ACUS11 KWNS 172255
SWOMCD
SPC MCD 172255
IAZ000-WIZ000-MNZ000-SDZ000-180030-
MESOSCALE DISCUSSION 1513
NWS STORM PREDICTION CENTER NORMAN OK
0555 PM CDT TUE JUL 17 2007
AREAS AFFECTED...PARTS OF ERN SD...SRN MN...NRN/ERN IA AND SW WI
CONCERNING...SEVERE POTENTIAL...WATCH LIKELY
VALID 172255Z - 180030Z
TIMING OF CONVECTIVE INITIATION IS STILL A BIT UNCERTAIN...BUT
TRENDS ARE BEING CLOSELY MONITORED FOR AN INCREASING SEVERE THREAT
AND THE POSSIBILITY OF ONE OR MORE WATCHES.
A LOW-LEVEL BAROCLINIC ZONE AND CONVECTIVELY GENERATED BOUNDARY
DOWNSTREAM OF NORTHERN PLAINS UPPER RIDGE INTO EASTERN CANADIAN/
GREAT LAKES UPPER TROUGH HAS BECOME A FOCUS FOR VERY STRONG
DESTABILIZATION FROM PARTS OF SOUTHEASTERN SOUTH DAKOTA THROUGH THE
IOWA/MINNESOTA BORDER AREA INTO EASTERN IOWA. DAYTIME HEATING INTO
THE MID/UPPER 90S ALONG AND TO THE SOUTH OF THESE BOUNDARIES...WHERE
SURFACE DEW POINTS ARE NEAR/ABOVE 70F...HAS CONTRIBUTED TO MIXED
LAYER CAPE IN EXCESS OF 4000 J/KG. AND...OBJECTIVE ANALYSIS
INDICATES INHIBITION HAS BECOME NEGLIGIBLE IN THE PEAK AFTERNOON
HEATING.
HOWEVER...NEAR TERM FORCING FOR CONVECTIVE DEVELOPMENT IS STILL
UNCLEAR. ONE WEAK IMPULSE ROUNDING CREST OF UPPER RIDGE IS JUST NOW
BEGINNING TO SLOWLY SHIFT EAST OF THE BLACK HILLS/NORTHERN HIGH
PLAINS AREA. AND...STRONGEST FORCING ASSOCIATED WITH 40-50 KT 500
MB JET STREAK DIGGING ACROSS THE UPPER MISSISSIPPI VALLEY WILL
LIKELY REMAIN TO THE EAST OF REGION.
WHILE INITIATION OF SCATTERED BOUNDARY LAYER BASED STORMS SEEMS
POSSIBLE BY 18/00-01Z TIME FRAME...IT APPEARS MOST PROBABLE THAT AT
LEAST A SUBTLE INCREASE IN FLOW...ABOVE A DECOUPLING BOUNDARY LAYER
WITH ONSET OF SURFACE COOLING...WILL SUPPORT STRENGTHENING LOW-LEVEL
CONVERGENCE ALONG AND TO THE NORTH OF THE BOUNDARIES BY/ SHORTLY
AFTER 18/02Z. AND...THIS FORCING...IN CONJUCTION WITH AT LEAST WEAK
COOLING ALOFT...WILL FINALLY SUPPORT A SIGNIFICANT INCREASE IN
CONVECTIVE DEVELOPMENT.
GIVEN THE EXTREME MAGNITUDE OF THE INSTABILITY...AND THE PRESENCE OF
STRONG DEEP LAYER SHEAR MORE THAN SUFFICIENT FOR UPDRAFT
ROTATION...STORMS SHOULD RAPIDLY BECOME SEVERE...WITH A FEW
SUPERCELLS LIKELY. THROUGH MID/LATE EVENING...HAIL...SOME POSSIBLE
VERY LARGE...WILL BE THE PRIMARY THREAT...BUT AN ISOLATED TORNADO
APPEARS POSSIBLE WITH AT LEAST INITIAL DEVELOPMENT.
.KERR.. 07/17/2007
..PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT...
ATTN...WFO...DVN...ARX...MPX...DMX...FSD...ABR...
43559749 43779918 44399858 44459604 44639546 44499399
44319264 43469131 42899097 41929088 41559164 41589226
41939275 42679310 43249408 43529581
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