Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 240257
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
957 PM CDT THU OCT 23 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 03Z...

* MVFR CIGS AND VSBY OVERNIGHT INTO FRIDAY MORNING. IFR CONDITIONS
  POSSIBLE LATE TONIGHT.

KMD

//DISCUSSION...UPDATED 03Z...

SHOWERS ARE GENERALLY FALLING APART OVER NE ILLINOIS...JUST SOME
ISOLATED -SHRA AT THIS POINT. CIGS ARE SLOWLY LOWERING FROM WEST
TO EAST. GUIDANCE IS BACKING OFF ON THE EASTWARD EXTEND OF LOW
VISIBILITIES GIVEN THAT THE CLOUD COVER WILL REMAIN PERSISTENT
THROUGH THE NIGHT...BUT WE DO EXPECT CONTINUED GRADUAL LOWERING OF
CIGS INTO MVFR EVERYWHERE....AND TO IFR IN WESTERN AREAS.
WIDESPREAD DENSE FOG IS NO LONGER EXPECTED...BUT THERE COULD BE
SOME MVFR OR POSSIBLY IFR VSBYS TOWARD DAYBREAK GIVEN THE MOIST
LOW LEVELS AND LIGHT FLOW AND THE LOWERING INVERSION. KMD

PREVIOUS DISCUSSION FROM 00Z...

SLOW MOVING BAND OF SCATTERED SHOWERS WILL CONTINUE TO MIGRATE
EAST THIS EVENING. EXPECT THE BEST CHANCE FOR A SHOWER WILL BE
ALONG AND NORTH OF A KRPJ-KDPA-KORD LINE OR GENERALLY NORTH OF
INTERSTATE 88.

WIDESPREAD MVFR CIGS BEHIND THE SHOWERS WILL SPREAD INTO NE
ILLINOIS AND NW INDIANA THROUGH THE NIGHT. EXPECT WITH LIGHT WINDS
AND NO DRY AIR PUSHING IN BEHIND THE FRONT THAT CONDITIONS WILL
DETERIORATE THROUGH THE NIGHT. AREAS TO THE WEST SUCH AS AROUND
KRFD AND WEST OF THE MAIN CHICAGOLAND TERMINALS HAVE THE BEST
CHANCE OF REALLY TANKING TONIGHT WITH LIFR CIGS AND VSBYS.
GUIDANCE IS HINTING AT IFR AND EVEN LIFR CONDITIONS EAST TO
CHICAGO LATE TONIGHT AS WELL...BUT WILL HAVE TO WATCH TRENDS AS WE
HAVE A HARD TIME BELIEVING CLOUDS WILL CLEAR AWAY FROM WESTERN
AREAS AND WOULD STILL GENERALLY EXPECT A GRADUAL LOWERING TO LOW
END MVFR. IF CLOUDS ARE ABLE TO CLEAR FARTHER EAST...THEN THE LIFR
CONDITIONS SUGGESTED BY MOS GUIDANCE THE HRRR WOULD BE POSSIBLE
ACROSS THE AREA.

EXPECT DECENT CLEARING TO BE SLOW GOING BUT EXPECT LIFTING TO VFR
BY THE MIDDAY OR SO. LAKE BREEZE LOOKS TO BE CONTAINED ALONG THE
LAKESHORE FRIDAY AFTERNOON.

KMD

KMD

//ORD AND MDW CONFIDENCE...UPDATED 03Z...

* HIGH IN CEILING TRENDS...BUT LOW-MEDIUM IN HOW LOW THE CIGS/VSBY
  WILL BE.

* LOW-MEDIUM IN SPECIFIC LIFTING TIMES ON FRIDAY.

KMD

//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...
244 PM CDT

HIGH PRESSURE WILL CONTINUE TO MOVE AWAY FROM LAKE MICHIGAN THROUGH
THE REMAINDER OF THE WEEK INTO THE WEEKEND AS LOW PRESSURE MOVES
EAST ACROSS THE CANADIAN PRAIRIES RESULTING IN GENERALLY SOUTH TO
SOUTHWEST FLOW OVER THE LAKE. THIS LOW WILL MOVE OVER JAMES BAY
SATURDAY DRAGGING A COLD FRONT ACROSS LAKE MICHIGAN AND TURNING
WINDS TO THE WEST/NORTHWEST. THE COLD AIR BEHIND THE FRONT IS SLOW
TO TRICKLE IN AND THE AIRMASS IS SHALLOW...SO WHILE THERE ARE
GALES JUST OFF THE DECK...IT DOESNT APPEAR THAT THEY WOULD MIX
DOWN TO THE LAKE SURFACE AT THIS TIME. WINDS WILL PEAK
OCCASIONALLY TO 30 KT SATURDAY AND SATURDAY NIGHT...THEN SHOULD
TREND DOWN THROUGH THE DAY SUNDAY AS HIGH PRESSURE BUILDS BACK
OVER THE REGION. ANOTHER AREA OF LOW PRESSURE IS PROGGED TO LIFT
ACROSS THE REGION AROUND MIDWEEK NEXT WEEK AND WILL ONCE AGAIN
BRING MODERATE WINDS TO LAKE MICHIGAN.

BMD

&&

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

$$

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