Weekly Weather Discussion, July 7 2012

[tenkiforecast]

Seeing comments of confusion in the previous topic has prompted my initial discussion and what meteorological concept to explain.

Previously I mentioned Pressure Levels, and here is where they will begin to come into play.

This week the topic is split into two features, Longwaves and Shortwaves. *Opens Synopic binder of doom*

For anyone who has taken a high school physics class, those two terms should sound familiar, as they are used to describe Electromagnetic Radiation. The same naming concept as EM Radiation is used in meteorology. Longwaves have long wavelengths (distance between two crests or two troughs--in Meteorology, ridges and troughs) and shortwaves have short wavelengths. Easy enough. The difference is that longwaves have wavelengths on the scale of 3000 kilometers. My discussion on Omega Blocks is in that sort of length scale, a block is just a longwave that has been cut off from the main flow. Longwaves are very stable, and adjust in size and amplitude over the course of days. They can move very quickly, but they usually move at a modest pace. A longwave can usually cross the United States within a day to two days during the Spring/Fall/Winter circulations. The equations are provided in the link below, and *this* is why meteorologists need to go through tons of math.

Welcome to Hell

The opposite of the Longwave is the Shortwave. Shortwaves have smaller wavelengths, about 1/3 the wavelength of a longwave, and move much faster than longwaves. In fact, the interaction between longwaves and shortwaves drives many meteorological phenomena.

Without a shortwave, a longwave will not tend to amplify. Longwaves prefer to propagate to the east without amplification. Shortwaves amplify longwaves when they move through a longwave.

There is a reason these are called waves, because they are transfers of energy. An easy way to visualize energy is the vorticity, or rotation, of the atmosphere. Longwaves have broad vorticity that is dispersed over a large area, however, when a longwave is very amplified (An example would be an Omega Block) the vorticity is very strong. The reason this is important is that vorticity high in the atmosphere causes air to rise or sink. Cyclonic vorticity, associated wiht low heights, causes air to rise. Anticyclonic vorticity, associated with high heights (the omega block), causes air to sink. Shortwaves have much smaller and more focused vorticity, and they can cause a lot of activity as a result. The massive system of storms that knocked out power across the East Coast on June 29/30 was fueled partly by a strong shortwave. Shortwaves tend to cause the most meteorological activity on their own during the summer when a source of lift is needed by the atmosphere. In the Fall, Winter, and Spring, amplified longwaves are the biggest source of activity. The lack of amplified longwaves in the United States is the reason that the winter in the Western Hemisphere was so mild and warm. The problem was that the amplified systems were over in Europe, which contributed to one of the coldest winters Europe has endured in decades.

Understanding longwaves, shortwaves, and how they relate to vorticity is one of the essential aspects of meteorology and forecasting.

Now let's look at the current situation, starting with the 500 mb global chart.

First thing we see are tons of longwaves. The omega block from last week is still there and looks to be just as strong. Not much to say there. The key difference is the eastern United States, as there appears to be a longwave trough developing over the East Coast. This will offer a *very* need reprieve in temperatures, some of the forecasts by the NWS show falls in temperature of about 10-20 degrees Fahrenheit--from 100 degrees to about 85 degrees in most cases.

Looking at the 500 mb chart over the US

There are a few shortwaves approaching the longwave trough, the strongest one is over Manitoba. A shortwave trough moving into a longwave trough will intensify the longwave trough and cause it to intensify. Looking at some of the other maps forecast by the NAM show much needed moisture heading into Colorado. There is not a whole lot to say at the moment involving this situation, only that the shortwaves are going to be the cause of the weather change.

The 850 plots show that there are weaker winds, about 30 knots over northern Michigan associated with the large low east of the Hudson Bay. The other notable feature is seen in the relative humidity plotted in the lower left hand corner, and shaded in green. Moisture is finally being funneled into the United States from the Pacific Ocean and Gulf of Mexico. The path moves through New Mexico and Colorado, providing them with desperately needed moisture. This, along with that shortwave, could cause storms in the northern half of the country once the shortwave starts providing the lift.

Skipping the 300 mb plot because nothing is going on. It's the summer, this usually happens, Jet dynamics shut down.

What exactly do I mean when I say that the trough with amplify? Here is the NAM 48 hour forecast from Jul 7, 12Z run:

The longwave trough is much more defined over the United States, and is oriented so that air will be advected from the north to the south. This will cause temperatures to fall because of cold air being placed further south. However, as the Omega block does not seem to be breaking down, I doubt that this will offer any dramatic changes in weather aside from much needed falls in temperature for every state under the 500 mb trough, and in the path of the colder air.

Any questions, please post them below and I will try to answer them.

Why was this a bit late, well...

[eclipse]

This is really interesting, thanks. Missed last week's one.