Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 221442 AAA
AFDLOT

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
942 AM CDT FRI AUG 22 2014

.DISCUSSION...
935 AM CDT

THE MAIN CORRIDOR OF HEAVY RAIN LATE LAST NIGHT INTO EARLY THIS
MORNING HAS SHIFTED INTO CENTRAL IL AND WEAKENED...HOWEVER AN AREA
OF THUNDERSTORM ACTIVITY HAS BUILT FROM EASTERN IA INTO WESTERN
IL AS OF 930 AM. THIS IS ASSOCIATED WITH A SHORT WAVE AND APPEARS
BY RADAR AND SATELLITE ALSO MAS AN MCV WITH IT. SHORT TERM
GUIDANCE VARIES WIDELY ON HOW THIS WILL UNFOLD BUT GIVEN ITS
ORGANIZATION WOULD EXPECT FOR IT TO PROGRESS EASTWARD ALONG AN
AXIS OF CONFLUENT FLOW AT 850-700MB. RAINFALL RATES HAVE TAPERED
IN THE PAST HALF HOUR...BUT STILL IN A VERY MOIST ATMOSPHERE
/PWATS OVER TWO INCHES/ WE ARE SEEING RAINFALL RATES IN EXCESS OF
ONE INCH PER HOUR AT TIMES WITH THIS. PARTS OF LASALLE AND
LIVINGSTON COUNTIES WERE HIT HARD LAST NIGHT INTO EARLY THIS
MORNING...SO THRESHOLDS FOR FLASH FLOOD WARNINGS WILL LIKELY BE
LOW IF THIS AREA WERE TO GET ANY MORE AGGRESSIVE OR SLOW DOWN.

BASED ON CALLS TO EMERGENCY MANAGEMENT AND LAW ENFORCEMENT
PARTNERS...EARLIER FLASH FLOOD WARNINGS WERE TRANSITIONED TO AREAL
FLOOD WARNINGS WHERE MAJOR IMPACTS WERE STILL OCCURRING. THE SAME
MAY BE DONE FOR PART OF LIVINGSTON/LASALLE/GRUNDY COUNTY DEPENDING
ALSO ON HOW EARLIER MENTIONED CONVECTION IS EVOLVING.

THE DENSE FOG ADVISORY FOR LAKE AND MCHENRY COUNTIES WILL BE
ALLOWED TO EXPIRE AT 10 AM AS OBSERVATIONS AND WEBCAMS HAVE SHOWN
IMPROVEMENT.

WILL LIKELY HAVE TO LOWER HIGH TEMPERATURE FORECAST SOME BASED ON
DEGREE OF CLOUD COVER OVER THE AREA. CONTINUED DEW POINTS OF 70 TO
75 WILL KEEP IT FEELING MUGGY.

MTF

&&

.PREV DISCUSSION...

349 AM...FORECAST CONCERNS INCLUDE HEAVY RAIN WITH THUNDERSTORMS
THROUGH SATURDAY NIGHT...IN ADDITION TO TIMING AND LOCATION OF
STORMS. ADDITIONAL CONCERNS AND CHALLENGES INCLUDE TEMPERATURES/
DEWPOINTS AND ASSOCIATED HEAT INDICES THROUGH EARLY NEXT WEEK.

ONE COMPLEX OF THUNDERSTORMS MOVING ACROSS FAR NORTHEAST IL AND
NORTHWEST INDIANA WITH A SECOND COMPLEX MOVING SOUTHEAST ACROSS
THE SOUTHERN CWA. THESE ARE VERY EFFICIENT RAIN PRODUCES WITH
DEWPOINTS IN THE 70S AND PRECIPITABLE WATER VALUES AROUND 2
INCHES. COOP OBSERVER 3SW OF MIDWAY AIRPORT MEASURED 3.60 INCHES
OF RAIN IN JUST 40 MINUTES WITH SEVERAL REPORTS OF FLOODED ROADS
IN THAT AREA. WHILE ADDITIONAL ACTIVITY CONTINUES ACROSS THE
AREA...THE GENERAL TREND IS FOR ALL THIS ACTIVITY TO MOVE
SOUTHEAST THIS MORNING. HOWEVER...ADDITIONAL ACTIVITY OVER
WESTERN IL AND SOUTHERN IOWA MAY CONTINUE MOVING EAST ACROSS THE
SOUTHERN CWA LATER THIS MORNING.

CONVECTIVE TRENDS ARE STILL SOMEWHAT UNCERTAIN OVER THE NEXT 36
HOURS OR SO. IF CURRENT ACTIVITY ACROSS SOUTHERN IA/WESTERN IL
DRIFTS SOUTHEAST...MUCH OF THE DAYLIGHT HOURS COULD END UP RATHER
DRY WITH TONIGHT POSSIBLY BEING MORE ACTIVE AS HINTED AT BY THE
ECMWF. HAVE TRENDED THIS DIRECTION BUT CONFIDENCE IS LOW. WITH
DEWPOINTS REMAINING IN THE LOWER/MID 70S AND PRECIPITABLE WATER
VALUES REMAINING AROUND 2 INCHES...ADDITIONAL TORRENTIAL RAIN WILL
BE POSSIBLE WITH ANY STORMS THAT DEVELOP. IF TRENDS BECOME MORE
CLEAR...ITS POSSIBLE A FLASH FLOOD WATCH MAY BE NEEDED FOR PART OF
THE AREA...BUT GIVEN THE UNCERTAINTY OF WHEN/WHERE ADDITIONAL
HEAVY RAIN WILL OCCUR...NO HEADLINES THIS MORNING.

CONVECTION THEN APPEARS TO SHIFT NORTHEAST DURING THE DAY ON
SATURDAY AS THE UPPER RIDGE BUILDS INTO THE AREA. THUS A DRY
PERIOD IS EXPECTED FROM LATER SATURDAY NIGHT THROUGH SUNDAY NIGHT
AND POSSIBLY INTO MUCH OF THE DAY ON MONDAY. CHANCE POPS THEN
ENTER BACK INTO THE FORECAST THROUGH THURSDAY. MEDIUM RANGE
MODELS ARE IN GOOD AGREEMENT WITH A COLD FRONT MOVING ACROSS THE
AREA MIDWEEK...THOUGH TIMING DIFFERENCES REMAIN. ADDITIONAL HEAVY
RAIN WILL BE POSSIBLE DURING THIS TIME AS WELL.

WHILE SUNDAY MAY END UP BEING COMPLETELY DRY AS THE RIDGE CAPS THE
AREA...IT DOES APPEAR THERE COULD BE A LAKE BREEZE ALONG THE IL
SHORE SUNDAY AFTERNOON. HOW MUCH CONVERGENCE OCCURS IS UNCERTAIN
BUT GIVEN HOW WARM/HUMID CONDITIONS WILL BE SUNDAY AFTERNOON...IF
THIS IS ENOUGH FOR THUNDERSTORMS TO DEVELOP...THERE COULD BE SOME
LOCALLY HEAVY RAIN ACROSS FAR NORTHEAST IL. ADDED SLIGHT CHANCE
POPS DURING THIS TIME TO ACCOUNT FOR THIS POTENTIAL.

DENSE FOG HAS FORMED ACROSS FAR NORTHEAST IL AND MAY SPREAD
SOUTHWEST INTO NORTHWEST IL EARLY THIS MORNING AS RAIN ENDS.
ISSUED A DENSE FOG ADVISORY FOR LAKE/MCHENRY COUNTIES EARLIER THIS
MORNING AND MAY NEED TO EXPAND THAT AS TRENDS EMERGE. DEPENDING ON
PRECIP COVERAGE TONIGHT...AREAS OF FOG POSSIBLY DENSE MAY DEVELOP
AGAIN INTO SATURDAY MORNING.

TEMPERATURES TODAY AND SATURDAY WILL BE DEPENDENT ON CLOUD COVER
AND PRECIP COVERAGE. MADE ONLY SMALL TWEAKS...BASICALLY NO
CHANGES. DID BUMP TEMPS UP A FEW DEGREES ON SUNDAY AND MONDAY WITH
EXPECTED LESS OR NO PRECIP AND MORE SUNSHINE. OVERNIGHT LOWS WILL
LIKELY REMAIN IN THE LOWER 70S MOST AREAS...POSSIBLY MID OR UPPER
70S DOWNTOWN CHICAGO SUNDAY NIGHT/MONDAY MORNING. FORECAST HEAT
INDICES WILL BE NEAR 100 TODAY...RETREAT JUST A LITTLE SATURDAY
AND THEN AGAIN NEAR 100 SUNDAY AND MONDAY. ACROSS THE SOUTHERN
CWA...HEAT INDICES APPROACH THE 105 MARK. IF CURRENT TRENDS
CONTINUE...ITS POSSIBLE PORTIONS OF THE SOUTHERN CWA MAY NEED A
HEAT ADVISORY SUNDAY/MONDAY. CMS

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 14Z...

* 900-1500 FT CIGS THROUGH AT LEAST 16Z.

* LIGHT SOUTHWEST WINDS TURNING NORTHEAST EARLY THIS AFTERNOON.
  SPEEDS EXPECTED TO BE UNDER 10 KT THROUGH THAT TIME.

* ISOLD-SCT SHRA/TSRA POSSIBLE THIS AFTERNOON. MAY DEVELOP WEST
  OF TERMINALS AND DRIFT BACK TO THE EAST.

* CHANCES FOR SHRA/TSRA TONIGHT AND SATURDAY MORNING...TIMING
  UNCLEAR.

MDB

//DISCUSSION...UPDATED 12Z...

WEAK LOW PRESSURE EXTENDS FROM SOUTHERN MINNESOTA/NORTHERN IOWA
SOUTHWEST INTO THE SOUTHERN PLAINS. A FRONTAL BOUNDARY EXTENDS
FROM EASTERN IOWA ACROSS NORTHERN ILLINOIS TO NORTHERN INDIANA
THIS MORNING WITH STRONG LOW LEVEL WINDS HELPING DRIVE
THUNDERSTORM ACTIVITY TO THE SOUTH OF THE TERMINALS. SOME SPOTTY
IFR IS IN PROXIMITY TO THE TERMINALS...WITH THE BULK NEAR RFD BUT
THIS SHOULD BE IMPROVING...WITH ORD/MDW/DPA/GYY LIKELY SEEING
LITTLE IF ANY IFR. LIGHT AND SOMEWHAT VARIABLE SOUTH TO SOUTHWEST
WINDS ARE IN PLACE BUT ARE EXPECTED TO TURN EAST AND NORTHEAST
THIS AFTERNOON AS THE FRONT RE-ESTABLISHES ITSELF PERHAPS WITH
THE HELP OF LAKE MICHIGAN. THIS BOUNDARY COUPLED WITH THE VERY
HUMID AIRMASS MAY TRIGGER ISOLD-SCT SHRA/TSRA THIS AFTERNOON
INLAND OF THE LAKE WHICH WOULD THEN DRIFT BACK TOWARDS THE LAKE
AFTER DEVELOPMENT. HAVE INCLUDED VCTS MENTION AT ORD/MDW/RFD/DPA
WITH DECENT CONFIDENCE THAT ACTIVITY WILL OCCUR BUT WITH QUESTION
MARKS SURROUNDING THE EXTENT.

WINDS WILL DIMINISH THIS EVENING/TONIGHT WHICH MAY ALLOW SOME
SHALLOW DENSE FOG OR MORE WIDESPREAD MVFR TYPE FOG. TSRA CHANCES
REMAIN A CHALLENGE BUT HAVE PRETTY LOW CONFIDENCE IN WHEN THE BEST
OPPORTUNITY WILL BE THIS EVENING THROUGH SATURDAY. WITH THE
FRONTAL BOUNDARY EXPECTED TO BE IN THE REGION IT WOULD LIKELY
SERVE AS A TRIGGER BUT IT MAY BE SOUTH OF THE AREA TONIGHT. UPPER
RIDGING WILL BUILD IN SATURDAY WHICH WOULD BRING ANOTHER PUSH OF
WARM AIR AND COUPLED WITH THE BOUNDARY WOULD PROBABLY DRIVE MORE
TSRA DEVELOPMENT WHICH MAY BE FAVORED SATURDAY MORNING. WILL LEAVE
THE TAF DRY BEYOND THIS AFTERNOON GIVEN THE UNCERTAINTY BUT HAVE
DECENT CONFIDENCE THAT TSRA WILL OCCUR AT SOME POINT.

MDB

//ORD AND MDW CONFIDENCE...UPDATED 14Z...

* LOW IN HOW LONG MVFR CIGS WILL LAST.

* HIGH IN LIGHT SW WINDS BECOMING NORTHEAST THIS AFTERNOON. MEDIUM
  IN SPEEDS UNDER 10 KT. MEDIUM IN TIMING OF SHIFT TO NORTHEAST.

* MEDIUM THAT THERE WILL BE SOME ISOLD/SCT SHRA/TSRA DEVELOPMENT
  THIS AFTERNOON. LOW IF THEY WILL OCCUR AT TERMINALS.

* LOW IN TIMING OF BEST SHRA/TSRA CHANCES THIS EVENING THROUGH
  SATURDAY MORNING BUT MEDIUM CONFIDENCE THAT ADDITIONAL ACTIVITY
  WILL OCCUR DURING THIS TIME.

MDB

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

* SATURDAY NIGHT THROUGH THURSDAY...PERIODIC CHANCES FOR
  TSRA/SHRA WITH OCNL MVFR CIGS/VIS.

MDB

&&

.MARINE...
349 AM CDT

AN ELONGATED AREA OF LOW PRESSURE STRETCHES FROM SOUTHERN MINNESOTA
INTO THE SOUTHERN PLAINS WITH A WEAK WARM FRONTAL BOUNDARY
STRETCHING FROM NORTHERN INDIANA BACK INTO CENTRAL/NORTHERN IOWA.
THIS FRONT WILL MOVE LITTLE INTO SATURDAY. MORNING STORMS HAVE
CAUSED WINDS TO BE A BIT CHAOTIC ACROSS THE FAR SOUTHERN PART OF THE
LAKE WHERE SOUTH TO SOUTHEAST WINDS WILL TURN MORE EAST AND
NORTHEAST INTO THE AFTERNOON. LOW PRESSURE WILL ORGANIZE ACROSS THE
CENTRAL AND NORTHERN PLAINS LATER SATURDAY AND MOVE TO WESTERN
ONTARIO SUNDAY ALLOWING WINDS TO TURN SOUTHERLY AND INCREASE IN
SPEED TOWARD MONDAY. THE LOW WILL THEN TRACK TO THE NORTHEAST EARLY
NEXT WEEK DRAGGING A FRONTAL BOUNDARY SOUTHWARD WHICH MAY STALL OUT
SOMEWHERE ACROSS THE LAKE ALLOWING FOR VARIABLE WINDS. ANOTHER LOW
IS EXPECTED TO LIFT NORTHEAST MID WEEK BRINGING A STRONGER COLD
FRONT ACROSS THE LAKE WHICH WOULD BRING A SHIFT TO NORTH AND
NORTHWEST WINDS. STILL EARLY FOR DETAILS BUT THIS SCENARIO WOULD
BRING A PERIOD OF STRONGER WIND SPEEDS THANKS TO COLD
ADVECTION...NOT UNLIKE SIMILAR COLD FRONTAL PASSAGES WE HAVE SEEN
THE PAST FEW WEEKS.

MDB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...DENSE FOG ADVISORY...ILZ005-ILZ006 UNTIL 10 AM FRIDAY.

IN...NONE.
LM...NONE.
&&

$$

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