Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 290558
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1258 AM CDT MON SEP 29 2014

.DISCUSSION...
259 PM CDT

SYNOPSIS...

THE LONG STRETCH OF WARM AND DRY WEATHER WILL COME TO AN
END THIS WEEK...BUT NOT BEFORE ONE MORE BEAUTIFUL DAY ON MONDAY. THE
FIRST SIGN OF A CHANGE IN THE WEATHER PATTERN WILL COME WITH A COLD
FRONT PASSAGE ON MONDAY NIGHT THAT WILL RESULT IN MUCH COOLER
TEMPERATURES ON TUESDAY...MOST NOTABLY NEAR THE LAKE. A STORM SYSTEM
LATE IN THE WEEK WILL BRING THE POTENTIAL FOR AT LEAST A PERIOD OR
TWO OF FAIRLY WIDESPREAD SHOWERS AND STORMS WEDNESDAY NIGHT INTO
THURSDAY NIGHT...THEN MUCH COOLER AIR WILL ARRIVE AREA WIDE FRIDAY
INTO NEXT WEEKEND.

TONIGHT AND MONDAY...

ANOTHER NIGHT OF LITTLE TO NO CHANGE IN AIR MASS AND CALM WINDS WITH
SFC HIGH OVER REGION...SO STAYED CLOSED TO OBSERVED LOWS FOR MINS
TONIGHT. THOUGH DEW POINTS DID MIX OUT IN SPOTS THIS
AFTERNOON...STATISTICAL AND HI-RES GUIDANCE IN GOOD AGREEMENT IN
PATCHY FOG AND HAZE OVERNIGHT...WITH AREAS OF FOG IN FAR NORTHEAST
IL. COULD SEE LOCALLY DENSE FOG IN SOME SPOTS. THIS SEEMS REASONABLE
GIVEN THAT FOG WAS ABLE TO DEVELOP EARLY THIS MORNING DESPITE FAIRLY
DENSE CIRRUS AND EXPECTING LESS CIRRUS TONIGHT. BOUNDARY LAYER FLOW
WILL SHIFT TO LIGHT WEST-NORTHWEST AHEAD OF COLD FRONT ON
MONDAY...BUT FLOW SHOULD STILL BE LIGHT ENOUGH FOR LAKE BREEZE TO
FORM. ANTICIPATING HIGHS GENERALLY IN THE LOWER 80S INLAND...WITH A
FEW MID 80S IN SW 1/3 OF CWA. LATER LAKE BREEZE FORMATION SHOULD
ENABLE SHORE TO REACH MID TO UPPER 70S.

MONDAY NIGHT AND TUESDAY...

SHARP BACKDOOR COLD FRONT WILL SWEEP SOUTH DURING EVENING WITH GUSTY
NORTHERLY WINDS AND FALLING TEMPS. THIS WILL BE FOLLOWED BY
EXTENSIVE STRATUS DECK OOZING SOUTHWARD THROUGH THE NIGHT. CONTINUE
TO FEATURE SLIGHT CHANCE POPS IN NORTHEAST ILLINOIS DURING EVENING
AND OVERNIGHT AS FORCING FROM SHARP UPPER TROUGH PASSING THROUGH THE
GREAT LAKES GIVES A GLANCING BLOW...THOUGH COULD BE MORE OF A
SPRINKLES/DRIZZLE SET-UP.

THE MODELS ARE IN GOOD AGREEMENT IN MOISTURE GETTING TRAPPED BENEATH
INVERSION AND POSSIBLY TAKING MUCH OF THE DAY ON TUESDAY TO SCOUR
OUT...ESPECIALLY NEAR THE LAKE. ONSHORE FLOW AND ONLY SLOW EROSION
OF CLOUD COVER EXPECTED TO KEEP LAKESHORE HIGHS IN THE UPPER 50S TO
LOWER 60S...WHILE AREAS WELL AWAY FROM THE LAKE SHOULD WARM INTO 60S
AND 70S WITH EXPECTATION OF CLOUD DECK ERODING FROM SOUTH/SOUTHWEST.
CONFIDENCE IS HIGH IN LOW STRATUS DECK TO START DAY...BUT LOWER IN
HOW LONG IT WILL TAKE TO ERODE...WHICH COULD IMPACT TEMPS IN SPOTS.

TUESDAY NIGHT THROUGH SUNDAY...

ATTENTION TURNS TO DEEP UPPER TROUGH OVER NORTHWESTERN CONUS THAT
WILL BRING AN ACTIVE PERIOD WEDNESDAY NIGHT THROUGH THURSDAY NIGHT
AFTER OUR LONG DRY STRETCH. DRY AND COOL TUESDAY NIGHT WITH HIGH
PRESSURE RIDGING OVER AREA WILL BE FOLLOWED BY A QUICK RECOVERY ON
WEDS IN WAA PATTERN WITH H85 TEMPS WARMING BACK TO MID TEENS
CELSIUS. CLOUDS WILL INCREASE FROM WEST...AND SOME SHOWER ACTIVITY
COULD SNEAK INTO FAR NW DURING AFTERNOON. MUCH BETTER CHANCE FOR
SHRA WEDNESDAY NIGHT AS LOW LEVEL JET RAMPS UP THOUGH INSTABILITY
LOOKS TO BE MINIMAL LIMITING THREAT FOR WIDESPREAD TSRA.

MAINTAINED LIKELY POPS ON THURSDAY...THOUGH THERE IS A DIFFERENCE IN
TIMING AND EVOLUTION OF SFC LOW EXPECTED TO DEVELOP OVER THE REGION
ON OPERATIONAL AND ENSEMBLE GUIDANCE...WITH GFS A STRONG OUTLIER.
THIS WILL LIKELY PLAY ROLE IN OVERALL INTENSITY OF SHRA/TSRA. COLD
FRONT WILL BE EAST OF CWA BY EARLY FRIDAY...THOUGH UPPER TROUGH AND
MUCH COOLER AIR MASS OVER AREA COULD TOUCH OFF SHRA DUE TO STEEP
LAPSE RATES. HAVE A DRY FORECAST FOR NOW ON SATURDAY...THOUGH SOME
GUIDANCE AT THAT RANGE INDICATING POTENTIAL FOR SHRA BEFORE SFC
RIDGE BUILDS OVERHEAD SATURDAY NIGHT. TEMPERATURES EXPECTED TO BE
NEAR TO BELOW SEASONABLE ON FRIDAY IN 60S...BUT THEN COOLER THAN
NORMAL ON SATURDAY WITH SOME AREAS POSSIBLY NOT GETTING OUT OF THE
50S. IF SKIES ARE CLEAR ON SATURDAY NIGHT...COOLNESS OF AIR MASS
COULD PRESENT POTENTIAL FOR FIRST FREEZE OF SEASON IN PARTS OF
CWA...RECOVERING TO SEASONABLE HIGHS ON SUNDAY.

RC

&&

.AVIATION...

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

* PATCHY FOG LATE TONIGHT THROUGH EARLY MORNING. MVFR VSBY LIKELY
  WITH IFR VSBY POSSIBLE.

* VARIABLE/CALM WINDS OVERNIGHT...BECOMING LIGHT WEST-NORTHWEST
  THIS AFTERNOON.

* COLD FRONTAL PASSAGE EARLY THIS EVENING WITH WIND SHIFT TO THE
  NORTHEAST AND GUSTS 15-20 KT DEVELOPING.

* MVFR/IFR CIGS LIKELY BEHIND COLD FRONT THIS EVENING.

MDB

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

HIGH PRESSURE REMAINS IN CONTROL ACROSS THE AREA BUT IS WEAKENING.
CALM TO LIGHT AND VARIABLE WINDS ARE IN PLACE ONCE AGAIN WITH
CLEAR SKIES AND DEWPOINTS SIMILAR TO LAST NIGHT. PATCHY FOG HAS
ALREADY BEGUN TO DEVELOP AND WILL CONTINUE TO DO SO THROUGH THE
OVERNIGHT. EXPECT A GENERAL MVFR VSBY FOR MOST OF THE AREA IN THE
NEXT FEW HOURS WITH DENSER PATCHY/SHALLOW FOG BRINGING SOME IFR
VSBY AT TIMES. VISIBILITY WILL IMPROVE WITH SUNRISE WITH VFR BY
MID MORNING CONTINUING THROUGH THE AFTERNOON WITH LIGHT WEST TO
WEST-NORTHWEST WINDS THROUGH THE DAY.

A SHARP COLD FRONT WILL PUSH ACROSS THE TERMINALS FIRST THING THIS
EVENING BRINGING A SHIFT TO NORTHEAST WINDS AND AN INCREASE IN
WIND SPEEDS/GUSTS. HAVE SPED UP THE TIMING SLIGHTLY WITH FRONTS
SUCH AS THIS OFTEN ACCELERATING DOWN LAKE MICHIGAN. COOLER AIR
WILL SPREAD IN BRINGING AN INCREASE IN LOW STRATUS. WILL CARRY LOW
END MVFR CIGS BEHIND THE FRONT THIS EVENING BUT IFR IS POSSIBLE.
VSBY MAY BE REDUCED IN MIST/HAZE AS THE COLDER AIR ARRIVES AND
THERE MAY BE SOME DRIZZLE THAT TRIES TO SET UP. MAY NEED TO LOWER
CIGS/VSBY FOR THE EVENING IF THE DRIZZLE ENDS UP BECOMING
WIDESPREAD.

MDB


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

* MEDIUM CONFIDENCE IN VSBY IN FOG LATE TONIGHT/EARLY AM.

* HIGH CONFIDENCE IN WIND TRENDS...THOUGH MEDIUM CONFIDENCE IN
  DIRECTION OF LIGHT WINDS THIS MORNING/EARLY AFTERNOON AND EXACT
  TIMING OF SHIFT TO NORTHEAST WINDS THIS EVENING.

* MEDIUM-HIGH CONFIDENCE IN LOW MVFR/HIGH IFR CIGS DEVELOPING THIS
  EVENING BEHIND COLD FRONT.

MDB

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

TUESDAY...MVFR CIGS LIKELY AND IFR CIG POSSIBLE IN THE MORNING.
NORTHEAST WINDS.

WEDNESDAY...VFR. LAKE BREEZE LIKELY.

THURSDAY...TSRA LIKELY...MVFR POSSIBLE. SOUTH WINDS.

FRIDAY...CHANCE SHRA...MVFR POSSIBLE. BREEZY WEST WINDS.

SATURDAY...VFR. WEST WINDS.

RATZER

&&

.MARINE...
213 PM CDT

QUIET WEATHER WILL CONTINUE OVER LAKE MICHIGAN REST OF TODAY INTO
EARLY MONDAY UNDER HIGH PRESSURE...HOWEVER A COLD FRONT WILL START
TO PUSH SOUTH ACROSS THE LAKE MONDAY WITH WINDS TURNING NORTHERLY
AND FRESHENING UP INTO THE 15 TO 25 KT RANGE. A SMALL CRAFT
ADVISORY WILL EVENTUALLY BE NEEDED FOR THE INDIANA AND ILLINOIS
NEARSHORE WATERS FOR MONDAY NIGHT THROUGH TUESDAY EVENING. WINDS
TURN BACK SOUTHEAST TUESDAY NIGHT AS A WARM FRONT LIFTS BACK
ACROSS LAKE MICHIGAN. BY THURSDAY LOW PRESSURE IS PROGGED TO LIFT
NORTH ACROSS THE UPPER MIDWEST INTO CANADA WHILE DEEPENING...
THOUGH THERE ARE SIGNIFICANT DIFFERENCES BETWEEN MODELS REGARDING
THE TRACK AND STRENGTH OF THIS LOW. THE GFS PAINTS THE WORST
CONDITIONS OVER THE LAKE AND SUGGESTS THE POSSIBILITY OF STRONG
GALES FOR A COUPLE PERIODS BETWEEN THURSDAY NIGHT THROUGH EARLY
SATURDAY. THE GFS FOR NOW THOUGH IS A BIT OF AN OUTLIER SO WILL
REMAIN CONSERVATIVE NOW WITH RESPECT TO THE WIND SPEED FORECAST
LATE IN THE WEEK THOUGH GALES DO APPEAR TO BE A DISTINCT
POSSIBILITY.

BMD

&&

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

$$

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