Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 212042
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
242 PM 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 20Z...

* WINDS GENERALLY FROM THE SOUTHEAST OR 140-160 BETWEEN 8-11KT
  THRU EARLY THIS EVE.

* FREEZING DRIZZLE INCREASING TO FREEZING RAIN LATE THIS
  EVE...MAINLY AFT 04Z.

* POSSIBLE IFR CIGS DEVELOPING OVERNIGHT THRU DAYBREAK SAT.

* SOUTHWEST WIND SHEAR ARND 2KFT AGL AFT 8Z WITH SPEEDS ARND 40KT.

BEACHLER

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

HIGH PRESSURE WILL CONTINUE TO PROVIDE MINIMAL CLOUDS AND LIGHT
SOUTHEAST WINDS THIS AFTN. AS THE LATE AFTN HOURS
PROGRESS...CLOUDS AND PRECIP WILL BEGIN TO APPROACH FROM THE
SOUTH/SOUTHWEST. VISIBLE SATELLITE IMAGERY SHOWS THE LEADING EDGE
OF THE CLOUDS CREEPING NORTH INTO NORTHCENTRAL MISSOURI/SOUTHWEST
ILLINOIS...AND EXPECT THIS TO ARRIVE ACROSS THE TAF SITES BETWEEN
2-4Z. GUIDANCE HAS BEEN CONSISTENT WITH CIGS QUICKLY LOWERING FROM
VFR TO MVFR. LGT PRECIP IS EXPECTED TO ARRIVE AFT 6Z...AND WITH
TEMPERATURES BELOW FREEZING THIS WILL LIKELY FALL AS FREEZING
DRIZZLE. THE STEADIER PRECIP IS EXPECTED TO ARRIVE ARND 8Z AND
LINGER THRU 12Z...LIKELY FALLING AS FREEZING RAIN. IF TEMPERATURES
CAN WARM QUICKER...THEN PRECIP MAY BEGIN TO TRANSITION OVER TO LGT
RAIN BEFORE DAYBREAK. CIGS SHUD HOVER ARND IFR CONDS TO LOW END
MVFR FOR THE BULK OF THE OVERNIGHT THRU SAT MORNING.

IN ADDITION TO THE FROZEN PRECIP EXPECTED...A CORE OF WINDS AT OR
ARND 2KFT AGL OF NEARLY 40KT WILL ARRIVE. SINCE WE WILL LIKELY
STRUGGLE TO MIX THIS TO THE SFC...HAVE INCLUDED A MENTION OF WS IN
THE CURRENT TAF.

BEACHLER

//ORD AND MDW CONFIDENCE...UPDATED 20Z...

* HIGH CONFIDENCE IN CIGS QUICKLY LOWERING TO MVFR THIS EVENING.

* HIGH CONFIDENCE IN FREEZING DRIZZLE BECOMING FREEZING RAIN
  TONIGHT...MEDIUM CONFIDENCE IN THE DURATION OF FREEZING RAIN.
  HIGH CONFIDENCE THAT FREEZING RAIN WILL REMAIN LIGHT.

* MEDIUM/HIGH CONFIDENCE IN WIND SHEAR FORECAST.

* MEDIUM/HIGH CONFIDENCE IN CIGS LOWERING TO LOW END MVFR...MEDIUM
  CONFIDENCE CIGS DIP TO IFR OVERNIGHT/EARLY SAT.

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...

242 PM...HIGH PRESSURE OVER THE SOUTHERN LAKES AND OHIO VALLEY
WILL MOVE SOUTHEAST TONIGHT REACHING THE CAROLINA COAST ON
SATURDAY AS LOW PRESSURE MOVES ACROSS NORTHERN ONTARIO AND JAMES
BAY TONIGHT. THE GRADIENT BETWEEN THESE TWO SYSTEMS WILL STEADILY
INCREASE OVER LAKE MICHIGAN TONIGHT WITH GALE FORCE WINDS
EXPECTED...POSSIBLY GUSTING AS HIGH AS 45KTS ACROSS THE NORTHERN
PORTION OF THE LAKE. THE GRADIENT WILL BEGIN TO WEAKEN LATER
SATURDAY AFTERNOON INTO SATURDAY EVENING...BUT ANOTHER LOW MOVING
ACROSS THE NORTHERN PLAINS SATURDAY NIGHT INTO SUNDAY WILL TIGHTEN
THE SOUTHERLY GRADIENT AGAIN. THIS LOW WILL BEGIN TO WEAKEN SUNDAY
EVENING AND THEN BE ABSORBED BY ANOTHER STRONGER LOW WHICH WILL
DEVELOP OVER THE SOUTHERN PLAINS SATURDAY NIGHT INTO SUNDAY...THEN
MOVE NORTH ACROSS LAKE MICHIGAN SUNDAY NIGHT INTO MONDAY MORNING.
THIS LOW HAS BEEN FORECAST TO DEEPEN TO AS LOW AS 975MB NEAR LAKE
SUPERIOR FOR SEVERAL DAYS. IF THIS OCCURS...WESTERLY HIGH END
GALES TO 45 KTS WOULD LIKELY DEVELOP MONDAY WITH THE POTENTIAL FOR
SOME STORM FORCE WINDS OR GUSTS. CMS

&&

.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 UNTIL 9 AM SATURDAY.

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

&&

$$

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