000
FXUS63 KLOT 280551
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1151 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 06Z...

* PERIOD OF LIGHT SNOW POSSIBLE LATE SATURDAY EVENING/OVERNIGHT.
  MVFR/BRIEF IFR CONDITIONS ASSOCIATED WITH PRECIP.

RATZER

//DISCUSSION...UPDATED 06Z...

HIGH PRESSURE OVER THE AREA EARLY THIS MORNING WILL START SATURDAY
OUT WITH SIMILAR QUIET CONDITIONS AS ON FRIDAY. ONLY A FEW MID-
LEVEL CLOUDS...AND THOSE MAINLY SOUTH OF THE TERMINALS...WILL BE
PRESENT WITH A WEAK DISTURBANCE MOVING THROUGH LARGELY DRY
ATMOSPHERE THROUGH MID-DAY. MOISTURE WILL INCREASE FROM THE SOUTH
LATE IN THE DAY...WITH VFR CEILINGS DEVELOPING AND LOWERING TO
4000-5000 FT AGL BY EARLY EVENING. MODEL FORECAST SOUNDINGS
INDICATE DEEPENING MID-LOW LEVEL SATURATION DEVELOPING MID-LATE
EVENING AS AN UPPER LEVEL DISTURBANCE TRACKS ACROSS THE REGION.
GUIDANCE INDICATES A PERIOD OF LIGHT SNOW BEGINNING 03Z-ISH AND
AND LINGERING PAST MIDNIGHT BEFORE A LULL DEVELOPS AS THE MID-
LEVEL WAVE MOVES AWAY TO THE EAST. ADDITIONAL PERIODS OF LIGHT
SNOW APPEAR LIKELY BEYOND END OF THE CURRENT TAF PERIOD SUNDAY
MORNING- EARLY AFTERNOON...THOUGH THE BEST FORCING/SATURATION
LOOKS TO REMAIN TO THE SOUTH OF THE TERMINALS.

AS FOR WINDS...SURFACE HIGH PASSING SOUTH OF THE TERMINALS EARLY
THIS MORNING RESULTING IN SOUTHWEST WINDS LESS THAN 5 KT BACKING
TO THE SOUTH-SOUTHEAST AFTER SUNRISE...WITH SPEEDS GENERALLY UNDER
10 KT THOUGH THE DAY. LIGHT SOUTHEAST WINDS EXPECTED SATURDAY
EVENING.

RATZER


//ORD AND MDW CONFIDENCE...UPDATED 06Z...

* MEDIUM WITH SNOW TIMING AND CIG/VIS IMPACTS.

* HIGH ALL OTHER ELEMENTS.

RATZER

//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.
&&

$$

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