Day 89: Meteorological Detective Work

Posted by pmarsh on 30 Mar 2010 | 7 comments

Warning:  This post does contain somewhat technical information.

In preparing tonight’s blog post, which was originally going to be a forecast discussion regarding late week severe weather potential, I came across something that I thought would end up making a better blog post.  Especially when you consider that the severe weather prospects for Friday didn’t look good based on this morning’s 12 UTC model forecasts.

From time to time, meteorologists have to play detective.  What I mean is that numerical model forecasts often do things that at first glance don’t seem all that extraordinary.  However, thorough meteorologists will investigate why a model did what it did, and relying on theory, attempt to glean useful information that isn’t exactly obvious.  A good example of this can be found in examining the 12 UTC Global Forecast System (GFS) forecast for 12 UTC Thursday (7 AM CDT Thursday) through 00 UTC Friday (7 PM CDT Thursday).

The image above is valid 12 UTC Thursday (7 AM CDT), and displays the forecast 500 milibar heights and vorticity (top half) and 500 milibar heights and wind speeds (bottom half).

Through the course of these images, pay attention to the area of southern Colorado and northern New Mexico.  Watch for changes in wind speed and vorticity (both color fills).

The image above is valid 18 UTC Thursday (1 PM CDT).

The image above is valid 00 UTC Friday (7 PM CDT Thursday)

Did you see it?

Notice the model’s forecast wind speeds for northern New Mexico increase dramatically between 12 UTC and 18 UTC and then decrease dramatically between 18 UTC and 00 UTC.  In fact, the maximum seen in wind speeds at 18 UTC are greater than the surrounding magnitudes at 12 UTC and a lot greater than the surrounding magnitudes at 00 UTC.  Did you also notice the same thing happen with the vorticity?  Why might this be?

First, let me explain why the two images are linked together in a single frame.  Typically, meteorologists look for kinks in the 500 milibar height field to identify potential shortwave troughs and/or ridges.  In fact, the dip in the 500 milibar heights throughout the western United States are associated with a (larger) trough.  Troughs are associated with increases in (relative) vorticity and ridges are associated with decreases in (relative) vorticity.  Because increases in (relative) vorticity are associated with troughs, whenever the colors on the top-half of the image increase, one could infer the presence of a trough.  Without going through all the explanation as to why, an increase in wind speed can also be attributed to the presence of a trough, espcially downstream (in this case on the east side) of a trough.  (It has to do with  strengthening lower-level temperature gradient(s)).

In the vorticity images, one can track an area of higher vorticity values from southern Utah (image 1) to northwest Colorado (image 2) to southeast Wyoming (image 3).  This is associated with a shortwave trough breaking off from the southern portion of the trough and rapidly ejecting northeast.  This is known as a lead-shortwave trough as it “leads” the main shortwave trough.  As the lead shortwave trough races through the flow of the longer wavelength trough, it helps to increase 500 milibar wind speeds in northern New Mexico.

If we think about the main shortwave trough, we would expect to find stronger winds located downstream (east) of the trough axis.  The same can be said of the lead-shortwave trough.  We’d expect to find an increase in wind speed downstream from the trough axis.  When the expected area of increased wind speed from both troughs overlap, we get a cumulative effect where the winds increase to more than what they would without interaction of the two troughs.

If a meteorologist had simply examined the noisy vorticity plot above, the presence of the lead shortwave could have been missed.  The same if a forecaster had only examined the forecast wind speeds.  However, by examining the two together, and understanding that it is possible to indirectly link increases in vorticity to increases in wind speed, we can determine the presence of a lead-shortwave trough.  Why is this important? Downstream from a shortwave trough, rising motion is often found.  When a “cap” (discussed more in upcoming blog posts) is present, the rising motion associated with a shortwave trough will often help thunderstorms initiate, when otherwise thunderstorms wouldn’t.  This often results in what chasers refer to as “the day before the day” events.  That is, thunderstorms and tornadoes developed on the day before the main trough moves through, which most people consider to be “the day”.

Now, I’m not saying that Thursday will be “the day before the day”, however, if moisture return is a little better than forecast, and a lead shortwave trough moves through during the afternoon hours, I wouldn’t be surprised if an isolated severe thunderstorm or two developed out in the Texas panhandle.  I also wouldn’t be surprised if nothing happened, either.