MESO: Mesoscale Discussion 0677

ACUS11 KWNS 211654
SWOMCD
SPC MCD 211654
MNZ000-IAZ000-NEZ000-SDZ000-211900-

MESOSCALE DISCUSSION 0677
NWS STORM PREDICTION CENTER NORMAN OK
1154 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

...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT...

ATTN...WFO...MPX...DMX...FGF...FSD...OAX...ABR...

44819691 45599683 45809598 44799534 43529510 42249530
41999613 42229674 43419694

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