks it down to 925mb shear, 850mb shear, and 700mb shear. If you go by sfc-6km that would pretty much cover them all. What if 700mb has solid shear and the others don't. Would that be conducive for thunderstorm development? Would shear in the levels below that determine how strong/super cells would form?

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I might be dumb, but I thought buoyancy and CAPE were the same thing.

ish. Buoyancy is how force is acting on the parcel in the z-direction, which is related to the delta between the parcel density and the ambient density, the integral of which is CAPE
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IIRC, 0-6km shear gives you a general t-storm outlook, but you need to look at 0-1km shear for tornadogenesis. Most of the people I follow on twitter use low level helicity (which measures both shear and instability) and the sigtor parameter

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IIRC, 0-6km shear gives you a general t-storm outlook, but you need to look at 0-1km shear for tornadogenesis. Most of the people I follow on twitter use low level helicity (which measures both shear and instability) and the sigtor parameter

The lower level shear makes sense for super cells as you would want turning of winds down to the surface.

Is that parameter the ehi? I know that takes your energy (cape) and helicity which makes up the index.

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The lower level shear makes sense for super cells as you would want turning of winds down to the surface.

Is that parameter the ehi? I know that takes your energy (cape) and helicity which makes up the index.

 

Around here a very low lcl is often useful for tornadoes. Last week's event was closer to midwestern type supercells.  Many of our tornadoes which have more to do with intersecting boundaries occur on days when flash flooding is also happening (Like Manhawkin last year).  Of course now that I typed intersecting boundaries, a bay/sea breeze did reach that tornadic supercell last week.

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  • 2 weeks later...

0-1 km helicity values > 100m2/s2 seems to work ok around our area too. The SPC site (SREF tool) has many shear products.

My storm chasing gene kicks in once it gets over 150 m2/s2 assuming adequate instability   ;)

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  • 3 weeks later...

Since I finally registered...  re: the shear question.

 

0-6 km or sfc-500 mb (essentially the same thing, if 500 mb is typically around 5.7 or 5.8 km in summer) is good for storm organization.  Typically want at least 35 kts bulk.  But bulk is just the vector difference between the top of the layer and the bottom, it doesn't account for entire wind profile, which is why high bulk shear values don't tell the whole story.

 

As others have mentioned, 0-1 km is key for tornadogenesis.  Typically want at least 20 kts (again bulk) but I've seen tornadoes, like the other day in Indy, with as little as 10 kts on mesoanalysis (though that may not have a high enough resolution to resolve a localized boundary).  In the Plains, you can use sfc-850 mb, since the surface is getting high enough that it isn't that more than 1 km between the two.  Out east, it's about 1.5 km between the two, so you might want to adjust those values upward.  Of course, that's to say nothing of localized climo, which any of our Mt. Holly friends would know more about.

 

If you want to actually know that you have a classic veering wind profile, use helicity.  It's the line integral of the hodograph, relative to the model-produced storm motion vector.  Don't know if it's different locally, but my threshhold values are 0-1 km of 150 m2/s2, or 0-3 km of 250 m2/s2.

 

Tombo asked about EHI.  EHI is Energy and Helicity (hence the E and H), multiplied and divided by some big number.  There's a paper out there that says 1.4 is the tornado threshhold for 0-1 km EHI, tough anecdotally I've noticed it seems that you really want at least a 2.  Strong tornadoes, like we had last week, are probably around a 6 of 0-1 km, and 10+ of 0-3 km.

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  • 3 weeks later...

0-1 km helicity values > 100m2/s2 seems to work ok around our area too. The SPC site (SREF tool) has many shear products.

 

Quick question. I was taking a look through the options in the "Severe" tab yesterday on the SPC SREFs page, and was wondering about Craven Brooks significant severe parameter. It is essentially defined as a product of mean layer CAPE and shear, so it appears to really just be a different view of the data, but I was curious if it's relevant at all from a forecasting perspective.

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  • 2 weeks later...
  • 2 weeks later...

Quick question. I was taking a look through the options in the "Severe" tab yesterday on the SPC SREFs page, and was wondering about Craven Brooks significant severe parameter. It is essentially defined as a product of mean layer CAPE and shear, so it appears to really just be a different view of the data, but I was curious if it's relevant at all from a forecasting perspective.

I look at shear and CAPE separately. Lumping them into a index can be misleading when dealing with a very high shear or CAPE event and little of the other. 

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