Area Forecast Discussion
Issued by NWS Chicago, IL

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000 FXUS63 KLOT 080314 AFDLOT AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL 1014 PM CDT TUE JUL 7 2015 .UPDATE... 823 PM CDT THE ONLY CHANGES TO THE GOING FORECAST WAS TO INCREASE POPS AND QPF AMOUNTS ON WEDNESDAY AFTERNOON AND EVENING WITH THE NEXT STORM SYSTEM EXPECTED TO IMPACT THE REGION. HEAVY RAINFALL APPEARS TO BE A GOOD BET WITHIN AND POSSIBLY EVEN A BIT NORTH OF THE CURRENT FLASH FLOOD WATCH...SO I HAVE ALSO INCLUDED THE MENTION OF HEAVY RAINFALL IN THE FORECAST FOR THE SOUTHERN HALF OR SO OF THE AREA. MODEL FORECAST GUIDANCE IS IN GOOD AGREEMENT IN FORECASTING A SYNOPTICALLY FORCED RAINFALL EVENT OVER THE AREA LATER WEDNESDAY THROUGH WEDNESDAY NIGHT. THE MAIN SYSTEM OF INTEREST IN EVIDENT IN WATER VAPOR IMAGERY OVER NORTHERN TEXAS INTO OKLAHOMA EARLY THIS EVENING. THIS SYSTEM IS EXPECTED TO INDUCE A SURFACE WAVE OF LOW PRESSURE TONIGHT ALONG A SOUTHWEST TO NORTHEAST ORIENTED SURFACE FRONTAL BOUNDARY...CURRENTLY STRETCHING FROM THE SOUTHERN PLAINS TO THE EASTERN GREAT LAKES REGION. THIS SYSTEM IS THEN FORECAST TO LIFT OVER NORTHERN ILLINOIS WEDNESDAY EVENING. THE TRACK OF THIS SYSTEM FAVORS AN AREA OF GOOD FRONTOGENESIS AND MOISTURE CONVERGENCE RIGHT ACROSS THE SOUTHERN HALF OF MY CWA WEDNESDAY EVENING...ROUGHLY FROM EAST CENTRAL ILLINOIS NORTHEASTWARD ACROSS NORTHWESTERN INDIANA. ALTHOUGH CONVECTIVE INSTABILITY WILL BE VERY LOW...THE PRESENCE OF STRONG MESOSCALE FORCING IN COMBINATION WITH THE GOOD SYNOPTIC SUPPORT...ALONG WITH THE PRESENCE OF AROUND 2 INCH PWATS...SHOULD PRODUCE A LARGE AREA OF MODERATE RAIN...WITH SOME EMBEDDED BANDING OF HEAVY RAINFALL AND POSSIBLE THUNDERSTORMS. CURRENTLY IT STILL APPEARS THE CURRENT FLASH FLOOD WATCH WILL COVER MUCH OF THE MOST SUSCEPTIBLE AREAS TO HEAVY RAINFALL AND POSSIBLE RENEWED FLOODING. HOWEVER...I DID CONSIDER ADDING A FEW COUNTIES TO THE NORTH OF THE CURRENT WATCH...ESPECIALLY LAKE AND PORTER IN NORTHWESTERN INDIANA. HOWEVER...GIVEN THIS IS STILL 24 HOURS AWAY I HAVE OPTED TO HOLD OFF ON ANY CHANGES TO THE WATCH...AS A FARTHER SOUTH TRACK OF THE LOW COULD MEAN LESS RAIN FARTHER NORTH. GIVEN ALL OF THE ABOVE...I HAVE UPPED POPS TO 80 TO 90 PERCENT SOUTH OF INTERSTATE 80...WITH LIKELY POPS EXTENDING OVER MUCH OF THE CHICAGO METRO AREA WEDNESDAY EVENING. TO COMPLEMENT THIS...I HAVE ALSO INCREASED QPF AMOUNTS WEDNESDAY EVENING. OVERALL...THESE CHANGES RESULT IN 1.5 TO 2 INCHES SOUTH OF INTERSTATE 80...THOUGH ISOLATED HIGH AMOUNTS ARE CERTAINLY POSSIBLE UNDER ANY MESOSCALE BANDS THAT DEVELOP. AS MENTIONED ABOVE...I HAVE OPTED TO NOT MAKE ANY CHANGES TO THE AREAS IN THE WATCH...HOWEVER...IF CURRENT MODEL TRENDS CONTINUE...THE WATCH MAY NEED TO BE EXPANDED TO THE SOUTHERN TIP OF LAKE MICHIGAN. KJB && .SHORT TERM... 234 PM CDT THROUGH WEDNESDAY... THE COLD FRONT THAT MOVED THROUGH EARLY TODAY NOW LIES ACROSS CENTRAL INDIANA AND SOUTHERN IL WITH THE RAIN ALSO LOCATED OVER THOSE REGIONS. EXPECTING THE DRY CONDITIONS TO CONTINUE OVERNIGHT AS HIGH PRESSURE MOVES OVER CENTRAL LAKE MI LATE TONIGHT. GUSTY NORTH WINDS WILL DIMINISH FIRST FOR AREAS WEST OF I-55 AS THE GRADIENT WILL WEAKEN QUICKEST THERE. GUSTY WINDS ALONG THE LAKE WILL TAKE THE LONGEST TO DIMINISH BUT THINKING GUSTS WILL FINALLY BE BELOW 20 MPH LATE THIS EVENING. CUMULUS CLOUDS HAVE FORMED IN THE COOL AIR AS THE STRATUS ASSOCIATED WITH THE FRONT SHIFTS TO THE SOUTHEAST. THE CU WILL BE DIURNAL IN NATURE SO EXPECTING SKIES TO CLEAR OVERNIGHT ALLOWING TEMPS TO FALL INTO THE LOW TO MID 50S. SOME LOCATIONS ALONG THE IL/WI STATE LINE MAY EVEN DROP INTO THE UPPER 40S. CHICAGO AND THE INDIANA SHORE LINE WILL BE THE WARMEST WITH LOWS AROUND 60 DUE TO FLOW OFF OF THE LAKE. ATTENTION THEN SHIFTS TO TOMORROW/S RAINFALL. SOME AREAS SOUTH OF I-80 RECEIVED UP TO 3 INCHES OF RAIN MONDAY AND MONDAY NIGHT...AND WHEN THAT IS COMBINED WITH THE RECENT WET PATTERN WE ARE IN...THERE ARE CONCERNS ABOUT FLOODING TOMORROW. BOTTOM LINE UP FRONT...ISSUED A FLASH FLOOD WATCH FOR LASALLE COUNTY...KANKAKEE COUNTY...THROUGH JASPER COUNTY SOUTH FROM WEDNESDAY AFTERNOON THROUGH WEDNESDAY NIGHT. A LOW PRESSURE SYSTEM WILL FORM OVER OKLAHOMA LATE THIS EVENING AND DEEPEN AS IT MOVES OVER THE MID IL/IN STATE LINE WEDNESDAY EVENING. AN UPPER LEVEL VORT STREAMER ASSOCIATED WITH THE LOW SHIFTS NORTH INTO NORTHERN IL AND INDIANA WEDNESDAY MORNING...AND THEN PARKS OVER THE AREA WEDNESDAY AFTERNOON. 12Z GUIDANCE IS IN BETTER AGREEMENT THAN 06Z RUNS ABOUT PRECIP FALLING ALONG AND SOUTH OF THE I-88 CORRIDOR...SO HAVE MEDIUM-HIGH CONFIDENCE IN HOW FAR NORTH PRECIP GETS WEDNESDAY. THE NAM IS THE MAIN OUTLIER WITH MUCH LESS PRECIP THAN THE GFS AND ECMWF. PWAT VALUES INCREASE TO 1.5-2 INCHES DURING THE DAY WEDNESDAY. HOWEVER...MODEL SOUNDINGS FEATURE A LOT OF DRY AIR ALOFT THAT THE RAIN WILL HAVE TO OVERCOME...SO HAVE MEDIUM CONFIDENCE IN HOW QUICKLY SHOWERS PUSH NORTH WEDNESDAY. GUIDANCE ALSO FEATURES VERY LITTLE CAPE...SO ONLY HAVE A SLIGHT CHANCE OF THUNDERSTORMS IN THE FORECAST. THINKING THE MAJORITY OF PRECIP WILL FALL AS STRATIFORM RAIN WITH A FEW EMBEDDED STRONGER RAIN SHOWERS OR THUNDERSTORMS. THE TOTAL AMOUNT OF RAIN IS THE CONCERN...WITH 0.75 TO AN INCH OF ADDITIONAL RAIN FORECAST FOR AREAS ALONG AND SOUTH OF I-80. LOCALLY HIGHER AMOUNTS ARE POSSIBLE. ONCE AGAIN...WE ISSUED A FLASH FLOOD WATCH FOR THE SOUTHERN PORTION OF THE CWA SINCE SIGNIFICANT RAINFALL AMOUNTS ARE POSSIBLE OVER AREAS THAT ALREADY RECEIVED AMPLE AMOUNTS OF RAIN FOR WEDNESDAY AFTERNOON THROUGH WEDNESDAY NIGHT. WITH THE INCREASED CLOUD COVER...LOWERED HIGH TEMPS FOR WEDNESDAY BY A DEGREE OR TWO. EXPECTING HIGH TEMPS AROUND 70 WITH UPPER 60S ALONG THE LAKE. JEE && .LONG TERM... 305 PM CDT WEDNESDAY NIGHT THROUGH TUESDAY... SURFACE LOW WILL TRACK FROM DOWNSTATE ILLINOIS INTO NORTHEASTERN INDIANA WEDNESDAY NIGHT WITH DEFOREMATION AXIS PIVOTING ACROSS THE SOUTHEASTERN TWO THIRDS OF THE CWA BRINGING STEADY RAINFALL OVERNIGHT. BASED ON THE LOW TRACK...CURRENT EXPECTATIONS ARE FOR THE HEAVIEST RAINFALL EAST OF THE I-55 CORRIDOR WHERE THREE QUARTERS TO AN INCH OF RAIN IS EXPECTED...WITH LOCALLY HIGHER AMOUNTS POSSIBLE. MOISTURE IS DEEP RESULTING IN FAIRLY POOR LAPSE RATES THROUGH THE COLUMN. GIVEN THE LACK OF ANY SIGNIFICANT INSTABILITY...WILL ONLY MENTION A SLIGHT CHANCE OF EMBEDDED THUNDER WITHIN THE CORRIDOR OF HIGHEST POPS. BY THURSDAY...SURFACE LOW BEGINS TO FILL AS IT MOVES TO NEW ENGLAND WITH A WEAK TRAILING SURFACE TROUGH SAGGING SOUTH INTO CENTRAL ILLINOIS. MEANWHILE...HIGH PRESSURE BUILDS FROM THE UPPER MIDWEST INTO NORTHERN ILLINOIS AND SHOULD PROVIDE QUIET WEATHER FOR MOST OF THE CWA...WITH TEMPERATURES IN THE MID 70S AWAY FROM THE LAKE. NORTHEAST WINDS OFF OF LAKE MICHIGAN THROUGH THE DAY WILL KEEP TEMPERATURES IN THE UPPER 60S TO AROUND 70 ALONG THE LAKE FRONT. MODELS DEVELOP VERY LIGHT QPF ACROSS THE SOUTHERN PORTIONS OF THE CWA NEAR THE AFOREMENTIONED TROUGH...HOWEVER SOUNDINGS SHOW FAIRLY SHALLOW MOISTURE AND THERE IS NO UPPER LEVEL SUPPORT WITH A BUILDING MID/UPPER LEVEL RIDGE...SO OPTED TO CAP POPS AT SILENT 14S THURSDAY AFTERNOON AND EVENING. FROM FRIDAY ON...BERMUDA HIGH WILL BEGIN TO SHIFT WEST WITH STRONG RIDGE DEVELOPING AND BECOMING CENTERED OVER TEXAS AND THE PLAINS OVER THE WEEKEND. MID LEVEL WARM ADVECTION WILL RESULT IN A WARMING TREND FRIDAY INTO THE WEEKEND WITH MID 80S EXPECTED CWA WIDE SATURDAY AND SUNDAY AND SHOULD CONTINUE INTO NEXT WEEK. WITH THE WARMER TEMPERATURES THOUGH WILL ALSO COME PERIODIC CHANCES FOR SHOWERS AND THUNDERSTORMS GIVEN THE PERSISTENT MID LEVEL WAA ALONG WITH SEVERAL WEAK UPPER LEVEL DISTURBANCES RIDING DOWN THE LEE SIDE OF THE RIDGE. DEUBELBEISS && .AVIATION... //ORD AND MDW CONCERNS...UPDATED 03Z... * RA/SHRA LIFTING NORTH ON WEDNESDAY AFTERNOON...WITH FALLING VSBY AND CIGS INTO THE EVENING. * EAST WINDS MAY BE AROUND/SLIGHTLY OVER 10 KT LATE WEDNESDAY AFTERNOON INTO WEDNESDAY EVENING. RC //DISCUSSION...UPDATED 00Z... QUIET CONDITIONS IN STORE THROUGH WEDNESDAY MORNING...WITH NORTH-NORTHEAST WINDS DIMINISHING THIS EVENING AS HIGH PRESSURE MOVES OVERHEAD. STRONG UPPER DISTURBANCE AND DEVELOPING SURFACE LOW OVER THE SOUTHERN PLAINS LIFTING NORTHEAST ON WEDNESDAY WILL SPREAD RAIN/SHOWERS FROM SOUTH TO NORTH WEDNESDAY AFTERNOON. STRENGTH OF FORCING AS SYSTEM TRACKS ACROSS CENTRAL IL WEDNESDAY EVENING SHOULD OVERCOME ANY DRYING INFLUENCE FROM EASTERLY FLOW TO BRING RAIN TO ALL TERMINALS EXCEPT PERHAPS RFD WHICH WILL BE ON NORTHERN FRINGE. STEADY/HEAVY RAIN IS POSSIBLE ON WEDNESDAY EVENING FOR AT LEAST MDW/GYY. CIGS AND VSBY WILL TREND DOWNWARD AS THE RAIN ARRIVES...AND IFR CIGS APPEAR POSSIBLE BEYOND THE ORD 30-HR TAF. THERE IS A VERY SLIM...BUT NON-ZERO CHANCE FOR ISOLATED EMBEDDED TSRA WEDNESDAY EVENING...MAINLY SOUTH OF INTERSTATE 80. RC //ORD AND MDW CONFIDENCE...UPDATED 03Z... * HIGH IN OVERALL WIND TRENDS...MEDIUM IN WIND SPEEDS WEDNESDAY AFTERNOON AND EVENING. * MEDIUM-HIGH IN RAIN ARRIVING LATE WEDNESDAY AFTERNOON...MEDIUM IN TIMING AND ASSOCIATED CIG AND VSBY IMPACTS. RC //OUTLOOK FOR ORD/MDW FOR 12Z THURSDAY-12Z TUESDAY...UPDATED 00Z... THURSDAY...MVFR CIGS PSBL EARLY...OTHERWISE VFR. NORTHEAST WINDS. FRIDAY...CHANCE SHRA/TSRA AND AND ASSOCIATED VSBY IMPACTS. MAINLY VFR CIGS. SATURDAY THROUGH MONDAY...PERIODIC CHANCE FOR SHRA/TSRA AND ASSOCIATED VSBY IMPACTS. MAINLY VFR CIGS. RC && .MARINE... 305 PM CDT WIND GUSTS HAVE BEGUN TO DIMINISH ALONG THE IL SHORE...BUT AM STILL SEEING GUSTS UP TO 30 KT OVER THE SOUTHEAST END OF THE LAKE. HIGH PRESSURE CURRENTLY OVER THE PLAINS WILL MOVE OVER THE LAKE BY WEDNESDAY MORNING WITH WINDS DIMINISHING THROUGH THE NIGHT. WAVE OBSERVATIONS ARE HIGHER THAN FORECAST WITH SITES ON THE SOUTHEAST END OF THE LAKE SHOWING 9 FT. THE LATEST WAVE MODEL RUN ALSO CAME IN WITH HIGHER WAVES ALONG THE NSH WATERS SO WILL EXTEND THE SMALL CRAFT ADVISORY FOR WINTHROP HARBOR TO WILMETTE HARBOR TO MID EVENING AS WAVES WILL REMAIN HAZARDOUS TO SMALL CRAFT LONGER THAN ORIGINALLY THOUGHT. NO CHANGES WILL BE MADE TO THE CURRENT SMALL CRAFT ADVISORY FOR THE OTHER NEARSHORE ZONES. WITH HIGH PRESSURE OVERHEAD TOMORROW...WINDS WILL BE LESS THAN 10 KT OUT OF THE NORTH TO NORTHEAST OVER THE SOUTHERN HALF AND VARIABLE OVER THE NORTHERN HALF. NORTHEAST WINDS INCREASE TO 10-15 KT OVER THE FAR SOUTHERN END OF THE LAKE AS A LOW PRESSURE SYSTEM PASSES OVER NORTHERN IL AND INDIANA WEDNESDAY NIGHT. WINDS THEN BECOME SOUTHWEST AS ANOTHER RIDGE SHIFTS EAST OF THE LAKE THURSDAY NIGHT INTO FRIDAY WITH SOUTHERLY WINDS INTO EARLY NEXT WEEK. JEE && .LOT WATCHES/WARNINGS/ADVISORIES... IL...FLASH FLOOD WATCH...ILZ023-ILZ032-ILZ033-ILZ039...1 PM WEDNESDAY TO 7 AM THURSDAY. BEACH HAZARDS STATEMENT...ILZ014 UNTIL 4 AM WEDNESDAY. IN...BEACH HAZARDS STATEMENT...INZ001-INZ002 UNTIL 4 AM WEDNESDAY. FLASH FLOOD WATCH...INZ010-INZ011-INZ019...1 PM WEDNESDAY TO 7 AM THURSDAY. LM...SMALL CRAFT ADVISORY...NEARSHORE WILMETTE HARBOR TO MICHIGAN CITY IN UNTIL 4 AM WEDNESDAY. && $$ VISIT US AT HTTP://WEATHER.GOV/CHICAGO (ALL LOWERCASE) FOLLOW US ON FACEBOOK...TWITTER...AND YOUTUBE AT: WWW.FACEBOOK.COM/NWSCHICAGO WWW.TWITTER.COM/NWSCHICAGO WWW.YOUTUBE.COM/NWSCHICAGO

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