The following additional imagery is attached to help document this interesting mesoscale banding: 

1. A 2010 UTC 15 Apr KBMX base reflectivity image centered on northern AL

2. A 2015 UTC 15 Apr KBMX base reflectivity image centered on AL and parts of adjacent states

3. A GR2Analyst base reflectivity cross section along a line from near Double Springs, AL to near Oneonta, AL

4. An 1800 UTC 15 Apr sounding from Jackson (JAN), MS

5. A map of surface observations centered on the Southeast for 2000 UTC 15 Apr

6. A US surface potential temperature analysis for 2000 UTC 15 Apr.

7. A visible satellite image centered on the Southeast for 1845 UTC 15 Apr

The radar loop and the attached imagery suggest that the 40-50 dBZ reflectivity cores in the precipitation bands were relatively shallow and mostly confined below 700 hPa. The surface potential temperature map indicates that the NNE-SSW oriented mesoscale precipitation bands were best defined in a persistent rain-chilled pocket of cold air centered over northern AL. The map of plotted surface observations shows that surface winds are from the ESE in the region of rain-chilled cold air where the mesoscale precipitation bands are observed, and broadly southerly to the west, south, and east. A mesoscale frontal boundary across central AL separates this rain-chilled cold-air mass from the warmer and more humid air to the west, south, and north. The nearest upstream sounding from JAN, located in the warm and humid air mass, at 1800 UTC 15 Apr is supportive of surface-based convection (MLCAPE is ~ 1650 J/kg). It also suggests that winds in the planetary boundary layer above the surface-based rain-chilled air over northern AL are from the SSW/SW. The visible satellite image for 1845 UTC shows that area where the precipitation mesoscale bands were observed over northern AL is obscured by cloud. Closer inspection of this satellite image suggests the presence of boundary layer rolls oriented from NNW-SSE over western GA, more N-S over southern and central AL, and NE-SW over southern MS.

A possible explanation for the observed mesoscale precipitation bands over northern AL is they are manifestations of remnant planetary boundary layer roll circulations that have been lifted over the mesoscale frontal boundary marking the southern edge of the rain-chilled cold air mass over northern AL. The shallow depth (< 700 hPa) of the precipitating cores in these mesoscale bands suggests that their NNE-SSW orientation is broadly consistent with the inferred mean wind direction in the surface to 700 hPa layer.

Bait dangled. Comments and alternative hypotheses for band origin and maintenance are welcome.

Lance Bosart and Kyle Griffin