Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 111615
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1115 AM CDT FRI JUL 11 2014

.DISCUSSION...
259 AM CDT

TODAY...
LARGEST FOCUS FOR THE DAY WILL BE ON THE GRADUAL DEPARTURE TO THE
SFC RIDGE...WHICH GUIDANCE HAS SLOWED SLIGHTLY. THIS SHUD ALLOW FOR
A MOSTLY DRY DAY...HOWEVER AS THE EVENING PROGRESSES AND HIGHER DEW
PTS STEADILY ADVECT NORTH ACROSS THE FORECAST AREA...PRECIP CHANCES
WILL STEADILY INCREASE. AT THIS TIME CONFIDENCE IS HIGH THAT ANY
PRECIP WILL NOW HOLD OFF UNTIL AFT 00Z...SO HAVE PUSHED THE TIMING
BACK A FEW HOURS. THE THERMAL TROUGH THAT HELPED TO KEEP TEMPS DOWN
A FEW DEGREES WILL ALSO DEPART TODAY...HOWEVER CLOUDS WILL BE
STEADILY ON THE INCREASE THROUGHOUT THE AFTERNOON. BUT EXPECT TEMPS
REBOUNDING INTO THE UPR 70S TO LOW 80S. WINDS SHUD BE GENERALLY
SOUTHERLY...THE ONLY EXCEPTION COULD BE FAR NORTHEAST IL WHERE WINDS
MAY BE SLIGHTLY TURNED FROM THE SOUTHEAST AND USHER IN SLIGHTLY
COOLER AIR INTO NORTHEAST LAKE COUNTY.

TONIGHT THROUGH SATURDAY...
AS PREVIOUSLY NOTED...HAVE PUSHED BACK THE TIMING OF THE PRECIP
UNTIL AFT 00Z. SOME WEAK DIFFLUENCE IN THE MID-LVLS IS STILL
PRESENT...ALTHOUGH STEADILY WEAKENING AS THE EVENING PROGRESSES.
WARM/MOIST AIR WILL CONTINUE TO ADVECT NORTH INTO THE FORECAST
AREA...WITH SKIES BECOMING MOSTLY CLOUDY. INSTABILITY AT THE ONSET
IS NOT SIGNIFICANT...SO EXPECT THE LEADING EDGE OF THE PRECIP TO
PERHAPS NOT HAVE MUCH CONVECTION WITH IT. THIS WILL HOWEVER CHANGE
AS THE OVERNIGHT HOURS PROGRESS INTO SAT MORNING. HI-RES LCL ARW8KM
HAS FOR A FEW CYCLES BEEN SUGGESTIVE OF A LOBE OF VORTICITY SLIDING
EAST/SOUTHEAST ACROSS IA/CENTRAL IL AFT 09Z SAT THEN TURNING
SOUTHEAST. PWAT VALUES CONTINUE TO BE PROGGED AT 3 TO 4 SIGMA...OR
HOVERING AT 2" FOR MUCH OF THE FORECAST AREA. SO ANY CONVECTION THAT
DEVELOPS COULD EASILY PRODUCE SOME BRIEF HEAVY DOWNPOURS.

SAT MORNING MAY FEATURE A BRIEF LULL IN THE ACTIVITY...DESPITE THE
HIGH DEW PTS THAT WILL CONTINUE TO ADVECT NORTH/NORTHEAST ACROSS THE
REGION. A FEW SOLUTIONS HAVE CONTINUED TO POINT TOWARDS DEW PTS
NEARING THE MID 70S BY SAT AFTN. SO A VERY HUMID ENVIRONMENT IS IN
SHAPE FOR SAT...FORTUNATELY ABUNDANT CLOUD COVER WILL KEEP TEMPS
FROM GETTING TOO OUT OF CONTROL. THUS HAVE ONLY MADE MINOR
ADJUSTMENTS TO THE AFTN HIGHS...WHICH WILL GENERALLY BE IN THE
LOW/MID 80S.

THE FOCUS FOR ADDTL CONVECTIVE DEVELOPMENT WILL BE ON THE SHORTWAVE
PROGGED TO BE LIFTING INTO THE CENTRAL PLAINS SAT AFTN. THE MAIN
TROUGH AXIS WILL BE STARTING TO DIG INTO THE NORTHERN PLAINS SAT
AFTN/EVE...WHICH WILL AID IN INCREASING THE SOUTHWEST LLVL FLOW SAT
EVE FOR THE FORECAST AREA. THIS COUPLED WITH INCREASING
SHEAR...COULD AID IN THE POTENTIAL FOR STRONG TO PERHAPS SEVERE
STORMS TO DEVELOP SAT EVE. GENERAL STORM MOTION SHOULD BE WEST-EAST.

CONFIDENCE...MEDIUM/HIGH.

SUNDAY THROUGH THURSDAY...
FOCUS FOR THE SECOND HALF OF THE WEEKEND INTO NEXT WEEK WILL BE THE
UNSEASONABLY COOL WEATHER EXPECTED FOR NEXT WEEK. ALTHOUGH PRIOR TO
THE ARRIVAL...SUN WILL FEATURE A COUPLE BOUNDARIES PUSHING EAST
ACROSS THE REGION. A WEAK BOUNDARY SHUD BE ORIENTED FROM NORTHERN MO
STRETCHING NORTHEAST THRU SOUTHERN MI...WITH A SECONDARY WAVE
APPROACHING THE REGION LATE SUN NGT/EARLY MON. THE 500MB VORT WILL
BE ARRIVING OVER THE UPPER MIDWEST LATE SUN...WITH A FEW WEAK LOBES
OF VORTICITY PIVOTING ARND THE AXIS. 850MB THERMAL TROUGH WILL ONLY
CONTINUE TO COOL AS TIME PROGRESSES...AND EXPECT BY TUE TEMPS ALOFT
TO NEAR -3 SIGMA. ENSEMBLES HAVE MAINTAINED A HIGH CONFIDENCE WITH
MINIMAL SPREAD FOR NUMEROUS CYCLES. BASED ON THE PATTERN OF A HIGH
AMPLITUDE RIDGE DEVELOPING ACROSS THE WESTERN CONUS/CANADA...THIS
WILL TRANSLATE INTO A DEEP/STRONG TROUGH FOR THE GREAT LAKES. EXPECT
TEMPS TUE TO STRUGGLE TO WARM BEYOND THE UPR 60S FOR SOME LOCATIONS
NORTH OF I-80...AND PERHAPS ARND 70 ELSEWHERE.

AS THE EXTENDED PERIODS PROGRESS...THE POTENCY OF THE TROUGH BEGINS
TO MODERATE AND TEMPS START TO WARM GENERALLY INTO THE MID 70S WITH
BROAD SFC RIDGING BUILDING IN.

CONFIDENCE...HIGH.

BEACHLER

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 16Z...

* POTENTIAL FOR A LAKE BREEZE THIS AFTERNOON.
* TSRA LATER TONIGHT WITH MVFR/IFR POSSIBLE.
* MVFR CONDITIONS AFTER TSRA NEAR DAYBREAK SATURDAY.

MDB

//DISCUSSION...UPDATED 16Z...

BAND OF SPRINKLES AND MAYBE A FEW SHOWERS CONTINUES TO MOVE ACROSS
NORTHWEST AND INTO NORTH-CENTRAL ILLINOIS. THIS MAY BRING
SPRINKLES OR A LIGHT SHOWER TO RFD BY MIDDAY BUT THAT MAY BE TOO
PESSIMISTIC. OTHERWISE...WINDS ARE STARTING TO INCREASE IN SPEED
ACROSS NORTH CENTRAL ILLINOIS WITH MORE VARIABILITY ACROSS THE
CHICAGO AREA AND NW INDIANA. STILL APPEARS THE TREND FOR THESE
AREAS WILL BE TO END UP SOUTH-SOUTHEAST WITH A LAKE BREEZE STAYING
EAST OF ORD/MDW...THOUGH GYY MAY SEE MORE OF AN EASTERLY SHIFT
THAN CURRENTLY INDICATED. MID AND HIGH CLOUD COVER WILL CONTINUE
TO INCREASE.

MDB

FROM 12Z...

BENIGN CONDITIONS THIS MORNING ACROSS THE TERMINALS AS HIGH
PRESSURE SHIFTS EAST OF THE AREA RESULTING IN LIGHT SSE WINDS.
WINDS WILL GRADUALLY PICK UP OUT OF THE SOUTH FROM WEST TO EAST AS
THE GRADIENT INCREASES. LAKE BREEZE IS THE FIRST CHALLENGE IN THIS
TAF PACKAGE. CONCERN IS THAT THE GRADIENT WILL NOT BE STRONG
ENOUGH TO PREVENT THE LAKE BREEZE FROM MAKING IT TO MDW AND
POSSIBLY ORD. THERE IS MIXED SIGNALS IN HIGH RESOLUTION
GUIDANCE...AS THE HRRR BRINGS THE LAKE BREEZE THROUGH BOTH ORD AND
MDW...WHILE MOST OTHER GUIDANCE BRINGS IT CLOSE BEFORE LOWER LEVEL
SOUTHERLY WINDS PICK UP. WILL CONTINUE WITH CURRENT THINKING THAT
IT WILL GET TO MDW AND NOT QUITE TO ORD BUT CONFIDENCE ON THIS IS
NOT SUPER HIGH.

ONCE SOUTHERLY WINDS PICK UP AND LIMIT THE LAKE BREEZE...CONCERN
SHIFTS TO THUNDERSTORM ACTIVITY LATER TONIGHT. HAVE GONE WITH
PROB30 TSRA AS CONFIDENCE IS NOT HIGH ON THE TIMING AT THIS POINT
AND IT COULD ACTUALLY BE A LITTLE SOONER THAN IN THE CURRENT
FORECAST. IT LOOKS LIKE SHOWERS WITH EMBEDDED THUNDER AS
INSTABILITY IS NOT ALL THAT HIGH. AFTER THUNDERSTORM ACTIVITY
CLEARS...EXPECT LINGERING LOW LEVEL MOISTURE AND A LIGHT GRADIENT
WILL RESULT IN MVFR CIGS FOR A TIME.

KMD

//ORD AND MDW CONFIDENCE...UPDATED 16Z...

* LOW CONFIDENCE IN LAKE BREEZE REACHING ORD TODAY...LOW TO
  MEDIUM CONFIDENCE IN GETTING VERY CLOSE TO MDW.

* MEDIUM CONFIDENCE IN TSRA IMPACTING THE TERMINALS LATE
  TONIGHT...BUT LOW CONFIDENCE ON TIMING.

* MEDIUM CONFIDENCE ON MVFR CIGS FOLLOWING TSRA...BUT LOW
  CONFIDENCE ON TIMING.

MDB

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

SATURDAY NIGHT...TSRA WITH MVFR/IFR POSSIBLE.

SUNDAY THROUGH TUESDAY...PERIODIC CHANCES FOR TSRA/SHRA. PRIMARILY VFR.

WEDNESDAY THROUGH THURSDAY...SMALL CHANCES FOR SHRA. PRIMARILY VFR.

KMD

&&

.MARINE...
332 AM CDT

HIGH PRESSURE HAS SHIFTED TO THE EASTERN GREAT LAKES EARLY THIS
MORNING...AND WINDS HAVE TURNED MORE SOUTHERLY ACROSS THE LAKE.
EXPECT SPEEDS TO INCREASE TODAY IN RESPONSE TO A STRONGER GRADIENT
CREATED BY LOW PRESSURE CENTERED IN NORTHERN MANITOBA AND INTO THE
NORTHERN PLAINS. THE STRONGER GRADIENT WILL BE OVER THE NORTHERN
HALF OF THE LAKE...WHERE SOUTH WINDS INCREASE TO 15 TO 25 KT...WHILE
THE SOUTH HALF WILL SEE S TO SE WINDS AT 10-20 KT. WINDS DIMINISH
SATURDAY AS A TROUGH OF LOW PRESSURE MOVES OVERHEAD AND BECOME
SOUTHWEST SATURDAY NIGHT.

A LOW PASSES WELL NORTH OF THE LAKE SATURDAY NIGHT AND SUNDAY AND
ITS COLD FRONT SWINGS OVER THE LAKE SUNDAY MORNING. WINDS BECOME
WEST BEHIND THE FRONT MAINLY FOR THE NORTHERN HALF...AND THEN A
SECOND FRONT PUSHES DOWN THE LAKE SUNDAY NIGHT INTO MONDAY BEFORE
FINALLY CLEARING THE LAKE MONDAY EVENING.  WINDS WILL BECOME NORTH
TO NORTHWEST 10-20 KT BEHIND THE FRONT.  WINDS DIMINISH LATE TUESDAY
NIGHT AND THEN HIGH PRESSURE MOVES OVER THE LAKE WEDNESDAY NIGHT.

KMD

&&

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

$$

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