Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 241802
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1202 PM CST SAT JAN 24 2015

.SHORT TERM...
308 AM CST

TODAY THROUGH SUNDAY NIGHT...

THE MAIN FORECAST ISSUE WILL BE ACCUMULATING SNOW AND GUSTY WINDS
STARTING LATER TONIGHT CONTINUING THROUGH MUCH OF SUNDAY. WATER
VAPOR IMAGERY SHOWS A LONGWAVE TROUGH STRETCHING FROM HUDSON BAY
SOUTH TO THE GULF COAST WITH BROAD RIDGING OVER THE EASTERN
PACIFIC AND MUCH OF WESTERN CANADA AND THE NORTHWEST U.S. A WEAK
COOL FRONT IS CROSSING THE AREA BEHIND THE PASSAGE OF THE LONGWAVE
TROUGH AXIS. COOL ADVECTION THIS MORNING WILL GIVE WAY TO MODEST
WARM ADVECTION LATE THIS AFTERNOON. CLOUD COVER WILL BE THE WILD
CARD WITH TEMPS. STRATUS IS SPREADING SOUTHEAST ACROSS THE AREA
THIS MORNING BUT SATELLITE SHOWS GAPS IN COVERAGE UPSTREAM.
GUIDANCE IS MIXED WITH THE EXTENT OF CLOUD COVER BUT SUSPECT THERE
WILL BE BREAKS IN THE CLOUDS...ESPECIALLY THIS AFTERNOON...WITH
ANY MEANINGFUL SUNSHINE DURING THE EARLY/MID AFTERNOON HELPING
TEMPS SOMEWHAT. THIS IS MOST FAVORED SOUTH AND WEST SO WILL NUDGE
TEMPS UP SLIGHTLY THERE WHILE CARRYING UPPER 30S ACROSS THE REST
OF THE AREA.

THE MAIN FEATURE OF CONCERN IS A WAVE MOVING EAST-SOUTHEAST ACROSS
SOUTHERN ALBERTA. EVENING UPPER AIR ANALYSIS SHOWS A STRONG JET
PUSHING INTO THE PACIFIC NORTHWEST AND PIVOTING SOUTHEASTWARD AS
ADDITIONAL ENERGY PUSHES EASTWARD AMPLIFYING THE RIDGE. THE WAVE OF
CONCERN WILL DIG SOUTHEASTWARD ACROSS THE NORTHERN PLAINS TOWARD THE
MID MISSISSIPPI RIVER VALLEY AS A RESULT AND AMPLIFY AS IT DOES SO
INTO TONIGHT. THE SOUTH/WEST TREND OF THE TRACK OF THE SURFACE LOW
HAS CONTINUED AND DIFFERENCES REMAIN...ESPECIALLY WITH SOME OF THE
HIGH RES GUIDANCE WHICH TAKES THE LOW MUCH FURTHER SOUTH AND WEST
THAN SOME OF THE LOWER RES OPTIONS. IT APPEARS THAT THE LOW WILL NOT
TRACK NORTH OF ROUGHLY A BURLINGTON IOWA TO TERRE HAUTE INDIANA LINE
AT THIS POINT. WITH THE WAVE STILL TAKING SHAPE AND THE UPPER JET
STILL COMING ASHORE OVER THE PAC NORTHWEST FEEL THAT CONFIDENCE HAS
ROOM FOR IMPROVEMENT WITH THE MORNING UPPER AIR ANALYSIS AND
SUBSEQUENT 12Z GUIDANCE. THE MAIN QUESTION WILL BE PLACEMENT BUT
HAVE BETTER CONFIDENCE IN THE TREND OF WHAT OCCURS. PRECIP IS ON
TRACK TO QUICKLY SPREAD SOUTHEAST ACROSS THE AREA OVERNIGHT/AFTER
06Z/ AND SOUTHWEST AREAS STILL LOOK TO SEE A MIX OF RAIN OR SNOW AND
EVEN SOME SLEET THROUGH MID MORNING WHEN COLD AIR WILL TAKE OVER
CHANGING THINGS TO SNOW. PEAK PRECIP INTENSITY WILL BE AFTER
DAYBREAK INTO THE EARLY AFTERNOON. WINDS WILL TURN NORTHEAST AND
INCREASE DURING THE MORNING SETTING UP A DECENT FETCH DOWN LAKE
MICHIGAN. LAKE EFFECT PARAMETERS ARE SUB PAR BUT WHEN COMBINED WITH
SYNOPTIC SCALE ASCENT ARE PROBABLY SUFFICIENT TO PROVIDE SOME
MINIMAL ENHANCEMENT...MAINLY DUE TO INCREASED MOISTURE FLUX.
GUIDANCE VARIES WITH LIQUID OUTPUT WITH SOME RATHER HIGH AMOUNTS
NOTED PROBABLY SUGGESTIVE OF STRONG BANDING WHICH IS CERTAINLY
POSSIBLE. GIVEN PRECIPITABLE WATER VALUES OF AROUND 0.5 INCHES AND
FORCING OF 6 TO MAYBE 9 HOURS PEAK LIQUID AMOUNTS OF 0.3 TO 0.4
INCHES STILL SEEMS REASONABLE. LOCATION OF THESE AMOUNTS IS EXPECTED
ACROSS THE SOUTHERN CWA WITH AMOUNTS OF ONLY AROUND 0.1 INCHES
ACROSS THE NORTH. WITH SOUTHERN AREAS ALSO SEEING A MIX NOT ALL OF
THE LIQUID WILL GO TOWARD SNOW ACCUMULATION. THUS HIGHEST SNOW
AMOUNTS WILL OCCUR JUST TO THE NORTH...THOUGH WITH SOMEWHAT LOW
LIQUID EQUIVALENT. WILL DEPICT THE PEAK SNOWFALL FROM ROUGHLY
LASALLE COUNTY EAST-SOUTHEAST ALONG AND JUST SOUTH OF INTERSTATE 80
TO ROUGHLY KANKAKEE TO RENSSELAER. WILL CARRY 2-4 INCHES FOR THIS
AREA WITH AMOUNTS TAPERING TO ABOUT AN INCH ALONG THE WI
BORDER...WITH 1-2 INCHES ACROSS THE FAR NORTHERN CHICAGO METRO TO
AROUND 3 POSSIBLY 4 INCHES ACROSS THE FAR SOUTHERN CHICAGO METRO.
THERE IS STILL A CHANCE THIS COULD SHIFT FURTHER SOUTH.
PRECIPITATION QUICKLY ENDS MID TO LATE SUNDAY AFTERNOON WITH LITTLE
IF ANY LAKE EFFECT LINGERING GIVEN POOR INVERSION HEIGHTS.

HEADLINE POTENTIAL...WILL HOLD OFF ON ANY HEADLINES FOR NOW GIVEN
THAT A FURTHER SOUTH SHIFT IS STILL PLAUSIBLE. ANOTHER CONCERN IS
WIND SPEEDS WHICH WILL INCREASE TO 20 TO 25 MPH WITH GUSTS OF 30-35
MPH POSSIBLE SUNDAY MORNING THROUGH THE AFTERNOON. HOWEVER...WITH
FAIRLY WET SNOW RATIOS BLOWING WILL BE MINIMIZED BUT PROBABLY STILL
OCCUR. THERE IS ALSO A WINDOW FROM LATE MORNING THROUGH EARLY
AFTERNOON WHERE THE FALLING SNOW AND WIND MAY PUSH VISIBILITIES DOWN
UNDER A HALF OR QUARTER MILE...BUT THIS MAY OCCUR OVER A RATHER
SMALL AREA. EXPECT TO NEED A WINTER WEATHER ADVISORY ACROSS THE PEAK
SNOW AREA BUT AGAIN THE LOCATION MAY SHIFT AND IF BANDING DEVELOPS
THE SWATH OF SNOW COULD REALLY NARROW SO WOULD LIKE TO SEE THE
SYSTEM EVOLVE A LITTLE BIT MORE BEFORE DECIDING WHICH AREAS WILL SEE
THE BIGGEST IMPACT. WILL ISSUE A SPECIAL WEATHER STATEMENT
HIGHLIGHTING THE POTENTIAL IMPACTS FOR NOW.

MDB

&&

.LONG TERM...
308 AM CST

MONDAY THROUGH FRIDAY...

A MUCH BETTER DEFINED RIDGE WILL BE OVER WESTERN NORTH AMERICA
WITH A DEEP TROUGH OVER THE EAST COAST BY MONDAY. ANOTHER WAVE
WILL BE TRAVERSING THE RIDGE THEN DROP SOUTHEAST ACROSS THE GREAT
LAKES MONDAY BRINGING LOW CHANCES FOR SOME LIGHT SNOW. HIGH
PRESSURE WILL BUILD IN FROM THE WEST MONDAY NIGHT TURNING WINDS
MORE NORTHERLY. LAKE PARAMETERS WILL LIKELY BE MARGINAL BUT COULD
SEE SOME FLURRY OR LIGHT SNOW ACTIVITY DEVELOP FOR TUESDAY. THE
UPPER RIDGE BEGINS TO SHIFT EASTWARD WEDNESDAY WITH THE SURFACE
HIGH SHIFTING EAST AS WELL. THIS WILL LEAD TO A SHIFT TO SOUTHERLY
WINDS AND WARMING TEMPS LATE IN THE WEEK. GUIDANCE IS IN AGREEMENT
ON A LOW CROSSING THE AREA FIRST THING THURSDAY BRINGING MIXED
PRECIP POTENTIAL OR EVEN JUST RAIN. COLDER AIR BUILDS IN BEHIND
THE SYSTEM FRIDAY AS HIGH PRESSURE BUILDS INTO THE PLAINS. OVERALL
EXPECT TEMPS TO BE AT OR ABOVE NORMAL FROM TUESDAY ONWARD.

MDB

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 18Z...

* MVFR CIGS WILL RETURN WITH BASES ARND 3KFT AGL...THEN LOWER
  TOWARDS 1000-1200FT AGL AFTER MIDNIGHT.

* WEST WINDS ARND 5-10KT...QUICKLY SHIFTING EAST AFTER MIDNIGHT
  AND INCREASING TO 10-18KT GUSTS TO 25KT BY DAYBREAK SUN. GUSTS
  INCREASE TOWARDS 30KT SUN MORNING.

* SNOW DEVELOPS JUST BEFORE DAYBREAK SUN...WITH PEAK TIMING
  BETWEEN 14-17Z FROM PERIODS OF STEADIER SNOW. CIGS WILL LOWER
  TOWARDS IFR CONDS WITH POSSIBLE IFR VSBYS FROM STEADY SNOW.

BEACHLER

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

VISIBLE IMAGERY INDICATES SOME CLEARING HAS TAKEN PLACE LATE THIS
MORNING ACROSS NORTHEAST IL/NORTHWEST IN...HOWEVER THERE IS A
WEDGE OF MVFR CIGS STARTING TO CREEP SOUTHEAST ACROSS NORTHWEST
IL. GUIDANCE AND TIMING WOULD INDICATE THE MVFR CIGS RETURNING TO
ORD/MDW ARND 19Z. THEN THE FOCUS WILL TURN TOWARDS THE APPROACHING
WINTER STORM SYSTEM. GUIDANCE CONTINUES TO SHIFT THE TRACK OF THE
SYSTEM FURTHER SOUTH/SOUTHWEST OF CHICAGO. THIS WILL LIKELY
DICTATE WHERE THE STEADIER/TIMING OF SNOW WILL OCCUR SUN MORNING.
AT THIS TIME HAVE ONLY MADE MINOR ADJUSTMENTS TO THE GOING
FORECAST...WITH SNOW DEVELOPING AFTER MIDNIGHT. POSSIBLY SEEING
REDUCED VSBYS FROM STEADIER SNOW BETWEEN 13-17Z. AS THE LOW PASSES
TO THE SOUTH...THE PRESSURE GRADIENT WILL RAPIDLY INCREASE ALONG
THE NORTHERN TIER OR NEAR THE TAF SITES. THIS WILL RESULT IN A
QUICK SHIFT TO EAST WINDS AND GUSTS APPROACHING 20KT ARND
DAYBREAK...THEN INCREASING TO NEAR 30KT BY MID TO LATE MORNING.

IFR CIGS WILL REMAIN ACROSS THE TAF SITES INTO SUN MIDDAY...THEN
SOME DRIER AIR WILL BEGIN TO FILTER INTO THE REGION AND BRING CIGS
BACK TO MVFR LATE SUN AFTN.

BEACHLER


//ORD AND MDW CONFIDENCE...UPDATED 18Z...

* HIGH CONFIDENCE IN CIGS RETURNING TO MVFR CONDS...MEDIUM
  CONFIDENCE IN PRECISE TIMING OF MVFR CIGS.

* MEDIUM CONFIDENCE IN TIMING OF CIGS LOWERING FURTHER TO IFR
  CONDS LATER TONIGHT.

* HIGH CONFIDENCE IN SNOW DEVELOPING AFTER MIDNIGHT...MEDIUM
  CONFIDENCE IN PRECISE TIMING OF STEADIER SNOW BETWEEN 13-17Z.

* HIGH CONFIDENCE IN WINDS QUICKLY TURNING EAST AND GUSTS
  DEVELOPING.

BEACHLER

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

SUNDAY NIGHT...MVFR CIGS POSSIBLE.

MONDAY...CHANCE OF LIGHT SNOW IN THE AFTERNOON. MVFR POSSIBLE.

TUESDAY...MVFR CIGS POSSIBLE.

WEDNESDAY...MAINLY VFR...CHANCE OF SNOW AND MVFR/IFR AT NIGHT.

THURSDAY...CHANCE OF RAIN/SNOW. MVFR/IFR POSSIBLE.

FRIDAY...PRIMARILY VFR.

RATZER

&&

.MARINE...
238 AM CST

MAIN MARINE CONCERNS FOCUS ON THE PERIOD FROM EARLY SUNDAY MORNING
THROUGH SUNDAY EVENING...AS NORTH-NORTHEASTERLY WINDS QUICKLY
INCREASE IN RESPONSE TO A FAST-MOVING DEEPENING ALBERTA-CLIPPER LOW
WHICH WILL DIG SOUTHEAST FROM IOWA INTO CENTRAL ILLINOIS AND
SOUTHERN INDIANA DURING THE DAY.

IN THE VERY NEAR TERM THIS MORNING...LOW PRESSURE WAS MOVING EAST OF
JAMES BAY ACROSS QUEBEC...WITH A COLD FRONT TRAILING FROM THE LOW
ACROSS LAKE SUPERIOR AND WEST ALONG THE INTERNATIONAL BORDER. THIS
FRONT WILL SAG SOUTH ACROSS NORTHERN LAKE MICHIGAN LATER TODAY WITH
WINDS SHIFTING NORTH AND INCREASING BEHIND IT AS HIGH PRESSURE
SPREADS SOUTHEAST ACROSS MANITOBA AND NORTHWEST QUEBEC. BY
TONIGHT...THE FAST-MOVING CLIPPER WILL ALSO ALREADY BE ENTERING
IOWA  AND MOVING INTO WEST-CENTRAL ILLINOIS BY SUNDAY MORNING WHILE
DEEPENING A BIT. THE TIGHTENING GRADIENT AROUND THE LOW WILL ALSO
HELP TO TURN WINDS NORTHEAST AND STRENGTHEN THEM ALL THE WAY TO THE
SOUTH END OF THE LAKE AS THE LOW EVENTUALLY TRACKS INTO THE OHIO
VALLEY IN THE AFTERNOON. MODEL FORECAST SOUNDINGS DEPICT A PERIOD OF
SOLID 30 KT NORTHEAST WINDS ACROSS THE SOUTH 2/3 OR 3/4 OF THE LAKE
SUNDAY...WITH A FEW GUSTS OVER 30 KTS (MARGINAL GALE FORCE) DURING
THE DAY. WITH THE LOW MOVING QUICKLY OFF TO THE SOUTHEAST SUNDAY
NIGHT...WINDS WILL DIMINISH AS QUICKLY AS THEY INCREASED EARLY IN
THE DAY...AS WEAK HIGH PRESSURE RIDGING MOVES ACROSS THE LAKE SUNDAY
NIGHT. WILL NEED A SMALL CRAFT ADVISORY FOR THE IL/NW IND NEARSHORE
WATERS FROM ABOUT 6 AM CST SUNDAY THROUGH ABOUT 6 AM CST MONDAY FOR
THE STRONG WINDS AND RESULTING WAVES.

EARLY NEXT WEEK THE PATTERN CONTINUES TO APPEAR RATHER
PROGRESSIVE...WITH THE WEEK HIGH PRESSURE RIDGE OVER THE LAKE SUNDAY
NIGHT QUICKLY BEING DISPLACED MONDAY BY ANOTHER LOW PRESSURE SYSTEM
WHICH MOVES INTO THE NORTHERN LAKES MONDAY AFTERNOON. THIS BRINGS
WINDS AROUND TO THE SOUTH AND INCREASING INTO THE 25 KT RANGE FOR A
TIME MONDAY AS THE LOW APPROACHES. THE LOW THEN WEAKENS AS IT MOVES
SOUTHEAST ACROSS THE LAKES MONDAY NIGHT...FILLING AS A DEEP LOW
DEVELOPS ALONG THE ATLANTIC COAST. WEAK HIGH PRESSURE SPREADS ACROSS
THE LAKE TUESDAY...BEFORE ANOTHER LOW DEVELOPS ACROSS THE CENTRAL
PLAINS AND MOVES INTO THE MID-MISSISSIPPI VALLEY REGION THROUGH LATE
WEDNESDAY.

RATZER

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...6 AM SUNDAY TO 9 AM MONDAY.

&&

$$

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