Questions?

[J Tyler]

Skip Talbot. Thank you. I had managed to figure out about 70% of that on my own (and by reading a lot on StormTrack), but you filled in those "grey areas" for me. I had NO IDEA how to figure the cap erotion, or strength of the cap. I did not know that was what the "CIN Forcast" was showing! So, huge help there. Thank you for explaining what values to look for also.

I also was unclear on what shear values were needed (what value was 'good' and what value was 'great').

I'll be rereading this post from you many times to make sure I absorb all the details, and I can't wait for the first storms to pop up so I can play with my new knowledge. Thank you again.

Andrew, your reply on the RFD question was a big help to me as well. You summed up in one paragraph what I've been trying to understand for quite a while now, and did so in easy to understand language. Thank you, sir.

 

[Jasson Dotts]

Danny... The information you have been sharing is fantastic for us new guys! Thank you so much!!

Skip... This is exactly what Iv'e been looking for! You just saved me a lot of time and frustration.

Thanks guys!

 

[Jim Parsons]

Great write up Skip....Thanks,
Do you use any satellite pictures when you are out on a chase and want to narrow your target area or to get a better idea of your storm initiation point?

 

[Jesse Risley]

I'll share a few additional ones that I always examine when making a forecast. A standard disclaimer would be that these are but yet another set of indices to consider after pouring over your basic shear, instability, moisture and lifting mechanisms. None of these indices, standing alone, are necessarily to be taken as "OMG..magic tornado bullseye" markers. Each parameter needs to be correlated with other environmental factors.

• 0-1 km EHI - This is called the Energy-Helicity Index, and it's essentially a mathematical computation that yields a composite parameter indicating where the best juxtaposition of CAPE and low-level wind shear exists. 0-1 km EHI values > 3 are generally considered to be strong indicators of where an environment may be ripe for significant supercell tornadoes. You can also review the 0-3 km EHI, but you would want to focus on how much of that turning is occurring in the lowest 1 km of the troposphere to better refine tornadogenesis possibilities. Composite parameters are often helpful as an additional tool to review, as opposed to simply trying to pour over simple wind shear or thermodynamic parameters to try and see where the better tornadic potential may lie. (ref: 0-1 km EHI)

• BL-6 km shear - This parameter is helpful in determining the storm mode. Strong deep layer wind shear vectors, which denote change in wind with height, that co-exist perpendicular to a boundary like the pacific front tend to favor discrete supercell structures. Stronger deep layer shear values tend to promote more vigorous updrafts, with supercells commonly associated with deep layer shear values AOA 38-44 kts. (ref: SFC- 500 mb Bulk Shear)

• LCL height - This is called the Lifting Condensation Level, and put simply, is the height, usually expressed in meters, at which a buoyant air parcel reaches saturation (dew point temperature) when cooled due to being lifted from the surface. It's a good estimate as to the height of the cloud base, since this is the point at which a saturated parcel condenses and forms a cloud. LCL heights < 1249m are considered decent in the context of general support for significant supercell tornadoes, though stronger tornadoes are often accompanied by LCL heights of <1000 m. However, sizable SRH, very ample deep layer shear and higher overall CAPE values have been noted to compensate for higher LCLs and still yield significant TORs. (ref: LCL height)

• LFC Height - Coined Level of Free Convection, is the point at which an air parcel exceeds the temperature of the surrounding air and rises freely. If the LFC level is closer to the surface, it indicates that the most buoyant potential energy is closer to the surface. LFC Heights <2000 m are considered decent values for tornadoes. (ref LFC height)

• 0-3 km Mixed-Layer (ML) CAPE & Surface Vorticity - CAPE values > 60 j/kg (vort > 1), when coupled with favorable shear profiles, can promulgate the evolution of surface-based convection with rotating updrafts.

 

[Skip Talbot]

Great write up Skip....Thanks,
Do you use any satellite pictures when you are out on a chase and want to narrow your target area or to get a better idea of your storm initiation point?

Yes, I look at a wide variety of data sources the morning of and during the chase. The ones I listed above are just the bare essentials. I use the satellite heavily the morning of and right up to initiation. The morning of, its useful for gauging which areas are going to realize their forecasted instability. Areas with more cloud cover than expected will have lower amounts of instability, and areas with less cloud cover will generally have more instability. This is great for reality checking yourself on big chase days and for preventing yourself from getting caught unaware by a surprise severe weather outbreak. I've noticed that some forecast sources including the SPC and NWS will often stick to their guns concerning outlooks. They had a lot of confidence going into the event and they often times will not back down, even as it becomes more obvious that conditions are not going to pan out. The satellite may be one of your first clues that things are awry. If you've got a 15 hatched tornado setup that was banking on moderate to strong instability, and the morning crapvection and clouds never clear out, you're probably not going to realize that instability, and, in the absence of strong upper level dynamics, the forecast may bust. Likewise, if there is no risk because one of your key ingredients was missing, like instability due to massive amounts of forecasted cloud cover, you can be caught off your guard if the clouds do clear. April 20, '04 comes to mind where we had more than sufficient moisture, ridiculous amounts of shear, and a lifting warm front that wasn't supposed to destabilize. The clouds cleared out behind and we had a tornado outbreak on a 0% risk day.

During the chase I watch visible satellite to follow where the cu fields are and where the the boundaries are. Your cu fields are the birthplaces of your future supercells. Follow the cu, and look for the most robust, agitated looking cu fields. One of the most valuable additions I made to my chase software data was getting a visible satellite data feed and underlaying it on my GrLevel3. Watching where the cu fields are before there are any blips on the radar keeps you ahead of the game. Also look for boundaries on the visible satellite. Outflow boundaries may show up as rings, and the cu will often track along other fronts. If you pick your target well and following the cu and progression of the surface features, you can be right under your storm before it even shows up on radar.

 

[Michael Riedl]

Here is a quick question that I have always wondered...

What is the advantage of using Mixed Layer CAPE as a parameter versus Surface Based CAPE?

Maybe I don't know the actual definition of Mixed Layer because wouldn't you want the most instability in the lower levels of the atmosphere?

 

[Jim Parsons]

What do you use for your sat feed on GeLevel3??? I have been using the vis sat placefile from the F5 Chaser site.

 

[Joshua Nall]

This is a pointless thread, it's all been discussed before! Just do a search, for crying out loud!... lol. I'm joking. I think a questions thread is a great idea. When I first joined seemed like a lot of the stuff that I had questions about, many were tired of talking about.

I don't do a lot of forecasting, but after several frustrating chases over the years, I'd like to mention one extra point that is important to me when picking a target... terrain. Say you've picked where you think the best storms will be, however you go about doing that, only problem is the roads or hilly and windy and have 70ft pine trees on both sides. 100 miles south of there is some flatter land dotted with fields and farmland. Sometimes I would just trudge on, but now I tend to hold up in the open areas, optimistic that storms will fire a little further south than predicted. Or if the terrain varies from west to east then hold out somewhere further east where you have a view and wait for the storms to come to you. I'll still brave the forests for some hail occasionally, even though eyeing a tornado is near impossible, but you don't want to be in a tunnel of trees near a severe storm. It just seems to average out so that staying in the open is more rewarding. Terrain is not such an issue for those who stay in the plains.

 

[Jesse Risley]

Here is a quick question that I have always wondered...

What is the advantage of using Mixed Layer CAPE as a parameter versus Surface Based CAPE?

Maybe I don't know the actual definition of Mixed Layer because wouldn't you want the most instability in the lower levels of the atmosphere?

It really depends on which "parcel" is being lifted. The surface-based CAPE (SBCAPE) computation involves the use of surface temps and dewpoints to determine a (surface) parcel's lifted path, whereas the MLCAPE formula involves lifting a parcel for some depth such as 100 mb of a well-mixed layer where temperature and mixing ratios are held constant. Perhaps we could say that offers a better average, but really all indices need to be considered because each convective situation is unique: SBCAPE, MLCAPE and MUCAPE.

 

[Skip Talbot]

What do you use for your sat feed on GeLevel3??? I have been using the vis sat placefile from the F5 Chaser site.

I use Allisonhouse's feeds including the visible satellite. http://allisonhouse.com/ I believe there are free sources out there as well.

Here is a quick question that I have always wondered...

What is the advantage of using Mixed Layer CAPE as a parameter versus Surface Based CAPE?

Maybe I don't know the actual definition of Mixed Layer because wouldn't you want the most instability in the lower levels of the atmosphere?

It really depends on which "parcel" is being lifted. The surface-based CAPE (SBCAPE) computation involves the use of surface temps and dewpoints to determine a (surface) parcel's lifted path, whereas the MLCAPE formula involves lifting a parcel for some depth such as 100 mb of a well-mixed layer where temperature and mixing ratios are held constant. Perhaps we could say that offers a better average, but really all indices need to be considered because each convective situation is unique: SBCAPE, MLCAPE and MUCAPE.

In simple terms for a our beginner forecasters and chasers, surface based CAPE assumes that the inflow going into your storm's updraft is coming up right off the ground by using the temperature and dewpoint readings at the surface. Mixed layer CAPE averages the temperature and dewpoint for a layer of air above the ground. The difference in temperature and dewpoint strongly influences the resulting CAPE value, or how unstable the inflow is. Surface based CAPE values are often higher than mixed layer CAPE values because the temp and dewpoint are higher at the surface. We are after surface based supercells so we assume our storm is going to be pulling some air up off the surface, but our storm is often pulling in air above the ground too, though, so a mixed layer CAPE value might be more realistic. Neither is going to exactly represent the instability of the air going into your storm's updraft, however, especially considering that we're often working with an approximation based on model data. The actual value may be somewhere in between or well outside either depending on the actual state of the storm and the environment. Look at both and see how they differ across your target area.

 

[Neal Wikner]

Outflow boundaries may show up as rings, and the cu will often track along other fronts.

Lot of great information here. If any of you could give a bit more in-depth discussion on outflow boundaries sometime that would be great! I read up on them but can't seem to grasp it. I also get confused because it seems like the term is also used when a racing squall line is kicking up a line of storms out in front of the main line.

 

[Skip Talbot]

Lot of great information here. If any of you could give a bit more in-depth discussion on outflow boundaries sometime that would be great! I read up on them but can't seem to grasp it. I also get confused because it seems like the term is also used when a racing squall line is kicking up a line of storms out in front of the main line.

There's a good thread about OFB's:
http://www.stormtrack.org/forum/showthread.php?3511-thunderstorm-outflow-boundary

Just think of them as a mini cold front created by the outflow of a thunderstorm. Just like a cold front they are marked by an advancing pool of cold air into a warmer air mass and a change in wind direction. On a surface chart you can look for a temperature gradient, but sometimes they are too small or subtle to notice other than maybe a single station with a wind barb off in a different direction that the rest. If there are thunderstorms in your target area or there were that morning, its a good assumption that there are some outflow boundaries hanging about. This is something to consider as an additional source of lift when comparing to other potential target areas that did not have recent thunderstorm activity.

 

[Rob H]

I hope I'm not crashing the thread..

Keep in mind that you want to know how various sites calculate CAPE, because some aren't kind enough to clearly tell you how it's computed. I wanted to briefly touch on how I use the various CAPE parameters in the real world, since that might be of value:

SBCAPE is useful for storm environments where only a low level of inflow is ingested into the storm, I personally don't use this much because it's tricky to identify situations that would benefit from this method. Use this when you think that MLCAPE might mask some short CAPE scenarios. The downside is that it might be a red herring if you have a very warm, moist layer right at the surface (evapotranspiration from crops comes to mind) and these types of scenarios aren't as common.

MLCAPE is the standard go-to measure of instability and can be used in most convective scenarios.

MUCAPE is useful for elevated convection (ie. likely non-tornadic) scenarios where the layer of inflow is not easily determined.

Also, to chime in on an earlier question - interpreting satellite imagery is extremely useful for things beyond drylines and outflow boundaries. One example we recently talked about in the ST chat room is 5/3/99, and how localized heating from a hole in the cirrus cover combined with lift from horizontal rolls potentially helped make that outbreak what it was. Satellite would have been the only clue to this, although I believe this was only recognized in post-event analysis.

 

[Danny Neal]

FWIW I started on night time chase strategy blog, I am trying to get statements from people who have had bad and good experiences with chasing at night so that is why it is taking a little bit to put together. I have rarely chased at night so it is good to have a wide sampling of strategies.

 

[Nicholas_Scherber]

Thank you everyone who is dedicating there time to help us newbies. I think it's amazing how many people are willing to take timebout of there lives to teach us beginners. There have been several times now where i have asked a small stupid question and ended up getting an hour long explanation on the subject. So once again, Thanks!