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Bob Chill

Mid Atlantic Met Class Thread

87 posts in this topic

[quote name='CoastalWx' timestamp='1329836709' post='1394166']

Well, I could spend all morning..lol, but I'll try my best. Frontogenesis simply means the genesis or formation of a front. It's an important process for precipitation to form. This basically means you have something separating different airmass or temperatures within an airmass.

So, deformation....we all love it as winter weenies. What deformation does, is to actually help cause frontogenesis. Deformation helps pack the isotherms tighter together. Packing isotherms tighter together leads to frontogenesis. You see this many times both and the cold and warm seasons..but mostly the cold season. Here is a textbook image of what the thermal field would look like. Instead of air flowing parallel to the isotherms, the wind is acting to deform the temp field and pack the isotherms together.


[attachment=57946:defm_frontogenesis.jpg]

Now in real life, it's not as pretty as that image. It's a little less obvious sometimes, but the idea is there. Even winds the aren't in a 90degree angle...as long as winds are at an angle and helping to converge or pack the isotherms together, you'll deform the thermal field and lead to frontogenesis. Here is an image from the SPC meso analysis site. This is from 12/26/10. Notice the closed 700mb low with a frontal looking feature cutting through SNE (in black). Now look at the winds to the east of the low. 70kts or so from the south, pumping up warmer air. Now, lets look north of the front in NNE. Notice the winds are more NE. You can even see some winds in se Canada that are more NNE and helping to bring down colder air at that level. While this image is not as classic as the image above, you can see how warmer air is rapidly moving north and converging with the colder air and associated ne winds over NNE. What you are doing, is stretching and deforming this thermal field, and helping to pack the isotherms closer together with time. Indeed you can see in purple, that we have frontogenesis over SNE..meaning the thermal field is becoming sharper and sharper over a smaller horizontal distance, with time.


[attachment=57945:7fnt1.gif]


So that's all fine and dandy, but why is this important? Well when you have isotherms packed together over a smaller horizontal distance, you have a much sharper frontal slope in the atmosphere. So, the air will rise faster along this sharper slope, than it would normally with a weaker slope. Also, the atmosphere is always wanting to stay in balance. It hates being out of balance. During times of strong frontogenesis, you disturb the thermal wind balance. A fancy way of saying...the temperature is changing to quickly over a small distance. So what happens is that you get this circulation that develops where you have rising air over the warmer, or southern side of the gradient and sinking air over the colder or northern side of this gradient. This is why many times you have that narrow, but enhanced band of snow or rain on the nw side of a storm. It's also a reason for sucker holes or subsidence in the region of sinking air. So if you think about it..it's a pretty cool way of balancing the atmosphere. What happens when air rises? It cools. So you have rising air helping to combat all the warming on the warm side of the gradient. What about sinking air? Sinking air warms, so you have that also trying to fight against the colder air moving in on the north or colder side of the gradient. Hope that helps.
[/quote]

Secondary ageostrophic circulations FTW! ;)

You have a knack for explaining things well and you having saved graphics from past storms really makes the posts enlightening. Good stuff.

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[quote name='chris87' timestamp='1329836844' post='1394174']
The price is a bit steep, but I highly recommend this newer book on synoptic meteorology:

[url="https://secure.ametsoc.org/amsbookstore/viewProductInfo.cfm?productID=81"]https://secure.amets...fm?productID=81[/url]

Even though intended for met students, I think the examples provided with mathematical/physics content makes it pretty easy to digest for someone with an interest in meteorology.
[/quote]

Seconded. Got this book for Christmas and it is great. I'd already taken Synoptic Meteorology as a class, and this book helped clarify a lot for me since, despite the math, the focus is not on derivations but is very conceptual. Dr. Lackmann seems great at explaining things like this. More specifically, he explains potential vorticity and its applications better than I've ever seen elsewhere. Great stuff.

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Another term that gets tossed around with the big storms or noreasters around here is vertically stacked. I only had a rough idea of what this meant but it totally hit home in 09-10. Generally speaking, most of our winter storms are tilted. Meaning the surface low (850) and upper level low (500) are not in the same place. This past weekend was a good example. The upper level low tracked though tn/ky/wv and the surface low tracked through ga, sc, and off the nc coast. This is probably the more common setup for us in the ma. Even though "energy transferred to the coast" it was not a miller B because the energy to our west was in the upper levels and not the surface.

We get our biggest storms with Miller A's when the ull (500mb) stacks directly above the surface (850mb) just south of our latitude (obx is generally the best spot I think). A vertically stacked low passing through our latitude can be distinguished by the classsic "eye" look of a noreaster. It's kinda the same principal of a hurricane but it's a cold core vs warm core system so there are sig differences. However, vertically stacked lows are spinning from the surface all the the way through the upper levels in the same place so the center is "hollow" for lack of a better term.


Miller B's are not the same irt to what happened this weekend so it's important not to confuse the 2. Miller B's have 2 surface lows. Primary will move west of us and then transfer it's energy to the "newer" surface low developing along the coast. Long story short, I see too many people confusing a miller B with a miller A when the ull tracks to the west and the surface tracks to the se of us. Miller B's can have a trailing upper level low too. We can totally whiff in the MA with the 850 transfer but still score a little something if the upper level low takes a favorable track.

With a little know how of what you're looking at on the model runs, it's very easy to distinguish between the two and set your back yard expectations accordingly.

Miller A = single surface (850) low tracking across the gulf and SE. Upper level low can track to the west into KY/TN but it is still a Miller A. Best case scenario is when the upper level low stacks over the suface low near or a bit south of our latitude. When this happens the amwx server melts down.

Miller B = 2 surface lows. One tracking into the oh valley and one tracking to our se. The initial stronger surface low will go to our west and then hand over all the goods to the newer low to our se and then that becomes dominent. NE loves these and the MA cries more often than not. Miller B's putting down good snow in our backyard is a function of latitude during the transfer. We want the secondary 850 well to our SE as the primary hands over the goods. Models have a really tough time with this as do our nerves. If we're near the southern fringe on the models, more often than not we will be dissappointed in this area. The Feb 10th 2010 was an excellent example of what needs to happen for us to score. Folks down in central VA weren't too happy though. Let's not even talk about 12/26/10.

Clicking through 850/slp maps on the models is the easiest way to understand whether we are going to have a Miller A or Miller B evolution. You don't even need to ask questions about Miller A vs Miller B if you can understand this simpleton explanation.

Met's, I may be screwing up the vertically stacked definition. I didn't really address anything @ 700mb because I don't really know enough about it other than RH. I know you can also have closed 700 lows but I generally only concentrate on the surface and 500 when looking at a coastal. Please correct any and all mistakes. I was trying to explain it in simpleton terms so anyone could understand.

I know the regular MA folks know alot of this stuff but I hope the "storm drop in" weenies read this post because they often don't get it.

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[quote name='Bob Chill' timestamp='1329838058' post='1394242']
Another term that gets tossed around with the big storms or noreasters around here is vertically stacked. I only had a rough idea of what this meant but it totally hit home in 09-10. Generally speaking, most of our winter storms are tilted. Meaning the surface low (850) and upper level low (500) are not in the same place. This past weekend was a good example. The upper level low tracked though tn/ky/wv and the surface low tracked through ga, sc, and off the nc coast. This is probably the more common setup for us in the ma. Even though "energy transferred to the coast" it was not a miller B because the energy to our west was in the upper levels and not the surface.

We get our biggest storms with Miller A's when the ull (500mb) stacks directly above the surface (850mb) just south of our latitude (obx is generally the best spot I think). A vertically stacked low passing through our latitude can be distinguished by the classsic "eye" look of a noreaster. It's kinda the same principal of a hurricane but it's a cold core vs warm core system so there are sig differences. However, vertically stacked lows are spinning from the surface all the the way through the upper levels in the same place so the center is "hollow" for lack of a better term.


Miller B's are not the same irt to what happened this weekend so it's important not to confuse the 2. Miller B's have 2 surface lows. Primary will move west of us and then transfer it's energy to the "newer" surface low developing along the coast. Long story short, I see too many people confusing a miller B with a miller A when the ull tracks to the west and the surface tracks to the se of us. Miller B's can have a trailing upper level low too. We can totally whiff in the MA with the 850 transfer but still score a little something if the upper level low takes a favorable track.

With a little know how of what you're looking at on the model runs, it's very easy to distinguish between the two and set your back yard expectations accordingly.

Miller A = single surface (850) low tracking across the gulf and SE. Upper level low can track to the west into KY/TN but it is still a Miller A. Best case scenario is when the upper level low stacks over the suface low near or a bit south of our latitude. When this happens the amwx server melts down.

Miller B = 2 surface lows. One tracking into the oh valley and one tracking to our se. The initial stronger surface low will go to our west and then hand over all the goods to the newer low to our se and then that becomes dominent. NE loves these and the MA cries more often than not. Miller B's putting down good snow in our backyard is a function of latitude during the transfer. We want the secondary 850 well to our SE as the primary hands over the goods. Models have a really tough time with this as do our nerves. If we're near the southern fringe on the models, more often than not we will be dissappointed in this area. The Feb 10th 2010 was an excellent example of what needs to happen for us to score. Folks down in central VA weren't too happy though.

Clicking through 850/slp maps on the models is the easiest way to understand whether we are going to have a Miller A or Miller B evolution. You don't even need to ask questions about Miller A vs Miller B if you can understand this simpleton explanation.

Met's, I may be screwing up the vertically stacked definition. I didn't really address anything @ 700mb because I don't really know enough about it other than RH. I know you can also have closed 700 lows but I generally only concentrate on the surface and 500 when looking at a coastal. Please correct any and all mistakes. I was trying to explain it in simpleton terms so anyone could understand.

I know the regular MA folks know alot of this stuff but I hope the "storm drop in" weenies read this post because they often don't get it.
[/quote]

I'm not positive on this, but I don't actually think it is the vertical stacking of the lows that causes the "eye" - or it is, but not always. When lows vertically stack it signals the occlusion process happening and the ceasing of much further strengthening. All cyclones eventually vertically stack and occlude, but clearly not all cyclones have "eyes". I think the eye formation process has more to do with the frontal structure and occlusion process of some cyclones vs. others. Shapiro-Keyser style cyclones are much more likely to develop an eye (and are also more likely to develop over water, and hence affect the EC region with snow) due to the progression of the warm front. Anyway, this may be a technicality from what you wrote, so I apologize if I'm simply repeating what you've said.

[url="http://weatherfaqs.org.uk/node/98"]http://weatherfaqs.org.uk/node/98[/url]

[img]http://upload.wikimedia.org/wikipedia/commons/5/50/Cyclonemodels.gif[/img]

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[quote name='LocoAko' timestamp='1329838641' post='1394269']
I'm not positive on this, but I don't actually think it is the vertical stacking of the lows that causes the "eye" - or it is, but not always. When lows vertically stack it signals the occlusion process happening and the ceasing of much further strengthening. All cyclones eventually vertically stack and occlude, but clearly not all cyclones have "eyes". I think the eye formation process has more to do with the frontal structure and occlusion process of some cyclones vs. others. Shapiro-Keyser style cyclones are much more likely to develop an eye (and are also more likely to develop over water, and hence affect the EC region with snow) due to the progression of the warm front. Anyway, this may be a technicality from what you wrote, so I apologize if I'm simply repeating what you've said.

[url="http://weatherfaqs.org.uk/node/98"]http://weatherfaqs.org.uk/node/98[/url]
[/quote]


Honestly, I'm not positive either. It's just what I've gleaned through years of reading and thinking and I'm probably off in my explanation. Great thread to be corrected in though because I don't know where else I can post this kind of stuff and discuss it. We're only as good as what we know and if what we know is wrong it doesn't help much.

There is a significance of where a noreaster becomes vertically stacked and how it impacts our area but I don't know the finer details.

This is a pretty good loop of the 12/19 storm. Maybe a met can chime in and explain it in more detail?


[attachment=57959:12.19 sat loop2.gif]

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[quote name='LocoAko' timestamp='1329838641' post='1394269']
When lows vertically stack it signals the occlusion process happening and the ceasing of much further strengthening.

[/quote]

Bingo.

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[quote name='Bob Chill' timestamp='1329839805' post='1394303']


Honestly, I'm not positive either. It's just what I've gleaned through years of reading and thinking and I'm probably off in my explanation. Great thread to be corrected in though because I don't know where else I can post this kind of stuff and discuss it. We're only as good as what we know and if what we know is wrong it doesn't help much.

There is a significance of where a noreaster becomes vertically stacked and how it impacts our area but I don't know the finer details.

This is a pretty good loop of the 12/19 storm. Maybe a met can chime in and explain it in more detail?


[attachment=57959:12.19 sat loop2.gif]
[/quote]

Fantastic discussion here. Also, that .gif is superb.

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[quote name='WxUSAF' timestamp='1329841052' post='1394352']


Bingo.
[/quote]

I only recently started understanding the vertically stacked evolution of cyclones so I'm pretty sure I'm missing some important details.

I do understand that when a noreaster becomes vertically stacked is has basically stopped strengthening and has become fully mature.

What stands out in my mind is the final evolution of an east coast storm becoming vertically stacked. Isn't the period when the ull is becoming stacked over the surface low when the most intense snows occur to the n & w of the storm?

Does the pivot that we often see during strong winter storms have to do with the final evolution before becoming vertically stacked or is this unrelated?

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[quote name='Bob Chill' timestamp='1329852185' post='1394800']
Does the pivot that we often see during strong winter storms have to do with the final evolution before becoming vertically stacked or is this unrelated?
[/quote]

The pivot action is counterclockwise so therefore, positive vorticity.
I'm thinking that the pivot indicates that the shortwave is developing
negative tilt.

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I don't have much to add other than repeating how good this thread is, and thank you for the information.

Hopefully as I learn more the weenie-ism of my posts in the model/obs threads will decrease. My most recent example of weenie-ism was trying to use http://tropic.ssec.wisc.edu/real-time/dlmmain.php?&basin=atlantic&sat=wg8&prod=dlm2&zoom=&time= to figure out where the weekend storm was going to go by looking at the 500-850 mb steering layer. Does this site have a purpose, and if so, when might you use it?

Coming from a software engineering background, I am curious what programming language most models are developed in? Also, what kind of computers are they using to process all of the algorithms?

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[quote name='snownerd' timestamp='1329866500' post='1395257']
[url="http://tropic.ssec.wisc.edu/real-time/dlmmain.php?&basin=atlantic&sat=wg8&prod=dlm2&zoom=&time="]http://tropic.ssec.w...lm2&zoom=&time=[/url]
[/quote]


At this time of year, we still have the prevailing westerlies. You want that chart when
the prevailing flow in the tropics is easterly, probably June until November.

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[quote name='CoastalWx' timestamp='1329835007' post='1394094']
It can get complicated real quick, so I'll try my best. You are right in that the strength of the low is in part, due to how strong the vortmax is, embedded in the shortwave trough. This is one of several factors in determining storm strength and position, but it's the most important one. 500mb is an important height level because in general this separates the atmosphere where about half the airmass lies below it and half above it..more or less. It's this level where features drive the surface pattern in terms of low and high pressure. If you want a low to form, you want convergence of air below this level, and the air to diverge or evacuate at that level and above. After all, in order to get low pressure to form, air must continuously be evacuated away from the area, so that more air rises in its place. This is a broad brushed idea, but I hope you understand what I mean.

So getting back to the original question, a strong vortmax embedded in the shortwave trough will really help kick things off. The basic idea, is that these features move at a certain phase speed....we'll say 40kts or so. Now the air moving through it, may be 60-80 kts. So, the air moving through this feature is moving faster than the actual feature itself. As the air leaves this area of spin, it begins to slow down and like a skater moving their arms away from their body...begins to diverge. So, if the air is diverging in the mid and upper levels, buy definition..air must converge below in order to help fill this void of air. So the tighter and stronger the vortmax..usually the stronger a storm will be. Now there are other feedback factors that start to come into play, but I'm just focusing on 500mb right now.

Here is an example of a powerful shortwave trough and associated strong vorticity maxima. Lets look at the Jan 26-27 2011 storm. Here is the 500mb prog for 06z on the 27th.

[attachment=57942:avn_06_50012711.gif]

Here is the surface low depiction. Notice that it's very close, if not under the 500mb vortmax. This is an indication of a mature low that is getting ready to occlude.

[attachment=57943:avn_06_sfc.gif]


Now remember we talked about rising air out ahead of the vorticity maxima? Look at this water vapor animation. Notice the higher cloud tops over SNE spiraling out ahead of the vortmax well to the south of SNE. This is an indication of rapidly rising air, and indeed there were 5" per hour snows in CT and we all know about your TSSN that you had in the evening.

[attachment=57944:SAT_ERG2_WVENH_ANI.gif]
[/quote]
Funny thing about this storm...NAM was by far (if I remember correctly) one fo the best guidance in terms of rapidly bombing that surface low and developing that warm sector moist convection and wrapping it into the deform band. That storm had a trop fold I believe.

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[quote name='CoastalWx' timestamp='1329835007' post='1394094']
It can get complicated real quick...(snip)

[/quote]

Coastal, extremely helpful post, thank you. You have a great way of explaining things in "dumbed" down terms and the images really bring the point home.

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[quote name='LocoAko' timestamp='1329838641' post='1394269']
I'm not positive on this, but I don't actually think it is the vertical stacking of the lows that causes the "eye" - or it is, but not always. When lows vertically stack it signals the occlusion process happening and the ceasing of much further strengthening. All cyclones eventually [b]vertically stack and occlude[/b], but clearly not all cyclones have "eyes". I think the eye formation process has more to do with the frontal structure and occlusion process of some cyclones vs. others. Shapiro-Keyser style cyclones are much more likely to develop an eye (and are also more likely to develop over water, and hence affect the EC region with snow) due to the progression of the warm front. Anyway, this may be a technicality from what you wrote, so I apologize if I'm simply repeating what you've said.

[url="http://weatherfaqs.org.uk/node/98"]http://weatherfaqs.org.uk/node/98[/url]

[/quote]
In general yes, but this can be deceiving because certain storms can exhibit what seems to be a vertically stacked config but are actualy undergoing rapid cyclogenesis. Instant occlusion storms are an example where the surface cyclone can undergo rapid intensification well after the storm has stacked/occulded at the surface.as the main surface low "bends" westward. Intense marine cyclones like the Shapiro-Keyser model with seclusions.

A great example is vertical stretching of a deep PV anomaly interacting with a moist/warm low level baroclinic zone. This type of cyclogenesis happens often on the lee side of the Rockies as deep PV's can stretch vertically rapidly intensify given the right low level thermal environment.
See the April 14-15th 2011 severe weather outbreak/blizzard.

[attachment=57996:april15thNARR.gif]

[attachment=57995:April_14,_2011_tornado_outbreak_satellite.png]

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[quote name='snownerd' timestamp='1329866500' post='1395257']
Coming from a software engineering background, I am curious what programming language most models are developed in? Also, what kind of computers are they using to process all of the algorithms?
[/quote]
Almost exclusively FORTRAN (some C++). The NCEP operational computer as well as those at both the UKMet Office and ECMWF are IBM power clusters (6 for us, and 7 for them, I believe). NCEP is getting a new supercomputer in 2013, but the contract was just recently awarded and the details haven't been made available.

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PVA is usually colocated with sthe strongest height falls. At least from what I estimate on the map. Sometimes there is a stray mesovortex that doesn't lower heights much.

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[quote name='TL97' timestamp='1329872587' post='1395433']
During severe season, what is meant by CAPE?
[/quote]

From wiki, though I am sure a met could explain for you further if needed

http://en.wikipedia.org/wiki/Convective_available_potential_energy

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[quote name='baroclinic_instability' timestamp='1329868976' post='1395324']
Funny thing about this storm...NAM was by far (if I remember correctly) one fo the best guidance in terms of rapidly bombing that surface low and developing that warm sector moist convection and wrapping it into the deform band. That storm had a trop fold I believe.
[/quote]


I think you are thinking of the Jan 11-12 storm perhaps?

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[quote name='TL97' timestamp='1329872587' post='1395433']
During severe season, what is meant by CAPE?
[/quote]

CAPE is an acronym for Convective Available Potential Energy, and is used as a metric when identifying the potential for convection (thunderstorm activity). It must be stressed that CAPE is the end all for determining severe weather risk, but is certainly important. Remember that air must have buoyancy (ability to rise) and then condense to form clouds, rain, hail, etc and CAPE provides a measure of this buoyancy. The higher this buoyancy, the quicker air will rise and begin the formation of thunderstorms. The CAPE value is measured in Joules per kilogram (J/Kg) and, generally, you need a minimum of 1,000j/kg of CAPE to see severe weather (Blanchard, 1998). CAPE can reach values of up to 5,000J/Kg, but that is reserved for rare events, such as large scale severe weather outbreaks. Here in the Mid Atlantic a good day is when CAPE gets to ~2,000J/Kg. There are two other types of CAPE talked about in forecast discussions and severe weather outlooks: MLCAPE (Mixed Layer CAPE) and SBCAPE (Surface-Based CAPE). Unfortunately I am not adept at explaining these two terms, but if you were to Google: SPC + MLCAPE or SPC + SBCAPE there are several good snippets of information. Hope this helps!


[u]Reference:[/u]

[color=#000000][font=Verdana, Arial, Helvetica, sans-serif][size=2]Blanchard, D. O. (1998). Assessing the Vertical distribution of Convective Available Potential Energy. [/size][/font][/color][i]American Meteorological Society[/i][color=#000000][font=Verdana, Arial, Helvetica, sans-serif][size=2], [/size][/font][/color][i]13[/i][color=#000000][font=Verdana, Arial, Helvetica, sans-serif][size=2], 870 - 877.[/size][/font][/color]:

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[quote name='snownerd' timestamp='1329866500' post='1395257']
I don't have much to add other than repeating how good this thread is, and thank you for the information.

Coming from a software engineering background, I am curious what programming language most models are developed in? Also, what kind of computers are they using to process all of the algorithms?
[/quote]

The computing systems are really a system of systems with Supercomputers doing a lot of the heavy lifting and mainframes and client/server stuff taking a significant amount of local load. Here's a high-level diagram of the overall architecture, with some stuff redacted out:
[attachment=58048:EOS App Arch.png]

Here's a slide deck that details most of the infrastructure and its tie in to numerical weather prediction modelling. There are some interesting computing systems links on slide 68.
[url="https://skydrive.live.com/?cid=f5a6540b544483e3#!/view.aspx?cid=F5A6540B544483E3&resid=F5A6540B544483E3%21258"]https://skydrive.liv...B544483E3%21258[/url]

The local office stuff is typical of a remote or regional office. Local, high-res models with smaller domains are run at these sites so that 1) the main computing systems are not impeded and 2) it offers the local offices more flexibility in ad-hoc runs for significant or hyper-local forecasting needs. Here's a pic of our local office:
[url="http://www.erh.noaa.gov/lwx/onlinetour/computers.htm"]http://www.erh.noaa....r/computers.htm[/url]

Here's a draft roadmap for the NextGen exascale computing systems that will likely power all sorts of government-facing numerical and graphical predictive analysis systems, including those in NOAA, going forward. I only have the draft on this workstation. Ping me via PM if you want the latest and I'll dig it up when I can.
[url="https://skydrive.live.com/?cid=f5a6540b544483e3#cid=F5A6540B544483E3&id=F5A6540B544483E3%21257"]https://skydrive.liv...B544483E3%21257[/url]

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Speaking of CAPE and following up about the skew-T discussion, the amount of CAPE can be directly calculated from the skew-T by adding up the area between the parcel trajectory (along a moist adiabat) and the environmental temperature profile. So, if there's a lot of room between the two, you can know the CAPE is high before looking at the calculated parameters directly.

The "opposite" of CAPE is CIN (convective inhibition) and is also measured in J/kg. CIN occurs when the parcel trajectory will lead it to be colder than the environmental temperature. If the parcel is colder, it's not buoyant in the free atmosphere and needs to be forced to a height in the atmosphere where it is bouyant before it can freely begin to rise. When people speak of a "cap", there means the atmospheric profile has a certain amount of CIN to overcome.

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[quote name='WxUSAF' timestamp='1329923164' post='1396417']
Speaking of CAPE and following up about the skew-T discussion, the amount of CAPE can be directly calculated from the skew-T by adding up the area between the parcel trajectory (along a moist adiabat) and the environmental temperature profile. So, if there's a lot of room between the two, you can know the CAPE is high before looking at the calculated parameters directly.

The "opposite" of CAPE is CIN (convective inhibition) and is also measured in J/kg. CIN occurs when the parcel trajectory will lead it to be colder than the environmental temperature. If the parcel is colder, it's not buoyant in the free atmosphere and needs to be forced to a height in the atmosphere where it is bouyant before it can freely begin to rise. [b] When people speak of a "cap", there means the atmospheric profile has a certain amount of CIN to overcome.[/b]
[/quote]

The easiest way for me to understand cap was to think of the atmosphere as a pot with a lid on it. The lid being the cap that needs to come off before any storms can develop and grow in size.

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[quote name='WxUSAF' timestamp='1329923164' post='1396417']
Speaking of CAPE and following up about the skew-T discussion, the amount of CAPE can be directly calculated from the skew-T by adding up the area between the parcel trajectory (along a moist adiabat) and the environmental temperature profile. So, if there's a lot of room between the two, you can know the CAPE is high before looking at the calculated parameters directly.

The "opposite" of CAPE is CIN (convective inhibition) and is also measured in J/kg. CIN occurs when the parcel trajectory will lead it to be colder than the environmental temperature. If the parcel is colder, it's not buoyant in the free atmosphere and needs to be forced to a height in the atmosphere where it is bouyant before it can freely begin to rise. When people speak of a "cap", there means the atmospheric profile has a certain amount of CIN to overcome.
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Very good explanation. I found this skew diagram while reading through the links that trix posted. I don't think there could be a better visual to go along with your explanation.


[attachment=58053:Skew T w cin + cape.png]

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[quote name='Eskimo Joe' timestamp='1329917553' post='1396273']
CAPE is an acronym for Convective Available Potential Energy, and is used as a metric when identifying the potential for convection (thunderstorm activity). It must be stressed that CAPE is the end all for determining severe weather risk, but is certainly important. Remember that air must have buoyancy (ability to rise) and then condense to form clouds, rain, hail, etc and CAPE provides a measure of this buoyancy. The higher this buoyancy, the quicker air will rise and begin the formation of thunderstorms. The CAPE value is measured in Joules per kilogram (J/Kg) and, generally, you need a minimum of 1,000j/kg of CAPE to see severe weather (Blanchard, 1998). CAPE can reach values of up to 5,000J/Kg, but that is reserved for rare events, such as large scale severe weather outbreaks. Here in the Mid Atlantic a good day is when CAPE gets to ~2,000J/Kg. There are two other types of CAPE talked about in forecast discussions and severe weather outlooks: MLCAPE (Mixed Layer CAPE) and SBCAPE (Surface-Based CAPE). Unfortunately I am not adept at explaining these two terms, but if you were to Google: SPC + MLCAPE or SPC + SBCAPE there are several good snippets of information. Hope this helps!


[u]Reference:[/u]

[color=#000000][font=Verdana, Arial, Helvetica, sans-serif][size=2]Blanchard, D. O. (1998). Assessing the Vertical distribution of Convective Available Potential Energy. [/size][/font][/color][i]American Meteorological Society[/i][color=#000000][font=Verdana, Arial, Helvetica, sans-serif][size=2], [/size][/font][/color][i]13[/i][color=#000000][font=Verdana, Arial, Helvetica, sans-serif][size=2], 870 - 877.[/size][/font][/color]:
[/quote]

MLCAPE and SBCAPE are just two different ways of initially lifting the parcel. With MLCAPE you average the temp and dewpoint in the lowest 100mb and lift from where they intersect. With SBCAPE, you lift from the surface temp and dewpoint to get your parcel trajectory. For example, in Bob's post above, the parcel looks to be SBCAPE where the traces start at the surface temp/dewpoint. If you try lifting from the average of the lowest 100mb temp/dewpoint, it looks like you would get slightly less CAPE.

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