Tuesday, September 27, 2011

A stacked low

We have an aesthetically beautiful storm system (at least, from a satellite perspective) over the midwest that has been lingering around for the last few days.
GOES-E IR satellite image from 1815Z, Sept. 27 2011
A nice spiral shape is visible over the Chicago area, indicative of what we call a stacked low--a situation where the center of low pressure (or the lowest heights of all pressure levels aloft) is vertically contiguous over the entire atmosphere.  That is to say, the troughs of lower pressure or geopotential height are all lined up over each other throughout the atmosphere.

We can see this on this morning's model analyses.  Here's the 12Z GFS 500mb analysis:

The height of the 500mb pressure surface off the ground is contoured in black.  You can see that the lowest heights are found right in the center of that bulls-eye over northeastern Illinois.  This is a closed low seen at 500mb.  Why is this a "closed" low?  Notice that the contours surrounding that very low height center are circular and completely closed off.  This is not an open wave in the mean flow--air is moving in full circles around this depression.  Thus this is a closed low in contrast with open waves that we often see moving through (like the broad open wave off the Pacific Northwest coast.

If we drop down to 850mb, we see the low in exactly the same place:

Once again, the lowest heights of the 850mb surface are centered over northeastern Illinois.  Finally, if we look at the surface pressure, we see the same thing:
Surface pressure is contoured in black on the above image.  You can see that there is an area of low pressure once again centered in northeastern Illinois.  Since all of these areas of depressed height or pressure are stacked right on top of each other, this is why we call this a stacked low.

One interesting feature of stacked lows is that they often indicate a weakening storm.  Look at the surface temperatures in the surface map above surrounding the center of low pressure.  See how the temperature is just about the same (in the greens and blues) all the way around the low pressure center?  The warmer air (the yellows) is well off to the east and south--far displaced from the center of the low.  A strengthening low pressure center draws its strength from these thermal contrasts--these fronts.  If the fronts move away from the center of low pressure, it gets more difficult for the surface low to strengthen.

Why is this?  Remember that the strong upper-air winds (and, consequently, the associated jet streak divergence that low-pressure centers need to form) are driven by temperature gradients below.  If the strong temperature gradients (usually fronts) move away from the center of the low, the strongest upper-level winds will move away with them.  This makes it less likely that the low will grow.

Another interesting feature of closed, stacked lows is that they don't usually move out quickly.  Since the temperature gradients and therefore the strong upper-level winds move away from the center of the trough, it's difficult to get enough strong motion to move the trough along.  Furthermore, when the low is closed instead of being an open wave, the flow around the upper-air low is more circular and less east-west.  This also doesn't help move the low off to the east.

So, in short, this means that the midwest has been and will continue to experience more rain and cloudiness for the next day or two as that low lingers around.  Most models have it transitioning to an open wave again by Friday, and with that it should begin to move out.

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