Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 220849
AFDLOT

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

.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 06Z...

* TSRA CLUSTERS AFFECTING ORD/MDW PRIOR TO 09Z. BRIEF IFR VSBY
  POSSIBLE...THEN LOWER CHANCES THROUGH DAYBREAK.

* MVFR VSBY POSSIBLE THROUGH DAYBREAK.

* LIGHT/VARIABLE EAST-SOUTHEAST WINDS TURN SOUTHWEST AFTER
  DAYBREAK...THEN NORTHEAST FOR THE AFTERNOON. SPEEDS EXPECTED TO
  BE LESS THAN 10 KT.

* SCATTERED TSRA POSSIBLE THIS AFTERNOON AND AGAIN TONIGHT/SATURDAY
  MORNING.

MDB

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

WEAK FRONTAL BOUNDARY APPEARS TO EXTEND FROM WEST TO EAST FROM
NEAR KMLI TO KJOT WITH LESS DEFINITION TO THE EAST ACROSS
NORTHWEST INDIANA. CLUSTERS OF THUNDERSTORMS HAVE DEVELOPED IN THE
VICINITY OF THIS BOUNDARY THANKS TO INCREASING FLOW ABOVE THE
SURFACE AND CONTINUED FEED OF MOISTURE. BASED ON LATEST TRENDS
EXPECT THAT THE TERMINALS WILL BE AFFECTED FOR THE NEXT FEW HOURS
BEFORE THE BULK OF THE CURRENT ACTIVITY SHIFTS TO THE EAST WITH
ADDITIONAL ACTIVITY LOOKING TO STAY TO THE SOUTH OVERNIGHT.
HOWEVER...WITH THE FRONTAL BOUNDARY NEARBY AND STRENGTHENING LOW
LEVEL FLOW ABOVE THE SURFACE MAY START TO SEE MORE ACTIVITY TO THE
WEST OF THE CURRENT ACTIVITY SO WILL NEED TO MONITOR. DID NOT HAVE
ENOUGH CONFIDENCE TO INCLUDE MORE THAN VCSH FROM AROUND 09Z
THROUGH THE MORNING. OTHERWISE LIGHT WINDS WILL BE IN PLACE AND
FAVOR A EAST TO SOUTHEAST DIRECTION BUT A SOUTHERLY DIRECTION MAY
TAKE OVER LATER THIS MORNING. THE ABUNDANT MOISTURE AND WEAK FLOW
WILL LIKELY RESULT IN SOME SHALLOW FOG IN OUTLYING AREAS AS WELL
AS MORE WIDESPREAD MVFR VSBY...WITH SOME IFR POSSIBLE...ESPECIALLY
AT RFD/DPA.

CANNOT RULE OUT SPOTTY SHRA/TSRA DURING THE DAY FRIDAY BUT THE
BEST CHANCE LOOKS TO BE IN THE AFTERNOON AS A FRONTAL BOUNDARY
MOVES ACROSS THE AREA. THIS WILL ALSO TURN WINDS EAST AND
NORTHEAST THOUGH SPEEDS WILL LIKELY REMAIN LIGHT. THE FRONT SHOULD
HAVE A BETTER CHANCE OF TRIGGERING CONVECTION SO WILL CONTINUE
WITH PROB 30 MENTION FOR TSRA IN THE AFTERNOON. TIMING MAY NEED TO
BE SHIFTED HOWEVER. A SIMILAR SITUATION TO TONIGHT MAY OCCUR
OVERNIGHT INTO SATURDAY MORNING IN TERMS OF STORMS. WILL LEAVE ANY
MENTION OUT FOR NOW BECAUSE HAVE VERY LITTLE CONFIDENCE IN TIMING
BUT AM EXPECTING ADDITIONAL STORMS AT SOME POINT FRIDAY NIGHT INTO
SATURDAY MORNING.

MDB

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

* HIGH CONFIDENCE IN TSRA NEXT FEW HOURS. MEDIUM CONFIDENCE THAT
  COVERAGE/CHANCES LOWER AFTER 09Z.

* MEDIUM CONFIDENCE IN MVFR VSBY THROUGH DAYBREAK.

* MEDIUM CONFIDENCE IN OVERALL WIND TRENDS AND SPEEDS LESS THAN 10
  KT OUTSIDE OF STORMS. MEDIUM-HIGH CONFIDENCE IN DIRECTION
  SHIFTING NORTHEAST...LOW CONFIDENCE IN TIMING.

* MEDIUM CONFIDENCE IN ADDITIONAL STORMS OCCURRING AT SOME POINT
  THIS AFTERNOON THROUGH SATURDAY MORNING BUT LOW CONFIDENCE IN
  TIMING AND EXTENT.

MDB

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

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

KJB

&&

.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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