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AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1113 AM CDT SAT MAY 18 2013

.DISCUSSION...
430 AM CDT

FOCUS OF FORECAST IS ON THE INCREASE IN CHANCES OF SHOWERS AND
THUNDERSTORMS BEGINNING LATE SUNDAY/SUNDAY NIGHT AND CONTINUING TO
GRADUALLY INCREASE THROUGH MONDAY...PERSISTING THROUGH THURSDAY.
TIME FRAME OF GREATEST THREAT OF PRECIP WILL BE MONDAY NIGHT
THROUGH TUESDAY NIGHT.

OTHER MAIN FORECAST ITEM IS THE CONTINUED WARM UP TODAY AND SUNDAY
WITH WARMTH PERSISTING MONDAY BEFORE HIGH TEMPERATURES START
FALLING OFF TUESDAY...WITH HIGHS ONLY IN THE MID 60S TO LOWER 70S
ON THURSDAY AND FRIDAY.

OTHER CONCERNS ARE POTENTIAL FOR SIGNIFICANT AMOUNTS OF RAINFALL
FROM MONDAY NIGHT INTO WEDNESDAY AND SOME POTENTIAL FOR SEVERE
CONVECTION...IN THE WESTERN PORTION OF THE FORECAST AREA LATE
SUNDAY AND SUNDAY NIGHT...AND ACROSS THE AREA MONDAY AND MONDAY
NIGHT.

CHANCES OF ANY PRECIP IN THE AREA ARE LOW FOR TODAY AND TONIGHT
AS AN UPPER RIDGE BUILDING OVER THE MID CONTINENT SHIFTS EASTWARD
TO OVER THE LOCAL AREA LATE TODAY AND TONIGHT RESULTING IN RISING
HEIGHTS AND LARGE SCALE SUBSIDENCE. THE CONVECTION THAT HAS BEEN
OCCURRING DURING THE NIGHT AND EARLY THIS MORNING OVER THE
NORTHERN PLAINS AND UPPER MS VALLEY WILL CONTINUE TO MOVE TO THE
EAST-NORTHEAST OVER THE TOP OF THE BUILDING RIDGE. BY SUNDAY NIGHT
THE UPPER RIDGE WILL HAVE REACHED THE CENTRAL GREAT LAKES AND THE OH
VALLEY REGIONS AS AN UPPER LOW SLOWLY MOVES EAST FROM THE CANADIAN
MARITIMES.

A VERY WARM AIR MASS WAS ACROSS THE HIGH PLAINS FRIDAY UNDER THE
UPPER RIDGE. THIS AIR MASS WILL SPREAD TO THE LOCAL AREA FOR
SUNDAY. DEPENDING ON CLOUD COVER...OF WHICH MUCH WOULD LIKELY BY
CONVECTIVE DEBRIS SPREADING EAST ACROSS THE MS RIVER FROM UPSTREAM
CONVECTION...TEMPERATURES MAY PEAK IN THE UPPER 80S IF THE CIRRUS
DEBRIS CLOUDS ARE NOT TOO THICK. EVEN WITH MORE CLOUDINESS MID 80S
LOOKS PRETTY LIKELY FOR SUNDAY AFTERNOON.

MEANWHILE...MULTIPLE STRONG UPPER WAVES DROPPING INTO AN UPPER
TROUGH EXTENDING FROM WEST OF THE B.C. COAST SOUTHEAST ACROSS THE
PACIFIC NORTHWEST TO THE DESERT SOUTHWEST ARE PROGGED TO CONGEAL
AND FORM AN UPPER LOW TO REACH THE CENTRAL HIGH PLAINS BY SUNDAY
EVENING. INCREASING SOUTH TO SOUTHWEST LOW AND MID LEVEL FLOW
AHEAD OF A SURFACE LOW ASSOCIATED WITH THE UPPER LOW WILL
TRANSPORT SIGNIFICANT MOISTURE FROM THE WESTERN GULF UP ACROSS THE
SOUTHERN PLAINS...MID MO VALLEY AND UPPER MS VALLEY DURING SUNDAY
WITH THE MOISTURE AXIS SHIFTING EAST OVER THE MID MS VALLEY AND
THE UPPER MIDWEST TO OVER IL AND THE WESTERN GREAT LAKES BY MONDAY.

WHILE THE UPPER LOW WILL STILL BE OUT OVER THE CENTRAL PLAINS
LATE SUNDAY AND NOT REACH THE MID MO VALLEY TIL LATER MONDAY
POTENTIAL INSTABILITY WILL BE INCREASING OVER THE LOCAL AREA AND
ANY SMALLER SCALE DISTURBANCES TRAVELING NORTHEASTWARD ACROSS
THE MID MS VALLEY MAY TRIGGER THUNDERSTORMS. GIVEN AMOUNT OF
AVAILABLE CAPE AND LOW LEVEL WIND SHEAR THERE IS THE POTENTIAL
FOR SOME OF THE THUNDERSTORMS THAT MAY DEVELOP TO BE STRONG TO
SEVERE.

WITH THE CORE OF THE WARM AIR DOME SHIFTING TO THE EAST MONDAY
AND MORE CLOUDS AND POSSIBLY MOIST CONVECTION OVER/NEAR THE AREA
TEMPERATURES WILL LIKELY BE HELD DOWN A FEW TO A COUPLE DEGREES
FROM THOSE REACHED SUNDAY.

THE SLOW PROGRESS OF THE UPPER LOW DUE TO BEING IN AN OMEGA BLOCK
TYPE UPPER PATTERN RESULTS IN IT NOT REACHING NEAR/OVER THE LOCAL
AREA UNTIL LATER WEDNESDAY AND WEDNESDAY NIGHT. THIS SLOW MOVEMENT
KEEPS THE THREAT OF THUNDERSTORMS GOING...WITH THE HIGHEST POPS
DURING MONDAY NIGHT THROUGH TUESDAY NIGHT AS THIS IS WHEN THE
HIGHEST THETA E AIR IS IN PLACE WITH SIGNIFICANT CAPE VALUES.

WHILE PRECIPITABLE WATER VALUES ARE NOT EXTREME...GENERALLY AROUND
1.5 INCHES FROM SUNDAY NIGHT INTO WEDNESDAY...THE POTENTIAL FOR
MULTIPLE RAINFALL EVENTS DURING THE PERIOD OF TIME IT TAKES THE
UPPER LOW TO REACH AND EVENTUALLY PASS OVER THE LOCAL AREA DOES
PRESENT THE POSSIBILITY OF SOME 2 TO 3 INCH RAINFALL TOTALS.
SPECIFICS ON WHICH PART OR PARTS OF THE FA WOULD BE MOST PRONE TO
RECEIVING SIGNIFICANT RAINFALL AMOUNTS IS NOT CLEAR AT THIS TIME
SO ALL THAT CAN BE SAID THAT THERE IS THE POTENTIAL.

CHANCES FOR SHOWERS AND SOME THUNDERSTORMS PERSIST WEDNESDAY
THROUGH THURSDAY AS THE UPPER LOW IS TO STILL BE SLUGGISHLY
MOVING EAST TO THE WESTERN GREAT LAKES AND MIDWEST. COOLER AIR
WILL BE DRAWN DOWN ACROSS THE REGION AS THE UPPER FLOW VEERS TO
OUT OF THE NORTHWEST THURSDAY. THIS RESULTS IN HIGHS OF ONLY MID
60S TO LOWER 70S FOR THURSDAY AND FRIDAY. BY THURSDAY NIGHT THE
THREAT OF ANY ADDITIONAL SHOWERS SHOULD HAVE ENDED WITH THE UPPER
LOW OPENING UP TO THE EAST OF THE AREA AND THE LOWER LEVEL FLOW
BECOMING SOMEWHAT ANTICYCLONIC AS A RIDGE OF HIGH PRESSURE
EXTENDING SOUTHWEST FROM ONTARIO DRIFTS EAST ACROSS THE UPPER
MIDWEST AND MID MO VALLEY.

TRS

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 16Z...

* SOUTHEAST WINDS /VARIABLE 130-170/ BECOMING EAST /VARIABLE
  080-120/ NEXT 1-3 HOURS AS LAKE INFLUENCE TAKES HOLD.

* MVFR VSBY POSSIBLE TONIGHT THROUGH DAYBREAK SUNDAY.

MDB

//DISCUSSION...UPDATED 16Z...

LAKE BREEZE/INFLUENCE STARTING TO APPEAR IN MESO OBS AND FAINTLY
IN RADAR DATA WITHIN A FEW MILES OF THE ILLINOIS SHORELINE...AND
SUGGESTED TO BE NEAR GYY WITH A CHANGE TO VARIABLE WINDS THIS PAST
HOUR. EXPECT THE CURRENT VARIABLE SOUTHEAST WINDS TO TURN MORE
EASTERLY WITH CONTINUED VARIABILITY OVER THE NEXT FEW HOURS AT
ORD/MDW/GYY. FEW-SCT CU HAS ALSO DEVELOPED AROUND 030 AND WILL
STEADILY LIFT THROUGH THE MID AFTERNOON. THE INFLUX OF LAKE AIR
MAY BE ABLE TO SCOUR THE CU NEAR ORD/MDW/GYY IF IT IS DEEP ENOUGH.

MDB

FROM 12Z...

IR IMAGERY EARLY THIS MORNING WAS INDICATING SOME LOW CLOUDS
SLIDING NORTHWEST ACROSS GYY...TOWARDS ORD/MDW. THESE CLOUDS HAVE
SINCE DISSIPATED. PATCHY FOG ALSO DEVELOPED...BUT REMAINED
SOUTHWEST OF THE TAF SITES. REDUCED VSBYS AT VARIOUS AIRFIELDS WAS
ARND 1-2SM. IT APPEARS THIS SHUD LIFT BY 14Z...HOWEVER WITH HIGH
CLOUDS BEGINNING TO APPROACH FROM A DECAYING COMPLEX OF
THUNDERSTORMS IN MINNESOTA OVERNIGHT...MAY SLOW THE RATE OF
IMPROVING VSBYS. WINDS HAVE REMAINED LIGHT EARLY THIS
MORNING...THRU DAYBREAK. GENERALLY VRB WITH SPEEDS LESS THAN 4 KT.
AS THE MORNING PROGRESSES...MIXING WILL INCREASE AND WINDS WILL
BRIEFLY TURN SOUTHEASTERLY. SPEEDS WILL GENERALLY BE 10KT OR LESS
THRU LATE MORNING. THEN IT APPEARS A LAKE BREEZE WILL DEVELOP AND
ALLOW WINDS TO TURN EASTERLY OR BETWEEN A 080-100 DEG RANGE. AT
THIS TIME IT APPEARS OCCASIONAL GUSTS MAY ACCOMPANY THE WIND
SHIFT. HOWEVER IT APPEARS THIS MAY ONLY AFFECT ORD/MDW. FOR GYY
WINDS WILL TURN MORE NORTH/NORTHEASTERLY OR A 010-030 DEG RANGE.

VFR CONDS ARE EXPECTED TO PREVAIL THRU THE DAY TODAY...WITH SCT CLOUDS
NOTED IN A 3-4KFT AGL DECK WITH A CIRRUS SHIELD OVERHEAD AT OR
AROUND 15KFT AGL. THEN AFT SUNSET WINDS DIMINISH AND THEN RELAX
BACK TO THE SOUTHEAST. GUIDANCE HAS BACKED OFF ON PRECIP CHCS
TODAY...HOWEVER SOME SPOTTY SHOWERS MAY STILL DEVELOP. THE
COVERAGE DOES NOT LOOK TOO GREAT...AND HAVE OPTED TO MAINTAIN A
DRY TAF. OVERNIGHT IT APPEARS CONDITIONS BECOME FAVORABLE FOR
PATCHY FOG TO DEVELOP. AT THIS TIME FEEL THE TAF SITES MAY ONLY BE
REDUCED TO ARND 3-4SM...HOWEVER ADDITIONAL MOISTURE WILL POOL INTO
THE NORTHERN IL/NORTHWEST IN AIRFIELDS. THIS MAY AID IN FURTHER
REDUCING VSBYS TO ARND 1SM IN LOCATIONS.

BEACHLER

//ORD AND MDW CONFIDENCE...UPDATED 16Z...

* MEDIUM-HIGH CONFIDENCE IN WIND DIRECTION TRENDS AND TIMING
  THROUGH THE AFTERNOON.

* MEDIUM CONFIDENCE IN MVFR VSBY OVERNIGHT/SUNDAY MORNING.

MDB

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

SUNDAY NIGHT...VFR TRENDING TO PATCHY MVFR. PERIODS OF TSRA.
MONDAY THROUGH WEDNESDAY...PERIODS OF TSRA
THURSDAY...CHC SHRA.
FRIDAY...VFR. NO SIG WX.

BEACHLER

&&

.MARINE...
221 AM CDT

MINIMAL CHANGES FROM PREVIOUS FORECAST. BROAD SFC RIDGE HAS BECOME
CENTERED ACROSS CENTRAL QUEBEC....HOWEVER THIS FEATURE CONTINUES
TO INFLUENCE THE WESTERN GREAT LAKES WITH A LIGHT EAST FLOW TODAY
ACROSS THE LAKE. AS THE SFC RIDGE WILL DRIFT FURTHER AWAY FROM
LAKE MICHIGAN...WINDS WILL GRADUALLY TURN SOUTHEASTERLY. A STALLED
FRONTAL BOUNDARY STRETCHES FROM CENTRAL MINNESOTA EXTENDING
SOUTHEAST THROUGH WESTERN ILLINOIS INTO CENTRAL INDIANA. IT
APPEARS THE BROAD SFC RIDGE WILL KEEP THIS BOUNDARY JUST SOUTH OF
LAKE MICHIGAN...AND MOST LIKELY WEAKEN. A TROUGH OF LOW PRESSURE
OVER THE NORTHERN PLAINS WILL WEAKEN...AS A SECOND AREA OF LOW
PRESSURE DEVELOPS ALONG A SLOW MOVING COLD FRONT IN THE CENTRAL
PLAINS SUNDAY EVENING. PRIOR TO THE FRONTAL BOUNDARIES
ARRIVAL...WINDS SPEEDS WILL GENERALLY BE 10 TO 20 KT. HOWEVER...AN
OCCASIONAL GUSTS TO 25 KT MAY OCCUR THROUGHOUT THE DAY AS A WEAK
GRADIENT IS FORECAST TO DEVELOP OVER THE CENTRAL PORTION OF THE
LAKE.

AS THE CENTRAL PLAINS LOW PRESSURE DEVELOPS...IT WILL LIFT A WARM
FRONT NORTH ACROSS THE CENTRAL PORTIONS OF THE LAKE SUNDAY
NIGHT/EARLY MON. WINDS WILL TURN SOUTHERLY AND INCREASE BEHIND
THIS BOUNDARY. THIS LOW PRESSURE SYSTEM WILL SLOWLY PUSH EAST INTO
THE MID-MISSISSIPPI VALLEY TUE/WED...THEN DRIFT EAST INTO THE OHIO
VALLEY. WITH THE SLOW MOVING FRONTAL BOUNDARY EXPECTED TO BE OVER
THE CENTRAL PORTIONS OF THE LAKE...WINDS NORTH WILL BE NORTHERLY
AND SOUTHERLY TO THE SOUTH. FORECAST GUIDANCE INDICATES THAT A
TIGHTENING GRADIENT ACROSS THE NORTHERN PORTIONS OF THE LAKE IS
LIKELY...WHICH COULD RESULT IN WINDS BETWEEN 15-25 KT MON/TUE. THEN
THE BOUNDARY WILL EVENTUALLY PUSH BACK TO THE SOUTH OF THE LAKE
TOWARDS THE MIDDLE OF THE UPCOMING WEEK.

BEACHLER

&&

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

$$


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