Area Forecast Discussion
Issued by NWS Chicago, IL

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000 FXUS63 KLOT 272338 AFDLOT AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL 538 PM CST FRI FEB 27 2015 .SHORT TERM... 157 PM CST THROUGH TONIGHT... QUIET CONDITIONS TO PREVAIL THIS AFTERNOON INTO TONIGHT AS HIGH PRESSURE DOMINATES THE REGION. SHEARED OUT SHORTWAVE WILL SHIFT EAST THROUGH THE REGION TONIGHT...WHILE UPSTREAM ENERGY OVER THE CENTRAL PLAINS LIFTS EAST/NORTHEAST THROUGH THE MID MISSISSIPPI VALLEY. THIS WILL HELP PROVIDE AN INCREASE IN MID/HIGH LEVEL CLOUD COVER TONIGHT...MAINLY ACROSS THE SOUTHEASTERN HALF OF THE CWA. THIS MOISTURE WILL REMAIN FAIRLY PROGRESSIVE WITH THE THICKER MOISTURE LIKELY EXITING BY EARLY SATURDAY MORNING...BUT WITH PARTLY CLOUDY TO POSSIBLY CLOUDIER SKIES FOR A TIME TONIGHT. MADE MINOR CHANGES TO FORECAST LOWS TONIGHT WITH SUBZERO TEMPS LIKELY ONCE AGAIN. HOWEVER...DONT FEEL THEY WILL BE AS COLD AS THIS MORNING BUT DO THINK SOME PLACES COULD APPROACH 10 BELOW. WINDS SHOULD RAPIDLY DROP OFF THIS EVENING AND ALTHOUGH WIND CHILLS WILL FALL TO AROUND 15 BELOW ZERO ONCE AGAIN TONIGHT...WONT BE LOW ENOUGH FOR ANY HEADLINES AT THIS TIME. RODRIGUEZ && .LONG TERM... 309 PM CST MAIN CONCERNS ARE WITH ACCUMULATING SNOW SATURDAY NIGHT THROUGH SUNDAY AND THEN A MESSY...STRONGER STORM SYSTEM MONDAY NIGHT INTO WEDNESDAY. HYDRO ISSUES ARE POSSIBLE ON TUESDAY FOR AT LEAST A PORTION OF THE CWA. SATURDAY... DAY WILL START OUT QUIET AND COLD WITH HIGH PRESSURE RIDGE AXIS OVERHEAD. AFTER EARLIER MID/HIGH CLOUDS IN MORNING PASS TO EAST... CLOUDS WILL THEN INCREASE IN THE AFTERNOON AS ISENTROPIC ASCENT BEGINS IN EARNEST. A LEAD MID LEVEL WAVE IN BROAD WEST-SOUTHWEST FLOW AT 500 MB COULD GENERATE SOME LIGHT SNOW IN LATE AFTERNOON IN THE FAR SOUTHWEST CWA...WHICH I INCLUDED SLIGHT POPS FOR. HIGH TEMPS SHOULD REACH THE LOWER-MID 20S THANKS TO STRONG NEARLY MARCH SUN AND WARM ADVECTION ALOFT...THOUGH A FEW TYPICALLY COLD SPOTS COULD BE HELD IN THE UPPER TEENS. SATURDAY NIGHT THROUGH SUNDAY... GUIDANCE IS IN GOOD AGREEMENT IN THE SYNOPTIC PATTERN OF BROAD LONGER DURATION ISENTROPIC ASCENT...BUT THERE ARE DIFFERENCES IN THE DETAILS...INCLUDING TIMING AND WHERE CUTOFF OF ACCUMULATING SNOW IS IN THE REGION. THERE WAS A SUBTLE TREND SOUTHWARD ON GFS/ECMWF WITH 12Z RUNS...AS WELL AS GEFS ENSEMBLE MEAN. FOCUS REMAINS ON SOUTHERN/SOUTHEAST CWA WITH BEST ACCUMS BY LATE SUNDAY. LEAD WAVE COULD BRING SOME LIGHT SNOW OR FLURRIES SATURDAY EVENING...ESPECIALLY SOUTHERN CWA...WITH LITTLE IF ANY ACCUMULATION. TIMING DIFFERENCES CREEP IN AFTER MIDNIGHT...BUT DID HEDGE DURING THIS PERIOD TOWARD SLIGHTLY FASTER ECMWF BECAUSE FORCING FOR SNOW MAINLY SOUTH OF I-80 CAN BE IMPLIED BY CONTINUED WARM ADVECTION WHICH SHOULD ENABLE COLUMN TO SATURATE. 1 TO 2 INCHES OF ACCUMULATION ARE POSSIBLE MAINLY SOUTH OF IL AND KANKAKEE RIVERS BY DAYBREAK SUNDAY. HAVE SOME CONCERN ON SUNDAY WITH HOW SHARP NORTHERN CUTOFF OF PRECIP WILL BE. DID NOT MAKE BIG CHANGES TO PREVIOUS FORECAST POPS...BUT LOWERED SNOW AMOUNTS IN NORTHERN TIER OF COUNTIES TO 1 TO 2 INCHES...WHICH COULD BE GENEROUS. WAA/ISENTROPIC ASCENT PATTERN WILL BRING A MOIST AIR MASS TO THE SOUTHERN CWA...WITH PWAT VALUES NEAR OR JUST OVER .7 INCHES. DECENT MOISTURE...WITH ROUGHLY HALF INCH PWATS...SHOULD MAKE IT UP TO CHICAGO AREA. FORECAST SOUNDINGS INDICATE DECENT MID LEVEL LAPSE RATES AS WELL IN THE SOUTHERN CWA...AND TIGHTENING THERMAL GRADIENT SHOULD YIELD STEADY LIGHT TO MODERATE SNOWFALL RATES. TEMPERATURES NEAR OR SLIGHTLY OVER 30 DEGREES WILL BRING SNOW RATIOS DOWN CLOSER TO AVERAGE AND HIGH MARCH SUN ANGLE MAY MAKE IT TOUGH FOR SNOW TO ACCUMULATE READILY ON WELL TREATED/HEAVILY TRAVELED ROADWAYS. OVERALL...SNOW AMOUNTS BY LATE SUNDAY FOR THE REMAINDER OF THE CWA LOOK TO FALL INTO 1 TO 3 INCH/LOCALLY 4 INCH RANGE ALONG I-88/I-90 CORRIDOR...2 TO 4 INCHES NEAR AND A BIT SOUTH OF I-80 AND 4 TO 6 INCHES SOUTH OF THE ILLINOIS AND KANKAKEE RIVERS. THERE COULD BE LOCALIZED AMOUNTS JUST OVER 6 INCHES IN FAR SOUTHEAST CWA...BUT LONGER DURATION AND LIGHT WINDS SHOULD MITIGATE IMPACTS. WILL LIKELY NEED A WINTER WEATHER ADVISORY FOR AT LEAST PART OF THE CWA SOUTH OF INTERSTATE 80 NONETHELESS. SUNDAY NIGHT AND MONDAY... EXPANSIVE HIGH PRESSURE WILL SETTLE OVER THE REGION SUNDAY NIGHT...WITH SKIES BECOMING PARTLY CLOUDY TO MOSTLY CLEAR AFTER MIDNIGHT. AIR MASS IS NOT ALL THAT COLD ALOFT...BUT STRONG INVERSION AND DIMINISHING WINDS AS HIGH NOSES IN SHOULD ENABLE CHILLY TEMPERATURES...ESPECIALLY IN TYPICAL COLD SPOTS. HAVE REMAINED CONSERVATIVE WITH LOWS FOR NOW...BUT WOULDNT BE SURPRISED IF LOCATIONS SUCH AS RPJ DIP BELOW ZERO. DEEP SOUTHWESTERN TROUGH WILL BRING RAPID MID LEVEL HEIGHT RISES AND INCREASING WARM ADVECTION DURING THE AFTERNOON...BUT LIMITED MIXING THROUGH MID DAY UNDER INFLUENCE OF HIGH PRESSURE SHOULD LIMIT TEMPS TO THE UPPER 20S TO AROUND 30. MONDAY NIGHT THROUGH FRIDAY... AFOREMENTIONED DEEP WESTERN TROUGH WILL RESULT IN STRONG SURFACE LOW MOVING NEAR OR OVER AREA ON TUESDAY. FAIRLY COMPLEX UPPER PATTERN DUE TO ANOMALOUS TROUGH EJECTING FROM SOUTHWEST AND DIFFERENCES ON GUIDANCE ON TRACK OF SURFACE AND MID/UPPER FEATURES AS WELL AS PHASING OF ALL THE PIECES. AIR MASS SHOULD START OUT COLD ENOUGH FOR SNOW MONDAY NIGHT...BUT THEN STRONG SOUTHWEST FLOW ALOFT SHOULD QUICKLY WARM COLUMN ABOVE FREEZING BY OR DURING TUESDAY MORNING. WONDERING IF AT LEAST NORTHERN HALF OF CWA COULD SEE A FRONT END OF ACCUMULATING SNOW...FOLLOWED BY QUICK TRANSITION TO MIX THEN PLAIN RAIN EVERYWHERE. UNSEASONABLY MOIST AIR MASS ADVECTING IN...ALONG WITH DEWPOINTS POSSIBLY APPROACHING OR EXCEEDING 40 DEGREES FOR A TIME TUESDAY INTO THE EARLY EVENING SOUTH OF I-80...ALONG WITH STEADY RAIN...COULD RESULT IN HYDROLOGIC ISSUES DUE TO FROZEN AND SNOW COVERED GROUND...AND ICED OVER RIVERS. WITH THIS SYSTEM STILL FAR OUT...THIS PERIOD NEEDS TO BE CLOSELY MONITORED FOR VARIETY OF POTENTIALLY SIGNIFICANT IMPACTS. STILL CAN SEE A WAY OUT FOR A MORE WINTRY SCENARIO OR ON THE OTHER HAND...HYDRO ISSUES FOR THE ENTIRE AREA IF SYSTEM TRACKS FARTHER NORTH. COLDER AIR WILL FILTER BACK IN AND CHANGE LINGERING PRECIP TO SNOW TUESDAY NIGHT...FOLLOWED BY A BLAST OF ARCTIC AIR WEDNESDAY INTO THURSDAY. POSSIBLE LARGE SCALE PATTERN CHANGE TO ZONAL PACIFIC FLOW COULD BRING MUCH MILDER WEATHER BY NEXT WEEKEND. RC && .CLIMATE... TEMPERATURES ARE FORECAST ONCE AGAIN TO BE CLOSE OR BREAK THE FOLLOWING DAILY RECORDS THROUGH THE END OF THE MONTH...INCLUDING ONE ON FRIDAY IN ROCKFORD THAT WAS SET JUST LAST WINTER: ROCKFORD... FRIDAY FEB 27TH LOW MAXIMUM: 15 IN 2014 SATURDAY FEB 28TH LOW: -7 IN 1962 SATURDAY FEB 28TH LOW MAXIMUM: 17 IN 1962 CHICAGO... SATURDAY FEB 28TH LOW: 0 IN 1884 && .AVIATION... //ORD AND MDW CONCERNS...UPDATED 00Z... * LIGHT SNOW COULD DEVELOP SATURDAY EVENING IZZI //DISCUSSION...UPDATED 00Z... WEATHER NIL THROUGH DAY SATURDAY. NEXT SYSTEM WILL APPROACH THE AREA SATURDAY NIGHT POTENTIALLY RESULTING IN LIGHT SNOW DEVELOPING SATURDAY EVENING DURING THE FINAL 6 HOUR FORECAST PERIOD AT O`HARE. IZZI //ORD AND MDW CONFIDENCE...UPDATED 00Z... * LOW-MEDIUM CONFIDENCE IN SATURDAY EVENING SNOW * HIGH CONFIDENCE IN EVERYTHING ELSE IZZI //OUTLOOK FOR ORD/MDW FOR 12Z SUNDAY-12Z FRIDAY...UPDATED 00Z... SUNDAY...IFR LIKELY WITH PERIODS OF LIGHT SNOW. MONDAY...VFR. TUESDAY...RAIN...FREEZING RAIN...OR SNOW. IFR PROBABLE. WEDNESDAY...MVFR CIGS POSSIBLE. THURSDAY...CHANCE OF MVFR CIGS EARLY. IZZI && .MARINE... 157 PM CST LARGE HIGH PRESSURE SITUATED JUST SOUTH OF THE LAKE THIS AFTERNOON WHILE LOW PRESSURE IS OVER HUDSON BAY. THIS IS ALLOWING FOR GENERALLY WESTERLY WINDS OVER THE LAKE IN THE 10 TO 20 KT RANGE. THIS HIGH WILL SLIDE TO THE EAST TONIGHT INTO SATURDAY...WITH WINDS BECOMING MORE SOUTHWEST OVERNIGHT AND WHILE WINDS SLIGHTLY INCREASE TO THE 15 TO 25 KT RANGE FOR A BRIEF PERIOD. A PERIOD OF SOUTHERLY WINDS WILL OCCUR ON SATURDAY AND SATURDAY NIGHT WITH 10 TO 20 KT RANGE INCREASING BACK UP TO THE 15 TO 25 KT RANGE. GENERALLY LIGHTER FLOW EXPECTED OVER THE LAKE TO END THE WEEKEND...BEFORE WINDS INCREASE ONCE AGAIN OUT OF THE WEST/NORTHWEST SUNDAY NIGHT INTO MONDAY. RODRIGUEZ && .LOT WATCHES/WARNINGS/ADVISORIES... IL...NONE. IN...NONE. LM...NONE. && $$ 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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