Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 121809
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
109 PM CDT SAT JUL 12 2014

.DISCUSSION...
1126 AM CDT

RAINFALL HAS DECREASED IN INTENSITY AND WILL BE EXITING THE
EASTERN CWA TOWARD 18Z OR SO. LOW LEVEL FLOW FIELDS HAVE BEEN
DISTURBED BY THE MORNING STORMS BUT SYNOPTIC FLOW IS STARTING TO
TAKE OVER ONCE AGAIN WITH SOUTHERLY WINDS RETURNING. IT APPEARS
THAT THE WARM FRONTAL BOUNDARY IS NEAR THE WI BORDER AND SHOULD BE
MOVING NORTHWARD AS LOW PRESSURE MOVES NORTHEAST ALONG THE
BOUNDARY THIS AFTERNOON. CONVECTION IS EXPECTED TO DEVELOP ALONG
THE BOUNDARY LATE THIS AFTERNOON AND AT THIS POINT LOOKS TO BE
ACROSS THE NORTHERN CWA AND PERHAPS MORE LIKELY ACROSS SOUTHERN
WISCONSIN. THE FRONT WILL THEN DROP SOUTHWARD THIS EVENING
BRINGING CONVECTION WITH IT. TIMING IS THE BIGGEST CHALLENGE AT
THIS POINT. HAVE ADJUSTED POPS THROUGH THE AFTERNOON AND MAINTAIN
HIGHEST POPS ACROSS THE NORTHERN CWA WITH LOWER CHANCES
SOUTHWARD...WITH THE EVENING PERIOD STILL LOOKING TO BE THE MOST
FAVORED TIME FOR STORMS.

MDB

//PREV DISCUSSION...
316 AM CDT

TODAY THROUGH TONIGHT...
MAIN FOCUS FOR TODAY CONTINUES TO REVOLVE AROUND CONVECTIVE
TIMING/COVERAGE AND INTENSITY. EARLY THIS MORNING AN AREA OF SHOWERS
WAS BEGINNING TO DEVELOP OVER THE CENTRAL/SOUTHERN PORTIONS OF THE
FORECAST AREA...MAINLY ASSOCIATED WITH WARM AIR ADVECTION. DEW PTS
WERE NUDGING UP TO THE MID 60S...WITH THE EXPECTED 70 DEG DEW PTS
JUST SOUTH/SOUTHWEST OF THE CWFA. PWAT VALUES LATER TDY ARE STILL
PROGGED TO NEAR 2"...AND THE LCL ARW8KM IS SUGGESTING THAT A FEW
POCKETS OF JUST OVER 2" PWAT VALUES FOR PORTIONS OF THE AREA SOUTH
OF I-80 LATE THIS AFTN/EARLY EVE. TEMPS TODAY WILL BE A
CHALLENGE...AS IF ANY BREAKS IN THE CLOUDS CAN OCCUR TEMPS COULD
NUDGE UP A FEW DEGREES. AT THIS TIME WILL MAINTAIN TEMPS IN THE LOW
TO NEAR MID 80S. WITH THE HIGH AMOUNT OF MOISTURE IN PARCELS...IT
WILL BE DIFFICULT FOR TEMPS TO SOAR AND LIKELY BE A SLOW DIURNAL
CURVE TDY. CONFIDENCE OF ANY BREAKS IN THE CLOUDS LOOKS
MINIMAL...AND MAY QUICKLY FILL BACK-IN.

THERE REMAINS A CONCERN ABOUT STRONG TO POSSIBLY SEVERE STORMS. THE
SPC CONTINUES TO HAVE THE ENTIRE FORECAST AREA OUTLOOKED FOR A
SLIGHT RISK OF SEVERE STORMS IN THE DAY 1. THE FOCUS FOR ACTIVITY
CONTINUES TO BE ALONG MOIST BOUNDARY THAT WILL EXTEND FROM CENTRAL
IOWA EAST THROUGH NORTHERN IL/NORTHWEST IN. GUIDANCE SOUNDINGS WOULD
SUGGEST RATHER UNIFORM WIND AT 30-40KTS...HOWEVER THIS IS ALL AHEAD
OF THE POTENT TROUGH THAT WILL BE STARTING TO DIG INTO THE UPPER
MIDWEST THIS EVENING. EXPECT AN MCS TO DEVELOP AND COULD BE SLOW
MOVING ACROSS THE REGION THIS EVENING.

THE PRESENCE OF A WARM LYR ALOFT WILL LIKELY
CAP THE CONVECTIVE COVERAGE UNTIL 21Z...THEN SHUD STEADILY DISSOLVE.
SO THE FOCUS FOR TIMING APPEARS TO BE MORE THIS EVENING INTO THE
OVERNIGHT HOURS. SEVERAL WEAK PIECES OF VORTICITY ARE PROGGED TO
SLIDE ALONG THE MOIST BOUNDARY AS IT SLOWLY SAGS SOUTH/SOUTHEAST
OVERNIGHT. THUS IT IS POSSIBLE SOME TRAINING OF STORMS MAY
OCCUR...AND GIVEN THE ANOMALOUSLY MOIST ENVIRONMENT FLOODING
CONCERNS COULD BE AN ISSUE.

LATE TONIGHT INTO EARLY SUN MORNING...THE BOUNDARY IS PROGGED TO BE
SLOWLY SLIDING SOUTHEAST ACROSS THE FORECAST AREA. GUIDANCE APPEARS
TO HAVE BEEN SLIGHTLY MORE PROGRESSIVE...AND COULD BRING AN END TO
THE CONVECTION A FEW HOURS AFT MIDNIGHT FOR THE NORTHWEST CWFA.

SUNDAY THROUGH MONDAY...
GUIDANCE HAS BEEN TRENDING DRY FOR MUCH OF THE DAY SUN...WITH WEAK
DIFFLUENCE NEAR THE SFC COVERING WISC/NORTHERN IL. LCL ARW8KM HAS
ALSO BEEN TRENDING DRY WITH THE LATEST CYCLES...SO CONFIDENCE HAS
INCREASED AND WILL LEVERAGE THIS WITH A DRY FORECAST SUN. THE
EXCEPTION WILL BE THE SOUTHERN CWFA WHERE THE BOUNDARY WILL STILL BE
RATHER CLOSE. THERMAL TEMPS WOULD STILL BE SUPPORTIVE OF A WARM
AFTN...SO HAVE HELD ONTO TEMPS IN THE LOW 80S SUN.

THE WEAK SFC RIDGE SHUD HOLD ACROSS THE REGION THRU THE EVE
HOURS...THEN BEGIN TO WEAKEN WITH THE MAIN FRONTAL BOUNDARY
APPROACHING FROM THE NORTHWEST EARLY MON. 500MB VORT WILL BE DIGGING
INTO THE UPPER MIDWEST EARLY MON. THE COMBINATION OF COOLER AIR
ALOFT AND SOME THIN CLOUD COVER MON MORNING WILL ALLOW LAPSE RATES
TO STEEPEN AND ADD THE POTENTIAL FOR ISOLATED STORM MON. SFC TEMPS
MON WILL BE STARTING THE COOL DOWN WITH HIGHS GENERALLY IN THE
LOW/MID 70S.

CONFIDENCE...MEDIUM/HIGH.

TUESDAY THROUGH FRIDAY...
ENSEMBLES CONTINUE TO DEMONSTRATE HIGH CONFIDENCE WITH MINIMAL
SPREAD AMONGST MEMBERS FOR THE BULK OF THE EXTENDED FORECAST
PERIODS. POTENT 500MB VORT WILL ARRIVE OVER THE GREAT LAKES
TUE...AND PRODUCE AN UNSEASONABLY COOL DAY WITH HIGHS STRUGGLING TO
WARM BEYOND THE 60S AS THE 850MB THERMAL TROUGH IS PROGGED AT 4 TO 6
DEG C. FORTUNATELY WITH BROAD SFC RIDGING ARRIVING TUE NGT AND
EXPECTED TO PERSIST THRU THE END OF THE WEEK...TEMPS WILL STEADILY
WARM INTO THE 70S.

CONFIDENCE...HIGH.

BEACHLER

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 18Z...

* BREEZY SOUTHWEST WINDS GUSTING 20-25 KT THROUGH AFTERNOON.

* LOW CHANCE OF SHRA/TSRA LATE AFTERNOON INTO EARLY
  EVENING...SHRA/TSRA EXPECTED SOME TIME AROUND MID EVENING. GUSTY
  WINDS AND HEAVY RAINFALL POSSIBLE.

* MVFR CIGS EXPECTED TO SCATTER TO VFR MID AFTERNOON.

MDB

//DISCUSSION...UPDATED 18Z...

SOUTHWEST WINDS ARE STARTING TO INCREASE IN SPEED WITH GUSTS IN
THE 20-25 KT RANGE. CIGS ARE STARTING TO IMPROVE WITH MAINLY MVFR
IN PLACE. A LOOK OUT THE WINDOW AT THE OFFICE SHOWS STEADY
SCATTERING OF THE LOWEST CLOUDS WHICH SUPPORTS THE THINKING THAT
IMPROVEMENT TO VFR WILL OCCUR OVER THE NEXT FEW HOURS. FOCUS TURNS
TO TIMING OF SHRA/TSRA INTO THE EVENING. NOT MUCH IN THE WAY OF
FOCUS FOR DEVELOPMENT THROUGH MID AFTERNOON BUT FORCING WILL
INCREASE FAR NORTHWEST LATE THIS AFTERNOON. SOME SCATTERED
ACTIVITY IS POSSIBLE UP TOWARDS RFD BY THEN BUT THE FOCUS FOR THE
MOST WIDESPREAD DEVELOPMENT WILL BE ALONG A FRONTAL BOUNDARY FROM
SOUTHERN WISCONSIN WEST-SOUTHWEST INTO EASTERN IOWA. THIS ACTIVITY
WILL SAG SOUTHWARD THIS EVENING. TIMING IS STILL IN QUESTION SO
FURTHER ADJUSTMENTS MAY BE NEEDED BUT AT THIS POINT EARLY EVENING
IS FAVORED AT RFD WITH MID EVENING IN THE CHICAGO AREA AND
SLIGHTLY LATER AT GYY AS THINGS SAG SOUTHWARD. THE FRONT WILL BE
FAIRLY SLOW TO MOVE SO TRAINING OF STORMS ALONG THE FRONT IS
EXPECTED LEADING TO A FAIRLY LONG DURATION OF THUNDER. TORRENTIAL
RAINFALL MAY BRING VSBY DOWN BELOW 1SM BUT WILL HOLD OFF ON THIS
DETAIL IN THE TAF UNTIL IT BECOMES MORE CLEAR. WINDS MAY BE
ERRATIC OVERNIGHT BEHIND THE FRONT BUT TREND TO WEST THEN
NORTHWEST INTO SUNDAY MORNING. MVFR MAY LINGER BEHIND THE FRONT AS
WELL BUT VFR SHOULD BUILD IN.

MDB


//ORD AND MDW CONFIDENCE...UPDATED 18Z...

* HIGH CONFIDENCE IN WINDS.

* HIGH CONFIDENCE THAT CHANCE FOR SHRA/TSRA IS LOW LATER THIS
  AFTERNOON AND SHRA/TSRA IS LIKELY THIS EVENING. MEDIUM
  CONFIDENCE IN TIMING THIS EVENING.

* MEDIUM-HIGH CONFIDENCE IN MVFR IMPROVING TO VFR THIS AFTERNOON.
  LOW-MEDIUM CONFIDENCE IN TIMING.

MDB

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

SUNDAY-MONDAY...LOW CHANCES OF TSRA.
TUESDAY...SLIGHT CHANCE OF SHRA.
WEDNESDAY-THURSDAY...VFR LIKELY.

KMD

&&

.MARINE...
244 AM CDT

STILL SEEING GUSTS TO NEAR 20 KT ON THE NORTHERN HALF OF THE
LAKE...BUT DO EXPECT THE PRESSURE GRADIENT TO RELAX TONIGHT INTO
SATURDAY AS HIGH PRESSURE ALONG THE ATLANTIC COAST CONTINUES EAST.
A TROUGH OF LOW PRESS SURE AND FRONTAL BOUNDARY WILL CROSS THE LAKE
ON SATURDAY NIGHT. CONFIDENCE IS LOWER WITH REGARDS TO OVERALL
WIND DIRECTION AND SPEEDS SATURDAY EVENING INTO SATURDAY NIGHT AS
PERIODS OF SHOWERS AND THUNDERSTORMS ARE EXPECTED ALONG THIS
FRONTAL BOUNDARY. A SERIES OF WEAK LOW PRESSURE SYSTEMS AND
ASSOCIATED COLD FRONTS WILL THEN PASS THROUGH THE LAKE THROUGH
MONDAY WHILE HIGH PRESSURE BUILDS INTO THE NORTHERN CENTRAL
PLAINS. WINDS SHIFT TO WEST- NORTHWEST BEHIND THE STRONGER COLD
FRONTAL PUSH MONDAY NIGHT AND REACH 20 KT OR POSSIBLY HIGHER
TUESDAY.

KMD

&&

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

$$

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