Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 280946
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
346 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 09Z...

* LOW CHANCE FOR 1500-2500 FT CIGS BEFORE 14Z.

* GUSTY SOUTH-SOUTHEAST SURFACE WINDS DEVELOPING BY 18Z.

* CHANCE FOR SCATTERED LIGHT RAIN SHOWERS BEGINNING LATE THIS
  EVENING.

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

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

AS HIGH PRESSURE MOVES EAST OVERNIGHT...SOUTHERLY WINDS WILL
START TO BRING CLOUDS ACROSS NORTHWEST AND WEST CENTRAL ILLINOIS
BACK NORTH TO NORTHEASTWARD. WHILE THE CLOUDS ARE THIN...THEY ARE
UNDER A STRONG INVERSION OF 8C/1000 FT...SO NOT ENVISIONING MUCH
MORE EROSION OVERNIGHT. RFD SHOULD GO BACK TO BROKEN OR EVEN
OVERCAST AS THESE CLOUDS EXPAND BACK NORTHWARD AND IT IS POSSIBLE
ORD AND DPA DO AS WELL.

WITH THE APPROACH OF LOW PRESSURE...SOUTH-SOUTHEAST WINDS WILL
INCREASE WEDNESDAY WITH GUSTS LIKELY IN THE AFTERNOON AND
PERSISTING INTO THE EVENING AS STRONG PRESSURE FALLS SPREAD OVER.
GIVEN THE CURRENT AND FORECAST PLACEMENT OF THE LOW PRESSURE...THE
WIND DIRECTION SHOULD BE EAST OF SOUTH MUCH OF WEDNESDAY AND EARLY
WEDNESDAY EVENING...BEFORE GRADUALLY VEERING DURING LATER
WEDNESDAY NIGHT. A COLD FRONT WILL PASS IN THE PRE-DAWN HOURS.
AHEAD...ALONG...AND BEHIND THIS FRONT WILL BE POTENTIAL FOR
SCATTERED RAIN SHOWERS...WITH THAT CHANGING TO A RAIN AND SNOW
SHOWER CHANCE DURING THE DAY THURSDAY.

//ORD AND MDW CONFIDENCE...UPDATED 09Z...

* HIGH THAT IF MVFR CIGS DO RETURN PRIOR TO 14Z THEY WOULD BE
  TEMPORARY.

* HIGH IN WIND SPEED AND IN WIND DIRECTION BEING SLIGHTLY EAST OF
  SOUTH THROUGH THE DAY. MEDIUM IN WIND DIRECTION TONIGHT.

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

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

FRIDAY...VFR. WX NIL.

SATURDAY...CHANCE OF SNOW IN THE AFTERNOON AND OVERNIGHT. VFR
EARLY THEN MVFR LIKELY/IFR POSSIBLE.

SUNDAY...CHANCE OF SNOW. IFR POSSIBLE.

MONDAY...VFR. WX NIL.

MTF/KREIN

&&

.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...NOON 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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