000
FXUS63 KLOT 230317
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1017 PM CDT WED MAY 22 2013

.DISCUSSION...
307 PM CDT

SYNOPSIS...TODAY AND TOMORROW MORNING LOOK LIKE THE LAST OF THE
ACTIVE WEATHER THROUGH AT LEAST LATE THIS WEEK AS A BLOCKING PATTERN
SETS UP ALOFT. BELOW NORMAL TEMPS ARE EXPECTED TOMORROW AS THE UPPER
LEVEL TROUGH PASSES OVERHEAD...BUT TEMPS RETURN TO NEAR NORMAL FOR
MEMORIAL DAY.

TODAY THROUGH THURSDAY...
THE SURFACE LOW STRETCHES FROM NE IOWA THROUGH MICHIGAN THIS
AFTERNOON WHILE ITS COLD FRONT LIES IN WESTERN MO AND EASTERN KS.
THE LOW AND THE UPPER LEVEL TROUGH WILL CONTINUE TO WEAKEN WITH THE
LOW PASSING THROUGH NORTHERN IL TONIGHT.  FOG IS EXPECTED ACROSS THE
LAKE AND MAY MOVE ONSHORE TONIGHT AS WINDS TURN NORTHEAST.  NOT
EXPECTING THE FOG TO MOVE TOO FAR INLAND...MAYBE AN MILE OR TWO.

FOR PRECIP...THE MAJORITY OF CONVECTION HAS MOVED OUT OF THE
CWA...BUT ANOTHER ROUND OF SHOWERS AND POSSIBLY SOME THUNDER IS
POSSIBLE THIS AFTERNOON AND OVERNIGHT.  A BREAK IN THE CLOUD COVER
HAS ALLOWED SOME ISOLATED CONVECTIVE SHOWERS TO FORM MENDOTA SOUTH
THROUGH SOUTHERN IL. LOW LEVEL LAPSE RATES HAVE INCREASED TO 7
DEGREES C PER KM OR GREATER IN THE AREAS THAT HAVE SEEN THE SUN.
THEREFORE EXPECTING SCATTERED SHOWERS AND A FEW THUNDERSTORMS TO
MOVE INTO AREAS ALONG AND SOUTHEAST OF I-55 THIS AFTERNOON.
DEWPOINTS ALONG THIS LINE ARE IN THE MID TO UPPER 50S...BUT THINKING
THESE SHOWERS WILL ALSO PRODUCE BRIEF HEAVY DOWNPOURS LIKE THE
SHOWERS AND STORMS WE EXPERIENCED THIS MORNING.

ANOTHER AXIS OF RAIN WILL STRETCH FROM NORTHERN MICHIGAN THROUGH
SOUTHERN WI AND NORTHEAST IA ON THE BACKSIDE OF THE LOW.  THE LINE
OF SHOWERS WILL GRADUALLY SINK SOUTH...BUT THINKING THE RAIN WILL
NOT INFILTRATE NORTHERN IL UNTIL AFTER MIDNIGHT TONIGHT.  THE RAIN
WILL LIKELY LINGER INTO THURSDAY MORNING AND GRADUALLY EXIT TO THE
SOUTHEAST AS THE LOW MOVES OUT OF THE REGION.  CLOUDS WILL QUICKLY
ERODE WITH IT LEADING TO A MOSTLY SUNNY LATE AFTERNOON/EVENING
THURSDAY.

FOR TEMPS...MUCH COLDER AIR FUNNELS IN TONIGHT AND TOMORROW AS THE
UPPER LEVEL TROUGH PASSES OVERHEAD.  HIGH TEMPS TOMORROW WILL BE
MUCH COOLER IN THE 50S AND 60S...AND COOLER BY THE LAKE DUE TO
ONSHORE FLOW.  TEMPS THURSDAY NIGHT ARE ANOTHER CONCERN GIVEN THE
LIGHT WINDS AND CLEAR SKIES.  WENT A DEGREE OR TWO COOLER...WITH
ONSHORE FLOW KEEPING DOWNTOWN AND AREAS ALONG THE LAKE A BIT
WARMER.  PATCHY FROST IS CERTAINLY POSSIBLE LATE THURSDAY NIGHT INTO
FRIDAY MORNING...ESPECIALLY NORTH OF I-80...BUT THINKING TEMPS WILL
REMAIN ABOVE FREEZING ACROSS THE CWA.

FRIDAY AND SATURDAY...
THE BLOCKING PATTERN BEGINS TO TAKE SHAPE FRIDAY WITH A TROUGH OVER
THE NORTHWEST COAST...THE RIDGE GROWS ACROSS THE GREAT PLAINS...AND
THE TROUGH REMAINS OVER THE EAST COAST.  THE UPPER LEVEL JET IS ALSO
INCREDIBLY WEAK DURING THIS TIME FRAME...WHICH HELPS THE BLOCKING
PATTERN REMAIN IN PLACE.  AT THE SURFACE...A HIGH PRESSURE RIDGE
SINKS SOUTH OVER THE WESTERN GREAT LAKES.  THE RIDGE HELPS PUSH SOME
OF THE COOLER CANADIAN AIR OUT OF THE REGION...BUT TEMPS LOOK TO
REMAIN BELOW NORMAL THROUGH THE END OF THIS WEEK.

GUIDANCE SUGGESTS A WEAK UPPER LEVEL DISTURBANCE MAY TRAVEL AROUND
THE UPPER LEVEL RIDGE AND LEAD TO A CHANCE OF SHOWERS AND
THUNDERSTORMS SATURDAY AND SUNDAY.

EXTENDED...SUNDAY THROUGH TUESDAY...
THE BLOCKING PATTERN BEGINS TO BREAK UP SUNDAY NIGHT INTO MONDAY AS
THE EAST COAST TROUGH FINALLY BEGINS TO SHIFT NORTHEAST.  THIS
ALLOWS THE RIDGE TO OPEN WITH SHORT WAVE TROUGHS PASSING THROUGH OUR
AREA. GUIDANCE DIFFERS ON WHERE AND WHEN THESE DISTURBANCES WILL
PASS...THEREFORE WENT WITH A CHANCE OF SHOWERS AND THUNDERSTORMS
THROUGHOUT THE EXTENDED.  THE BEST DAY FOR PRECIP APPEARS TO BE
TUESDAY OR WEDNESDAY AS AN UPPER LEVEL PASSES THROUGH SOUTHERN WI.

NEXT WEEK BEGINS WITH A WARMING TREND.  NEAR NORMAL TEMPS ARE
EXPECTED FOR MEMORIAL DAY AND BACK INTO THE UPPER 70S OR LOW 80S BY
MID WEEK.

JEE

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 03Z...

* IFR/LIFR CIGS DEVELOPING LATE THIS EVENING.
* IFR/LIFR CIGS LIFTING THROUGH MVFR LATE THURSDAY MORNING INTO
  THURSDAY AFTERNOON.
* CHANCE OF SHOWERS THROUGH OVERNIGHT WITH A PERIOD OF SHOWERS
  AFTER SUNRISE.
* WIND SHIFT/COLD FRONT LATE THIS EVENING.
* STRONG/GUSTY NORTH/NORTHEAST WINDS DEVELOPING THURSDAY MORNING.
  CMS

//DISCUSSION...UPDATED 03Z...

COLD FRONT/WIND SHIFT FINALLY PUSHING THROUGH ORD AND SHOULD
SLOWLY CONTINUE PUSHING SOUTH ACROSS THE REST OF THE CHICAGO AREA
TERMINALS LATE THIS EVENING. LOW CLOUDS LAG THE WIND SHIFT BY AN
HR OR SO WITH CIGS AT PWK NOW 500FT AND THESE TOO ARE EXPECTED TO
OVERSPREAD THE TERMINALS OVER THE NEXT FEW HRS. WHILE ISOLATED
SHOWERS WILL BE POSSIBLE THROUGH THE OVERNIGHT HOURS...IT NOW
APPEARS THAT THE BEST CHANCE FOR A PERIOD OF WIDESPREAD SHOWERS
WILL BE LATER THURSDAY MORNING...AFTER SUNRISE AND PERHAPS JUST
BEFORE THE WINDS SIGNIFICANTLY INCREASE. CONFIDENCE STILL NOT THAT
HIGH BUT HAVE INCLUDED PREVAILING SHOWERS DURING THIS TIME PERIOD
WITH THIS UPDATE. CMS

PREVIOUS 00Z DISCUSSION...

MULTIPLE FORECAST CONCERNS/CHALLENGES THIS EVENING. A BROAD TROUGH
OF LOW PRESSURE EXTENDS FROM THE EASTERN LAKES BACK THROUGH
SOUTHERN WI. THIS TROUGH WILL SLOWLY SHIFT EAST/SOUTHEAST TONIGHT.
AS THIS OCCURS...WINDS WILL SHIFT NORTHERLY...THEN INCREASE IN
SPEEDS THURSDAY MORNING AS THE GRADIENT TIGHTENS. PRIOR TO THAT...
A WIND SHIFT/COLD FRONT...ENHANCED BY THE LAKE IS SLOWLY WORKING
ITS WAY SOUTH INTO FAR NORTHEAST IL. WINDS HAVE TURNED NORTHEAST
AT UGN WITH 400FT CIGS. ITS SOMEWHAT DIFFICULT TO SEE THIS
BOUNDARY ON VARIOUS RADARS. BUT IT DOES APPEAR TO BE MOVING
STEADILY SOUTH AND THUS COULD IMPACT ORD AS EARLY AS 02Z. TRENDS/
TIMING WILL NEED TO BE MONITORED OVER THE NEXT FEW HOURS. WHILE
WIND SPEEDS ARE NOT THAT STRONG BEHIND THIS FRONT...THEY DO SHIFT
TO THE NORTHEAST WITH IFR/LIFR CIGS. PRIOR TO THIS FRONT/S
ARRIVAL...WINDS WILL REMAIN WEST/SOUTHWEST IN THE 10-12KT RANGE...
WITH A FEW HIGHER GUSTS POSSIBLE.

SCATTERED SHOWERS MAINLY SOUTH OF I-80 WITH SHORT TERM GUIDANCE
SUGGESTING THESE COULD DEVELOP FURTHER NORTH ACROSS THE TERMINALS
OVER THE NEXT FEW HRS BUT CONFIDENCE REMAINS LOW AND HAVE
MAINTAINED VICINITY MENTION. LATER TONIGHT...AS THE TROUGH SWINGS
THROUGH...FAIRLY DECENT AGREEMENT FOR ADDITIONAL SHOWERS SPREADING
OR DEVELOPING ACROSS THE TERMINALS BUT DURATION/INTENSITY
UNCERTAIN ENOUGH TO JUST INTRODUCE VICINITY MENTION HERE AS WELL.
INSTABILITY LOOKS RATHER MEAGER FOR MUCH THUNDER POTENTIAL. WHILE
AN ISOLATED TSRA IS STILL POSSIBLE THIS EVENING...EXPECTED
COVERAGE TOO LOW TO MENTION.

AS THE TROUGH PASSES THE REGION TONIGHT...POSSIBLE FOR CIGS TO
DROP FURTHER INTO LIFR AND PERHAPS EVEN VLIFR. DEPENDING ON HOW
FAST IT EXITS THURSDAY MORNING...COULD BE A SLOWER PROGRESSION
FROM LIFR OR IFR INTO MVFR MID/LATE THURSDAY MORNING. BUT ONCE
CIGS BEGIN TO LIFT...SHOULD BE FAIRLY STEADY IMPROVEMENT INTO VFR
BY EARLY/MID AFTERNOON WITH CLEARING SKIES BY THURSDAY EVENING.

WINDS REMAIN NORTHERLY THURSDAY MORNING UNTIL THE GRADIENT
TIGHTENS AND AT THAT POINT WINDS WILL QUICKLY INCREASE TO 20KTS
WITH GUSTS 30KTS...POSSIBLY HIGHER ESPECIALLY NEAR THE LAKE AND AT
GYY...THOUGH SPECIFIC TIMING WILL NEED FURTHER REFINEMENT. WINDS
WILL SLOWLY DIMINISH DURING THE MID/LATE AFTERNOON AND THEN SLOWLY
DIMINISH THURSDAY EVENING BUT LIKELY STILL REMAIN SOMEWHAT GUSTY
PAST SUNSET AS MUCH COOLER AIR KEEPS THE LOW LEVELS MIXED. CMS

//ORD AND MDW CONFIDENCE...UPDATED 03Z...

* HIGH CONFIDENCE FOR IFR/LIFR CIGS DEVELOPING LATE THIS EVENING
  OR EARLY OVERNIGHT.
* HIGH CONFIDENCE CIGS WILL LIFT THROUGH MVFR LATE THURSDAY
  MORNING/EARLY AFTERNOON...LOW CONFIDENCE ON TIMING.
* LOW CONFIDENCE REGARDING SHOWERS THROUGH THE OVERNIGHT...MEDIUM
  CONFIDENCE FOR SHOWERS THURSDAY MORNING.
* HIGH CONFIDENCE FOR WIND SHIFT/COLD FRONT THIS EVENING...MEDIUM
  CONFIDENCE ON TIMING.
* HIGH CONFIDENCE FOR STRONG/GUSTY NORTH/NORTHEAST WINDS THURSDAY.
  CMS

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

* FRIDAY...DRY. VFR.
* SATURDAY...SLIGHT CHANCE OF TSRA AFTERNOON/EVENING.
* SUNDAY THROUGH TUESDAY...CHANCE OF SHRA/SLIGHT CHANCE OF TSRA.

&&

.MARINE...
1250 PM CDT

SURFACE LOW WILL MOVE EAST OF THE LAKE TONIGHT WITH STRONG HIGH
PRESSURE BUILDING INTO THE REGION. TIGHT PRESSURE GRADIENT WITH
FAIRLY STRONG LATE SEASON HIGH PRESSURE WILL COMBINE WITH A HEALTHY
SURGE OF COLD AIR LOOKS LIKELY TO RESULT IN A PERIOD OF GALE FORCE
WINDS LATE TONIGHT INTO THURSDAY...PARTICULARLY ACROSS SOUTHERN
PORTIONS OF THE LAKE. WILL BE UPGRADING GALE WATCH TO A WARNING WITH
THE AFTERNOON FORECAST AND INCLUDE THE NSH IN A WARNING...WITH
EXPECTATION THAT A SMALL CRAFT ADV WILL BE NEEDED TOMORROW NIGHT
INTO FRIDAY FOR LINGERING WAVES. BEYOND THIS WIND EVENT AN EXTENDED
PERIOD OF FAIRLY BENIGN CONDITIONS EXPECTED ON THE LAKE THROUGH THE
WEEKEND WITH HIGH PRESSURE OVER THE AREA.

IZZI

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...DENSE FOG ADVISORY...LMZ080-LMZ366-LMZ565-LMZ567-LMZ669-LMZ671-
     LMZ673-LMZ675-LMZ777-LMZ779-LMZ868-LMZ870-LMZ872-LMZ874-
     LMZ876-LMZ878 UNTIL 3 AM THURSDAY.

     GALE WARNING...LMZ080-LMZ671-LMZ673-LMZ675-LMZ740-LMZ741-LMZ742-
     LMZ743-LMZ744-LMZ745-LMZ777-LMZ779-LMZ872-LMZ874-LMZ876-
     LMZ878...7 AM THURSDAY TO 9 PM THURSDAY.

     GALE WARNING...LMZ366-LMZ565-LMZ567-LMZ669-LMZ868-LMZ870...4 AM
     THURSDAY TO 3 PM THURSDAY.

&&

$$

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