Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 282012
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
212 PM 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 20Z...

* GUSTY SOUTH-SOUTHEAST SURFACE WINDS THIS AFTERNOON...POSSIBLY STRENGTHENING
  INTO EVENING.

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

* SOUTH-SOUTHWEST WINDS AT 2000 FT OF AT LEAST 40-45 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 18Z...

QUITE A FEW AVIATION CONCERNS FOR THIS TAF CYCLE. THE FIRST
CENTERS AROUND WINDS. SE WINDS WILL GRADUALLY RAMP UP THIS
AFTERNOON AND EVENING WITH SUSTAINED WINDS HOLDING IN THE MID
TEENS AND GUSTS INTO 22-24 KT RANGE AHEAD OF LOW PRESSURE MOVING
EAST THROUGH THE PLAINS. THE LOW MOVES THROUGH LATE TONIGHT WITH A
TRAILING COLD FRONT INTO TOMORROW MORNING. AFTER A PERIOD OF
LIGHTER WINDS...NW WINDS RAMP UP QUICKLY MID MORNING THROUGH THE
AFTERNOON. NAM/GFS FORECAST SOUNDINGS SHOW GOOD MIXING THAT WILL
SUPPORT 30 KT OR SO GUSTS THROUGH THE AFTERNOON. AT THIS POINT
LLWS HAS NOT BEEN INTRODUCED AS WINDS AT 2000 FT WILL REACH 40-45
KT WHICH IS ON THE THRESHOLD...BUT WITH THE CONTINUED GUSTINESS IT
IS MARGINAL AT THIS POINT. DURING PERIODS OF LESS FREQUENT GUSTS
THERE COULD BE SOME LLWS.

OTHER CONCERNS REVOLVE AROUND PRECIPITATION EVOLUTION AND TYPE
THROUGH EARLY THURSDAY. SEVERAL MODELS SHOW SOME DECENT LIFT IN
THE WARM ADVECTIVE REGIME THIS EVENING. IN SPITE OF
THIS...GUIDANCE IS MIXED AS TO HOW MUCH MOISTURE THERE WILL
ACTUALLY BE TO SUPPORT PRECIPITATION DURING THIS INITIAL TIME
FRAME. IF PRECIP WERE TO DEVELOP IN THE 4Z-8Z TIME FRAME...SURFACE
TEMPERATURES COULD SUPPORT FZRA. HAVE ONLY GONE WITH A VCSH GIVEN
THE LACK OF MOISTURE ON SOME GUIDANCE...BUT WILL BE A PERIOD TO
WATCH. BETTER CHANCES OF PRECIPITATION COME WITH THE COLD FRONT
LATE TONIGHT INTO EARLY THURSDAY. PRECIP LOOKS TO BE A RAIN OR
DRIZZLE BEFORE TAPERING TO A RAIN SNOW MIX. SURFACE TEMPS HOLD AT
OR ABOVE 32 FOR MOST OF THE PRECIP THUS LITTLE TO NO ACCUMULATION
IS EXPECTED.

KMD

//ORD AND MDW CONFIDENCE...UPDATED 20Z...

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

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

* MEDIUM THAT LLWS WILL REMAIN MARGINAL THROUGH THE EVENING.

* HIGH IN MVFR CIGS LATE TONIGHT INTO THURSDAY MORNING THOUGH
  MEDIUM ON TIMING. 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...
211 PM CST

I HAVE UPGRADED THE GALE WATCH TO A WARNING FOR THURSDAY
AFTERNOON AND NIGHT. AN AREA OF LOW PRESSURE APPROACHING THE LAKES
REGION FROM THE PLAINS WILL PRODUCE A PERIOD OF STRONG SOUTHERLY
WINDS AROUND 30 KT TONIGHT...THEN A PERIOD OF NORTHERLY GALE FORCE
WINDS THURSDAY AFTERNOON AND NIGHT AS MUCH COLD AIR SPILLS IN OVER
THE LAKES IN THE WAKE OF THIS LOW. I HAVE INCLUDED MY ILLINOIS
NEAR SHORES IN THIS GALE WARNING AS IT APPEARS THE OFF SHORE
COMPONENT OF THE WIND WILL BE STRONG ENOUGH TO SUPPORT SOME LOW
END GALES.

SURFACE HIGH PRESSURE WILL GRADUALLY BUILD EASTWARD ACROSS THE PLAINS
AND UPPER MIDWEST LATER THURSDAY AND THURSDAY NIGHT...AND THIS
SHOULD ALLOW FOR SOME STRONGER WINDS TO CONTINUE INTO EARLY
FRIDAY...BEFORE DIMINISHING BY FRIDAY AFTERNOON. ANOTHER COLD FRONT
LOOKS TO DROP ACROSS THE LAKES ON SUNDAY...AND A PERIOD OF STRONGER
NORTHERLY WINDS (25 TO 30 KT) APPEAR LIKELY IN ITS WAKE AS A VERY
COLD AIRMASS MOVES SOUTHWARD OVER THE LAKE.

KJB

&&

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