Severe Thunderstorm Warning 97, NWS Chicago, IL

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
252 PM CDT SAT JUL 26 2014

.DISCUSSION...

246 PM...FORECAST CONCERN AND CHALLENGE REMAINS CONVECTIVE TRENDS
THROUGH SUNDAY MORNING.

18Z SOUNDINGS FROM ILX AND DVN THIS AFTERNOON SHOW STRONG CAPPING
IN PLACE ACROSS THE AREA. RADAR IMAGERY SHOWS LIGHT RETURNS WHICH
ARE LIKELY ELEVATED SHOWERS/WEAK CONVECTION ROOTED FROM 800MB...
ABOVE THE CAP. NOT EXPECTING MUCH ACTIVITY ACROSS NORTHERN IL WITH
THIS CAP IN PLACE BUT ISOLATED SHOWERS/SPRINKLES WILL BE POSSIBLE.

THUNDERSTORMS OVER NORTHERN MO CONTINUE MOVING EAST AND INCREASING
IN COVERAGE BUT NOT MUCH ORGANIZATION YET. AS THIS ACTIVITY MOVES
EAST INTO THE MORE CAPPED AIR...IT MAY CONTINUE TO STRUGGLE TO
ORGANIZE INTO A SEVERE LINE/COMPLEX. THUS CONFIDENCE REGARDING
CONVECTIVE TRENDS ACROSS THE SOUTHERN CWA/CENTRAL IL/CENTRAL
INDIANA ARE DECREASING AT THE MOMENT. BUT WITH THE WARM FRONTAL
BOUNDARY NEARBY AND MID 70S DEWPOINTS POOLING SOUTH OF THE
FRONT...HAVE NOT MADE ANY CHANGES TO COVERAGE/TIMING. AND WHILE
SEVERE POTENTIAL REMAINS UNCERTAIN...SHOULD STORMS BECOME SEVERE
EARLIER THOUGHTS REMAIN ON TRACK FOR WIDESPREAD DAMAGING WINDS TO
BE THE MAIN THREAT...MAINLY SOUTH OF THE KANKAKEE RIVER.

THE GRADIENT MAY BECOME WEAK ENOUGH FOR LIGHT/CALM WINDS OVERNIGHT
ESPECIALLY OVER THE SOUTHERN HALF OF THE CWA. WHILE PRECIP
COVERAGE REMAINS UNCERTAIN...LOW LEVELS WILL REMAIN QUITE MOIST
AND FOG WILL BE POSSIBLE...AND WHERE ANY RAIN DOES FALL...COULD
BECOME LOCALLY DENSE.

A COLD FRONT WILL MOVE ACROSS THE AREA SUNDAY MORNING...LIKELY
MOVING EAST OF THE CWA BY 18Z. COULD BE ISOLATED/SCATTERED
THUNDERSTORMS ALONG/AHEAD OF THIS FRONT DURING THE MID MORNING
HOURS BUT WILL BE SHORT-LIVED WITH A PERIOD OF DRY CONDITIONS
EXPECTED MIDDAY/EARLY AFTERNOON POST FRONTAL. AS COOLER AIR
SPREADS ACROSS THE AREA...DIURNAL SHOWERS AND PERHAPS A FEW
THUNDERSTORMS WILL BE POSSIBLE MID/LATE SUNDAY AFTERNOON AND THIS
WOULD BE MOST LIKELY ACROSS NORTHERN IL. AS THIS COOLER AND DRIER
AIR ADVECTS INTO THE AREA...LOW LEVEL MIXING WILL BECOME QUITE
EFFICIENT. FORECAST SOUNDINGS SHOW 25-30 KTS IN THE MIXED LAYER
BY LATE AFTERNOON...THUS WINDY CONDITIONS ARE EXPECTED SUNDAY
AFTERNOON WITH WIND GUSTS AS HIGH AS 35 MPH.

A SECONDARY COLD FRONT WILL MOVE SOUTH ACROSS THE AREA SUNDAY
NIGHT USHERING IN EVEN COOLER AIR WITH HIGHS ON MONDAY ONLY IN THE
LOWER/MID 70S. A LARGE UPPER TROUGH WILL THEN REMAIN ACROSS MUCH
OF THE GREAT LAKES REGION AND EASTERN U.S. THROUGH THE END OF NEXT
WEEK. HIGH TEMPS WILL REMAIN BELOW NORMAL...GENERALLY IN THE MID/
UPPER 70S THROUGH THE END OF NEXT WEEK. IN ADDITION THERE WILL BE
AFTERNOON/EARLY EVENING CHANCES FOR ISOLATED SHOWERS/THUNDERSTORMS
JUST ABOUT EVERYDAY. CMS

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 20Z...

* TSRA POTENTIAL EXPECTED TO REMAIN SOUTH OF TERMINALS ACROSS
  CENTRAL IL LATE THIS AFTERNOON/EVENING.

* VARIABLE SFC WINDS...WITH W-S-SE DIRECTION FAVORED IN NEAR TERM
  BUT LESS THAN 10 KT. LAKE BREEZE HAS STALLED EAST OF ORD/MDW AT
  20Z AND IS EXPECTED TO REMAIN EAST OF AIRFIELDS...THOUGH COULD
  DRIFT WEST AS WINDS DIMINISH FURTHER EARLY EVENING.

* POTENTIAL FOR MVFR VIS IN FOG DEVELOPMENT OVERNIGHT.

* LOW CHANCE SHRA/TSRA AROUND SUNRISE SUNDAY...AND AGAIN SUNDAY
  AFTERNOON.

* WINDS BECOMING WEST-NORTHWEST SUNDAY MORNING AND INCREASING IN
  STRENGTH. GUSTS 25-30 KT LIKELY BY EARLY/MID AFTERNOON.

RATZER

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

COMPLICATED AND RATHER LOW CONFIDENCE FORECAST THROUGH TONIGHT
ESPECIALLY WITH RESPECT TO WIND DIRECTION AND SHRA/TSRA COVERAGE.

DISSIPATING SURFACE COLD FRONT WAS ACROSS THE FORECAST AREA FROM
FAR NORTHEAST IL TO FAR NORTHERN MISSOURI THIS AFTERNOON. WEAK
PRESSURE GRADIENT IN THE VICINITY OF THIS BOUNDARY WAS PRODUCING
RELATIVELY LIGHT/VARIABLE SURFACE WINDS...THOUGH LIGHT WESTERLY
WINDS WERE AT THE TOP/ABOVE SHALLOW SURFACE MIXED LAYER. LAKE
BREEZE WAS DEVELOPING ACROSS EASTERN LAKE/NORTHEAST COOK COUNTY...
THOUGH TDWR IMAGERY INDICATES THAT IT HAS SLOWED ITS WEST-
SOUTHWESTWARD PUSH NORTH OF DOWNTOWN CHICAGO...AND MOST HIGH-RES
MODEL GUIDANCE DOES NOT BRING IT AS FAR INLAND INTO ORD/MDW.

MVFR CIGS HAD DEVELOPED LATE MORNING WITH DIURNAL MIXING OF
SHALLOW MOIST BOUNDARY LAYER...THOUGH SOME SCATTERING AND LIFTING
OF THIS LOWEST LAYER OCCURRING PER SATELLITE/SFC OBS. FORECAST
SOUNDINGS ALSO DEPICT DRYING AND MIXING ABOVE BASE OF STRONG
INVERSION NEAR 2500 FT THROUGH THE AFTERNOON SUGGESTING MVFR CIGS
WILL NOT PERSIST. OTHER CONCERN IS THE POTENTIAL FOR SHRA/TSRA
DEVELOPMENT AS A MID-LEVEL DISTURBANCE TRACKS SOUTHEAST ACROSS THE
AREA THIS AFTERNOON. FORECAST SOUNDINGS INDICATE AFOREMENTIONED
INVERSION WOULD PROVIDE STRONG CAPPING FOR SURFACE BASED
PARCELS...THOUGH STEEP MID/UPPER LEVEL LAPSE RATES WOULD SUPPORT
SUBSTANTIAL INSTABILITY IF TAPPED. HIGH-RES GUIDANCE DOES DEPICT
SOME LIMITED DEVELOPMENT ACROSS THE AREA DURING THE AFTERNOON WITH
SUPPORT FROM RECENT RADAR TRENDS...THOUGH CONVECTION IS EXPECTED
TO STRUGGLE ACROSS TERMINALS WITH LOW CONFIDENCE IN COVERAGE.
STRONG/SEVERE THUNDERSTORMS ARE MUCH MORE LIKELY TO AFFECT
PORTIONS OF CENTRAL IL LATE THIS AFTERNOON/EVENING...EVOLVING EAST
FROM CURRENT TSRA COMPLEX SEEN OVER NORTHERN MO. MOIST LOW LEVEL
ENVIRONMENT AND LIGHT WINDS MAY SUPPORT FOG OR LOW STRATUS
DEVELOPMENT LATER TONIGHT...THOUGH MID/HIGH CLOUD COVER IS
EXPECTED TO LIMIT DEVELOPMENT SOMEWHAT.

STRONGER MID-LEVEL SYSTEM DIGS SOUTHEAST ACROSS THE REGION EARLY
SUNDAY ACCOMPANIED BY A STRONGER COLD FRONTAL PUSH. SOME POTENTIAL
WILL EXIST FOR SHRA/TSRA AROUND 12Z WITH THIS FRONT...THOUGH POOR
DIURNAL TIMING SUGGESTS LOW COVERAGE AT THAT HOUR AND PRECLUDES
MENTION IN TAF AT THIS TIME. SAME WITH POTENTIAL FOR INSTABILITY
SHOWERS LATER SUNDAY AFTERNOON. COLD ADVECTION BEHIND STRONGER
FRONT DOES DEEPEN THE MIXED LAYER SUNDAY...WITH WINDS SHIFTING
WEST-NORTHWEST AND BECOMING FAIRLY GUSTY NEAR 30 KT IN THE
AFTERNOON.

RATZER

//ORD AND MDW CONFIDENCE...UPDATED 20Z...

* LOW CONFIDENCE IN SHRA/TSRA DIRECTLY AFFECTING ORD/MDW THROUGH
  PERIOD.

* LOW CONFIDENCE IN WIND DIRECTION THIS AFTERNOON AND TONIGHT...
  THOUGH HIGH CONFIDENCE IN PREVAILING SPEEDS LESS THAN 10 KT.

* LOW CONFIDENCE IN FOG AND LOW CIGS TONIGHT.

* HIGH CONFIDENCE IN WEST-NORTHWEST WINDS BECOMING GUSTY SUNDAY.

RATZER

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

SUNDAY NIGHT...SLIGHT CHANCE OF SHRA IN THE EVE.
MONDAY...VFR. GUSTY NORTH-NORTHEAST WINDS.
TUESDAY...VFR.
WEDNESDAY-FRIDAY...SLIGHT CHANCE OF TSRA.

MTF

&&

.MARINE...
230 PM CDT

AN ACTIVE PERIOD OF WEATHER IS SETTING UP FOR LAKE MICHIGAN FOR
THE NEXT FEW DAYS. A DIFFUSE AREA OF LOW PRESSURE OVER THE LAKE
THIS AFTERNOON WILL KEEP WINDS RELATIVELY LIGHT AND VARIABLE...BUT
A DEEPENING LOW PRESSURE SYSTEM DROPPING THROUGH THE UPPER
MISSISSIPPI VALLEY WILL LEAD TO STRENGTHENING SOUTHERLY WINDS
OVERNIGHT TONIGHT. THE LOW IS EXPECTED TO TRACK ACROSS CENTRAL
LAKE MICHIGAN LATE TONIGHT OR EARLY SUNDAY MORNING...DRAGGING A
STRONG COLD FRONT ACROSS THE LAKE AND TURNING WINDS NORTHERLY.
WINDS SHOULD QUICKLY INCREASE TO ARND 30 KT FOLLOWING THE LOW
PASSAGE AND FROPA AND DURING THE PERIOD OF STRONGEST COLD
ADVECTION...THE SEVERAL HOURS AFTER THE PASSAGE OF THE LOW AND
COLD FRONT...OCCASIONAL GALE FORCE GUSTS WILL BE POSSIBLE. WILL
ISSUE A SMALL CRAFT ADVISORY FOR THE NEAR SHORE WATERS FOR SUNDAY
MORNING THROUGH LATE MONDAY NIGHT FOR THE ILLINOIS NEARSHORE
WATERS AND SUNDAY MORNING THROUGH EARLY TUESDAY MORNING FOR THE
INDIANA WATERS AS WINDS AND WAVES WILL BOTH EXCEED ADVISORY
CRITERIA...WITH WAVES IN THE INDIANA NEAR SHORE WATERS REACHING AS
HIGH AS 7 TO 10 FT WITH OCCASIONAL HIGHER WAVES. A NORTHWESTERLY
TO NORTHERLY FETCH OVER THE ILLINOIS WATERS WILL KEEP WAVES A BIT
LOWER...BUT SHOULD EXCEED 4-5 FT BY THE END OF THE 5NM NEARSHORE
FETCH. HIGH PRESSURE BUILDING ACROSS CENTRAL PLAINS TO THE WESTERN
GREAT LAKES BY MONDAY AFTERNOON AND EVENING SHOULD ALLOW WINDS
AND WAVES TO SUBSIDE OVER THE LAKE. AS THE HIGH SETTLES INTO THE
OHIO VALLEY TUESDAY NIGHT AND WEDNESDAY...ANOTHER LOW PRESSURE
SYSTEM DROPPING SOUTHEAST THROUGH ONTARIO DRAGGING ANOTHER COLD
FRONT DOWN THE LAKE. THIS COMBINATION SHOULD KEEP MODEST WEST TO
NORTHWEST WINDS OVER THE LAKE THROUGH MIDWEEK. ANOTHER AREA OF
SPRAWLING HIGH PRESSURE IS FORECAST TO SPREAD ACROSS THE WESTERN
GREAT LAKES REGION BY LATE IN THE WEEK...BRING BACK LIGHT AND
VARIABLE WINDS AND LOW WAVES FOR LATE IN THE WEEK AND POSSIBLY
INTO NEXT WEEKEND.

KREIN

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...BEACH HAZARDS STATEMENT...INZ001-INZ002...NOON SUNDAY TO 5 AM
     TUESDAY.

LM...SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742...NOON SUNDAY TO 11
     PM MONDAY.

     SMALL CRAFT ADVISORY...LMZ743-LMZ744-LMZ745...NOON SUNDAY TO 5
     AM TUESDAY.

&&

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