Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 131921
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
221 PM CDT SUN APR 13 2014

.DISCUSSION...
1153 AM CDT

FOR MORNING UPDATE...

HAVE MADE SOME ADJUSTMENTS TO FORECAST THROUGH THIS AFTERNOON BASED
ON OBS/RADAR TRENDS THIS MORNING. MOST NOTABLY...COLD FRONTAL
BOUNDARY HAS SETTLED ACROSS FAR NORTHERN/NORTHEAST IL ESPECIALLY
NEAR LAKE MICHIGAN IN THE WAKE OF WEAK LOW PRESSURE WAVE MOVING OUT
OF LOWER MICHIGAN. WINDS VEERING TO NORTHEAST OFF THE CHILLY LAKE
WATERS WERE DROPPING TEMPS INTO THE MID-40S ALONG THE SHORE AND
PRODUCING AREAS OF FOG WITH VISIBILITY DROPPING BELOW A MILE. HAVE
THEREFORE ADJUSTED MAX TEMPS DOWN ACROSS FAR NORTHEAST IL AND HAVE
TRIED TO CAPTURE THE EXPECTED STEADY/COOLING TREND IN THE HOURLY
TEMP FORECAST ALONG THE LAKE THROUGH THIS AFTERNOON.

OTHERWISE...HAVE ALSO BEEN MONITORING AREA OF RAIN/EMBEDDED THUNDER
ACROSS A GOOD PORTION OF IA/MO AND WESTERN IL. AFTER EARLY
SUBSIDENCE IN THE WAKE OF EARLY MORNING SHORT WAVE AND MCS...THIS
AREA OF PRECIP SHOULD CONTINUE TO SPREAD SLOWLY NORTHEAST ACROSS THE
CWA THIS AFTERNOON BRINGING RAIN BACK TO THE AREA. MID-LEVEL LAPSE
RATES ARE A LITTLE MUSHY...THOUGH EMBEDDED THUNDER EXPECTED AT TIMES
ESPECIALLY LATER THIS AFTERNOON/TONIGHT AS LARGE SCALE FORCING
BECOMES STRONGER WITH APPROACH OF MID-LEVEL SHORT WAVE FROM THE
OK/KS PLAINS REGION. PERIODS OF RAIN/THUNDER AND POTENTIALLY SOME
HEAVY RAINFALL STILL LOOKS TO BE ON TRACK FOR TONIGHT AS SFC LOW AND
FRONTAL SYSTEM MOVE THROUGH THE AREA ALONG WITH MID-LEVEL TROUGH.

RATZER

//PREV DISCUSSION...
417 AM CDT

TODAY...
EARLY THIS MORNING LOW PRESSURE LIES OVER IOWA WITH A WARM FRONT
EXTENDING EAST ACROSS FAR NORTHERN ILLINOIS. THE BOUNDARY WAS DRIVEN
SOUTH OVERNIGHT BY CONVECTION/OUTFLOW. ON THE WARM SIDE OF THE
FRONT...TEMPERATURES REMAINED IN THE 60S OVERNIGHT...BUT NORTH OF
THE FRONT TEMPS QUICKLY FALL OFF INTO THE 40S AND THIS TIGHT THERMAL
GRADIENT WILL HOVER ACROSS NORTHERN COUNTIES OF THE STATE THIS
MORNING. AS FOR CONVECTION THIS MORNING...THUNDERSTORMS HAVE MOVED
EAST OVER LAKE MICHIGAN THIS MORNING WITH STABLE CONDITIONS IN THEIR
WAKE AND EXPECT QUIET CONDITIONS TO PERSIST THROUGH THE REMAINDER OF
THE MORNING AND EARLY AFTERNOON. COUPLED JET STILL PROGGED TO
DEVELOP THIS AFTERNOON WITH MODERATE TO STRONG UPPER LEVEL
DIVERGENCE RESULTING IN CYCLOGENENSIS ALONG THE MID MISSISSIPPI
VALLEY. ISENTROPIC ASCENT WILL INCREASE THROUGH THE LATE MORNING
INTO THE AFTERNOON WITH PRECIP OVERSPREADING NORTHERN ILLINOIS FROM
THE WEST. SOUNDINGS STILL INDICATE DEEP SATURATED LAYER WITH PWATS
PUSHING TO AROUND 1.4-1.5 INCHES. MODELS IN DECENT AGREEMENT ON
PLACEMENT OF AXIS OF HEAVIEST RAIN THIS EVENING AND TONIGHT...
STRETCHING FROM EASTERN IOWA TO EAST-CENTRAL WISCONSIN AND CLIPPING
NORTH CENTRAL ILLINOIS INCLUDING THE ROCKFORD AREA. HEAVY PRECIP
REMAINS A CONCERN THOUGH PRECIP SO FAR HAS UNDERPERFORMED FROM PRIOR
EXPECTATIONS AND A PROLONGED PERIOD OF DRY WEATHER OVER NORTH
CENTRAL ILLINOIS THE PAST SEVERAL WEEKS WILL HELP MITIGATE THE FLOOD
THREAT TO SOME DEGREE.

MONDAY...
LONGWAVE TROUGH AXIS WILL TRANSLATE EAST ACROSS THE PLAINS MONDAY
REACHING THE MISSISSIPPI VALLEY BY THE EVENING. MEANWHILE...COLD
ADVECTION INTO THE AREA WILL DRIVE H85 TEMPS DOWN TO AROUND -11C AND
SEVERAL SMALLER SCALE DISTURBANCES LIFTING UP THE LEE SIDE OF THE
TROUGH WILL RESULT IN RAIN CHANGING OVER TO SNOW ACROSS THE AREA.
MODELS HAVE TRENDED MORE BULLISH WITH QPF MONDAY AND CANNOT RULE OUT
LIGHT ACCUMULATIONS AT THIS TIME...MAINLY DURING THE LATE AFTERNOON
INTO THE EVENING WHEN THE AREA IS BETTER ENTRENCHED IN THE COLD AIR.
HIGHS MAY STRUGGLE TO REACH INTO THE LOW 40S FOR MUCH OF THE CWA AND
A GUSTY NORTH WIND WILL MAKE IT FEEL CLOSER TO THE FREEZING MARK.

TUESDAY THROUGH SATURDAY...
GENERALLY QUIET WEATHER EXPECTED TUESDAY AND WEDNESDAY UNDER THE
INFLUENCE OF HIGH PRESSURE. BY WEDNESDAY...LOW PRESSURE OVER THE
PLAINS WILL HELP SPREAD WARM AIR BACK INTO THE MIDWEST WITH A
WARMING TREND THROUGH THURSDAY WHEN TEMPERATURE SHOULD BE CLOSER TO
NORMAL. MORE ACTIVE WEATHER MAY RETURN LATE IN THE WEEK INTO THE
WEEKEND AS ANOTHER SOUTHWEST LOW LIFTS ACROSS THE MIDWEST BRINGING
CHANCES FOR PRECIP AND FLUCTUATING TEMPERATURES.

BMD

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 18Z...

* EAST WINDS OF 10 TO 15 KT THROUGH THE EARLY TO MID EVENING.

* IFR CIGS ARRIVING THIS HOUR AND VSBY EXPECTED TO LOWER TO IFR BY
  19Z...THOUGH TIMING COULD BE DELAYED.

* PERIODS OF SHRA/TSRA THIS AFTERNOON AND EVENING...HEAVIEST COULD PRODUCE
  IFR VSBY.

* WINDS TURN NORTH AND GUST TO 25-30 KT LATE TONIGHT INTO MONDAY MORNING.

* RAIN POSSIBLY MIXING WITH OR CHANGING TO SNOW ON MONDAY AND
  ASSOCIATED CIG/VSBY IMPACTS.

RC

//DISCUSSION...UPDATED 18Z...

MULTIFACETED AVIATION CONCERNS THROUGH THE PERIOD. DUE TO WEAKNESS
IN PRESSURE PATTERN OVER THE TERMINALS...A LAKE ENHANCED FRONT IS
SURGING SOUTHWESTWARD AND HAS PUSHED THROUGH ORD/MDW AND GYY WITH
A WIND SHIFT TO EAST AND NORTHEAST. WARM MOIST AIR ALOFT ABOVE THE
DENSE MOIST MARINE LAYER HAS RESULTED IN IFR CIGS AND LOWERING
VSBY...WITH DENSE FOG ALONG THE LAKEFRONT. EXPECTING IFR VSBY TO
REACH ORD/MDW BY AROUND 19Z...THOUGH CONFIDENCE IS STILL ONLY
MEDIUM IN EXACT TIMING AND OCCURRENCE BASED ON UPSTREAM TRENDS AT
PWK. A WAVE OF SHRA AND EMBEDDED TSRA ALSO CURRENTLY LIFTING
NORTHEAST...WITH ADDITIONAL WAVES OF SHRA AND TSRA LATE THIS
AFTERNOON AND EVENING AND AT LEAST OCNL IFR VSBY IN THE HEAVIER
RAIN. AS LOW PRESSURE OVER THE SOUTHERN PLAINS LIFTS
NORTHEAST...WINDS WILL PICK UP OUT OF THE SOUTH-SOUTHWEST SOUTH OF
THE FRONTAL BOUNDARY...BUT THE CONVECTION IS LIKELY TO KEEP WINDS
OUT OF THE EAST AT ORD/MDW/DPA...POSSIBLY GOING BACK TO SOUTH AT
GYY AROUND 00Z. WINDS ARE EXPECTED TO SHIFT BACK TO GUSTY SOUTH-
SOUTHWEST AT ORD/MDW AND DPA BY MID TO LATE EVENING...WHICH COULD
TEMPORARILY LIFT CIGS BACK ABOVE IFR...THOUGH THIS IS NOT
CURRENTLY INDICATED IN TAF.

MUCH COLDER AIR WILL SURGE INTO THE AREA ON GUSTY NORTHERLY WINDS
LATE TONIGHT WHILE PERIODS OF RA/SHRA CONTINUE AS WELL AS IFR CIGS AND
VSBY REDUCTIONS. THE RAIN MAY LIGHTEN IN INTENSITY AND CIGS LOWER
BY MID MORNING MONDAY...THOUGH NORTHERLY WIND GUSTS COULD STILL BE
AS HIGH AS 30 KT ESPECIALLY FOR THE EASTERN TERMINALS. THE NEXT
CONCERN IS FOR ANOTHER UPPER LEVEL DISTURBANCE MOVING IN FROM THE
SOUTHWEST TOWARD MIDDAY MONDAY AS EVEN COLDER AIR ARRIVES WHICH
COULD RESULT IN A RAIN/SNOW MIX OR A CHANGE TO SNOW. UNLESS PRECIP
RATES ARE HEAVY AND THERE IS AN ACTUAL CHANGEOVER TO ALL SNOW AT
THE TERMINALS...WOULD EXPECT IMPACTS TO PRIMARILY BE TO VSBY AND
POSSIBLY LOWER CIGS THAN CURRENTLY INDICATED. FOR NOW HAVE A PROB
30 RASN MENTION IN THE ORD AND RFD TAFS...WITH SUBSEQUENT UPDATES
LIKELY TO ADD SOME BETTER DETAIL TO THIS POTENTIAL ON MONDAY.
WINDS ARE EXPECTED TO REMAIN CLOSE TO DUE NORTHERLY AND GUSTY
THROUGH THE AFTERNOON...THOUGH A SLOW DIMINISHING TREND IN SPEEDS
AND GUST MAGNITUDES IS PROBABLE.

RC

//ORD AND MDW CONFIDENCE...UPDATED 18Z...

* HIGH CONFIDENCE IN EAST WINDS THROUGH EARLY TO MID EVENING...MEDIUM
  CONFIDENCE IN TIMING OF SHIFT BACK TO SOUTH-SOUTHWEST THIS
  EVENING.

* HIGH CONFIDENCE IN IFR CIGS MOVING IN BY 19Z.

* MEDIUM IN VSBY LOWERING TO MVFR OR IFR IN BR...MEDIUM IN
  TIMING...AND MEDIUM HIGH IN POTENTIAL FOR IFR VSBY IN HEAVIEST
  SHRA/TSRA.

* MEDIUM CONFIDENCE IN OVERALL SHRA/TSRA TRENDS AND TIMING THIS
  AFTERNOON AND EVENING...WITH HIGH CONFIDENCE THERE WILL BE AT
  LEAST OCNL TSRA AT THE TERMINALS THIS AFTERNOON AND EVENING.

* HIGH CONFIDENCE IN SHIFT TO GUSTY NORTHERLY WINDS LATE
  TONIGHT...MEDIUM IN TIMING.

* LOW CONFIDENCE IN POTENTIAL FOR RASN MIX OR CHANGE TO SN AND
  ASSOCIATED VSBY/CIG IMPACTS MONDAY AFTERNOON.

RC

//OUTLOOK FOR ORD/MDW FOR 00Z TUESDAY-12Z SUNDAY...UPDATED 12Z...

MONDAY NIGHT...-RASN PSBLY BCMG -SN AFTER MIDNIGHT. MVFR TO IFR
CIGS IMPROVING TO VFR.

TUESDAY...VFR.

WEDNESDAY...VFR.

THURSDAY...CHANCE OF RAIN OVERNIGHT...MVFR/IFR CIGS PSBL OVERNIGHT.

FRIDAY...CHANCE OF RAIN...MVFR TO IFR CIGS PSBL.

SATURDAY...CHANCE OF RAIN.

JEE

&&

.MARINE...
218 PM CDT

A FRONTAL BOUNDARY IN CURRENTLY IN PLACE ACROSS THE SOUTHERN TIP OF
LAKE MICHIGAN EARLY THIS AFTERNOON. LAKE INFLUENCES HAVE ALLOWED THE
WINDS TO SHIFT ONSHORE ACROSS THE INDIANA AND ILLINOIS NEAR
SHORES...WITH DENSE FOG ALSO RESULTING DUE TO THE HIGH DEW POINT AIR
ACROSS THE SOUTHERN END OF THE LAKE IN THE VICINITY OF THE FRONTAL
BOUNDARY.

WINDS WILL SHIFT BACK OUT OF THE SOUTH ACROSS FAR SOUTHERN TIP OF
THE LAKE EARLY THIS EVENING...AND THIS SHOULD IMPACT MAINLY THE
INDIANA NEAR SHORE WATERS. THIS WILL HAPPEN IN RESPONSE TO THE
APPROACHING SURFACE LOW...NOW IN PLACE ACROSS KANSAS AND OKLAHOMA.
SO...ALTHOUGH WINDS ARE CURRENTLY LIGHT OUT OF THE NORTH ACROSS THE
INDIANA NEAR SHORES...EXPECT THAT TO CHANGE EARLY THIS
EVENING...WITH WINDS LIKELY TO BECOME GUSTY FOR A PERIOD UP TO 30 KT
OUT OF THE SOUTH. WE CURRENTLY HAVE A SMALL CRAFT ADVISORY IN PLACE
TO COVER THESE STRONGER SOUTHERLY WINDS...HOWEVER...I DID PUSH OFF
THE START TIME OF THE SMALL CRAFT ADVISORY A FEW HOURS UNTIL THIS
EVENING WHEN THE WINDS ARE EXPECTED TO SHIFT.

AREAS OF FOG WILL LIKELY CONTINUE TO IMPACT SOUTHERN PORTIONS OF
THE LAKE THROUGH MUCH OF TONIGHT AS THE FRONTAL BOUNDARY CONTINUES
TO MEANDER ACROSS THIS AREA. WITH THIS IN MIND I WILL CONTINUE THE
DENSE FOG ADVISORY THROUGH LATE TONIGHT.

OTHERWISE...EXPECT WINDS TO INCREASE OUT OF THE NORTH...FIRST ACROSS
THE NORTHERN 1/2 OR SO OF THE LAKE THIS EVENING...WITH NORTHERLY
GALES LIKELY DEVELOPING BY LATE THIS EVENING. THESE NORTHERLY GALES
WILL THEN OVERSPREAD THE REST OF THE LAKE BY DAYBREAK MONDAY.
OVERALL...IT STILL APPEARS THAT THE MAGNITUDES OF THE WINDS WILL BE
UP AROUND 40 KT...WITH A FEW SHORT PERIODS OF SOME SLIGHTLY HIGHER
WINDS POSSIBLE TOWARDS DAYBREAK MONDAY. THESE GALES WILL GRADUALLY
EASE TO AROUND 30 KT BY EARLY MONDAY EVENING AND MONDAY NIGHT.

A PERIOD OF LIGHTER WINDS ARE EXPECTED TUESDAY INTO EARLY WEDNESDAY
AS A SURFACE RIDGE OF HIGH PRESSURE MOVES OVER THE LAKE.
HOWEVER...THE WINDS ARE EXPECTED TO INCREASE AGAIN OUT OF THE
SOUTHEAST BY THE MIDDLE OF NEXT WEEK AS OUR NEXT AREA OF LOW
PRESSURE BEGINS TO MOVE ACROSS THE MIDWEST.

KJB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...DENSE FOG ADVISORY...INDIANA NEARSHORE WATERS UNTIL 4 PM SUNDAY.

     SMALL CRAFT ADVISORY...INDIANA NEARSHORE WATERS UNTIL 4 AM MONDAY.

     GALE WARNING...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745...4 AM
     MONDAY TO 4 PM MONDAY.

     DENSE FOG ADVISORY...ILLINOIS NEARSHORE WATERS UNTIL 4 AM MONDAY.

     GALE WARNING...LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565-LMZ567-
     LMZ669-LMZ671-LMZ673-LMZ868-LMZ870-LMZ872-LMZ874...10 PM
     SUNDAY TO 3 PM MONDAY.

     GALE WARNING...LMZ080-LMZ675-LMZ777-LMZ779-LMZ876-LMZ878...3 AM
     MONDAY TO 3 PM MONDAY.

&&

$$

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