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000
FXUS63 KLOT 170841
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
341 AM CDT FRI MAY 17 2013

.DISCUSSION...
339 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.

TRS

&&

.AVIATION...

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

* ISOLATED SHOWER REMAIN SOUTHWEST OF TERMINALS THRU MID-MORNING.
* PRECIP COVERAGE INCREASES BY MIDDAY...POSSIBLE THUNDERSTORM ARND
  19-22Z.
* WINDS REMAIN EASTERLY WITH SPEEDS GENERALLY ARND 10KT OR
  LESS...TURNING SOUTHEASTERLY AFT 05Z SAT.
* CIGS REMAIN VFR...POSSIBLE MVFR CIGS IN OR AROUND TSRA/SHRA.

BEACHLER

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

IR IMAGERY EARLY THIS MORNING SHOWS AN AREA OF CLOUDS ACROSS THE
WESTERN GREAT LAKES INTO THE UPPER MIDWEST. SCT SHOWERS/THUNDERSTORMS
ACROSS NORTHCENTRAL IOWA/SOUTHCENTRAL MINNESOTA. THIS PRECIPITATION WAS LOCATED
ALONG A STALLED FRONTAL BOUNDARY THAT STRETCHES EAST/SOUTHEAST
INTO CENTRAL IL/IN. AIRFIELDS ACROSS NORTHERN IL/NORTHWEST IN
REMAIN VFR...WITH SOME CLOUDS OBSERVED AT 8KFT AGL OR HIGHER.
GUIDANCE CONTINUE TO SUGGEST VFR CONDS WILL CONTINUE THROUGH
DAYBREAK...WITH A CONTINUED LIGHT EAST WIND ARND 8KT OR LESS. THE
DIRECTION WILL GENERALLY REMAIN BETWEEN A 060-090 DEG
RANGE...HOWEVER SOME VRB DIRECTIONS MAY OCCUR THRU DAYBREAK AS
SPEEDS WILL BE LESS THAN 5 KT.

THEN IT APPEARS THE BEST CHC FOR PRECIP WILL OCCUR SOUTHWEST OF
THE TAF SITES THRU MUCH OF THE MORNING...WITH MINIMAL COVERAGE. BY
MIDDAY COVERAGE SHUD BEGIN TO INCREASE IN
SHOWERS/THUNDERSTORMS...AND GUIDANCE IS BEGINNING TO SHOW A HIGHER
CONFIDENCE IN TIMING OF 18-22Z. HAVE OPTED TO PLACE A VCSH TO
HANDLE THE BEGINNING COVERAGE AT 16Z...WITH A TEMPO IN A 19-22Z
TIMEFRAME. THIS APPEARS TO BE THE BEST TIME THAT A THUNDERSTORM
WOULD OCCUR. THEN IT APPEARS ANY PRECIP WILL PUSH BACK TO THE
WEST/SOUTHWEST OF THE AIRFIELDS...WITH A SMALL CHC OF PRECIP
CONTINUEING THROUGH THE EVENING HOURS. WINDS WILL EVENTUALLY BEGIN
TO TURN SOUTHEASTERLY ONCE THE BOUNDARY LIFTS OVERHEAD AND TO THE
NORTH OF THE TAF SITES...HOWEVER THIS DOES NOT APPEAR TO OCCUR
UNTIL AFT 05Z SAT.

BEACHLER

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

* HIGH CONFIDENCE IN WIND DIR/SPEED.
* MEDIUM CONFIDENCE IN CIGS FORECAST.
* LOW CONFIDENCE IN PRECIP TIMING/COVERAGE.
* MEDIUM CONFIDENCE IN THUNDERSTORMS.
* MEDIUM/HIGH CONFIDENCE IN WIND DIRECTION BECOMING SOUTHEAST
  TIMING.

BEACHLER

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

SATURDAY THROUGH WEDNESDAY...PERIODIC CHANCES FOR SHRA/TSRA...
OTHERWISE...VFR.

KREIN

&&

.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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