Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 211614
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1014 AM CST FRI NOV 21 2014

.SHORT TERM...
311 AM CST

THROUGH TONIGHT...

PRIMARY FORECAST CHALLENGE IS WITH FREEZING RAIN EVENT TONIGHT.
SURFACE HIGH WILL MOVE EAST OF THE REGION TODAY ALLOWING FOR
SOUTHERLY WINDS TO DEVELOP TODAY AND BEGIN THE SLOW PROCESS OF
DISPLACING THE ARCTIC AIR THAT HAS BEEN ENTRENCHED OVER THE AREA.
COMPARING THE WRF-NAM MODEL TO VISIBLE SATELLITE SUGGESTS THERE IS
LESS SNOW COVER THAN INITIALIZED IN THE NAM AND HAVE GENERALLY
TRENDED TEMPS TODAY AND TONIGHT A BIT TOWARD THE WARMER GUIDANCE
WITH HIGHS MOSTLY IN THE LOWER 30S.

CLOUDS RAPIDLY INCREASE TONIGHT AND ONLY ALLOW FOR A MODEST DROP IN
TEMPS AT SUNSET...BUT STILL ANTICIPATE EVERYONE TO DROP INTO THE MID
TO UPPER 20S THIS EVENING. A STRONG LOW LEVEL JET IS PROGGED TO
DEVELOP TONIGHT IN ADVANCE OF A LEAD SHORTWAVE TROUGH JUST NOW
EMERGING OUT ONTO THE CENTRAL HIGH PLAINS. THE STRONG LOW LEVEL JET
WILL RESULT IN THE RAPID NORTHWARD TRANSPORT OF HIGHER THETA-E AIR
MASS ABOVE THE SURFACE WHILE SOME MODEST COOLING OF THE MID LEVELS
RESULTING IN STEEPENING LOW/MID LEVEL LAPSE RATES ABOVE THE LOW
LEVEL INVERSION. SREF PLUMES SHOW SOME WEAK MUCAPE VALUES SO WITH OR
WITHOUT LIGHTING EXPECT PRECIP TO BE CONVECTIVE IN NATURE TONIGHT
WITH SOMEWHAT VARIABLE AMOUNTS. SOME OF THE CONVECTIVE ALLOWING
MODELS CRANK OUT ALMOST A QUARTER INCH OF QPF AND WITH EXPECTED
CONVECTIVE NATURE OF QPF WOULD NOT BE SURPRISED TO SEE ISOLATED
AREAS SEE UP TO TWO TENTH OF AN INCH OF ICE ACCUMULATION WITH THE
HEAVIEST SHOWERS. SUNSHINE THURSDAY ALLOWED PAVEMENT SURFACE TEMPS
TO WARM INTO THE MID-UPPER 30S AND SUNSHINE TODAY SHOULD ALLOW FOR
SIMILAR READINGS BUT AFTER SUNSET THEY SHOULD FALL BELOW FREEZING
AGAIN FAIRLY QUICKLY...SO ANTICIPATE TRAVEL PROBLEMS WITH ANY
PRECIP. THE AMOUNT OF FREEZING RAIN NEEDED TO CAUSE MAJOR TRAVEL
PROBLEMS IS MINIMAL AND HAVE OPTED TO HOIST A FREEZING RAIN ADVISORY
FOR TONIGHT TO COVER THIS THREAT.

IZZI

&&

.LONG TERM...
311 AM CST

IN THE WAKE OF THE LEAD SHORTWAVE LOOK FOR A CONTINUED CHANCE OF
SOME PERIODIC LIGHT RAIN OR DRIZZLE SATURDAY INTO SATURDAY NIGHT
BEFORE THE MAIN SOUTHERN STREAM SHORTWAVE LIFTS NORTHEASTWARD INTO
THE REGION LATER THIS WEEKEND PRODUCING MORE WIDESPREAD AND HEAVIER
RAIN VERY LATE SAT NIGHT THROUGH EARLY SUNDAY EVENING. HIGH PWATS
CERTAINLY FAVOR SOME RESPECTABLE RAINFALL TOTALS...POTENTIALLY IN
EXCESS OF AN INCH. THE LONG DURATION AND MOSTLY LIGHT-MODERATE
NATURE OF THE RAIN SHOULDN`T RESULT IN ANY BIG FLOODING PROBLEMS
BEYOND PERHAPS SOME NUISANCE STANDING WATER IN THE TYPICAL LOW
SPOTS.

SYSTEM IS FORECAST TO PHASE WITH AMPLIFYING NORTHERN STREAM TROUGH
OVER THE UPPER GREAT LAKES SUNDAY NIGHT INTO MONDAY RESULTING IN SFC
LOW DEEPENING TO POTENTIALLY SUB-980MB. DESPITE THE VERY DEEP NATURE
OF THE SFC LOW THE LATEST GUIDANCE IS NOT AS STRONG WITH THE HIGH IN
ITS WAKE SO WHILE BREEZY AND COLDER WITH SLOWLY FALLING TEMPS MONDAY
LATEST MODELS SUGGEST WINDS WON`T GET TOO OUT OF HAND. SOME SPORADIC
LIGHT SNOW SHOWERS OR FLURRIES ARE POSSIBLE MONDAY AFTERNOON THROUGH
TUESDAY IN THE COLD MOIST CYCLONIC FLOW AROUND THE UPPER TROUGH.
WHILE MUCH MORE TEMPERED THAN THE RECENT RECORD COLD...STILL LOOKING
AT A HEALTHY SHOT OF ARCTIC AIR WITH 850MB TEMPS AROUND 2 SIGMA
BELOW AVG. LOCAL 850MB/925MB CLIMO SUPPORTS HIGHS IN THE 20S AGAIN
BY TUESDAY.

ANOTHER NORTHWEST FLOW SHORTWAVE IS PROGGED TO DIVE DOWN THE
BACKSIDE OF THE MEAN EASTERN NOAM TROUGH WEDNESDAY WITH CLIPPER
SYSTEM FORECAST TO DIVE SOUTHEAST TOWARD THE REGION. MEDIUM RANGE
MODELS HAVE SHOWN SOME RUN TO RUN CONSISTENCY IN DROPPING SOME
SYSTEM INTO THE AREA WEDNESDAY WITH A CHANCE OF SOME LIGHT
ACCUMULATING SNOW SOMEWHERE IN THE MIDWEST. A LOT COULD CHANGE
BETWEEN NOW AND THEN BUT DID COORDINATE A MODEST INCREASE IN POPS
WITH NEIGHBORING OFFICES FOR WEDNESDAY GIVEN THE POTENTIAL HIGH
IMPACT OF ANY SNOW ON A BUSY TRAVEL DAY. FOR GOOD MEASURE MEDIUM
RANGE MODELS FOLLOW THAT CLIPPER UP WITH MORE ARCTIC AIR OF COURSE
IN TIME FOR THANKSGIVING.

IZZI

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 16Z...

* CROSS WINDS THIS AFTERNOON...NOT OVERLY STRONG.

* FREEZING RAIN DEVELOPING OVERNIGHT.

* POSSIBLE WIND SHEAR DEVELOPING AFTER 8Z ARND 2KFT AGL AND SPEEDS
  OF 40KT FROM THE SOUTHWEST.

LENNING/BEACHLER

//DISCUSSION...UPDATED 12Z...

HIGH PRESSURE MOVING OFF TO THE SOUTHEAST WILL ALLOW WINDS TO TURN
SOUTH THIS AFTERNOON AND INCREASE IN SPEED. THESE SOUTH WINDS WILL
BRING INCREASED MOISTURE TO THE AREA OVERNIGHT ALONG WITH SLOWLY
RISING TEMPERATURES...BUT TEMPERATURES ARE LIKELY TO REMAIN BELOW
FREEZING FOR AT LEAST A SHORT PERIOD AFTER THE RAIN BEGINS. NOT
HIGHLY CONFIDENT ABOUT TIMING AND LOCATION OF FZRA IN THE MDW/ORD
AREAS...SO ONLY INCLUDED PROB30 FOR NOW. HIGHER CONFIDENCE TOWARD
RFD...SO WENT WITH TEMPO THERE. EXPECT CHANGE TO RAIN EARLY SATURDAY
BUT MAYBE NOT BEFORE START OF THE MORNING RUSH.

LENNING

//ORD AND MDW CONFIDENCE...UPDATED 16Z...

* HIGH CONFIDENCE IN WIND...CEILING...VSBY TRENDS.

* MEDIUM CONFIDENCE IN FZRA TIMING AND LOCATION OVERNIGHT.

* HIGH CONFIDENCE IN WIND SHEAR DEVELOPING AFT 8Z.

LENNING/BEACHLER

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

SATURDAY NIGHT...RAIN LIKELY WITH DENSE FOG AND IFR POSSIBLE
OVERNIGHT. SOUTH TO SOUTHWEST WINDS 10-15 KT.

SUNDAY...RAIN. HEAVY AT TIMES. IFR LIKELY. GUSTY S TO SE WINDS.

MONDAY...CHANCE -RASN EARLY AND -SN LATER. MVFR CIGS POSSIBLE.
GUSTY W TO SW WINDS.

TUESDAY...CHANCE OF FLURRIES. MVFR CIGS PSBL. GUSTY WEST WINDS.

WEDNESDAY...LIGHT SNOW POSSIBLE. MVFR CIGS/VIS POSSIBLE. WEST TO
SOUTHWEST WINDS 10 KT OR LESS.

THANKSGIVING DAY...VFR LIKELY.

LENNING

&&

.MARINE...
254 AM CST

IN THE TIGHT GRADIENT BETWEEN A DEEPENING LOW TO OUR NORTH AND
HIGH PRESSURE TO OUR SOUTHEAST...WINDS START WEST TODAY THEN
BACK SOUTH BY TONIGHT WHILE INCREASING TO STRONG GALES. THE NORTH
END OF THE LAKE LIKELY WILL SEE STRONGER AND EARLIER GALES THAN
THE SOUTH END...THUS THE TWO DIFFERENT START TIMES ON THE
WARNINGS. FOR SIMPLICITY THEY BOTH END AT THE SAME TIME SATURDAY
MORNING BUT LIKELY THE SOUTH WILL SEE GALES DIMINISH SOONER.

THE NEXT LOW PRESSURE SYSTEMS LIFT THROUGH ON MONDAY AND LIKELY
BRING ANOTHER PERIOD OF WEST GALES THAT AFTERNOON AND EVENING.

LENNING

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...FREEZING RAIN ADVISORY...ILZ019-ILZ021-ILZ023-ILZ032-ILZ033-
     ILZ039...10 PM FRIDAY TO 6 AM SATURDAY.

     FREEZING RAIN ADVISORY...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-
     ILZ010-ILZ011-ILZ012-ILZ013-ILZ014-ILZ020-ILZ022...11 PM
     FRIDAY TO 9 AM SATURDAY.

IN...FREEZING RAIN ADVISORY...INZ001-INZ002...11 PM FRIDAY TO 9 AM
     SATURDAY.

     FREEZING RAIN ADVISORY...INZ010-INZ011-INZ019...10 PM FRIDAY TO
     6 AM SATURDAY.

LM...GALE WARNING...LMZ080-LMZ671-LMZ673-LMZ675-LMZ777-LMZ779-LMZ872-
     LMZ874-LMZ876-LMZ878...9 PM FRIDAY TO 9 AM SATURDAY.

     GALE WARNING...LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565-LMZ567-
     LMZ669-LMZ868-LMZ870...3 PM FRIDAY TO 9 AM SATURDAY.

     SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...10 PM FRIDAY TO 3 PM SATURDAY.

&&

$$

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