Central Plains Severe Weather Outbreak of April 27th – 28th

Keith Roberts led a discussion titled, “Central Plains Severe Weather Outbreak of April 27th – 28th”
The discussion was focused on the severe weather outbreak on the 27th and 28th of April, 2014 in the southeast United States. There were 108 reported tornadoes over the two-day period starting on April 27th.  This severe weather caused 32 fatalities and 42 injured in Mississippi, Arkansas, and Georgia. Surface analysis charts valid for April 27 16 UTC and April 28th 12 UTC from the WPC were shown. There was a warm occluded low over the central Great Plains with a trailing cold front extending from northern OK to southeastern Texas on the 27th of April. However, this cold frontal boundary appeared weak and a well-defined temperature gradient could not easily be found at the surface nor a wind shift. At upper levels, the cold frontal boundary was more diffuse. 500 mb heights showed two main cut off lows over the CONUS and a blocky, stagnant pattern.  The frontal structure resembled more of a dry line given the moisture gradient.
Radar reflectively showed precipitation initiation around local noon in the southeast US around Mississippi and Georgia.  A sounding from Jackson field airport (JFD) in Mississippi (ahead of the cold front by about 200 km’s) revealed a strong capping layer around 950 mb with a significant dry layer aloft (inverted v-sounding).  Nearby soundings showed a similar inverted v vertical thermal structure. Some severe weather parameters were analyzed. Sfc-6 km shear was around 60 kts with more than half of this shear contained in the lowest 3 km’s. Dewpoint depressions were on the order of 10-15 C ahead of the surface frontal boundary. CAPE was on the order of 3500 J/kg. Given the dewpoint depressions and low-level shear, it was suggested that linear structures would be the likely convective mode. Indeed the bulk Richardson number was in agreement with an average value around 40. Pure supercells are often supported with values around 20. Reflectivity revealed quasi-linear structures with embedded super cells initiated around 18 UTC. To the north in southeastern Illinois and Iowa, the convective regime was different. In this region, the majority of damage was associated with penny to quarter sized hail and heavy precipitation (HP) supercells.  No tornadoes were reported in the northern Great Plains.
Thereafter the discussion focused briefly on the forecast for the local region. 18 UTC NAM and 12 UTC GFS combo showed that the weekend in the Northeast would feature a weak cold frontal passage Sunday afternoon associated with a northern stream 500 mb shortwave. Precipitation looked diffuse and mostly driven by large scale QG PVA. There was no forecasted severe weather potential for anywhere in the CONUS over the following three days.