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000
FXUS63 KLOT 171903
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
203 PM CDT FRI MAY 17 2013

.MESOSCALE DISCUSSION...
201 PM CDT

A CLUSTER OF STORMS IS TRYING TO TAKE SHAPE ACROSS THE SOUTHERN
PORTION OF THE CWA THIS AFTERNOON ALONG A STATIONARY SURFACE
BOUNDARY AND MOISTURE CONVERGENCE AXIS...BUT AT THIS TIME WE ARE NOT
EXPECTING ANYTHING SEVERE TO DEVELOP.  LOCALIZED FLOODING WOULD BE
MORE OF A CONCERN.  SFC-BASED CAPE VALUES HAVE REACHED SEVERAL
HUNDRED J/KG...BUT DEEP LAYER SHEAR IS VIRTUALLY NONEXISTENT...WITH
BOTH FORECAST AND LATEST ACARS SOUNDINGS FROM ORD SHOWING WINDS
THROUGH MOST OF THE TROPOSPHERE AT 20 KTS OR LESS.

STORMS THAT DO PULSE UP IN THIS LOW SHEAR AND MODERATE CAPE
ENVIRONMENT COULD POSE A HAIL OR WIND THREAT...BUT ACARS -20C
HEIGHTS AROUND 23 KFT PROVIDE AN IDEA OF HOW HIGH 60 DBZ ECHOES
WOULD HAVE TO GET TO SUPPORT SEVERE HAIL...AND STORMS SO FAR HAVE
ONLY BEEN REACHING HALF THAT LEVEL.  FAIRLY SATURATED LOW LEVELS IN
FORECAST SOUNDINGS ALSO SUGGEST THE THREAT OF STRONG DOWNBURST WINDS
WOULD NOT BE HIGH.  AGAIN...LOCALIZED FLOODING WOULD BE MORE OF A
CONCERN GIVEN THE COMBINATION OF SATURATED SOUNDINGS SOMEWHAT
TROPICAL IN NATURE...SLOW STORM MOTION...MINIMAL SHEAR...THE
PRESENCE OF A FOCUSING BOUNDARY...AND PRECIPITABLE WATER VALUES
AROUND 1.4 INCHES.  AT THIS POINT THE STORMS ARE REMAINING GENERALLY
SCATTERED AND DISCRETE...AND HAVE NOT BEEN TRAINING.  THESE WILL
CONTINUE TO BE MONITORED FOR ANY CHANGES AS WE PROGRESS THROUGH PEAK
MIDAFTERNOON HEATING.

LENNING

&&

.DISCUSSION...
428 AM CDT

EARLY THIS MORNING A STATIONARY FRONT RAN FROM NE IA...FAR NW AND
NE IL...AND NORTHERN IND. ND...NORTHEAST AND FAR NORTHWEST IL.
BOUNDARY POSITION IS WELL DEFINED BY BOTH WIND PATTERN AND LARGE
DIFFERENCE IN DEW POINTS ON OPPOSITE SIDES OF THE FRONT. AN UPPER
LOW WAS OVER THE EASTERN PORTION OF THE MO-AR BOARDER WHILE
A FEW DISTURBANCES...LIKELY CONVECTIVELY INDUCED...WERE JUST
UPSTREAM OF THE LOCAL FORECAST AREA.

THE FIRST DISTURBANCE WAS MOVING ACROSS FAR NW AND N CENTRAL IL
WITH A SECOND ALONG THE MN-IA BORDER AND A THIRD MOVING OVER THE
MID MO VALLEY FROM NEB E OVER FAR WESTERN IA. THESE DISTURBANCES
PRECEDED AN UPPER LOW MOVING E ALONG THE SD-NEB BOARDER.

THE FIRST DISTURBANCE DID PRODUCE A FEW SHOWERS AND THUNDERSTORMS
AS FAR E AS OVER THE FAR SW PORTION OF THE FA LAST EVENING AND
EARLIER THIS MORNING BUT HAD DISSIPATED AS THEY MOVED E INTO A
CAPPED AREA. THE SECOND DISTURBANCE. THE MAIN AREA OF CONVECTION
THAT HAD BEEN MOVING ACROSS IA HAD ALSO DISSIPATED AS IT MOVED
AWAY FROM THE BETTER ISENTROPIC LIFT RESULTING FROM MODERATELY
STRONG MID LEVEL WAA. THE THIRD DISTURBANCE HAS GENERATED AN
ELONGATED AREA OF THUNDERSTORMS ALONG THE MN-IA BORDER WHERE 30-35
KT SOUTHERLY FLOW IN THE 1.5 TO 3 KM AGL LAYER WAS MAXIMIZING LIFT
IN AN AREA OF RELATIVELY LARGE ELEVATED CAPE N OF THE STATIONARY
FRONT.

LATER TODAY THE CAP IS PROGGED TO BE REMOVED THOUGH FORECAST
TEMPERATURE PROFILES ARE NOT INDICATING AND SIGNIFICANT CAPE
AVAILABLE TO BE REALIZED. IN ADDITION...THE BEST ISENTROPIC LIFT
IS PROGGED TO MOVE ACROSS WI DURING THE DAY. STILL...THERE WILL BE
INCREASING CHANCES OF SHOWERS AND THUNDERSTORMS SPREADING W TO E
ACROSS THE LOCAL AREA DURING THE MORNING AS PERSISTING THROUGH THE
AFTERNOON AND EARLY EVENING AS THE SD-NEB UPPER LOW AND THE MO-AR
UPPER LOW MERGE AND MOVE TO OR SLIGHTLY E OF THE MID MS VALLEY BY
00Z. BEYOND THE FIRST PART OF THIS EVENING CHANCES OF PRECIP ARE
SMALL EXCEPT FOR THE FAR SE PORTION OF THE FA WHICH WILL STILL BE
UNDER THE INFLUENCE OF THE SLOW MOVING UPPER LOW WHICH IS PROGGED
TO BE OVER/NEAR THE LOWER OH VALLEY AT 12Z SAT.

THE UPPER PATTERN WILL BECOME MORE AMPLIFIED OVERNIGHT AND SAT
BEFORE BECOMING ZONAL E OF THE ROCKIES MID NEXT WEEK. WHILE THE
FLOW WILL BE AMPLIFIED THE FLOW WILL BE SLUGGISH AS AN UPPER
TROUGH MOVING OVER THE PACIFIC COAST EARLY THIS MORNING REACHES
THE ROCKIES LATE SAT-SAT NIGHT WHILE ANOTHER UPPER TROUGH OVER THE
N PACIFIC OFF THE B.C. COAST MOVES OVER PAC NW COAST. BY LATE SUN-
SUN NIGHT THE PAIR OF TROUGHS ARE PROGGED TO FORM AN UPPER LOW
OVER THE NORTHERN HIGH PLAINS. THIS UPPER LOW IS TO THEN VERY
SLOWLY MOVE E TO THE UPPER MIDWEST BY WED EVENING WHILE ALSO
GRADUALLY WEAKENING. HOWEVER...A FAIRLY STRONG SURFACE LOW IS
PROGGED TO MOVE E ACROSS THE NORTHERN PLAINS AND UPPER MS VALLEY
BEFORE THE UPPER SYSTEM WEAKENS AS DOES THE SURFACE REFLECTION.
DUE TO THE VERY SLOW EASTWARD PROGRESS OF THE SYSTEM A PROLONGED
PERIOD OF PERSISTENT RELATIVELY STRONG S TO SW LOWER LEVEL FLOW
WILL BE TRANSPORTING A CONTINUOUS STREAM OF MOISTURE FROM THE
WESTERN GULF UP ACROSS THE MID MS VALLEY AND TO THE UPPER MIDWEST
AND WESTERN GREAT LAKES.

WHILE THE MOST LIKELY TIME FRAME FOR RAIN IS LATER MON AND TUE AS
THIS IS WHEN SIGNIFICANT WARM AIR ADVECTION AND RESULTING UVV
ALONG WITH THE ARRIVAL OF ABUNDANT MOISTURE OCCURS...THERE WILL BE
CHANCES OF RAIN IN THE FORECAST FROM SUNDAY NIGHT RIGHT THROUGH
END OF NEXT WEEK...FIRST AS THE UPPER LOW APPROACHES THERE LIKELY
WILL BE SMALLER SCALE SHORT WAVES ROTATING AROUND TO AFFECT THE
LOCAL AREA EARLY IN THE WEEK BEFORE THE UPPER LOW IN OVER THE
AREA...AND THEN AS AN UPPER TROUGH TAKES UP SEMI PERMANENT
RESIDENCE OVER THE GREAT LAKES LATE WEEK.

STRONG WAA OCCURS AHEAD OF THE UPPER LOW DURING SAT AND SUN AND
WITH THE CORE OF THE WARM AIR OVER THE REGION ON SUN. WITH PARTLY
TO MOSTLY SUNNY SKIES TEMPERATURES ARE EXPECTED TO BE INTO THE
MID AND UPPER 80S. INCREASED CLOUDINESS WILL LIMIT HIGH
TEMPERATURES MON TO THE LOWER AND MID 80S. MIDWEEK AS THE UPPER
LOW/TROUGH MOVE OVER AND PAST THE AREA COOLER AIR WILL SPREAD S
AND SE OVER THE AREA. CONSIDERABLE CLOUDINESS AND SHOWERS WILL
KEEP AFTERNOON TEMPERATURES LIMITED TO LOWER AND MID 70S BY WED
AND UPPER 60S AND LOWER 70S ON THU.

TRS

&&

.AVIATION...

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

* ISOLD-SCT SHRA/TSRA DEVELOPING SOUTH OF THE TERMINALS NEXT 1-2
  HRS...POSSIBLE AT OR NEAR TERMINALS MID AFTERNOON...ESPECIALLY
  MDW.

* NORTHEAST WINDS 10-12KT WITH SOME GUSTS NEAR 20 KT AT
  TIMES...TRENDING MORE EASTERLY LATE AFTERNOON.

* MVFR CIGS/VSBY POSSIBLE LATE TONIGHT/EARLY SATURDAY MORNING.

MDB

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

COMPLEX FORECAST CONTINUES WITH LAKE MICHIGAN HELPING TO PUSH A
FRONTAL BOUNDARY SOUTHWESTWARD ACROSS THE CHICAGO/NW INDIANA METRO
AREA. THIS HAS RESULTED IN A SHIFT TO STEADY NORTHEAST WINDS AT
ORD/MDW/DPA AND GYY SHORTLY WITH SOME GUSTS TO NEAR 20 KT. WINDS
SHOULD STAY NORTHEASTERLY FOR THE NEXT FEW HOURS AND TREND MORE
EASTERLY THROUGH THE AFTERNOON BUT THIS MAY BE SLOWER TO OCCUR
THAN FORECAST. SHRA/TSRA HAS BEGUN TO DEVELOP SOUTH OF THE
TERMINALS AND WILL BE MOVING NORTHWARD THROUGH THE AFTERNOON. ONE
QUESTION IS HOW FAR NORTH IT WILL BE ABLE TO MAKE IT NOW THAT THE
FRONTAL BOUNDARY HAS PUSHED THROUGH THE TERMINALS. AM THINKING
THAT THE BOUNDARY ITSELF MAY ACT TO TRIGGER MORE STORMS BUT THAT
IT WILL NEED TO MOVE FURTHER SOUTH TOWARD/PAST JOT/KC09 WHERE THE
NORTHERN EDGE OF DECENT INSTABILITY EXISTS IN ORDER TO DO SO. WILL
CONTINUE WITH THE IDEA OF THE HIGHEST POTENTIAL AT GYY AND MDW AND
INCLUDE TEMPO TS THERE BY MID AFTERNOON. ORD/DPA/RFD HAVE A CHANCE
AS WELL BUT IT IS MUCH LESS CLEAR WHETHER OR NOT THEY WILL BE
DIRECTLY AFFECTED. WHERE SHRA/TSRA DO PASS OVER THEN BRIEF IFR
VSBY IS POSSIBLE IN HEAVY DOWNPOURS.

SHRA/TSRA ACTIVITY WILL WANE THIS EVENING AND WINDS WILL LIKELY
REMAIN EAST OR EAST-NORTHEAST FOR THE MOST PART BUT MAY TURN
SOUTHEAST OVERNIGHT. BETTER LOW LEVEL MOISTURE WILL BE IN
PLACE..ESPECIALLY IF RAIN CAN FALL...AND MVFR VSBY IS A DECENT
POSSIBILITY TOWARD DAYBREAK. SCT-BKN MVFR CLOUDS MAY ALSO DEVELOP
DURING THIS TIME. EXPECT THAT ANY MVFR VSBY/CIGS WOULD IMPROVE
AFTER DAYBREAK. WINDS WILL CONTINUE TO BE TRICKY WITH A EASTERLY
DIRECTION CONTINUING WITH VARIABILITY LIKELY.

MDB

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

* HIGH CONFIDENCE IN SHRA/TSRA DEVELOPING SOUTH OF THE TERMINALS
  NEXT 1-2 HRS...LOW-MEDIUM CONFIDENCE IN ACTIVITY NEAR MDW THIS
  AFTERNOON...LOW CONFIDENCE IN POTENTIAL NEAR ORD.

* MEDIUM CONFIDENCE IN WIND TRENDS THROUGH THE PERIOD.

* MEDIUM CONFIDENCE IN MVFR VSBY LATE TONIGHT...LOW CONFIDENCE IN
  MVFR CIG POTENTIAL.

MDB

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

SATURDAY NIGHT...VFR.
SUNDAY THROUGH WEDNESDAY...VFR. PERIODIC CHANCES FOR SHRA/TSRA.
THURSDAY...VFR. PSBL MVFR WITH CHANCE FOR SHRA.

BEACHLER

&&

.MARINE...
341 AM CDT

RIDGE OF HIGH PRESSURE HAS BECOME CENTERED NEAR JAMES BAY EARLY THIS
MORNING...EXPECTED TO DRIFT SOUTHEAST AND BECOME CENTERED NEAR
WESTERN QUEBEC TONIGHT. SOUTH OF THIS HIGH PRESSURE WAS A STALLED
FRONTAL BOUNDARY THAT STRETCHING ACROSS THE CENTRAL PLAINS INTO THE
OHIO VALLEY. ACROSS THE CENTRAL PLAINS AN AREA OF LOW PRESSURE IS
EXPECTED TO DEVELOP...THIS LOW PRESSURE WILL DRIFT NORTH AND BEGIN
TO LIFT THE STALLED FRONTAL BOUNDARY TO THE NORTH. THE GRADIENT
REMAINS RELATIVELY WEAK ACROSS THE LAKE...AND THIS APPEARS TO REMAIN
THROUGH THIS EVENING AS THE BOUNDARY BEGINS TO APPROACH THE SOUTHERN
PORTION OF THE LAKE. WINDS WILL GENERALLY BE NORTH/NORTHEAST OVER
THE LAKE...FURTHER SOUTH WINDS MAY START EASTERLY THEN BECOME
NORTHEASTERLY. THEN AS THE BOUNDARY SLIDES NORTH ACROSS THE SOUTHERN
PORTION OF THE LAKE...WINDS WILL BECOME EASTERLY. THE GRADIENT DOES
INCREASE MARGINALLY...AND MAY PRODUCE SPEEDS BETWEEN 10 TO 20 KT
SAT. LOW PRESSURE ACROSS THE CENTRAL PLAINS WILL DRIFT NORTH OVER
THE WEEKEND...AND THIS WILL ALLOW THE BOUNDARY TO STEADILY LIFT
NORTH ACROSS THE LAKE. AS THIS OCCURS...WINDS TO THE SOUTH OF THE
BOUNDARY WILL TURN SOUTHEASTERLY. MEANWHILE THE FAR NORTHERN
PORTIONS OF THE LAKE WILL BE NORTHEAST/EAST. EVENTUALLY THE NORTHERN
PORTIONS OF THE LAKE WILL BECOME SOUTHEAST WINDS...BUT THIS DOES NOT
OCCUR UNTIL CLOSER TO SUNDAY MORNING. THE SLOW MOVING LOW PRESSURE
SYSTEM WILL EVENTUALLY REACH THE WESTERN GREAT LAKES...HOWEVER THIS
DOES NOT APPEAR TO OCCUR UNTIL CLOSER TO WED OF NEXT WEEK. AS THE
LOW FINALLY LIFTS NORTHEAST OF LAKE MICHIGAN MID-WEEK...THE FRONTAL
BOUNDARY WILL PUSH SOUTH ACROSS THE LAKE. THIS WILL ALLOW WINDS TO
BECOME NORTHWEST TO NORTH.

BEACHLER

&&

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

$$

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