Mesoscale Discussion 677

MESOSCALE DISCUSSION 0677
NWS STORM PREDICTION CENTER NORMAN OK
11:54 AM CDT MON APR 21 2008
AREAS AFFECTED
PARTS OF WRN MN...EXTREME ERN SD...NE NEB/NW IA
CONCERNING...SEVERE POTENTIAL...WATCH UNLIKELY
VALID 211654Z - 211900Z
STORMS ARE EXPECTED TO DEVELOP/INCREASE ACROSS REGION THROUGH THE
EARLY TO MID AFTERNOON HOURS. WHILE SOME COULD CONTAIN HAIL...THE
SEVERE THREAT GENERALLY IS NOT EXPECTED TO BECOME SUFFICIENT TO
REQUIRE A WW. BUT...TRENDS WILL BE MONITORED.
SURFACE COLD FRONT CONTINUES TO ADVANCE EASTWARD INTO THE UPPER
MISSISSIPPI VALLEY...WELL OUT IN ADVANCE OF STRONGER MID/UPPER
COOLING ASSOCIATED WITH LARGE-SCALE NORTHWESTERN U.S. UPPER LOW.
HOWEVER...THE LEADING EDGE OF A WEAK INITIAL MID-LEVEL COOL SURGE
HAS SPREAD THROUGH MUCH OF THE EASTERN DAKOTAS. WHILE THIS APPEARS
TO LAG THE SHALLOW SURFACE FRONT...FORCING IS PROVIDING THE FOCUS
FOR ONGOING POST-FRONTAL CONVECTIVE DEVELOPMENT NOW SLOWLY SPREADING
IN A BAND EAST OF THE NORTH DAKOTA/MINNESOTA BORDER.
ABOVE THE FRONTAL INVERSION...MOISTURE ADVECTION ALONG THE AXIS OF A
NARROW LOWER/MID TROPOSPHERIC THERMAL RIDGE IS CONTRIBUTING TO WEAK
TO MODERATE DESTABILIZATION. STRONGEST INSTABILITY IS NOW
DEVELOPING NORTH OF THE MID MISSOURI VALLEY INTO THE SOUTH DAKOTA/
MINNESOTA BORDER AREA. THIS IS BENEATH STEEPEST MID-LEVEL LAPSE
RATES...WHERE MODEL GUIDANCE IS SUGGESTIVE THAT FORCING WILL BECOME
INCREASINGLY SUPPORTIVE OF NEW CONVECTIVE DEVELOPMENT THROUGH THE
18-21Z TIME FRAME.
LATEST RUC GUIDANCE INDICATES THAT INSTABILITY FOR LIFTED PARCELS
BASED AROUND THE 700 MB LEVEL COULD REACH 1000-1500 J/KG...
SUFFICIENT FOR VIGOROUS UPDRAFTS CAPABLE OF PRODUCING HAIL. MODEST
CLOUD BEARING LAYER SHEAR PROBABLY WILL MINIMIZE MAXIMUM HAIL
SIZES...BUT GENERALLY SHORT-LIVED ROTATING UPDRAFTS COULD BECOME
CAPABLE OF PRODUCING SOME HAIL APPROACHING OR BRIEFLY EXCEEDING
SEVERE LIMITS.
KERR 04/21/2008