East Tennessee Storms

[welkin87]

So I've been on twisterdata.com watching the storm systems come through Tennessee and I'm doing this to learn more about severe weather (had my first post recently, going on my first chase in May).

I've been reading about CAPE and helicity recently. The helicity seems to be very high for East Tennessee the next 6-8 hours but the CAPE seems to be low. The reason I ask is that they are forecasting high winds, hail, and possible tornadoes. The helicity makes sense but I'm trying to tie in other factors like CAPE, humidity, temperatures, etc., and I'm getting lost

So, I guess my question is, what ingredients should I be focused on when forecasting for potential severe weather?

I know a fair amount about weather in general but an understanding of severe weather is a new frontier for me.

Thanks for the help!

 

[Timothy Finn]

This is a great question. I look forward to seeing some replies on this.

Tim

 

[Donnie Beasley]

I'm fairly new to forecasting myself so I can't explain in detail the way some around here can. Basically what it is, is helicity is a parameter that has to do with wind shear. In the colder months you typically see high wind shear and low cape. In the spring and summer you typically see the opposite, high cape with lower shear numbers. The reason the 4/27 Tuscaloosa event last year was so dangerous is that it had very high amounts of both. So, what you are seeing today is that the very high shear is making up for the lack of instability.

 

[Chip Redmond]

Without getting into great detail and beating a dead horse, I recommend that if you want to learn what factors go into predicting severe weather, go to the past forecasts in the Target Area or Historical Chases forums on here. Also read up a bit on the internet. There are countless sources for this information.

Here are a few links:
http://www.theweatherprediction.com/
http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/svr/home.rxml
http://www.nwas.org/committees/professionaldevelopment/links.php
https://www.meted.ucar.edu/

And as a quick and dirty resource, obviously Tim's books are a great resource http://www.weathergraphics.com/

Chip

 

[Wes Carter]

For a quick answer, when the atmosphere is highly sheared it takes less instability (CAPE) to form a supercell capable of producing a tornado. We tend to get high shear/low CAPE severe events earlier in the severe weather season. As the season progresses the systems can have more instability and less shear. Either one can produce a rotating updraft and separate the updraft from the downdraft/rain core enough for the cell to achieve a steady state.

This was the quick answer from a fellow novice hack. I hope I gave you enough to help. If not, I'm sure someone will be able to add to it.

 

[Skip Talbot]

So, I guess my question is, what ingredients should I be focused on when forecasting for potential severe weather?

I spent some time writing up this post on basic ingredients based forecasting:
http://www.stormtrack.org/forum/showthread.php?26607-Questions&p=298977&viewfull=1#post298977

Moisture, lift, and shear are certainly in place on today's setup. You're right that the instability seems lacking. 500 J/Kg SBCAPE can squeak out a supercell on it's own, but you're getting a lot of help from the upper level dynamics. Also of note is that there is a good deal of low level instability. So while you're not getting gigantic summer time storms, you're low topped convection can still whip up a mesocyclone and put down tornadoes, it just happens in a lower/shorter layer.

Wes points out what makes up for our lacking instability. We still need a way for updrafts to rise as one of our key ingredients, and here the buoyancy is being supplemented by mechanical lift. You've got a 100+ knot midlevel jet. That's extreme. The dynamics from upper air divergence will actually force your updrafts upwards.

When you have an extreme excess of an ingredient, all bets are off. Extreme upper air support can make up for lacking instability, and likewise extreme instability can produce gigantic supercells that modify their own environment to make up for a lack of shear.

 

[Bob Schafer]

...I'm trying to tie in other factors like CAPE, humidity, temperatures, etc.

The other posters answered your questions very well, and I have little to add, but I thought I'd toss in that I, personally, almost never look at humidity or temperatures anymore. Dewpoints, yeah, but even better than Td's is LCL's. Most of the time I'll take a quick glance at Td's, then just look at the forecast LCL's and the forecast sounding to see if there's any weirdness. I don't care if the Td is 40 if there's (at least some) CAPE and the LCL is 750 meters.

My obsession for the last couple years, though, has been inflow characteristics. Storms gotta breathe.

 

[Erik Perozo]

So I've been on twisterdata.com watching the storm systems come through Tennessee and I'm doing this to learn more about severe weather (had my first post recently, going on my first chase in May).

I've been reading about CAPE and helicity recently. The helicity seems to be very high for East Tennessee the next 6-8 hours but the CAPE seems to be low. The reason I ask is that they are forecasting high winds, hail, and possible tornadoes. The helicity makes sense but I'm trying to tie in other factors like CAPE, humidity, temperatures, etc., and I'm getting lost

So, I guess my question is, what ingredients should I be focused on when forecasting for potential severe weather?

I know a fair amount about weather in general but an understanding of severe weather is a new frontier for me.

Thanks for the help!

You have been given some really good answers. Instability and Shear are the main ingredients to focus on for Tornado development. You could go into more detail and look for 0-3km Cape, Sfc Vorticity, 0-1 Shear, MLCAPE....etc but as a good rule of thumb you need a good mixture of both. Too much shear and not enough CAPE and supercells don't last as long. Too much CAPE and not enough shear and you tend to get water loaded storms. Air Mass Thunderstorms are a great example of High CAPE, Low Shear. Other factors are the depth of the CAPE (fat CAPE vs. skinny CAPE), the depth of the Shear within the CAPE or vice versa. Obviously if you throw in a boundary with just high CAPE you can get some interesting stuff. But to apply the KISS method Instability and Shear.

Chip gave some great links for you to look at.
I love Meted's Principles of Convection III: Shear and Convective Storms
Jon Davies has some good stuff as well.
http://www.jondavies.net/DaviesWAF_coldcoretors_dec06.pdf
This is a good read for tornadoes in low CAPE environments.

 

[welkin87]

Can't too high of shear ruin a storm though? I might have mis-read/heard.

Also, would Tim's Weather Analysis and Forecasting be a good book to start with?

 

[brice.coffer]

The other posters answered your questions very well, and I have little to add, but I thought I'd toss in that I, personally, almost never look at humidity or temperatures anymore. Dewpoints, yeah, but even better than Td's is LCL's. Most of the time I'll take a quick glance at Td's, then just look at the forecast LCL's and the forecast sounding to see if there's any weirdness. I don't care if the Td is 40 if there's (at least some) CAPE and the LCL is 750 meters.

My obsession for the last couple years, though, has been inflow characteristics. Storms gotta breathe.

High dewpoint depressions will dictate the temperature of the storm's downdraft. Water evaporates in dry air, evaporation leads to cooling, and cold downdrafts can potentially cut off the storms inflow.

Storms can't breathe if they are being choked.

Can't too high of shear ruin a storm though? I might have mis-read/heard.

Also, would Tim's Weather Analysis and Forecasting be a good book to start with?

High shear can cause a weak storm to tilt so much it topples over. However CAPE over 1000 J/KG will usually be able to sustain itself.

And yes, Tim's books are great for learning.