Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 222208
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
508 PM CDT FRI AUG 22 2014

.DISCUSSION...
259 PM CDT

WE ARE LOOKING AT AN EXTENDED PERIOD OF MUGGY CONDITIONS OVER THE
COMING WEEK WITH OFF AND ON CHANCES FOR SHOWERS AND THUNDERSTORMS.
FORECAST FOCUS WAS TRYING TO PIN DOWN BETTER TIME FRAMES FOR
THUNDERSTORM ACTIVITY. THIS IS COMPLICATED BY TRACKING VERY SUBTLE
WAVES THAT MAKE ALL THE DIFFERENCE IN WHEN AND WHERE ADDITIONAL
HEAVY RAIN WILL FALL.

THE OVERALL PATTERN IS CHARACTERIZED BY A VERY BROAD UPPER RIDGE
ACROSS THE MIDWEST WITH THE CENTER OF HIGH PRESSURE OVER THE
ALABAMA/MISSISSIPPI WITH TROUGHING OFF THE EAST COAST...AN UPPER
LEVEL TROUGH OVER THE PACIFIC NORTHWEST WITH A SHORTWAVE TROUGH
FEEDING INTO THE BACK SIDE OF THE RIDGE IN ARIZONA. CLOSER TO
HOME...WE ARE SOMEWHAT IN THE RING OF FIRE ON THE NORTH SIDE OF THE
UPPER RIDGE AXIS WHERE SMALLER SCALE PERTURBATIONS CONTINUED TO
ROTATE THROUGH THE AREA. THE UPPER LEVEL FLOW IS WEAK...AND WITH THE
PATTERN AMPLIFIED...THE PATTERN DOES NOT LOOK TO CHANGE MUCH SOON.
MEANWHILE...THE LOWER LEVEL BAROCLINIC ZONE REMAINS DRAPED JUST TO
THE SOUTH OF OUR CWA THIS AFTERNOON. THERE IS A SURFACE BOUNDARY
POSITIONED ALONG THE NORTHERN TIER OF OUR AREA...BUT THE SURFACE
PATTERN IS A BIT MORE MUDDLED. WIDESPREAD DEWPOINTS IN THE 70S
SUMS THINGS UP PRETTY WELL.

IN THE NEAR TERM...CONVECTION FROM EARLIER HAS PUSHED SOUTH HAS
CONTINUED TO WEAKEN AND PUSH SOUTHWEST AS LOW LEVEL FLOW IS TURNING
NORTHWESTERLY...PUSHING BETTER MOISTURE TRANSPORT AND LIFT INTO THE
LOW LEVEL BAROCLINIC ZONE TO THE SOUTH OF OUR CWA. WE CONTINUE TO
WATCH SMALLER SCALE WAVES IN IOWA THAT WILL TRAVERSE THE RIDGE AS
MOISTURE CONTINUES TO FEED FROM THE GULF INTO THE CENTER OF THE
COUNTRY AND TOWARDS THE AREA. SOME HIGH RESOLUTION GUIDANCE IS
HINTING AT SOME CONVECTION ALONG A LAKE BREEZE THIS AFTERNOON. THIS
COULD BE AN AREA OF FOCUS AS A WEAK WAVE IN IOWA APPROACHES. IF
ANYTHING FORMS...IT WOULD BE ISOLATED. CONFIDENCE ON THIS HAPPENING
IS LOW AS ONLY A HANDFUL OF GUIDANCE MEMBERS ARE SHOWING THIS
SOLUTION AND FEEL THAT IT WOULD ONLY BE ALONG SUBTLE CONVERGENCE
BOUNDARIES. MUCH OF THE LARGER SCALE MODELS KEEP ANY PRECIPITATION
WITH THIS WAVE ALONG OUR SOUTH AND WESTERN TIER OF COUNTIES WHERE WE
KEEP THE HIGHEST POPS. EVEN THIS IS IN QUESTION AS THE MAIN FORCING
IS HAVING A HARD TIME GETTING OUT OF IOWA WHERE MORE ORGANIZED
THUNDERSTORM ACTIVITY IS ONGOING...BUT IT IS THESE AREAS THERE IS
SOME 925 MB FLOW STILL INTO THE LOWER LEVEL BAROCLINIC ZONE THAT MAY
AID IN SOME ENHANCEMENT TONIGHT. AREAS IN AN AROUND LIVINGSTON
COUNTY HAD SOME PRETTY SIGNIFICANT RAINFALL..SO THIS WOULD BE AN
AREA TO WATCH IF WE GET SOME ADDITIONAL DEVELOPMENT THIS EVENING AS
THE GFS/EC SUGGEST. IF STUFF GETS GOING IT MAY LINGER INTO THE
MORNING HOURS. ADMITTEDLY THE EVOLUTION OVER THE NEXT 12-18 HOURS IS
LOWER THAN I WOULD LIKE.

MODELS CONTINUE TO DIFFER WITH THE HANDLING OF THINGS ON SATURDAY AS
WELL BUT STILL GENERALLY AGREE WITH THE BEST CHANCES SOUTH AND WEST
OF THE MAIN METROPOLITAN AREA. FEEL LIKE THERE WILL BE MANY DRY
PERIODS BUT THAT THUNDER IS POSSIBLE AND THEREFORE SOME DEGREE OF
CHANCE POPS FOR NOW...WITH AREAS CLOSER TO THE LAKE HAVING SMALLER
CHANCES. CONFIDENCE IN THIS PERIOD IS LOW-MEDIUM.

SATURDAY NIGHT INTO SUNDAY...THE UPPER LEVEL RIDGE AXIS WILL AMPLIFY
CAUSING THE MAIN MOISTURE TRANSPORT AXIS TO REMAIN OFF TO THE WEST.
THIS WILL LARGELY KEEP MUCH OF NORTHEAST ILLINOIS AND NORTHWEST
INDIANA UNDER DRY BUT MUGGY CONDITIONS.

CONCERNS ON SUNDAY AND MONDAY WILL BE MORE SO ON THE HOT AND HUMID
CONDITIONS WHERE HEAT INDICES WILL BE WELL INTO THE
90S...APPROACHING 100 IN THE SW CWA.

SHORTWAVE ENERGY CURRENTLY OVER ARIZONA WILL MERGE WITH THE LARGE
SCALE TROUGH OVER THE NORTHWEST AS THE TROUGH MOVES EASTWARD. THE
RIDGE LOOKS TO HOLD AND SHUNT MOST OF THE INITIAL SHORTWAVE ENERGY
WITH THE TROUGH NORTH AND WEST...WHICH WOULD SUGGEST THAT MONDAY
CHANCES OF SHOWERS AND STORMS REMAINS LOWER THAN THE REMAINDER OF
THE WEEK...THOUGH WE MAY GET GRAZED WITH IT SO WILL CONTINUE THE
LOWER POPS FOR MONDAY. ON TUESDAY THE TROUGH/MOISTURE TRANSPORT AXIS
GETS CLOSER. IT APPEARS THAT WEDNESDAY AND WEDNESDAY NIGHT WILL BE
THE DAY WHEN WE TRANSITION OUT OF THE CURRENT AIR MASS. A COLD FRONT
ASSOCIATED WITH THE UPPER TROUGH WILL BRING THE BEST CHANCE FOR
ORGANIZED SHOWERS AND THUNDERSTORMS. WHILE PRECIPITABLE WATER VALUES
IN THE EXTENDED TEND TO DROP SOME...WE ARE STILL LOOKING AT 2-3
STANDARD DEVIATIONS ABOVE NORMAL WHICH MEANS ANY PERIOD OF
THUNDERSTORM ACTIVITY WILL POSE A THREAT FOR HEAVY RAINFALL. ON
WEDNESDAY WE WILL ALSO HAVE BETTER WIND SHEAR...SO ADDITIONAL
HAZARDS EXISTS INCLUDING THE POTENTIAL FOR STRONG/SEVERE WINDS AND
HAIL.  COOLER AND DRIER CONDITIONS ARE ON TAP FOR THE LATTER HALF OF
THE WEEK WITH THURSDAY BEING A BIT BREEZY BEFORE HIGH PRESSURE MOVES
OVERHEAD. KMD

KMD

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 22Z...

* IFR CIGS AND POSSIBLY VISIBILITY DEVELOPING LATER TONIGHT AND
  PERSISTING INTO SATURDAY MORNING.

* SMALL CHANCE FOR SHRA/TSRA LATE TONIGHT THROUGH THE DAY
  SATURDAY.

JEE

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

A STATIONARY FRONT REMAINS DRAPED OVER THE AREA THIS AFTERNOON
WITH WEAK ASCENT OVERSPREAD IT. SCATTERED STORMS ARE LIKELY TO
CONTINUE TO DEVELOP ACROSS EASTERN IA/NORTHEAST MO/WESTERN IL AND
PROBABLY INTO SOUTHERN WI. HOW FAR EAST SOME OF THIS DEVELOPS IS A
BIT CHALLENGING. CLOUD COVER HAS KEPT THE ATMOSPHERE FAIRLY STABLE
OVER NORTH CENTRAL AND NORTHEAST IL THROUGH EARLY AFTERNOON.
CONVERGENCE WILL INCREASE WITH AN EXPECTED LAKE BREEZE LATE THIS
AFTERNOON AND MAY BE ENOUGH OF AN IMPETUS TO SPARK A FEW STORMS.
CONFIDENCE ON THIS IS LOW THOUGH AS THE PATTERN IS BASICALLY
MESSY...WITH DEEP MOISTURE AND SUBTLE UPPER LEVEL DISTURBANCES
MOVING OVER THE REGION. SO WHILE TIMING CAN NOT BE LOCKED
DOWN...BELIEVE ANY COVERAGE WILL BE ISOLATED TO SCATTERED AT
BEST.

SOUTHWEST WINDS THUS FAR HAVE KEPT A LAKE BREEZE AT BAY BUT ONE
SHOULD BEGIN TO CREEP INLAND EARLY THIS AFTERNOON. LIGHT FLOW IN
THE COLUMN AIDED BY LIMITED MIXING SHOULD KEEP EAST-NORTHEAST WIND
SPEEDS UNDER 10 KT.

THE MOIST CONDITIONS AND LIGHT WIND FLOW GOING INTO TONIGHT SHOULD
SUPPORT LOW CLOUDS AND POTENTIALLY FOG. EXPECTING IFR CONDITIONS
TO BE SEEN LATE TONIGHT AND PROBABILISTIC GUIDANCE IS KEYING IN ON
THIS. THESE CLOUDS MAY LIFT SLOWER SATURDAY MORNING THAN INDICATED
IN THE TAF. EASTERLY WINDS SHOULD PREVAIL SATURDAY WITH A LITTLE
BIT OF A LAKE PUSH COME AFTERNOON...POSSIBLY REACHING 10 KT.

//ORD AND MDW CONFIDENCE...UPDATED 22Z...

* HIGH CONFIDENCE IN TIMING OF EAST WINDS AND EAST WINDS WILL BE
  ARND 8 KT OR LESS.

* HIGH CONFIDENCE IN IF ANY MVFR CIGS OCCUR LATE THIS AFTERNOON
  THEY WILL BE ABOVE 1500 FT.

* HIGH CONFIDENCE IN IFR CONDITIONS DEVELOPING LATER TONIGHT. LOW
  IN HOW LONG CONDITIONS LAST SATURDAY MORNING.

* LOW CONFIDENCE IN ANY TSRA OCCURRING TONIGHT THROUGH SATURDAY.
  MEDIUM THAT IF ANY TSRA OCCUR DURING THAT TIME THEY WILL BE
  ISOLATED.

* HIGH CONFIDENCE IN ALL OTHER FORECAST ELEMENTS.

JEE

//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.

MTF

&&

.MARINE...
246 PM CDT

WINDS WILL REMAIN FAIRLY LIGHT AND FREQUENTLY CHANGING ACROSS THE
LAKE OVER THE NEXT SEVERAL DAYS. A BROAD AREA OF LOW PRESSURE OVER
THE PLAINS WILL CONSOLIDATE AND MOVE TO WESTERN ONTARIO SUNDAY.
THIS WILL TURN WINDS SOUTHERLY AND INCREASE THEM SLIGHTLY TOWARD
SUNDAY AFTERNOON INTO MONDAY. THE LOW WILL THEN TRACK NORTH OF THE
LAKES AND PULL A FRONTAL BOUNDARY DOWN OVER LAKE MICHIGAN...LIKELY
STALLING OUT OVER THE LAKE ALLOWING FOR VARIABLE WINDS TOWARD
MIDWEEK.

LENNING

&&

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

$$

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