Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 270606
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1206 AM CST THU NOV 27 2014

.UPDATE...
650 PM CST

MONITORING AN AREA OF CONVERGENCE OVER CENTRAL/SOUTHERN LAKE
MICHIGAN THIS EVENING WHERE A NE-SW ORIENTED WEAK BAND IS FORMING.
LAKE TEMPS ARE AROUND 6 DEG C...BUT WITH 850 TEMPS DROPPING THROUGH
THE NEGATIVE SINGLE DIGITS AND FORECASTED TO DROP SOME MORE THIS
EVENING...THIS IS RESULTING SOME WEAK CAPE. INVERSION LEVELS ARE
NOT VERY HIGH...AROUND 5-6KFT...BUT THIS AREA OF CONVERGENCE MAY
LAST LONG ENOUGH TO ALLOW SOME OF THIS SNOW SHOWER ACTIVITY TO
SNEAK INTO NE ILLNOIS THIS EVENING. WITH THE MODEST LAKE
PARAMETERS IN PLACE...NOT EXPECTING THIS TO AMOUNT TO MUCH AS LOW
LEVEL WINDS WILL BEGIN TO SHIFT FROM NE TO N THAN NW...BUT COULD
SOME ADDITIONAL LIGHT SNOW SHOWERS/FLURRIES MID TO LATE EVENING
BEFORE THE CONVERGENCE AXIS SHIFTS INTO NW INDIANA. COULD SEE A
DUSTING CLOSE TO THE LAKE IN COOK COUNTY. MORE LIKELY TO SEE SOME
ACCUMULATION OVERNIGHT IN NW INDIANA WITH THE COMBINATION OF THE
WEAK SYNOPTIC FORCING AND THE LAKE EFFECT PROCESSES. CURRENT
FORECAST ONLY CALLS FOR A SMALL CHANCE OF ACCUMULATION IN PORTER
COUNTY...BUT ITS POSSIBLE THAT SOME LIGHT SNOW MAY SNEAK INTO LAKE
COUNTY INDIANA AS WELL.

KMD

&&

.SHORT TERM...
311 PM CST

THROUGH FRIDAY...

REST OF TODAY INTO THIS EVENING...

IN IMMEDIATE NEAR TERM...SNOW ASSOCIATED WITH CLIPPER IS LIKELY TO
REMAIN ALMOST COMPLETE SOUTHWEST OF CWA...BUT COULD BE CLOSE TO
FAR SOUTHWEST FOR A FEW HRS INTO EARLY EVENING WHERE MAINLY SLIGHT
POPS HAVE BEEN CARRIED.

TONIGHT THROUGH FRIDAY...

THE BIG STORY OVER THE NEXT FEW DAYS WILL BE A HUGE TEMPERATURE
SWING FROM ONE OF THE COLDEST THANKSGIVING DAYS ON RECORD TO HIGHS
POTENTIALLY WELL INTO THE 50S FOR MUCH OF THE AREA OVER THE
WEEKEND. HIGHLY AMPLIFIED PATTERN OVER THE CONUS TODAY...WITH A
150+ KT UPPER JET CARVING OUT CURRENT DEEP TROUGH OVER THE EASTERN
HALF. 12Z RAOB SAMPLED -17 C AT 850 MB AT INL TODAY AND THIS AIR
MASS WILL SURGE SOUTH TOWARD AREA FOR BRIEF BLAST OF ARCTIC AIR
THAT WILL ARRIVE LATE TONIGHT INTO THANKSGIVING MORNING. THERE
SHOULD BE ENOUGH LIFT AS SHEARED OUT VORT/TROUGH AXIS CROSSES THE
AREA TONIGHT FOR FLURRIES TO BE SQUEEZED OUT FOR MAINLY
NORTH/NORTHEAST CWA OVERNIGHT INTO VERY EARLY THURSDAY.

TEMPERATURES WILL LIKELY BE FALLING INTO MID TEENS-LOW 20S BY MID
THURSDAY MORNING...FOLLOWED BY ONLY MINOR RECOVERY INTO LOW
20S/AROUND 20 NORTHWEST TO UPPER 20S FAR SOUTHEAST. PARAMETERS FOR
LAKE EFFECT SNOW WILL BE QUITE FAVORABLE ON THURSDAY...BUT WEST-
NORTHWEST FLOW IS EXPECTED TO KEEP FOCUS TO THE EAST OF THE CWA.
HAVE ONLY LOW CHANCE POPS IN NORTHEAST PORTER COUNTY...BUT IT MAY
BE VERY CLOSE FOR THAT AREA BEING CLIPPED BY AT LEAST PERSISTENT
LIGHT LAKE SNOWS.

THERE SHOULD BE A PERIOD OF PARTLY CLOUDY SKIES THURSDAY EVENING
INTO THE FIRST PART OF THE NIGHT AS SFC HIGH CROSSES THE AREA.
THIS WOULD PROVIDE SEVERAL HOURS OF GOOD RADIATIONAL COOLING AND
POTENTIAL FOR UPPER SINGLE DIGITS TO LOW TEENS WEST AND NW OF
CHICAGO.

INCREASING MID/HIGH CLOUDS AND WAA THEN SHOULD RESULT IN RISING
TEMPS TOWARD DAYBREAK. ISENTROPIC ASCENT AND UPPER FORCING WILL
RESULT IN PRECIP BREAKING OUT MAINLY NORTH OF CWA ON
FRIDAY...THOUGH FAR NORTHEAST COULD BE CLIPPED WHERE I MAINTAINED
SLIGHT CHANCE POPS. CONTINUE TO SUSPECT THAT MODELS ARE TOO COLD
WITH TEMPS ON FRIDAY DUE TO TOO MUCH SNOW COVER...SO STAYED
SIMILAR TO PREVIOUS FORECAST WITH UPPER 30S-NEAR 40 FOR SOUTHEAST
2/3 OF CWA.

RC

&&

.LONG TERM...
311 PM CST

FRIDAY NIGHT THROUGH THURSDAY...

AS FLOW  RAPIDLY TURNS ZONAL ON FRIDAY NIGHT...ANY MINOR COOLING
IN THE EVENING WILL LIKELY BE FOLLOWED BY RISING TEMPERATURES
OVERNIGHT. THIS WILL OCCUR AS VERY MILD AIR MASS RAPIDLY ADVECTS
INTO AREA...WITH BOTH ECMWF AND GFS SUGGESTING LOW-MID TEENS
CELSIUS AT 850 BY LATE DAY SATURDAY. LOCAL CLIMO SUGGESTS AT LEAST
LOW-MID 50S FOR HIGHS ON SATURDAY AND PROBABLY WARMER EVEN WITH
SOME CLOUD COVER. GUIDANCE CONTINUES TO SLOW FROPA ON SUNDAY...SO
EVEN WARMER TEMPS SATURDAY NIGHT WILL PROVIDE CHANCE FOR SOUTHERN
CWA ESPECIALLY TO NEAR 60 DEGREES FOR HIGHS ON SUNDAY! LIGHT
RAIN MAY BREAK OUT AS FRONT CROSSES THE AREA BUT FORCING IS
LIMITED. MODIFIED ARCTIC AIR WILL OOZE BACK IN ON SUNDAY
NIGHT...WITH CHILLY CONDITIONS MONDAY AND TUESDAY UNDER EXPANSIVE
HIGH PRESSURE. OPERATIONAL AND ENSEMBLE GUIDANCE VARIES LARGELY
BEYOND THIS...SO STAYED CLOSE TO A CONSENSUS BLEND...YIELDING
CLOSE TO AVERAGE TEMPERATURES. PRECIP CHANCES WILL DEPEND ON HOW
CLOSE BAROCLINIC ZONE CAN GET TO OUR AREA...WITH A SIGNIFICANT
TEMPERATURE GRADIENT PERSISTING ACROSS THE MIDWEST BUT EITHER
NEARBY OR SHUNTED WELL SOUTH.

RC

&&

.AVIATION...

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

* MVFR CEILINGS EXITING THIS MORNING...BUT RETURNING LATER THIS
  MORNING.

* BRIEF PERIOD OF LAKE EFFECT LIGHT SNOW SHOWERS EARLY...THEN
  ANOTHER CHANCE FOR LIGHT SNOW SHOWERS LATER THIS MORNING.

RODRIGUEZ

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

MVFR CEILINGS REMAIN OVER THE TERMINALS AT THIS HOUR...WITH THE
EXCEPTION OF RFD. EXPECT CLEARING TO OCCUR FOR THE REMAINING
TERMINALS OVER THE NEXT SEVERAL HOURS...BUT WITH GYY LIKELY
HOLDING ONTO THE CEILINGS AS AN AREA OF LAKE EFFECT CLOUDS AND SNOW
SHOWERS MOVES SOUTH DOWN THE LAKE. THESE SNOW SHOWERS AR CURRENTLY
CLIPPING THE ORD AND MDW TERMINALS BUT SHOULD ONLY LAST 1-2 HOURS
WITHOUT MUCH OF ANY VIS RESTRICTION. GYY WILL LIKELY OBSERVE A
PERIOD OF BETTER SNOWFALL WITH MORE PREVAILING MVFR VIS LIKELY.
THIS AREA OF LAKE EFFECT SNOW SHOWERS SHOULD REMAIN
PROGRESSIVE...EXITING GYY BY THE 9Z TIME FRAME. AFTER A PERIOD OF
VFR CONDITIONS THIS MORNING...UPSTREAM MVFR CEILINGS WILL LIKELY
MOVE BACK OVER THE TERMINALS WHILE SOME LIGHT SNOW SHOWERS ALSO
DEVELOP. DONT ANTICIPATE MUCH WITH THESE SHOWERS AS THEY SHOULD
REMAIN SCATTERED AND LIGHT...WHILE THE MVFR CEILINGS HOLD THROUGH
AT LEAST MID DAY. VFR CONDITIONS WILL THEN PREVAIL LATER THIS
AFTERNOON INTO THE EVENING AS HIGH PRESSURE BUILDS ACROSS THE
AREA. CURRENT NORTHWEST WINDS WILL REMAIN IN PLACE FOR
TODAY...INCREASING THROUGH MID AND LATE MORNING.

RODRIGUEZ


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

* HIGH CONFIDENCE WITH MVFR CEILINGS AND TRENDS IN THE NEAR
  TERM...MEDIUM HIGH CONFIDENCE OF MVFR CEILINGS RETURNING WITH
  MEDIUM CONFIDENCE OF EXACT TIMING AND DURATION.

* HIGH CONFIDENCE WITH LAKE EFFECT SNOW SHOWERS AND
  TRENDS...MEDIUM CONFIDENCE WITH SNOW SHOWERS LATER THIS MORNING.

RODRIGUEZ

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

FRIDAY...VFR. WX NIL.

SATURDAY...VFR. WX NIL.

SUNDAY...SLIGHT CHANCE -RA EARLY. VFR/WX NIL OVERNIGHT.

MONDAY...VFR. WX NIL.

TUESDAY...VFR/WX NIL EARLY. CHANCE RAIN OR FREEZING RAIN
OVERNIGHT.

KREIN

&&

.MARINE...
335 PM CST

A STALLED WEST-EAST FRONTAL BOUNDARY STRETCHED ACROSS NORTHERN LAKE
MICHIGAN WILL GET A GOOD PUSH SOUTH TONIGHT AS COLDER AIR SURGES IN
BEHIND DEPARTING LOW PRESSURE OVER CENTRAL ILLINOIS.  HIGH PRESSURE
OVER THE SOUTHERN CANADIAN PRAIRIE PROVINCES WILL HELP DRIVE THE
COLDER AIR SOUTH. WINDS WILL INCREASE QUICKLY BEHIND THE FRONT EARLY
THURSDAY FROM NORTH TO SOUTH... WITH A BRIEF PERIOD OF GALE FORCE
GUSTS IS EXPECTED. HIGH PRESSURE RIDGING THEN QUICKLY BUILDS EAST
ACROSS THE LAKE THURSDAY NIGHT ALLOWING WINDS AND WAVES TO THEN
QUICKLY SUBSIDE.  WINDS AND WAVES THEN QUICKLY INCREASE AGAIN FRIDAY
AS THE PRESSURE GRADIENT TIGHTENS BEHIND THE HIGH AND AHEAD OF
DEEPENING LOW PRESSURE OVER THE PLAINS.

THIS LOW PRESSURE AREA WILL MOVE TO THE U.P. OF MICHIGAN BY EARLY
SUNDAY.  A COLD FRONT ASSOCIATED WITH THIS LOW WILL MOVE EAST ACROSS
THE LAKE DURING THE DAY SUNDAY... WITH BRISK NORTHWEST FLOW
INTENSIFYING BEHIND THE FRONT INTO MONDAY.

ED F

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...LMZ743-LMZ744-LMZ745...6 AM THURSDAY TO
     MIDNIGHT FRIDAY.

     SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742...9 AM THURSDAY TO 6
     PM THURSDAY.

&&

$$

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