Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 220212
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
912 PM CDT MON JUL 21 2014

.DISCUSSION...
911 PM CDT

EVENING UPDATE...

HAVE MADE NO MAJOR CHANGES TO THE GOING FORECAST FOR TONIGHT. MAIN
FOCUS IS ON THE HEAT AND HUMIDITY ON TUESDAY AND THE THREAT FOR A
FEW SEVERE STORMS MAINLY TUESDAY EVENING. THERE IS SOME POTENTIAL
FOR DEBRIS CLOUD COVER FROM UPSTREAM CONVECTION TUESDAY
MORNING...BUT OVERALL SHOULDNT BE SIGNIFICANT IMPACTS ON FORECAST
HIGHS IN THE LOWER 90S. DEW POINTS POOLED WELL INTO THE 70S OUT TO
OUR WEST TODAY...SO THE LOW-MID 70S IN GRIDS BY AFTERNOON APPEARS
ON TRACK. THE TEMPERATURES AND DEW POINTS YIELD HEAT INDEX VALUES
RANGING FROM 95 TO 105 DEGREES...HIGHEST IN THE NW CWA. GIVEN THE
FAIRLY COOL WEATHER OF LATE AND THE OVERALL LACK OF HIGH HEAT AND
HUMIDITY THIS SUMMER...HAVE ISSUED AN SPS FOR AN EXTRA HEADS UP.
AT THIS TIME...NOT APPEARING WE WILL NEED AN ADVISORY FOR ANY
PARTS OF THE AREA...THOUGH WILL NEED TO MONITOR FOR HIGHER TEMPS
AND/OR DEWPOINTS THAT COULD PUT FAVORED NW AREAS AT OR SLIGHTLY
ABOVE 105 DEGREE HEAT INDEX CRITERIA FOR A HEAT ADVISORY.

REGARDING THE THUNDERSTORM/SEVERE WEATHER POTENTIAL TOMORROW
EVENING...STILL A FAIRLY CONDITIONAL THREAT DUE TO PRESENCE OF
VERY STOUT CAP AS AN EML IS ADVECTED OVER THE REGION. SOME
GUIDANCE SUCH AS 18Z GFS ACTUALLY DEVELOPS LITTLE OR NO QPF OVER
THE AREA DURING THE EVENING...BUT THIS COULD BE OWING TO SFC TEMPS
BEING COOLER THAN WHAT WILL LIKELY OCCUR. BEST UPPER FORCING WILL
PASS TO THE NORTH...THOUGH INCREASING BOUNDARY LAYER CONVERGENCE
WITH APPROACH OF COLD FRONT AND SLIGHT MIDLEVEL COOLING SHOULD BE
SUPPORTIVE OF SCATTERED TSRA COVERAGE. FOR ANY STORMS THAT ARE
ABLE TO OVERCOME CAPPING AND BECOME MORE INTENSE...HIGH CAPE AND
STEEP MIDLEVEL LAPSE RATES ALONG WITH MODEST DEEP LAYER SHEAR WILL
FAVOR A THREAT OF LARGE HAIL...AND STEEP LOW LEVEL LAPSE RATES A
THREAT FOR DAMAGING WIND GUSTS. AS ALLUDED TO IN THE AFTERNOON
AFD...HEAVY RAIN RATES WILL ALSO BE A CONCERN IN MORE INTENSE
CONVECTION DUE TO PWAT VALUES INCREASING TO CLOSE TO OR ABOVE 2
INCHES.

RC

&&

.PREV DISCUSSION...
257 PM CDT

WEATHER HIGHLIGHTS INCLUDE THE HEAT AND HUMIDITY THAT WILL BE ON TAP
THROUGH TOMORROW. A COLD FRONT WILL BRING A CHANCE FOR THUNDERSTORMS
TUESDAY EVENING INTO EARLY WEDNESDAY. COOLER AND DRY CONDITIONS WILL
RETURN FOR THE LATTER HALF OF THE WEEK...FOLLOWED BY A RETURN TO
UNSETTLED THOUGH STILL GENERALLY COOLER WEATHER FOR THE EXTENDED.

IT IS FEELING A BIT MORE SUMMERLIKE THIS AFTERNOON AS DEWPOINTS
ACROSS THE AREA HOVER IN THE MID 60S AND TEMPERATURES INTO THE 80S.
CLOUD COVER OFF THE N IS FADING AS THE STRONG UPPER RIDGE IN THE
NATIONS HEARTLAND EXPANDS NORTH AND EASTWARD INTO NORTHERN
ILLINOIS...AND MUCH LESS IN THE WAY OF DIURNAL CUMULUS CLOUDS.  THE
LAKE BREEZE HAS BEEN HELD LARGELY IN CHECK TODAY DUE TO THE
STRENGTHENING SW WINDS THOUGH IS MAKING AN APPEARANCE ALONG THE
NORTH SHORE.

OUR MAIN FEATURE OF INTEREST IS A SHORTWAVE PUSHING INTO THE DAKOTAS
WORKING ITS WAY ALONG THE PERIPHERY OF THE UPPER RIDGE OUT AHEAD OF
A DIGGING UPPER TROUGH OFF THE PACIFIC NW COAST. THUNDERSTORMS ARE
ONGOING ACROSS THE DAKOTAS AHEAD OF THIS WAVE. THERE IS ADDITIONAL
ACTIVITY SPREADING INTO NEBRASKA WHERE WEAKER FORCING IS IN PLACE.
THIS
DISTURBANCE IS FORECAST TO STRENGTHEN AS IT ENTERS MINNESOTA LATE
TONIGHT WHILE THE UPPER RIDGE FLATTENS SOMEWHAT TO OUR WEST.
THUNDERSTORMS LOOK TO BE ONGOING LATE TONIGHT AND LIKELY IN A
DECAYING PHASE AS THEY PASS BY TO THE NORTH AND POSSIBLY DRIFT
TOWARD OUR FAR WESTERN COUNTIES. SOME HI-RES GUIDANCE SUGGESTS
THAT SOME ACTIVITY MAY SNEAK INTO THESE AREAS ESPECIALLY IF THE
ACTIVITY PUSHING INTO NEBRASKA HOLDS TOGETHER...BUT STILL FEEL THAT
MOST/ALL OF THE AREA WILL REMAIN DRY INTO TUESDAY MORNING WITH ALL
SREF/GEFS MEMBERS KEEPING THINGS OUT OF OUR AREA.

ON TUESDAY THIS SHORTWAVE WILL CONTINUE SLIDING SOUTHEAST AND WILL
DRIVE A COLD FRONT AND SURFACE WAVE THROUGH NE IL/NW IN TUESDAY
NIGHT. BEFORE THAT HAPPENS...THE 850H THERMAL RIDGE BUILDS OVERHEAD.
GUSTY SW WINDS AND MOISTURE POOLING ARE EXPECTED AHEAD OF THE COLD
FRONT WITH THE MOST UNCOMFORTABLE DAY OF THE WEEK. HEAT INDEX VALUES
LOOK TO TOP OUT AROUND 100 OR SO DO TO AN UPTICK IN TEMPERATURES AND
A MORE SIGNIFICANT INCREASE IN DEWPOINTS OVER THAT EXPERIENCED TODAY.

MODELS ARE STILL AT ODDS WITH HOW WIDESPREAD THUNDERSTORM ACTIVITY
WILL BE WITH THE FRONTAL PASSAGE DUE TO A STRONG CAP IN
PLACE...MAKING FOR A CHALLENGING FORECAST. I PREFER NOT TO BACK OFF
ON LIKELY POPS BUT FEEL SLIGHTLY LESS CONFIDENT THAN YESTERDAY ON
MORE THAN SCATTERED COVERAGE. WITH THE GENERAL PATTERN CANNOT
RULE OUT ISOLATED ACTIVITY IN THE AFTERNOON WITH CONVERGENCE AHEAD
OF THE FRONT IN THE MOIST WARM SECTOR. BUT THE MORE ORGANIZED TIME
PERIOD WOULD BE IN THE EVENING HOURS. CONFIDENCE IS MODERATE TO
HIGH THAT THERE BE AT LEAST SCATTERED THUNDERSTORM ACTIVITY.
INSTABILITY AND MOISTURE VALUES ARE VERY HIGH...AND BULK SHEAR HAS
SHOWN SIGNS OF INCREASING IN RECENT MODELS SPC HAS HIGHLIGHTED SE
WISCONSIN AND NE ILLINOIS IN THE SLIGHT RISK CATEGORY FOR SEVERE
STORMS TUESDAY NIGHT IN THE REGION WHERE STRONG INSTABILITY/HIGHER
SHEAR WILL BE THE BEST CO- LOCATED...GENERALLY ALONG AND NORTH OF
INTERSTATE 80. SEVERE WIND GUSTS/LARGE HAIL MAY BE POSSIBLE...BUT
THE GREATEST THREAT WILL BE HEAVY RAINFALL RATES GIVEN PWAT VALUES
ABOVE 2" WHICH MEANS STORMS WILL BE VERY EFFICIENT RAIN MAKERS
WITH 1"/HR TOTALS POSSIBLE. STORMS MAY STILL BE SURFACE BASED EVEN
WITH THE FRONTAL PASSAGE COMING IN THE EVENING AS TEMPERATURES AND
DEWPOINTS REMAIN ON THE HIGH SIDE. NO CLEAR SIGNAL OF ANY TRAINING
GIVEN THE PROGRESSIVE NATURE OF THE FRONT. CAPPING ISSUES DECREASE
CONFIDENCE IN COVERAGE FARTHER SOUTH AND EAST...BUT IRONICALLY AS
THE FRONT AND UPPER FORCING HAVE A LONGER TIME TO ACT ON THE CAP
AREAS SOUTH MAY ACTUALLY HAVE A BETTER CHANCE FOR MORE WIDESPREAD
ELEVATED STORMS...THOUGH CONFIDENCE ON SEVERE IN THESE AREAS IS
LOWER.

850 TEMPERATURES DROP SOME 8-10C ON WEDNESDAY WITH THE LOW LEVEL
FLOW TURNING ONSHORE. HIGH PRESSURE BUILDS SOUTH INTO THE UPPER
MIDWEST. THIS WILL KEEP A MODEST PRESSURE GRADIENT DRAPED OVER THE
AREA WEDNESDAY BEHIND THE DEPARTING SURFACE WAVE. THEREFORE EXPECT
GUSTY N/NE WINDS AND HIGHS IN THE 70S. THE SURFACE HIGH BUILDS
OVERHEAD THURSDAY FOR ANOTHER SEASONABLY COOL AND DRY DAY...THEN THE
HIGH DRIFTS EAST ON FRIDAY BRINGING SOME RECOVERY TO TEMPS CLOSER TO
80 OR SO WITH MORE OF SOUTHERLY COMPONENT TO LOWER LEVEL WINDS.

BEYOND THIS TIME FRAME...AN UPPER TROUGH WILL REMAIN PARKED OVER
EASTERN CANADA WHILE ANOTHER UPPER LOW MIGRATES THROUGH THE CANADIAN
PRAIRIES. WEAK RIPPLES AROUND THE CENTRAL U.S. UPPER RIDGE COMBINED
WITH WARM AIR ADVECTION FRIDAY NIGHT INTO SATURDAY. THE UPPER AIR
PATTERN EVOLUTION GETS A BIT MESSIER BEYOND THAT...BUT GENERALLY THE
REGION WILL REMAIN UNDER THE INFLUENCE OF THE EASTERN U.S. TROUGH TO
SOME DEGREE CREATING SOMEWHAT COOLER AND UNSETTLED CONDITIONS.

KMD

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 00Z...

* GUSTY SOUTH-SOUTHWEST WINDS TUESDAY AND TUESDAY EVENING.

* SCATTERED TSRA PROBABLE TUESDAY EVENING.

//DISCUSSION...UPDATED 00Z...

LOW PRESSURE IS DEVELOPING INTO WESTERN MN EARLY THIS EVE WITH A
COMPLEX OF SEVERE STORMS MOVING EASTWARD JUST NORTH OF THIS LOW
CENTER. SOUTHERLY WINDS WILL INCREASE IN SPEED TUESDAY MORNING
WITH GUSTS EXCEEDING 20 KT MUCH OF THE AFTERNOON. THE ATMOSPHERE
LOOKS TO BE WARM AND PROVIDING A CAP TO ANY THUNDERSTORM
DEVELOPMENT DURING MUCH OF THE DAY. AS THE SYSTEM COOL FRONT
APPROACHES FROM THE NORTH...CONVERGENCE WILL INCREASE AND LIKELY
SERVE AS A FOCUS FOR AT LEAST SCATTERED EVENING INTO EARLY
OVERNIGHT STORMS IN THE REGION. ONE POSSIBILITY TO STILL CONSIDER
IS IF AN OUTFLOW WERE TO EMANATE SOUTHWARD FROM THE
AFOREMENTIONED STORM COMPLEX LATE TONIGHT....AS THAT COULD CAUSE
AN EARLIER FOCUS AND WEAKENING OF THE CAP FOR STORMS. ON THE OTHER
HAND...THE CAP LOOKS TO BE QUITE STRONG. SO CONFIDENCE IN PRECISE
TIMING OF STORMS IN THE AREA FOR THE TUESDAY EVENING PERIOD IS
LOW. SOME STORMS IF THEY CAN DEVELOP COULD BE STRONG GIVEN THE
AMOUNT OF INSTABILITY THAT WILL BE PRESENT AND BUILDING THROUGH
THE AFTERNOON. BEHIND THE COLD FRONT...LATE TUESDAY NIGHT INTO
WEDNESDAY MORNING...A PERIOD OF AT LEAST MVFR CIGS IS PROBABLE FOR
CHICAGO AREA TAF SITES AS FLOW TURNS OFF OF LAKE MICHIGAN.

//ORD AND MDW CONFIDENCE...UPDATED 00Z...

* MEDIUM IN WINDS.

* LOW ON SPECIFIC TSRA TIMING TUE EVE. IT IS LIKELY TO BE A 2-3
  HOUR WINDOW FOR THE MOST PROBABLE TIME.

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

WEDNESDAY...MORNING LOW CLOUDS. GUSTY NORTHEAST WINDS.
THURSDAY...NORTHEAST WINDS.
FRIDAY...CHC OF TSRA DURING THE NIGHT.
SATURDAY-SUNDAY...CHC OF TSRA.

MTF

&&

.MARINE...
306 PM CDT

FOR THE NEARSHORE WATERS...WINDS WILL INCREASE TOMORROW AFTERNOON
AND BECOME HAZARDOUS TO SMALL CRAFT OVER THE IL NEARSHORE WATERS. A
COLD FRONT PUSHES DOWN THE LAKE TUESDAY EVENING AND NIGHT...WITH
WINDS INCREASING TO 30 KT OVER ALL OF THE NEARSHORE WATERS AND WAVES
BUILDING TO 9 FT.  ANOTHER SMALL CRAFT WILL BE NEEDED FOR AN
EXTENDED PERIOD...RIGHT NOW EARLY WEDNESDAY MORNING THROUGH AT LEAST
THURSDAY MORNING.

HIGH PRESSURE CONTINUES TO HOLD ON OVER THE EASTERN GREAT LAKES
WHILE LOW PRESSURE IS OVER THE DAKOTAS.  SOUTH WINDS ARE ACROSS THE
LAKE WITH THE HIGHEST GUSTS UP TO 25 KT OVER THE NORTHERN HALF OF
THE LAKE.  THE LOW WILL DEEPEN AS IT MOVES OVER LAKE SUPERIOR
TOMORROW AND EXPECTING SOUTH GUSTS TO INCREASE TO 30 KT OVER THE
NORTHERN TWO THIRDS OF THE LAKE. MAY SEE OCCASIONAL GALES TO 35 KT
OVER THE NORTHERN HALF OF THE LAKE TOMORROW AFTERNOON.  FORECAST
SOUNDINGS HAVE WINDS 40-50 KT JUST 200 FT ABOVE THE SURFACE SO WE
WILL LIKELY SEE OCNL GALE GUSTS. HOWEVER NOT EXPECTING ENOUGH GUSTS
TO JUSTIFY A GALE WARNING.

THE LOWS COLD FRONT PASSES OVER THE LAKE TOMORROW EVENING AND WINDS
WILL TURN NORTH BEHIND THE FRONT. GUSTS WILL INCREASE TO 30 KT OVER
THE SOUTHERN HALF OF THE LAKE THROUGH THE NSH WATERS.  THE LOW
DEEPENS AS IT MOVES NORTHEAST INTO NEWFOUNDLAND...AND NORTH WINDS
SLOWLY DIMINISH OVER THE SOUTHERN HALF OF THE LAKE AS HIGH PRESSURE
SPREADS OVER THE LAKE WEDNESDAY NIGHT. WINDS REMAIN NORTH TO
NORTHEAST THROUGH THURSDAY EVENING AND THEN BECOME VARIABLE UNDER
THE HIGH THURSDAY NIGHT INTO FRIDAY.

JEE

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742...10 AM TUESDAY TO
     10 PM TUESDAY.

&&

$$

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