The Role of Polar/Subtropical Jet Interaction in the South-Central US Flood of March 2016

Interaction between the usually separate polar and subtropical jets occasionally results in the production of high impact synoptic-scale weather events.  In early March 2016 such an interaction conspired to produce an anomalously deep upper tropospheric cyclonic disturbance at an unusually low latitude over Mexico.  The development of this feature from March 8-12^th   instigated a severe flooding event in the south-central United States in which 12-18 inches of rain fell in northern Louisiana and far eastern Texas, with some localized areas receiving more than 24” of rain in what was characterized as a “1-in-500 to 1-in-1000 year” event. In addition to the rainfall, 3 EF1 tornadoes and multiple reports of baseball-sized hail also characterized this event.

The analysis traces the synoptic evolution of the separate polar and subtropical jets, along with the development of the anomalous upper level trough, over the period March 5-15^th.  Employing both quasi-geostrophic omega and semi-geostrophic frontal circulation diagnostics, we find that differential tilting across the upper tropospheric baroclinicity associated with the subtropical jet was instrumental in lowering the tropopause over unusually low latitudes.  Simultaneously, a developing upper-level jet/front system in the northwesterly flow of the polar stream provided additional development in the region through downward advection of high values of stratospheric potential vorticity.  The modification of these initially separate developmental processes, occasioned by the encroachment of the two jets on one another, is discussed.