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000
FXUS63 KLOT 041703
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1103 AM CST WED DEC 4 2013

.DISCUSSION...
1046 AM CST

MAIN FORECAST CONCERNS FOR THIS MORNING UPDATE ARE WITH THE DENSE
FOG THAT REMAINS IN PLACE...CONTINUING AND EVEN EXPANDING THE
ADVISORY INTO 00Z TONIGHT.

HAVE EXTENDED THE DENSE FOG ADVISORY THRU 00Z TONIGHT AS DENSE FOG
PERSISTS THIS MORNING. AS LOW PRESSURE LIFTS NORTHEAST ACROSS THE
REGION...THE CWA CONTINUES TO OBSERVE STRONG MOISTURE AND TEMP
ADVECTION. THIS IS RESULTING IN WELL ABOVE NORMAL TEMPS...AND AN
ABUNDANCE OF MOISTURE ACROSS A GOOD PORTION OF THE REGION. INITIAL
THOUGHTS WERE TO LET THE ADVISORY EXPIRE THIS MORNING WITH
INCREASING DEW POINT DEPRESSIONS. ALTHOUGH WITH SLOWLY RISING
TEMPS THIS MORNING...DEW POINTS HAVE RISEN ALONG WITH THEM. AREAS
LIKELY TO OBSERVE PERSISTENT LOW DEW POINT DEPRESSIONS AND DENSE
FOG WERE FOR AREAS IN THE CURRENT ADVISORY. LATEST TRENDS THIS
MORNING AS WELL AS WEBCAMS SHOWED AREAS JUST OUTSIDE OF THE
ADVISORY FROM LA SALLE NORTHEAST TO COOK DID INDICATE THE MORE
DENSE FOG WAS PERSISTING...AND SO HAVE INCLUDED THEM IN THE
ADVISORY. ALTHOUGH FOR COOK...THE MORE LIKELY AREAS TO OBSERVE
THIS DENSE FOG WOULD BE ACROSS THE EAST AND NORTHEAST SIDES OF THE
COUNTY...WITH VERY MOIST CONDITIONS OVER THE LAKE AND A SOUTHEAST
WIND ALLOWING DENSE FOG TO AFFECT AREAS ALONG THE SHORE. WITH
SIMILAR CONDITIONS TO PERSIST ACROSS THE ENTIRE AREA UNTIL FROPA
AROUND THE 00Z TIME FRAME...FOG SHOULD PERSIST BUT WILL THEN
IMPROVE THIS EVENING WITH DRIER AIR FILTERING IN ON THE BACKSIDE
OF THIS SYSTEM.

RODRIGUEZ

&&

.PREV DISCUSSION...
406 AM CST

FORECAST CONCERNS THIS MORNING FOCUS ON DENSE FOG TRENDS THIS
MORNING...AND TRENDS INTO THIS AFTERNOON WITH MILD TEMPS UNTIL COLD
FROPA LATE AFTERNOON/EVENING. COLDER AIR COMES IN TWO PUSHES...
FIRST BEHIND THE FRONT TONIGHT AND THEN A SECOND PUSH FRIDAY
NIGHT-SATURDAY AS ARCTIC HIGH BUILDS MORE SOLIDLY EAST ACROSS THE
GREAT LAKES REGION. COUPLE OF CHANCES OF SOME LIGHT ACCUMULATING
SNOW IN THE COMING DAYS...INITIALLY ACROSS FAR SOUTHEAST CWA LATE
THURSDAY INTO FRIDAY...AND AND ACROSS THE CWA SUNDAY-SUNDAY NIGHT.
EVEN COLDER PUSH OF ARCTIC AIR APPEARS HEADED FOR THE AREA BY THE
MIDDLE OF NEXT WEEK.

VERY MILD AND MOIST AIR MASS IN PLACE ACROSS THE REGION EARLY THIS
MORNING...WITH BOTH SFC TEMPS AND DEW POINTS IN THE 40S WITH VERY
SMALL DEPRESSIONS. DENSE FOG WITH VISIBILITY LESS THAN 1/2SM
CONTINUES ACROSS THE FAR NORTHERN IL COUNTIES ROUGHLY NORTH OF A
STERLING TO DEKALB AND WAUKEGAN LINE AS OF 330 AM CST...WITH A
SMALLER AREA OF DENSE FOG OVER EAST CENTRAL IL. HIGH-RES GUIDANCE
INCLUDING HOPWRF AND NARRE TIME-LAGGED ENSEMBLES INDICATE THERE MAY
BE SOME IMPROVEMENT IN VISIBILITY ACROSS THE NORTHERN COUNTIES THIS
MORNING...WHILE SOME ENSEMBLE MEMBERS EXPAND DENSE FOG ACROSS
CENTRAL IL PRIOR TO ABOUT 15Z AND THEN DECREASE COVERAGE. OBS AT
PNT-IKK AND RZL HAVE NOT SHOWN A SIGNIFICANT DOWNWARD TREND AT THIS
TIME. WILL LEAVE ONGOING DENSE FOG ADVISORY ACROSS THE NORTH AS
IS...AND WILL CONTINUE TO MONITOR TRENDS ACROSS THE SOUTH FOR THE
POSSIBILITY OF NEEDING A HEADLINE THERE. WHILE VISIBILITY MAY
IMPROVE SOMEWHAT...AREAS OF FOG WILL LIKELY STICK AROUND INTO THE
AFTERNOON BEFORE AN INCREASE IN SCATTERED SHOWER COVERAGE...WARMER
DAYTIME TEMPERATURES AND THE EVENTUAL COLD FRONTAL PASSAGE CLEAR
THINGS OUT THIS EVENING.

WHILE COLDER MORE TYPICAL EARLY WINTER WEATHER WILL SPREAD INTO THE
AREA TONIGHT/THURSDAY...THE DEPARTING COLD FRONT WILL SLOW AS IT
BECOMES PARALLEL WITH THE MID/UPPER FLOW AND MOVES INTO THE EASTERN
LAKES AND OHIO VALLEY OVER THE NEXT 24-36 HOURS. THIS WILL LIMIT THE
EXTENT OF THE COLD AIR INTRUSION BEHIND THE FRONT...AND WILL LEAVE
THE ELEVATED FRONTAL ZONE ACROSS CENTRAL IL AND NORTHERN INDIANA
INTO FRIDAY. GUIDANCE REMAINS FAIRLY SIMILAR IN BRINGING TWO
SEPARATE DISTURBANCES ALONG THIS BAROCLINIC ZONE WITH THE FIRST LATE
THURSDAY/THURSDAY NIGHT...AND THE OTHER DURING THE DAY ON FRIDAY.
REASONABLY SIMILAR QPF OUTPUT FROM THE VARIOUS MODELS OF THE
AVAILABLE GUIDANCE INDICATE THE POTENTIAL FOR GENERALLY LESS THAN AN
INCH OF SNOW ACROSS THE FAR SOUTHEAST TIER OF COUNTIES OF THE CWA
BEFORE THE SECOND SHORT WAVE DEPARTS LATE FRIDAY...WITH FLURRIES
PERHAPS AS FAR NORTH AS A PONTIAC-WHITING LINE. AFTER THIS SECOND
SLIGHTLY MORE AMPLIFIED WAVE DEPARTS...ARCTIC HIGH PRESSURE BUILDS
MORE ROBUSTLY EAST ACROSS MINNESOTA AND WISCONSIN FRIDAY NIGHT AND
SETS THE STAGE FOR CHILLY TEMPS IN THE SINGLE DIGITS AWAY FROM THE
CITY FRIDAY NIGHT...AND ONLY IN THE TEENS TO LOW 20S SATURDAY.

LONGER RANGE GUIDANCE ALSO REMAINS IN DECENT AGREEMENT IN BRINGING
ANOTHER SHORT WAVE INTO THE REGION SUNDAY-SUNDAY NIGHT...WITH SOME
SPREAD IN THE MODEL SOLUTIONS LOOKING A LITTLE MORE IN AGREEMENT
WITH THE NEW 00Z ECMWF SIMILAR TO THE GFS IN BRINGING LOW PRESSURE
THROUGH THE OHIO VALLEY DURING THAT TIME. LIGHT SNOW LOOKS TO COVER
MOST/ALL OF THE CWA WITH THIS SYSTEM...AND HAVE BUMPED POPS TO
LIKELY IN MOST AREAS.

ANOTHER SURGE OF ARCTIC AIR THEN LOOKS TO SPREAD INTO THE AREA
BEHIND THIS SYSTEM MONDAY-TUESDAY. BOTH ECMWF/GFS MOS GUIDANCE
SHOWING LOWS IN THE SINGLE DIGITS BY TUESDAY MORNING...WITH HIGHS
ONLY AROUND 12-15 TUESDAY. THIS IS IMPRESSIVE GIVEN STRONG CLIMO
WEIGHTING IN THE MOS BY DAY 7...SOME 22-25 DEGREES BELOW NORMAL FOR
HIGHS FOR EARLY DECEMBER. BRRR.

RATZER

&&

.AVIATION...

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

* CEILINGS...CONTINUED PREVAILING LOW END IFR NTO LATE AFTERNOON-
  EARLY EVENING...THEN SLIGHT IMPROVEMENT TO LOW END MVFR FOR
  REST OF NIGHT.

* VISIBILITY...PREVAILING HIGH END IFR CAT-LOW END MVFR CAT INTO
  EARLY EVENING IMPROVING TO MVFR INTO LATE AFTERNOON-EARLY
  EVENING...THEN VFR MID EVENING AND BEYOND.

* PATCHY -DZ/-RA THROUGH EARLY AFTERNOON THEN BECOMING MORE
  WIDESPREAD FOR MID AFTERNOON TO EARLY EVENING.

* WIND SHIFT THIS EVENING WITH COLD FRONTAL PASSAGE.

TRS

//DISCUSSION...UPDATED 16Z...

SURFACE LOW BECOMING BETTER ORGANIZED JUST NE OF DSM WITH PRES
FALL CENTER OVER SE NM AND W CENTRAL TO NW WI. MODELS TAKE THIS
LOW TO FAR SE MN BY 18Z AND N WI BY 00Z. SE-SSE LOW LEVEL WINDS
ACROSS THE LOCAL AREA ARE ADVECTING IN WARMER BUT ALSO HIGHER DEW
POINT AIR. WITH SURFACE DEW POINT DEPRESSIONS OF 0-1 DEG F
CURRENTLY MOVING OVER THE LOCAL AREA AND EXTENDING WELL UPSTREAM
ACROSS ALL BUT FAR S PORTIONS OF IL AND IN. THIS IS EXPECTED TO
MAINTAIN RESTRICTED VSBY AND LOW CIGS UNTIL LOW REACHES N WI AND
LOCAL SURFACE WIND VEERS A BIT TO S-SSE IN RESPONSE WHICH WILL
DELIVER SOMEWHAT GREATER DEW POINT DEPRESSIONS TO THE LOCAL AREA.

A FEW PATCHES OF -DZ/-RA HAVE RECENTLY MOVED OVER AND N OF ORD AND
MDW. MODELS CONTINUE TO SHOW SOME WEAK BUT PERSISTENT UVV DOWN
INTO THE LOW LEVEL MOISTURE TIL EARLY EVENING. FEW MORE WEAK RADAR
ECHOES MOVING N AND APPROACHING I-80. EXPECT LOW COVERAGE BEFORE
INCREASING MID AFTERNOON.

WITH LOW PROGGED TO MOVE TO OVER/NEAR CENTRAL U.P. OF MI BY 06Z
THE TRAILING COLD FRONT IS EXPECTED TO PUSH W TO E ACROSS THE
LOCAL AREA EARLY-MID EVENING.

TRS

PREVIOUS DISCUSSION...

THIS MORNING...A WARM FRONT LIES JUST NORTH OF THE TERMINALS WITH
SOUTHEAST FLOW AND IFR CONDITIONS IN PLACE ACROSS NORTHERN ILLINOIS
AND NORTHWEST INDIANA. WILL DISCUSS EACH FORECAST ELEMENT
INDIVIDUALLY...

CEILINGS...AS MENTIONED MOST OF THE REGION IS UNDER IFR CIGS THIS
MORNING...THOUGH THERE IS A HOLE IN THE LOWER CIGS THAT STRETCHES
FROM SW MICHIGAN INTO THE SOUTH CHICAGO METRO. GYY HAS MAINTAINED
VFR LEVEL CIGS FOR THE PAST SEVERAL HOURS...AND MDW HAS BEEN IN AND
OUT OF THE IFR CIGS. NAM/HRRR GUIDANCE TO SOME DEGREE HINTING AT
THIS HOLE...WHICH COULD BRIEFLY MOVE OVER ORD THIS MORNING. BOTH
MODELS DO SHOW THE HOLE FILLING IN THOUGH LATE THIS MORNING...SO ANY
TEMPO HIGHER CIGS WILL BE SHORT LIVED THIS MORNING. A SECONDARY WARM
FRONTAL LIKE FEATURE WILL LIFT ACROSS THE AREA THIS AFTERNOON...WITH
WINDS VEERING TO THE S OR SSW. MODELS INDICATE SOME IMPROVEMENT IN
CIGS POSSIBLE AFTER IT PASSES NORTH OF THE TERMINALS...THOUGH THE
DEGREE OF IMPROVEMENT IS UNCERTAIN. IT IS POSSIBLE WE MAY LIFT TO
MVFR AT THIS POINT...BUT WILL ERR ON THE SIDE OF LOWER CIGS FOR NOW
WHICH FITS WELL WITH LAV/MET GUIDANCE. BETTER CHANCE TO LIFT TO MVFR
WILL BE LATER IN THE EVENING AS A COLD FRONT SWEEPS ACROSS THE AREA.
EXPECT THE CIGS TO GRADUALLY RISE OVERNIGHT AS THE COLD AIR ERODES
THE CLOUD BASES.

VISIBILITY...VSBY RANGES FROM NEAR ZERO TO AROUND 4-5SM. DENSEST FOG
IS GENERALLY ALONG AND NEAR THE WARM FRONT AND THIS AREA SHOULD STAY
NORTH OF THE CHICAGO TERMINALS...THOUGH WILL CONTINUE TO IMPACT RFD
THROUGH THE MORNING. MODEL GUIDANCE IS ALL OVER THE PLACE WITH
RESPECT TO VSBY TRENDS TODAY. SEVERAL MODELS HINT THAT AS THE
SECONDARY BOUNDARY SOUTH OF THE AREA LIFTS NORTH...LOWER VSBY WILL
ALSO SPREAD NORTH INTO THE CHICAGO METRO. THERE IS ALREADY DENSE FOG
ACROSS PORTIONS OF EAST CENTRAL ILLINOIS...AND WILL HAVE TO MONITOR
THIS AREA CLOSELY AS IT MAY SPREAD NORTH OVER THE TERMINALS...BUT
FOR NOW GIVEN THE LOW CONFIDENCE...WILL STICK CLOSE TO MAV/LAV
TRENDS WHICH HOLD VSBY PRIMARILY MVFR FOR THE CHICAGO TERMINALS
TODAY. ANY PRECIP THAT DEVELOPS THIS AFTERNOON MAY ALSO CAUSE THE
VSBY TO DROP TO IFR.

WINDS...PRIMARILY SOUTHEASTERLY WINDS 6-10KTS ARE EXPECTED THIS
MORNING BEHIND THE FRONT NEAR THE STATE LINE THIS MORNING. WINDS
WILL EVENTUALLY VEER TO THE SOUTH/SSW AS THE SECOND BOUNDARY LIFTS
ACROSS THE AREA MID TO LATE THIS AFTERNOON. LATE THIS EVENING A COLD
FRONT WILL SWEEP ACROSS THE AREA WITH WINDS TURNING WESTERLY...AND
INCREASING WITH GUSTS INTO THE HIGH TEENS EXPECTED OVERNIGHT.

BMD

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

*MEDIUM CONFIDENCE IN CIG TRENDS.

*MEDIUM CONFIDENCE IN VSBY TRENDS.

*HIGH CONFIDENCE IN WINDS INTO MID EVENING...THEN MODERATE
 CONFIDENCE REST OF PERIOD.

*MEDIUM CONFIDENCE IN -SHRA/-RA OCCURRENCE THIS AFTERNOON.

TRS

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

THURSDAY NIGHT THROUGH SATURDAY...VFR.
SUNDAY...CHC SNOW...WITH SNOW LIKELY SUNDAY NIGHT.
MONDAY...SLT CHC SNOW. MVFR POSSIBLE.
TUESDAY...PRIMARILY VFR.

BMD

&&

.MARINE...
239 AM CST

MODELS ARE FINALLY SYNCED UP ON DEEPENING LOW PRESSURE WHICH IS
EXPECTED TO LIFT ACROSS WISCONSIN LATE THIS AFTERNOON...AND ACROSS
LATE SUPERIOR TONIGHT. MODERATE EASTERLY FLOW AROUND 30 KTS IS
EXPECTED NORTH OF A WARM FRONT STRADDLING THE MID SECTION OF THE
LAKE THIS MORNING...WITH STABLE SOUTHEASTERLY FLOW ACROSS THE SOUTH
OF THE FRONT. MID/UPPER 40 DEWPOINTS ADVECTING OVER THE LOW/MID 40
DEGREE WATER TEMPERATURES IS EXPECTED TO RESULT IN WIDESPREAD DENSE
FOG. A DENSE FOG ADVISORY IS IN EFFECT ACROSS THE SOUTH HALF OF THE
LAKE...BUT MAY NEED TO BE EXTENDED NORTH AS THE FRONT LIFTS ACROSS
THE REMAINDER OF THE LAKE. OVERNIGHT A COLD FRONT WILL SWEEP ACROSS
THE LAKE WITH WINDS TURNING WESTERLY BEHIND. STRONG COLD ADVECTION
WILL HELP TO MIX DOWN GALES...PRIMARILY ACROSS THE NORTH END OF THE
LAKE WHERE THE GRADIENT IS TIGHTEST...THOUGH APPEARS FOR NOW TO BE
LOW END GALE EVENT. A GALE WATCH REMAINS IN EFFECT FOR THE NORTH
PORTIONS OF THE LAKE. A STRONG HIGH WILL SETTLE ACROSS THE NORTHERN
PLAINS FRIDAY THOUGH LATE SATURDAY RESULTING IN WINDS STAYING
ELEVATED IN THE 20 TO 25 KT RANGE...WITH A FEW PERIODS OF 30 KTS.
ANOTHER LOW IS EXPECTED TO LIFT FROM THE LOWER MISSISSIPPI VALLEY
INTO THE EASTERN LAKES REGION EARLY NEXT WEEK.

BMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...DENSE FOG ADVISORY...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     ILZ011-ILZ012-ILZ013-ILZ014-ILZ019-ILZ020 UNTIL 6 PM
     WEDNESDAY.

IN...NONE.
LM...DENSE FOG ADVISORY...LMZ080-LMZ669-LMZ671-LMZ673-LMZ675-LMZ740-
     LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-LMZ779-LMZ870-
     LMZ872-LMZ874-LMZ876-LMZ878 UNTIL 3 PM WEDNESDAY.

     GALE WATCH...LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565-LMZ567-
     LMZ868...3 AM THURSDAY TO 3 PM THURSDAY.

     SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742-LMZ743...11 PM
     WEDNESDAY TO 3 PM THURSDAY.

     SMALL CRAFT ADVISORY...LMZ744-LMZ745...1 AM THURSDAY TO 4 AM
     FRIDAY.

&&

$$

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