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AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1236 PM CDT SAT MAY 25 2013

.DISCUSSION...
1048 AM CDT

SOME CHANGES MADE EARLIER TO THE GOING FORECAST. DID FURTHER
TIGHTEN THE GRADIENT IN POPS FROM WEST TO EAST ACROSS THE FORECAST
AREA THIS AFTERNOON INTO EARLY THIS EVE...WHILE BUMPED UP POPS IN
THE FAR SOUTHWEST LATER THIS AFTERNOON INTO TONIGHT.

WELL-DEFINED SHORT WAVE ON SATELLITE AND RADAR MOSAIC IMAGERY IS
TRAVERSING EAST ACROSS IOWA ON A 50 KT MID-LEVEL JET PER RAP
ANALYSIS. UPPER AND SURFACE RIDGING ACROSS THE IMMEDIATE AREA HAVE
HELPED TO STIFLE THE EASTWARD PROGRESSION OF MOISTURE TRANSPORT AND
ASSOCIATED DEEPER SATURATION AND RAIN. THIS WILL CONTINUE TO SERVE
AS BASICALLY A WALL FOR THE EASTERN FORECAST AREA...INCLUDING
CHICAGO...THROUGH TODAY AND LIKELY INTO THE EVE. SHORT TERM
GUIDANCE AGREES WITH OBSERVATIONAL TRENDS IN INCHING THE RAIN INTO
THE WESTERN FORECAST AREA AS THE AFTERNOON PROGRESSES. THIS RAIN
LOOKS TO BE LIGHT FOR MOST OF THE TIME INTO EARLY THIS EVE. RAIN
INTENSITY MAY PICK UP SOME LATER THIS EVE AS DEEPER MOISTURE
CONVERGENCE WORKS OVER THE AREA...INCLUDING EVEN SOME ELEVATED
INSTABILITY. THERE WAS QUITE A BIT OF LIGHTNING ACROSS IA THIS
MORNING. WHILE NOT ANTICIPATING ANYWHERE NEAR THAT MUCH IN THE
FORECAST AREA TONIGHT INTO SUNDAY...CERTAINLY COULD SEE SOME
SCATTERED THUNDER.

TEMPERATURES HAVE BEEN SLOW TO CLIMB AND CERTAINLY LOOK TO REMAIN
AS SUCH IN THE WESTERN FORECAST AREA WITH THICKER CLOUDS AND LIGHT
RAIN MOVING IN. HAVE ADJUSTED DOWN HIGHS A COUPLE DEGREES TOWARD
LOCAL ARW GUIDANCE WHICH SEEMS TO HAVE A GOOD HANDLE ON OBSERVED
SLOW CLIMBING TRENDS. LAKE INFLUENCE WILL KEEP NEARSHORE AREAS
PLENTY COOL ONCE AGAIN...WITH HIGHS IN THE LOWER TO MID 50S. THIS
IS IMPRESSIVELY COOL FOR THIS LATE IN THE MONTH. LOOKING AT LOCAL
RESEARCH FOR DOWNTOWN CHICAGO /SINCE 1982/...FOR THE LAST TEN DAYS
OF MAY WHEN THE WINDS ARE PRIMARILY OFF THE LAKE...THE 25TH
PERCENTILE IS 59 DEGREES AND THE 10TH PERCENTILE IS 53
DEGREES...WHICH WE WILL BE RIGHT AROUND TODAY.

MTF

//PREV DISCUSSION...
300 AM CDT

BELOW NORMAL TEMPERATURES AND PERIODIC CHANCES FOR SHOWERS OR
THUNDERSTORMS THROUGH THE WEEKEND WILL SET THE STAGE FOR A RAPID
WARMING TREND FOR NEXT WEEK.

FOR THE REMAINDER OF THE NIGHT AND INTO SATURDAY MORNING...A WEAK
SHORTWAVE OVERTOPPING A BUILDING RIDGE OVER THE CENTRAL CONUS WILL
BRING INCREASING CLOUDINESS AND A CHANCE FOR SOME LIGHT SHOWERS OR
SPRINKLES...PARTICULARLY FOR LOCATIONS WEST OF THE FOX VALLEY AND
THE I-55 CORRIDOR.  LATEST RADAR IMAGERY SHOWS SCATTERED SHOWERS
CROSSING THE MS RIVER INTO NWRN IL...BUT AS THEY MOVE INTO AN
ENVIRONMENT NOTED BY DRY LOW LEVEL CONDITIONS...THE SHOWERS SHOW A
DIMINISHING TREND.  HOWEVER...REPEATED SHOWER DEVELOPMENT WILL
EVENTUALLY MOISTEN UP THE LOWER LEVELS ENOUGH FOR SOME LIGHT PCPN TO
REACH THE GROUND.  TEMPERATURES HAVE ALREADY REACHED A MINIMA ACROSS
THE AREA AND SHOULD BEGIN TO SLOWLY RISE DURING THE REMAINDER OF THE
OVERNIGHT HOURS AS DENSER CLOUD COVER OVERSPREADS THE REGION AND
SHUTTING OFF ANY FURTHER RADIATIVE COOLING.

FOR THE REMAINDER OF SATURDAY AND INTO SATURDAY NIGHT...WILL
CONTINUE TO FOCUS ON PCPN CHANCES.  THE UPPER LEVEL PATTERN REMAINS
GENERALLY UNCHANGED OVER THE PAST 24-36 HRS WITH A HIGH
AMPLITUDE...SLOWLY PROGRESSIVE PATTERN CONTINUING.  THE MAIN CULPRIT
IS A DEEP UPPER LOW MAKING VERY LOW PROGRESS LIFTING NEWD THROUGH
NEW ENGLAND.  THIS IS HELPING BUILD THE UPPER RIDGE OVER THE CENTRAL
CONUS WHILE A SERIES OF SYSTEM MOVE INTO THE PACIFIC NORTHWEST.
WHILE THE UPPER LOW REMAINS OVER NEW ENGLAND...AND THE UPPER RIDGE
AXIS REMAINS TO THE WEST OF THE LOCAL AREA...PERSISTENT WEAK COLD
ADVECTION THROUGH DEEP LEVELS OF THE ATMOSPHERE WILL HELP KEEP
TEMPERATURES ON THE COOLER SIDE.  ALSO...AT THE SURFACE...HIGH
PRESSURE EXTENDS FROM HUDSON BAY...THROUGH THE EASTERN GREAT LAKES
TO THE TENNESSEE VALLEY.  THIS HAS KEPT NELY FLOW OFF OF LAKE
MICHIGAN...WITH THE RELATIVELY COLD LAKE WATERS HELPING TO KEEP
LAKEFRONT TEMPERATURES IN THE 50S.  THERE WILL BE LITTLE CHANGE
TODAY AS TEMPS WILL STILL REMAIN IN THE 50S ALONG THE LAKEFRONT.
THE INCREASING CLOUD COVER WILL ALSO HELP TO LIMIT DAYTIME
HEATING...WITH HIGHS REMAINING IN THE LOWER TO MIDDLE 60S.  A WARM
FRONT IS BEGINNING TO DEVELOP SEWD FROM A SFC LOW OVER THE WESTERN
PLAINS.  THIS WARM FRONT WILL INCREASINGLY BECOME THE FOCUS OF
FURTHER PCPN DEVELOPMENT THROUGH THE REMAINDER OF THE WEEKEND AND
INTO EARLY NEXT WEEK.  THE WARM FRONT WILL GRADUALLY BECOME MORE
FOCUSED...AND EXTEND EWD...THROUGH NEBRASKA...ALONG THE IA/MO
BORDER TO SRN IL/IN BY SUNDAY MORNING.  WHILE THE BOUNDARY REMAINS
TO THE SOUTH...INCREASING WARM ADVECTION AND ISENTROPIC LIFT THROUGH
THE LOWER LEVELS WILL HELP TO SPREAD PCPN FROM THE SOUTHWEST TO THE
NORTHEAST ACROSS THE CWA THROUGH THE WEEKEND...THOUGH PCPN SHOULD
GENERALLY BE   PCPN SHOULD BEGIN TO BECOME MORE WIDESPREAD ACROSS
THE CWA BY SUNDAY AND SUNDAY NIGHT AS THE WARM FRONT SLOWLY LIFTS
NWD. IT IS A LITTLE TOUGH TO SAY IF THE WEEKEND AND THE MEMORIAL DAY
HOLIDAY WILL BE A TOTAL WASHOUT...BUT PERIODS OF PCPN WILL LIKELY
IMPACT MUCH OF AREA.  AND WHILE THUNDERSTORMS WILL...SIMILARLY
LIKELY NOT BE WIDESPREAD...THERE WILL BE A PERSISTENT CHANCE FOR
ELEVATED CONVECTION IN THE PERSISTENT ISENTROPIC LIFT.

WHILE MEMORIAL DAY WILL LIKELY NOT BE DRY...AT LEAST TEMPERATURES
WILL BE ON THE INCREASE.  BY MONDAY...THE UPPER LOW WILL FINALLY
LIFT NEWD INTO NEW BRUNSWICK AND THE UPPER RIDGE WILL BEGIN TO
PROGRESS EWD.  THE MODELS ARE BEGINNING TO LOOSE FOCUS AND
CONTINUITY IN HANDLING THE EVOLUTION OF THE UPPER RIDGE AS THERE IS
LITTLE CONSENSUS ON THE TIMING AND STRENGTH OF ADDITIONAL SHORTWAVES
OVERTOPPING THE RIDGE...WHICH WILL IN TURN...LEAD TO SOME
UNCERTAINTY TO THE PCPN FORECAST.  THE ONE GENERAL CERTAINTY IS THAT
THERE WILL BE A SERIES OF SHORTWAVES TRACKING ACROSS THE
REGION...HELPING TO FOCUS THE PCPN INVOF THE WARM FRONT.

BY MONDAY NIGHT...THE WARM FRONT SHOULD LIFT WELL NORTH OF THE
AREA...INITIATING THE RAPID WARMING TREND AS PERSISTENT SWLY FLOW
DEVELOPS AT THE SFC WHILE THE UPPER RIDGE BUILDS ACROSS THE
MIDWEST.  HOWEVER...WITH A SFC RIDGE EXTENDING THROUGH THE SERN
CONUS...GULF MOISTURE WILL BE LIMITED THROUGH TUESDAY.  BUT BY
WEDNESDAY...THE RIDGE WILL RETREAT TO THE EAST...OPENING UP THE GULF
AND ALLOW MORE MOISTURE TO STREAM NWD.  SO...BY MID WEEK...EXPECT
THAT DEWPOINTS SHOULD REACH INTO THE MIDDLE TO UPPER 60S TO
ACCOMPANY THE HIGHER TEMPERATURES WHICH SHOULD REACH INTO THE MIDDLE
80S.  THE GFS/ECMWF ARE STILL BRINGING 850MB TEMPS AROUND 18C ACROSS
THE MIDWEST...WHICH WOULD IMPLY THAT TEMPS IN THE UPPER 80S ARE NOT
OUT OF THE QUESTION GIVEN ENOUGH SUNSHINE TO ALLOW FOR DEEP DEEP
MIXING.  AS USUAL...THE INCREASE IN GULF MOISTURE ACCOMPANYING THE
INCREASED HEAT WILL ALSO KEEP THE CHANCES FOR THUNDERSTORMS OVER THE
AREA.  SO...THE UNCERTAINTY IN THE CLOUD COVER WILL BE THE MAJOR
QUESTION MARK RIGHT NOW AS TO WHETHER THE REGION WILL SEE A REAL
TASTE OF SUMMER NEXT WEEK.

KREIN

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 18Z...

* LIGHT NORTHEAST TO EAST WINDS.

* AREAS OF SHRA EXPECTED TO STAY TO THE SOUTH AND WEST OF ORD/MDW
  THROUGH THIS EVENING.

PAW

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

A HIGH PRESSURE RIDGE REMAINS IN PLACE OVER THE AREA. THIS WILL
MAINTAIN A LOW LEVEL EASTERLY FLOW OF RELATIVELY DRY AIR THROUGH
THIS EVENING. AREAS OF SHRA TO THE WEST MAY GET TO THE VICINITY OF RFD
THIS AFTERNOON AND EVENING...BUT SHOULD STAY WELL TO THE WEST OF
THE REST OF THE TERMINALS. ON SUNDAY THE RIDGE WILL MOVE TO THE
EAST AND ALLOW AREAS OF SHRA TO SPREAD INTO RFD DURING THE MORNING
AND THE REST OF THE TERMINALS DURING THE AFTERNOON.

PAW

//ORD AND MDW CONFIDENCE...UPDATED 18Z...

* HIGH CONFIDENCE IN WIND TREND.

* MEDIUM CONFIDENCE THAT SHRA WILL REMAIN WEST AND SOUTH OF THE
  TERMINALS THROUGH THIS EVENING.

PAW

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

SUNDAY NIGHT...SHRA/TSRA LIKELY. MVFR POSSIBLE.

MONDAY THROUGH FRIDAY...MAINLY VFR...PERIODIC SHRA/TSRA ESPECIALLY
MONDAY AND AGAIN FRIDAY. BREEZY SOUTH WINDS WEDNESDAY AND
THURSDAY.

MDB

&&

.MARINE...
242 AM CDT

QUIET CONDITIONS PERSIST ACROSS LAKE MICHIGAN WITH AN AXIS OF HIGH
PRESSURE CENTERED JUST EAST OF THE LAKE. THE HIGH WILL BE SLOW TO
MOVE TO THE EAST WHICH WILL KEEP LIGHT AND SOMEWHAT VARIABLE WINDS
ACROSS THE LAKE TODAY...WITH AN ONSHORE COMPONENT DEVELOPING ALONG
THE SHORELINES. IT WILL LIKELY TAKE UNTIL MONDAY FOR A MORE
PERSISTENT SOUTHEAST FLOW TO SET UP AS LOW PRESSURE GRADUALLY
ORGANIZES ACROSS THE PLAINS AND THE HIGH SHIFTS TO THE EAST. WARMER
AIR WILL BE MOVING INTO THE REGION WHICH WILL LIMIT MIXING AND GUST
POTENTIAL BUT THE PRESSURE GRADIENT MAY BE STRONG ENOUGH AT TIMES TO
ALLOW FOR PERIODS OF 15-25 KT WINDS BY WEDNESDAY. WIND DIRECTION
WILL SHIFT FROM SOUTHEAST TO SOUTH THEN SOUTHWEST THROUGH THE WEEK.

MDB

&&

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

$$


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