531 FXUS63 KILX 262358 AFDILX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE LINCOLN IL 558 PM CST SAT JAN 26 2013 .DISCUSSION... ISSUED 329 PM CST SAT JAN 26 2013 MAIN CONCERN THIS PACKAGE REVOLVES AROUND THE FREEZING RAIN EVENT FORECAST FOR TOMORROW AND HOW QUICKLY THE SFC TEMPS WARM...CHANGING THE PCPN FROM FREEZING RAIN TO ALL RAIN. WARMING TEMPS AFTER THIS SYSTEM ALSO ARE AN ISSUE ALONG WITH CHANCES OF THUNDERSTORMS...ALL IN THE SHORT TERM. LONG TERM CONCERNS ARE WITH ENDING OF PCPN AND TEMPS GETTING COLDER AGAIN. MODELS LOOK SIMILAR OVERALL WITH UPPER LEVEL PATTERN AND SFC FEATURES. DIFFERENCES ARE WITH SPEED OF WARMING IN THE VERY LOW LEVELS TOMORROW. 12Z NAM APPEARS TO BE TOO AGGRESSIVE SO WILL FOLLOW CLOSER TO THE GFS AND ECMWF FOR TEMP PROFILES. LONG RANGE MODELS LOOK VERY SIMILAR THROUGH REST OF THE WEEK AND INTO THE WEEKEND....SO FOLLOWED A BLEND CLOSELY. SHORT TERM...TONIGHT THROUGH TUESDAY AFTER A BRIEF DRY PERIOD TONIGHT...CLOUDS/MOISTURE/PCPN WILL ADVECT INTO THE AREA FROM THE WEST TOMORROW. FOLLOWING THE GFS/ECMWF GUIDANCE...PCPN SHOULD BEGIN IN THE WESTERN PART OF THE CWA AROUND 12Z AND THEN SPREAD EAST DURING THE MORNING HOURS. 850MB TEMPS SHOULD BE 4C BY TIME PCPN STARTS...BUT SFC TEMPS WILL STILL BE BELOW FREEZING SO PCPN WILL BEGIN AS FREEZING RAIN. THERE WILL BE SOME DRY AIR INITIALLY WHEN THE PCPN BEGINS SO SLEET COULD BE MIXED IN WITH THE FREEZING RAIN AT THE BEGINNING. THE FREEZING RAIN WILL CONTINUE INTO THE AFTERNOON ACROSS THE AREA AND THEN GRADUALLY CHANGE TO LIQUID RAIN AS THE SFC TEMPS WARM TO ABOVE THE FREEZING MARK. TIMING OF THIS CHANGE OVER IS UNCERTAIN SINCE MANY FACTORS WILL CONTRIBUTE TO DEGREE/SPEED OF WARMING DURING THE AFTERNOON. EVAPORATIVE COOLING WILL COMPETE WITH CLOUDS AND WAA ACROSS THE AREA. SO FORECAST ACROSS THE CENTRAL AND NORTHERN PARTS OF THE CWA WILL BE FREEZING RAIN CHANGING TO LIQUID RAIN DURING THE AFTERNOON...WITH CENTRAL AREAS CHANGING EARLIEST IN THE AFTERNOON...FOLLOWED BY NORTHERN AREAS DURING THE LATE AFTERNOON...TOWARD EVENING. FREEZING RAIN ADVISORY HAS BEEN ISSUED FOR TOMORROW MORNING THROUGH TOMORROW AFTERNOON/EVENING. QPF AMOUNTS COULD RANGE FROM .10 TO .30 BUT BELIEVE CONDITIONS WILL NOT BE IDEAL FOR ACCUMULATIONS OVER ONE QUARTER INCH. SO THINKING IS ICE ACCUMULATIONS SHOULD BE FROM ONE TO TWO TENTHS OF AN INCH THROUGH THE PERIOD. ONCE SFC TEMPS WARM ALL PCPN WILL BE LIQUID RAIN FROM SUNDAY NIGHT THROUGH TUESDAY. THE SFC FRONT SHOULD REMAIN IN NORTHWEST ILLINOIS THROUGH THE REMAINDER OF THE FORECAST PERIOD SO TEMPS WILL REMAIN WELL ABOVE THE FREEZING MARK. POPS WILL FALL BACK INTO THE CHC CATEGORY FOR MONDAY BUT THEN RISE BACK UP TO LIKELY FOR MONDAY NIGHT AND TUE. WITH STRONG WAA AND LOTS OF MOISTURE...THUNDERSTORMS WILL BE POSSIBLE OVER MOST OF THE CWA TUESDAY INTO TUE EVENING. TEMPS WILL START COOL AND THEN WARM TOMORROW TO WELL ABOVE NORMAL FOR END OF JAN. MON TEMPS WILL BE IN THE 50S WITH TUE BEING WARMER...WITH SOME AREAS REACHING INTO THE 60S...WHICH IS JUST BELOW RECORDS. LONG TERM...TUESDAY NIGHT THROUGH SATURDAY THE FRONTAL SYSTEM WILL FINALLY PUSH THROUGH THE CWA TUE NIGHT INTO WED WITH COLDER AIR ADVECTING INTO THE AREA BEHIND THE FRONT. A FEW THUNDERSTORMS WILL REMAIN POSSIBLE IN THE EAST AND SOUTHEAST TUE EVENING/NIGHT...WITH ONLY A CHANCE OF RAIN IN THE SOUTHEAST FOR WED. AS THE PCPN IS ENDING...COLDER AIR SHOULD CAUSE SOME OF THE PCPN TO BE A MIX OF RAIN AND SNOW...BUT CHANCES WILL BE SLIGHT SO THIS WILL ONLY SHOW UP IN THE GRIDS AND NOT IN THE WORDED FORECAST. REMAINDER OF THE FORECAST PERIOD LOOKS TO BE DRY THOUGH THERE IS A WEAK SYSTEM FORECAST TO MOVE ACROSS THE NORTHERN PART OF THE STATE LATE IN THE WEEK WITH A SLIGHT CHANCE OF SNOW...AGAIN THIS WILL NOT SHOW UP ON WORDED FORECAST BUT ONLY IN GRIDS. TEMPS WILL COOL AGAIN AND BE THE COLDEST THUR NIGHT AND FRI BUT FORECAST MODELS SHOW TEMPS WARMING BACK UP AGAIN TO AROUND NORMAL FOR THE BEGINNING OF THE WEEKEND. AUTEN && .AVIATION... ISSUED 558 PM CST SAT JAN 26 2013...FOR THE 00Z TAFS MAIN CONCERN WILL BE ONSET OF FREEZING RAIN SUNDAY MORNING. WE EXPECT COLD AIR IN THE NEAR GROUND AIRMASS... AND COLD GROUND/ROAD SURFACE CONDITIONS TO CAUSE MAJOR TRAVEL IMPACTS AT THE START OF THE FREEZING RAIN. FREEZING RAIN FALLING ON DRY AND COLD GROUND WILL STICK QUICKLY AS A GLAZE OF ICE AT THE ONSET...TURNING TRAVEL SURFACES TO A SHEET OF ICE...EVEN THOUGH ICE THICKNESS WILL BE VERY THIN INITIALLY. THE THICKNESS OF GLAZING MAY REACH UP TO 0.10"-0.20" BY MID AFTER NOON FOR THE NORTHERN TERMINALS OF PIA AND BMI...WITH CMI/SPI/DEC CLOSER TO 0.10". MVFR CLOUD HEIGHTS ARE LIKELY BY SUNRISE FOR SPI AND MID MORNING FOR THE REMAINDER OF THE TAF SITES. MVFR VIS IN STEADY FZRA IS POSSIBLE AS WELL. SLEET OR SNOW MIXING IN AT THE ONSET WOULD LIKELY LIMIT VIS TO MVFR RANGES AS WELL. STRONG WARMING AT 3-6K FT ABOVE THE GROUND WILL TURN THE PRECIP TO ALL RAIN IN THE AFTERNOON. SPI AND DEC BY 19Z...CMI BY 21Z...AND PIA/BMI BY 22Z. STEADY RAINS WILL CONTINUE OVERNIGHT...AS SURFACE TEMPS STEADILY RISE ABOVE FREEZING SUNDAY NIGHT INTO MONDAY MORNING. WINDS WILL START OFF LIGHT EAST-SOUTHEAST THIS EVENING...THEN INCREASE TO 10-14KT WHEN PRECIP BEGINS AT EACH TERMINAL SITE. WINDS SHOULD REMAIN SE THROUGH THE END OF THIS TERMINAL FORECAST. SHIMON && .ILX WATCHES/WARNINGS/ADVISORIES... FREEZING RAIN ADVISORY FROM 6 AM TO 3 PM CST SUNDAY FOR ILZ036- 040>042-047>054-061. FREEZING RAIN ADVISORY FROM 9 AM TO 6 PM CST SUNDAY FOR ILZ027>031-037-038-043>046-055>057-062-063. && $$