A potentially dangerous synoptic scenario

Update: 4:12 pm. Latest "Supercell Composite" image from SPC, showing the greatest threat of tornadic T-storms (for now) over Iowa, and the Brainerd-Wadena area of Minnesota. The longer the sun stays out, the more unstable the atmosphere over central and southern MN will become. Factor in strong wind shear and ample moisture near the ground and you have all the necessary ingredients for a significant tornado outbreak. In the coming hours we hope to zero in on the counties in Minnesota most at risk.




This is a technical post, but I want to err on the side of providing more, rather than less information about today's tornado risk. Here is the very latest from SPC in Norman, OK.

MESOSCALE DISCUSSION 1210
NWS STORM PREDICTION CENTER NORMAN OK
0315 PM CDT THU JUN 18 2009

AREAS AFFECTED...SRN MN INTO NRN

CONCERNING...SEVERE POTENTIAL...WATCH LIKELY

VALID 182015Z - 182145Z

THE THREAT FOR SIGNIFICANT SUPERCELLS CAPABLE OF VERY LARGE
HAIL...DAMAGING WINDS AND POTENTIALLY STRONG TORNADOES IS EXPECTED
TO INCREASE ACROSS DISCUSSION AREA LATE THIS AFTERNOON INTO EVENING.
A TORNADO WATCH HAS BEEN ISSUED ACROSS MN AND ANOTHER MAY WELL BE
REQUIRED WITH TIME OVER PARTS OF IA.

LATEST SURFACE ANALYSIS INDICATES A PERSISTENT MESOLOW NE OF TQE
ALONG THE NOSE OF LOW-LEVEL THERMAL AXIS WHICH HAS DEVELOPED THROUGH
ERN NEB INTO W-CNTRL IA.  MEANWHILE...HIGH-RESOLUTION RADAR AND
SURFACE DATA INDICATE COMPOSITE OUTFLOW BOUNDARY RETREATING NWD AS A
WARM FRONT THROUGH CNTRL IA.  AIR MASS TO THE E OF THIS BOUNDARY
REMAINS QUITE MOIST WITH DEWPOINTS IN THE LOWER 70S.  MOREOVER...THE
PRESENCE OF CONSIDERABLE DIABATIC HEATING SHOULD CONTINUE TO MODIFY
THE CHARACTER OF THE BOUNDARY LAYER THROUGH THE REMAINDER OF THE
AFTERNOON WITH ENVIRONMENT BECOMING MODERATELY TO STRONGLY UNSTABLE
WITH MLCAPE OF 2000-4000 J/KG AND DECREASING CONVECTIVE INHIBITION.

AFTERNOON WATER VAPOR IMAGERY INDICATES A COUPLE OF WEAK MIDLEVEL
IMPULSES LOCATED UPSTREAM...ONE OVER SD AND THE OTHER OVER WRN/CNTRL
NEB.  FORCING FOR ASCENT ASSOCIATED WITH THESE FEATURES COUPLED WITH
SUSTAINED LOW-LEVEL CONVERGENCE ALONG COMPOSITE BOUNDARY SHOULD BE
SUFFICIENT TO INITIATE SCATTERED STORMS BY LATE AFTERNOON OR EARLY
EVENING.

GIVEN THE DEGREE OF INSTABILITY...50-60 KT OF EFFECTIVE BULK SHEAR
AND RATHER STRONG LOW-LEVEL SHEAR ON COOL SIDE OF
BOUNDARY...ENVIRONMENT WILL SUPPORT INTENSE SUPERCELL DEVELOPMENT
WITH THE THREAT FOR VERY LARGE HAIL...DAMAGING WINDS AND POSSIBLY
STRONG TORNADOES.  THOUGH FORECAST STORM MOTIONS ARE LARGELY NORMAL
TO ORIENTATION OF COMPOSITE BAROCLINIC ZONE...AIR MASS ON COOL SIDE
OF BOUNDARY SHOULD BE SUFFICIENTLY MOIST/UNSTABLE TO SUSTAIN STORMS
AND ATTENDANT TORNADO THREAT WELL TO THE E.