Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 220544
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1144 PM CST SUN DEC 21 2014

.UPDATE...
914 PM CST

WHILE THERMAL PROFILES ARE STILL A BIT TRICKY IN OUR WESTERN CWA
OVERNIGHT...FEELING IS THAT IF SOME PRECIP SNEAKS IN OVERNIGHT...SURFACE
TEMPS WOULD BE AROUND FREEZING OR POSSIBLY SLIGHTLY WARMER WHICH
WOULD MITIGATE IMPACTS SIGNIFICANTLY.  MLI IS REPORTING UNKNOWN
PRECIP THIS EVENING BUT WITH A SURFACE TEMP IN THE MID 30S. WHEN
PRECIP ARRIVES IN THE MORNING IN THE WEST AND SOUTH...TEMPS SHOULD
LARGELY STILL BE ABOVE FREEZING. WHILE THE CHANCE IS NOT ZERO FOR
SOME LIGHT FZDZ OR SLEEET...CONFIDENCE IS LOW.

KMD

&&

.SHORT TERM...
246 PM CST

THROUGH TUESDAY...

TONIGHT THROUGH TUESDAY...
BROAD SFC RIDGE HAS BECOME CENTERED OVER THE NORTHEAST CONUS...AND
CONTINUES TO SLOWLY DRIFT EAST/NORTHEAST. THE GRADIENT ACROSS THE
GREAT LAKES HAS REMAINED LIGHT THIS AFTN...WITH A MAINLY SOUTHERLY
FLOW. CLOUDS REMAIN LOCKED IN ACROSS THE FORECAST AREA...WITH SOME
CLEARING JUST SOUTHEAST OF THE CWFA IN CENTRAL/NORTHERN IN. SFC
TEMPS HAVE BEEN ABLE TO WARM TO THE MID/UPR 30S...WITH DEW PTS
HOVERING AROUND 30-33 DEG. MOST THE EVENING WILL REMAIN DRY...WITH
THE WEAK MID-LVL WAVE SLIDING NORTHEAST ACROSS IOWA/WISC WITH SOME
LGT SNOW...OTHERWISE MOST OF THE FORECAST AREA SHUD REMAIN DRY THRU
THE OVERNIGHT PERIOD.

500MB VORT BEGINS TO LIFT NORTHEAST FROM THE CENTRAL PLAINS BEFORE
DAYBREAK MON...PUSHING THE MOIST CHANNEL FURTHER EAST ACROSS THE
FORECAST AREA. WITH SOUNDINGS SUGGESTING A WARM WEDGE DEVELOPING
ALOFT...P-TYPE COULD BE A CHALLENGE ARND DAYBREAK. GUIDANCE WOULD
SUGGEST TEMPS COULD BE WARM ENOUGH TO LIMIT PRECIP TO LGT
RAIN...WITH NEAR SFC TEMPS HOLDING ARND THE FREEZING MARK...SO HAVE
CONTINUED THE MENTION OF FZRA MAINLY SOUTH OF ROCKFORD. CLOSER TO
ROCKFORD THE THERMAL PROFILE IS A LITTLE MORE TRICKY AND COULD ALLOW
SOME LGT SNOW/SLEET AT THE ONSET CHANGING TO LGT RA/FZRA BEFORE
MIDDAY. THROUGHOUT THE AFTN TEMPS WILL STEADILY WARM...NEARING 40
DEG IN MOST LOCATIONS AS WARM/MOIST AIR CONTINUES TO ADVECT NORTH
ACROSS THE CWFA. MOST GUIDANCE INDICATES LGT RAIN WILL PERSIST THRU
THE AFTN/EVE HOURS...WITH THE DRY WEDGE NOT ARRIVING UNTIL CLOSER TO
DAYBREAK TUE. LOCAL SOLUTION SUGGESTS SLIGHTLY EARLIER WITH BRINGING
A TEMPORARY END TO THE PRECIP...JUST AFT MIDNIGHT MON NGT.

TEMPS OVERNIGHT WILL REMAIN MILD IN THE UPR 30S/NEAR 40. WITH THE
TROUGH TAKING ON A NEGATIVE TILT MIDDAY TUE...THE SFC LOW WILL BE
WRAPPED UP ACROSS CENTRAL WISC. GUIDANCE HAS BOUNCED AROUND POSSIBLY
KEEPING EITHER DRY/LOW CHC POPS OVER NORTHERN IL/NORTHWEST IN
TUE/TUE AFTN...BUT HAVE OPTED TO HOLD ONTO A SLT CHC OF RAIN. WITH
WARM AIR CONTINUING TO ADVECT NORTH...SFC TEMPS COULD PUSH INTO THE
MID/UPR 40S WITH A FEW LOCATIONS IN THE SOUTHEAST REACHING 50 DEG.
BY TUE EVENING THE TROUGH AXIS IS PROGGED TO EJECT ANOTHER SFC LOW
ACROSS THE GULF STATES...THEN QUICKLY LIFT NORTH ACROSS THE TENN
VALLEY LATE TUE NGT. MOISTURE WILL FILL BACK IN ACROSS THE FORECAST
AREA TUE NGT...WITH RAIN BECOMING MIXED WITH SNOW OVERNIGHT. TEMPS
WILL SLOWLY COOL...SO THE PERIOD OF RAIN/SNOW MIX COULD LINGER
AWHILE.

BEACHLER

&&

.LONG TERM...
246 PM CST

WEDNESDAY THROUGH SUNDAY...

MID-WEEK ENSEMBLES CONTINUE TO PROG A SLIGHT STRENGTHENING TO THE
DOWNSTREAM RIDGE OVER THE CANADIAN MARITIME...WHICH WILL LIMIT HOW
FAR EAST THE SECONDARY LOW WILL PROGRESS. OPERATIONAL SOLUTIONS
CONTINUE TO LIFT THE STRENGTHENING LOW NORTH ACROSS THE OHIO VALLEY
WED AFTN...AND QUICKLY NORTH OF LAKE HURON BY LATE WED NGT. WRAP
AROUND MOISTURE AND COOLER AIR WILL BRING SOME LGT SNOW TO NORTHERN
IL WED.

FOR CHRISTMAS DAY THE DOWNSTREAM RIDGE WILL BEGIN TO
WEAKEN...ALLOWING THE MID-LVL TROUGH TO BECOME PROGRESSIVE AND PUSH
EAST. THIS SHUD BRING AN END TO THE LGT SNOW/FLURRIES BY DAYBREAK.
TEMPS SHUD RETURN TO THE MID/UPR 30S...ALONG WITH PERHAPS SOME
BREAKS IN THE CLOUD COVER BY THE AFTN. MID-LVL HEIGHTS SHUD START TO
INCREASE THUR NGT/FRI...HOWEVER THIS LOOKS TO BE SHORT-LIVED AS YET
ANOTHER TROUGH IS PROGGED TO DIG SOUTH FROM THE CANADIAN ROCKIES
INTO THE NORTHERN PLAINS. THE STRONGER PUSH OF COLDER AIR LOOKS TO
ARRIVE SAT...ALTHOUGH THIS MAY ONLY BE A SHORT DURATION WITH THE
SEMI-ZONAL PATTERN SETTING UP FOR THE SECOND HALF OF THE WEEKEND.

BEACHLER

&&

.AVIATION...

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

* VFR OR BRIEF HIGHER END MVFR CIGS THIS MORNING EXPECTED TO LOWER
  TO IFR THIS AFTERNOON AND POSSIBLY LIFR THIS EVENING.

* SPOTTY LIGHT RAIN/SNOW WEST/NORTHWEST OF ORD/MDW EARLY THIS
  MORNING.

* RAIN DEVELOPING MIDDAY AND CONTINUES INTO TONIGHT.

* SOUTH-SOUTHEAST WINDS AROUND 10 KT BECOMING SOUTHEAST NEAR 15 KT
  BY AFTERNOON.

RATZER

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

FORECAST AREA REMAINS ALONG SOUTHEASTERN PERIPHERY OF LOW STRATUS
WITH CIGS HAVING IMPROVED TO VFR DURING THE LATE EVENING HOURS.
COMPACT MID-LEVEL DISTURBANCE NOTED LIFTING NORTHEAST ACROSS WEST-
CENTRAL IL PRODUCING SPOTTY LIGHT PRECIP IN THE FORM OF LIGHT
RAIN OR RAIN/SNOW AND SOME PATCHY 2500-3000 FT MVFR CIGS...AND
WILL LIKELY AFFECT RFD AND POSSIBLY CHICAGO TERMINALS FOR A FEW
HOURS DURING THE PRE-DAWN HOURS. OTHERWISE...MORE STEADY RAIN IS
EXPECTED TO OVERSPREAD THE AREA MIDDAY AND CONTINUE INTO TONIGHT.
CIGS AND VSBY EXPECTED TO DETERIORATE TO IFR WITH THE RAIN EARLY
THIS AFTERNOON...AND LIKELY TO LIFR TONIGHT AS A WARM FRONT
APPROACHES.

SOUTH-SOUTHEAST WINDS 8-10 KT ARE EXPECTED TO BECOME EAST-
SOUTHEAST NEAR 15 KT BY EARLY AFTERNOON...DECREASING AND BACKING
SOUTH AGAIN TOWARD TUESDAY MORNING.

RATZER


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

* MEDIUM CONFIDENCE IN CIGS THROUGH PERIOD.

* MEDIUM-HIGH IN GENERAL PRECIP TIMING.

* HIGH CONFIDENCE OTHER ELEMENTS.

RATZER

//OUTLOOK FOR ORD/MDW FOR 12Z TUESDAY-12Z SUNDAY...UPDATED 00Z...

TUESDAY...MVFR CIGS LIKELY. IFR CIGS POSSIBLE...MAINLY EARLY.

WEDNESDAY...MVFR CIGS LIKELY. CHANCE OF RA/SN.

THURSDAY...CHANCE OF MVFR CIGS...MAINLY EARLY.

FRIDAY...CHANCE OF MVFR CIGS. CHANCE OF -RA/-SN

SATURDAY...MVFR CIGS LIKELY. CHANCE OF FLURRIES.

IZZI

&&

.MARINE...
204 PM CST

SMALL CRAFT ADVISORY...KEPT THE CURRENT SMALL CRAFT AS IS AND
EXPANDED IT TWO ZONES SOUTH THROUGH CALUMET HARBOR.  WAVES WILL
BECOME HAZARDOUS OVERNIGHT NORTH OF WILMETTE HARBOR AND THEN WINDS
AND WAVES WILL BECOME HAZARDOUS TOMORROW AFTERNOON. MARGINAL SMALL
CRAFT CONDITIONS ARE POSSIBLE ALONG THE INDIANA NEARSHORE WATERS AND
DO NOT HAVE ENOUGH CONFIDENCE TO ISSUE A SMALL CRAFT ADVISORY AT
THIS TIME.

HIGH PRESSURE IS OVER THE EAST COAST WHILE THE NEXT LOW IS OVER THE
NORTHERN PLAINS.  SOUTH WINDS HAVE INCREASED TO 15-25 KT ACROSS THE
LAKE AND WILL CONTINUE TO INCREASE TO 30 KT THIS AFTERNOON. WINDS
BECOME SOUTHEAST TOMORROW AS THE LOW MOVES OVER THE EASTERN PLAINS
AND WEAKENS.  THE LOW THEN WEAKENS OVER WISCONSIN TUESDAY AFTERNOON
THROUGH TUESDAY NIGHT. WINDS WEAKEN ACROSS THE LAKE MONDAY NIGHT AND
BECOME WEST TO NORTHWEST 10-15 KT TUESDAY NIGHT.  A SECOND LOW FORMS
OVER THE SOUTHERN MISSISSIPPI VALLEY TUESDAY NIGHT AND QUICKLY
DEEPENS AS IT MOVES NORTH.  GUIDANCE DIFFERS ON HOW FAR TO OUR EAST
THE LOW WILL PASS...BUT THINKING IT WILL BE OVER THE EASTERN GREAT
LAKES. GUIDANCE HAS ALSO BACKED OFF ON WIND SPEEDS BEHIND THE LOW
AND WILL ONLY CARRY OCCASIONAL GALES IN THE GLF AT THIS TIME. SO
HAVE WEST TO NORTHWEST WINDS INCREASING TO 30 KT WEDNESDAY NIGHT.
THE BEST WINDOW FOR GALES WOULD BE LATE WEDNESDAY NIGHT.  THE LOW
THEN CONTINUES NORTHEAST INTO QUEBEC THURSDAY AND HAVE SOUTHWEST
WINDS DECREASING THURSDAY.  ANOTHER LOW SLIDES ACROSS THE PLAINS
THURSDAY AFTERNOON AND WILL IMPACT THE LAKE FRIDAY.  GUIDANCE
DIFFERS ON HOW STRONG THE LOW WILL BE WHEN IT MOVES OVER THE
LAKE...SO HAVE LOW CONFIDENCE IN THE FORECAST LATE NEXT WEEK.

JEE

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...NEARSHORE WINTHROP TO WILMETTE HARBORS UNTIL 3 AM TUESDAY.

     SMALL CRAFT ADVISORY...LMZ741-LMZ742...NOON MONDAY TO 3 AM
     TUESDAY.

&&

$$

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