Area Forecast Discussion
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000
FXUS63 KLOT 240919
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
319 AM CST WED DEC 24 2014

.SHORT TERM...
317 AM CST

TODAY AND TONIGHT...

WE MADE SOME CHANGES TO THE WINTER HEADLINES FOR THE EVENT TODAY.
FIRST...WITH THE LIKELIHOOD OF THE SYSTEM BEING FARTHER EAST THAN
ORIGINALLY THOUGHT...IT APPEARS THAT THE WESTERN PORTIONS OF THE
ADVISORY CAN BE DROPPED. THE WINTER STORM WATCH IS NO LONGER IN
EFFECT. INSTEAD...WE HAVE OPTED TO GO WITH A WINTER WX ADVISORY
BASICALLY ALONG AND EAST OF A LINE FROM CHICAGO TO NEAR PAXTON.
COUNTIES WEST OF THIS LINE HAVE BEEN DROPPED FROM THE ADVISORY.

THIS STORM SYSTEM HAS BEEN A SIGNIFICANT FORECAST CHALLENGE OVER THE
PAST SEVERAL DAYS TO SAY THE LEAST. YESTERDAY EVENINGS RAOBS AND
AREA VWP DATA INDICATE THAT A 140 KT SOUTHWESTERLY JET HAS DEVELOPED
ACROSS ILLINOIS IN RESPONSE TO SHARPENING TROUGH...WHICH IS DIGGING
ALL THE WAY DOWN INTO CENTRAL AND SOUTHERN TEXAS. WATER VAPOR IMAGERY
INDICATES THAT THE MAIN BACH OF ENERGY IS BEGINNING TO SHIFT
NORTHEASTWARD ACROSS WESTERN ARKANSAS...AND AS SUCH...THE SURFACE
ANALYSIS IS SHOWING AN ELONGATED AREA OF LOW PRESSURE NOW
STRETCHING FROM THE CENTRAL GULF COAST TO THE OHIO VALLEY. THE BEST
PRESSURE FALLS ARE OCCURRING ACROSS WESTERN OHIO...WHERE THE
INVERTED SURFACE TROUGH CURRENTLY RESIDES. THIS CURRENT SURFACE
SETUP CERTAINLY ADDS CREDIBILITY TO THE LATEST MODEL GUIDANCE
SHIFTING THE SURFACE LOW FARTHER EAST TOWARDS TOLEDO OHIO BY EARLY
THIS EVENING. THEREFORE...THE POTENTIAL AREA FOR HEAVY SNOWFALL
LOOKS TO BE SHIFTED EASTWARD...MORE OVER NORTHWESTERN INDIANA.

RAINFALL IS ALREADY BEGINNING TO SPREAD NORTHWARD OVER THE
AREA...THEREFORE IT LOOKS LIKE RAIN WILL BE THE PRIMARILY
PRECIPITATION TYPE FOR THE MORNING...ESPECIALLY CONSIDERING SURFACE
WET BULB TEMPERATURES ARE WELL ABOVE FREEZING ACROSS THE AREA. THE
MAIN WINDOW FOR A POTENTIAL OF HEAVY WET SNOW LOOKS TO BE FROM EARLY
THIS AFTERNOON THROUGH EARLY THIS EVENING...PRIMARILY ACROSS THE
SOUTHEASTERN 1/3 OR SO OF MY CWA. THE STORM SYSTEM STILL WILL HAVE
VERY IMPRESSIVE DYNAMICS ASSOCIATED WITH IT AS THIS LEAD IMPULSE OF
ENERGY QUICKLY SHIFTS NORTHEASTWARD THIS AFTERNOON. AS A
RESULT...THE SURFACE LOW WILL DEEPEN SIGNIFICANTLY TO NEAR 988 MB BY
EARLY THIS EVENING...THEN SUB 980 MB BY MIDNIGHT TONIGHT. A WELL
DEFINED TROWAL WILL DEVELOP THIS AFTERNOON...AND THIS WILL SET THE
STAGE FOR A BAND OF STRONG FGEN WITHIN THE DEFORMATION ZONE OF THIS
LOW. THE COMBINATION OF STRONG Q-VECTOR CONVERGENCE WITH THE
NEGATIVELY TILTED TROUGH...AND FGEN SHOULD PROVIDE FOR SOME STRONG
DYNAMIC FORCING FOR ASCENT. COMBINE THIS WITH THE PRESENCE OF VERY
STEEP MID LEVEL LAPSE RATES AND THIS APPEARS TO BE THE RECIPE FOR A
BAND OF HEAVY PRECIPITATION THIS AFTERNOON. THE MAIN QUESTIONS IS
WILL THIS BE ENOUGH TO OVERCOME THE WARM BOUNDARY LAYER
TEMPERATENESS. I STILL THIS THE ANSWER TO THIS IS YES....ESPECIALLY
UNDER THE HEAVIEST PRECIPITATION AREAS. AS SUCH...AS THE UPPER
SYSTEM MOVES OVER THE AREA THIS AFTERNOON...THIS WOULD BE THE
PRIMARY TIME FOR A CHANGE OVER TO HEAVY SNOW.

IT APPEARS THAT MANY AREAS ACROSS NORTHERN ILLINOIS MAY SEE LITTLE
IF ANY SNOW OUT OF THIS SYSTEM...AND THIS COULD POSSIBLY BE THE CASE
INTO PORTIONS OF THE CHICAGO METRO AREA AS WELL. THE BEST CHANCES
FOR SNOW NOW LOOK TO BE ACROSS NORTHWESTERN INDIANA AND ADJACENT
AREAS OF EASTERN ILLINOIS. IT APPEARS THAT UNDER SOME OF THE
HEAVIEST SNOW...SHOULD IT MATERIALIZE...UP TO 4 INCHES OF WEST SLUSHY
SNOW IS POSSIBLE. THEREFORE...THE ADVISORY HAS BEEN DROPPED FOR THE
WESTERN AREAS...BUT REMAINS FOR THE SOUTHEASTERN HALF OF THE CWA. IF
TRENDS CONTINUE TO SHIFT THIS BETTER DEFORMATION ZONE
EAST...ADDITIONAL COUNTIES MY BE ABLE TO BE CHOPPED OUT OF THE
ADVISORY. THE MAIN IMPACT FROM THIS SNOW WILL BE THE HEAVY NATURE OF
IT...WITH SNOW AMOUNTS POSSIBLY UP TO AN INCH AN HOUR. GIVEN THIS IS
BUSY TRAVEL DAY...I FELT IT PRUDENT TO KEEP AN ADVISORY FOR MY
EASTERN AREAS...EVEN IF TOTAL SNOW AMOUNTS REMAIN ON THE LOW SIDE.

PRECIPITATION WILL COME TO AN END FROM WEST TO EAST THIS EVENING AS
THE LOW QUICKLY RACES NORTHWARD TOWARDS SOUTHEASTERN ONTARIO.

KJB

&&

.LONG TERM...
317 AM CST

THURSDAY/CHRISTMAS DAY THROUGH TUESDAY...

CHRISTMAS MORNING SHOULD SEE THE DEEPENING STORM SYSTEM MOVING WELL
NORTHEAST OF THE REGION INTO QUEBEC. BROAD UPPER RIDGING DEVELOPS
ACROSS THE MIDWEST IN ITS WAKE...WITH THE SURFACE RIDGE AXIS PASSING
QUICKLY ACROSS THE FORECAST AREA DURING THE DAY. SUBSIDENCE SHOULD
HELP SCATTER OUT CLOUDS FOR A WHILE...ALLOWING AT LEAST SOME PARTIAL
SUNSHINE DURING THE MIDDAY AND AFTERNOON HOURS BEFORE MID AND HIGH
CLOUDS SPREAD BACK ACROSS THE AREA WITH A QUICK RETURN OF WARM
ADVECTION AND ASCENT ASSOCIATED WITH UPPER JET STREAK LIFTING
NORTHEAST AHEAD OF THE NEXT UPSTREAM UPPER TROUGH DEVELOPING OVER
THE WESTERN CONUS. BREEZY SOUTH-SOUTHWEST WINDS WILL HELP PUSH TEMPS
BACK INTO THE 40-45 DEGREE RANGE DURING THE AFTERNOON...AS
DEVELOPING SURFACE LOW OVER THE HIGH PLAINS OF COLORADO AND
SOUTHWEST KANSAS TIGHTENS THE PRESSURE GRADIENT DOWNSTREAM INTO THE
MIDWEST. SOUTH WINDS CONTINUE THURSDAY NIGHT AND HELP KEEP TEMPS
NEAR OR SLIGHTLY ABOVE FREEZING AS A WEAK SURFACE LOW LIFTS
NORTHEAST FROM THE PLAINS TO THE UPPER MISSISSIPPI VALLEY. THIS
PATTERN HOLDS INTO FRIDAY AS WELL WITH MID 40S TO NEAR 50 DEGREE
WARMTH...BEFORE SURFACE TROUGH AND COLD FRONT EVENTUALLY DRIFTS INTO
THE AREA WITH SOME WEAK MID-LEVEL SHORT WAVE ENERGY ARRIVING FRIDAY
NIGHT. GUIDANCE IS IN GENERAL AGREEMENT IN TRYING TO DEVELOP SOME
RELATIVELY LIGHT PRECIPITATION ACROSS THE AREA FRIDAY NIGHT AS A
WARM FRONT LIFTS NORTHEAST AHEAD OF THE SURFACE WAVE...AND LARGE
SCALE LIFT INCREASES IN ASSOCIATION WITH THE SHORT WAVE AND
INCREASING LOW-LEVEL MOIST CONVERGENCE AHEAD OF/ALONG THE SURFACE
COLD FRONTAL TROUGH AND WEAK SFC WAVE. FORECAST SOUNDINGS IN THE
WARM SECTOR AHEAD OF THE FRONT DEPICT THAT MOISTURE IS FAIRLY
SHALLOW...WITH GFS/NAM DEPICTING CLOUD TOPS 800-700 MB AND GENERALLY
WARM ENOUGH TO NOT INCLUDE ICE NUCLEI...WITH SLIGHTLY DEEPER
MOISTURE FARTHER WEST ALONG THE FRONT PERHAPS ABLE TO GENERATE SOME
ICE CRYSTALS AT -5 TO -8 C AND ALLOW FOR SOME WET SNOW FLAKES.
GENERALLY LOOKS LIGHT A DRIZZLE/LIGHT RAIN SCENARIO WITH OVERNIGHT
WITH THE BEST CHANCE FOR A RAIN/SNOW MIX ACROSS NORTH CENTRAL IL
ALONG AND WEST OF THE I-39 CORRIDOR. SURFACE TEMPS LOOK TO REMAIN
ABOVE FREEZING ACROSS MUCH OF THE CWA SO NOT EXPECTED ANY FZRA/FZDZ
PROBLEMS...AND NOT MUCH IN THE WAY OF PRECIPITATION IS NOTED BEHIND
THE COLD FRONT.

SHORT WAVE AND ITS ACCOMPANYING SURFACE LOW LIFT OFF TO THE
NORTHEAST OF THE FORECAST AREA SATURDAY MORNING...WITH LIGHT PRECIP
LINGERING ALONG THE DEPARTING LOW-LEVEL FRONTAL ZONE AT LEAST INTO
THE MORNING SOUTHEAST OF CHICAGO. SOME MODEL DIFFERENCES BEGIN TO
SHOW UP WITH THE SPEED OF THE FRONT BY SATURDAY 12Z/84 HOURS...WITH
THE GEFS/GFS QUICKER TO MOVE THE FRONT THROUGH THE AREA WHILE THE
00Z ECMWF AND CMC ARE SLOWER AND LINGER THE POTENTIAL FOR LIGHT
PRECIP ACROSS THE AREA INTO SATURDAY NIGHT ESPECIALLY ACROSS OUR
SOUTHEAST CWA. LEANING TOWARD THE SLOWER CMC/EC SOLUTIONS...HAVE
MAINTAINED CHANCE POPS FOR RAIN/SNOW SATURDAY AND LIGHT SNOW
SATURDAY NIGHT ACROSS MUCH OF THE CWA. TEMPS COOL BEHIND THE FRONT
SATURDAY...THOUGH STILL ABOVE FREEZING DURING THE DAY ACROSS MOST
AREAS. EVEN IN THE SLOWER INTERNATIONAL MODELS...THE UPPER TROUGH
EVENTUALLY SHEARS OUT TO THE EAST-NORTHEAST BY EARLY SUNDAY...WITH
THE FRONT FINALLY MOVING SOUTHEAST OF THE AREA AND HIGH PRESSURE
SPREADING IN WITH COLDER AIR FROM THE NORTHWEST.

BY THE END OF THE PERIOD...MODELS INDICATE UPPER PATTERN TO BECOME
SOMEWHAT ZONAL ACROSS THE SOUTHERN HALF OF THE CONUS...WITH GENERAL
TROUGHINESS FROM EASTERN CANADA TO THE WESTERN CONTINENTAL U.S. FAST
WEST-SOUTHWEST TO EAST-NORTHEAST JET FEATURES A FEW RIPPLES
PARTICULARLY LATER MONDAY INTO TUESDAY WHICH BRING THE POTENTIAL FOR
SOME LIGHT PRECIPITATION...LIKELY IN THE FORM OF LIGHT SNOW AS
COLDER LOW-LEVEL AIR IS TAPPED BY NORTH-NORTHEAST LOW LEVEL WINDS.
LOW CONFIDENCE IN THESE DETAILS BY DAYS 6-7...BUT OVERALL A TREND
TOWARD COLDER TEMPS AND PERHAPS A PERIOD OR TWO OF LIGHT PRECIP
APPEAR REASONABLE AT THIS DISTANCE FOR EARLY NEXT WEEK.

RATZER

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 09Z...

* RAIN OVERSPREADING THE TERMINALS INTO MID MORNING. CHANGE TO
  RAIN/SNOW WITH INTERMITTENT PERIODS OF ALL SNOW POSSIBLE MIDDAY
  INTO THE AFTERNOON...TAPERING OFF INTO THE EVENING.

* CIGS LOWER TO MVFR BY MID MORNING...THOUGH SPORADIC MVFR IS
  POSSIBLE UNTIL THEN...LOWERING TO IFR/LIFR TOWARD MIDDAY.

* MAINLY MVFR VSBY EXPECTED FROM MID MORNING ONWARD WITH
  INTERMITTENT IFR VSBY POSSIBLE...ESPECIALLY IF ALL SNOW CAN
  OCCUR AT TIMES.

* N/NNE WINDS TURN NORTHWEST TOWARD MIDDAY AND BECOME GUSTY.

MDB

//DISCUSSION...UPDATED 09Z...

NOT MUCH CHANGING IN THINKING THUS FAR OVERNIGHT. RAIN SHOWERS
ARE QUICKLY SPREADING NORTHWARD ACROSS NORTHEAST IL/NORTHWEST IN
WHICH IS ALSO BRINGING SPORADIC MVFR CIGS. BAND OF IFR TO THE WEST
HAS PUSHED INTO RFD AND LOOKS TO REMAIN THERE THROUGH DAYBREAK..IF
NOT LONGER. HAVE LESS CONFIDENCE IN WIND SPEEDS THROUGH THE
MORNING WITH LATEST TRENDS SUPPORTING LOWER SPEEDS AND PERHAPS
EVEN MINIMAL GUSTINESS THIS AFTERNOON. WILL TREND DOWN A FEW KNOTS
FOR NOW.

FROM 06Z...

UPCOMING STORM SYSTEM CONTINUES TO BE TRICKY BUT LATEST OBSERVED
TRENDS AND FORECAST GUIDANCE ARE HELPING TO ESTABLISH AT LEAST
MARGINALLY INCREASED CONFIDENCE IN HOW THINGS EVOLVE. LOW PRESSURE
IS DEVELOPING FROM THE MISSISSIPPI COAST ACROSS CENTRAL TENNESSEE
INTO FAR NORTHWEST OHIO EARLY TONIGHT. LOW PRESSURE IS EXPECTED TO
CONSOLIDATE AND TRACK ACROSS EASTERN INDIANA OR FAR WESTERN OHIO
THROUGH THE DAY WHILE UPPER LEVEL LOW PRESSURE LIFTS NORTHEAST
ACROSS INDIANA. THIS WILL SPREAD RAIN NORTHWARD ACROSS THE
TERMINALS THROUGH THE MORNING. A NARROW CORRIDOR OF VERY STRONG
FORCING IS EXPECTED TO DEVELOP ALONG OR JUST EAST OF THE IL/IN
BORDER INTO MIDDAY AND SHOULD LEAD TO A BAND OF ALL SNOW WHICH MAY
BE VERY INTENSE. PLACEMENT OF THIS BAND IS LOOKING TO BE TO THE
EAST OF EARLIER THINKING WITH ORD AND MDW FORECAST TO BE ON THE
FRINGE OR JUST WEST OF IT...WITH GYY LIKELY TO BE MOST AFFECTED
WITH MODERATE/HEAVY SNOW. HAVE THUS ADJUSTED THE TAFS AT
ORD/MDW/DPA TO REFLECT THIS THINKING AND BRING IMPROVED
CIGS/VSBY...THOUGH IFR/LIFR CIGS ARE STILL EXPECTED. ORD/DPA AND
ESPECIALLY MDW MAY STILL SEE INTERMITTENT PERIODS OF ALL SNOW AND
IFR VSBY BUT THIS COULD BE PESSIMISTIC. WILL CARRY PREVAILING
-RASN FOR THE AFTERNOON WITH TEMPO -SN FOR NOW. IF THE SNOW BAND
DOES DEVELOP THEN MAY NOT BE LOW ENOUGH AT GYY AS 1/4SM +SN IS A
POSSIBILITY. RFD IS MORE OF A CHALLENGE AND MAY END UP SEEING MORE
OF A DRIZZLE SCENARIO FOR A TIME VS. STEADIER LIGHT RAIN AND/OR
SNOW SO ADJUSTMENTS WILL PROBABLY BE NEEDED.

WINDS WILL TURN N/NNE OVERNIGHT AND BEGIN TO INCREASE IN SPEED
LATER IN THE MORNING AS THEY TURN NORTHWESTERLY. EXPECT GUSTS IN
THE 20-25 KT RANGE TO DEVELOP BUT THESE MAY HOLD OFF UNTIL WINDS
TURN NW. IFR CIGS AND MVFR VSBY LOOK TO PREVAIL THROUGH THE
EVENING BUT THERE MAY BE SOME IMPROVEMENT OF CIGS INTO THURSDAY
MORNING.

MDB

//ORD AND MDW CONFIDENCE...UPDATED 09Z...

* HIGH CONFIDENCE IN RAIN OVERSPREADING THE TERMINALS INTO MID
  MORNING. MEDIUM CONFIDENCE IN CHANGE TO RAIN/SNOW AND ASSOCIATED
  TIMING. LOW CONFIDENCE IN PERIODS OF ALL SNOW OCCURRING AND
  TIMING.

* HIGH CONFIDENCE IN CIG TRENDS.

* MEDIUM CONFIDENCE IN MAINLY MVFR VSBY. MEDIUM-HIGH CONFIDENCE
  THAT INTERMITTENT IFR VSBY WOULD OCCUR IF ALL SNOW CAN FALL AT
  TIMES.

* HIGH CONFIDENCE IN WIND DIRECTION TRENDS BUT MEDIUM CONFIDENCE
  IN SPEEDS AND TIMING OF ARRIVAL OF GUSTS.

MDB

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

THURSDAY...MVFR/IFR IMPROVING TO VFR.
FRIDAY...MVFR/IFR POSSIBLE.
SATURDAY THROUGH MONDAY...MVFR POSSIBLE.

BMD

&&

.MARINE...
211 AM CST

A TROUGH OF LOW PRESSURE STRETCHING FROM THE MISSISSIPPI COAST
NORTHWARD INTO NORTHWEST OHIO WILL CONSOLIDATE INTO A QUICKLY
DEEPENING CLOSED LOW WHICH WILL TRACK FROM TENNESSEE TO GEORGIAN BAY
INTO TONIGHT. WEAK HIGH PRESSURE CURRENTLY OVER THE LAKE IS
PROVIDING LIGHT AND VARIABLE WINDS BUT THESE WILL QUICKLY TURN NORTH
TO NORTHEAST THROUGH THE MORNING WITH SPEEDS STEADILY INCREASING.
SPEEDS WILL INTENSIFY MORE RAPIDLY THIS AFTERNOON AS THE LOW TRACKS
ACROSS FAR NORTHWEST OHIO WITH THE DIRECTION TRENDING NORTH AND
NORTHWEST BEFORE TURNING WEST TONIGHT. PEAK SPEEDS LOOK TO OCCUR
THIS EVENING AND TONIGHT. THE SYSTEM IS TRACKING FURTHER TO THE EAST
WHICH WILL LIKELY LEAD TO SLIGHTLY WEAKER WINDS ACROSS THE LAKE BUT
SHOULD STILL SEE MARGINAL GALES SO WILL CONTINUE WITH THE GALE
WARNING FOR THE OPEN WATERS...BUT DELAY THE START UNTIL 21Z ACROSS
THE SOUTHERN PART OF THE LAKE. GALES LOOK MUCH LESS LIKELY ACROSS
THE IL/IN NEARSHORE WATERS SO WILL TRANSITION TO A SMALL CRAFT
ADVISORY.

HIGH PRESSURE WILL BUILD ACROSS THE GULF COAST AND SOUTHEAST
THURSDAY WITH A RIDGE AXIS MOVING ACROSS THE LAKE. THIS WILL TURN
WINDS SOUTHWESTERLY THURSDAY WITH SPEEDS EASING UP INTO THE EVENING.
A WEAK TROUGH AXIS WILL SET UP FROM THE TEXAS PANHANDLE TO LAKE
SUPERIOR INTO FRIDAY WITH THE HIGH STILL TO THE SOUTHEAST ALLOWING
FOR A STEADY SOUTHWEST WIND TO PERSIST ACROSS THE LAKE INTO
SATURDAY. A MORE CONSOLIDATED AREA OF LOW PRESSURE IS THEN EXPECTED
TO CROSS THE WESTERN LAKES OVER THE WEEKEND BRINGING A COLD FRONT
WITH IT AND A SHIFT TO WEST AND NORTHWEST WINDS. HIGH PRESSURE THEN
BUILDS IN EARLY NEXT WEEK.

MDB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...WINTER WEATHER ADVISORY...ILZ014-ILZ022-ILZ023-ILZ032-ILZ033-
     ILZ039...NOON WEDNESDAY TO 9 PM WEDNESDAY.

IN...WINTER WEATHER ADVISORY...INZ001-INZ002-INZ010-INZ011-
     INZ019...NOON WEDNESDAY TO 9 PM WEDNESDAY.

LM...GALE WARNING...LMZ080-LMZ669-LMZ671-LMZ673-LMZ675-LMZ777-LMZ779-
     LMZ870-LMZ872-LMZ874-LMZ876-LMZ878...3 PM WEDNESDAY TO 3 AM
     THURSDAY.

     GALE WARNING...LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565-LMZ567-
     LMZ868...3 PM WEDNESDAY TO 6 AM THURSDAY.

     SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742-LMZ743...NOON
     WEDNESDAY TO 9 AM THURSDAY.

     SMALL CRAFT ADVISORY...LMZ744-LMZ745...NOON WEDNESDAY TO 3 PM
     THURSDAY.

&&

$$

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