Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 241950
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
150 PM CST MON NOV 24 2014

.UPDATE...

NO CHANGES TO GOING WINTER WEATHER ADVISORY. DUAL POL HAS SHOWN
PROGRESSION OF RAIN/SNOW LINE NICELY THIS MORNING AS COLD AIR HAS
WRAPPED IN FROM SOUTHWEST. HAVE OBSERVED SLUSHY ACCUMULATION ON
PAVEMENT HERE AT OFFICE AND ALSO RECEIVING SIMILAR REPORTS VIA
SPOTTERS AND SOCIAL MEDIA. AS SECONDARY DEFORMATION AXIS LIFTS
NORTH...CONDITIONS WILL LIKELY IMPROVE FOR SOUTHWEST CHI METRO TO
DOWNTOWN WITHIN NEXT ONE TO TWO HOURS. MEANWHILE...WORST OF
CONDITIONS WILL SETTLE INTO FAR NORTHERN IL...INCLUDING NORTH AND
NORTHWEST CHICAGO METRO AND ROCKFORD AREA. GIVEN VISIBILITY IN
HEAVIEST PORTION OF BAND DOWN AROUND 1/4 MILES AS OF 11AM...WIND
AND SNOW ACCUMS ON ROADWAY WILL RESULT IN HAZARDOUS TRAVEL AS
TEMPERATURES CONTINUE TO FALL TO AND BELOW FREEZING. IT APPEARS
SOME PORTIONS OF NORTHWEST/NORTH CENTRAL IL HAVE ALREADY RECEIVED
ONE INCH OR SO OF ACCUMULATION...SO STILL LOOKING AT ACCUMULATIONS
GENERALLY IN THE 2 TO 4 INCH RANGE IN THE ADVISORY AREA.

STRONG WESTERLY WINDS ARE AN ISSUE AREA WIDE...BUT PARTICULARLY IN
SOUTH/SOUTHEAST CWA WHERE MAX 3 HR PRESSURE RISES ~8MB ARE
FOCUSING. EXPECT WIND GUSTS TO 40 TO 45 MPH THROUGH EARLY TO MID
AFTERNOON. WILL HOLD OFF ON ISSUING AN ADVISORY...AS EXPECTING
ONLY SPORADIC GUSTS TO 45 MPH.

RC

&&

.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 20...

* CIGS REMAINING LOW END MVFR.

* MVFR VSBYS WITH LGT SNOW...LIFTING TO VFR AS SNOW BEGINS TO
  DIMINISH TO FLURRIES.

* WEST WINDS 20-24KT WITH GUSTS 30-35KT.

BEACHLER

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

RAIN QUICKLY CHANGED OVER TO ALL SNOW LATE THIS MORNING...WITH A
FEW BANDS OF MODERATE SNOW REDUCING VSBYS TO 1/2SM TEMPORARILY AT
ORD/MDW/RFD. BASED ON RADAR TRENDS THE HEAVIEST PRECIP WAS QUICKLY
LIFTING NORTHEAST OF ORD/MDW...AND SHUD ALLOW VSBYS TO RETURN TO
ABOVE 1SM BY 19Z. CIGS WILL REMAIN ARND IFR CONDS...BUT SHUD
IMPROVE TO LOW END MVFR CONDS SHORTLY AFT THE HEAVIER SNOW ENDS.
THEN MVFR CIGS WILL LIKELY PERSIST THRU THIS EVE UNTIL MUCH DRIER
AIR ARRIVES AND CIGS BEGIN TO SCATTER OUT TO BKN DECK
OVERNIGHT/EARLY TUE. WINDS WILL CONTINUE TO BE GUSTY FROM THE
WEST. SUSTAINED WINDS HAVE REMAINED ARND 20-24KT WITH GUSTS
APPROACHING 34-38KT AT TIMES.

AS THE PRESSURE BEGINS TO INCREASE TONIGHT...WINDS WILL STEADILY
DIMINISH BUT REMAIN WESTERLY THROUGH THE OVERNIGHT HOURS. CURRENT
INDICATIONS ARE THAT SCATTERED FLURRIES WILL CONTINUE UNTIL ARND
MIDNIGHT.

BEACHLER

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

* HIGH CONFIDENCE IN THE VSBYS STAYING AT OR ABOVE 3SM.

* MEDIUM CONFIDENCE IN TIMING OF CIGS LIFTING FROM LOW END MVFR TO
 VFR CONDS OVERNIGHT.

* HIGH CONFIDENCE IN THE STEADIER HEAVIER SNOW ENDING BEFORE
  20Z...WITH MEDIUM/HIGH CONFIDENCE IN THE ACCUMULATING SNOW
  ENDING BY 23Z.

* LOW CONFIDENCE IN PRECISE ENDING TIME OF LGT SNOW/FLURRIES ARND
  6Z.

* HIGH CONFIDENCE IN WIND DIR/SPEED/GUSTS FORECAST. MEDIUM/HIGH
  CONFIDENCE IN TIMING OF GUSTS ENDING.

BEACHLER

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

TUESDAY NIGHT...VFR.

WEDNESDAY...SCHC SNOW. OTHERWISE VFR. SOUTHEAST TO EAST WINDS.

THURSDAY...A FEW FLURRIES POSSIBLE...OTHERWISE VFR. BREEZY NORTHWEST WINDS.

FRIDAY...SCHC SNOW. SSW WINDS.

SATURDAY...VFR. WEST-SOUTHWEST WINDS.

SUNDAY...VFR. NORTH WINDS.

KMD

&&

.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 UNTIL 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 UNTIL 3 AM TUESDAY.

&&

$$

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