Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 240913
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
313 AM CST MON NOV 24 2014

.SHORT TERM...
313 AM CST

THROUGH TONIGHT...

COMPLEX STORM SYSTEM WILL CONTINUE TO INFLUENCE OUR WEATHER THROUGH
TONIGHT WITH A TRANSITION FROM MILD AND RAINY TOWARD COLD AND
POTENTIALLY SNOWY LIKELY TO TAKE PLACE LATER THIS MORNING.

DEEPENING SURFACE LOW FROM CENTRAL LAKE MICHIGAN EAST INTO LOWER
MICHIGAN WILL CONTINUE TO LIFT NORTHEAST TODAY AND DEEPEN FURTHER.
COLD FRONT TRAILING THIS BROAD LOW IS CROSSING THE MISSISSIPPI RIVER
NOW AND WILL STEADILY MARCH EAST ACROSS OUR CWA THIS MORNING
RESULTING IN STEADILY FALLING TEMPERATURES FROM THE POINT OF FROPA
THROUGH TONIGHT.

THE PRIMARY DEFORMATION ZONE WITH THIS SYSTEM IS MOVING OUT OF THE
AREA AT THIS TIME...HOWEVER THERE ARE A COUPLE ADDITIONAL VORTICITY
MAXIMA UPSTREAM. THE FIRST WELL DEFINED VORT IS NEAR STL WITH A SLUG
OF RAIN AND EVEN A COUPLE OF ISOLATED THUNDERSTORMS. THIS VORT IS
PROGGED TO SHEAR ENE CENTRAL ILLINOIS INTO NORTHERN OHIO TODAY. A
SECOND VORT MAX IS LOCATED UPSTREAM OVER KANSAS AND PIVOTING AROUND
THE BASE OF THE TROUGH AND EVENTUALLY PHASE WITH THE SHEARING OUT
VORT OVER INDIANA AND OHIO THIS AFTERNOON. AS THIS PROCESS TAKES
PLACE...THE RAIN ASSOCIATED WITH VORT ONE WILL SPREAD NORTHEAST
ACROSS OUR CWA THIS MORNING. AS THE SECOND VORT BEGINS TO PHASE WITH
THE FIRST A ZONE OF MID-LEVEL FRONTOGENESIS WILL DEVELOP ACROSS THE
LOWER LAKES THIS AFTERNOON AND ALLOW PRECIP WITH FIRST VORT TO
MAINTAIN ITSELF AND EVEN DEVELOP UPSTREAM (WESTWARD) INTO FAR
NORTHERN ILLINOIS AND ESPECIALLY INTO SOUTHERN WISCONSIN.

GUIDANCE HAS ALL SHIFTED NORTHWARD WITH THIS PROCESS COMPARED WITH
EARLIER RUNS AND ITS DIFFICULT TO FIND ANY MODEL THAT STILL SUPPORTS
THE 3-5" FORECAST TOTALS WE HAVE GOING IN THE ADVISORY AREA. AM VERY
LEARY ABOUT OVER CORRECTING THINGS SO DID A LOT OF FUZZY MATH AND
FUDGING OF GRIDS TO MAINTAIN AMOUNTS IN THE 2-5" RANGE IN THE
ADVISORY AREA...THOUGH SUSPECT MAJORITY OF THE ADVISORY AREA WILL
END UP WITH TOTALS ON THE BOTTOM END OF THE RANGE. SOUTH OF THE
ADVISORY AREA STILL THINK THERE COULD BE A CHANGEOVER TO SNOW BEFORE
THE DEFORMATION AXIS SHIFTS NORTHWARD AND PRECIP TAPER OFF THIS
AFTERNOON. IF CHANGE OVER ISNT FAST ENOUGH THEN COULD END UP WITH
JUST A TRACE OF SNOW...THOUGH FASTER CHANGE OVER AND BRIEF HEAVIER
INTENSITY COULD SUPPORT AMOUNTS AROUND AN INCH. BEFORE THE ENTIRE
CHANNELED VORT MAX TRANSLATES NORTHEAST OUT OF THE AREA COULD SEE
WESTERN EXTENT OF THE AXIS PIVOT BACK SOUTHWARD A BIT THIS AFTERNOON
INTO OUR NORTHWEST CWA...AND THIS WOULD BE NECESSARY TO GET FORECAST
AMOUNTS.

STRONG WESTERLY WINDS DEVELOP THIS MORNING BEHIND THE COLD FRONT AND
WILL CONTINUE INTO THIS EVENING. THE WINDS WILL WHIP AROUND ANY
FALLING SNOW LEADING TO GREATER REDUCTIONS IN VSBY. IN ADDITION...AS
TEMPS CONTINUE TO FALL FARTHER BELOW FREEZING LATER THIS AFTERNOON
SOME OF THE SNOW ON THE GROUND COULD GET BLOWN AROUND AS SNOW LIKELY
BECOMES MORE POWDERY IN NATURE.

IZZI

&&

.LONG TERM...
313 AM CST

TUESDAY ONWARD...

EXPANSIVE STRATUS DECK LIKELY TO BECOME ESTABLISHED OVER THE AREA
TUESDAY AND WHILE LOW LEVEL FLOW WILL BEGIN TO TRANSITION TO
ANTICYCLONIC TUESDAY AFTERNOON AHEAD OF THE NEXT CLIPPER...NOT SURE
HOW MUCH CLEARING THERE WILL BE AS DRY AIR ADVECTION WILL BE WEAK
WITH DIMINISHING FLOW. IN ADDITION...SUBSIDENCE WILL FURTHER SHARPEN
THE INVERSION CLOUDS ARE TRAPPED BENEATH SO HAVE INCREASED SKY COVER
TUESDAY AND NO LONGER SHOW ANY SUBSTANTIAL CLEARING BEFORE THE MID
WEEK CLIPPER COMES ZIPPING DOWN. GIVEN THE ANTICIPATED CLOUD COVER
HAVE GONE WITH SMALLER DIURNAL TEMP SWINGS WITH CLOUDS PROBABLY
KEEPING TEMPS UP A SMIDGE OVER GUIDANCE AT NIGHT AND A BIT LOWER
DURING THE DAY.

SPEAKING OF THE MID WEEK CLIPPER...GUIDANCE REMAINS INCONSISTENT RUN
TO RUN AND MODEL TO MODEL. THERE HAS BEEN A DISTINCT SOUTHWARD SHIFT
IN GUIDANCE WITH THIS SYSTEM WHICH IF IT WERE TO PAN OUT WOULD PLACE
THE ACCUMULATING SNOW OVER MAINLY OUR SOUTHERN CWA AND POTENTIALLY
BYPASS NORTHERN CWA. THIS CLIPPER IS ORIGINATING AS A PIECE OF
ENERGY BREAKING OFF AN UPPER LOW NEAR THE NORTHERN ALEUTIANS AND
SEEING AS IT HADNT EVEN BROKEN OFF THE LOW AND BEEN WELL SAMPLED YET
AM NOT READY TO THROW MY EGGS IN THE MORE SOUTHERLY TRACK BASKET
JUST YET. BY 12Z WEDNESDAY THE GFS ENSEMBLES HAVE SURFACE LOW
POSITIONS RANGING FROM WESTERN WISCONSIN TO NORTHWEST ARKANSAS...SO
NEEDLESS TO SAY FURTHER CHANGES ARE LIKELY THE NEXT SEVERAL GUIDANCE
CYCLES WITH THIS SYSTEM AND IT IS TOO SOON TO WRITE OFF THE THREAT
OF LIGHT ACCUMULATING SNOW ANYWHERE IN OUR CWA.

ANOTHER INTERESTING TWIST IN THE LATEST MEDIUM RANGE GUIDANCE IS
LESS OF A COLD INTRUSION IN THE WAKE OF THIS CLIPPER FOLLOWED BY A
RAPID TRANSITION TO ZONAL FLOW BY FRIDAY AND INTO THE WEEKEND.
GFS...GEM...AND ECMWF ALL TRACK A SFC LOW OF VARYING STRENGTHS
ACROSS THE UPPER GREAT LAKES SATURDAY PLACING OUR AREA IN WHAT WOULD
BE A VERY MILD SOUTHERLY FLOW. THE 00Z ECMWF THERMAL FIELDS WOULD
SUPPORT HIGHS WELL INTO THE 50S BY SATURDAY...BUT OBVIOUSLY DIDN`T
MAKE THAT DRASTIC OF A CHANGE TO THE GOING FORECAST BUT IF MODELS
REMAIN CONSISTENT WITH THIS LATEST TREND THEN A MILD WEEKEND COULD
BE ON TAP FOR THE REGION.

IZZI

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 08Z...

* IFR CIGS PERSIST THROUGH THE NIGHT AND MAY LOWER TO 300-400 FT
  AT TIMES.

* PERIODS OF LIGHT TO MODERATE RAIN OVERNIGHT...THEN STEADIER RAIN RETURNING
  AFTER DAYBREAK/MID MORNING.

* IFR VSBY LIKELY OVERNIGHT AT TIMES

* SOUTHWEST WINDS TO 10 KT SHIFT WEST MONDAY MORNING THEN SOUTHWEST
  WITH GUSTS TO 30 KT WITH A PERIOD OF 35 KT GUSTS POSSIBLE
  MIDDAY/EARLY AFTERNOON.

* RAIN CHANGES TO SNOW MIDDAY/EARLY AFTERNOON...SOME LIGHT
  ACCUMULATION POSSIBLE...ESPECIALLY LATER IN THE DAY.

* IFR CIGS LIFTING TO MVFR LATE THIS AFTERNOON

KMD

//DISCUSSION...UPDATED 08Z...

VSBYS ARE TEMPORARILY RISING AHEAD OF THE NEXT BATCH OF RAIN BUT
EXPECT ADDITIONAL PERIODS OF IFR VSBYS AS THIS MOVES NORTHWARD.
OTHERWISE IFR CIGS CONTINUE THROUGH THE NIGHT...OCCASIONALLY
DROPPING TO 200-300 FEET ANYWHERE IT IS CURRENTLY NOT THAT LOW.

PREVIOUS DISCUSSION FROM 06Z...

BAND OF LIGHT/OCCASIONALLY MODERATE RAIN IS LIFTING NORTHWEST
THROUGH THE TERMINALS EARLY THIS MORNING WITH WIDEPREAD IFR CIGS
AND LARGELY IFR/LIFR VSBYS WITH A SMALL CORRIDOR IN THE SOUTHWEST
FROM KIIK TO KMDW SOUTHEASTWARD. EXPECT VSBYS TO DROP ACROSS THE
AREA AS THE FIRST ROUND OF RAIN DISSIPATES SOME AND LIGHTER WINDS
CONTINUE.

THE NEXT BATCH OF PRECIPIATION IS FORECAST TO MOVE THROUGH JUST
AFTER DAYBREAK WITH INITIAL PRECIPITATION TYPE AS -RA. EXPECT CIGS
TO REMAIN IFR BUT PROBABLY LIFT SOME AS WEST WINDS BEGIN TO PICK
UP WITH LOW PRESSURE MOVING NORTHEASTWARD WITH -RASN MIX AROUND
15Z AT KRFD AND A FEW HOURS LATER IN THE CHICAGO AREA...TURNING TO
ALL -SN. MOISTURE WIL BE DECREASING FROM SOUTH TO NORTH AS THE
TRANSITION FROM -RA TO -SN OCCURS. THE HIGHEST LIKELIHOOD FOR
ACCUMS IS WEST OF THE MAIN CHICAGO TERMINALS TOWARD ROCKFORD...BUT
SOME ACCUMS ARE CERTAINLY STILL PLAUSIBLE EVERYWHERE. LOOKS LIKE A
MIX OF SNOW SHOWERS OR POSSIBLY MORE FLURRIES OVERNIGHT WITH AN
IFR TRANSITION TO MVFR.

KMD

//ORD AND MDW CONFIDENCE...UPDATED 08Z...

* HIGH IN IFR CONTINUING WITH SOME LOWERING OVERNIGHT.

* HIGH CONFIDENCE IN STEADIER RAIN DIMINISHING TO DRIZZLE
  OVERNIGHT WITH STEADIER RAIN RETURNING AFTER DAYBREAK/MID
  MORNING.

* MEDIUM-HIGH CONFIDENCE IN VSBY FALLING TO IFR OVERNIGHT AT
  TIMES. LOW- MEDIUM CONFIDENCE IN POTENTIAL TO GO BELOW 1SM.

* HIGH CONFIDENCE IN WIND TRENDS TONIGHT BUT MEDIUM CONFIDENCE IN
  DIRECTION/SPEED DETAILS.

* HIGH CONFIDENCE IN WIND FORECAST MONDAY AND 30+ KT GUSTS. MEDIUM
  CONFIDENCE IN SPORADICALLY REACHING 35 KT OR SO.

* HIGH CONFIDENCE IN RAIN CHANGING TO SNOW BUT LOW CONFIDENCE IN
  HOW MUCH PRECIP COVERAGE THERE WILL BE THROUGH THE
  AFTERNOON/EVENING.

* HIGH CONFIDENCE ON CIG TRENDS...LOW-MEDIUM ON WHEN IFR LIFTS TO
  MVFR AND HOW LONG MVFR LASTS.

KMD

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

TUESDAY...MVFR CIGS POSSIBLE WITH A FEW FLURRIES IN THE MORNING.
WEST WINDS.

WEDNESDAY...CHANCE OF SNOW. MVFR/IFR PSBL. SSE WINDS TURNING WEST.

THURSDAY...SCHC SNOW...OTHERWISE VFR. BREEZY NORTHWEST WINDS.

FRIDAY...SCHC SNOW. SSW WINDS.

SATURDAY...VFR. WEST WINDS.

MDB

&&

.MARINE...
154 AM CST

DEEPENING LOW PRESSURE IS PASSING OVER THE LAKE EARLY THIS MORNING
BRINGING A LULL IN WINDS AND WAVES. THE LOW WILL CONTINUE NORTHEAST
INTO SOUTHWEST QUEBEC LATER THIS AFTERNOON THEN REMAIN STEADY AS IT
PUSHES OFFSHORE OVER NOVA SCOTIA. A STRONG SOUTHWEST TO NORTHEAST
ORIENTED PRESSURE GRADIENT WILL REACH THE SOUTH TIP OF THE LAKE BY
15Z...THEN SPREAD OVER THE REMAINDER OF THE LAKE IN THE AFTERNOON.
GALES LOOK SOLID FOR THE SOUTH HALF OF THE THE LAKE...BUT DUE TO A
WEAKENING PRESSURE GRADIENT WITH TIME TODAY AND A MODEST HIGH
BUILDING INTO THE CANADIAN PRARIES AND UPPER MIDWEST...FEEL THAT
WINDS WILL LIKELY PEAK JUST UNDER GALES FOR THE NORTH HALF. A FEW
GALE FORCE GUSTS ARE POSSIBLE...ESPECIALLY LATE AFTERNOON/EARLY
EVENING.

WEAK HIGH PRESSSURE EXTENDING FROM THE NORTH TIP OF LAKE SUPERIOR TO
THE OHIO VALLEY PASSES OVER THE LAKE TUESDAY AFTERNOON. ANOTHER
WEAKENING LOW EMERGING FROM THE CANADIAN ROCKIES WILL PASS JUST
SOUTH OF THE TIP OF LAKE MICHIGAN WITH THE REMNANT INVERTED TROUGH
CLEARING THE LAKE BY THURSDAY MORNING. THIS WILL BE FOLLOWED BY WEAK
HIGH PRESSSURE FOR A FEW DAYS.

KMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...WINTER WEATHER ADVISORY...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-
     ILZ010-ILZ011-ILZ012...9 AM MONDAY TO MIDNIGHT TUESDAY.

IN...NONE.
LM...GALE WARNING...LMZ080-LMZ669-LMZ671-LMZ673-LMZ675-LMZ740-LMZ741-
     LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-LMZ779-LMZ870-LMZ872-
     LMZ874-LMZ876-LMZ878...9 AM MONDAY TO 3 AM TUESDAY.

&&

$$

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