AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1025 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 16Z...
* SCATTERED SHOWERS WITH A FEW THUNDERSTORMS LATE THIS MORNING AND
EARLY AFTERNOON...SHOWERS CONTINUING THROUGH THE EVENING.
* LIGHT SOUTHWEST WINDS SHIFTING WEST TONIGHT AND NORTHWEST BY
MORNING.
* VARIABLE MVFR/VFR CIGS BECOMING IFR AFTER 06Z TONIGHT.
* STRONG GUSTY NNE WINDS DEVELOPING THURSDAY.
ALLSOPP/BMD
//DISCUSSION...UPDATED 12Z...
MESSY WEATHER THIS MORNING WITH A NEARLY STATIONARY BOUNDARY FROM
CENTRAL LAKE MICHIGAN TO NORTHWEST IL. SOUTH OF THE BOUNDARY DEW POINTS
IN THE LOW TO MID 60S WITH LIGHT SW WINDS. THIS VERY MOIST LOW LEVEL
AIR HAS LED TO DEVELOPMENT OF MVFR CIGS AND LOCALLY IFR CIGS. A
SHORT WAVE IS ROTATING NORTH ACROSS IL WITH WIDESPREAD LIGHT RAIN
AND A FEW EMBEDDED HEAVIER SHOWERS ACROSS NORTHERN IL. NO THUNDER
HAS BEEN REPORTED OUT OF THIS FOR THE LAST SEVERAL HOURS...SO HAVE
LEFT THUNDER OUT OF THE TAFS FOR NOW. ANY THUNDER THIS MORNING
SHOULD BE ISOLATED. THIS AREA OF SHOWERS SHOULD LIFT NORTH OF THE
TERMINALS BY MID MORNING.
EXPECT CIGS/VSBYS TO IMPROVE TO MOSTLY VFR MID TO LATE MORNING AND
THERE MAY BE A BREAK IN THE PRECIP. HOWEVER A BROAD WEAK LOW
PRESSURE AREA...NOW OVER EASTERN IA...WILL DRIFT SLOWLY EAST
ACROSS NORTHERN IL THIS AFTERNOON. VARIOUS NUMERICAL GUIDANCE
SUGGESTS SCATTERED SHOWERS WILL BE POSSIBLE THROUGH THE DAY AND
INTO TONIGHT. AN ISOLATED THUNDERSTORM IS ALSO POSSIBLE TODAY. DONT
HAVE A GOOD FEEL FOR TIMING OR COVERAGE OF SHOWERS. CIGS MAY VARY
FROM MVFR IN SHOWERS TO VFR AT TIMES.
THIS EVENING THE LOW WILL DRIFT EAST AND COOLER AIR WILL MOVE IN
ON LIGHT NORTHWEST WINDS. SOME LIGHT RAIN OR DRIZZLE IS POSSIBLE
TONIGHT BUT THUNDER THREAT WILL END. MODEL GUIDANCE SUGGEST CIGS
WILL LOWER TO IFR BY AROUND MIDNIGHT. SO HAVE TRENDED THIS WAY IN
TAFS. BY DAYBREAK WINDS WILL VEER AROUND TO NW THEN NNE AND
INCREASE. EXPECT LOW CIGS TO BEGIN LIFTING TO MVFR AS COLDER DRIER
AIR BEGINS MIXING IN THURSDAY MORNING.
ALLSOPP
//ORD AND MDW CONFIDENCE...UPDATED 16Z...
* MEDIUM CONFIDENCE IN CIG TRENDS THIS MORNING.
* MEDIUM TO HIGH CONFIDENCE IN WIND FORECAST.
* LOW CONFIDENCE IN THUNDERSTORMS IMPACTING TERMINALS DIRECTLY.
* MEDIUM CONFIDENCE IN IFR CIGS DEVELOPING AFTER 06Z.
* HIGH CONFIDENCE IN STRONG NNE WINDS DEVELOPING THURSDAY MORNING.
ALLSOPP/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...
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.
&&
$$
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