Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 280603
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1203 AM CST FRI NOV 28 2014

.UPDATE...
914 PM CST

NO MAJOR CHANGES MADE TO THE FORECAST THE REMAINDER OF THIS
THANKSGIVING NIGHT AND INTO FRIDAY. DID ISSUE A SPECIAL WEATHER
STATEMENT FOR THE FAR NORTHERN FOUR COUNTIES OF THE FORECAST AREA
TO HIGHLIGHT THE LIKELY BRIEF PERIOD OF SNOW BETWEEN 4 AND 9 AM
FRIDAY MORNING.

A STRONG 1032MB HIGH PRESSURE RIDGE IS MOVING OVER NORTH CENTRAL
IL AND WILL SHIFT EAST OVER CHICAGO BY MIDNIGHT. THE BREAK IN THE
CLOUDS HAS LED TO THE EXPECTED SHARP DROP IN TEMPERATURES...WITH
SOME SINGLE DIGIT VALUES IN NORTH CENTRAL ILLINOIS. WARM AIR
ADVECTION MID CLOUDS SHOULD SPREAD OVER THE WESTERN FORECAST AREA
ALREADY IN THE NEXT 1-2 HOURS SO EXPECTED TEMPERATURES TO BEGIN TO
SLOWLY RECOVER FROM WEST TO EAST. JUST SOME MINOR TOUCH-UPS MADE
TO THE TEMPERATURE FORECAST THE REST OF TONIGHT.

THE 00Z RAOBS INDICATED THE UPSTREAM WARM ADVECTION TEMPERATURE
GRADIENT TAKING SHAPE WITH JUST A 20C 850MB GRADIENT ACROSS SOUTH
DAKOTA /RAP TO ABR/. SOME SPOTTY SNOW HAS ALREADY BEEN SEEN AS FAR
SOUTH AS DES MOINES WITHIN THIS ZONE OF ASCENT AND DEVELOPING
FRONTOGENETIC VERTICAL CIRCULATION. WHILE THERE IS SUBSTANTIAL DRY
AIR NOTED ON THE DVN SOUNDING OVER OUR AREA...THE 00Z NAM CAPTURED
THIS DRYNESS WELL AND DOES INDICATE AS THE SHEARED SHORT WAVE IN
WESTERN MN DROPS SOUTHWARD THE NARROW BUT STRONG LIFT SHOULD
OFFSET THIS LATE TONIGHT INTO EARLY FRIDAY MORNING. FORECAST
TIMING AND SPATIAL COVERAGE IN SNOW POTENTIAL LOOKS GOOD. HAVE JUST
BUMPED UP POPS IN THE FAR NORTHERN FOUR COUNTIES...BUT EXPECT
SOUTH OF THERE ANY SNOW TO MAINLY JUST BE FLURRIES. THOUGHT AN SPS
NEAR THE WI BORDER TO HIGHLIGHT THE TEMPORARY SNOW DURING THE FIRST
THING IN THE MORNING MAY BE BENEFICIAL GIVEN THE EXTRA AMOUNT OF
PEOPLE OUT AT THE VERY EARLY HOUR FOR BLACK FRIDAY.

MTF

&&

.SHORT TERM...
320 PM CST

THROUGH SATURDAY...

THE PRIMARY FORECAST CONCERNS WILL BE:
-TEMPERATURES THROUGH MUCH OF THE PERIOD...WITH A BIT OF A SEE SAW
EXPECTED.
-THE POTENTIAL FOR A QUICK HIT OF SNOW VERY LATE TONIGHT INTO EARLY
FRIDAY MORNING.

A COLD 1034 MB HIGH IS BUILDING SOUTH-SOUTHEASTWARD ACROSS THE
MISSISSIPPI VALLEY THIS AFTERNOON. TEMPERATURES UNDER THIS HIGH HAVE
REMAINED IN THE SINGLE DIGITS. HOWEVER...FARTHER EAST ACROSS
NORTHERN ILLINOIS MOSTLY CLOUDY SKIES AND SCATTERED FLURRIES HAVE
BEEN THE RULE. THIS SURFACE HIGH WILL SET UP OVER THE TENNESSEE VALLEY
LATER THIS EVENING...AND THIS SHOULD SET UP A VERY COLD NIGHT ACROSS
THE REGION...WITH SINGLE DIGIT TEMPERATURES LIKELY ACROSS MOST OF
FAR NORTHERN ILLINOIS OUTSIDE OF CHICAGO. IT APPEARS THAT CLOUD
COVER WILL DECREASE FOR A COUPLE HOURS THIS EVENING...WHICH SHOULD
ALLOW TEMPERATURES TO TUMBLE QUICKLY. HOWEVER...CLOUDINESS WILL
QUICKLY INCREASE LATER THIS EVENING AND OVERNIGHT AHEAD OF OUR NEXT
WEATHER SYSTEM.

THIS CURRENT SURGE OF COLD AIR WILL BEGIN TO QUICKLY SHIFT EASTWARD
LATER TONIGHT INTO FRIDAY AS THE LARGE SCALE FLOW PATTERN REMAINS
PROGRESSIVE. UPPER LEVEL HEIGHT RISES...ASSOCIATED WITH UPPER LEVEL
RIDGING BUILDING EASTWARD OVER THE PLAINS...WILL SHIFT A MUCH WARMER
AIRMASS EASTWARD TOWARDS THE UPPER MIDWEST AND GREAT LAKES
AREA OVER THE NEXT 24 HOURS. AS THIS OCCURS...A RATHER STOUT
BAROCLINIC ZONE BETWEEN THE TWO AIRMASS WILL SET UP RIGHT ACROSS THE
AREA INTO FRIDAY. AS A RESULT OF THIS...THE INCREASING SOUTHWESTERLY
FLOW ALOFT WILL ALLOW SOME GOOD ISENTROPIC ASCENT/WARM AIR ADVECT TO
RAMP UP QUICKLY OVER THE UPPER MIDWEST AND INTO THE WESTERN LAKES
REGION TONIGHT AND FRIDAY. ENHANCED MESOSCALE FORCING FOR
ASCENT...WITHIN THE VICINITY OF THE BEST FRONTOGENESIS BAND...SHOULD
FOCUS A FAIRLY QUICK MOVING BAND OF MODERATE TO POSSIBLY BRIEFLY
HEAVY SNOWFALL LATE TONIGHT INTO FRIDAY OVER THE UPPER MIDWEST. THE
MAIN QUESTION IS HOW FAR SOUTH WILL THIS SNOWFALL BAND DEVELOP. IT
APPEARS NORTHERN ILLINOIS WILL BE ON THE SOUTHERN END OF THE AREA
FAVORED FOR THE DEVELOPMENT OF THIS BAND. THE MAIN ISSUE WE WILL BE
FIGHTING LOCALLY...IS A DECENT AMOUNT OF DRY AIR. HOWEVER...I THINK
THAT THE ENHANCE FORCING FOR ASCENT ALONG THE STRENGTHENING BAROCLINIC
ZONE WILL BE ENOUGH TO SATURATE THE COLUMN LATE TONIGHT INTO FRIDAY
MORNING BEFORE THE BEST FORCING QUICKLY MOVES EAST OF THE AREA.
THEREFORE...IT APPEARS THAT WE WILL HAVE A SHORT WINDOW OF TIME TO
SUPPORT A QUICK MOVING BURST OF SNOW ACROSS NORTHERN ILLINOIS DURING
THE EARLY MORNING HOURS FRIDAY.

IT APPEARS THAT THIS BAND OF SNOW COULD RESULT IN A BRIEF PERIOD OF
MODERATE TO POSSIBLY HEAVY SNOW. MID LEVEL LAPSE RATES ARE EXPECTED
TO BE AROUND 6.5 DEGREES C PER KM WITHIN THE 500-700 MB
LEVEL...WHICH SUGGESTS THAT SOME HIGHER SNOW RATES MAY BE POSSIBLE.
THE GOOD NEWS IS THAT THIS BAND OF SNOW SHOULD NOT LAST TOO LONG
OVER ANY GIVEN AREA. THEREFORE...ONLY LIGHT ACCUMULATIONS UNDER AND
INCH ARE POSSIBLE EARLY FRIDAY MORNING...WITH THE BEST CHANCES FOR
THIS ACROSS FAR NORTHERN ILLINOIS...MAINLY ALONG AND NORTH OF I88.

FOLLOWING THIS POTENTIAL BAND OF SNOW...THINGS LOOK TO WARM UP
NICELY BY SATURDAY. TEMPERATURES AT 925 MB ARE PROGGED TO REACH +5
TO +6 DEGREES C BY MIDDAY SATURDAY. CLIMATOLOGY THIS TIME OF YEAR
SUGGESTS TEMPERATURES COULD REACH INTO THE LOWER 50S FOR HIGH
TEMPERATURES. THIS IS CURRENTLY WHAT WE HAVE GOING...SO I MADE NO
CHANGES TO TEMPS ON SATURDAY. THE ONLY POSSIBLE THING THAT COULD
KEEP TEMPERATURES FROM WARMING THIS MUCH IS IF LOW LEVEL
CLOUDINESS AND FOG REMAIN MORE EXTENSIVE DURING THE DAY.

KJB

&&

.LONG TERM...
342 PM CST

SATURDAY NIGHT THROUGH FRIDAY...

VERY MILD CONDITIONS WILL CONTINUE THROUGH SUNDAY AS WE REMAIN WELL
ENTRENCHED IN THE WARM SECTOR OF A STORM SYSTEM ACROSS THE UPPER
MIDWEST. SOUTH-SOUTHWESTERLY FLOW WILL LIKELY KEEP LOW TEMPERATURES
WELL INTO THE 40S SATURDAY NIGHT...AND THUS A VERY WARM START IS
EXPECTED FOR SUNDAY. THIS WARM START COULD ALLOW TEMPERATURES TO BE
THE WARMEST ON SUNDAY BEFORE A COLD FRONT PUSHES OVER THE AREA. THE
MAIN QUESTION THAT REMAINS...IS THE TIMING OF THIS COLD FRONTAL
PASSAGE. NCEP MODEL GUIDANCE SUGGESTS THIS FRONT MAY MOVE ACROSS
THE AREA EARLY IN DAY ON SUNDAY...WHICH WOULD NOT GO TO WELL TO
SUPPORT VERY WARM CONDITIONS. HOWEVER...THE 12 UTC ECMWF AND GEM
SOLUTIONS SUGGEST THE FRONT WILL NOT ARRIVE UNTIL VERY LATE SUNDAY
AFTERNOON OR EVENING...AND AS SUCH THEY SUGGEST VERY WARM CONDITIONS
AREA-WIDE ON SUNDAY. AT THIS TIME I AM LEANING TOWARDS A SLOWER
FRONTAL TIMING AND HENCE WARMER CONDITIONS ON SUNDAY THEN THE GFS AND
NAM WOULD SUGGEST.

IN THE WAKE OF THIS COLD FRONT...TEMPERATURES WILL LIKELY DROP BACK
INTO THE UPPER 20S FOR MONDAY. HOWEVER...TEMPERATURES ARE EXPECTED
TO MODERATE THROUGH THE WEEK AS THE HEART OF THIS NEXT PUSH OF COLD
AIR LOOKS TO REMAIN NORTHEAST OF THE REGION. MODEL GUIDANCE IS NOT
IN GOOD AGREEMENT LATER IN THE PERIOD. THE 12 UTC GFS SUGGESTS THAT
A PACIFIC SYSTEM WILL SPREAD RAIN ACROSS THE AREA WEDNESDAY NIGHT
THROUGH THURSDAY. HOWEVER...THE ECMWF WEAKENS TO SYSTEM AS IT MOVES
ONSHORE ACROSS THE WEST COAST...AND THEREFORE REMAINS DRY ACROSS THE
REGION DURING THIS PERIOD. AT THIS TIME WE WILL CONTINUE TO RUN WITH
LOW END POPS UNTIL A BETTER CONSENSUS CAN BE REACHED.

KJB

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 06Z...

* LIGHT AND VARIABLE WINDS ARND 2-4KT SLOWLY TURNING
  SOUTHWEST/SOUTH BEFORE DAYBREAK AND INCREASING.

* POSSIBLE LIGHT SNOW DEVELOPING ARND 12Z THRU 14Z. COULD PRODUCE
  PATCHY MVFR CIGS AND VSBYS TO ARND 3-5SM.

* INCREASED MIXING MIDDAY WITH WIND GUSTS NEARING 20KT FROM THE
  SOUTH THRU 00Z SAT.

BEACHLER

//DISCUSSION...UPDATED 06Z...

IR IMAGERY SHOWS A WEDGE OF CLOUDS ARRIVING ACROSS NORTHERN IL
FROM THE NORTHWEST. THESE CLOUDS APPEAR TO BE STEADILY
THICKENING. ACROSS SOUTHEAST MINNESOTA SFC OBS ARE INDICATING LGT
SNOW...AND BASED ON LATEST GUIDANCE THIS AREA OF LGT SNOW WILL
ARRIVE AT NORTHERN IL TAF SITES ARND DAYBREAK. THERE CONTINUES TO
BE SOME DIFFERENCES ON IF THE SNOW WILL BE STEADY ENOUGH TO LIMIT
VSBYS/CIGS. HAVE HELD ONTO THE MENTION OF A SLIGHT REDUCTION TO
THE CIGS DOWN TO HIGH END MVFR/LOW END VFR CONDS...AND ALSO ONLY
BRINGING VSBYS DOWN TO 5SM. SNOW SHUD QUICKLY DEPART ARND
14-15Z...WITH CIGS LIFTING BACK TO VFR CONDS. WINDS WILL REMAIN
SOUTHERLY THRU THE AFTN/EVE...WITH BETTER MIXING ALLOWING GUSTS TO
20KT TO DEVELOP. SOME CLOUDS WILL HOLD IN PLACE THRU THE AFTN/EVE
BUT SHUD REMAIN AT OR ABOVE 7KFT AGL.

BEACHLER


//ORD AND MDW CONFIDENCE...UPDATED 06Z...

* HIGH CONFIDENCE IN WIND DIR/SPEED/GUSTS.

* MEDIUM/HIGH CONFIDENCE IN LGT SNOW TIMING. MEDIUM CONFIDENCE IN
  SNOW LIMITING TO VSBYS/CIGS TO MVFR CONDS.

* HIGH CONFIDENCE IN ALL ELEMENTS AFT 16Z.

BEACHLER

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

FRIDAY NIGHT...WX NIL. CHANCE MVFR VIS LATE.

SATURDAY...WX NIL. CHANCE MVFR VIS EARLY MORNING.

SUNDAY...SLIGHT CHANCE OF RAIN EARLY. OTRW...VFR/WX NIL.

MONDAY...VFR/WX NIL.

TUESDAY...SLIGHT CHANCE OF LIGHT SNOW LATE...WITH MVFR CIGS/VIS.

WEDNESDAY...SLIGHT CHANCE OF LIGHT SNOW WITH MVFR CIGS/VIS
OVERNIGHT.

KREIN

&&

.MARINE...
239 PM CST

HIGH PRESSURE STRETCHING FROM WESTERN ONTARIO TO THE LOUISIANA COAST
WILL MOVE EAST ACROSS LAKE MICHIGAN TONIGHT BEFORE SHIFTING TO THE
EAST COAST BY SATURDAY MORNING. GUSTY NORTHWEST WINDS WILL DIMINISH
THROUGH THE EVENING AND TURN SOUTHWESTERLY OVERNIGHT AS THE RIDGE
AXIS CRESTS THE LAKE. LOW PRESSURE DEVELOPING OVER MONTANA WILL MOVE
EASTWARD ACROSS THE NORTHERN PLAINS AND TURN NORTHEASTERLY ACROSS
LAKE SUPERIOR LATER SATURDAY. THIS WILL TIGHTEN THE PRESSURE
GRADIENT ACROSS THE LAKE FRIDAY MORNING LEADING TO AN INCREASE IN
SOUTH WINDS. CENTRAL AND SOUTHERN SECTIONS OF THE LAKE SHOULD PUSH
30 KT FRIDAY AFTERNOON BEFORE EASING FRIDAY NIGHT. WINDS LOOK TO
INCREASE AGAIN SATURDAY MORNING AS THE GRADIENT IS REINFORCED AS THE
LOW PASSES TO THE NORTH. THE LOW WILL DRAG A COLD FRONT ACROSS THE
LAKE SUNDAY SHIFTING WINDS TO THE NORTHWEST WHILE VERY STRONG HIGH
PRESSURE DEVELOPS AND MOVES ACROSS THE NORTHERN PLAINS. COLD
ADVECTION WILL COMBINE WITH A STRONG PRESSURE GRADIENT TO KEEP A
PERIOD OF STRONG NORTHWEST WINDS ACROSS THE LAKE FROM SUNDAY
AFTERNOON INTO MONDAY AFTERNOON WITH LOW TO MID RANGE GALES A
POSSIBILITY. WINDS WILL QUICKLY DROP OFF MONDAY NIGHT AS THE HIGH
MOVES RIGHT OVERHEAD. ANOTHER LOW IS EXPECTED TO TRACK FROM THE
CANADIAN PRAIRIES TOWARD JAMES BAY NEXT TUESDAY/WEDNESDAY ALLOWING
SOUTHERLY WINDS TO RETURN TO THE LAKE WHICH MAY BE STRONG GIVEN THE
STRENGTH OF THE DEPARTING HIGH.

MDB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...INDIANA NEARSHORE WATERS UNTIL 3 AM FRIDAY.

&&

$$

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