Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 272359
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
559 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...
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 00Z...

* POTENTIAL FOR A PERIOD OF LIGHT SNOW EARLY FRIDAY MORNING.

* GUSTY SOUTHERLY WINDS MID FRIDAY MORNING THROUGH THE AFTERNOON.

KREIN

//DISCUSSION...UPDATED 00Z...

LATEST SATELLITE AND RADAR TRENDS INDICATING THAT CLOUD COVER AND
SNOW SHOWERS ARE BECOMING MORE RAGGED AND DECREASING IN COVERAGE
WITH SUNSET AND AS HIGH PRESSURE BUILDS ACROSS THE REGION. ANY
LINGERING SNOW SHOWERS SHOULD COME TO AN END BY ARND 01Z...EXCEPT
FOR GYY...WHICH IS STILL UNDER A LAKE FETCH WITH NWLY WINDS. AS
WINDS BECOME MORE WLY LATER THIS EVENING...ANY LAKE INFLUENCE WILL
SHUT OFF AND CONDITIONS AT GYY SHOULD BECOME QUIET AS WELL.

THE NEXT CONCERN IS FOR THE POTENTIAL FOR ANOTHER BRIEF PERIOD OF
SNOW TOMORROW MORNING. THE LATEST MODEL GUIDANCE STILL INDICATES A
MID LEVEL IMPULSE DROPPING OUT OF THE NRN PLAINS WHILE A WEAK WARM
FRONT SETS UP ACROSS THE UPPER MISSOURI VALLEY TO NEAR THE IL/WI
BORDER. THERE WILL BE A SHORT WINDOW OF OPPORTUNITY FOR SOME LIGHT
SNOW IF THERE IS A DEEP ENOUGH LAYER OF MOISTURE IN PLACE AT THE
TIME OF OPTIMAL UPPER LEVEL FORCING. THE LATEST HIGH RESOLUTION
MODEL GUIDANCE SUGGESTS THAT THERE COULD BE A PERIOD OF LIGHT SNOW
DURG THE MORNING HOURS FOR PERHAPS 2 HOURS...BUT TIMING THE SNOW
IS STILL UNCERTAIN. SO...WILL MAINTAIN THE GOING FORECAST OF A 3
HOUR TEMPO GROUP...THOUGH THIS IS LIKELY TOO LONG MORE PRECISE
TIMING SHOULD BE DETERMINED IN LATER UPDATES. REGARDLESS OF EXACT
TIMING...ONLY LIGHT ACCUMULATIONS...PERHAPS A HALF INCH OR LESS IS
EXPECTED.

FOR THE REMAINDER OF THE DAY TOMORROW...FORECAST CONCERNS SHIFT TO
GUSTY SOUTHERLY WINDS DEVELOPING IN THE MID/LATE MORNING AS HIGH
PRESSURE SETTLES OVER THE SERN CONUS AND LOW PRESSURE DEVELOPS
OVER THE NRN PLAINS. OCCASIONAL GUSTS TO ARND 20KT ARE
LIKELY...AND POTENTIALLY HIGHER ARE POSSIBLE WITH DEEPER MIXING IF
ENOUGH SUNSHINE CAN BREAK THROUGH THE CLOUD COVER.

KREIN

//ORD AND MDW CONFIDENCE...UPDATED 00Z...

* MEDIUM CONFIDENCE IN CIG TRENDS.

* LOW TO MEDIUM CONFIDENCE IN TIMING AND COVERAGE OF A SHORT 1-2
  HR PERIOD OF SNOW OCCURRING FRIDAY MORNING.

* HIGH CONFIDENCE IN WIND TRENDS.

KREIN

//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 4 AM FRIDAY.

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

&&

$$

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