000 FXUS63 KLOT 041828 AFDLOT AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL 128 PM CDT SAT JUL 4 2009 .DISCUSSION... 344 AM CDT MAIN FEATURE OF FORECAST REMAINS NEAR TERM...WITH FOCUS ON PRECIPITATION TRENDS THROUGH TODAY. 00Z RAOB ANALYSIS DEPICTS UPPER RIDGE ACROSS TX...WITH SERIES OF MID LEVEL SHORT WAVE TROUGHS NOTED RIPPLING THROUGH NEARLY ZONAL FLOW ACROSS THE PLAINS. GOES WATER VAPOR IMAGERY INDICATES MOST SIGNIFICANT OF THESE DISTURBANCES WAS THE CENTRAL PLAINS AT 07Z WITH ASSOCIATED AREA OF SURFACE LOW PRESSURE. SYSTEM HAS BEEN SLOWER THAN MODEL RUNS INDICATED YESTERDAY...WITH BEST FORCING/LOW LEVEL JET FORCING FOCUSED ACROSS EASTERN KS/MO PER H8/H9 PROFILER WIND PLOTS. RESULTING ELEVATED CONVECTION ALONG I-70 CORRIDOR A BIT TO THE SOUTH OF PREVIOUS MODEL FORECASTS...THOUGH AREA OF LIGHTER SHOWERS EXTENDS NORTH INTO IA/MN/WESTERN IL IN ADVANCE OF MID LEVEL TROUGH. WHILE PRECIP HAS BEEN TAKING ITS TIME PUSHING INTO IL OVERNIGHT...RAIN WILL SPREAD ACROSS THE FORECAST AREA THIS MORNING AS SHORT WAVE APPROACHES. OTHER THAN SLOWER PROGRESSION ACROSS THE AREA TODAY... GENERAL IDEA OF PREVIOUS FORECAST REMAINS VALID WITH POPS AND QPF AMOUNTS HIGHEST ACROSS FAR SOUTHERN COUNTIES AND DIMINISHING TO THE NORTH AND EAST. FORECAST SOUNDINGS CONTINUE TO DEPICT RATHER WEAK MID LEVEL LAPSE RATES ACROSS THE NORTHERN PARTS OF THE CWA...THUS EXPECT MAINLY RAIN/SHOWERS WITH SOME ISOLATED THUNDER MAINLY SOUTH OF I-80. QPF AXIS FROM VARIOUS MODELS SUGGESTS HEAVIEST RAIN AMOUNTS JUST SOUTH OF CWA ACROSS CENTRAL IL...THOUGH AMOUNTS MAY APPROACH AN INCH ACROSS FAR SOUTHERN PARTS OF FORECAST AREA WHERE MORE CONVECTIVE PRECIP IS LIKELY. MID LEVEL TROUGH AXIS MOVES ACROSS AREA DURING THE AFTERNOON/EARLY EVENING HOURS...WITH SURFACE LOW TRACKING INTO LOWER OHIO VALLEY THIS EVENING. SUBSIDENCE IN WAKE OF TROUGH AXIS SHOULD ALLOW SCATTERED SHOWERS TO DIMINISH FROM THE NORTH/WEST TOWARD EVENING...THOUGH PRECIP THREAT WILL LINGER ACROSS SOUTHERN PARTS OF FORECAST AREA INTO THE EVENING HOURS AS TROUGH BECOMES ELONGATED IN A MORE EAST/WEST ORIENTATION. GIVEN CLOUDS/PRECIP TEMPS EXPECTED TO REMAIN RATHER COOL THROUGH THE DAY WITH READINGS GENERALLY IN THE LOWER 70S. WEAK HIGH PRESSURE RIDGING DEVELOPS LATER TONIGHT/SUNDAY AS TROUGH CONTINUES TO PULL SLOWLY AWAY TO THE EAST. WRF/GFS TIME/HEIGHT SECTIONS DEPICT SUBSIDENCE AND DRYING INTO SUNDAY MORNING WHICH SHOULD ALLOW A GOOD DEAL MORE SUNSHINE. LOW LEVEL THERMAL PROGS SUPPORT TEMPS REBOUNDING INTO THE LOWER 80S...THOUGH WEAK GRADIENT ASSOCIATED WITH RIDGE AXIS WILL ALLOW LAKE BREEZE COOLING NEAR LAKE MICHIGAN. BOTH WRF AND GFS DO DEPICT ANOTHER SHORT WAVE ROTATING THROUGH THE NORTHERN LAKES ON SUNDAY...WITH WEAK SURFACE TROUGH REFLECTION SAGGING ACROSS WI DURING THE DAY. GFS A LITTLE FASTER THAN THE WRF...AND ACTUALLY BRINGS TROUGH/FRONT INTO NORTHERN IL DURING THE AFTERNOON. SREF/WRF-NAM AND 4 KM NCEP SPC WRF MAINTAIN SLOWER SOLUTION WITH FRONT OVER WI...AND AT THIS POINT PREFER TO MAINTAIN DRY FORECAST BASED ON THOSE SOLUTIONS. MEDIUM RANGE GUIDANCE INDICATES THAT THIS TROUGH/BOUNDARY WILL EVENTUALLY STALL/WASH OUT ACROSS IL/IN MONDAY. WHILE SOME LOW PROBABILITY POTENTIAL EXISTS FOR CONVECTION TO DEVELOP ALONG THIS FEATURE...CONFIDENCE IS LOW GIVEN VERY WEAK LOW LEVEL CONVERGENCE AND AGAIN PREFER TO MAINTAIN DRY FORECAST AT THIS TIME. LITTLE CHANGE TO FORECAST IN MEDIUM/EXTENDED RANGE THIS MORNING. GOING FORECAST OF GRADUAL WARMING TREND APPEARS APPROPRIATE WITH TEMPS CLIMBING BACK ABOVE NORMAL AS UPPER RIDGE BUILDS ACROSS THE MIDWEST LATER IN THE WEEK. RATZER && .AVIATION... 125 AM CDT 1800 UTC TAFS...NORTHERN PORTION OF AREA OF MORE SOLID LIGHT RAIN CONTINUES TO SPREAD EAST-SOUTHEAST ACROSS NORTHERN IL AND NORTHWEST IN AT MIDDAY AS MAIN SHORT WAVE TROUGH MOVES FROM SOUTHEAST IA AND EASTERN MO OVER CENTRAL AND SOUTHERN IL. AS THIS MORE WIDESPREAD RAIN IN AND MOISTENS THE LOWER LAYERS AREAS OF MVFR AND LOCAL IFR CEILINGS SPREADING IN. EXTRAPOLATION OF BACK EDGE OF PREVAILING RAIN ARRIVES AT CHI VICINITY AROUND 22Z AND LINGERING SHOWERS END AROUND 00Z. SHOWERS HAVE BEEN DEVELOPING ACROSS SOUTHEAST MN INTO WEST CENTRAL WI ALONG FRONTAL BOUNDARY DRAPED ACROSS THIS AREA AND AS A SECOND SHORT WAVE DROPS FROM THE EASTERN DAKOTAS TOWARD SOUTHWEST MN AND NORTHWEST IA. MODELS KEEP THE SHOWERS IN THE VICINITY OF THE BOUNDARY ACROSS CENTRAL WI WHILE DAMPING OUT THE SHORT WAVE AS IT MOVES FURTHER EAST DURING THE AFTERNOON AND EARLY EVENING. SUBSIDENCE FOLLOWING MAIN SHORT WAVE TROUGH PASSAGE WILL LEAD TO CLEARING OVERNIGHT. LINGERING SHALLOW MOIST LAYER ALONG WITH EXPECTED ADDITION OF SIGNIFICANT AMOUNTS OF SMOKE FROM FIREWORKS...LEGAL AND OTHERWISE...TONIGHT WILL SHOULD LEAD TO MVFR VISIBILITY IN WITH BR..FU..HZ WITH SURFACE BASED INVERSION FORMING IN LIGHT SURFACE FLOW IN WEAK SURFACE RIDGE. TRS && .MARINE... 318 AM CDT A RELATIVELY WEAK GRADIENT WILL PERSIST ACROSS THE WESTERN GREAT LAKES OVER THE NEXT SEVERAL DAYS AND NEW GLF ISSUED THIS MORNING CONTINUES TO TOP OUT WAVE HEIGHTS AT 2 FEET THROUGH THE FORECAST PERIOD. SURFACE LOW PRESSURE ACROSS THE PLAINS WILL MOVE ACROSS SOUTHERN ILLINOIS THIS EVENING. A RATHER SHARP CUT OFF IN WIND SPEEDS WILL ACCOMPANY THIS LOW...AS SFC GRADIENT WILL DROP OFF SUBSTANTIALLY ACROSS SOUTHERN LAKE MICHIGAN DUE TO THE PRESENCE OF AN ELONGATED WEST-EAST ORIENTED SFC RIDGE AXIS. WIND DIRECTIONS TODAY WILL BE TRICKY WITH SUCH A WEAK GRADIENT ALTHOUGH MAY TEND TO FAVOR LIGHT SOUTHERLY DUE TO PRESENCE OF WEAKER SFC LOW ACROSS WISCONSIN. THE RIDGE AXIS WILL DROP SOUTH OF LAKE EARLY SUNDAY AS ANOTHER SFC TROUGH APPROACHES FROM ONTARIO. A BRIEF PERIOD OF WEST WINDS ARE EXPECTED IN ADVANCE OF THIS SFC TROUGH THAT WILL TAKE WINDS BACK TO THE NORTH NORTHWEST AS IT TRACKS ACROSS SOUTHERN LAKE MICHIGAN LATE SUNDAY NIGHT...BUT AGAIN SPEEDS WILL GENERALLY TOP OUT AROUND 10 TO 15 KNOTS. MONDAY SHOULD START OUT WITH NORTH WINDS AROUND 10 KNOTS BUT THEN BECOME VARIABLE OVER THE OPEN WATERS AS NEXT SFC HIGH BUILDS IN...WITH FLOW LIKELY BECOMING ONSHORE DUE TO LAKE BREEZE EFFECTS. NOT MANY CHANGES TO THE REMAINDER OF THE FORECAST...WITH ALL IN ALL A QUIET PERIOD SHAPING UP WITH A RETURN TO SOUTHEAST WINDS BY LATER TUESDAY AS LOW PRESSURE DEVELOPS ACROSS THE UPPER MIDWEST. NO MARINE HEADLINES APPEAR TO BE IN THE CARDS IN THE FORESEEABLE FUTURE. MARSILI && .LOT WATCHES/WARNINGS/ADVISORIES... IL...NONE. IN...NONE. LM...NONE. && $$