Area Forecast Discussion
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FXUS63 KLOT 242023
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
223 PM CST WED DEC 24 2014

.UPDATE...
1024 AM CST

HAVE CANCELED THE WINTER WEATHER ADVISORY FOR THE ENTIRE AREA. IN
A NUTSHELL...THE POTENTIAL PITFALLS WITH THIS SYSTEM IN A VERY
MARGINAL THERMAL SETUP ALL CAME TO FRUITION. ITS LIKELY THAT THE
PLENTIFUL CONVECTION NEAR THE GULF COAST YESTERDAY INTO LAST NIGHT
PLAYED A ROLE IN A FARTHER EAST LOW PRESSURE TRACK THAN
EXPECTED...AS WELL AS DISRUPTING THE MORE RAPID INSENSIFICATION OF
THE UPPER LEVEL AND SURFACE CIRCULATION. THIS IS KEY BECAUSE THE
DRIVER OF WHAT WAS ANTICIPATED TO BE A FLIP TO WET SNOW WAS
DYNAMIC COOLING IN AN INTENSE FGEN DRIVEN DEFORMATION BAND TO
OVERCOME THE MILD LOW LEVEL TEMPERATURES. GIVEN SURFACE
TEMPERATURES WHERE IT IS ACTUALLY RAINING NOW IN THE UPPER 30S TO
LOWER 40S...EVEN A GRADUALLY COOLING COLUMN WILL AT BEST
EVENTUALLY SUPPORT A MIX OR MAYBE A BRIEF CHANGE TO SNOW
ESPECIALLY TOWARD THE FAR EAST CWA LATE IN THE DAY. THERE WILL BE
LITTLE IF ANY ACCUMULATIONS AND THUS NO TRAVEL IMPACTS OTHER THAN
WET ROADS SO FELT COMFORTABLE CANCELING THE WINTER WEATHER
ADVISORY.

OUTSIDE OF THIS...THE WEAKER SURFACE LOW KEPT A MUCH BAGGIER
PRESSURE GRADIENT IN PLACE OVER THE AREA AND LIGHTER WINDS. THIS
ALLOWED AREAS OF DENSE FOG TO DEVELOP ACROSS PARTS OF NORTH
CENTRAL IL AND THE FAR WEST/NW CHICAGO BURBS DUE TO LOW DEW POINT
DEPRESSIONS. LATEST TRENDS HAVE BEEN FOR IMPROVING CONDITIONS AS
A STEADY LIGHT NORTHWEST WIND KICKS IN AND SLIGHTLY HIGHER
DEWPOINT DEPRESSIONS. SO LIKELY WILL NOT NEED TO ISSUE ANOTHER SPS
AFTER IT EXPIRES AT 18Z.

RC

&&

.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 THE WINTER STORM WATCH IS NO LONGER IN
EFFECT.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 20Z...

* MVFR CEILINGS LOWERING ONCE AGAIN EARLY THIS EVENING...TO IFR.

* POSSIBLE LIGHT SNOW SHOWERS THROUGH THIS EVENING.

RODRIGUEZ

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

IN THE NEAR TERM...HAVE REMOVED ANY MENTION OF SNOW FOR ALL THE
TERMINALS AS THE CURRENT PRECIP IN PLACE WILL STAY ALL RAIN THIS
AFTERNOON. IT DOES APPEAR THAT THE ONLY TERMINALS TO LIKELY
OBSERVE THIS PRECIP WILL BE MDW AND GYY...BEFORE THIS SLOWLY
SHIFTS TO THE EAST LATER THIS AFTERNOON. AS AN UPPER LEVEL SYSTEM
PROGRESSES THROUGH...SCATTERED LIGHT SNOW SHOWERS WILL ONCE AGAIN
BE POSSIBLE BRIEFLY LATER THIS EVENING. CURRENT MVFR CEILINGS
OVERHEAD WILL REMAIN...AS A LOWERING TREND IS ANTICIPATED. CURRENT
UPSTREAM LOWER CEILINGS OVER NORTH CENTRAL ILLINOIS AFFECTING
RFD...WILL LIKELY SHIFT EAST TOWARDS THE REMAINING TERMINALS LATER
TODAY AND HAVE THAT REFLECTED IN THE TAFS. CEILINGS WILL APPROACH
IFR DURING THIS PERIOD BUT AM NOT OVERLY CONFIDENT THEY WILL REACH
IFR...OR AT LEAST REMAIN THIS HEIGHT FOR AN EXTENDED PERIOD OF
TIME. WILL NEED TO CONTINUE TO MONITOR TRENDS FOR THE POSSIBILITY
OF IFR AND EVEN A LONGER DURATION OF THESE LOWER CEILINGS INTO
TONIGHT.

RODRIGUEZ

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

* HIGH CONFIDENCE WITH MVFR CEILINGS AND THE LOWERING
  TREND...MEDIUM CONFIDENCE OF HOW LOW CEILINGS WILL FALL WHICH
  INCLUDES IFR POTENTIAL.

* LOW MEDIUM CONFIDENCE WITH SNOW SHOWER TRENDS THROUGH THIS
  EVENING.

RODRIGUEZ

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

THURSDAY NIGHT...MAINLY VFR.

FRIDAY..MVFR/IFR POSSIBLE.

SATURDAY...CHANCE OF RAIN OR SNOW. MVFR POSSIBLE.

SUNDAY...GENERALLY VFR.

MONDAY AND TUESDAY...MVFR/IFR POSSIBLE. CHANCE OF SNOW TUESDAY.

RATZER

&&

.MARINE...

223 PM...LOW PRESSURE NEAR DETROIT IS A BIT WEAKER AND FURTHER
EAST THAN PREVIOUSLY EXPECTED THIS AFTERNOON. THIS LOW WILL
CONTINUE TO DEEPEN AS IT REACHES GEORGIAN BAY TONIGHT. AS IT DOES
THE GRADIENT WILL TIGHTEN AND NORTHWEST WINDS WILL INCREASE UP TO
30 KTS DURING THE EVENING AND CONTINUE INTO THURSDAY MORNING. GALE
POTENTIAL APPEARS TO BE RATHER MARGINAL...AND MAINLY CONFINED TO
THE NORTHERN PORTIONS OF THE LAKE AND WILL BE CANCELING THE GALE
WARNING. THIS LOW WILL CONTINUE TO DEEPEN AS IT MOVES ACROSS
QUEBEC THURSDAY.

ANOTHER WEAKER LOW WILL TRACK FROM MANITOBA TO WESTERN ONTARIO ON
THURSDAY AND BECOME ABSORBED BY THE LOW OVER NORTHERN QUEBEC
THURSDAY NIGHT. HIGH PRESSURE MOVING ACROSS THE GULF COAST WILL
MOVE TO THE CAROLINAS AND MID ATLANTIC FRIDAY MORNING. THE
COMBINATION OF ALL THESE SYSTEMS WILL MAINTAIN STEADY 15-25KT
GRADIENT OVER MUCH OF THE LAKE THROUGH FRIDAY...WHEN A LARGE
TROUGH/COLD FRONT WILL SLOWLY MOVE INTO THE WESTERN LAKES REGION.
A NEW LOW WILL DEVELOP ALONG THIS TROUGH SATURDAY MORNING AND MOVE
NORTHEAST INTO QUEBEC SATURDAY NIGHT. WINDS WILL TURN NORTHWEST
BEHIND THIS TROUGH SATURDAY NIGHT INTO SUNDAY WITH HIGH PRESSURE
BUILDING ACROSS THE PLAINS AND OHIO VALLEY SATURDAY NIGHT INTO
SUNDAY. CMS

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...NEARSHORE GARY TO MICHIGAN CITY IN UNTIL
     3 PM THURSDAY.

     SMALL CRAFT ADVISORY...NEARSHORE WINTHROP HARBOR TO GARY IN
     UNTIL 9 AM THURSDAY.

&&

$$

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