South Lousiana Bookend Vortex

This past December 29, Southern Louisiana experienced a severe storm.  Little damage was done, with the only damage reports coming from the Intracoastal City area in the form of downed trees.  An 80 mph wind gust was recorded from a private weather station in the Mouton Cove area and a 60 mph wind gust was recorded at the KATC-TV3 station in Lafayette, LA.  Tornado warnings were issued by the NWS offices in Lake Charles and Slidell, due to the threat of brief tornadoes along the front of the MCS and associated with the bookend vortex on the north side of the MCS.

The 0Z sounding from WFO LCH showcases several interesting features that led to the initiation of this event.  While surface based CAPE values were only ~550 J/Kg and surface temperatures were in the lower 60's, several factors led to the destabilization of the atmosphere.  First, the area's in the blue circle and red square show significant dry air layers.  This is important, because these dry layers steepen lapse rates (temperature decrease with height) aloft, thus allowing the mid levels of the troposphere to destabilize.  The significance of the second dry layer ~650mb is not known due to the failure of the carbon hygristor (measures atmospheric humidity) in the radiosonde at this level.  The stratification of this particular layer (shown by the orange circle at 450mb) suggests the end of this dry layer.  However, due to the absence of data at this point, this statement cannot be made with confidence.  Finally, backing of the winds in the upper levels (as denoted by the green box in the wind profile) suggests cold air advection aloft.  This is significant, because this increases lapse rates aloft, thus further destabilizing the atmosphere (a warm air parcel rises more rapidly with colder air surrounding it).

The radar images above shows the MCS bowing out around the Intracoastal City area.  The rear inflow jet (RIJ) can clearly be seen in both base reflectivity and storm relative velocity products.  The RIJ is the reason why an 80 mph wind gust was recorded around the Gator Cove area.  The RIJ is also directly responsible for the bookend vortex (denoted by the blue circle) that formed on the north side of the MCS.  It was this feature that prompted a tornado warning for parts of Vermillion, Iberia, and St. Martin Parishes.  Although no tornado occured in this case, many tornadoes that occur with derechos occur in these bookend vorticies, therefore the warnings from the WFO LCH were justified.