Day 113: Severe Weather Event Underway

Posted by pmarsh on 23 Apr 2010 | 0 comments

As mentioned in last night’s (err, early this morning’s) post, a significant severe weather event was forecast for today.  Unlike most events that take start during the day and wind down during the evening and overnight, this event will actually begin to ratchet up overnight and peak tomorrow during the day.

Below are three separate water vapor images from today.  The top image is the original image and bottom image is an annotated equivalent.  Blue lines represent the outline of the jet stream (thick) and jet streaks (thin).  Yellow X’s indicate areas of vorticity maximums.  (This is frequently referred to incorrectly as “energy”.)  The size of the X is a rough approximation of the vorticity maximum’s relative magnitude.

This satellite image is from 12 UTC (7 AM CDT) and indicates a very complicated pattern.  A large, complex mid-to-upper level low was slowly redeveloping eastward from the western United States to eastern Colorado.  Ahead of the large low, divergence aloft was aiding the development of widespread showers and thunderstorms across Kansas, Nebraska, and northwest Missouri.  These thunderstorms originally developed Thursday afternoon in the Texas panhandle and western Kansas as part of an initial shortwave trough that moved through this area.  (Yes, some of these storms produced tornadoes!)

To the southwest of the eastern Colorado maturing cyclone, a strong vorticity maximum was located in northern Mexico, south of the Arizona – Mexico border.  A smaller, weaker vorticity maximum was located in far eastern New Mexico.  Also, an inferred jet streak/potential vorticity anomaly/shortwave trough was ejecting northeast from Mexico through central Texas.

By 18 UTC (1 PM CDT), the eastern Colorado cyclone had not moved much.  However, the jet stream continued to progress eastward over an unstable airmass throughout the south central US.  The eastern New Mexico vorticity maximum was ejecting northeast along the western periphery of the mid-to-upper-level jet stream.  The vorticity maximum south of Arizona began moving eastward and was located just south of the Arizona-New Mexico border in Mexico.

Of note is the development of showers and thunderstorms across portions of northern Louisiana and Arkansas.  These showers and thunderstorms were being aided by the ascent associated on the nose of the inferred embedded jet streak/shortwave trough.  These showers and thunderstorms developed fairly early in the day, preventing wide-spread low-level destabilization of the atmosphere.  This limited the amount of convective available potential energy (CAPE) that thunderstorms were able to use to support vigorous updrafts.  This helped to keep the severe threat and tornado threat a lot lower than it could have been.  If the thunderstorms had developed 3-6 hours later, they would have most likely been even more severe than they were.

Areas to the south and west of today’s convection in Arkansas and Louisiana were experience descent in the wake of the lead shortwave trough that was lifting northeast.  This detrimentally impacted thunderstorm development across a large portion of the warm sector, and also helped limit the severe threat.

By 00 UTC, 24 April 2010 (7 PM CDT, 23 April 2010). the eastern Colorado cyclone had yet to move any distance of significance.  The lead shortwave (embedded jet streak) continued to progress northeastward into Arkansas and thunderstorms persisted across the weakly capped warm sector, which was under the broad divergence aloft.  The thunderstorms in Arkansas and Texas were stronger than they were at 18 UTC (1 PM CDT), but still lacked radar appearances of more robust convection typical in the plains states during severe weather events.  In fact, as the sun continues to set, the intensity of these storms should continue to wane.

The relatively weak vorticity maximum that was located in eastern New Mexico to start the day was now into Nebraska, and was aiding ascent with thunderstorms up there.

The big change in the image is that the vorticity maximum that started the day in northwest Mexico was now located near the Big Bend of Texas and beginning to eject into the United States.  The strong ascent associated with this “primary” shortwave trough was beginning to overcome the earlier descent and aid in the development of convection across portions of central Texas.  As this primary shortwave trough continues to lift northeast during the overnight tonight and into tomorrow, the strong ascent will spread north and east as well.  This will help to ignite new thunderstorms in places that have already seen thunderstorms today as well as in places that have not seen thunderstorms.

Thus, if you live across the south or south east United States, please be aware that even though thunderstorms may not be threatening you at that moment, the threat will continue throughout the overnight hours and into tomorrow.