This week in Weather

[tenkiforecast]

I thought I would try this out, so here goes. Like I mentioned in my intro thread (and my avatar) I am a meteorologist who will be attending Graduate School. I guess that I may as well do a discussion on what is happening in the weather, probably once a week, or if something is very interesting. I will also be explaining some aspect of Meteorology to help people understand what the heck I am talking about or an appreciation of the atmosphere as a whole.

These take about an hour to an hour and a half to put together, so this will most definitely be a weekly thing unless there's a tropical storm developing or the SPC issues severe forecasts.

The first thing I should mention is the coordinate I will be using to describe the atmosphere. The most common vertical coordinate used by Meteorologists is Pressure. Pressure is measured in multiple ways in science, but in Meteorology two units are used. The first is to establish context, or one Atmosphere (atm). This is the baseline or the pressure of the full atmosphere, but it is not that effective. The most common unit is the millibar (mb), which at one atmosphere or Mean Sea Level Pressure (MSLP) is 1013.3 mb.

The atmosphere is also huge, tens of kilometers high. (Sorry, with the exception of temperature I will almost exclusively be using metric units). However, various phenomena occur at levels in the atmosphere, and these correspond to the most frequent levels referenced by meteorologists. These would be the surface (usually measured in MSLP, the only non-pressure coordinate commonly used), 850 mb (1/4 of the way through the atmosphere), 500 mb (halfway through the atmosphere), and 250 mb (3/4 of the way through the atmosphere). The various phenomena are measured at these levels: Surface, fronts and temperature variations; 850 mb, temperature advection and winds; 500 mb, vorticity (the spin of the atmosphere) and height variations; 250 mb, the jet streams. I will be referencing these and explaining them later.

Why is pressure used? The thickness of the atmosphere varies with temperature and phenomena in the atmosphere follow surfaces of constant pressure. In this sense, pressure is the ideal coordinate. If you want the actual equations, let me know.

So, now for the actual discussion. This will pertain to the wildfires in Colorado, Utah, and Wyoming, as well as to why it is so hot in the United States right now. If you're in Europe, Asia, Australia, or any other part of the globe, I'm sorry. I don't know the climatology of those areas or how various phenomena in the atmosphere affect the weather. A "500 mb trough" will have different effects over Western Europe than over the central Plains because of the geography. I'm a US Citizen, and I don't know how other parts of the world are affected by the same phenomena, and I don't want to appear as a douche, so I'm not going to try anything.

Please take note: All of these images are not mine, but generated by the Meteorology Department at Penn State University. If you want to see the site itself, here is the link:

http://www.meteo.psu...mski/ewall.html

All the images I am using are the 00Z or 12Z model runs.

First let's look at the 500 mb Northern Hemispheric chart, as progged by the, I believe this is the NAM, I could be wrong.

First, I'll explain what the left image is, that is the important one. This is the hemispheric 500 mb heights, with the colors being the amount of deviation from the climatological normal. The first thing you can see is that the pattern of blue (lower heights) and red (higher heights) is very frequent, implying a high wavenumber (number of atmospheric waves). They are also very high amplitude, with two blocking patterns. What I mean by blocking patterns are the two circular pattern of heights over the Atlantic, and off the West Coast of the United States. The pattern over the Atlantic is called an Omega Block, a large area of high heights and warm temperatures that, like the name implies, prevents the flow of air. Omega blocks, once they set up, are very stable and can last for two to three weeks, if not longer. This blocking pattern just recently transitioned into an Omega block from the kind that is starting to dissipate over the West Coast, a Rex Block. A Rex Block is a circular pattern of high heights just north of a circular pattern of low heights. Rex blocks are more unstable, but they still halt the flow of air. What is starting to form is the shortest-lived block, a cutoff low. A cutoff low when separated from any other dynamics moves against the usual flow--it moves from East to West. This does not appear to be the case here, but it still succeeds in blocking the flow of air.

What this means is that the airflow over the United States right now is very stagnant. No new air is being introduced into the system, and the nature of blocking patterns will only cause the current situation--a ridge, an area of high heights over the Great Plains--to intensify. This means warmer temperatures for those under the ridge, and more potential energy in the system. It is also possible that the ridge could develop into an Omega block, but I find that unlikely.

Rather than this being the output from a Numerical Model, this map shows the results from the weather balloons (radiosondes) launched by the National Weather Service. Look at the reports over the US South. The winds are 25 knots from Texas up through Kansas. These winds are coming directly from the desert, uninfluenced by the upper level of the atmosphere. These winds are extremely dry and are not transporting moisture into the United States. They are also extremely warm. This does not bode well for control of the Colorado Wildfires because there is no moisture being introduced to help control the flames. On the northern half of the country, there are winds to help moderate the heat of the atmosphere, but they are not that strong, and the low heights system over Hudson Bay is not strong enough to force enough cold air into the United States to moderate the temperatures.

These are the main reasons that the weather over most of the USA is so warm. A ridge of high heights over most of the country, and winds at 850 mb coming directly out of Mexico, very hot and dry air.

I'll attach the surface analysis from the National Weather Service:

You can see two cold fronts, but they are relatively weak. They don't have enough strength to drastically change the temperature, only induce possible storms and lower the temperatures by about 5 to 10 degrees Fahrenheit. This won't last too long, because the winds from 850 mb will keep heating the US.

What exactly will happen on a broad sense over the next few days? Well, the models are showing a good transfer of energy at 500 mb, small packets of vorticity moving across the major ridge to help cause small surface lows and weaker fronts. The atmosphere over most of the US will be unstable due to the high temperatures, but without a source of lift, no organized convection will occur. Also, until the 850 mb pattern shifts so that the winds are coming from the Gulf instead of the Mexican deserts there won't be significant convection either. Until this shifts, nothing will be able to help the Colorado Firefighters and the temperatures over the US will stay this warm, or even increase in temperature. The problem is the Omega Block. Until the Omega Block breaks down over the Atlantic, there won't be any drastic changes into the atmosphere.

Any questions, please let me know, and I hope I explained this well enough.

[Strawman]

Wow, meteorology is incredibly more complex than they make it seem on the news.

This is awesome, btw.

What makes the Omega Blocks and the like form like that?

[Melonhead]

Uhhhhhhhhhhhhhh....... *drools over keyboard*

I like weather too, and I'm about to read most of this... Seems pretty cool. Almost 100 degrees where I live. :/

EDIT: So this is pretty cool. I'm gonna have to look more into the airflow with the highs and lows and such. So without them, it's more unstable, and weather changes more often, and with Omega blocks and Rex blocks, it will cause the weather and tempeture to stay around the same? I found that part pretty intresting.

Edited June 30, 2012 by Melonhead215

[Rewjeo]

Well, I found this interesting, but I would be lying if I said I really understood it. I think I got the gist of it, but trying to match your words with the pictures didn't entirely make sense. Maybe after a few more weeks I'll have some "Aha!" moments.

[Melonhead]

Living on the eastern seaboard, we got slammed by some big storms yesterday. Knocked the power out until a couple of hours ago, and we might have another big one tonight. Any reason for this?

[Raven]

i like the bit where the sun shines on my face

but the clouds are all like "not today, bitch"

fuck you, clouds

[Esau of Isaac]

No fuck you man. Clouds are awesome. They protect my face from that vengeful jerk the Sun. Why can't you shine somewhere else you broiling, blinding bastard?! Here I am trying to enjoy walking around and looking at the beautiful blue sky and you gotta get all up in my grill bro.

[tenkiforecast]

Yes, I know. Believe me, it takes a while to understand this that well... I still dont' understand much completely, and I'm going to grad school. 0_O;

The whole premise of blocking flow is that the atmosphere becomes much more stable and predictable. One example I can think of from this year was a cutoff low over Ohio causing constant partly cloudy, 60 degree Days in Michigan for a week and a half. During september.

Omega blocks tend to form when the 'troughs' (areas of low heights) on either side of a ridge (areas of high heights) start amplifying. Due to wave dynamics (Don't even ask me to explain this... I don't even know how to or understand it that well. Dynamics are really messy... ugh) this will cause the ridge to amplify. If it expands so much that it becomes cut off from the rest of the atmospheric flow, it becomes an omega block.

As for the East Coast, it's first pointed out on the 500 mb map. There is the presence of a longwave trough (areas of low heights, this is the main topic I'll explain in the next one), and the areas effected by storms were on the right side of it, which is a very good environment for convective activity. I looked at the wind shear, and that was also high--40 to 50 knots, which is very good for convective development. There's dynamical support, there is the shear, there is obviously the temperature and surface instability, the question is a source of lift. That's provided by a weak cold front. You have moisture, instability, lift, and favorable dynamics. You are going to get storms.

As for why you will probably get more, it's the blocking pattern. Usually the trough would move with the flow of the atmosphere, but the atmosphere is not flowing. It's blocked off, and relatively stationary. The fronts are not moving much either. The environment is the same, so storms will form again.

If I was looking at the east Coast in detail, I should have noticed that. I live in Michigan right now, and as a result, that tends to be where I focus. I'll try to look into more areas of the country in the future.

Edited June 30, 2012 by tenkiforecast