000
FXUS63 KLOT 181852
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
152 PM 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 18Z...

* VARIABLE SOUTHEAST WINDS TURNING EAST OR EAST-NORTHEAST TOWARD
  20Z WITH LAKE BREEZE PASSAGE.

* MVFR VSBY POSSIBLE LATE TONIGHT THROUGH DAYBREAK SUNDAY.

MDB

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

LAKE BREEZE BOUNDARY HAS BECOME BETTER DEFINED VIA MESO OBS AND
VISIBLE SATELLITE IMAGERY EXTENDING FROM NEAR PWK TO GYY AND IS
MAKING A SLOW INLAND PUSH. HAVE DECENT CONFIDENCE IN WINDS TURNING
EAST OR NORTHEAST AT ORD/MDW TOWARD MID AFTERNOON BUT AM NOT SURE
HOW FAR PAST THE TERMINALS THE BOUNDARY WILL PUSH. TIMING MAY NEED
TO BE REFINED. SCT-BKN LOWER END VFR CU WILL CONTINUE TO LIFT
THROUGH THE AFTERNOON AND AREAS EAST OF THE LAKE BREEZE WILL
LIKELY SCOUR OUT BASED ON LATEST TRENDS. WINDS WILL DIMINISH THIS
EVENING AND BECOME LIGHT AND VARIABLE OVERNIGHT. THERE IS A BIT
MORE MOISTURE IN THE AIR SO VSBY REDUCTION LOOKS MORE LIKELY THAN
THIS MORNING SO WILL CONTINUE WITH MVFR VSBY TOWARD DAYBREAK. SOME
IFR IS POSSIBLE AT RFD/DPA BUT WILL LEAVE MVFR FOR NOW. WINDS WILL
INCREASE FROM THE SOUTHEAST THROUGH SUNDAY MORNING ALLOWING VSBY
TO IMPROVE. SOME VFR CU MAY ONCE AGAIN DEVELOP. SHRA/TSRA CHANCES
WILL INCREASE FROM THE WEST LATE IN THE DAY AND EVENING.

MDB

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

* MEDIUM-HIGH CONFIDENCE IN WINDS SHIFTING TO EAST OR EAST-
  NORTHEAST AND TIMING.

* MEDIUM CONFIDENCE IN MVFR VSBY POTENTIAL EARLY SUNDAY.

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...
151 PM CDT

A SURFACE RIDGE OF HIGH PRESSURE CONTINUES TO BUILD
EASTWARD ACROSS EASTERN QUEBEC AND THE NORTHEASTERN CONUS THIS
AFTERNOON. HOWEVER...ATTENTION IS QUICKLY TURNING OUT WEST ACROSS
THE CENTRAL HIGH PLAINS...WHERE AN AREA OF LOW PRESSURE IS TAKING
SHAPE THIS AFTERNOON. THIS STORM SYSTEM IS EXPECTED TO MOVE
EAST-NORTHEASTWARD TOWARDS THE MID MISSOURI VALLEY BY SUNDAY
EVENING. AN ASSOCIATED SURFACE WARM FRONT...EXTENDING EASTWARD
ACROSS THE MID MISSISSIPPI VALLEY...WILL MOVE NORTHWARD ACROSS AT
LEAST THE SOUTHERN HALF TO TWO THIRDS OF THE LAKE SUNDAY NIGHT AND
MONDAY. BEHIND THIS FRONT WINDS LOOKS TO INCREASE OUT
OF THE SOUTH IN THE 15 TO 25 KT RANGE SUNDAY NIGHT INTO MONDAY.
HOWEVER...TO THE NORTH OF THIS FRONT...WINDS WILL PRIMARILY BE
MORE EASTERLY. THERE IS SOME UNCERTAINTY IN HOW FAR NORTH UP THE LAKE
THIS WARM FRONT WILL REACH. IT IS POSSIBLE THAT IT NEVER REACHES THE
NORTHERN THIRD OF THE LAKE...WHICH WOULD RESULT IN EASTERLY WINDS
REMAINING IN PLACE THERE DURING THE PERIOD.

THE OTHER CONCERN WILL BE THE THREAT FOR PERIODS OF DENSE MARINE FOG
SUNDAY NIGHT AND INTO AT LEAST THE FIRST PART OF THE COMING WEEK. AS THIS
WARM FRONT LIFTS OVER THE LAKE...MUCH HIGHER DEW POINTS WILL MOVE
OVER THE RELATIVELY COLD WATER...POSSIBLY RESULTING IN SOME FOG
DEVELOPMENT.

IT APPEARS THAT THE SURFACE LOW WILL SLOWLY MOVE EASTWARD ACROSS THE
MID MISSISSIPPI VALLEY THROUGH MID WEEK AND THEN ACROSS THE SOUTHERN
LAKES AND THE OHIO VALLEY LATER IN THE WEEK. THIS SHOULD ALLOW WARM
FRONT TO SHIFT BACK SOUTHWARD WITH TIME DURING THE WEEK. DURING THIS
TIME...THE WIND DIRECTIONS WILL PRIMARILY BECOME EAST NORTHEASTWARD
15 TO 25 KT NORTH OF THE FRONT. LATER IN THE WEEK...AS THE SURFACE
LOW PASSES EAST OF LAKE MICHIGAN...THE WINDS LOOK TO BE RATHER STOUT
UP TO 25 TO POSSIBLY AS HIGH AS 30 KT OUT OF THE NORTH FOR A PERIOD
AS A GOOD AREA OF HIGH PRESSURE BUILDS EASTWARD ACROSS ONTARIO AND
THE UPPER GREAT LAKES REGION.

KJB

&&

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

$$

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