Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 310602
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
102 AM CDT SUN AUG 31 2014

.DISCUSSION...
221 PM CDT

SYNOPSIS...SCATTERED THUNDERSTORMS ARE POSSIBLE NW OF I-55 WHILE A
LINE OF STORMS WILL IMPACT AREAS ALONG AND EAST OF I-55 THROUGH THIS
EVENING. TOMORROW WILL BE DRY BUT UNSETTLED WEATHER RETURNS MONDAY
WITH MULTIPLE CHANCES FOR THUNDERSTORMS.  SEVERE STORMS ARE POSSIBLE
MONDAY. TEMPERATURES WILL BE ABOVE NORMAL THROUGH THE PERIOD WITH A
COUPLE DAYS GETTING INTO THE MID 80S.

THROUGH TONIGHT...
AN UPPER LEVEL TROUGH IS OVER THE UPPER MISSISSIPPI RIVER VALLEY AND
IT WILL PASS OVER THE REGION THROUGH THIS EVENING. A SURFACE LOW IS
OVER THE NORTHERN END OF LAKE MI WITH ITS COLD FRONT EXTENDING
THROUGH WI AND EASTERN IA.  THE LOW WILL CONTINUE NE THROUGH THE
NIGHT.

HAVE MUCH HIGHER CONFIDENCE IN THE CONVECTIVE FORECAST TODAY
COMPARED TO YESTERDAY DUE TO CLEAR FORCING MECHANISMS.  EXPECTING TO
HAVE TWO AREAS TO MONITOR. THE FIRST IS NORTHWEST OF I-55 WHERE
ISOLATED TO SCATTERED STORMS WILL CONTINUE TO FESTER THROUGH THIS
AFTERNOON AHEAD OF UPPER LEVEL WAVE AND COLD FRONT.

THE BIGGER CONCERN WILL BE ALONG AND SE OF I-55 WHERE A LINE OF
SHOWERS AND STORMS HAS ALREADY DEVELOPED.  THE LATEST RAP ANALYSIS
SHOWS A NICE CORRIDOR OF MOISTURE CONVERGENCE WHERE THE CONVECTION
HAS FIRED.  THE LATEST AMDAR SOUNDINGS ARE CAPLESS WITH 1000-1500
J/KG OF CAPE.  HOWEVER...LAPSE RATES ARE POOR AND WIND SHEAR IS
MINIMAL AT BEST.  THE AMDAR SOUNDINGS DO HAVE LOW LEVEL INVERTED
V/S AND THE RAP ANALYSIS HAS AROUND 1000 J/KG OF DOWNDRAFT CAPE.
THEREFORE NOT EXPECTING WIDESPREAD SEVERE WEATHER...BUT A STORM OR
TWO MAY PULSE UP AND PRODUCE SEVERE WINDS.

HEAVY DOWNPOURS ARE ALSO EXPECTED WITH PWAT VALUES OF 1.6-1.8
INCHES.  BASED ON SHORT TERM GUIDANCE...THE AREA MOST CONCERNING IS
SOUTH OF A VALPARAISO IN TO GIBSON CITY IL LINE. STORMS WILL
CONTINUE TO DEVELOP AND MOVE OVER THIS REGION WITH AMPLE FORCING.
THEREFORE IF STORMS TRAIN OVER THE SAME AREA FOR AN EXTENDED PERIOD
OF TIME...FLASH FLOODING MAY OCCUR.

STORMS DIMINISH AND PUSH EAST OVERNIGHT WHILE HIGH PRESSURE SPREADS
OVER THE REGION. WINDS WILL DIMINISH AND FOG IS POSSIBLE.

SUNDAY...
SUNDAY LOOKS DRY WITH HIGH PRESSURE OVER THE REGION.  WE WILL BE IN
THE NEXT LOW/S WARM SECTOR WITH 850MB TEMPS AROUND 18C. THEREFORE
RAISED HIGH TEMPS INTO THE MID 80S AWAY FROM THE LAKE. THE WEAK
PRESSURE GRADIENT WILL ALLOW A LAKE BREEZE TO FORM AND KEEP THE IL
LAKEFRONT AROUND 80. GUIDANCE HAS SLOWED THE NEXT LOW SO PUSHED
BACK POPS SUNDAY NIGHT.

JEE

MONDAY...
UPPER WAVE DIGGING TOWARDS THE INTER-MOUNTAIN WEST THIS AFTERNOON
WILL LIFT ACROSS THE PLAINS INTO THE UPPER MIDWEST ON MONDAY WITH
ATTENDANT SURFACE LOW DEEPENING AS IT LIFTS ACROSS ONTARIO TO NEAR
JAMES BAY BY LATE MONDAY. A SURFACE FRONT WILL SLOWLY PUSH ACROSS
THE REGION MONDAY AND GUIDANCE HAS TRENDED TOWARDS A SLOWER SOLUTION
OVER THE LAST 24 HOURS. MODERATE TO STRONG INSTABILITY IS PROGGED TO
DEVELOP ALONG AND AHEAD OF THE FRONT WHERE THE GFS ADVERTISES A
CORRIDOR OF JUST OVER 2000 J/KG MLCAPE JUST TO THE WEST OF THE CWA
MONDAY AFTERNOON. THE BETTER CONVERGENCE ASSOCIATED WITH THE COLD
FRONT MAY ARRIVE JUST PAST THE MOST FAVORABLE DIURNAL TIMING IF
TIMING TRENDS HOLD TRUE...BUT THE RIGHT ENTRANCE REGION TO THE UPPER
JET IS PROGGED TO MOVE OVER THE AREA BY MID TO LATE IN THE EVENING
WITH DEEP LAYER SHEAR VALUES INCREASING TO AROUND 40 KT OR SO WHICH
WOULD BE SUFFICIENT FOR AN ORGANIZED SEVERE WEATHER THREAT...THOUGH
THIS THREAT SHOULD DIMINISH WEST TO EAST AS WE GET LATER INTO THE
OVERNIGHT HOURS AND THE BOUNDARY LAYER STABILIZES.

TUESDAY THROUGH THURSDAY...
A SECONDARY SHORTWAVE TROUGH IS PROGGED TO PUSH ACROSS THE REGION ON
TUESDAY BRINGING ADDITIONAL CHANCES FOR SHOWER AND THUNDERSTORMS
MAINLY ACROSS SOUTH/SOUTHEASTERN PORTIONS OF THE CWA WHERE THE
FRONTAL BOUNDARY MAY STILL SLOWLY BE MAKING A PUSH. MODERATE TO
STRONG HEIGHTS RISES QUICKLY BUILD IN BEHIND THIS FEATURE THOUGH AND
SHOULD RESULT IN A DRY WEATHER FOR MOST OF THE FORECAST AREA
TUESDAY. THE GFS IS PARTICULAR IS HOLDING ONTO SOME POPS NORTH OF
THE BOUNDARY WEDNESDAY INTO THURSDAY WHERE PERSISTENT WARM ADVECTION
WILL RESULT IN MID LEVEL ASCENT...THOUGH THE ECMWF REMAINS DRY THIS
PERIOD. WILL MAINTAIN SLIGHT CHANCE POPS IN THE MEANTIME.

FRIDAY THROUGH SATURDAY...
THERMAL RIDGE IS EXPECTED TO MOVE BACK OVERHEAD LATE IN THE WEEK
WITH H85 TEMPS PUSHING INTO THE LOW 20C RANGE WHICH WILL LEAD TO
ANOTHER WARM UP WITH TEMPS INTO THE UPPER 80S. ANOTHER SLOW MOVING
COLD FRONT WILL THEN SAG SOUTH ACROSS THE REGION BRINGING RENEWED
CHANCES FOR SHOWERS AND THUNDERSTORMS.

DEUBELBEISS

&&

.AVIATION...

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

* SLIGHT CHANCE OF FOG/STRATUS EARLY SUNDAY MORNING.

* CHANCE FOR A LAKE BREEZE/WIND SHIFT TO EASTERLY LATE SUNDAY
  AFTERNOON.

KREIN

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

SCT TSRA WILL CONTINUE TONIGHT WELL TO THE SOUTHEAST OF THE
TERMINALS OVER EAST CENTRAL IL AND WEST CENTRAL INDIANA. AN OLD
BOUNDARY LIES JUST SOUTH OF THE TERMINALS WITH SLIGHTLY DRIER/LESS
HUMID AIR TO THE NORTH...MOSTLY NOTABLE IN THE DEWPOINT GRADIENT.
GIVEN THE LACK OF RAIN TODAY AND MIXING OUT OF LOW LEVEL
MOISTURE...PLUS THE URBAN HEAT ISLAND EFFECTS...NOT ANTICIPATING
FOG/STRATUS TO BE MUCH OF AN ISSUE FOR THE CHICAGO TERMINALS.
HOWEVER...SHORT RANGE MODEL GUIDANCE IS STILL INDICATING A HIGH
POSSIBILITY OF IFR CIGS/VSBY AWAY FROM THE HIGHLY URBANIZED
AREAS...SO WILL MAINTAIN MVFR/IFR FOG AT THE MORE FOG PRONE
OUTLYING TERMINALS...PARTICULARLY GYY/DPA/RFD. ANY VSBY
RESTRICTIONS THAT DO FORM OVERNIGHT WILL QUICKLY DISSIPATE BY MID
MORNING SUNDAY WITH VFR CONDITIONS EXPECTED THE REMAINDER OF THE
DAY. WINDS SHOULD REMAIN LIGHT ENOUGH TO ALLOW A LAKE BREEZE TO
DEVELOP...BUT THERE APPEARS TO BE LITTLE PUSH BEHIND THE
BOUNDARY...SO AT THIS POINT...FEEL THAT CHANCES ARE LOW THAT THE
LAKE BREEZE WILL PUSH INLAND OF ORD/MDW. HOWEVER...IF THE LAKE
BREEZE DOES MANAGE TO PUSH WEST OF ORD/MDW...IT WOULD NOT BE UNTIL
ARND 22-23Z AND ELY WINDS WOULD BE ARND 5-7KT.

KREIN

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

* MEDIUM-HIGH CONFIDENCE IN NO IFR AT ORD/MDW OVERNIGHT.

* MEDIUM TO HIGH CONFIDENCE IN LAKE BREEZE SUNDAY AFTERNOON...AND
  MEDIUM CONFIDENCE THAT THE BOUNDARY WILL NOT PUSH THROUGH
  ORD/MDW.

* HIGH CONFIDENCE IN ALL OTHER FORECAST ELEMENTS.

KREIN

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

MONDAY...PERIOD OR TWO OF SHRA/TSRA...STRONG TSRA POSSIBLE.

TUESDAY AND WEDNESDAY...PREDOMINANTLY VFR.

THURSDAY...SLIGHT CHANCE TSRA...OTHERWISE VFR.

FRIDAY...CHANCE TSRA.

IZZI

&&

.MARINE...
221 PM CDT

MODERATE SOUTHWEST FLOW IS CURRENTLY IN PLACE ACROSS LAKE MICHIGAN
WHERE SOUTHWEST WINDS ARE GUSTING 20 TO 25 KTS WITH OCCASIONAL 30 KT
WINDS. A SMALL CRAFT ADVISORY HAS BEEN ISSUED FOR THE NEARSHORE
WATERS THROUGH LATE THIS EVENING. THIS IS AHEAD OF LOW PRESSURE
CURRENTLY CENTERED NEAR GREEN BAY. THE LOW WILL DRAG A COLD FRONT
SOUTH ACROSS LAKE MICHIGAN TONIGHT TURNING WINDS TO THE NORTHWEST.
WINDS WILL ABATE BRIEFLY AS TRANSIENT HIGH PRESSURE PUSHES ACROSS
THE NORTHERN LAKES REGION...BUT BY MIDDAY SUNDAY WINDS ARE EXPECTED
TO BEGIN TO TURN SOUTHEAST AND START TO RAMP UP AGAIN AHEAD OF
ANOTHER AREA OF LOW PRESSURE. THIS NEXT WILL LOW LIFT FROM THE
NORTHERN PLAINS...DEEPENING AS IT HEADS TO HUDSON BAY MONDAY NIGHT
INTO TUESDAY. SOUTH WINDS ARE EXPECTED TO PEAK MONDAY OVER THE LAKE
WITH GUSTS SOLIDLY INTO THE 20 KT RANGE...APPROACHING 30 KT OVER THE
NORTH END OF THE LAKE. ANOTHER COLD FRONT WILL SAG SOUTH ACROSS LAKE
MICHIGAN MONDAY NIGHT.

DEUBELBEISS

&&

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

$$

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