Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 240730
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
230 AM CDT FRI OCT 24 2014

.DISCUSSION...
903 PM CDT

THE CHANGES TO THE FORECAST WERE TO DISSIPATE POPS AT A QUICKER
RATE...HOLD ONTO MORE CLOUD COVER THROUGH THE NIGHT...AND ADD
DENSE MENTION TO THE FOG WORDING OVER NORTH CENTRAL ILLINOIS.

THE SURFACE TROUGH THAT HAD BEEN APPROACHING STEADILY TOWARD THE
AREA THE PAST 24 HOURS HAS DIMINISHED GREATLY SINCE MID-
AFTERNOON...LOSING MUCH OF ITS LOW-LEVEL CONVERGENCE. FORCING FOR
RAIN SHOWERS HAS ALSO BEEN DIMINISHING WITH MORE LIMITED MOISTURE
CONVERGENCE WITHIN THE AREA OF ASCENT...SO SHOWERS HAVE QUICKLY
BEEN WANING. THE SPOTTY LIGHT SHOWERS SURVIVING SHOULD CONTINUE
SLOWLY EAST WITH A GRADUAL FADE INTO OVERNIGHT.

WITH THE HIGHER DEW POINT PLUME OF UPPER 40S TO LOWER 50S BROUGHT
UP TO EASTERN IA AND NORTHWEST/NORTH CENTRAL IL BY THE
TROUGH...CONDITIONS ARE IN PLACE FOR FOG. ONE THING AGAINST FOG
DEVELOPMENT IS EXISTING CLOUDS WHICH AS SEEN ON THE 00Z DVN RAOB
ARE FAIRLY DEEP /SEVERAL THOUSAND FEET SATURATED LAYER/. THIS AREA
LOOKS TO SLOW ITS MOVEMENT EAST OVER THE CWA THUS NOT ALLOWING
MUCH CLEARING FOR NORTHEAST IL AND NORTHWEST IN...INCLUDING
CHICAGO. MORE PROBLEMATIC WILL BE POSSIBLY SOME CLEARING OF THIS
CLOUD LAYER IN NORTH CENTRAL IL LATE ALLOWING FOR NEARBY VERY LOW
STRATUS AND SOME FOG ALREADY BEING OBSERVED TO JUST EXPAND IN AND
WORSEN THROUGH DAYBREAK. FORECAST SOUNDINGS AND HIGH-RES GUIDANCE
FIELDS HAVE TRENDED THIS ROUTE IN THE GENERAL ROCKFORD AND DIXON
AREAS. HAVE BUMPED UP FOG WORDING THERE AND COULD SEE A DENSE FOG
ADVISORY BEING NEEDED DEPENDING ON HOW AGGRESSIVE THE LOWERING IS.
AGAIN FURTHER EASTWARD TOWARD CHICAGO EXPECT THE CLOUDS TO LIMIT
ANY FOG THAT GRADUALLY DEVELOPS FROM GETTING TOO BAD.

HAVE BUMPED UP MINS A DEGREE OR TWO WARMER WITH THE CLOUDS
EXPECTED TO HANG ON A GOOD PART OF THE NIGHT IN MOST PLACES.

MTF

//PREV DISCUSSION...
327 PM CDT

AFTERNOON GOES VAPOR IMAGERY DEPICTS A MID-LEVEL SHORT WAVE TROUGH
PROPAGATING THROUGH THE UPPER MISSISSIPPI VALLEY...WITH A WEAK
SURFACE LOW PRESSURE REFLECTION OVER EAST CENTRAL IOWA. AS EXPECTED
THE MOISTURE TRANSPORT AHEAD/INTO THIS SYSTEM IS RATHER SPARSE...
WITH A FAIRLY NARROW RIBBON OF LOW LEVEL MOISTURE AHEAD OF THE
TROUGH INTO PORTIONS OF NORTHWEST IL AND SOUTHERN WI. HIGH-RES
GUIDANCE CONTINUES TO INDICATE THE POTENTIAL OF SCATTERED SHOWERS
SPREADING EAST ACROSS THE CWA THROUGH THIS EVENING...THOUGH WITH
COVERAGE ANTICIPATED TO REMAIN SOMEWHAT LIMITED AS LOW LEVEL
MOISTURE TRANSPORT CONTINUES TO WEAKEN WITH LOSS OF DEFINITION OF
THE SURFACE WAVE AND LOW PRESSURE TROUGH. IN ADDITION TO THE
EXTENSIVE MID-HIGH LEVEL CLOUD COVER ACROSS THE AREA THIS
AFTERNOON...WHICH WILL LIKELY OBSCURE PARTIAL SOLAR ECLIPSE LATE
THIS AFTERNOON...LOWER CLOUDS ALSO EXPECTED AS LOW LEVEL MOISTURE
AXIS ARRIVES. SOME POTENTIAL FOR FOG DEVELOPMENT ESPECIALLY ACROSS
THE WESTERN PORTION OF THE CWA WHERE LIGHT WINDS AND 45-50 DEGREE
SURFACE DEW POINT TEMPS WILL RESULT IN HIGH LOW LEVEL RH...THOUGH
CLEARING SKIES WILL BE MORE LIKELY WEST OF THE CWA WHERE SIGNIFICANT
DENSE FOG/STRATUS DEVELOPMENT IS POSSIBLE LATE TONIGHT/EARLY FRIDAY
MORNING. AT LEAST CLOUD COVER AND MORE HUMID AIR WILL KEEP TEMPS A
LITTLE MILDER OVERNIGHT.

SHORT-WAVE WILL MOVE QUICKLY SOUTHEAST OF THE FORECAST AREA FRIDAY
MORNING...WITH MODEST HEIGHT RISES AND A SUBTLE SURFACE RIDGE AXIS
DEVELOPING OVER THE AREA. ANOTHER TRAILING SHORT WAVE CURRENTLY OVER
THE ROCKIES IS PROGGED TO MOVE SOUTHEAST ACROSS MISSOURI AND
CENTRAL/SOUTHERN ILLINOIS FRIDAY MORNING/EARLY AFTERNOON WHICH MAY
KEEP A LITTLE CLOUD COVER AROUND. OTHERWISE...DOMINANT WEATHER
FEATURE BECOMES A BROAD UPPER RIDGE CENTERED OVER WEST TEXAS WHICH
COVERS MUCH OF THE CENTRAL CONUS INTO THE WEEKEND. A STRONG 150+ KT
UPPER JET FLATTENS THE TOP OF THE RIDGE AS IT RIDES ALONG THE
CANADIAN BORDER FRIDAY...THOUGH THE MAIN AREA OF SFC LOW PRESSURE
REMAINS WELL NORTH OF THE GREAT LAKES REGION THROUGH SATURDAY. A
COLD FRONT DOES TRAIL INTO THE REGION HOWEVER...MOVING ACROSS THE
CWA EARLY SATURDAY MORNING. SOME GUIDANCE DOES INDICATE SOME VERY
SPOTTY/VERY LIGHT QPF WITH FROPA DURING THE PRE-DAWN HOURS SATURDAY
ALTHOUGH MODELS ARE MUCH LESS IMPRESSIVE WITH PRECIP THAN WITH
CURRENT WAVE...WITH SREF POPS LESS THAN 20 PERCENT. COLD FRONT
COMPLETES THE TRANSIT OF THE CWA SATURDAY MORNING...THOUGH PACIFIC
NATURE OF THE FRONT PRODUCES ONLY MODEST COOLING AS IT SHIFTS WINDS
TO THE WEST-NORTHWEST DURING THE DAY. LOW LEVEL THERMAL PROGS AND
POTENTIAL FOR A GOOD DEAL OF SUNSHINE SUPPORT TEMPS IN THE 65-AROUND
70 RANGE DESPITE THE SLIGHT COOL ADVECTION BEHIND THE FRONT...AS
UPPER RIDGE BEGINS TO AMPLIFY AND DEVELOP INTO THE CENTRAL CONUS
THROUGH SATURDAY NIGHT. UPPER RIDGE CONTINUES TO AMPLIFY SUNDAY AS
LARGE SCALE UPPER TROUGH PUSHES ASHORE IN THE WEST...WITH A LARGE
LEE TROUGH DEVELOPING EAST OF THE CENTRAL AND NORTHERN ROCKIES. LOW
PRESSURE BEGINS TO DEVELOP ACROSS THE PLAINS...WITH A WARM FRONT
LIFTING ACROSS IA-IL SUNDAY NIGHT. SOUTHEAST LOW LEVEL FLOW AHEAD OF
THE WARM FRONT...IN ADDITION TO CLOUD DEVELOPMENT NORTH OF THE WARM
FRONT LOOK TO TEMPER WARM-UP ONLY SLIGHTLY...WITH 60-66 FOR HIGHS
WITH COOLEST AIR ALONG THE IL LAKE SHORE. SOME POTENTIAL FOR
ELEVATED SHOWERS/ISOLATED THUNDERSTORMS NORTH OF THE WARM FRONT
SUNDAY NIGHT...PARTICULARLY ACROSS FAR NORTHERN IL.

WARM FRONT LIFTS NORTH OF THE CWA MONDAY...WITH BREEZY/WARM AND MORE
HUMID AIR IN THE WARM SECTOR SUPPORTING TEMPS IN THE 70S AREA-WIDE.
ECMWF/GFS AND GEM ALL INDICATE VERY LITTLE PRECIP IN THE WARM SECTOR
WITH THE MAIN FOCUS ALONG THE TRAILING COLD FRONT TO OUR WEST. RAIN
AND THUNDERSTORM CHANCES THUS INCREASE MONDAY NIGHT AND TUESDAY AS
THE SURFACE LOW PRESSURE CENTER LIFTS NORTHEAST ACROSS THE UPPER
MIDWEST AND TRAILING COLD FRONT PUSHES EAST THROUGH THE REGION.
BREEZY AND COOLER CONDITIONS THEN RETURN AS THE LOW PULLS AWAY TO
THE NORTHEAST TUESDAY NIGHT AND WEDNESDAY. SOME DIFFERENCES REMAIN
WITH THE HANDLING OF THE LOW DEVELOPMENT AND TRACK...THOUGH THIS
MAINLY AFFECTS HOW QUICKLY WE DRY OUT ON TUESDAY. OTHERWISE GUIDANCE
IS ACTUALLY QUITE REASONABLE IN AGREEMENT THROUGH THE PERIOD.

RATZER

&&

.AVIATION...

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

* MVFR CIGS/VIS DURG THE LATE NIGHT/EARLY MORNING HOURS. CHANCE
  CIGS COULD DROP TO IFR LEVELS.

KREIN

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

FOLLOWING THE PASSAGE OF A WEAK SFC TROUGH...ANY RESIDUAL PCPN
REMAINS ISOLD AND SOUTH OF THE TERMINALS. HOWEVER...THERE IS SOLE
RESIDUAL LOW LEVEL MOISTURE AND AS HIGH CLOUD PUSHES AWAY...A BAND
OF STRATUS CAN BE SEEN IN THE LATEST SATELLITE IMAGERY. AS A
NOCTURNAL SUBSIDENCE INVERSION LOWERS OVERNIGHT...CLOUD BASES
SHOULD GRADUALLY LOWER TO LOWER END MVFR LEVELS. THERE IS A CHANCE
FOR SOME PATCHY IFR CIGS DURG THE EARLY MORNING HOURS...ESPECIALLY
OVER NWRN IL WHERE SOME RAIN FELL EARLIER...WHICH WOULD MOST
LIKELY AFFECT RFD AND POSSIBLY DPA. AS WINDS BECM LGT/VRBL
OVERNIGHT...VIS SHOULD DROP TO 3-5SM DURG THE EARLY MORNING
HOURS...AND IF THE WESTERN EDGE OF THE STRATUS DECK CAN ERODE OVER
NWRN IL BEFORE SUNRISE...THERE IS A CHANCE FOR RFD TO DROP INTO
IFR OR EVEN LIFR VIS LEVELS. HOWEVER...THAT WOULD BE HIGHLY
DEPENDENT ON THE CIG TRENDS.

THROUGH THE DAY ON FRIDAY...ANOTHER SFC TROUGH WILL TRACK ACROSS
THE CNTRL PLAINS TO THE UPPER MISSISSIPPI VALLEY BY EARLY EVENING.
WINDS WILL BECOME SSWLY THOUGH THE DAY...THOUGH WITH A WEAK
PRESSURE GRADIENT...WIND SPEEDS SHOULD BE LESS THAN 10KT THROUGH
THE DAY. FOR THE EARLY EVENING HOURS...SOME OF THE MODELS ARE
SUGGESTING THAT THERE COULD BE SOME LIGHT PCPN WITH THE PASSAGE OF
THE NEXT SFC TROUGH. HOWEVER...SINCE FORCING WITH THIS TROUGH AXIS
LOOKS TO BE RATHER WEAK...CONFIDENCE IS TOO LOW TO ADD PCPN TO THE
TAFS AT THIS TIME.

KREIN

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

* MEDIUM CONFIDENCE IN CIG/VIS TRENDS OVERNIGHT.

* LOW CONFIDENCE IN SPECIFIC TIMING OF IMPROVEMENT TO VFR FRIDAY.

* HIGH CONFIDENCE IN ALL FORECAST ELEMENTS THROUGH THE DAY ON
  FRIDAY.

* LOW CONFIDENCE IN THE CHANCES FOR PCPN FRIDAY EVENING.

KREIN

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

* FRIDAY NIGHT...MAINLY DRY. MVFR/FOG POSSIBLE.
* SATURDAY...DRY/VFR. GUSTY WEST/NORTHWEST WINDS.
* SUNDAY...DRY/VFR. SOUTHEAST WINDS.
* MONDAY...DRY/VFR. GUSTY SOUTH WINDS.
* TUESDAY...CHANCE OF SHRA. STRONG/GUSTY WESTERLY WINDS.
* WEDNESDAY...CHANCE OF SHRA. WEST-SOUTHWEST WINDS.

KMD

&&

.MARINE...
215 AM CDT

A WEAK RIDGE OF HIGH PRESSURE EXTENDS FROM WESTERN QUEBEC
SOUTHWEST THROUGH THE CENTRAL GREAT LAKES...INTO THE TENNESSEE
VALLEY. THIS RIDGE WILL CONTINUE TO DRIFT EAST OF THE REGION THIS
AFTERNOON...MEANWHILE A TROUGH OF LOW PRESSURE OVER CENTRAL CANADA
CONTINUES TO SLIDE EAST TOWARDS HUDSON BAY SATURDAY. THE GRADIENT
WILL REMAIN LIGHT THROUGH THIS AFTERNOON...THEN SLOWLY TURN
SOUTH/SOUTHWESTERLY. AS THE LOW NEARS HUDSON BAY ON SATURDAY...A
FRONTAL BOUNDARY WILL TRAIL AND SLOWLY SLIDE SOUTH ACROSS LAKE
MICHIGAN. THIS WILL RESULT IN WINDS TURNING WEST/NORTHWEST
SATURDAY AFTERNOON/EVENING. COOLER AIR WILL FLOW ACROSS THE LAKE
ALONG WITH A TIGHTENING GRADIENT...RESULTING IN INCREASING WINDS
NEARING 30KT AT TIMES. THEN ANOTHER RIDGE OF HIGH PRESSURE WILL
QUICKLY SLIDE SOUTHEAST FROM THE UPPER MIDWEST/NORTHERN PLAINS
SUNDAY...STEADILY DIMINISHING THE GRADIENT. ANOTHER TROUGH OF LOW
PRESSURE IS EXPECTED TO DEVELOP EARLY NEXT WEEK ACROSS THE CENTRAL
PLAINS...THEN APPROACH LAKE MICHIGAN CLOSER TO MID-WEEK. THIS WILL
BRING WINDS BACK TO THE SOUTHWEST WITH OCCASIONALLY WINDS GUSTING
TO 30KT.

BEACHLER

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...NONE.
&&

$$

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