000
FXUS63 KLOT 220746
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
246 AM 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 06Z...

* SCATTERED THUNDERSTORMS THROUGH THE EARLY MORNING HOURS.

* PERIODS OF SHOWERS MID MORNING THROUGH EARLY EVENING.

* MVFR CIGS/VSBYS...POSSIBLE IFR CIGS...DEVELOPING LATE TONIGHT.

ALLSOPP

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

PRECIP/CLOUD TRENDS...BOUNDARY HAS SET UP ACROSS NORTHEAST
ILLINOIS WITH MID TO UPPER 50S DEW POINTS ACROSS NORTHWEST IL AND
SOUTHERN WI AND MID TO UPPER 60S CENTRAL AND NORTHEAST IL. POTENT
SHORT WAVE COMING AROUND THE BASE OF THE CENTRAL US TROF...NOW
OVER THE LOWER MS VALLEY...PROVIDING LIFT WHICH IS SUPPORTING
SCATTERED THUNDERSTORM CLUSTERS OVER NORTHEAST IL. BUMPED UP
COVERAGE FROM VCTS TO TEMPO IN LIGHT OF THE RECENT INCREASE IN
ACTIVITY. COULD SEE SOME BRIEF MVFR VSBY IN HEAVIER
SHOWERS/THUNDERSTORMS. THIS MAY TAPER OFF IN THE NEXT COUPLE
HOURS. BUT AS THE SHORT WAVE CONTINUES TO LIFT NORTHEAST AND THE
WEAK SURFACE LOW DRIFTS EAST FROM IOWA TO NORTHERN IL THIS
AFTERNOON AND EVENING...EXPECT MORE WIDESPREAD SHOWERS TO DEVELOP
BY MIDDAY. WITH CLOUD COVER AND COOLER SURFACE TEMPERATURES...MODEL
SOUNDINGS NOT INDICATING MUCH INSTABILITY AND THUS DID NOT MENTION
THUNDER IN TAFS FOR LATE MORNING-AFTERNOON. LIGHT RAIN OR DRIZZLE
COULD LINGER THROUGH THE NIGHT BUT COVERAGE AND INTENSITY SHOULD
BE ON THE WANE. AS COLDER AIR WORKS ITS WAY INTO THE AREA EXPECT
CIGS TO LOWER. MODEL GUIDANCE SUGGESTING IFR CIGS AFTER 06Z THU
AND HAVE TRENDED IN THIS DIRECTION IN THE 30 HOUR PORTION OF ORD
TAF.

WINDS...WEAK PRESSURE GRADIENT ACROSS THE AREA EARLY THIS MORNING
WITH WEAK DISORGANIZED LOW OVER IA/NORTHWEST IL. EXPECT GENERALLY
LIGHT SOUTH TO SOUTHEAST WINDS EARLY THIS MORNING...THOUGH WINDS
MAY VARY SOME IN AND AROUND SHOWERS. THIS LOW WILL DRIFT SLOWLY
EAST TODAY AND EXPECT WINDS WILL BE GENERALLY SSW AND WEAK THROUGH
THE DAY...WITH WINDS GRADUALLY SHIFTING SW THEN W THIS EVENING AS
THE LOW PASSES. AS THE LOW CONTINUES EASTWARD OVERNIGHT...WINDS WILL
EVENTUALLY COME AROUND TO NNW BY THU MORNING.

ALLSOPP

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

* MEDIUM CONFIDENCE IN WIND TRENDS.

* LOW TO MEDIUM CONFIDENCE IN THUNDERSTORM TRENDS EARLY THIS MORNING.

* MEDIUM CONFIDENCE IN CIG/PRECIP TRENDS MID MORNING THROUGH EARLY
  EVENING.

* MEDIUM CONFIDENCE IN LOWER CIGS DEVELOPING LATE TONIGHT.

ALLSOPP

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

THURSDAY...CHC SHRA IN THE DAY. VFR OVERNIGHT.

FRIDAY...VFR. WX NIL.

SATURDAY - MONDAY...PERIODIC CHANCES FOR SHRA/TSRA.

KREIN

&&

.MARINE...
205 AM CDT

A WEAK BROAD AREA OF LOW PRESSURE WAS OVER THE WESTERN GREAT LAKES
EARLY THIS MORNING. THE LOW WILL MOVE SLOWLY EAST TO LOWER GREAT
LAKES BY THURSDAY MORNING. MEANWHILE HIGH PRESSURE CENTERED OVER
MANITOBA WITH A RIDGE EXTENDING SOUTH INTO NORTHERN PLAINS WILL
MOVE SOUTHEAST. THIS WILL SET UP A PERIOD OF GALE FORCE NORTH
WINDS ON LAKE MICHIGAN LATE TONIGHT THROUGH THURSDAY. THE LOW
WILL MOVE TO THE EAST COAST BY FRIDAY AS THE HIGH MOVES TO LAKE
MICHIGAN. THE RIDGE OF HIGH PRESSURE WILL BE NEARLY STATIONARY
FROM HUDSON BAY TO THE LOWER OHIO VALLEY THROUGH THE WEEKEND. THIS
WILL SET UP A PROLONGED PERIOD OF LIGHT EAST TO SOUTHEAST WINDS
OVER LAKE MICHIGAN.

ALLSOPP

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...GALE WATCH...LMZ080-LMZ669-LMZ671-LMZ673-LMZ675-LMZ777-LMZ779-
     LMZ870-LMZ872-LMZ874-LMZ876-LMZ878...3 AM THURSDAY TO 9 PM
     THURSDAY.

     GALE WATCH...LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565-LMZ567-
     LMZ868...3 AM THURSDAY TO 7 PM THURSDAY.

     GALE WATCH...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745...10 AM
     THURSDAY TO 9 PM THURSDAY.

&&

$$

VISIT US AT HTTP://WEATHER.GOV/CHICAGO (ALL LOWERCASE)

FOLLOW US ON FACEBOOK...TWITTER...AND YOUTUBE AT:
WWW.FACEBOOK.COM/US.NATIONALWEATHERSERVICE.CHICAGO.GOV
WWW.TWITTER.COM/NWSCHICAGO
WWW.YOUTUBE.COM/NWSCHICAGO