Reading a cap (cinh) map

[Jay Press]

Hey Guys,

This is Brian Press posting under my brothers account. I am in the process of reactivating it..

Anyway, I am pretty clueless when it come to alot of this weather forecasting but I am finally getting better understanding of the important severe elements.

One major question I have is how do you read the cinh (cap) map? I live in Southern California and have a friend who gets me super cheep buddy passes so I can jump on a plane with just a day notice.. Anyway, I have been thinking about coming out for the Monday / Tesday setup but now SPC and others are talking about a cap.. I understand its a warm layer of air, but how do you know when it's gona break. In the attached photo for this 0Z Tuesday (Monday Evening) it shows -10 just SE of OKC, and -40ish near Wichita Falls. Everything else look OK for this setup, yes, I know the winds are veering a bit at 850, but it does seem the surface winds back, but what about that cap.. (Sorry MOD's don't mean for this to turn into a forecast thread, just want some insight on these maps). Any comments would be greatly appreciated.

Thanks All,

B.p.

 

[Andrew Ryan]

I would save your money and time for another day, IMO. I'm going to school in OK and I'm having serious doubts about chasing on Monday!

 

[Derrick Herndon]

Hey Guys,

This is Brian Press posting under my brothers account. I am in the process of reactivating it..

Anyway, I am pretty clueless when it come to alot of this weather forecasting but I am finally getting better understanding of the important severe elements.

One major question I have is how do you read the cinh (cap) map? I live in Southern California and have a friend who gets me super cheep buddy passes so I can jump on a plane with just a day notice.. Anyway, I have been thinking about coming out for the Monday / Tesday setup but now SPC and others are talking about a cap.. I understand its a warm layer of air, but how do you know when it's gona break. In the attached photo for this 0Z Tuesday (Monday Evening) it shows -10 just SE of OKC, and -40ish near Wichita Falls. Everything else look OK for this setup, yes, I know the winds are veering a bit at 850, but it does seem the surface winds back, but what about that cap.. (Sorry MOD's don't mean for this to turn into a forecast thread, just want some insight on these maps). Any comments would be greatly appreciated.

Thanks All,

B.p.
View attachment 1061

CINH is basically the amount of negative energy that needs to be overcome in order for convection to grow (above/through the cap). In the example you provided -10 CINH is not very much and could likely be overcome by sufficient surface heating. Even a strong cap can be broken if the low-level forcing is strong enough, for example a cold front. Some amount of capping is desirable as this allows the instability to increase beneath the capping layer. Keep in mind that the CINH is an integral that represents the area between the environmental temperature and the parcel temperature on a Skew-T. Take a look at these examples:

http://www.directwx.com/files/active/1/682_large.jpg

Both examples have the same amount of CINH. However the cap strength in example 1 is stronger than example 2. In other words the environmental temperature is higher at the level of the cap. There are measures of cap strength (cap or lid strength). This is separate though often related to CINH. The cap in example 1 would likely be more difficult to break. So look at both CINH and cap strength when assessing convective potential.

As to how people know that the cap is going to break they don't. There are many days when the cap does not break despite what appears to be sufficient forcing. You will want to try and find areas where forcing may be maximized such as the intersection of boundaries (assuming other parameters are favorable in that area as well). Keep in mind you are looking at model output which is limited by resolution and the physics of the model. That forecast may not verify. The cap could be stronger than forecast (or weaker). I agree that saving your flight for a another day might be best. Fly out during an event that might provide 2 or more days of possible chase opportunities. This way you maximize your chances. While Mon has potential it is by no means the best looking day. There will be better days ahead.

here is a ST discussion on the subject:

http://www.stormtrack.org/forum/archive/index.php/t-7807.html

 

[Skip Talbot]

Here's another plot I like to check out:
http://68.226.77.253/models/eta/central/CENTRAL_ETA212_ATMOS_LSI_60HR.gif
That's the cap index for Monday night (from this morning's 12z run).

The CINH essentially represents the area of the inversion region on the Skew-T as CAPE represents the area of the instability. The Cap Index is more like the Lifted Index in that its the widest part of the inversion area, whereas the Lifted Index is the widest part of the instability area.

Here's some excellent information and values you can reference when looking at that plot:
http://www.theweatherprediction.com/habyhints/308/

You'll notice that there is a rather small hole in the cap over OK, but if you look at the plots 3 hours either side of this plot, you'll notice that the window of that hole is very brief. The cap is stout all day, and then fills back in rapidly with sun down. Days like that are always a big gamble. Another thing to keep in mind, however, is that these plots are very sensitive, and rarely represent what actually happens the day of the event.

 

[Jay Press]

Hey Guys,


Thanks so much for your input.. Amazing knowledge here. I keep getting those "light bulb" uh,hu moments. Totally makes sense.

I may go out this Wednesday.. See how it goes.. Today was a great example of a cap. Storm finally got going right at dark..

Thanks all and see ya out on the Plains..

B.p.

 

[Andy Elliott]

It is important to have some CINH when looking for severe weather. The CINH acts as a lid to store energy - and once the CINH is overcome by a source of lift, the stored energy can be rapidly released. Think of a lid on a pot of boiling water. As soon as you remove the lid, the steam rapidly rises upward. If the CINH is too high, it is almost impossible to overcome. As a rough rule of thumb, if the CINH is higher than 300 J/kg then you will not get storms. Of course there are exceptions. With a good source of lift (front, vorticity, divergence) 150-300 J/kg is probably the sweet spot, given enough instability and moisture. I hope this is somewhat helpful.

Andy

 

[Paul Knightley]

The best proven way to tell if the cap will break is to look at NWP output. If every available model shows no precip, you can be fairly confident that convection won't develop. This doesn't work on every single occasion, but it's a pretty good rule of thumb!