Minimum criteria for tornado genesis?

[calvinkaskey]

I'm wondering if anyone has a good idea of what the minimums for helio, shear, and cape combinations for tornado genesis. For example in NE pa NAM is predicting 450 parcel cape with helio of 709 and BSHR of 149 at 2am EDT this Thursday. At 4 pm EDT in northern VA this Wednesday the numbers are 329 parcel cape, helio of 683 and BSHR of 193. Now the hodographs are pretty much off the charts from other times I've chased and been near tornado and severe weather outbreaks. Here is the hodo for northern VA http://www.twisterdata.com/index.ph...ng=y&sndclick=y&sounding.x=716&sounding.y=315

This one if for ne pa http://www.twisterdata.com/index.ph...nding=y&output=image&view=large&archive=false

Climatologically in New York 2 am would not be unprecedented as 4 out of 10 tornadoes to occur in New York have occurred at odd hours ie. 12am, 2:45am and 2 between 9 and 10 am.
Thursday afternoon has a 244 BSHR but only 307 Helio and 274 parcel cape and has this hodograph http://www.twisterdata.com/index.ph...nding=y&output=image&view=large&archive=false Maine has had 6 tornadoes which is pretty impressive considering the lack of population, terrain and that N.Y. has had only 10 and PA only 13.

 

[Tim Supinie]

First of all, it's never a good idea to assign hard minimums to anything, because eventually something will come along and break your minimums. Unless your minimums are 0 CAPE and 0 SRH.

So it sounds like you're trying to figure out why there wasn't a higher tornado threat in the northeast US the other day. The soundings you posted had very large SRH (600-700 m^2/s^2 SRH), but pretty meager instability (200-500 J/kg). Just from looking at the soundings, CAPE is the limiting factor. Sub-500 J/kg CAPE will not cut it in most cases; the updrafts won't be strong enough.

The other issue that you can't really pick out from single soundings is surface lifting mechanisms and/or storm mode (they're related). The issue with the afternoon stuff is where the boundaries are: the cold front is still back in Indiana and Ohio, and the warm front across central/southern New York is not oriented well to initiate surface-based supercells. The issue with the overnight stuff is storm mode. Your cold front is oriented NNE-SSW over much of the region, which is right along the upper-level winds: meaning a linear storm mode. The exception is in southern New York, where it's more NNW/SSE. This in itself is not enough to prevent tornadoes, but what really discourages them is the low-level shear. I've attached a hodograph from the 29/00Z KOKX sounding to describe what I'm talking about. The brown line is the boundary motion in southern NY (guesstimated from the WPC surface analyses here: http://www.wpc.ncep.noaa.gov/html/sfc-zoom.php). You really want the 0-3 km shear to be more perpendicular to the boundary (there's a graphic here: https://cimmse.wordpress.com/2015/0...st-with-qlcs-tornado-warning-decision-making/ that describes that) for QLCS tornadoes.

 

[calvinkaskey]

Ok, thank you. Yeah I was thinking that the Cape was pretty low. A lot of times the conditions change so that they aren't as favorable as in previous models. I'm not sure how to interpret the graph above. So if the Blue arrow in the top right quadrant would be more favorable if it was the "brown line"? It seems like this involves some manual calculation. I was hoping to get some help on the skew t's that I posted and the BSHR and helio numbers. I thought the wind directions as plotted on the skew ts showed wind shear in the low levels. Thanks for you help Tim.