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000
FXUS63 KLOT 202020
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
320 PM CDT MON MAY 20 2013

.MESOSCALE DISCUSSION...
303 PM CDT

SPECIAL 1800 UTC SOUNDING FROM DVN CONFIRMED WHAT VISIBLE SATELLITE
IMAGERY HAS SUGGESTED ALL AFTERNOON...A VERY STRONG CAPPING
INVERSION. THE MCV MOVING INTO NORTHERN ILLINOIS IS SURVIVING BUT
WITH CONVECTION HAS COMPLETELY DISSIPATED AS IT HAS MOVED INTO THE
DEEPLY MIXED/DRIER/MORE CAPPED ENVIRONMENT OVER NORTHERN ILLINOIS.

VISIBLE SATELLITE IMAGERY SHOWS AN INCREASING CUMULUS FIELD
SOUTHEAST OF INTERSTATE 55 OVER EAST CENTRAL ILLINOIS INTO NORTHWEST
INDIANA. SURFACE STREAMLINE ANALYSIS INDICATES A REGION OF FAIRLY
GOOD LOW LEVEL CONVERGENCE FROM NORTHWEST INDIANA SOUTHWEST INTO
FORD COUNTY ILLINOIS WHERE CUMULUS FIELD IS BECOMING A BIT MORE
ENHANCED. COMBINATION OF SLOW DESTABILIZATION...LOW LEVEL
CONVERGENCE AND LIKELY SOME ASCENT ALOFT FROM PASSING MCV COULD BE
ENOUGH TO ALLOW SOME CONVECTION TO DEVELOP OVER OUR SOUTHEASTERN CWA
LATE THIS AFTERNOON.

ELSEWHERE HAVE REMOVED POPS NORTHWEST OF I-55 WHERE CONDITIONS LOOK
TO REMAIN TOO HOSTILE FOR ANY PRECIPITATION UNTIL AT LEAST MID
EVENING.

IZZI

&&

.DISCUSSION...
257 PM CDT

SHORT TERM (TONIGHT)...

POTENTIAL WILL EXIST FOR CONVECTION TO DEVELOP THIS EVENING OVER
SOUTHEASTERN CWA...BUT BETTER CHANCES FOR THUNDERSTORMS TONIGHT
LOOKS TO BE LATER THIS EVENING INTO THE OVERNIGHT HOURS OF LEFT OVER
CONVECTION THAT DEVELOPS OVER THE EASTERN PLAINS SPILLS EASTWARD
INTO THE AREA. SUBSIDENCE IN THE WAKE OF THIS EVENING`S MCV WILL
LIKELY RESULT IN MUCH OF THE EVENING HOURS REMAINING DRY NORTHWEST
OF INTERSTATE 55 WITH PRECIP CHANCES LIKELY HOLDING OFF UNTIL LATE
EVENING OR MORE LIKELY AFTER MIDNIGHT.

IZZI

LONG TERM (TUESDAY AND BEYOND)...

TUESDAY AND TUESDAY NIGHT...TRYING TO TIME THE MOST LIKELY
PERIODS FOR THUNDERSTORMS WILL BE THE MAIN CHALLENGE THROUGH
WEDNESDAY. THE UPPER LOW WILL CONTINUE TO SPIN OVER WESTERN
MINNESOTA AND SURROUNDING AREAS TUESDAY. CONVECTION MAY BE ONGOING
TUESDAY MORNING BUT SHOULD PRIMARILY BE SHIFTING OFF TO THE NORTH
AND EAST. IT APPEARS THAT SOME MID LEVEL DRYING WILL WORK INTO THE
AREA WHICH SHOULD ALLOW FOR A BREAK IN ORGANIZED CONVECTIVE
CHANCES THROUGH EARLY IF NOT LATE AFTERNOON. HOWEVER...AFTERNOON
HEATING MAY BE ENOUGH TO ENHANCE THE THREAT FOR STORM DEVELOPMENT
AS WILL ANY WEAK WAVE THAT CAN COME THROUGH THE FLOW...WHICH AT
THIS TIME IS INDISCERNIBLE. IN ADDITION...SHOULD CONVECTION CROSS
THE AREA TONIGHT THEN OUTFLOW BOUNDARIES LEFT BEHIND MAY SERVE AS
A TRIGGER DURING THE AFTERNOON AS WELL. HAVE BETTER CONFIDENCE IN
POPS TUESDAY EVENING AND TUESDAY NIGHT. A CLOSER LOOK AT CURRENT
WATER VAPOR IMAGERY SHOWS A TROUGH PIVOTING SOUTHWARD INTO THE
FOUR CORNERS. THIS IS EXPECTED TO ROUND THE BASE OF THE UPPER LOW
AND TRACK NORTHEASTWARD ACROSS THE LOCAL AREA TUESDAY NIGHT. THIS
WILL HELP NEW SURFACE LOW PRESSURE TO DEVELOP AHEAD OF THE UPPER
TROUGH ACROSS CENTRAL ILLINOIS AND MOVE RIGHT ACROSS THE AREA
EARLY WEDNESDAY MORNING. WILL TIE THE HIGHEST POPS WITH THE
PASSAGE OF THIS FEATURE...GIVEN THAT IT IS THE MOST COHERENT. THIS
MAY LEAVE MUCH OF THE EVENING WITH MINIMAL PRECIP COVERAGE BEFORE
MORE WIDESPREAD ACTIVITY ARRIVES FROM SOUTH TO NORTH LATE EVENING
OR AFTER. WITH THE EXPECTATION OF LOWER PRECIP COVERAGE DURING THE
DAY HAVE RAISED HIGHS SEVERAL DEGREES AND MAY NEED TO GO HIGHER IF
ENOUGH SUNSHINE CAN DEVELOP AND MIXING CAN DEEPEN ENOUGH.

WEDNESDAY AND THURSDAY...THE UPPER TROUGH NOW APPROACHING THE
BRITISH COLUMBIA COAST WILL DROP SOUTHWARD AND CLOSE OFF OVER THE
PACIFIC NORTHWEST AND HELP TO PUSH THE MINNESOTA UPPER LOW EASTWARD.
THE LOW WILL OPEN UP IN TO MORE OF A TROUGH WITH THE AXIS CROSSING
THE AREA WEDNESDAY NIGHT. EXPECT THAT SHOWERS/THUNDERSTORMS WILL BE
EXITING THE AREA WEDNESDAY MORNING WITH A POTENTIAL BREAK IN THE
ACTIVITY BEFORE ANOTHER ROUND OF SHOWERS AND MAYBE SOME STORMS
ARRIVE WEDNESDAY EVENING/NIGHT WITH THE TROUGH AXIS. THESE LOOK TO
EXIT QUICKLY THURSDAY MORNING. BY THIS TIME THE UPPER AIR PATTERN
WILL HAVE EVOLVED INTO A LONGWAVE OVER THE AREA WITH A SHARP RIDGE
OVER THE HIGH PLAINS INTO THE CANADIAN PRAIRIES AND A BIG CLOSED
UPPER LOW OVER THE FAR WESTERN U.S. THE PASSAGE OF THE TROUGH WILL
MARK THE ARRIVAL OF NOTICEABLY COOLER AIR AS LOW LEVEL FLOW TURNS
NORTHERLY AND HIGH PRESSURE BUILDS IN.

FRIDAY THROUGH MONDAY...QUIET WEATHER IS EXPECTED FRIDAY AND AT
LEAST EARLY SATURDAY AS HIGH PRESSURE REMAINS IN CONTROL THEN SHIFTS
EAST. THE UPPER RIDGE WILL PUSH EASTWARD ALLOWING WARM ADVECTION TO
RETURN SOMETIME SATURDAY WHICH MAY RESULT IN SOME PRECIPITATION
THROUGH THE WEEKEND. TEMPERATURES WILL SLOWLY MODERATE THROUGH THE
WEEKEND...WITH HIGHS MAINLY IN THE LOW TO MID 60S FRIDAY AND WARMING
A FEW DEGREES EACH DAY...WITH 70S BY MONDAY. HIGHS ALONG THE LAKE
WILL LIKELY BE HELD IN THE 50S THROUGH THE WEEKEND.

MDB

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 20Z...

* POTENTIAL FOR THUNDERSTORMS IN THE VICINITY THIS EVENING.
* GUSTY SOUTH TO SOUTHWEST WINDS.

PAW

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

INCREASING INSTABILITY OVER NORTHERN ILLINOIS AND NORTHWEST
INDIANA THIS AFTERNOON AND EVENING...AHEAD OF A SLOW-MOVING
FRONTAL SYSTEM IN THE PLAINS. POTENTIAL FOR THUNDERSTORMS TO MOVE
INTO THE TERMINAL AREAS BY MID TO LATE AFTERNOON AND THREAT
CONTINUING INTO THE OVERNIGHT HOURS. THEN THERE IS STILL SOME
CHANCE OF THUNDERSTORMS IN THE AREA AGAIN ON TUESDAY...THOUGH AT
THIS TIME THE BEST POTENTIAL APPEARS TO BE TO THE EAST OF THE
TERMINALS AND LATER IN THE DAY. STRONG AND GUSTY SOUTH TO
SOUTHWEST WINDS THIS AFTERNOON WILL DIMINISH A LITTLE OVERNIGHT
BUT THEN INCREASE AGAIN ON TUESDAY.

PAW

//ORD AND MDW CONFIDENCE...UPDATED 20Z...

* LOW TO MEDIUM CONFIDENCE IN THUNDERSTORM POTENTIAL.
* MEDIUM CONFIDENCE IN WIND TREND.

PAW

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

TUESDAY...PERIODS OF TSRA. GUSTY SW WINDS.
WEDNESDAY...LIKELY TSRA DURING THE DAY. CHC DURING THE NIGHT.
            POTENTIAL FOR MVFR CIGS.
THURSDAY...CHANCE OF SHRA.
FRIDAY...VFR LIKELY. GUSTY NE WINDS.
SATURDAY...CHC OF TSRA DURING THE NIGHT.

MTF

&&

.MARINE...
314 PM CDT

LOW PRESSURE OVER THE NORTHERN PLAINS WILL MOVE VERY LITTLE
TONIGHT BEFORE SLOWLY SHIFTING EAST TOWARDS THE UPPER MIDWEST AND
WESTERN GREAT LAKES ON TUESDAY. WITH THIS IN PLACE...STRONGER
SOUTHERLY WINDS WILL OCCUR OVER MUCH OF THE LAKE TONIGHT BEFORE
DIMINISHING ON TUESDAY WITH THIS LOW WEAKENING WHILE MOVING ACROSS
THE GREAT LAKES. LIGHTER WINDS WILL BE IN PLACE BRIEFLY TUESDAY
NIGHT AS THE LOW MOVES OVERHEAD BUT THEN INCREASE ONCE AGAIN ON
WEDNESDAY WHILE TURNING MORE NORTHERLY. EXPECT BOTH WINDS AND
WAVES TO INCREASE/BUILD WEDNESDAY NIGHT INTO THURSDAY...BEFORE
HIGH PRESSURE DROPS SOUTH THROUGH THE PLAINS AND UPPER MIDWEST
INTO THURSDAY NIGHT AND WINDS DIMINISH ONCE AGAIN INTO FRIDAY.

RODRIGUEZ

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...NEARSHORE WATERS
     UNTIL 10 PM MONDAY.

&&

$$

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