850-700 mb capping/convective temperatures

[David Conaway]

I have a question regarding the temperatures(cap) at the 850 to 700 mb level and what is the favored range for convective development?

I know that there are many factors such as synptic or mesoscale boundries that can cause rising motion and allow surface based particles to break through the cap if there is enough heating/moisture. Also, an upper level disturbance can lower the temperatures at these levels.

I guess the question I'm really asking is what is the ratio from forecasted surface temperatures to the 850-700mb level temepratures that make you decide weather you think that there will be some storms as opposed to nothing at all?

I know that if there was a simple answer, there would be many more chasers that don't waste an entire day and all of the expenses just to witness a nice sunset. However, I have learned a lot from this site and I know that there are some here who have preferences on this aspect of making a forecast.

Thanks in advance to anyone who can help me expand my knowledge on this topic.

David Conaway

 

[Jason Boggs]

It's not all about just the cap. Yes, the cap has a lot to do with it, but there are scenarios that come into play. Take today for example. The SPC has most of northern OK in a risk for hail and a slight chance for a tornado or two. I got up today and decided not to make the trip. The reasons...

There really is no dynamic system that's coming in to give a lot of forcing. It's mostly daytime heating along with some very weak impulses coming in. On top of that, the temp at 700mb right now is anywhere from 10-13 degrees celsius. That being said, there was a very very small chance storms would form in northern OK. Nothing has initiated up to this point.

So, it all depends on a lot of things.

The temp at 700mb. Generally anything over 12 degrees celsius is pretty stout. There will have to be other factors come into play to help weaken the cap. Below are a few of the factors:

The amount of forcing can help weaken the cap. The more forcing, the better chance of weakening the cap.

Moisture and temp advection can help weaken the cap. I think moisture is a little more important than warm air advection.

Jet streaks can help weaken the cap.

Shortwaves can help weaken the cap.

There are other ingredients that can help weaken the cap, but these are some of the main ones.

EDIT: It's always good to have a moderate cap on a possible convective day. If the cap is weak, thunderstorms form everywhere, especially if there is a lot of linear forcing such as a cold front.

 

[Jon Miller]

Jon Davies has some good info on 700mb cap temps:

http://www.jondavies.net/700mbTcap/700mbTcap.htm

 

[David Conaway]

I appreciate the information. I had saved the document from Jon Davies some time ago but I do not know what I have done with it. This is the information I was looking for.

I understand the aspect of a cold front causing enough convergence or an upper disturbance weakening the cap to cause convection. But many times, we expect development along the dry line during the afternoon and evening only for it to not happen. Then later on during the evening and night, storms will develope along a surface cold front or other boundry because of forcing.

When storms do not develope along the dry line because of the presence of the cap being too strong, I would like to know the reason why the cap did not break. Weather it be because of the surface heating not being warm enough or because of the 850-700mb temperatures being too warm.

I have studied severe weather outbreaks and busts over the years but the cap is one aspect that I still have trouble with. Again, if there was a simple cut and dry answer then chasers would have much more success.

I have seen days where the cap would seem impossible to break only to have severe weather. However, on other days that it appeared that the cap would easily break, the cap would hold firm.

I thank you for the responses and any information helps.

David Conaway

 

[Jon Miller]

Jeff Haby also has a topic on capping temps found on upper air soundings:

http://www.theweatherprediction.com/habyhints/308/

 

[Rich Thompson]

David,

There are several complicating factors when dealing with the so-called "cap". I'll list a few of them below:

1. Is the sounding in the right place at the right time to judge the cap? Many observed soundings give a rough idea of cap strength, but they're not located exactly where you'd like to see them. Narrow zones of lift along fronts and drylines can change the profiles quite a bit in a small area.

2. What's the path of the air parcels from the LCL to their LFC? The LCL is often assumed to be the cloud base, where clouds first form due to lift. On the way to the LFC (level of free convection, or the beginning of the layer in the sounding with CAPE), what happens to the air parcels? In a "loaded gun" environment, it's often quite dry between the LCL and the LFC. Dry environmental air can mix with the air parcels in the updraft and dilute them to the point where there's no CAPE left (a true "turkey tower" with no more upward growth).

This topic is pretty complicated by itself. All I can tell you is stronger lift makes it easier for air parcels to reach their LFC because the environmental lift results in a deeper moist layer with less opportunity for dry entrainment. Strong vertical shear in the LCL to LFC layer also makes it tougher for an air parcel to reach the LFC due to mixing with drier environmental air. If you think about what the soundings look like in FL during the summer compared to KS in the spring, you can see why so many more storms tend to form in FL each day.

3. Storms don't just automatically form because the cap=zero. Strong surface heating often reduces convective inhibition during the afternoon and brings the LCL and LFC closer together, which requires much less lift to get a storm started. You still need some form of lift, though. It can be subtle terrain features, weak outflow boundaries, differences between land use, changes in soil moisture, etc., can all contribute to storm formation if the cap is negligible and the air parcels have little chance of entraining dry air from the environment (very moist from the LCL to the LFC).

Some people think that mid-upper features provide the lift to initiate storms. Outside of elevated convection that's based in the mid levels, the source for lift for chase storms is down close to the ground. An approaching mid-upper trough can condition the background environment through rising motion and moistening which make it easier for storms to form. Likewise, sinking air aloft results in warming and drying, which makes it harder for storms to form.

I know I threw a bunch of stuff in my reply, but it's important to think about all of this if you want to improve at forecasting storm formation. My reply also hints at why probabilities are the "language" of weather forecasting - there's just no way to actually know all of this in each case with no doubt!

Rich T.

 

[Jeff Duda]

Another thing to consider is the convective temperature. It gives you an idea of how warm the low-mid levels are. The method to determine convective tempreature on a skew-T (as I've been taught) is to take the surface mixing ratio value up a constant mixing ratio line until you hit the environmental temperature curve (the main one...don't count notches...this is called the convective condensation level (CCL) and is analogous to the LCL), then bring that down dry-adiabatically to the surface. The warmer it is at 700mb (or in that vicinity), the farther to the right you go before coming back down, and hence the warmer you are at the surface. Also, the higher you have to go along the constant mixing ratio curve before hitting the temperature curve, the higher the CCL and the more warming you'll get going down a dry adiabat.

It can be humorous to look at profiles on days where the convective temperatures are in the upper 90s or > 100, because it's highly unlikely that storms would ever form on such days. However, if convective temperatures are more manageable, say in the 80s, then you'll have enough resistance (capping) to let instability build, but also enough leeway to get storms to form just from daytime heating. Then they'd have plenty of instability to work with.