This page is made with anyone in mind, not just chasers(since most site visitors are not chasers). This would be considered very basic stuff. I'm not a meteorologist(I've never stayed at a Holiday Inn either), but think I know just enough to add this stuff here.
I guess a good place to start this is at the surface(sfc). For a chaser there is a tremendous amount of information here. I'll try and make it make sense.
The drawing to the right(I failed art...not really, but I was close) shows a basic low pressure system with the fronts that come with them.
Here is a drawing showing the wind direction. There's really not a lot to the basics of a surface low pressure. Sooner or later the cold front will often overtake the dryline. At that time you'll just have the two fronts basically(cold front and warm front). The fronts can be marked by tempture, dew points(measure of the amount of moisture in the air) as well as wind direction. If a warm front stops lifting northward and stalls it would then be called a stationary front. There's no one set way to define all the changes fronts can go through. There can be all fashions of tempertature differences on either side, as well as degree of wind direction change. Basically though, it's like the image to the right.
So why are fronts so important? Well storms need a source of lift. Imagine two air masses moving into one another, or an area of easterly winds colliding with an area of westerly winds. Look at the image to the right showing soutwest winds behind the dryline/sfc trough and the southeast winds ahead of it. If air comes together it will tend to want to go up...it has to. Storms simply have to have that help. It's important to understand that.
They also need moisture and instability. Moisture comes from the Gulf of Mexico for those of us in tornado alley...at least the far majority of it. Instability comes from cooler air aloft overspreading an area and/or heating of the ground as the day progresses. Upper level storm systems move east(generally...they can come from any direction), bringing in the cooler air aloft and they also have effects on any surface low pressure systems. Cold air is heavier and so the colder aloft the more unstable it gets. The other aspect to this is the amount of moisture. Dry air is heavier than moist air(crazy as that may sound) and so the drier the air aloft, the better for storms. But if the winds are strong aloft, and there's strong heating, it is possible to be too dry aloft and mix this down to the ground before storms fire. Generally it doesn't seem that too dry aloft is a problem. More often, being too moist aloft is the problem.
So you have a good front, low level moisture, and instability and manage to get a storm up and going. If the storm has any chance of living long it will need one more thing...wind shear.
Before going any further it would be wise to understand some heights given in millibars(mb). One way air pressure is measured is in millibars(mb). As you get higher the air pressure decreases. Well, meteorologists will often go by a few mb readings for height. Here are the main ones going up.
Surface = near/around 1000mb at/near sea level
6,000 feet = 850 mb
10,000 feet = 700 mb
18,000 feet = 500 mb
30,000 feet = 300 mb
If you ever plan to look at forecast models for chasing or whatever, knowing those will be a big help.
Back to wind shear. There are two kinds, both important. There is speed shear and directional shear. Speed shear is when the winds increase with height. Directional shear is where the wind changes direction with height. Why is it important? If you have a storm, your warm air is rising and cooling, forming rain. It is a huge help to the storm if this rain is blown out away from the updraft. If it is not, well your updraft becomes a downdraft of rain and dies. The outflow of cold air crashing to the ground can go out as a front and trigger some new storms, but the whole process is very unorganized and doesn't lead to long-lived or very severe storms. The other thing is that if there is no wind aloft to create the shear, you aren't able to feed the storm with new cold, dry air aloft. And wind shear isn't just a thing way up in the air. Think of low level winds near the ground while thinking of this too. No winds at the surface can mean no good inflow into a storm.
Supercell storms really do need everything to come together. Here(to the right) are some images showing an example of some good shear. Much of the rain is being blown away from the storm up there between 700mb and 300mb(and higher than that). 850mb is considered the low level jet level and there down to the surface is your more moist air being fed into the storm.
Not to get too confusing yet, but you can actually have too much turning with height as well. I've seen it many times in the more northwest flow events(systems where the flow aloft at 300mb or so is from the northwest). More descriptions on the image...
Here are a couple images of a real event with around 180 degrees of turning with height. I'm looking to the southwest. You can see the flow depicted on the images.
Often a good visual way to tell this happening is the size of the base of the storm. It will get smaller and smaller until it is gone, leaving the updraft to die/vanish from the bottom up. I have been on some frustrating chases where I watched storm after storm doing this, always having the same outcome. Often a forecast model will depict the winds aloft one way and what actually happens is they are more veered up there than they were supposed to be. Veering winds are winds that turn clockwise with time. Say a south wind becomes more from the southwest. Or a northwest wind becomes more northerly. Backing winds are the opposite, turning counter clockwise with time. Say a south wind becomes more southeasterly. But anyway, I'd avoid the days with 180 degrees of turning with height. I'd say if it is 160 degrees or more I'd worry. This seems to occur the most frequently with summer patterns. It does a good job of showing that accidents really don't happen with storms. If the environment isn't right, it's not going to go bonkers.
I guess I can go back to the surface for a bit here. To the right is a surface plot during the afternoon of a big tornado day in Nebraska. They are pretty darn simple to read. Well most are, where I got this one from it sort of overlays numbers at some stations. Just look at the one with the blue dot. The top number next to it, 87, is the tempture in F. The number below it is the dewpoint...66. It is always like that, temp over dewpoint or T(temperature) over TD(temperature dewpoint). The number just above it and to the right is the surface pressure. That might be the hardest part about reading it. If it starts with a zero you have to add a 10 to the front and a decimal point for the last number. So that 028 would become 1002.8mb. If it starts with a 9 you have to add a 9 to the front of it and a decimal point for the last spot again. So that one in northern Nebraska reading 987 would be 998.7mb.
The other thing to look at are the sticks extending off of circles. The blue dot one is pointing up and to the left. That is how you tell the wind direction being reported. There is a southeasterly wind there. At the end of the stick are other barbs/sticks/half sticks. A full barb means 10 knots while a half means 5 knots. So the blue dot one has a 10 and a 5 for 15 knots. The one below and to the right with the temperature of 81 has 2 full barbs so it is reporting 20 knots. If you see a triangle looking barb that stands for 50 knots(you'll see them for things aloft...or damn windy days at the surface). 1 knot is equal to 1.15 mph. Ignoring my red and blue dots I markered in on there, look at the other dots/circles. If it is all white it is clear, all black and it is cloudy, half and half then it's half cloudy/sunny. Those are the basics to a surface chart. Different sources will plot many more weather symbols and whatnot. At the red dot see the line with the arrow at the end of it kind of pointing down? That is the symbol for thunderstorm. They have symbols for pretty much anything imagineable.
Now back to surface convergence. Can you find any on the surface plot? Some that are more sparsely plotted like this one can be tough to narrow it down, but you can at least tell about where the front is. Notice the westerly winds in far southwest Nebraska and far northwest Kansas. Now look at the wind direction to the east/right of those. They are southeast. So you know the front is between those somewhere. You can also note the change in dewpoint west of the boundary. That one in northwest KS has a temp of 90 and a dewpoint of 46, while the dewpoint at the blue dot one(Hill City I believe) is 66. That ob in northern NE is also behind the trough/dryline since it is westerly. If you keep watching these as they update during the day you can monitor where the front has gone through. I think that one in northern NE just recently changed direction.
Incase it's not obvious where it is I drew it in here with the line.
Thanks to Al Pietrycha for letting me use some of the satellite images below, from COD, which he had saved on his site. He has a great write up from this very day on his site HERE. I used this day as an example on my video, Storm Structure 101, since I had footage from it and since it was a fairly classic example. Thanks to Plymouth State and the SPC for the other data from the day, the surface and the upper air stuff.
The other thing a chaser will monitor is the visible satellite image. Towering cumulus will start to show up along the boundaries as the heating continues. In the morning you can find and track the boundaries on surface maps(some on satellite in the morning but best later) and then add in satellite obs as the day goes on. It's hard to see the towers on the boundary north of I-80 in Nebraska, but they are there. They have some higher clouds moving overhead. They really stand out along it in southwest Nebraska. The surface low center with this storm was up in southwest South Dakota. The best convergence is going to be near the low. Just north of the Nebraska border where that top arrow is, you can see a thicker area of agitated clouds. That is where the dryline/sfc trough intersected the warm front up there. The moist axis in the middle of Nebraska was nosing up in there too. It's essentially a triple point of convergence with the main surface low just west or northwest of it. It's a fine area of storm initation since there is so much convergence at it(convergence along the warm front being pinched with the dryline/sfc trough).
Same day but one hour later. See the white puffs directly east/right of the lower two arrows? Those are happening now because a towering updraft has grown high enough to start producing an anvil. For hours you can have just towering cumulus or even the lesser cumulus on the boundaries. Once they start to produce an anvil they are getting ready to become storms. The idea is to be in the remote area or on them at this time. This is how a chaser gets to the cool storms. If I waited till something was on radar to leave, I'd still be at home in eastern Nebraska(over 3 hours away). So knowing some basic meteorology is needed to chase. I'm currently nearing the updraft producing the anvil on that middle arrow on the South Dakota/Nebraska border. None of this is really rocket science or hard, but being on the right boundary in the right area can be tough. This dryline continued down through Kansas and there was also the warm front option this day, further east(it's pretty much draped on the SD/NE border then arcing se as it goes east.
Some transverse roll clouds along and just north of the warm front.
1 hour and 40 minutes later. You can see the one in northern Nebraska is a mature supercell now, with a big anvil shield. This is from June 9, 2003 for anyone curious.
I hope this has shown the use of surface plots as well as satellite images for chasing storms.
This is a streamline plot of the winds from this same day. The black lines are the surface winds while the white lines are the 500mb winds(18,000 feet). You can see the windshift in NE at the surface with those black lines being westerly in the western half and southerly east of there. Then up above(white lines) you can see the flow is westerly. I like that amount of turning between the surface and 500mb(90 degrees). Up there near that storm it was probably closer to 120 degrees with the surface winds backed near the warm front.
One other quick thing to understand if you ever get into forecast models and meteorology, is the use of zulu time. They don't change with the spring and fall so they never set it forward or backwards. Here is how it works, at least for the central time zone. If it is after setting it forward in spring you simply add 5 hours to get the zulu time(fall and winter add 6). All the meteorology stuff is given in this, so it's useful to understand.
Changing to Zulu in the central time zone after spring time change.
12z = 7 a.m.
1630z = 11:30 a.m.
2000z = 3 pm
0z = 7pm
Back to shear and whatnot. Here is a map for 0z(7pm) of the storm day above. It's at the 850mb level or the low level jet area(6,000 feet). The national weather service sends up instruments on balloons twice a day at 12z and 0z. They record all this weather data. There is an office at each of the blue wind direction/speed sticks. There are actually more office than it shows on here. Anyway, you can see the 35 knots in eastern NE. I'm pretty sure it was a little stronger west of there. The west edge of the low level jet is out near North Platte(note the west nature there behind the front). In southern and central NE it was more like 40-45 knots out of the south. That would be considered a pretty healthy low level jet. Again on here the red numbers(top numbers) are the temperatures(in C now) with the number below them the dew point(also in C now). The green shading is for the higher dew points and the red dashed lines are for the temps. The green lines really show the moisture tongue searching north on the low level jet(you can see the 17+ C shading by that number in sw Oklahoma). You could actually see this richer, deep moisture channel on those satellite images. It shows up as those dotted clouds in the middle of Nebraska.
Here are the 0z obs the same day at 500mb(18,000 feet). 500mb is often considered the steering level, but really they all work together to dictate that. Their orientation and speed can change the actual storm speed, so using 500mb alone isn't the greatest idea. But if it is 100 knots you can be sure it'll be hard to chase the storms in that. This day shows 40 knot westerly winds at North Platte. I think there was a core between there and Rapid City around 45-50 knots. That would be considered rather ideal flow at that level. Often 40 knots is used as some kind of threshold for supercells at this level(well 40 knots of 0-6km shear). I've seen some cases where 20 knots of flow at 500mb worked, but the storm was extremely deviant to the mid-level(500mb) flow. The flow at 500mb was west at 15-20 knots and the storm anchored on a ne-sw boundary and moved sw down the boundary. This deviation increased the storm relative shear(important term as the storm's direction and speed is factored into the flow to get the relevant shear). It's kind of like driving with a strong wind. If you drive a car 50mph WITH a 50mph wind you'll feel no wind with your hand out the window. Turn the car around with the same exact wind and it'll feel like 100 mph wind. So storm speed and direction are very important when looking for the storm relative shear. Also I mentioned earlier zero surface winds can mean weak storm inflow. Well, if they are zero and your storm is moving east at 30 mph, well you'll have 30 mph of storm relative inflow. Anyway, note the temps in red at this level, -14C...getting much colder up here.
Another map from this day, a 300mb(30,000 feet) map. This is getting up there near the anvil level(a bit below this day). Rapid City has a 70 knot westerly wind while North Platte has a 55 knot westerly wind. There was probably 55 knots where the storm was, especially during the time after 0z(7pm). One thing to note up here is diverging winds. In the exit region of a strong jet you'll see this divergence(opposite of convergence). Divergence aloft leads to rising below. If you look at the North Platte reading you see it is westerly and even somewhat north of due west. Then look up in southern North Dakota. That is southwest. This isn't a great example of it but that is what one looks for. If there is strong convergence aloft that leads to sinking, which is not good.
So if you go from the surface up, you can see some of the shear this day. The surface flow ahead of the storm was 20-25 knots from the southeast(gusting much higher very near the storm at one point). Then you have the low level jet(850mb) from the south at 40-45 knots. Then up to the mid-levels(500mb) there was a 40-50 knot core moving in with a westerly component. On up to the upper-levels(300mb) and there was the 55 knot(or so) jet from the west. That could have been a bit stronger, but it works. The wonderful thing was the amount of low level turning and the speed of it. 25 knots se at the sfc, to 40-45 knots s at 850mb, to 40-50 knots west at 500mb is simply awesome. That is 135 degrees of turning with height and at high speeds. It can obviously be more insane, but that alone has big tornado day written all over it given there is good instability.
Instability is calculated a few different ways, but one main way is called CAPE. CAPE stands for Convective Available Potential Energy. Basically the higher the dewpoint in the low levels and the colder and drier it is aloft the higher the CAPE. Here is a CAPE map from the SPC(Storm Prediction Center) for the same day I've been using as an example. The red lines are the CAPE values. It's hard to see but the highest is in the middle of Oklahoma at 4000 and 3000 extending up into Nebraska. It gets lower as you move away from that center. 3000 is considered pretty high or as strong instability. I think 5000 is when it is considered Extreme Instability(funny I'm not sure of that). Often when it gets into those extreme values it is summer and the shear is less. It's just very hard to get good shear along with such high instability. Often with extreme shear you are battling the other end, low instability. This mix of 3000 CAPE and very strong shear values(mentioned previously above) are what a chaser dreams of. Most aren't going to ask for more than this.
This is also from the SPC and shows 2 measures of shear. The blue areas are for the 0-6km shear, given in knots. Notice the only areas it covers are those greater than 40 knots. 40 knots is just kind of the threshold looked for for supercells. So you can see the 40-50 knots of 0-6km shear over Nebraska. The orange and red shadings are for effective helicty. Helicty is another term for shear, but you'll just have to google if you really want to understand how it works. There is a 500 bullseye in se SD which is quite high. It's ramping up to 500 right there thanks to the backing winds along the warm front in the area(if low level winds veer that will drop off). Warm fronts are great as that's what they will do to the surface flow, they'll back it. Look at the previous image and you can see how this shear and that cape overlap. That's often another challenge, getting shear and instability together.
Then we have the lovely CAP that can make or break chase days. This will be a little harder to explain and cover. Remember I'm not a meteorologist and never went to school for this. I also never read much outside of chatroom talk. But here goes. If you go from the ground up and the temperture is falling the whole trip, then there's no cap. What happens the majority of the time is that it cools for the first few thousand feet, then will begin to rise for a short bit, then will start to fall again.
The image to the right is a sounding from Omaha/Valley Nebraska. The bottom of the graph represents the temperature in C(-30 to +50). The left side represents height up to 100mb. The green line is the measured dewpoint and the solid red is the measured temperature...as the sounding went up via a balloon. The red dashed line is the expected temperature of a rising parcel of air. The area between the red dashed line and the red solid line is how they figure CAPE. But anyway, to the cap. To tell temperture you don't go straight down on the image but you follow the dashed blue lines to the left as you go down. So if you find where the red line crosses the 500mb height level and follow that point down the path of the blue dotted line you should come to almost -10 C(that then is the temp at 500mb...about 6 km). Since this goes up and reads the actual atmosphere you can see right where the cap is. If the red line moves right as you go up then the temp is rising. I made a black arrow to point to the cap in this case. Now since you have to go down at an angle to see the temp, going right as it goes up doesn't mean it has to be warming. In this case it looks to be just barely warming, almost the same temp(since if you had a red jog perfectly parallel to the dashed blue line that portion would mean it's the same temp). There is another small one up around 700mb(700mb temps are often used as a guess on the cap, especially after early spring)). The cap acts as a lid on all that low level juice. None of that low level air will be able to rise until the cap is gone.
How to get rid of the cap? 2 ways. Cool the air at that level, or warm the air below. See the red below the cap? That all needs to move right(warm) to make that area of rising temps go away. It's either that or get strong cooling aloft to mix down and move that area of the cap left(cool it). Most chase days will use a combo of both, cooling aloft and warming of the surface. The cap is good and bad, all depending on how strong or weak it is. Too weak and too many storms competing against one another, too strong and no storms.
To measure or guage the strength of this cap the term CIN is used. It stands for "convective inhibition". The amount of CIN describes the amount of energy preventing air from rising. The stronger the cap is the more CIN there will be. Now remember the area between the red line and the red dashed line is the amount of cape. If you go way down low you'll see where the red dashed line crosses the solid red line. That smaller gap is the amount of CIN. That needs to be erased or overcome to release that cape above it. Fronts can actually help to force the low level air up through that if heating of the surface or cooling aloft can't completely erase the cap on their own. So during the afternoon of storm days that's one thing to keep tabs on, where is your CIN being eroded. This is most likely to be in the late afternoon near peak heating. Remember you can heat the surface more and cool things down from above. So on that sounding, the red line showing the actual tempture will move right near the bottom(ground level) as the surface temperture rises. If it moves far enough(warms enough) the cap will be removed(basically).
Now what happens after sunset? The air will start to cool and the near ground temps will begin to fall/move left on the graph. Your cap will now start to come back, and you can watch the CIN numbers climb. These aren't the only things affecting this either. That level where the warming is can increase at the same time as the peak heating, and just as much...or quicker and moreso. You'll wind up capped out for sure. It can happen if the storm system is advecting even warmer air in the capping height.
While I'm thinking about it. Ever wonder why the low level jet increases after dark? During the day the ground is heating, causing vertical mixing/motion. This tends to impede flow some. At night this vertical mixing lets up, since the sun has set. This allows the flow to pick up at the height of the low level jet.
Another thing chasers watch is the temperature dewpoint depressions. Say you have a temperature of 90 and your depoint is 50. The T/TD depression(Temperature Dewpoint Depression) is 40. It's the difference between the air tempertature and the dewpoint temperature. It will directly relate(for the most part) to the height the base of the storm is off the ground. The lower the base the better for tornadoes...basically(I'm not a huge fan of that wording, since so much else is involved and so many higher based storms produce tornadoes just fine). It would be given this way, 90/50. That is a horrible dewpoint depression and you'll have a very very high base for sure. Tornadoes are going to start getting very unlikely in that kind of air. Something more optimal might be 85/72(85 temp, 72 td/dewpoint). That'd only be a 13 dewpoint depression. You still need the right shear and whatnot, but it's just another important piece of the puzzle. I guess over 25 for a dewpoint depression and the odds start to go down for tonradoes. If you do get them they'll just be "longer/taller" tornadoes. It seems I've seen some good examples of strong tornadoes in the 20-30 degree dewpoint depression ranges. Ok, so maybe over 30 and the odds really drop off. 40 and you are very unlikely to see much for a tornado.
The image to the right was probably in air with a 25 degree dewpoint depression. It seems like I can think of many good tornado examples in that 25-30 area, but come up pretty blank above 30. With 25 degree dewoint depressions your base will be fairly high, like the tornadic supercell to the right.
This image here is from the May 28, 2004 supercell. Where it started it was in a good 30-40 dewpoint depression environment(probably closer to 40). The base was very high and flat. Many times storms will fire in a bit drier environment further west, and move east, often into better moisture. If they can survive long enough they'll surely lower as they encounter that moisture.
Here is a storm from May 16, 2004. I think this was in 10-15 degree dewpoint depression air. You can see the base is much lower, and there is actually a tornado on the ground in this image. It was actually pretty hard for me to track down a decent example of the much lower dewpoint depressions. It seems the far majority of cooler supercells I've seen were closer to the 20-30. If I had any threshold to worry over I'd say it would be 30 and above, but one can still find cool high based storms with that. And it is important to remember low bases, low depressions alone don't mean tornadoes will happen...obviously. I've see far more tornadoes from more higher based storms than I have ones as low as this one. And again that is up to a point of around 30 for a temp/td depression. Some people don't want to chase when they are even much over 20. Give me 20-30 and I'm happy any day....and some shear!
Here is another term one might want to understand. Boundary layer. What is the boundary layer. The boundary layer is the layer where Earth's surface no long causes friction. Well that level is the top of the boundary layer. So where there are no more effects from Earth's surface friction is the top of the boundary layer, and the surface is the bottom.
The image to the right here is a 500mb forecast from the GFS model. Might be hard to see, but see how the black lines dip in the middle of the US? There's colored shading in the same area. There's also a circle over Nebraska. That is a trough of low pressure moving through the US. There's another big one off the east coast. There is a smaller one in se Canada. There's yet an even smaller one in central Canada(no colored shading around that one). Those colored areas are higher wind speeds at that height. Red starts at 70 knots as seen on the graph at the bottom. Now see where the black lines rise or poke northward over the western states? That's a ridge of higher pressure. Best area of severe weather will be upstream from a ridge and down stream from a trough, or between the two. In this case your surface cold front or dryline, associated with the central US storm system, would likely lie from eastern IA or Illinois on south through eastern Missouri and Arkansas. That's just a guess, you would be able to see right where it is on the surface plots or if it is still a forecast, where the model expects the surface pressure and boundaries. The thing is, you'll often have those higher wind speeds in the jet stream spreading over your surface boundaries. So then your storm fires on the boundaries, and use that shear/flow aloft.
Same image as above obviously, but I drew in a few lines. See how the lower middle one is almost straight up and down. If the trough axis is north south, it is considered a neutral tilt trough. It's not quite neutral but it is close. The top left one has its axis running sw to ne and is considered a positive tilt trough. The one to its right runs nw to se and is considered a negative tilt trough. Neutral and negative are generally prefered. With a positive tilt trough you will likely be dealing with a cold front, not good for good severe storms. Most times the cold front is out ahead of that shear aloft....not good. With a neutral tilt that is less likely, as well as a negative tilt. Also you will want your winds at this level to cross your surface boundary(be perpendicular to it) more than run parallel to it. Parallel will lead to linear storms(squal lines), while perpendicular will want to give you a more discrete and hopefully supercellular mode. If you get a trough with too much of a negative tilt you can get this paralleling flow to your surface front, as the winds aloft will often back to the south too much. So, too positive tilt and you get a cold front pushed out ahead of the winds aloft, too negative tilt and you may get the winds aloft to back too much. Cut off lows are lows/troughs of low pressure that have cut off from the main jet stream. They generally aren't great either and tend to stack themselves above the surface low pressure, leading to unidirectional flow over the boundaries
Anyway, I somehow need to end this page. It could go on and on. If you want to play around trying to track forecasted systems/troughs, use this link to RAP's models. It starts you on the RUC model, which only goes out 12 hours. It's obviously used more for the day of stuff. You can click on the ETA or GFS links for those two models. ETA is actually the NAM now, not sure why they don't change the label. ETA/NAM goes out 84 hours, while GFS goes out 180 on there(other places it goes out over 300 hours, but that's getting silly). If you see a big spinning upper low system on satellite one day, and want to see it compared to these kind of charts, just click on 00 hr for a forecast time. Realize though that the ETA/NAM and GFS both update at 12z and 0z, so 00 hr forecast would be for either of those depending on the time. When the image comes up it has a date and time in the upper right, that's the model run time. Say it says 0z May 15th. That would be the run from 7pm May 14th. ETA/NAM is available generally about 2 hours after it started, so 9pm in the spring(central time). GFS is out about 3-4 hours after it started. They start after they recieve the data from all the soudings sent up around the US.
