Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 281605
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1005 AM CST WED JAN 28 2015

.SHORT TERM...
345 AM CST

TODAY THROUGH FRIDAY NIGHT...

MAIN FORECAST CONCERNS IN THE SHORT TERM ARE CLOUD AND TEMP TRENDS
TODAY AND TONIGHT...AND TIMING...COVERAGE AND TYPE OF PRECIPITATION
TONIGHT AND THURSDAY AS LOW PRESSURE ORGANIZING OVER THE PLAINS
MOVES EAST.

RELATIVELY QUIET WEATHER THIS MORNING AS SURFACE HIGH PRESSURE
DRIFTS SLOWLY EAST OF THE AREA. SUBSIDENCE ASSOCIATED WITH HEIGHT
RISES ALOFT HAS LED TO A STRONG INVERSION AT AROUND 900 HPA HOWEVER
PER DVN 00Z SOUNDING...WHICH HAS TRAPPED LOW CLOUDS ACROSS THE
WESTERN PORTIONS OF THE FORECAST AREA OVERNIGHT. CLOUDS WERE SLOWLY
ERODING FROM THE SOUTHEAST THANKS TO 950-900 HPA WINDS GRADUALLY
VEERING TO THE SOUTHEAST AND SOUTH...AND LOWERING SUBSIDENCE
INVERSION. RAP GUIDANCE INDICATES SOUTH WINDS WILL INCREASE THIS
MORNING AND SHOULD CONTINUE TO LIFT THESE LOWER CLOUDS NORTH TOWARD
THE WI BORDER...THOUGH OVERALL NORTH CENTRAL ILLINOIS IS LIKELY TO
BE CLOUDIER THAN AREAS SOUTH AND EAST TODAY EVEN AS MID AND HIGH
CLOUDS BEGIN TO INCREASE AS WARM ADVECTION DEVELOPS ALOFT.

SURFACE LOW PRESSURE IS ALREADY TAKING SHAPE ACROSS THE PLAINS TO
OUR WEST THIS MORNING...IN RESPONSE TO A SHORT WAVE TROUGH EMERGING
FROM THE CENTRAL/NORTHERN ROCKIES. SURFACE PRESSURE FALLS AHEAD OF
THIS DEVELOPING LOW WILL LEAD TO INCREASING SOUTH-SOUTHEAST WINDS
NEAR 25 MPH THIS AFTERNOON. DESPITE SOUTHERLY FLOW...LOW LEVEL
PARCEL TRAJECTORIES WILL BE LARGELY FROM INDIANA/OHIO WHERE STRONG
COOLING HAS OCCURRED NEAR THE SURFACE HIGH THIS MORNING. COMBINED
WITH SOME MID/HIGH CLOUD COVER THIS MAY MAKE FOR HIGH TEMPERATURES
COMING LATER IN THE DAY WHEN LOW LEVEL WINDS VEER MORE SOUTHERLY AND
TAP MILDER AIR CURRENTLY ACROSS THE MISSOURI VALLEY. STRONG SOUTH
FLOW WILL CONTINUE TONIGHT AHEAD OF THE APPROACHING SURFACE
LOW...ALLOWING TEMPS TO STAY ABOVE FREEZING IN MOST AREAS.

GUIDANCE IN GOOD AGREEMENT IN DEVELOPING SOME LIGHT SPOTTY
PRECIPITATION ACROSS THE AREA LATE THIS EVENING...EXPANDING
OVERNIGHT ESPECIALLY ACROSS NORTHERN/NORTHEASTERN PARTS OF THE
FORECAST AREA. FORECAST SOUNDINGS INDICATE WARM LAYER TEMPS ALOFT IN
THE +5C RANGE OR BETTER...WITH SATURATION INITIALLY OCCURRING BELOW
700 HPA AND SUPPORTING LIGHT RAIN. WITH BREEZY SOUTH SURFACE WINDS
KEEPING TEMPS ABOVE MAINLY ABOVE FREEZING...PRECIPITATION TYPE IS
EXPECTED TO REMAIN LIQUID IN MOST LOCATIONS. LOW PROBABILITY DOES
EXIST FOR LIGHT FREEZING RAIN ESPECIALLY ACROSS FAR NORTH/NORTHEAST
IL...BUT AT THIS TIME FEEL IT IS FAIRLY UNLIKELY. THE SURFACE
LOW...AND A PAIR OF UN-PHASED SHORT WAVES....MOVE ACROSS THE CWA
DURING THE MORNING AND RESULT IN WINDS SHIFTING TO THE NORTHWEST.
THIS BRINGS COLDER AIR INTO THE REGION WITH COLLAPSING THICKNESS
FIELDS SUPPORTING LINGERING PRECIPITATION CHANGING OVER FROM LIGHT
RAIN TO A BRIEF RAIN/SLEET/SNOW MIX BEFORE ENDING FROM WEST-EAST.
MOST PRECIPITATION SHOULD END BY NOON WEST OF ABOUT THE IL/IN STATE
LINE...AND SHORTLY AFTER ACROSS NORTHWEST INDIANA. HOWEVER...GUSTY
NORTH-NORTHWEST WINDS AND COLDER AIR SPREADING ACROSS LAKE MICHIGAN
WILL LIKELY SUPPORT LAKE EFFECT SNOW SHOWERS ACROSS PARTS OF EASTERN
LAKE AND PORTER COUNTIES ESPECIALLY THURSDAY EVENING INTO EARLY
FRIDAY MORNING. THE REMAINDER OF THE AREA WILL JUST BE BLUSTERY AND
COLDER...WITH NORTH-NORTHWEST WINDS GUSTING ABOVE 30 MPH AT TIMES
THURSDAY BEFORE SLOWLY DIMINISHING THURSDAY NIGHT. AFTER MORNING
HIGHS IN THE MID-UPPER 30S OR EVEN NEAR 40 IN SOME SPOTS...THURSDAY
NIGHT LOOKS TO BE COLD WITH LOWS IN THE TEENS TO LOW 20S AND SINGLE
DIGIT WIND CHILLS.

SURFACE HIGH PRESSURE SPREADS INTO THE AREA AGAIN ON FRIDAY...WITH
TEMPERATURES STRUGGLING INTO THE MID-UPPER 20S BUT WITH MUCH LIGHTER
WINDS. LAKE SNOW SHOWERS WILL DIMINISH ACROSS NORTHWEST INDIANA AS
INVERSION HEIGHTS LOWER AND LOW LEVEL WINDS DIMINISH...WITH SUNSHINE
ELSEWHERE.

RATZER

&&

.LONG TERM...
345 AM CST

SATURDAY THROUGH TUESDAY...

THE MAIN FOCUS IN THE EXTENDED PERIOD IS ON A STORM SYSTEM WHICH
LOOKS TO LIFT THROUGH THE OHIO VALLEY LATE SATURDAY NIGHT THROUGH
SUNDAY. ECMWF/GFS ARE IN RELATIVELY GOOD AGREEMENT IN TAKING SURFACE
LOW TRACK ALONG THE OHIO RIVER...THOUGH GFS IS A LITTLE SLOWER AND
GEFS ENSEMBLE MEMBERS STILL SHOW QUITE A BIT OF SPREAD IN THEIR
SOLUTIONS AT THE SURFACE. GEM ALSO SLOWER AND A BIT FARTHER SOUTH
THAN THE OTHER GLOBAL MODELS. DEPENDING UPON HOW THIS SYSTEM
EVENTUALLY TRACKS...IT COULD BE A SNOW PRODUCER FOR ALL OR PART OF
THE CWA...AS IT TAPS GULF MOISTURE DEFORMATION BAND WOULD LIKELY
CROSS CENTRAL IL/IN. GLOBAL MODELS ALSO INDICATE A STRONG SURGE OF
ARCTIC AIR IN THE WAKE OF THIS SYSTEM...WITH SIGNIFICANTLY COLDER
WEATHER I.E. LOWS +/- SINGLE DIGITS AND HIGHS PERHAPS ONLY IN THE
TEENS BY MONDAY/TUESDAY.

RATZER

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 16Z...

* GUSTY SOUTH-SOUTHEAST SURFACE WINDS DEVELOPING BY EARLY
  AFTERNOON AND STRENGTHENING INTO EVENING.

* SCATTERED LIGHT RAIN SHOWERS LIKELY AND MVFR CIGS DEVELOPING
  OVERNIGHT...AND THE POSSIBILITY OF A BRIEF MIX WITH SNOW AND IFR
  VISIBILITY BEFORE ENDING THURSDAY MORNING.

* SOUTH-SOUTHWEST WINDS AT 2000 FT OF AT LEAST 50 KT THIS EVENING
  INTO OVERNIGHT THAT MAY CREATE LOW-LEVEL WIND SHEAR.

* NORTHWEST WINDS GUSTING TO AROUND 30 KT DURING THE DAY THURSDAY.

KMD

//DISCUSSION...UPDATED 12Z...

CURRENT BATCH OF MVFR CIGS ACROSS NORTHWEST ILLINOIS EARLY THIS
MORNING IS BEING STEERED ON SOUTHERLY WINDS EARLY THIS
MORNING...WITH RFD LIKELY TO EXPERIENCE AT LEAST TEMPORARY 2000 FT
CIGS OR SO THROUGH MID-MORNING. WITH THE APPROACH OF LOW
PRESSURE...SOUTH-SOUTHEAST WINDS WILL INCREASE WITH GUSTS LIKELY
IN THE AFTERNOON AND PERSISTING...AND LIKELY STRENGTHENING...INTO
THE EVENING AS STRONG PRESSURE FALLS ENVELOP THE REGION. GIVEN THE
CURRENT AND FORECAST PLACEMENT OF THE LOW PRESSURE...THE WIND
DIRECTION SHOULD BE EAST OF SOUTH MUCH OF TODAY AND EARLY THIS
EVENING...BEFORE GRADUALLY VEERING AHEAD OF A FRONT LATE TONIGHT.
A CHANNEL OF STRONG SOUTHWEST WINDS ALOFT...AROUND 2000-3000
FT...ARE LIKELY TO BE 50 PLUS KNOTS TONIGHT AND COULD RESULT IN
SOME LOW-LEVEL WIND SHEAR...ALTHOUGH SURFACE WINDS WILL REMAIN
ALSO INCREASED IN SOMEWHAT MIXED CONDITIONS.

THE COLD FRONT WILL PASS IN THE PRE-DAWN HOURS THURSDAY. AHEAD OF
THIS FRONT SATURATION SHOULD SLOWLY OCCUR NEAR THE INVERSION
LEVEL...WHICH IS EXPECTED TO BE AROUND 2000-3000 FT SO CIGS OF
THAT HEIGHT SHOULD GRADUALLY DEVELOP AS TONIGHT PROGRESSES. IN
ADDITION...THERE WILL BE POTENTIAL FOR SCATTERED RAIN
SHOWERS...WITH ANY RAIN POSSIBLY CHANGING TO A RAIN AND SNOW MIX
AFTER SUNRISE THURSDAY. ANY MIX IS NOT LIKELY TO LAST LONG BEFORE
ENDING...SO ACCUMULATIONS SHOULD BE ONLY A TRACE.

//ORD AND MDW CONFIDENCE...UPDATED 16Z...

* HIGH IN WIND DIRECTION AND SPEED TODAY AND THIS
  EVENING...INCLUDING THE WIND DIRECTION REMAINING SLIGHTLY EAST
  OF SOUTH.

* MEDIUM IN PRECIPITATION OCCURRING AT THE AIRFIELD LATER TONIGHT
  AND EARLY THURSDAY MORNING.

* HIGH IN MVFR CIGS LATE TONIGHT INTO THURSDAY MORNING. MEDIUM IN ANY
  IFR OCCURRING.

KMD

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

THURSDAY NIGHT...MAINLY VFR. NORTH-NORTHWEST WINDS.
FRIDAY...VFR. NORTHWEST WINDS.
SATURDAY...CHANCE OF SNOW AND IFR DURING THE NIGHT. SOUTHWEST
           WINDS.
SUNDAY...SNOW AND IFR LIKELY. NORTHEAST WINDS.
MONDAY...CHANCE OF SNOW WITH MVFR LIKELY. IFR POSSIBLE. NORTHWEST
         WINDS.
TUESDAY...CHANCE OF SNOW DURING THE NIGHT. SOUTHWEST WINDS.

MTF

&&

.MARINE...
158 AM CST

AN ACTIVE PERIOD WILL UNFOLD OVER LAKE MICHIGAN THROUGH THIS
FORECAST THANKS TO THE PROGRESSIVE NATURE IN THE WEATHER PATTERN.
HIGH PRESSURE EARLY THIS MORNING WILL EXIT THE PICTURE AND A QUICKLY
APPROACHING LOW PRESSURE FROM THE WEST WILL BRING INCREASING
SOUTHERLY WINDS LATER TODAY INTO THIS EVENING. DURING THE EVENING
THERE IS A PERIOD WHERE OCCASIONAL GALES SHOULD OCCUR ACROSS THE
SOUTHERN HALF AS SOME INSTABILITY WILL BE PRESENT WITH GALE FORCE
WINDS ONLY 1000-1500 FT OFF THE WATER SURFACE. THE LOW SHOULD BE
ELONGATED RIGHT DOWN THE AXIS OF THE LAKE AROUND 6 AM THURSDAY
BEFORE IT QUICKLY MOVES EAST. THIS WILL RESULT IN STOUT NORTHWEST
WINDS AND COLD ADVECTION ACROSS THE LAKE WITH WHAT LOOKS LIKE
DEFINITE GALES FOR AT LEAST THE OPEN WATER...INCLUDING 40 KTS
WITH 45 KTS EVEN POSSIBLE ESPECIALLY IN THE NORTH. THIS LIKELY
WILL BRING SOME FREEZING SPRAY THURSDAY NIGHT TO THE OPEN
WATER. AT THIS TIME FORECAST INGREDIENTS ARE A LITTLE SHY FOR
HEAVY FREEZING SPRAY SO NO WATCH FOR THAT ISSUED BUT WILL NEED TO
BE MONITORED.

HAVE OPTED TO GO WITH A GALE WATCH FOR BEHIND THE LOW PRESSURE AS
THAT TIME PERIOD WILL BE THE HIGHEST WINDS WITH THE SYSTEM. WILL
JUST MENTION OCCASIONAL GALES ON THE FRONT SIDE DURING THIS
EVENING ACROSS MAINLY THE SOUTH HALF. SMALL CRAFT ADVISORY
CONDITIONS WILL OBVIOUSLY EXIST A GOOD AMOUNT OF THIS TIME ON THE
ILLINOIS AND INDIANA NEAR SHORES. WITH A SHORT 4-6 HOUR RESPITE
CENTERED ON DAYBREAK THURSDAY AS THE LOW PASSES...HAVE OPTED TO NOT
EXTEND THE CURRENT SMALL CRAFT ADVISORY...BUT ONE WILL DEFINITELY
BE NEEDED FOR THURSDAY AFTERNOON AND NIGHT...AND FOR THE INDIANA
SHORE INTO FRIDAY. THE GALE WATCH DOES INCLUDE PARTS OF THE
INDIANA NEARSHORE THAT WILL FEEL THE GREATER FETCH OF ONSHORE
WINDS AND THUS COULD REALIZE GALES THURSDAY AFTERNOON AND EVENING.

MTF

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...3 PM WEDNESDAY TO 4 AM THURSDAY.

     GALE WATCH...LMZ080-LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565-
     LMZ567-LMZ669-LMZ671-LMZ673-LMZ675-LMZ744-LMZ745-LMZ777-
     LMZ779-LMZ868-LMZ870-LMZ872-LMZ874-LMZ876-LMZ878...NOON
     THURSDAY TO 3 AM FRIDAY.

&&

$$

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