000
FXUS63 KLOT 222003
AFDLOT

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

.DISCUSSION...
245 AM CDT

TODAY THROUGH TONIGHT...
MAIN FORECAST CHALLENGE FOR TODAY REVOLVES AROUND PRECIP
CHANCES/COVERAGE/TIMING.

IR IMAGERY EARLY THIS MORNING SHOWS A CHANNEL OF THICKER CLOUDS
STRETCHING FROM THE OZARKS NORTH/NORTHEAST THROUGH THE OHIO VALLEY
INTO THE WESTERN GREAT LAKES. WATER VAPOR IMAGERY INDICATES A NARROW
DRY WEDGE THAT EXTENDS FROM WISC SOUTHWEST THROUGH NORTHWEST
IL...INTO NORTHEAST MISSOURI. THIS IS REFLECTED AT THE SFC WITH DEW
POINTS IN THE MID 50S...WHILE TO THE EAST AND INFLUENCED BY DEEPER
MOISTURE...DEW POINTS ARE MAINLY IN THE LOW/MID 60S. REGIONAL RADAR
MOSAIC INDICATES SOME SPOTTY SHOWERS AND AN ISOLATED THUNDERSTORM
FROM ST LOUIS TO PEORIA EARLY THIS MORNING. A LOBE OF VORTICITY
ROUNDED THE TROUGH AXIS LAST EVENING...AND WAS LIFTING
NORTH ALONG THE LEE-SIDE OF THE 500MB TROUGH. AS THIS FEATURE
ARRIVES MIDDAY ACROSS THE WESTERN OHIO VALLEY...COUPLED WITH THE
500MB TROUGH DRIFTING EAST...A WEAK UPTICK IN THE LOW/MID LVL
FORCING WILL OCCUR. MID-LVL HEIGHT FIELDS SHOW SOME INDICATIONS OF
TIGHTENING...WHICH DESPITE GUIDANCE SOUNDINGS SHOWING VERY MINIMAL
CAPE...COULD BE JUST ENOUGH TO PRODUCE SOME CONVECTION. THE
ENVIRONMENT DOES REMAIN RATHER MOIST...WITH PWAT VALUES STILL
HOVERING ARND 1-1.5". SO WITH THE MARGINAL INSTABILITY...WILL TRIM
BACK THE CONVECTIVE THREAT TO A CHANCE THUNDER. THE BEST TIMING
APPEARS TO BE THE MIDDAY THROUGH EVENING HOURS.

TOWARDS THE LATE AFTN/EVE HOURS...GUIDANCE SUGGESTS THE WAVE WILL
BEGIN TO WEAKEN AND POSSIBLY BEGIN TO FORWARD PROPAGATE SLIGHTLY
QUICKER THIS EVENING. A NARROW CHANNEL OF PRECIP SHUD BEGIN TO
DEVELOP...ASSOCIATED WITH SOME UPGLIDE/ISENTROPIC LIFT. THIS CHANNEL
OF PRECIP APPEARS TO SETUP ACROSS FAR NORTHERN IL/SOUTHERN WISC
STRETCHING EAST INTO SOUTHWEST LOWER MI. EXPECT THAT SHORTLY AFT
SUNSET...THE CONVECTIVE THREAT WILL COME TO AN END. SOUNDINGS
SUGGEST MINIMAL INSTABILITY AT BEST TONIGHT...SO WILL JUST RIDE WITH
RAIN AFT 00Z.

TEMPS TODAY WILL BE MORE SEASONAL TODAY...WITH THE THERMAL TROUGH
PUSHING EAST AND TEMPS AT 850MB COOLING TO 8-10 DEG C. THIS APPEARS
TO TRANSLATE TO TEMPS AROUND 70 TO THE MID 70S ACROSS THE CWFA. THEN
FOR TONIGHT TEMPS SHOULD SLOWLY COOL INTO THE LOW/MID 50S...AS THE
DEEPER MOISTURE SLOWLY SLIDES EAST.

CONFIDENCE IN PRECIP...HIGH.
CONFIDENCE IN TEMPERATURES...MEDIUM.

THURSDAY THROUGH FRIDAY...500MB TROUGH AXIS PASSES OVERHEAD EARLY
THUR WITH THE BACK EDGE OF THE MOISTURE APPROACHING THE UPPER
MIDWEST. WINDS WILL BE TURNING NORTHERLY IN ADVANCE OF THE BROAD
ANTI-CYCLONIC FLOW THAT WILL ARRIVE LATER IN THE DAY THUR. MUCH
COOLER AIR WILL STEADILY ADVECT ACROSS THE CWFA...WHICH WILL HELP
KEEP TEMPS IN CHECK AND MUCH COOLER THAN THE PAST FEW DAYS. GUIDANCE
HAS TRENDED SLIGHTLY COOLER FOR THUR...AND HAVE REFLECTED THIS WITH
TEMPS STRUGGLING TO REACH THE LOW 60S FOR AREAS WEST OF A BOONE TO
LIVINGSTON COUNTY LINE. FURTHER EAST AND CLOSER TO LAKE
MICHIGAN...THESE AREAS WILL BE HEAVILY INFLUENCED BY THE COOLER
MARINE AIR AND TEMPS MAY REMAIN IN THE MID/UPR 50S.

PRECIP WILL BE PEELING OFF TO THE EAST BY THE AFTN HOURS...WITH
CLOUD COVER SLOWLY ERODING FROM WEAK SUBSIDENCE. THIS WILL SET THE
STAGE THUR NIGHT FOR VERY COOL CONDITIONS. SFC HIGH PRESSURE WILL
BECOME CENTERED OVER CENTRAL WISC...WHICH COULD BE CLOSE ENOUGH TO
PROVIDE A NEAR CLOUDLESS SKY AND LIGHT WINDS. THE COMBINATION OF
THESE ELEMENTS AND VERY COOL TEMPS ALOFT...COUPLED WITH RECENT
RAINFALL...SHOULD ALLOW TEMPS TO READILY RADIATE TO ARND 40 IN MANY
AREAS. THE EXCEPTION WILL LIKELY BE DOWNTOWN CHICAGO...WHERE TEMPS
SHOULD REMAIN THE MID/UPR 40S. A FEW AREAS WEST OF A CRYSTAL LAKE TO
KANKAKEE LINE MAY SEE PATCHY FROST DEVELOP LATE THUR NIGHT AS A
RESULT OF THE COOL TEMPS.

THEN FOR FRI DRY WEATHER IS IN STORE...AS ANTI-CYCLONIC FLOW WILL
REMAIN IN CONTROL. WINDS WILL REMAIN EASTERLY...WITH TEMPS REMAINING
IN THE LOW/MID 60S. AREAS ADJACENT TO LAKE MICHIGAN WILL SEE TEMPS
REMAIN IN THE MID/UPR 50S.

CONFIDENCE...HIGH.

SATURDAY THROUGH MONDAY...
EXTENDED FORECAST CONTINUES TO SHOW DISCREPANCIES AMONGST
OPERATIONAL SOLUTIONS. ENSEMBLES CONTINUE TO SHOW A MODEST WESTERN
ATLANTIC RIDGE IN PLACE SAT/SUN...WHICH WILL MOST LIKELY SLOW THE
EASTWARD PROGRESSION OF THE 500MB TROUGH. THUS IT WILL MOST LIKELY
HAVE AN EFFECT OVER THE GREAT LAKES REGION AS WELL...IN SLOWING THE
EROSION OF THE BROAD ANTI-CYCLONIC FLOW. THIS ULTIMATELY WOULD
SUGGEST CONTINUED DRY WEATHER THROUGH AT LEAST SAT.
HOWEVER...OPERATIONAL SOLUTIONS DO SHOW DIFFERENCES. THUS WILL
MAINTAIN A SLIGHT CHC FOR PRECIP SAT/SUN. THE ECMWF DOES CONTINUE TO
FAVOR A DRY SOLUTION...WHICH AGREES WITH THE LONG WAVE PATTERN
FORECAST FROM THE ENSEMBLES. GFS SOLUTIONS FAVOR A WET STRETCH
WITH CONVECTIVE CHANCES THROUGH EARLY NEXT WEEK. WOULD NOT BE
SURPRISED OVER THE NEXT DAY OR TWO THAT SATURDAYS PRECIP TRENDS
DRY.

THEN HEADING INTO THE SECOND HALF OF THE WEEKEND INTO EARLY NEXT
WEEK...A FRONTAL BOUNDARY TRIES TO LIFT NORTH AS MID-LVL HEIGHTS
INCREASE. ENSEMBLES AND OPERATIONAL SOLUTIONS DO AGREE ON AN
AMPLIFYING RIDGE ACROSS THE CENTRAL CONUS...WITH A SLOW WARMING
TREND. THE ECMWF DOES HOLD THE DRY WEATHER THROUGH MON NGT...WITH A
CHANNEL OF MOISTURE STRETCHING FROM THE TENN VALLEY NORTHWEST
THROUGH THE NORTHERN PLAINS...OR ALONG THE WESTERN PERIPHERY OF THE
PROGGED SFC RIDGE.

TEMPS BY MON WILL BE ARND 70...PSBLY LOWER 70S. WINDS SHOULD BE SLOW
TO TURN SOUTHERLY...AND WITH A PROLONGED PERIOD OF
EAST/SOUTHEASTERLY WINDS...DOWNTOWN WILL REMAIN ONE OF THE COOLER
LOCATIONS IN THE CWFA.

CONFIDENCE...MEDIUM.
CONFIDENCE IN PRECIP...LOW.

BEACHLER

&&

.AVIATION...

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

* VARIABLE CIGS...VFR...THEN POSSIBLY DROPPING TO MVFR LATE THIS
  AFTERNOON...AND IFR LIKELY TONIGHT.
* ADDITIONAL SHRA EARLY EVENING POSSIBLE...LOW CHC TSRA.
* WIND SHIFT WITH FROPA THIS EVENING.

BMD

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

FIRST ROUND OF THUNDERSTORMS ARE PASSING OVER THE TERMINALS EARLY
THIS AFTERNOON...BUT SHOULD BE OUT OF THE CHICAGO AREA AROUND 19Z.
CIGS BEHIND PRECIP QUICKLY RISE BACK TO VFR. OFF TO THE WEST...CU
FIELD IS GROWING OVER THE MISSISSIPPI RIVER AS UPPER LEVEL
DISTURBANCE LIFTS ACROSS THE AREA. FORECAST SOUNDINGS SHOW VERY
MODEST INSTABILITY REDEVELOPING MID TO LATE THIS AFTERNOON...WITH
ADDITIONAL SHOWERS POSSIBLE...AND CANT RULE OUT A THUNDERSTORM.

SURFACE LOW CURRENTLY OVER NORTHEASTERN IOWA WILL TRANSLATE EAST
TODAY WITH WINDS BECOMING LIGHT AND VARIABLE THIS EVENING...THEN
NORTH TO NORTHEAST BEHIND THE DEPARTING LOW LATE THIS EVENING AND
TONIGHT. LOWER CIGS ARE LIKELY BEHIND THE FRONT AS WELL AS LIGHT
RAIN OR DRIZZLE. THERE IS ALSO POTENTIAL FOR VERY LOW CIGS //LESS
THAN 005// OR FOG OFF THE LAKE AT THE CHICAGO TERMINALS. THE
GRADIENT TIGHTENS UP EARLY ON THURSDAY AND EXPECT GUSTS TO COMMENCE
EARLY IN THE DAY...TOPPING OUT POTENTIALLY AROUND 30 KTS THURSDAY
LATE MORNING AND AFTERNOON.

BMD

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

* MEDIUM CONFIDENCE IN ADDITIONAL SHRA THIS EVENING BUT LOW
  CONFIDENCE ON COVERAGE. LOW CONFIDENCE IN TSRA.
* LOW CONFIDENCE IN MVFR CIGS THIS AFTERNOON/EVENING...MEDIUM TO
  HIGH CONFIDENCE IN CIGS DROPPING TO IFR TONIGHT.

BMD

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

THURSDAY NIGHT AND FRIDAY...VFR. WX NIL

SATURDAY THROUGH TUESDAY...VFR. CHC SHRA SLIGHT CHC TSRA.

ALLSOPP

&&

.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.

     DENSE FOG ADVISORY...LMZ261-LMZ362-LMZ364-LMZ563 UNTIL 9 PM
     WEDNESDAY.

&&

$$

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