Day 13: Active Southern Stream

Posted by pmarsh on 13 Jan 2010 | 2 comments

In a typical El Nino year meteorologists expect the storm track to favor the southern United States.  This results in a lot of rain across southern California, the desert southwest, southern Texas, and the southeast United States.  As many may have heard, we are currently in the midst of an El Nino winter.  What is interesting about this is that the southern storm track has failed to materialize for most of the winter.  (There have been a few exceptions, but for most of the winter so far, southern California has remained dry!)  This may be about to change.

Today’s weather graphics are water vapor imagery from this evening.  These products come from weather satellites that the United States government launches in support of the National Weather Service.  What this image does is capture the amount of water contained in the middle-to-upper troposphere.  The brighter the colors, the more water and vice-versa.  Why do meteorologists care about water in the middle-to-upper troposphere? The answer lies in the fact that there must be rising motion to get the water vapor up that high.  Thus, the brighter the colors the more moisture in the mid-to-upper troposphere which means the stronger the upward vertical motion!  Meteorologists are actually (mostly) interested in identifying where there is rising motion because this is a necessary ingredient for precipitation.  A good rule of thumb is that all things equal, more rising motion equals more intense precipitation.

In the image below I have circled two upper-level lows that are currently affecting the southern portion of the United States.  The first upper-level low is currently located over central Texas and is lifting to the north and east as a result of the second upper-low behind (west) it.  The United States has been very cold as of late and since cold air holds less moisture than warm air there is not a lot of moisture across much of the United States.  Because of this lack of moisture, the first upper-level low is having a hard time creating precipitation (notice how the colors in the first upper-low are not as bright as the colors in the second upper-low).

What the first upper-level low is doing is “priming the pump” for the second upper-level low by inducing moisture over the Gulf of Mexico to begin moving northward into the southern United States.  As the second upper-level low moves into Texas over the next 24-48 hours, heavy rain and thunderstorms are expected to develop across much of central and eastern Texas.  As much as 5 inches of rain is possible in the Corpus Christie, TX are over the next few days.  If most of this rain falls in a short period of time, flash flooding will be possible.

Eventually this upper-level low will move toward the southeast United States.  For those attending the 90th annual meeting of the American Meteorological Society in Atlanta, GA, I would expect rain during the first part of the conference!

Also, as I alluded to in the opening of this post, the long range models are indicating that several mid-to-upper level lows will be tracking along the southern United States over the next few weeks.  As this storms move onshore, heavy rain will be possible across dry southern California.  Mudslides will certainly be possible in the next few weeks on hillsides that have been scarred by any recent wildfires.