Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 210520
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1220 AM CDT MON JUL 21 2014

.DISCUSSION...
210 PM CDT

FORECAST CONCERNS REMAIN CENTERED ON INCREASING HEAT AND HUMIDITY
FOR MONDAY AND TUESDAY...FOLLOWED BY CHANCES FOR THUNDERSTORMS AND
POTENTIALLY HEAVY RAINFALL TUESDAY NIGHT. AFTER A COLD FRONT BRINGS
A RETURN TO DRIER AND COOLER WEATHER...A MORE ACTIVE PATTERN LOOKS
TO SET UP FOR THE WEEKEND.

THE OVERALL WEATHER PATTERN ACROSS THE MIDWEST HAS CHANGED SLIGHTLY
OVER THE LAST 24 HOURS. BROAD UPPER LEVEL TROUGHING REMAINS IN PLACE
ACROSS THE EASTERN THIRD OF THE COUNTRY. A FEW WEAK WAVES ON THE
BACK SIDE OF THIS TROUGH ARE MOVING SOUTHWEST THROUGH THE AREA. A
STRONG UPPER LEVEL RIDGE CONTINUES TO STRENGTHEN IN THE SOUTHERN
PLAINS AND EXTENDING INTO THE ROCKY MOUNTAIN REGION. THE UPPER LEVEL
JET IS FOCUSED NORTH OF THE CANADIAN BORDER AND A STRONGER WAVE IS
PASSING THROUGH THE CENTRAL AND NORTHERN MANITOBA...LEAVING FAIRLY
ZONAL FLOW ACROSS THE NORTHERN TIER OF THE COUNTRY. AT THE
SURFACE...WIDESPREAD HIGH PRESSURE EXTENDS FROM THE ATLANTIC AND
THROUGH THE EASTERN GREAT LAKES. A TROUGH AXIS EXTENDS THROUGH THE
NATIONS HEARTLAND. THIS LEAVES A PRESSURE GRADIENT DRAPED TO OUR
NORTH AND WEST.

THE LEADING EDGE OF THE THERMAL RIDGE IS STILL LARGELY WEST OF THE
AREA THIS AFTERNOON...SO TEMPERATURES HAVE WARMED JUST SLIGHTLY INTO
THE 80S WITH JUST A SLIGHT INCREASE IN HUMIDITY LEVELS FROM
SATURDAY. DIURNAL CUMULUS AND THE LAKE BREEZE IS AGAIN KEEPING
THINGS IN CHECK TODAY. A FEW WEAK WAVES ARE TRAVERSING THE UPPER
FLOW ON THE BACK SIDE OF THE EASTERN U.S. TROUGH BRINGING VARIABLE
CLOUDINESS AT TIMES...BUT GENERALLY UPPER LEVEL HEIGHTS CONTINUE TO
BUILD KEEPING CLOUD COVER SOMEWHAT LIMITED OUTSIDE OF THE DAYTIME
CUMULUS DEVELOPMENT. THAT SAID...A FAIRLY EXTENSIVE AREA OF HAZE FROM
FIRES IN THE WESTERN UNITED STATES EXTENDS BACK TO THE IOWA BORDER
THIS AFTERNOON...AND THUS EXPECT THE HAZY CONDITIONS TO PERSIST
THROUGH THE EVENING HOURS AND POSSIBLY INTO TOMORROW. DEWPOINTS REMAIN IN
THE UPPER 50S TO LOW 60S TONIGHT...SO EXPECT LOWS TO REMAIN IN THE
60S. ANY POTENTIAL CONVECTION TONIGHT INTO MONDAY MORNING WOULD
REMAIN WELL NORTH AND WEST CLOSER TO THE LOW LEVEL JET AXIS AND
UPPER FORCING.

ON MONDAY THE UPPER RIDGE IN THE CENTER OF THE COUNTRY STRENGTHENS
AND EXPANDS EXPANDS EASTWARD OVER THE AREA. IT ALSO AMPLIFIES IN
RESPONSE TO A DIGGING TROUGH OFF THE PACIFIC COAST. AS THIS OCCURS
THE THERMAL RIDGE AXIS EXPANDS EASTWARD WITH A MORE NOTICEABLE
INCREASE IN TEMPERATURES AND HUMIDITY VALUES. 850 TEMPS REACH FROM
18-21C ON MONDAY...POSSIBLY COOLER EAST INTO NW INDIANA. LOCAL
CLIMATOLOGY WOULD SUPPORT AROUND 90 IN SPOTS...BUT MOS GUIDANCE HAS
BEEN PERFORMING WELL THE PAST FEW DAYS. STILL EXPECTING TEMPS
GENERALLY IN THE MID TO UPPER 80S.

MONDAY NIGHT INTO TUESDAY SEVERAL WAVES WILL BE RIDING ALONG THE
NORTHERN SIDE OF THE STRONG UPPER LEVEL HIGH. MODELS ARE IN GOOD
AGREEMENT IN CONVECTION BREAKING OUT IN THE NORTHERN PLAINS EARLIER
IN THE DAY WHICH MAY POTENTIALLY MORPH INTO A FORWARD PROPAGATING MCS
OVERNIGHT. THE LOW LEVEL JET AXIS AND INSTABILITY AXIS AGAIN REMAINS
FOCUSED NORTH AND WEST OF THE AREA. STILL SOMETHING TO KEEP AN EYE
ON. BY TUESDAY MORNING SOME OF THE SHORTWAVE ENERGY GETS MERGED WITH
THE NORTHERN STREAM UPPER LOW IN NORTHERN CANADA. THE CONVECTION
MONDAY NIGHT INTO TUESDAY WILL GET MUCH CLOSER...THOUGH MOST OF OUR
AREA REMAINS LARGELY CAPPED EVEN INTO TUESDAY. OVERNIGHT LOWS WILL
BE IN THE MID TO UPPER 60S...TO THE LOW 70S DOWNTOWN.

TUESDAY THE HEAT AND CERTAINLY HUMIDITY WILL BE IN FULL EFFECT. ONCE
ANY MORNING DEBRIS CLOUDS CLEAR SW WINDS. LOCAL 925/850 CLIMATOLOGY
AND CURRENT GRIDS ARE IN LINE IN SHOOTING FOR THE LOW 90S...WHICH IS
CERTAINLY PLAUSIBLE GIVEN THAT THE THERMAL RIDGE AXIS WILL BE RIGHT
OVER US AND SW FLOW REMAINS IN PLACE AHEAD OF THE SOUTHWARD MOVING
COLD FRONT. CONCERNS ARE ANY LINGERING DEBRIS CLOUDS AND THE HIGH
END OF SREF/GEFS ENSEMBLE GUIDANCE TOPPING OUT NEAR 90...FEEL WITH
THE SW WINDS THAT THE POTENTIAL IS STILL THERE. REGARDLESS IT WILL
FEEL QUITE UNCOMFORTABLE WITH DEWPOINTS INTO THE 70S RESULTING IN
HEAT INDICES AROUND 100. SW WINDS WILL PUSH THE MUGGY CONDITIONS ALL
THE WAY TO THE LAKEFRONT. THERE IS A FAIRLY STRONG CAP IN PLACE THAT
LOOKS TO REMAIN IN PLACE ACROSS MOST OF THE AREA...BUT KEPT A CHANCE
POP IN FOR THE NORTHERN TIER SHOULD THE WAVE GET CLOSER SOUTH.

THE POSITIVELY TILTED SHORTWAVE WILL PUSH SE THROUGH THE AREA
SOMETIME TUESDAY EVENING/NIGHT...BUT SOME DISAGREEMENTS AS TO HOW
STRONG THE WAVE WILL BE. THE ASSOCIATED SURFACE COLD FRONT WILL BE
PUSHING INTO THE UPPER MIDWEST BY EARLY AFTERNOON/EVENING AND
APPEARS ENOUGH COMBINATION OF UPPER LEVEL DYNAMICS AND LOW LEVEL
CONVERGENCE FOR SHOWERS AND THUNDERSTORMS.  ISOLATED SEVERE
WIND/HAIL REMAINS POSSIBLE WITH THE THREAT FOR HEAVY RAINFALL/FLASH
FLOODING BEING THE MOST LIKELY HAZARD AS FORECAST SOUNDINGS SUGGEST
PRECIPITABLE WATER VALUES IN EXCESS OF 2 INCHES. MODELS GENERALLY
KEEP THE HEAVIEST PRECIPITATION NORTH BUT A LOT DEPENDS ON THE TIMING
AND HOW INTACT THE FRONT/WAVE REMAINS AS IT SLIDES THROUGH OUR AREA.
SEEMS THERE WOULD BE ENOUGH FORCING/CONVERGENCE TO WARRANT LIKELY
POPS FOR A GOOD PORTION OF THE AREA.

THE WARM UP WILL BE SHORT LIVED AS WEDNESDAY WE WILL BE BACK INTO
THE COOLER AND DRIER REGIME AS A COLD FRONT BRINGS THE RETURN TO DRY
AND COOL NORTHERLY TO ONSHORE FLOW. EXPECT GENERALLY CLEAR SKIES FOR
SEVERAL DAYS BEHIND THE FRONT WITH READINGS BACK INTO THE 70S TO
AROUND 80 WITH COOLER CONDITIONS AT THE LAKEFRONT. THE NEXT CHANCES
FOR PRECIP DOES NOT LOOK TO BE UNTIL FRIDAY NIGHT OR POSSIBLY
SATURDAY AS AN UPPER LOW ALONG THE CANADIAN PRAIRIES APPROACHES FROM
THE NORTHWEST WITH ACTIVE AND COOLER THAN NORMAL CONDITIONS REMAINING IN
PLACE AS THE LOW CUTS OFF AND SLIDES THROUGH WISCONSIN ON SUNDAY.

KMD

&&

.AVIATION...

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

* NONE.

BMD

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

HIGH PRESSURE REMAINS EAST OF THE TERMINALS WITH A WEAK GRADIENT
MAINLY OVER NORTH CENTRAL ILLINOIS PROVIDING LIGHT SOUTHERLY WINDS.
PATCHY FOG IS ALREADY STARTING TO DEVELOP...MAINLY ACROSS PORTIONS
OF EAST CENTRAL ILLINOIS AND NORTHWEST INDIANA THIS EVENING WHERE
THE GRADIENT IS WEAKER. AS SOME LINGERING CIRRUS CLEARS OUT OVER THE
NEXT COUPLE HOURS...EXPECT FOG TO EXPAND BUT FOR NOW EXPECT IT TO BE
CONFINED TO GYY. SOUTH WINDS WILL PICK UP SOME MONDAY MORNING TO
AROUND 10 KT OR SO AND SHOULD KEEP ANY LAKE BREEZE FROM PUSHING
ACROSS THE TERMINALS. OUTSIDE OF THE FOG POTENTIAL
OVERNIGHT...EXPECT VFR CONDITIONS TO PREVAIL THROUGH THE REMAINDER
OF THE PERIOD.

BMD


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

* HIGH CONFIDENCE IN ALL FORECAST ELEMENTS.

BMD

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

TUESDAY...TSRA LIKELY DURING THE NIGHT.
WEDNESDAY...CHC OF TSRA EARLY THEN INCREASING GUSTY NE WINDS.
THURSDAY...VFR. NORTHEAST WINDS.
FRIDAY...SLGT CHC OF TSRA LATE.
SATURDAY...CHC OF TSRA.

MTF

&&

.MARINE...
235 PM CDT

LOW PRESSURE MOVING OUT OF SASKATCHEWAN INTO ONTARIO WILL PUSH A
COLD FRONT DOWN THE LAKE TUESDAY NIGHT.  FROM TONIGHT THROUGH LATE
TUESDAY THE LAKE WILL INCREASINGLY BECOME INFLUENCED BY THE
SOUTHERLY FLOW AHEAD OF THIS FRONT AS IT WORKS ITS WAY TOWARD THE
AREA.  THE RELATIVELY LIGHT PRESSURE GRADIENT TODAY IS STILL
ALLOWING SOME PRONOUNCED ONSHORE FLOW TO DEVELOP AROUND THE
PERIMETER OF THE LAKE.  THIS CAN BE SEEN AS POCKETS OF CLEARING
EVIDENT IN VISIBLE SATELLITE IMAGERY...AND OF COURSE IN THE WIND
OBSERVATIONS AT THE COASTAL PLATFORMS.  WINDS WILL TURN QUICKLY TO
THE NORTH TUESDAY NIGHT AND INCREASE AS THE FRONT MOVES THROUGH.
WINDS MAINTAIN A NORTHERLY COMPONENT THROUGH LATE IN THE WEEK BUT
GRADUALLY RELAX.

DEPENDING ON THE STRENGTH OF THE PRESSURE GRADIENT AND THE COOLNESS
OF THE POST FRONTAL AIRMASS...GUSTS COULD APPROACH LOW END GALES FOR
A WHILE ON WEDNESDAY.

LENNING

&&

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

$$

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