AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL 347 PM CDT MON JUN 15 2015 .MESOSCALE DISCUSSION... 347 PM CDT CLOSELY MONITORING WEAK SUPERCELL MOVING ACROSS KANE COUNTY INTO THE WESTERN SUBURBS WITH WELL DEFINIED BWER. LOW LEVEL SHEAR REMAINS WEAK...THOUGH WITH SUCH LOW LCL HEIGHTS AND MODERATE DEEP LAYER SHEAR CANNOT COMPLETELY RULE OUT A BRIEF TORNADO. UPDRAFT INTENSIFIED AND CORE BRIEFLY SPIKED UP A BIT BUT GIVEN THE SMALL SIZE OF THE STORM THINKING THE WIND DAMAGE THREAT IF ANY WOULD BE VERY LOCALIZED. WILL CONTINUE TO CLOSELY MONITOR RADAR TRENDS FOR ANY SIGNS OF AN INCREASED SEVERE THREAT...BUT FOR NOW HAIL...WIND...AND TORNADO THREAT APPEAR TOO LOW FOR ANY WARNING. GREATER POTENTIAL HAZARD FOR METRO AND POINTS WEST NEAR THE I-88 CORRIDOR WILL LIKELY END UP BEING THE HEAVY RAIN AND FLASH FLOOD POTENTIAL ASSUMING THESE CELLS BEGIN TO FILL IN A BIT MORE AND CONTINUE TO TRAIN. IZZI && MONDAY, JUNE 15 2015 .SHORT TERM... 255 PM CDT THIS AFTERNOON AND TONIGHT... ATMOSPHERE REMAINS REMARKABLY FAVORABLE FOR VERY HEAVY RAINFALL WITH VERY HIGH FREEZING LEVELS AND RECORD/NEAR RECORD HIGH PWATS ALLOWING FOR WARM RAIN PROCESSES (TROPICAL-LIKE) TO DOMINATE. THE RESULT THIS MORNING HAS ALREADY BEEN THUNDERSTORMS WITH EXTREME RAIN RATES...IN SOME CASES OVER AN INCH IN LESS THAN 20 MINUTES. ATMOSPHERE WILL CHANGE VERY LITTLE THROUGH THE AFTERNOON AND INTO THIS EVENING...SO CONTINUE TO BE VERY CONCERNED ABOUT LOCALIZED VERY HEAVY RAINFALL AND THE POTENTIAL FOR FLASH FLOODING. GIVEN THE EFFICIENCY OF RAINFALL PRODUCTION AND EXTREME RAINFALL RATES...ANY TRAINING OF THUNDERSTORM ACTIVITY COULD RESULT IN A SERIOUS FLASH FLOOD SITUATION DEVELOPING IN SOME LOCATIONS THIS AFTERNOON INTO THIS EVENING. THE AREAS MOST SUSCEPTIBLE TO THIS WOULD BE AREAS THAT HAVE ALREADY SEEN FLOODING IN RECENT DAYS WHERE THE GROUND CAN TAKE NO MORE WATER AND ALSO IN URBANIZED AREAS THAT CAN NEVER TOLERATE EXTREME RAINFALL RATES. VISIBLE SATELLITE IMAGERY LATER THIS MORNING SHOWS SOME POCKETS OF FILTERED SUNSHINE ALLOWING TEMPERATURES TO WARM INTO THE LOW/MID 80S WITH DEWPOINTS IN THE LOW TO MID 70S. MODIFYING ILX`S MORNING SOUNDINGS FOR CURRENT SURFACE OBS YIELD MLCAPE VALUES OVER 1000 J/KG NOW WITH NO REMAINING CONVECTIVE INHIBITION AND PWATS NEARING 2 INCHES. AREA VWPS SHOW DEEP LAYER OF 30-40KT WEST-SOUTHWESTERLY WINDS FROM 850-500MB...WHICH IS RESULTING IN EFFECTIVE DEEP LAYER SHEAR VALUES OF AROUND 30KT. WHILE DEEP LAYER SHEAR AND VERY WEAK LAPSE RATES ARE NOT PARTICULARLY FAVORABLE FOR SEVERE WEATHER...WE HAVE ALREADY SEEN SOME TRANSIENT OUTFLOW DOMINANT HP SUPERCELL-ISH CONVECTION THIS MORNING. WITH SLOWLY BUILDING INSTABILITY...THE THREAT DOES EXIST FOR SOME ADDITIONAL WET MICROBURSTS AND SMALL BOWING SEGMENTS TO RESULT IN AN ISOLATED WIND DAMAGE THREAT THIS AFTERNOON AND PERHAPS EARLY THIS EVENING. ONE SMALL SCALE BOWING SEGMENT TRACKED ACROSS LEE...SOUTHERN DEKALB/KANE COUNTIES LATE THIS MORNING AND HAS LAID OUT AN EAST-WEST OUTFLOW BOUNDARY IN VICINITY OF I-80. SHOULD THIS OUTFLOW OR ANY OTHER BECOME A FOCUS FOR CONVECTIVE DEVELOPMENT THIS AFTERNOON THE THREAT OF TRAINING AND A MORE SUBSTANTIAL FLASH FLOOD THREAT WILL GROW. NOTE TO WSR-88D USERS: THE VERY HIGH EFFICIENCY OF RAIN IN THE STORMS AND LACK OF HAIL IS RESULTING IN LEGACY PRECIPITATION ESTIMATES ON THE RADAR TO BE RUNNING TOO LOW. USE SUCH PRODUCTS WITH CAUTION...WE ARE UTILIZING THE DUAL POL RADAR ESTIMATES WHICH HAVE BEEN PERFORMING MUCH BETTER IN THIS ENVIRONMENT. IZZI &&
