000
FXUS63 KLOT 210108
AFDLOT

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

.MESOSCALE DISCUSSION...
808 PM CDT

VERY LITTLE CHANGE IN THINKING FROM PREVIOUS MESO-AFD WITH REMNANT
OUTFLOW BOUNDARY/PSEUDO WARM FRONT CONTINUING TO MOVE QUICKLY
NORTHWARD. THE 00 UTC SOUNDING FROM ILX LIKELY GIVES A GOOD
REPRESENTATION OF THE POST BOUNDARY AIR MASS WITH MLCAPE VALUES
NEARING 1500 J/KG AND VERY LITTLE INHIBITION. WILL LIKELY SEE SOME
NOCTURNAL COOLING THIS EVENING WHICH COULD LESSEN THE INSTABILITY A
BIT...BUT SUFFICIENT CAPE/SHEAR LOOK LIKELY TO PERSIST TO MAINTAIN A
SEVERE THREAT AT LEAST THE NEXT COUPLE/FEW HOURS. AS
ANTICIPATED...STORM MODE SO FAR CONTINUES TO LOOK MORE LINEAR AT
THIS POINT SUGGESTING MORE OF A WIND/HAIL THREAT...PARTICULARLY
GIVEN THE MODERATELY LARGE TEMP/TD DEPRESSIONS.

DEVELOPING MCV CROSSING THE MS RIVER COULD RESULT IN INCREASING
MESOSCALE ORGANIZATION OF THE RATHER SLOPPY LOOKING QLCS CROSSING
THE RIVER NOW. IF COLD POOL CONTINUES TO DEVELOP AND EXPAND EARLY
THIS EVENING THEN QLCS EVOLVE INTO A MORE WELL ORGANIZED BOWING LINE
SEGMENT WITH A GREATER WIND DAMAGE THREAT ACROSS MAINLY OUR
SOUTHERN/SOUTHEASTERN CWA...PARTICULARLY IN LIGHT OF FORWARD
PROPAGATING CORFIDI VECTORS WHICH ARE PROGGED TO INCREASE TO 50KT+
AS LOW LEVEL JET INTENSIFIES EARLY THIS EVENING.

IZZI

544 PM CDT

LATEST VISIBLE SATELLITE IMAGERY IS PAINTING A MUCH CLEARER PICTURE
OF THE POTENTIAL CONVECTIVE EVOLUTION AND THREAT THIS EVENING.
OUTFLOW BOUNDARY FROM LAST NIGHT`S CONVECTION SHOWS UP NICELY IN
FORM OF CUMULUS CONGESTUS FIELD EXTENDING FROM NEAR KRZL WEST TO
KAAA TO KUIN. SATELLITE SHOWS THIS BOUNDARY LIFTING RAPIDLY
NORTHWARD AT OVER 20KT WITH AN IMPRESSIVE JUMP IN DEWPOINTS NOTED
WITH THE PASSAGE OF THIS BOUNDARY. SFC OBS SHOW ABOUT A 10F JUMP IN
TDS AS THE BOUNDARY PASSES WITH A CORRESPONDING INCREASE IN SFC
BASED CAPE. WINDS IN THE WAKE OF THIS BOUNDARY HAVE BACKED MORE
SOUTH TO EVEN SOUTHEASTERLY RESULTING IN STRONGER DEEP AND LOW
LEVEL SHEAR PROFILES COMPARED TO IN THE MORE STABLE ENVIRONMENT
NORTH OF THE FRONT.

RAP GUIDANCE SUGGESTS THAT THE LOW LEVEL FLOW WILL INCREASE THIS
EVENING INCREASING LOW LEVEL THETA-E ADVECTION AS WELL AS
STRENGTHENING MOISTURE CONVERGENCE ALONG AND NORTH OF THIS
RETREATING BOUNDARY. SEVERAL SCATTERED SUPERCELLS ARE ONGOING OVER
CENTRAL MISSOURI AT THIS TIME AND WOULD ANTICIPATE THESE STORMS TO
CONGEAL INTO A QLCS AND POTENTIALLY ACCELERATE NORTHEASTWARD INTO
AND ACROSS CENTRAL AND LIKELY NORTHERN ILLINOIS LATER THIS EVENING.
WHILE BOUNDARY WILL LIKELY BE LIFTING NORTH ACROSS THE CWA THIS
EVENING RESULTING IN FAIRLY QUICK DESTABILIZATION...THIS COULD BE
OFFSET SOMEWHAT BY NOCTURNAL COOLING. EVEN SO...THERE IS A VERY REAL
THREAT THAT THE SEVERE STORMS OVER MISSOURI COULD GAIN SUFFICIENT
MESOSCALE ORGANIZATION TO MAINTAIN A PRETTY DECENT SEVERE THREAT
INTO OUR CWA MID TO LATE THIS EVENING...ESPECIALLY THE FARTHER SOUTH
AND WEST YOU GO OVER OUR AREA.

EXPECTATION IS THAT THE PRIMARY STORM MODE WILL BE LINEAR BY THE
TIME STORMS ARRIVE INTO OUR AREA...GIVEN THIS AND THE LOSS OF
DAYTIME HEATING THINK THE PRIMARY THREAT WILL BE DAMAGING WINDS WITH
A SOMEWHAT LOWER SECONDARY THREAT BEING LARGE HAIL. CURRENTLY...SFC
OBS SHOW A LESSER DEFINED BUT SECONDARY PUSH OF MORE MOIST AIR
LAGGING THE INITIAL OUTFLOW BOUNDARY BY ABOUT 100MI TO THE SOUTH
WITH TDS CLOSER TO 70F THERE. ASSUMING THIS REALLY GOOD MOISTURE
DOES NOT CATCH UP WITH THE RETREATING OUTFLOW BOUNDARY THIS EVENING
THEN THE TORNADO THREAT WILL LIKELY BE PRETTY LOW. GIVEN THE
STRENGTHENING LOW LEVEL JET AND POTENTIAL FOR THIS OUTFLOW BOUNDARY
TO STILL BE ALIGNED SOMEWHAT PERPENDICULARLY TO THE QLCS THE TORNADO
THREAT IS NOT TOTALLY ZERO LATER THIS EVENING.

CANNOT RULE OUT ISOLATED THUNDERSTORMS DEVELOPING ALONG THE
NORTHWARD MOVING OUTFLOW/PSEUDO WARM FRONT THIS EVENING...BUT THE
MORE ORGANIZED THREAT OF CONVECTION WILL LIKELY NOT ARRIVE UNTIL
01-02Z OVER FAR SOUTHWEST CWA SPREADING NORTHEAST ACROSS THE AREA
THROUGH THE LATE 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 00Z...

* TSRA...POSSIBLY GUSTY...AT TIMES LATE THIS EVE INTO OVERNIGHT.
* 35-40 KT SW FLOW AT AROUND 1500 FT BETWEEN 02Z AND 09Z.
* GUSTY SW WINDS ON TUE.
* A CHANCE FOR TSRA LATE IN TUE AFTERNOON AND EVE.

//DISCUSSION...UPDATED 00Z...

MOIST AND UNSTABLE AIR IS MOVING BACK NORTHWARD ACROSS CENTRAL
ILLINOIS AND WILL BE THE LEADING EDGE OF THUNDERSTORM POTENTIAL
THROUGH THE EVE. THE ACTIVITY ACROSS MO AND FAR WESTERN IL EARLY
THIS EVE WILL EVOLVE NORTHEAST WITH THIS UNSTABLE AIR. THE STORMS
ARE LIKELY TO MORPH INTO MULTIPLE CLUSTERS/SEGMENTS AND THUS COULD
PROPAGATE AT A QUICKER FORWARD SPEED...ALTHOUGH WHERE EXACTLY
THEY TRACK AND HOW FAR NORTH THAT OCCURS IS TOUGH TO SAY YET. THE
TAF TIMING IS THE BEST ESTIMATE...BUT EACH AIRPORT COULD SEE
ACTIVITY AROUND 1 HR EARLIER IF STORMS BEGIN TO PICK UP PACE. THE
HIGHER POTENTIAL FOR STRONG TO SEVERE WINDS IS JUST SOUTH OF THE
CHICAGO TAF SITES...HOWEVER ONCE AGAIN ANY ONE ORGANIZED CLUSTER
WILL LIKELY HAVE STRONG WINDS. THERE IS SOME POTENTIAL...THOUGH
LESS...FOR HAIL UNDER STRONG STORM CORES. TEMPORARY IFR CONDITIONS
MAY BE OBSERVED...WITH THE POSSIBILITY OF LINGERING MVFR CIGS AND
SHOWERS INTO THE MORNING RUSH.

WINDS WILL REMAIN GUSTY INTO THE EARLY TO MID EVE WITH CONVECTION
TEMPORARILY SHIFTING WINDS AT TIMES IF THEY PASS OVER THE
AIRFIELDS. THE LOW-LEVEL JET AT AROUND 35-40 KT WILL BE POISED
AROUND 1500 FT AND ABOVE. WHILE THERE WILL BE AN INVERSION...THE
PRESSURE GRADIENT WILL DRIVE ENOUGH WIND SPEED TO PREVENT TRUE
LLWS CRITERIA.

IN THE WAKE OF TONIGHTS STORMS THE ATMOSPHERE IS LIKELY TO BE
WORKED OVER FAIRLY WELL FOR REDEVELOPMENT ON TUE MORNING AND EVEN
POTENTIALLY INTO THE AFTERNOON. TUE NIGHT CHANCES LOOK HIGHER AS
SIMILAR ELEMENTS COME TOGETHER AS TONIGHT FOR NOCTURNAL
CONVECTION.

//ORD AND MDW CONFIDENCE...UPDATED 00Z...

* HIGH CONFIDENCE IN AREAS OF TSRA REACHING AT LEAST NEAR THE TAF
  SITES. MEDIUM CONFIDENCE IN TSRA START TIME AND LOWEST
  CONDITIONS.
* MEDIUM CONFIDENCE IN WIND SPEEDS AT 1500 FT REMAINING BELOW LLWS
  CRITERIA.
* HIGH CONFIDENCE ON SFC WIND FORECAST...HOWEVER STORMS LATE THIS
  EVE AND OVERNIGHT WILL POTENTIALLY DISRUPT THE WIND FLOW AND
  LEAD TO TEMPORARY HIGHER GUSTS.
* MEDIUM CONFIDENCE ON TSRA TUE NIGHT.

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

WEDNESDAY...SCT SHRA/TSRA AND POSSIBLE MVFR CONDITIONS.
THURSDAY...CHC OF SHRA. GUSTY NE WINDS.
FRIDAY...VFR LIKELY.
SATURDAY...SLT CHC OF TSRA.
SUNDAY...CHC OF TSRA.

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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