The Cap and What It Takes to Break It

[Bob Hartig]

I have to confess that The Cap is still a bit of a mystery to me, moreso after a couple observations on some of the recent chase scenarios.

In determining whether the cap is going to be an issue, I've been using the 700mb chart to see what temps are like at that level, and/or whether colder (or warmer) air is advecting in. My threshold has been 12 degrees C, but I've picked up that to others here on ST, that's a bit high.

I'm also gathering that the cap kind of sloshes around between the 850 mb and 700 mb levels. That puzzles me, since I've relied on the 850 mb chart as an indicator of deep moisture, so it seems to me that higher temps at that level combined with good moisture would indicate good boundary layer CAPE, not CINH. Good Tds, higher temps, and reasonable Td depression at 850 mbs, with 12 degrees or less temps sitting on top or moving in have been part of my criteria, though now I'm starting to think maybe 10 degrees at 700 ought to be my threshold.

I understand that these things aren't hard, fast rules; I'm just looking to get a better understanding of the cap so I can fine-tune my forecasts a little more.

I could ask a number of questions, but what I've written above is probably all that those of you who respond require in order to give some helpful input. So...how do YOU work with the maps to determine whether the cap is going to be an issue, or perhaps an asset?

 

[Scott Weberpal]

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

Also when you get above roughly 3-4k' ASL in the high plains, it's not that uncommon for H7 temps to be above the normal 12-13c breakable range and still have the cap breached for surface based convection.

 

[Jeff Snyder]

Bob,

Using a 700mb temp as an indicator of cap strength is not a terribly good idea, since the "magic number" (10C, 13C, etc) depends on elevation (700mb is much farther above the surface over Tulsa than it is over Limon, CO) and season (warmer boundary layer is June and July compared to March and April). If we assume some model accuracy, you may well be better off looking at CINH, which is a vertically-integrated quantity that is quite physically meaningful. Similar to the reason why many meteorologists look at CAPE instead of the older Lifted Index (LI), it's often better to look at CINH instead of looking at the temp at a single level.

And actually, using a 700 mb lifted index (which would be positive if 700 mb is stable relative to the parcel) would be better than using 700 mb temp alone, since at least a 700 mb lifted index would include the surface temperature data. Otherwise, using 700 mb temp alone (or the temp at ANY particular level) for cap info would be like using 500 mb temp alone for potential instability info. For the nuclear bomb updrafts (i.e. extreme instability), do we want 500 mb temp of -15, -20, -8, or something else? Well, we can't really say, since it depends upon the temp of the parcel (at least initially, before we get any vertical perturbation pressure gradients that give supercells their more intense updraft).

Another prob w/ using 700 mb temp alone as your measure of "cap strength" is that that base of the environmental mixed layer (EML, the base of which is often warmest relative to the ascending parcel) isn't necessarily at any particular level. Sometimes, depending upon particular wind direction and other factors, the base of the EML is at 800 mb, in which case using 700 mb temp (or even a 700 mb lifted index) wouldn't give you the entire perspective.

This is all similar to the reason why one should be slightly cautious about using 850 mb Td charts as an the sole indicator of the presence of "deep" moisture. Indeed, if you are chasing in western Kansas, 850 mb isn't too far off the surface, so high 850 mb Tds don't tell you too much about the depth of the quality moisture. In addition, you may have moisture hanging out immediately below the 850 mb level that is missed on the 850 mb charts. The benefit of a sounding because very evident here, as you have considerably more data in the vertical compared to standard charts you find on the net (sfc, maybe 925, 850, 700, 500, etc). Examining isentropic surfaces typically is a better way to look at moisture (particularly moisture advection!), but very few sites on the internet provide much in the way of isentropic charts. Physically, using isentropic charts for moisture advection makes sense since air (and moisture) will advect on (i.e. stay on) an isentropic surface when unsaturated).

EDIT: Ooops, I'm not saying 700 mb temp and 850 mb Td info aren't worth looking at! In the absence of a lot of isentropic charts and accurate sounding data (or forecast sounding output), data from these two levels can be quite revealing. In fact, I often use 850 mb Td charts to get a quick glance / rough idea as to where moisture will be sufficiently deep, and using 700 mb T forecasts can give a very quick view as to whether a temp of 70 F or 95 F will likely be necessary for sfc-based convection. However, it is very important to keep the caveats in mind (elevation, time of year, the fact that the most stable or highest-moisture air may reside just below 700 or 850 mb, etc).

 

[Bob Hartig]

Ooops, I'm not saying 700 mb temp and 850 mb Td info aren't worth looking at! In the absence of a lot of isentropic charts and accurate sounding data (or forecast sounding output), data from these two levels can be quite revealing.

First, Jeff, thanks to you for your very thoughtful insights, and also to you, Scott, for your input and the link to Jon Davies' article. I haven't interacted with it all yet; after I've done so, I'll respond again.

Right now, though, I should clarify that, given a full suite of indices and other forecast tools, I'd definitely be looking at CINH. But in the absence of those tools, I'd like to be able to forecast with reasonable accuracy based on surface obs and upper air charts, along with remote sensing and perhaps morning soundings. It's kind of like learning alchemy to me.

 

[Andrew Ryan]

Also when you get above roughly 3-4k' ASL in the high plains, it's not that uncommon for H7 temps to be above the normal 12-13c breakable range and still have the cap breached for surface based convection.

Sorry, don't mean to thread jack, but does H7 = 700mb? I've seen H5, H4, etc being thrown around on this forum, forecast discussions, etc and have had no clue what it meant. And how exactly do you format it into the H#?

 

[Bob Hartig]

That's one I can answer: H5 = 500 mb, H7 = 700 mbs, H85 = 850 mb, and so forth. It's shorthand. I'm guessing that the H stands for "hectopascals," which is the same thing as millibars; but instead of saying H500, it just gets shortened to H5 for convenience.

 

[Bob Hartig]

I've read through the responses. Thanks very much, guys!

Scott, that's an extremely helpful article by Jon Davies, and I appreciate your note on high plains chasing.

Jeff, again, thanks for your in-depth commentary. It looks like you've opened up the next phase of my education, namely, getting my arms around isentropic charts. No end to the learning, is there? Right now, it's helpful just to get a better picture of the uses and limits of 700 mb and 850 mb maps. Some good points about elevation relative to the H85 for determining deep moisture; I suppose that as elevation increases, the 700 mb can sometimes become a proxy for the 850 mb--true?

I understand that there's a lot more to this thing than the maps in question can reveal by themselves; they're just a part of the picture, and the picture isn't paint-by-numbers. It's more a matter of relationships between parcels and environmental air, and the thresholds are relative, dependent on different factors. Sheesh...I'd consider going back to school for meteorology, except I suck at math.

 

[Chris Nuttall]

Make sure you also incorporate the use of soundings in your analysis. Looking at constant height charts only give you a horizontal perspective on things, which is good when considering things over a large area. But, soundings will give you a better perspective of how things are lining up vertically.

 

[Skip Talbot]

Check out Earl Barker's weather graphics:
http://wxcaster.com/smallfiles_central_svr.htm

I like using his Cap Index plots for quick glimpses at what the cap is up to during the day. The plot is like the opposite of the Lifted Index as it gives you the span of the inhibition as opposed to the area of the inhibition (CINH). Generally a cap index over three will ruin the day unless there is a super sharp front or very strong jet. I also like the plot for comparing targets and seeing how the cap will change as the day progresses to get an idea of initiation time. Stuff usually goes up when the index is between 0 and 1.

 

[Steve Miller TX]

Great discussion!

In addition to analyzing the thermodynamic profiles as Jeff discussed, I also carefully consider surface convergence and looking "upstream" at the 850-500mb levels.

Surface convergence - I have seen what were considered to be "thermonuclear" capping inversions breached with very strong surface convergence. I'm talking about the dryline in this respect.

I look for dryline bulges first with an eastward moving dryline. Since dry air is more dense than moist air, this will help get a parcel above the capping inversion as it in effect wedges under the moist airmass. I watch the area along the apex of the bulge and to the north. Looking at a surface chart, the classic example will have moderate to strong westerly winds behind the dryline and southerly to southeasterly winds ahead of it with about a 30-40F drop in dewpoints...or more!

For stationary or very slow moving drylines, I still look for convergence, but more on a mesoscale level (or perhaps even misoscale). Satellite, clear air radar, and careful surface analysis are useful here. I look for "kinks" along the dryline indicating a small scale circulation or even a meso low. The more pronounced it is, the better. I prefer radar analysis myself when possible. Of course, in Oklahoma or in West Texas, the mesonet surface observations are valuable tools. I wish we could get something like that across more of North Central Texas and Kansas.

On the flipside, I also look for "crawfishing" drylines...those that retreat westward just about in time for convective initiation. I've seen many a red box and anticipated severe weather events bust because of that. This is another very difficult forecast, but I always look "upstream" for any approaching energy as a vortmax of jet exit region which would induce pressure falls to the west of the dryline.

Another big bust factor along the dryline is what is happening about 5000 feet off the surface which along the I-35 corridor is 850mb. SW winds at this level are always a "death knell" to me for a bust. This indicates to me that the convergence along the dryline isn't deep enough....too shallow. Plus, it usually indicates drier, warmer air at that level above the shallow moist layer at the surface. So, this is pretty much summed up as the "depth" of convergence.

Another aspect is looking "upstream" from 5000 feet up to the mid levels...typically 700-500mb. Is there any approaching impulse or disturbance to enhance lifting and slight cooling of the capping layer? Any approaching cooler air at these levels? What about jet dynamics such as the left exit region or right entrance region? All of these factors will help erode a cap.

Of course, everything I mentioned above is not itself a "magic bullet" in busting the cap. I've seen the most skilled and admired forecasters bust either way regarding the cap. I contend it is THE most difficult part of forecasting. My best advice is alot of careful analysis and translating that into a "probability" based on your own knowlege and experience. I usually weigh that against the distance involved in driving to the target area.

So, in summary, forecasting whether a cap can be busted or not is a pretty complex and formidable forecasting effort. Simply looking at 700mb "magic number" temperatures isn't going to work. In looking back at alot of my forecasts, I'd say forecasting the cap is by far the most challenging aspect of chasing. After all, we want SOME capping inversion to keep storms more isolated and preventing a convective melee, but not too strong to squash everything altogether.

My best advice is to dive into it and gain some experience. I've learned more about forecasting the cap when I've busted the worst. Nothing like a bad experience being such a great teacher.