Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 210302
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
902 PM CST THU NOV 20 2014

.UPDATE...
858 PM CST

HIGH PRESSURE CONTINUES TO STEADILY MOVE SOUTHEAST ACROSS IOWA
THIS EVENING AND WILL MOVE RIGHT OVER THE LOCAL AREA OVERNIGHT.
WINDS HAVE REMAINED STEADY FOR MOST OF THE AREA SO FAR THIS
EVENING BUT THEY WILL BE DROPPING OFF AS THE HIGH COMES ACROSS.
THIS WILL COMBINE WITH THE CLEAR SKIES TO ALLOW TEMPERATURES TO
QUICKLY FALL OVERNIGHT. CURRENT DEWPOINTS IN THE 3 TO 10 DEGREE
RANGE SUPPORT LOWS IN THE LOWER SINGLE DIGITS TO AROUND 10 DEGREES
WITH THE COLDEST READINGS NORTH. HAVE LOWERED LOWS IN THE NORTH A
FEW DEGREES AND WOULD NOT BE SURPRISED IF A FEW OF THE
TRADITIONALLY COLDER SPOTS FROM ROCHELLE TO AURORA SEE LOWS FALL
TO ZERO.

MDB

&&

.SHORT TERM...

TONIGHT THROUGH SATURDAY...

305 PM...MAIN FORECAST CONCERN AND CHALLENGE REMAINS LIGHT
FREEZING RAIN POTENTIAL FRIDAY NIGHT INTO EARLY SATURDAY MORNING.

ANOTHER BLUSTERY DAY WITH WINDS GUSTING 30-35 MPH WITH TEMPS
REACHING THE UPPER 20S TO LOWER 30S ACROSS SOUTHERN AREAS. CLEAR
SKIES WILL CONTINUE INTO TONIGHT AS A RIDGE OF HIGH PRESSURE
BUILDS ACROSS THE REGION ALLOWING WINDS TO BECOME LIGHT AND
VARIABLE OVERNIGHT. ONE LAST NIGHT OF VERY COLD TEMPS...UNTIL
NEXT WEEK...WITH LOW TEMPS TANKING TO THE SINGLE DIGITS NORTH TO
PERHAPS LOWER TEENS ACROSS THE SOUTH.

AS THE RIDGE SHIFTS EAST FRIDAY...FLOW TURNS LIGHT SOUTHERLY AND
WITH QUITE A BIT OF SUN EXPECTED THRU EARLY AFTERNOON ALONG WITH
WARMING TEMPS ALOFT...SHOULD AT LEAST BE ABLE TO REACH SAME TEMPS
AS TODAY AND PERHAPS A FEW DEGREES WARMER.

WARMER AIR CONTINUES TO SPREAD NORTH FRIDAY NIGHT AS WEAK LIFT
BEGINS TO SQUEEZE OUT SOME LIGHT PRECIPITATION. AIR TEMPERATURES
WILL INITIALLY REMAIN BELOW FREEZING AS PRECIPITATION BEGINS TO
FALL. DRY LOW LEVELS MAY SUPPORT SOME LIGHT SLEET AT ONSET...BUT
APPEARS THE MAIN PRECIP TYPE WILL BE LIGHT FREEZING RAIN AS A VERY
DEEP WARM LAYER DEVELOPS OVERNIGHT. QPF AMOUNTS HAVE GENERALLY
RANGED FROM A FEW HUNDREDTHS ACROSS THE NORTHERN CWA TO PERHAPS AS
MUCH AS A TENTH OF AN INCH ACROSS THE SOUTHEAST CWA INTO NORTHWEST
INDIANA. SO A PERIOD OF LIGHT FREEZING RAIN APPEARS LIKELY.

MAIN QUESTION WILL THEN BE HOW LONG DOES IT PERSIST. GROUND
SURFACES HAVE BEEN SUB FREEZING FOR OVER A WEEK WITH HIGH TEMPS
MAYBE REACHING NEAR THE FREEZING MARK FRIDAY. THUS IT WILL TAKE
SOME AMOUNT OF TIME FOR AIR TEMPERATURES ABOVE FREEZING TO WARM
THESE COLDER SURFACES AND END FREEZING RAIN POTENTIAL. CONFIDENCE
ON THIS TIMING IS FAIRLY LOW. HOURLY TEMP TRENDS HAVE TEMPS INTO
THE MID 30S ACROSS THE SOUTHERN CWA AND TO AROUND FREEZING
NORTHERN AREAS BY 12Z SATURDAY. THESE TEMPS ARE IN LINE AND
PERHAPS A TAD COOLER THAN THE GFS/ECMWF. TEMPS CONTINUE TO
STEADILY RISE THROUGH THE MID/UPPER 30S SATURDAY MORNING...WITH
PRECIPITATION BECOMING ALL LIQUID. ANY AMOUNT OF LIGHT FREEZING
RAIN OR FREEZING DRIZZLE WILL BECOME HAZARDOUS ON UNTREATED
SURFACES AND PLAN TO ISSUE AN SPS THIS AFTERNOON TO HIGHLIGHT THE
THREAT.

TEMPERATURES CONTINUE TO WARM INTO THE 40S SATURDAY WITH A CHANCE
OF LIGHT RAIN OR PERHAPS SOME DRIZZLE. CMS

&&

.LONG TERM...

SATURDAY NIGHT THROUGH FRIDAY...

305 PM...MAIN FORECAST CONCERNS ARE HEAVY RAIN SUNDAY/SUNDAY
EVENING AND THEN ANOTHER PROLONGED PERIOD OF COLDER/BELOW NORMAL
TEMPS NEXT WEEK.

NOT MUCH CHANGE IN TEMPS IS EXPECTED SATURDAY NIGHT AS LOW LEVEL
MOISTURE STREAMS NORTH. TEMPS MAY HOLD STEADY IN THE LOWER TO MID
40S. EXTENT OF FOG IS STILL A BIT UNCERTAIN BUT POTENTIAL FOR FOG
CONTINUES HIGH ENOUGH TO MAINTAIN IN THE FORECAST WITH AT LEAST
THE CHANCE OF SOME DENSE FOG.

LOW PRESSURE WILL DEVELOP OVER THE SOUTHERN PLAINS SATURDAY NIGHT
AND MOVE NORTH ACROSS THE AREA SUNDAY AFTERNOON OR SUNDAY EVENING.
WHILE THE LOW IS STILL EXPECTED TO DEEPEN TO AS LOW AS 976MB OVER
THE NORTHERN LAKES MONDAY...IT MAY DEEPEN A BIT SLOWER AND ITS
OVERALL TRACK COULD BE A BIT SLOWER AND A TAD FURTHER EAST...FROM
EASTERN IL TO SOUTHERN MICHIGAN SUNDAY NIGHT. NET AFFECT OF THIS
WOULD BE TO BRING THE AXIS OF HEAVY RAIN RIGHT OVER THE CWA FROM
SUNDAY AFTERNOON INTO SUNDAY EVENING. PRECIPITABLE WATER VALUES
STILL PROGGED INTO THE 1.0 TO 1.2 INCH RANGE DURING THIS TIME
PERIOD. STORM TOTAL RAINFALL OF 1 TO 2 INCHES STILL LOOKS POSSIBLE
BUT NOT ENOUGH CERTAINTY TO TRY TO PIN DOWN A PARTICULAR AREA OF
THE CWA...IT MAY END UP BEING MUCH OF THE AREA.

TRACK OF THE LOW WILL ALSO LIKELY RESULT IN A RATHER SHARP TEMP
GRADIENT...PERHAPS ONLY LOW/MID 40S WESTERN AREAS AND LOW/MID 50S
FAR EASTERN AREAS. A COLD FRONT WILL MOVE ACROSS THE AREA SUNDAY
NIGHT BRINGING COLD AIR BACK TO THE AREA WITH FALLING TEMPS
THROUGH THE 30S MONDAY. LINGERING PRECIP WILL CHANGE FROM LIGHT
RAIN TO LIGHT SNOW WITH SNOW SHOWERS/FLURRIES POSSIBLE MONDAY INTO
MONDAY NIGHT. STILL LOOKS QUITE BREEZY EARLY NEXT WEEK WITH
SPECIFICS DEPENDING ON THE TRACK/STRENGTH/TIMING OF THE LOW.

CONFIDENCE REGARDING MID/LATE NEXT WEEK IS LOW. A WEAKER CLIPPER
LIKE SYSTEM IS SHOWN BY THE GFS ON WEDNESDAY AND THE ECMWF ON
THURSDAY WITH A STRONG COLD FRONT MOVING ACROSS THE AREA AND
ANOTHER PUNCH OF VERY COLD AIR. WHILE THEY ARE SIMILAR THE TIMING
DIFFERENCES ARE LARGE ENOUGH WITH LITTLE CONTINUITY FROM THE 00Z
ECMWF...TO NOT MAKE ANY BIG CHANGES AT THIS TIME. CMS

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 03Z...

* WINDS BECOMING SOUTHERLY BY LATE MORNING AND INCREASING TO
  AROUND 10KT THROUGH THE DAY.

KREIN

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

WITH HIGH PRESSURE SLIDING ACROSS THE REGION OVERNIGHT TONIGHT...
SKIES WILL BE CLEAR WITH WINDS DECREASING TO 5KT OR LESS. THROUGH
THE MORNING HOURS TOMORROW...THE CENTER OF THE HIGH WILL SETTLE
INTO THE OHIO VALLEY WHILE A TROUGH OF LOW PRESSURE DEVELOPS OVER
THE NORTHERN PLAINS. WINDS WILL BECOME SOUTHERLY AND WITH A MODEST
PRESSURE GRADIENT SETTING UP...WINDS SHOULD INCREASE TO ARND 10KT
BY LATE MORNING AND REMAIN AT THAT LEVEL THROUGH THE DAY. A MID
LEVEL IMPULSE LIFTING OUT OF THE SRN ROCKIES AND ACROSS THE
CENTRAL PLAINS TOMORROW WILL APPROACH THE REGION TOMORROW
EVENING. INCREASING CLOUD COVER IS EXPECTED THROUGH THE LATE
EVENING...WITH ANY ASSOCIATED PCPN LIKELY NOT MOVING INTO THE AREA
UNTIL AFTER MIDNIGHT TOMORROW NIGHT.

KREIN

//ORD AND MDW CONFIDENCE...UPDATED 03Z...

* CONFIDENCE IN ALL FORECAST ELEMENTS.

KREIN

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

SATURDAY...CHANCE OF RAIN OR FREEZING RAIN EARLY...THEN RAIN
LIKELY WITH DENSE FOG POSSIBLE OVERNIGHT. MVFR CONDITIONS LIKELY
THROUGH MUCH OF THE DAY...IFR CONDITIONS LIKELY OVERNIGHT. SOUTH-
SOUTHWEST WINDS 10-15 KT.

SUNDAY...RAIN. HEAVY AT TIMES. IFR LIKELY. GUSTY SOUTHEAST WINDS.

MONDAY...CHC -RASN BCMG CHC -SN. MVFR CIGS PSBL. STRONG SOUTHWEST
WINDS WITH GUSTS ARND 30 KT PSBL.

TUESDAY...SCHC -SN DURING THE DAY. MVFR CIGS PSBL. GUSTY WEST
WINDS.

WEDNESDAY...VFR/WX NIL DURING THE DAY. SCHC -SN OVERNIGHT. MVFR
CIGS/VIS PSBL. WEST TO SOUTHWEST WINDS 10 KT OR LESS.

KREIN

&&

.MARINE...
200 PM CST

LOW PRESSURE OVER SOUTH CENTRAL QUEBEC WILL CONTINUE TO SLIDE
EAST. AN ELONGATED RIDGE OF HIGH PRESSURE STRETCHES FROM THE
NORTHERN PLAINS SOUTHEAST THROUGH THE TENNESSEE VALLEY...AND WILL
SLOWLY MOVE EAST TOWARDS LAKE MICHIGAN TONIGHT. HIGH PRESSURE WILL
BRING MUCH LIGHTER WINDS TO LAKE MICHIGAN BY FRIDAY
MORNING...HOWEVER HIGH PRESSURE IS EXPECTED TO CONTINUE TO SLIDE
EAST TOWARDS THE MIDDLE ATLANTIC REGION BY LATE FRIDAY NIGHT. AS
HIGH PRESSURE DEPARTS LAKE MICHIGAN...WINDS WILL TURN SOUTHWEST TO
SOUTH FRIDAY NIGHT. AN AREA OF LOW PRESSURE WILL SLIDE EAST ACROSS
MANITOBA TOWARDS JAMES BAY...WITH A TIGHTENING GRADIENT EXPECTED
TO DEVELOP BY LATE FRIDAY NIGHT ACROSS CENTRAL/NORTHERN LAKE
MICHIGAN. HAVE HELD IN PLACE THE GALE WATCH...HOWEVER THIS MAY
NEED TO BE UPGRADED TO A GALE WARNING. THE MAIN CONCERN IS THAT
WITH MUCH WARMER AIR POISED TO ARRIVE FRIDAY NGT...THAT THIS MAY
CREATE LESS MIXING TO THE SURFACE OF THE HIGHER WIND GUSTS.
GUIDANCE SUGGESTS GALES MAY BE BETWEEN 34-37KT SO ALONG THE
MARGINAL LINES.

DURING THE DAY SATURDAY THE GRADIENT WILL DIMINISH ONCE AGAIN AND
TURN SOUTHEASTERLY...HOWEVER THIS WILL ONLY BE TEMPORARY AS A MORE
POTENT LOW PRESSURE SYSTEM IS POISED TO SLIDE SOUTHEAST FROM CENTRAL
CANADA SATURDAY NIGHT. A WEAKER LOW PRESSURE SYSTEM WILL DEVELOP
ACROSS THE SOUTHERN PLAINS EARLY SUNDAY...THEN QUICKLY LIFT NORTH
TOWARDS LAKE MICHIGAN. GUIDANCE DIFFERS ON THE EXACT STRENGTH AND
PATH OF THIS LOW...BUT MODELS INDICATE THE LOW WILL DEEPEN TO AROUND
975 MB/28.8 INCHES AT THE NORTH END OF THE LAKE MONDAY AFTERNOON.
EXPECTING SOUTHEAST WINDS TO INCREASE TO AT LEAST 30 KT THROUGH
SUNDAY EVENING AND THEN BECOME SOUTHWEST BEHIND THE LOW. GALES ARE
EXPECTED BY LATE SUNDAY NIGHT AND DEPENDING ON THE PATH AND STRENGTH
OF THE LOW...STORM FORCE WINDS MAY BE INCLUDED IN FUTURE FORECASTS.
GUIDANCE INDICATED A BRIEF PERIOD OF HIGH END GALES OVER THE
NORTHERN END OF THE LAKE MONDAY AFTERNOON.  THE LOW MOVES OVER
QUEBEC TUESDAY WITH WEST WINDS DIMINISHING TUESDAY MORNING.

BEACHLER

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...INDIANA NEARSHORE WATERS UNTIL 10 AM FRIDAY.

     SMALL CRAFT ADVISORY...ILLINOIS NEARSHORE WATERS UNTIL 4 AM FRIDAY.

     GALE WATCH...LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565-LMZ567-
     LMZ669-LMZ868-LMZ870...6 PM FRIDAY TO 9 AM SATURDAY.

&&

$$

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