MESOSCALE DISCUSSION 0357
NWS STORM PREDICTION CENTER NORMAN OK
0503 PM CDT SAT APR 17 2004

AREAS AFFECTED...PORTIONS NRN IA AND EXTREME SRN MN

CONCERNING...SEVERE THUNDERSTORM POTENTIAL

VALID 172203Z - 180030Z

RAPID DEVELOPMENT OF SEVERE TSTMS IS POSSIBLE BETWEEN 22Z-00Z IN
NWRN IA -- ESPECIALLY N OF I-70 BETWEEN SUX-MCW.  RESULTING TSTMS
THEN SHOULD MOVE/EXPAND EWD WITH DAMAGING HAIL/WIND
POSSIBLE...PERHAPS A TORNADO EARLY IN SUPERCELL LIFESPAN.  WW MAY BE
REQUIRED IN NEXT 1-2 HOURS.

SFC MESOANALYSIS SHOWS WARM FRONT E-W ACROSS IA FROM NEAR SUX TO
JACKSON COUNTY...LIFTING NWD AROUND 15-20 KT TOWARD MN BORDER.
AIR MASS ALONG AND WITHIN 20 NM N OF WARM FRONT WILL BE MOST
SUITABLE FOR SEVERE.  IN THAT REGIME HIGHER DEW POINTS ARE ADVECTING
INTO AREA BUT SFC FLOW REMAINS BACKED...OPTIMIZING SHEAR.
PROFILER/VWP DATA N OF FRONT SHOWS 250-450 J/KG SRH IN 0-3 KM
LAYER...AND 50-60 KT SHEAR THROUGH 6 KM AGL.  ALTHOUGH BOUNDARY
LAYER FLOW WILL VEER TO SLY WITH FROPA...HODOGRAPHS STILL WILL BE
FAVORABLE FOR BOTH SUPERCELLS AND EVENTUAL BOW DEVELOPMENT.

MODIFIED RUC SOUNDINGS INDICATE CINH NEARLY GONE OVER NW IA BY
23Z...CONSISTENT WITH VIS IMAGE TRENDS OF GRADUALLY DEEPENING CU
ELEMENTS NEAR WARM FRONT.  SFC DEW POINTS ARE LOCALLY ENHANCED OVER
REGION BY SWATH OF WET GROUND FROM OVERNIGHT MCS...BUT MEAN MIXING
RATIOS SUPPORT USE OF 61-64 F SFC DEW POINTS OBSERVED AT MOST
STATIONS AS MOST REPRESENTATIVE LIFTED PARCEL. WITH 8-9 DEG C
MIDLEVEL LAPSE RATES THIS STILL GENERATES AROUND 3000 J/KG
MLCAPE...INDICATING EXPLOSIVE UPDRAFTS POSSIBLE ONCE CAP DOES BREAK.
 SOME CONVECTION OVER NRN IA MAY AFFECT AREAS NEAR MN BORDER
..PERHAPS AS FAR N AS SRN-MOST MN COUNTIES.

.EDWARDS.. 04/17/2004