Topic: Air masses and Fronts from Weather Underground Maps

[p3aul]

I am still learning meteorology.I use WunderMap, and the Front's Maps to try and get a handle on fronts moving in to my area. It is my understanding that fronts occur between airmas boundaries, as between an area of high pressure and an area of low pressure. But looking on the Fronts Maps right now: http://www.wunderground.com/maps/?&MR=1

at 01:24am Eastern Time. I see a boundary abut the middle of Arkansas between a High on the right and a low on the left but there is no front marked!

Just above Oklahoma there is the center of a low pressure area. There are three fronts eminating from the middle of this low pressure area and passing through it at no boundary points! Could someone more weatherwise than me please explain this? It would have been easier to explain this had I been able to upload an attachment but the forum won't let me for reasons know but to God and the moderators!
Thanks,
Paul

[alexstaar]

These maps may be generated solely by a computer algorithm with little to no human interaction. Computers are generally not very good at analyzing fronts and other surface boundaries. Based on your description of the surface analysis map you saw, there may have been some things that a human would have caught that the computer didn't catch. Here's the current map as of this post, what would you like to know about it? I'm a senior meteorology student and would be willing to help you understand basic surface analysis.

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[p3aul]

Ah just the person I was looking for! Thanks Alex. and thanks for posting that map. As a new member I can't post attachments, but this will do very well. My confusion lies with air masses and high/low pressure areas. Let's start by my telling you what i think I know. I know that a front occurs along the boundary where a cold air mass meets a warm air mass. I don't think their moisture content matters, correct me if I'm wrong. The only evidence I see of the warm and cold air masses is the front where they meet. The air masses themselves don't seem to be drawn on the map. The highs and lows are drawn along with their isobars spreading outward. This is the point where I'm confused. How do these Highs and lows interact with the air masses? Are they contained within them? I know that at a cold front the cold air mass is behind the front and the warm air mass is in front. I guess a warm front is just the opposite. We don't seem to get many warm fronts passing through Georgia. Is their any instrument that can detect the cold and warm air masses? Can you get an idea of the general shape and size of an air mass?

I have also read that there is a high pressure area behind the front where the cold air mass is. I'm guessing that that would be the high that is centered at the "H" in Kansas? But why is there not a low pressure area in front of the front  around Georgia or off the coast?

Thanks,
Paul

[alexstaar]

Hopefully this won't confuse you even more, but I have posted the current 00z run of the GFS surface temp and dewpoint forecast for 123 hours (which is Tuesday night). The temp map shows isobars in black, wind barbs, and surface temperature.  Perhaps this will help you see where the fronts are in relation to the colder/warmer air. You can also see the sharp contrast in the dewpoint map. Fronts are not analyzed on these maps, but you can easily see where the cold front is located by the sharp temperature gradient right through AL, TN, KY, etc. (temps are in the 60s ahead of it and in the 40s behind it) as well as the troughing or "kinks" in the isobars. Now you will also typically see a significant change in wind direction behind the front, but this system is a bit complicated so I won't go into why it's not on this one. You can also see that behind this strong low pressure system is a high pressure system with much colder air that is moving down from Canada.

By definition, warm fronts and cold front must extend from an area of low pressure. The reason is because it is the dynamics related to the low pressure system (as well as upper level forcing) that drives the progression of the fronts; a portion of this is the fact that low pressures are areas of lower level convergence (air wanting to move toward their center counterclockwise). High pressures are not associated with any large scale (synoptic scale) surface boundaries as they are areas of lower level divergence (air wanting moving away from their center clockwise). So basically you could think of especially a strong cold front as the leading edge of the high pressure system, which extends away from the center of low pressure. So as a high is moving off the coast, the wind on it's western or left side is southerly. This would then precede the next coming cold front and so on and so forth.

You are correct about the fronts separating 2 air masses. A cold front means cold air is advancing behind it (cold air advection). A warm front is when warm air is advancing behind it (warm air advection). But this is atmospheric and it's not always textbook pretty and all. Sometimes they are sharp and strong, other times they are weak and hard to pick out. I also have posted the graphs from my weather station from a strong cold front we had pass though last March. The temp plummeted after its passage; you can also note the quick jump in pressure and abrupt change in wind direction. To answer your other question of how you see the different air masses, really the only way to do that is to look at a surface temperature or dewpoint map, kind of like what I have posted from the GFS.

I would suggest reading through this article on wikipedia about cold fronts for a better understanding of them. https://en.wikipedia.org/wiki/Cold_front

I know this was a lot Let me know if I need to clarify anything!

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[p3aul]

 Looking at the first two maps, It looks like the front is at the concentration of the blue lines in the 2nd map. On the 1st map it looks like the same front is at the border of yellow and green air masses( I assume the different colors represent the temps of the two air masses that have collided.)

If this is so it makes my understanding a bit easier. I also notice that the front begins at the center of a low pressure area. Is this a coincidence or is that what happens normally?

Where did you get these maps? Can I get them also?

[alexstaar]

The lines you are pointing out on the second map is Lifted Index (LI) which is an instability parameter. Obviously, instability rapidly decreases behind the front, but the main reason I posted that map was for you to see the drop in dewpoint behind the front. Notice that ahead of it, dewpoints are in the 60s and behind it they are around 40º. You are correct about the first map. Each color shade represents 5ºF in temp, the color bar/key is located at the bottom of the map. Ahead of the front, temps are in the 60s and behind it they are in the 40s.

You are correct about the low and the front. A front by definition must always extend from and area of low pressure. This is true for both warm fronts and cold fronts.

The maps are from the College of DuPage (COD) webpage. They have free model output available. Here is the model website: http://weather.cod.edu/forecast/ You can click on each of the model tabs a view any of the data from the list on the left side of the page and scroll through with the bar at the top.

[p3aul]

Thanks for the reply. Boy these maps can make your head spin! Right now I'm looking at a surface map from the COD site like your first map you posted. I didn't realize it was part of an animation.

As I move my mouse across the page, the time changes and so do the Isobars! I didn't know the center of a pressure area could move so fast! In 6 hours it seems to move from Missouri to what looks like Michigan. Is this possible?

Still not allowed attachments! Are you a member of another forum where I could upload attachments?
thanks,
Paul
OOPs well I wrong. I have attachments!

[alexstaar]

Yeah, lows can move that fast! They don't always move that quickly though, some are faster than others depending upon what's going on in the mid and upper levels.

Glad to see you know can now post attachments! Let me know if you have anymore questions. I would highly suggest playing around with the NAM and GFS products on that site so you can see the differences in the models. Things get really tricky with forecasting high impact events.

[p3aul]

Thanks Alex, you are a great help. I see some differences between Weather underground  pressure areas and those at COD.EDU.
I don't think WU is as complete or as correct as CODs.

Question: I'm looking at the 2m Dewpoint chart and I see a map with Isobar-like lines but the numbers don't correspond to millibars, but the are concentric and converge to a small center. Now I suppose these are lines of the same dewpoint but the resemble the Hi/Lo pressure gradients. Do these correspond in some way to the pressure areas are is this just co-incidental? Also what do they tell me?

I'll let you off the hook in a little bit, but this is so interesting I hate to let you go!

[alexstaar]

No problem, I'm happy to help!

Those line you see on the dewpoint map are NOT isobars. They are lines of constant Lifted Index (LI) which is an instability parameter found from a sounding (in this case, a forecasted sounding). You can see a forecast sounding for any point you click on map. LI is found by subtracting the parcel temp and environmental temp from the sounding at the 500mb level (in ºC). I'm not sure if you know what that means, but basically if the parcel is warmer than the environment, it is positively buoyant and wants to rise (supportive of convection). Therefore if this number is negative (red on the map), a surface based air parcel is unstable at 500mb (about 18,000 feet above the ground); if it's positive (blue on the map), the air parcel is stable at 500mb. So you can see that ahead of the cold front, the air is unstable and behind it it's stable.

If you click on the map for a sounding ahead of and behind the front, you will see a red, green, and dashed yellow line. Environmental temperature is red, dewpoint is green, and parcel temp is dashed yellow. Ahead of the front, the parcel temp is warmer than the environmental temp and just the opposite behind the front.

I know that was a lot, sorry for throwing that to you all at once! Keep the questions coming!