Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 272128
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
328 PM CST THU NOV 27 2014

.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...
337 AM CST

SATURDAY NIGHT THROUGH THURSDAY...

COMING SHORTLY.

&&

.AVIATION...

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

* MVFR CIG TRENDS THROUGH MID-LATE AFTERNOON.

* POTENTIAL FOR A PERIOD OF SNOW EARLY FRIDAY MORNING.

* GUSTY SOUTHERLY WINDS MID FRIDAY MORNING THROUGH THE AFTERNOON.

RC

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

MAIN AVIATION CONCERN IS POTENTIAL FOR A SHORT PERIOD OF SNOW
EARLY FRIDAY MORNING. PRIOR TO THAT...STILL EXPECTING MVFR CIGS TO
SCATTER THIS AFTERNOON AND GUSTY NORTHWEST WINDS TO DIMINISH EARLY
THIS EVENING AS HIGH PRESSURE MOVES OVERHEAD. REGARDING THE PERIOD
OF SNOW...MAIN QUESTION IS IF SATURATION OCCURS WHILE GOOD FORCING
PASSES ACROSS THE AREA IN THE EARLY MORNING HOURS. IF IT
DOES...LOOKING AT A FAIRLY NARROW BAND OF SNOW TO TRACK WEST TO
EAST...WITH GYY POSSIBLY ON THE SOUTHERN FRINGE. HAVE INTRODUCED
A 3-4 HR PROB 30 IN THE TAFS...BUT SNOW IS ONLY FAVORED TO LAST
1-2 HOURS AT MOST UPON ONSET...WITH UNDER 1 INCH OF ACCUMULATION.
SHOULD THE ORGANIZED BAND OF SNOW MATERIALIZE...THEN IFR TO LOWER
MVFR VSBY WOULD BE LIKELY.

AFTER ANY SNOW ENDS...CONCERN SHIFTS TO GUSTY SOUTHERLY WINDS
(160-180 DEG) DEVELOPING IN THE MID/LATE MORNING AND CONTINUING
THROUGH THE AFTERNOON ON FRIDAY. GUSTS COULD BE IN THE LOW 20KT
RANGE AND POTENTIALLY HIGHER IF MIXING IS A BIT DEEPER. VFR
CONDITIONS ARE EXPECTED DURING THE STRONGER SOUTHERLY WINDS.

RC

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

* MEDIUM IN MVFR CIG TRENDS THROUGH MID-LATE AFTERNOON...HIGH IN
  CIGS SCATTERING BY EARLY EVENING.

* LOW IN SHORT 1-2 HR PERIOD OF STEADY SNOW OCCURRING EARLY
  FRIDAY...MEDIUM IN TIMING IF IT OCCURS.

* HIGH IN WIND TRENDS.

RC

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

FRIDAY NIGHT...VFR.

SATURDAY...VFR.

SUNDAY...SLIGHT CHANCE OF RAIN.

MONDAY...VFR.

TUESDAY...SLIGHT CHANCE OF LIGHT SNOW LATE.

WEDNESDAY...SLIGHT CHANCE OF LIGHT SNOW.

RODRIGUEZ

&&

.MARINE...
424 AM CST

FRONTAL BOUNDARY PUSHING DOWN THE LAKE EARLY THIS MORNING AND AS
THIS IS OCCURRING...NORTHWEST WINDS ARE RAPIDLY INCREASING UP TO
30 KT. THIS TREND WILL CONTINUE IN THE NEAR TERM WITH ALL AREAS
ACROSS THE OPEN WATERS OBSERVING NORTHWEST 30 KT WINDS TODAY.
ALTHOUGH IT DOES APPEAR TO BE BRIEF...OCCASIONAL GALE FORCE GUSTS
STILL APPEAR TO BE POSSIBLE TODAY AND SO HAVE CONTINUED THE
MENTION IN THE OPEN WATERS FORECAST. FOR THE NEARSHORE
TODAY...NORTHWEST WINDS WILL PREVAIL WITH THE STRONGEST LIKELY
OBSERVED IN THE INDIANA NEARSHORE. THIS IS ALREADY OCCURRING AND
SO HAVE STARTED THE SCA FOR INDIANA NOW AND THEN EXTENDED IT
FURTHER INTO TONIGHT...AS HIGHER WAVES WILL LINGER FOR MOST OF
TONIGHT. AFTER A BRIEF LULL IN SPEEDS TONIGHT AS HIGH PRESSURE
MOVES ACROSS THE REGION...WINDS WILL TURN TO THE SOUTHWEST AND
SOUTH ON FRIDAY. SOUTHERLY WINDS ARE EXPECTED TO INCREASE OVER THE
LAKE AND NEARSHORE WATERS...WITH SPEEDS UP TO 30 KT LIKELY OVER
THE OPEN WATERS FRIDAY AND FRIDAY EVENING. THERE ONCE AGAIN
APPEARS TO BE A POSSIBILITY FOR SPEEDS APPROACHING GALE
FORCE...ESPECIALLY IN THE AFTERNOON AND EARLY EVENING OVER THE
ENTIRE LAKE. HAVE MADE MENTION OF GALE FORCE GUSTS ONCE AGAIN
DURING THIS TIME...BUT CONFIDENCE OF THESE HIGHER SPEEDS IS LOW.
WILL NEED TO MONITOR THESE PERIODS...WITH THE POSSIBILITY OF THESE
HIGHER SPEEDS BEING OBSERVED.

RODRIGUEZ

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...INDIANA NEARSHORE WATERS UNTIL 4 AM FRIDAY.

     SMALL CRAFT ADVISORY...ILLINOIS NEARSHORE WATERS UNTIL 6 PM THURSDAY.

&&

$$

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