ACUS11 KWNS 190101
SWOMCD
MESOSCALE DISCUSSION 1713
NWS STORM PREDICTION CENTER NORMAN OK
0800 PM CDT TUE AUG 18 2015
AREAS AFFECTED...SOUTHERN WI AND NORTHEAST IL
CONCERNING...TORNADO WATCH 494...
VALID 190100Z - 190230Z
THE SEVERE WEATHER THREAT FOR TORNADO WATCH 494 CONTINUES.
SUMMARY...DAMAGING WINDS AND A TORNADO THREAT CONTINUE ACROSS
NORTHEAST IL INTO PARTS OF SOUTHEAST WI...WITH THE POTENTIAL FOR
ADDITIONAL WFO-LOT COUNTIES TO BE ADDED ACROSS FAR NORTHWEST
INDIANA. WEAKENING INSTABILITY AND SHEAR WITH EASTWARD EXTENT INTO
NORTHERN INDIANA SUGGEST NEW WATCH ISSUANCE EAST OF THE WFO-LOT
COUNTY WARNING AREA IS UNCERTAIN...THOUGH TRENDS WILL CONTINUE TO BE
MONITORED. MEANWHILE...THE COUNTIES IN THE WAKE OF THE QLCS PASSAGE
CAN CONTINUE TO BE CANCELLED FROM THIS WATCH.
DISCUSSION...AT 0040Z...MOSAIC RADAR IMAGERY SHOWED A QLCS
CONTINUING TO ADVANCE TO THE EAST-NORTHEAST ACROSS NORTHEAST IL AND
SOUTH-CENTRAL INTO SOUTHEAST WI. THE ENVIRONMENT IMMEDIATELY
DOWNSTREAM OF THESE STORMS HAS DESTABILIZED INTO THE EARLY EVENING
WITH OBJECTIVE ANALYSES INDICATING MODERATE INSTABILITY EXTENDING
FROM THE QLCS INTO WESTERN INDIANA AND THE FAR SOUTHERN EXTENT OF
LAKE MICHIGAN. AREA WSR-88D VWPS SHOWED A FAIRLY DEEP LAYER OF
CONVERGENCE GENERALLY ATTENDANT TO THE QLCS...WHILE A S/SWLY LLJ
EXTENDING INTO THE CHICAGO AREA HAD STRENGTHENED TO 40 KT. THE
STRONGER LLJ HAS BOOSTED SURFACE-1-KM SRH TO 225 M2/S2 SUGGESTING
ANY DISCRETE STORMS AHEAD OF THE QLCS AND EMBEDDED CIRCULATIONS
WITHIN THE QLCS WILL BE IN A KINEMATIC ENVIRONMENT FAVORABLE FOR A
TORNADO THREAT THROUGH THE EARLY-MID EVENING. FARTHER NORTH INTO
SOUTHERN WI...THE ENVIRONMENT HAS BEEN STABILIZING WITH OBJECTIVE
ANALYSES SHOWING SURFACE-BASED INHIBITION ALSO INCREASING ACROSS FAR
SOUTHEAST WI. THIS MAY TEND TO LIMIT THE SEVERE WEATHER THREAT
ACROSS SOUTHEAST WI...THOUGH AN INCREASE IN SRH SUGGESTS A NON-ZERO
SEVERE THREAT REMAINS IN THE SHORT TERM.
AS INDICATED IN THE SUMMARY...DEEP-LAYER SHEAR AND INSTABILITY
WEAKEN WITH EASTWARD EXTENT INTO NORTHERN INDIANA...SUCH THAT
PERHAPS ONLY THE WFO-LOT COUNTIES IN NORTHWEST INDIANA MAY NEED TO
BE INCLUDED IN WW 494. THIS SCENARIO IS SUPPORTED BY THE 23Z HRRR
AND ESRL HRRR EACH SUGGESTING THE ONGOING QLCS WILL BEGIN TO WEAKEN
AS IT EXITS IL.
..PETERS.. 08/19/2015
...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT...
ATTN...WFO...IWX...GRB...LOT...ILX...MKX...
LAT...LON 40408848 41548869 42188922 42698919 43418960 43838958
44028884 43568830 42798824 42488785 41678735 40888705
40518718 40408848
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