Hi Everyone,

I used Tom Galarneau's real-time standardized anomaly page (link is provided below) to do a quick and dirty assessment of just how anomalous the synoptic-scale flow was between 1200 UTC 27 Apr and 0000 UTC 28 Apr. Standardized anomaly maps for 850/500/200 hPa for these times were extracted from Tom's web site and are appended below. Inspection of these maps suggests the following inferences:

1. The ~ 6 sigma difference in 850 hPa heights between the trough over the southern Plains and the ridge, and the near Bermuda, the equivalent 850 hPa u-/v- standardized wind anomalies, and the observed 850 hPa winds suggests that "supercell city" (aka as Alabama and Mississippi) was situated beneath the corridor of the most anomalous and strongest SSW flow (low-level jet; LLJ). PW values in "supercell city" were generally between 40-45 mm (+2 to +3 sigma) and indicative of the unusually moist air mass (not shown; see Tom's web site).

2. The -3 and -4 sigma 500 hPa height anomalies in the trough centered over Oklahoma at 1200 UTC 27 Apr, along with near-zero 500 hPa height anomalies over extreme northeastern Mexico are consistent with the observed strong midlevel jet, the nose of which is crossing eastern Texas, Arkansas, Louisiana, and extreme western Mississippi at this time. The ~ +2 sigma 500 hPa ridge near Bermuda when combined with the deep upstream trough and relatively short downstream half wavelength helps to ensure an anomalously strong (+3 to +4 sigma) southerly flow from Mississippi and Alabama northeastward to the east of the nose of the jet to the west. This synoptic-scale flow configuration ensures that any synoptic-scale disturbances rotating through the base of the upstream trough will turn northeastward through the Tennessee and Ohio Valleys. 

3. The 200 hPa height and standardized height anomaly maps are consistent with the 500 hPa analyses and clearly show the existence of a negatively tilted trough over the lower Mississippi Valley and the aforementioned downstream half wavelength.

For an additional perspective, I have also attached maps for the same times of dynamic tropopause potential temperature/winds and layer-mean 925-850 hPa relative vorticity, SLP/1000-500 hPa thickness and 250 hPa winds, 700 hPa heights/isotherms, Q-vectors and Q-vector divergence, and 250 hPa wind speed/PV/%RH (shaded), 600-400 hPa layer-averaged vertical motion (ascent only contoured), and 300-200 hPa layer-averaged irrotational wind (vectors), all images courtesy of Heather Archambault.

Finally, I have attached images of the 1800 UTC 27 Apr soundings from Jackson (JAN), MS, and Birmingham (BMX), AL, and the 0000 UTC 28 Apr sounding from BMX (sounding images courtesy of the SPC).

These additional maps and soundings enable us to make these further points:

1. A moderately strong PV hook on the DT helps to organize a quasi-linear band of layer-mean 925-850 hPa relative vorticity over and immediately upstream of "supercell city" in the 12 h ending 0000 UTC 28 Apr.

2. A moderately intense, but non-deepening sub-996 hPa surface cyclone moves from southwestern Arkansas to extreme southern Indiana in the 12h period ending 0000 UTC 28 Apr. Even though this cyclone does not deepen it is associated with a strengthened SLP gradient on its southern and eastern side that is consistent with the observed strong LLJ. The highest PW values (> 45 mm) are colocated with the strong SLP gradient on the southern and eastern side of the aforementioned cyclone.

3.  

Bottom line: J-E-T-S! Jets! Jets! Jets!