The ridge and the week ahead

Last night we had two MCCs merge together over Wisconsin to form a very potent MCC with a leading convective line in southern Michigan this morning.  Here's the IR satellite image from earlier this morning:

GOES-E IR image from 1015Z, July 11, 2011.

You can see the characteristic circular "blob" pattern in the IR imagery that is the hallmark of MCCs.  It seems that the particular route these storm complexes took--from the central and northern plains out through the central and upper midwest--is going to be the favored storm track for the next several days.  Here's today's SPC convective outlook:

SPC Day 1 convective outlook as of 1626Z, July 11, 2011.

And the day 2 convective outlook for Tuesday:

SPC ay 2 convective outlook as of 0600Z, July 11, 2011.

So what's causing the storms to move this way?

It turns out that there is a big ridge aloft over the southeastern US--and it's not going anywhere.  At least, according to the models it's not going anywhere.  Here's this morning's GFS analysis at 300mb:

GFS Analysis of 300mb height (contours) and winds (barbs and colors) at 12Z, July 11, 2011.

Note the high-height area over the southeast with winds flowing clockwise around it.  The jet streaks within the jet stream are somewhat weak and well to the north--mostly in southern Canada.  You can visualize with clockwise flow around this ridge that the prevailing wind pattern would be a big arc stretrching from the desert southwest, up through the central and northern plains, across the central midwest, then out over the mid-Atlantic states.  It's no coincidence that that is the preferred storm path over the next few days.  These prevailing winds aloft carry the storms along with them.

Let's look forward in the GFS model to the forecast pattern on Wednesday morning:

GFS 48-hour forecast of 300mb height (contours) and winds (barbs and colors) at 12Z, Wednesday, July 13, 2011.

The main jet stream is still pretty far to the north.  However, there is a strong, sharp ridge whose axis is forecast to stretch across the northern plains.  If you follow the wind barbs, you can still see that the prevailing flow is up out of the desert southwest, across the northern plains, and down through the central and upper midwest.  So I'd expect a similar storm track to continue.

Now on to Friday morning:

GFS 96-hour forecast of 300mb height (contours) and winds (barbs and colors) at 12Z, Friday, July 15, 2011.

It looks like the ridge has grown even more in this forecast.  Same predominant flow pattern, however.  I'm also interested to see what's going to happen with the trough that has been continually trying to move onshore in the Pacific Northwest all week.  It looks like this trough could be dampening the early summer glory here in the Seattle area by bringing more cloudiness and showers.  In fact, the longer this ridge sticks around the central part of the country, the longer the weather is going to be drizzly in Seattle...

With weak jet streaks moving through, it's clear that storms that fire aren't going to have a lot of support of the flow aloft.  In general, a ridge is not the most conducive area for storm growth and development--at least in a baroclinic sense.  With weak synoptic-scale forcing, I'd continue to expect to see MCCs as the dominant form of storm as this pattern continues.  Remember from my previous posts that MCCs tend to modify the environment more than the environment works to develop them.  So, in the absence of strong forcing from the ambient environment, any instability is probably going to manifest itself in MCC form.

Why should we expect any instability at all?  Two reasons--heat and moisture.  As most people in the central part of the country already know, it's been hot this summer--and sinking, warming air under the prevailing ridge is not going to do much to diminish that.  Heating at the ground level inherently works to destabilize the atmosphere.

The humidity is also a factor.  Not only are we seeing muggy conditions near the ground, but the moist layer can extend to be rather deep.  A lot of this is due to that same, clockwise flow pattern aloft.  It turns out that this particular pattern (with a ridge over the southern US) helps drive the flow of "monsoon" moisture up from the southwest.  This moisture doesn't originally come from the desert southwest--it actually has its origins in the tropical east Pacific.  But the upper-air winds advect that moisture up and around.  It's brought a lot of storminess to the southwest over the last few days.  Here's the RUC analysis of 500mb relative humidity from this morning:

RUC Analysis of 500mb  relative humidity (greens) winds (barbs) and heights (contours) at 15Z, Wednesday, July 13, 2011.

Note a broken stream of high relative humidities stretching up along the western Mexican coast, through the desert southwest, into the central plains and across the upper midwest.  It's that same pattern that we were seeing for the storm track.  So, storms forming in this region have both heat and deep moisture to work with.

It will be interesting to see just how this week plays out.  With such a dominant ridging pattern, the small-scale features will make all the difference.  For instance--what about that small upper-level low analyzed over northwest Texas at 300mb in the first GFS image that I showed?  It actually shows up on the water vapor imagery from this morning as a nice swirl over northwest Texas, so we know this isn't an error in the analysis:

GOES-E water vapor image from 1645Z, July 11, 2011.

That upper-level low has been drifting northwestward overnight and it looks like it's about to start interacting with that plume of higher "monsoonal" mositure coming up from the desert southwest.  Will the added vorticity of this upper-level feature be enough to spin up a warm-core MCC?  What will happen?  We'll have to wait and see.

Ridges don't always have to be boring...

Aha! An OBSERVED MCV--just the other day

I was pleasantly surprised to see the University of Wisconsin -- Cooperative Institute for Mesoscale Satellite Studies (CIMSS) blog post on July 6th.  It discusses a mesoscale convective vortex that just occured over the southwestern US on July 5th and 6th.  Moreover, they have a stunning GIF animation of visible and IR satellite images that shows how the storms first organized as a mesoscale convective complex then later evolved to a mesoscale convective vortex.

The blog post is at:
http://cimss.ssec.wisc.edu/goes/blog/archives/8409

And that GIF animation (which is the first image in the blog post) is here.

I encourage you to watch the animation to really get a good idea of the evolution of these sorts of storms.  The synoptic-scale flow aloft was relatively weak, indicating that this complex of storms organized away from any strong upper-level forcing.  What began as clusters of thunderstorms during the day (with very distinct outflow boundaries) congealed into an organized mass as night fell.  You can see in the overnight hours (when the animation switches to the lower-resolution IR images) how the cloud structure does indeed take on the characteristic round shape of a MCC. 

By the morning hours (when the animation returns to the higher-resolution visible images), the remaining circular cloud shield (and a fair amount of the convection going on) dissipates.  But, it leaves behind the warm-core vortex that lies at the heart of the complex.  Internal interactions within the cyclonic flow about this vortex are able to maintain some convection about the vortex center.  However, because this is a vortex, the convective structure has the characteristic "swirl" shape we see in MCVs.  Once again, not unlike a hurricane...

It's incredible how fragile MCVs actually are.  As I mentioned in my last post, studies indicate that on average only three or four MCVs are observed in the US each year.  Any strong upper level winds will tend to shear apart that warm-core vortex long before it gets very organized.  You can tell in the animation that the synoptic flow is weak, though, and atypical of a progressive, strong jet pattern.  The MCV itself drifts westward, which is not the typically direction we see storms move--this shows the relatively "mild" conditions aloft.

Anyhow, I just thought it was pretty amazing that the day after I do a post about MCVs (a relatively rare phenomenon), one happened to be observed.  So, please enjoy this fascinating type of storm...

The Mesoscale Convective Vortex (MCV)

For my last post in this series about different kinds of mesoscale convective systems (MCSs), I'm going to briefly talk about one of the rarer types of MCSs--the mesoscale convective vortex (MCV).  These sorts of storms usually form from parent mesoscale convective complexes (MCCs), so we have a full line of mesoscale convective classification:

MCS --> MCC --> MCV

In trying to trace the history of MCVs in the meteorological literature, it seems that the MCV was recognized very soon after Maddox's 1980 paper describing the features of MCCs. It was noted that in some cases, strong mesoscale low-pressure centers would develop in the wake of MCCs.  Johnston (1981) talks about mesoscale vorticity maxima induced by mesoscale convective compexes. Such maxima were theorized to have helped organize the storm that caused the 1977 Johnstown, Pennsylvania, floods by  Zhang and Fritsch (1987).  However, the first actual mention I could find of the term "mesoscale convective vortex" being used to describe this phenomenon came from a paper by Menard and Fritsch in 1989, where they describe a "mesoscale, convectively-generated vortex (MCV)" over Oklahoma and Arkansas.  So you can see that our understanding of this particular phenomenon is pretty young, having only been recognized over the past 20-30 years of reseach.

So what is a mesoscale convective vortex?  In my discussion of MCCs, I talked about how the large area of rising air in the circular MCC had lots of condensation going on.  That condensation of water vapor into rain releases a lot of latent heat, warming the core of the storm.  That warming causes the air to expand, reducing the pressure relative to the surrounding environment.  This, in effect, creates a "warm-core low" with a structure analogous to the structure of a hurricane.  Sometimes, after convection and rainfall associated with an MCC dissipates, this area of low-pressure can actually linger on.  The cyclonic flow associated with the low-pressure center can later help organize new areas of rising motion and convection, often with a charactaristic cyclonic "swirl".

Here's an example of a visible satellite image of this organization going on from the CIMSS satellite page:

GOES visible satellite image from 1615Z, Jul 8, 1997 showing a MCV over western Missouri.  From http://cimss.ssec.wisc.edu/goes/misc/970708.html.

Sometimes you'll see MCVs described as a "mesoscale vorticity center".  This is an apt description, and it means essentially the same thing--it just changes the acronym to MVC. You can see on that example image how the MCC has organized itself into a charactaristic swirl-shape.  The analogies between MCVs and tropical cyclones really start to make sense when you see this kind of organization.

Here's a radar image from Patrick Marsh's blog last year when an MCV was observed moving through the Houston area:

NEXRAD base reflectivity composite of an MCV near Houston at 2220Z on June 3, 2010.  From http://www.patricktmarsh.com/2010/06/day-154-mesoscale-convective-vortex-near-houston/.  Annotated by Patrick Marsh.

Patrick has annotated that radar image with streamlines showing the wind patterns and an L marking the center of this mesoscale vortex.

You can see in these two images why these features are considered to be "mesoscale".  Unlike the banded "swirls" or comma-shapes we see with strong synoptic-scale low-pressure centers, these bands do not mark the locations of frontal boundaries.  They are much more akin to hurricane rain bands than frontal bands.  Furthermore, the size of these vortexes is only at most a few hundred kilometers across--often less.  You can see above that these storms are about the size of a state or less.  This still puts them in the "mesoscale" category--smaller than the synoptic scale.

It turns out that, though mesoscale convective complexes are relatively common across the central US, only a very few actually give way to the eventual development of a mesoscale convective vortex.  Bartels and Maddox (1991) did a survey over seven and a half years of visible satellite data and only observed 24 cases of MCVs, working out to around three or four per year.  So, they're a fun feature to see when you can actually find them.

That's just a quick look at MCVs, a rarer, but still important type of mesoscale convective system.  In my next blog, I think I'll finally move away from talking about MCSs and get back to talking about the weather that's currently going on.

TS Arlene -- another kind of circular MCS

Fittingly enough, as I began this discussion on mesoscale convective systems (MCSs), our first tropical storm of the Atlantic hurricane season formed off the east coast of Mexico.  Tropical Storm Arlene made landfall this morning near Cabo Rojo, Mexico.  Here's the an IR satellite image shortly after landfall:

GOES IR satellite image of TS Arlene at 1715Z, Jun 30, 2011.

I find it fascinating comparing the satellite presence of a tropical storm with the satellite presence of a mesoscale convective complex (MCC) (or a mesoscale convective vortex--but that's another post).  In Maddox (1980)'s classification scheme, the two types of recognized "circular" MCSs were tropical cyclones and MCCs.  As odd as it might sound, I believe that the origin of the "circular" structure is actually very similar in both cases, even if the both types of storms form in vastly different ways.

One main difference between circular MCSs (like tropical cyclones and MCCs) and more linear MCS features (like squall lines) is the environment in which they form.  Remember from my previous discussion that squall lines usually form along or near frontal boundaries.  Their structure and maintenence are dictated from the surrounding environment.  In fact, a lot of their energy is derived from a baroclinic environment.  That's a fancy way of describing an environment where there are strong horizontal temperature contrasts like we see around fronts.  But, remember that a MCC tends to form away from fronts--off by itself, more or less.  Tropical cyclones, too, don't have fronts in the picture--even if they are powerful low-pressure centers, they don't have fronts associated with them.  This is because both MCCs and tropical cyclones tend to form in a barotropic environment--an environment where there are not strong horizontal temperature contrasts.

This makes sense if you think about it in a qualitative sense.  Fronts are often depicted on weather maps as long, "straight" lines.  When we consider the polar front separating the cold, arctic air in the north from the warm, subtropical air to the south, it's a long, linear boundary that's asymmetrical on either side--one side is cold and the other side is warm.  Storms that form along these boundaries between warm and cold air (or moist and dry air) are going to inherit this asymmetry--the parts of the storms in the warmer, moister air are going to behave differently than the parts of the storms in the cooler, drier air.  This works against having a circular, symemtrical structure.

In contrast, storms that form away from strong frontal boundaries (i.e. in a barotropic environment) don't have these contrasting areas of warm and cold to deal with.  This is particularly true with hurricanes--they form over the tropical oceans where everything is very warm and very moist all the time (more or less).  As such, anything that spins (or has a vorticity maximum) in such an environment should (theoretically) assume a more symmetrical, circular shape.  If all parts of the storm are experiencing similar environments, they should theoretically all look the same.  Granted, tropical cyclones do often move into areas where there is differential wind shear aloft or cooling sea-surface temperatures and this disrupts that symmetry and weakens the storm.  But still--the circular structure makes more sense.

What about MCCs?  They're not usually associated with a powerful center of low-pressure.  However, remember what I said about MCCs in my last post--MCCs tend to modify their surrounding environment more than the surrounding environment drives them.  At the heart of any MCC is an area of rising motion and a lot of latent heat release as moisture evaporates when air is lifted and cooled.  That rising motion and heating lowers the pressure in the center, creating a warm-core-like vortex.  It may be weak and subtle, but it's the same kind of mechanism that drives tropical cyclone formation and growth.  And it implies that there still is some (however weak) vorticity and circulation going on in the storm.  Is this enough to really promote a circular structure?  That's my guess for now.  Though if anyone has a better idea, I'd welcome any thoughts...

Going back to TS Arlene, it was interesting to note that our global models were forecasting a tropical cyclone to develop in the western Gulf of Mexico a full week ago--an impressive lead time for tropical cyclone genesis.  Here's the ECMWF 144 hour forecast of sea-level pressure for 00Z yesterday:

Note that the model forecasted a small, compact low in the Bay of Campeche just off the eastern Mexican coast.  It's a little further south than where the storm actually impacted the coast, but still rather impressive with such a long lead time.

Here's the same forecast from the GFS model:

The forecast low-pressure center was a significantly more pronounced, though still a little far south.

And here's the CMC model (Canadian GEM):

The CMC model was predicting a much deeper low.  For some reason the Canadian model always seems to predict tropical cyclones that are much stronger and much more frequent than actually happens.  I'm not entirely sure why this is, but it's an interesting question.

What is a mesoscale convective complex?

As I mentioned in my last post, I want to go into some detail about the definition and structure of several classes of storms.  We use the term mesoscale convective system to describe any collection of storms (or other convective activity) that is organized on a scale larger than a single storm.  Today I want to focus on one particular type of mesoscale convective system--the mesoscale convective complex.

Much of this discussion is going to follow Robert Maddox's 1980 paper "Mesoscale Convective Complexes," which is the first paper to define and describe these types of storms.  You can read the full paper here (which is actually very understandable if you can follow the kinds of things I write in my blog).

In Maddox (1980), he describes a breakdown of different types of large-end mesoscale convective phenomena using this convenient chart which follows nicely from my last blog post:

Table -- Describing a classification of meso-alpha (large mesoscale) convective phenomena, broken down by geometric shape and location.  From Maddox (1980).

The main point from this chart is that these mesoscale convective complexes are "circular" in structure and they can occur in both the tropics and the midlatitudes.  Maddox proposed a definition that still stands today (at least it still matches the American Meteorological Society's definition of a mesoscale convective complex) though there have been proposed amendments over the years:

Table -- A definition of a mesoscale convective complex from Maddox (1980).

MCCs are mostly determined by the shape and size of their infrared satellite signature.  This has always struck me as somewhat arbitrary--what's so significant about the cloud returns being more than 100,000 sq. kilometers in area?  Why does the structure have to have a near-circular shape (eccentricity greater than 0.7)?  It turns out that these parameters themselves are indeed arbitrary--they exist as a way of describing MCCs in the context of the observations that we have and in as succinct of a way as possible. 

From Maddox's paper and several subsequent descriptions, there are several key points that describe MCCs uniquely and separate them from other features such as squall lines:

1. MCCs do not require significant large-scale forcing to form.  Whereas squall lines often form along fronts and are associated with deep troughing and strong upper-air support,  MCCs can form in areas where there is relatively weak support for convection--only mild lifting motion and no significant support aloft.
2. MCCs require a very warm and moist layer that extends from the surface relatively deeply up into the atmosphere.  Because MCCs typically form in areas with weak upper-level support (as mentioned above), they need a lot of latent heat energy to sustain themselves.  Basically, with structures like squall lines along a front, you have an ambient, synoptic-scale environment that works to force and maintain thunderstorms.  With MCCs, it's more like you have storms that work to change and modify the synoptic-scale environment.
3. Because of the strong dependence on latent heat release to maintain the structure of a MCC, they tend to share many charactaristics in common with a warm-core system as opposed to a cold-core system. Typically we think of hurricanes as the strongest warm-core-type storms.  However, MCCs also exhibit this kind of structure.

Let's look at a more direct comparisons between MCCs and squall lines.  Maddox provides some examples from 1979 of both in an attempt to illustrate the difference in structure.

Figures taken from Maddox (1980).  The left side illustrates a MCC on July 12, 1979 and the right side illustrates a squall line on June 20, 1979.  The top panels show infrared satellite images, the middle panels show radar returns and the lower panels show the surface analysis.

The left-hand side of the above image shows an MCC whereas the right-hand side shows a squall line event.  In the satellite images (the top panels), the shape difference is clear--the MCC is very round in shape and covers a large, continuous area.  The squall line, in contrast, has a much more linear shape and lots of variation in the infrared satellite returns--it's not just one big blob.

In the middle panels showing the radar returns, we can see that in the squall line, the bulk of the radar return (and therefore most of the precipitation) is confined to the immediate area of the squall line (ignoring the other precipitation off to the northwest associated with the surface low).  In the MCC, however, there's a rather large area of radar returns underneath the big circular infrared satellite feature.  Furthrmore, there are embedded areas of deeper convection in the MCC--some of which do look "linear".  But, the satellite return and other radar return areas are very large and aren't just confined to that deep convective area.  This sets the MCC apart from the squall line.

Finally, in the lower panels, for the squall line case on the right we can see that the squall line is out ahead of a deep surface low to the west.  The only way you would get a deep surface low like that is to have significant upper-level support (i.e., a strong jet streak with its exit region above).  We can see a lot of pressure contours, and they're fairly close together, indicating strong pressure gradients.  The squall line itself has moved out ahead of the main cold front, but the fact that its structure so well mirrors the main cold front hints that the original storms probably formed along the cold front before strong winds aloft started moving them out ahead of the front.  This is a classic scenario that I've talked about in this blog multiple times.

However, in the MCC case, we notice a very different synoptic setup.  There's no strong surface low nearby at all.  The pressure contours are rather few and far between, indicating weak pressure gradients.  There is a cold front analyzed off to the northwest, but it doesn't appear to be very strong.  Furthermore, the shape of the convective area doesn't really match well with the shape of the cold front.  This implies that while the front may have forced initial storm development, it was not the primary mechanism for organization in these storms.  You can see that in the MCC surface analysis, they've drawn outflow boundaries on the southern and eastern edge of the complex, with a mesoscale area of high pressure underneath the middle of the complex.  This contrasts greatly with the squall line case, and gets back to the difference I mentioned earlier--

Squall lines--their forcing and maintenance--are a product of the surrounding synoptic environment whereas MCCs tend to force and change the surrounding synoptic environment.

In the surface analyses above, the MCC analysis is dominated by features (like the outflow boundaries and meso-high pressures) created by the MCC itself.  In the squall line case, the dominant features (the surface low, the strong fronts and pressure gradients) were NOT created by the squall line.  They worked to create the squall line.  This is a fundamental difference.

Let's explore some of the other features of MCCs by looking at some potential MCC candidates from the other day.  Remember this radar image from Sunday night?

NEXRAD composite base reflectivity from 0228Z, June 27, 2011.

Now--do we call these features squall lines or MCCs?  On radar the deep convection does look rather linear, but we saw even in Maddox's example MCC that we could see linear-like convective elements within an MCC.  There's also extended areas of precipitation to the north and the west of these convective lines in the radar image.  However, the technical definition revolves around the infrared satellite image, so I'll bring that back too.

GOES Infrared satellite image from 0245Z, June 27, 2011.

Immediately we can see that the shapes of these storm complexes on the satellite image is more rounded--there's no linear feature or "comma shape" like we'd expect if there was a deep surface low or a squall line. I haven't measured the area of the satellite returns explicitly, but knowing that the state of Iowa has an area of about 147,000 square kilometers seems to imply that these are large enough to qualify as MCCs (the exact definition(s) of area needed is in the first table above).  So just from these satellite images, I'd call these MCCs.

But lets look at some analyses of the surrounding environment.  Let's go back to 18Z (around the middle of the day) to when these storms were just forming.  Here was the radar image then:

NEXRAD composite radar reflectivity from 18Z, June 26, 2011.

Scattered storms were beginning to develop throughout the Dakotas and into northern Nebraska.  Other storms were beginning to fire in southeastern Nebraska.  Now, let's look at the 300mb chart for this time:

SPC mesoanalysis 300mb winds (barbs and blue colors), divergence (pink) and height (black) for 18Z, June 26, 2011.

Is there a deep trough moving in?  Not really.  Are we in the exit region of a strong jet streak?  Kind of--you can see the blue shadings highlighting the core of the jet winds off to the west.  There's some divergence aloft (shown by the pink contours) but this doesn't seem to be tied to the jet streak.  This divergence is right above the area where storms were developing on radar.  This makes sense--the rising motion in the convection implies that air above has to get out of the way, so we expect divergence aloft above the storms.  But the fact that this divergence is so localized to just the areas where the storms are seems to indicate that this is a storm-induced phenomenon--the divergence is not significantly coming from the overall synoptic pattern.  This agrees nicely with what I said about MCCs before--they modify the surrounding environment more than it forces them.

Continuing to the time when the MCCs had matured, here's the new 300mb map.

SPC mesoanalysis 300mb winds (barbs and blue colors), divergence (pink) and height (black) for 2Z, June 27, 2011.

We can see the divergence aloft (once again, the pink contours) associated with the MCCs in eastern and western Iowa--they're nowhere near any significant jet at 300mb.  There are some wiggles in the height contours, but no troughs or anything supporting these storm clusters.  Once again, the storms are modifying the environment. 

Also in this 300mb image, I find it interesting how the jet aloft has changed surrounding the storms that were developing in central South Dakota at this time.  Notice how the jet streak seems to curve up and around the top of the strong area of divergence associated with those storms in central South Dakota.  If you follow that curvature, you'd see that the jet streak is starting to become anticyclonically curved--it curves clockwise.  Interestingly, one of the features of a warm-core storm is that it promotes an area of anti-cyclonic flow (and "higher" pressure) aloft.  Is this anticyclonic curvature of the jet evidence of a warm-core structure in the storms in central South Dakota?  Remember, in my list of features of an MCC, I said that these storms often exhibited warm-core charactaristics.  So it could be...

Finally, some more interesting facts about MCCs:

1. Most MCCs develop in the afternoon hours, then strengthen through the evening and perisist overnight until the next morning.  Thus they provide a lot of nocturnal rainfall.
2. In fact, MCCs are thought to be one of the largest contributors to annual rainfall for much of the central US (several studies have looked at this, including Fritsch (1986), McAnelly and Cotton (1986, 1989) and Kane et al. (1987)).  Fritsch (1986) claims that most states in the plains and midwest receive 30-70% of their annual rainfall from MCC events.
3. Because of their heavy rainfall contribution (remember--MCCs have precipitation over a much larger area than squall lines and usually occur when there is a very deep moist layer), MCCs are responsible for many flash flood events across the central US.
4. Severe weather is definitely possible within a MCC.  Severe wind and hail are frequently found with the stronger convective elements.  If those elements can organize along one of the MCC's outflow boundaries, the wind threat can get even greater.  Tornadoes are also possible from stronger convective elements within the MCC.  Here's the SPC storm reports for that group of MCCs on Sunday night:

SPC storm reports for June 26, 2011.

And with that I'm going to wrap up my discussion of what makes a mesoscale convective complex.  It's a peculiar type of storm that is very frequent in the late spring/early summer months.  While superficially it can look like a squall line, it's a unique type of organized storm that:

• Doesn't need strong forcing aloft
• Actually works to change the synoptic pattern aloft instead of being driven by the synoptic pattern
• Usually forms with a very deep layer of moisture in the low-levels
• Has warm-core charactaristics
• Brings a lot of rainfall, usually at night, to the central part of the US
• Has a very recognizable circular-type structure on infrared satellite images.

Now you'll know what to look for the next time that the term "MCC" comes up.