000
FXUS63 KLOT 201946
AFDLOT

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

.MESOSCALE DISCUSSION...
113 PM CDT

CENTRAL MISSOURI CONVECTIVE CLUSTER HAS ADVANCED INTO WC IL WITH
APPARENT SMALL SCALE MCV EVIDENT IN RADAR/SATELLITE IMAGERY.

VISIBLE SATELLITE IMAGERY SHOWS COMPLETE LACK OF CUMULUS OVER MUCH
OF NORTHERN/CENTRAL ILLINOIS...EVIDENCE OF THE SUBSIDENCE IN THE
WEAK OF LEAD SHORTWAVE AND MORE SHALLOW MOISTURE WHICH IS QUICKLY
MIXING OUT. DEEPER MOISTURE FIELD DOES EXTEND FROM LOWER OH VALLEY
NORTH INTO SE IL AND THEN INTO INDIANA AND LOWER MICHIGAN IN ADVANCE
OF THE LEAD SHORTWAVE. SMALL MCV LOOKS LIKELY TO TRACK NW OF THIS
MORE MOIST/BUOYANT ENVIRONMENT...SO IT WILL HAVE QUITE AN UPHILL
BATTLE TO INTENSIFY. LOW LEVEL WINDS IN ADVANCE OF THE CONVECTION
ARE ALL SOUTHWESTERLY WHICH WILL LIMIT CONVERGENCE AS WELL...BUT
DESPITE THE WEAK LOW LEVEL CONVERGENCE AND SLUGGISHNESS OF THE
DOWNSTREAM ENVIRONMENT TO DESTABILIZE AM GROWING MORE SKEPTICAL THAT
THESE STORMS WILL BECOME SEVERE. STILL...GIVEN THE WELL DEVELOPED
NATURE OF THE MCV AND ITS ABILITY TO SURVIVE THUS FAR IT DOES
REQUIRE MONITORING. VERY LARGE T/TD SPREADS OUT AHEAD OF THE
CONVECTION HAVE GREATLY REDUCED THE TORNADO THREAT AND WILL BE
UPDATING THE HWO TO REFLECT THIS THINKING SHORTLY.

IZZI

&&

.DISCUSSION...
249 AM CDT

TODAY THROUGH TONIGHT...
MAIN CONCERN FOR THE FIRST COUPLE PERIODS OF THE FORECAST RESIDE
WITH CONVECTIVE CHANCES/TIMING AND INTENSITY. FIRST THINGS
FIRST...SPC HAS ALL OF NORTHERN IL/NORTHWEST IN OUTLOOKED FOR A
SLIGHT RISK OF SEVERE WEATHER TODAY. THE MAIN HAZARDS TODAY WILL BE
DAMAGING WINDS/1" HAIL...AND HAS BEEN THE CASE TO OUR WEST...A
TORNADO RISK. THE TIMING FOR SEVERE WEATHER DEVELOPMENT APPEARS TO
BE FOCUSED IN THE AFTERNOON/EVENING TIME FRAME...POSSIBLY CONTINUING
INTO THE OVERNIGHT HOURS.

EARLY THIS MORNING A DECAYING COMPLEX OF THUNDERSTORMS WAS LIFTING
E/NE ACROSS CENTRAL WISC STRETCHING SOUTH INTO FAR NORTH CENTRAL IL.
CLOUD TOPS HAVE BEEN SHOWING SOME WARMING...AND EXPECT AROUND
SUNRISE OR JUST AFTER...CLOUDS WILL BEGIN WARMING AT A FASTER RATE.
A TONGUE OF UNSEASONABLY WARM/MOIST AIR CONTINUES TO ADVECT INTO THE
CWFA THIS MORNING. THIS CONVEYOR LIKE MECHANISM HAS ALLOWED DEW
POINTS TO RISE INTO THE UPR 60S...WITH A FEW SITES REACHING 70 DEG
DEW POINTS. FORTUNATELY SOME DRYING WILL TAKE PLACE BEFORE SUNRISE
AND DEW POINTS SHUD LOWER BACK INTO THE MID/UPR 60S. WITH
CONSIDERABLE DEBRIS CLOUDS FLOATING OVERHEAD THIS MORNING...TEMPS
MAY NOT COOL MUCH. THIS WILL ALLOW THE DAY TO START VERY MILD IN THE
UPR 60S TO ARND 70 DEGREES.

IT APPEARS THE FIRST HALF OF TODAY WILL BEGIN RATHER QUIET...AS A
WEAK CAP IS PROGGED TO DEVELOP AND LIMIT THE VERTICAL GROWTH
POTENTIAL OF PARCELS. HOWEVER...THIS CAP AS NOTED WILL BE WEAK AND
WILL QUICKLY DISSOLVE BY 16-18Z...OPENING THE GATES FOR STRONG
DESTABILIZATION TO TAKE PLACE. WITH THE 500MB MID-LVL VORT REMAINING
MORE NEGATIVELY TILTED...THIS HAS A TENDENCY TO BE AN INDICATOR IF
THE ENVIRONMENT IS UNSTABLE...TO LEAD TO SEVERE WEATHER POTENTIAL.
GIVEN THE PAST COUPLE OF DAYS HAVE BEEN VERY ACTIVE WITH SEVERE
WEATHER ACROSS THE CENTRAL/SOUTHERN PLAINS...THAT AIR MASS AND
INGREDIENTS WILL BE OVER THE OZARKS STRETCHING NORTH THROUGH IL INTO
SOUTHEAST WISC MON. THIS COUPLED WITH SUBSTANTIAL ENERGY AT GREATER
THAN 2000J/KG...PWATS AT 1.5-2.0"...AND DECENT DIRECTIONAL SHEAR IN
THE 0-6KM LAYER...SUGGEST CONVECTIVE POTENTIAL WILL BE PRESENT. THE
CHALLENGE IS TRYING TO FIND THE TRIGGER. WITH THE EARLIER CONVECTION
THIS MORNING...SEVERAL OUTFLOW BOUNDARIES HAVE PUSHED OVER THE CWFA.
EXPECT THIS TO SERVE AS A FOCAL POINT FOR CONVECTIVE AREAS LATER
TDY. EARLIER GUIDANCE WAS HINTING THAT A MESO LOW WOULD LIFT INTO
CENTRAL IL LATE IN THE AFTN...WHICH WOULD MOST LIKELY BE THE
MECHANISM NEEDED. IN ADDITION TO THIS SCENARIO IS THAT SFC WINDS MAY
BEGIN TO BACK FURTHER TO THE SE...WHICH WILL ENHANCE THE LVL
HELICITY. WHICH SUGGESTS THE POTENTIAL FOR ISOLATED ROTATING
UPDRAFTS...THAT COULD LEAD TO TORNADIC DEVELOPMENT OF SOME OF THE
STRONGER THUNDERSTORMS.

TEMPERATURES TDY ARE ALSO A CONCERN...AS DEBRIS CLOUDS/CIRRUS SHIELD
MAY TAKE TIME TO BURN OFF. AT THIS POINT HAVE MAINTAINED TEMPS
WARMING INTO THE UPR 80S TO NEAR 90...ALTHOUGH IF THE CLOUD SHIELD
IS SLUGGISH TO DEPART...THIS WOULD MOST LIKELY RESULT IN COOLER
TEMPS. GUIDANCE HAS BEEN LEANING IN THE COOLER DIRECTION...HOWEVER
CONFIDENCE REMAINS RELATIVELY HIGH THAT TEMPS WILL EASILY REBOUND
PRIOR TO CONVECTION INTO THE UPR 80S.

ONGOING CONVECTION THIS EVENING WILL MOST LIKELY CONTINUE INTO THE
LATE EVENING HOURS. THE MOIST TONGUE/AXIS WILL BECOME ORIENTED FROM
SOUTHERN IL STRETCHING NORTH/NORTHEAST THROUGH THE CWFA INTO LOWER
MICHIGAN. THIS WILL MOST LIKELY BE THE CONTINUED CHANNEL FOR
MAINTENANCE OF CONVECTION. PWATS HOVER ARND 1.5" TONIGHT. EXPECT
ANOTHER MILD NIGHT...WITH TEMPS HOLDING IN THE UPR 60S TO NEAR 70
DEG.

CONFIDENCE IN SEVERE WEATHER TIMING...MEDIUM.

TUESDAY THROUGH WEDNESDAY...
500MB TROUGH AXIS WILL CONTINUE TO PIVOT EAST TOWARDS THE
MID-MISSISSIPPI VALLEY TUE. GUIDANCE CONTINUES TO SUGGEST THAT A FEW
WEAK WAVES OF ENERGY WILL EJECT NORTHEAST ACROSS THE CWFA...WHICH
MAY AID IN KEEPING A SEVERE WEATHER THREAT INTO TUE. THE SPC
CONTINUES TO HAVE THE CWFA OUTLOOKED FOR THE DAY 2 IN A SLIGHT RISK
FOR SEVERE WEATHER. THE CHALLENGE WILL BE SFC DESTABILIZATION...WITH
CONSIDERABLE CLOUD COVER EXPECTED TO LINGER FROM OVERNIGHT MON
NGT/EARLY TUE MORNING CONVECTION AND DEBRIS CLOUDS. LLVL NOCTURNAL
JET WILL PERSIST INTO TUE MORNING AT 30-40KT...HOWEVER IT APPEARS
WINDS WILL BE MORE UNIFORM. THUS THE MAIN CONCERNS WOULD BE FOR WIND
DAMAGE AND HAIL. TEMPS ALOFT WILL BE COOLING AS THE THERMAL RIDGE
PUSHES EAST TOWARDS THE MID-ATLANTIC REGION TUE AFTN/EVE.

TEMPS TUE WILL WARM TO ARND 80 DEG...PSBLY THE LOW 80S FOR THE FAR
EASTERN CWFA. THEN FOR WED CONVECTIVE CHANCES WILL BE SLOWLY ENDING
FROM THE WEST TO EAST...AS THE TROUGH AXIS FINALLY ARRIVES OVERHEAD.
TEMPS WILL BE NOTICEABLY COOLER IN THE LOW/MID 70S...WITH SCT
SHOWERS. HOWEVER AS MENTIONED ABOVE...THERE WILL BE A CHANCE FOR
CONVECTION IN THE FIRST HALF OF THE DAY WED. IF ANY THINNING OF THE
CLOUD COVER CAN OCCUR WED AFTN...LAPSE RATES MAY QUICKLY STEEPEN AND
LLVL DESTABILIZATION COULD ALLOW FOR AN ISOLATED LOW TOPPED
CONVECTION WED AFTN.

CONFIDENCE...MEDIUM.

THURSDAY THROUGH SUNDAY...
THE EXTENDED PERIODS WILL FEATURE A BRIEF RETURN TO UNSEASONABLE
COOL TEMPS FOR THE END OF THE WEEK. ENSEMBLES CONTINUE TO PROG A
DOWNSTREAM RIDGE ACROSS THE WESTERN ATLANTIC...WHICH SHOULD SLOW THE
EASTWARD PROGRESSION OF THE 500MB SHORTWAVE. THIS WILL MOST LIKELY
RESULT IN ALLOWING MUCH COOLER AIR TO SLIDE SOUTH FROM CENTRAL
CANADA INTO THE CWFA LATE IN THE WEEK. MEANWHILE UPSTREAM...A
MID-LVL RIDGE WILL BEGIN TO AMPLIFY. THIS WILL PROVIDE SOME
MAINTENANCE TO THE SETUP OF COOLER THAN NORMAL TEMPS FOR THE GREAT
LAKES REGION. ANTI-CYCLONIC FLOW SHUD ARRIVE IN THE LLVLS
THUR/FRI...ALLOWING FOR DRY WEATHER BOTH OF THESE DAYS. THIS SFC
RIDGE WILL THEN SLIDE EAST AS LLVL FLOW BEGINS TO TURN SOUTHEASTERLY
SAT...AND THE NEXT CHC FOR SHOWERS/THUNDERSTORMS ARRIVE FOR THE
SECOND HALF OF THE HOLIDAY WEEKEND. TEMPS THUR/FRI WILL STRUGGLE TO
REACH THE MID/UPR 60S...AND FOR AREAS ADJACENT TO LAKE
MICHIGAN...TEMPS MAY ACTUALLY STRUGGLE TO REACH THE UPR 50S AS THE
COOLER MARINE ENVIRONMENT FLOWS INLAND FROM A NORTHEAST WIND. THEN
TEMPS WILL BEGIN TO WARM FOR THE WEEKEND INTO THE UPR 60S TO NEAR 70
DEG.

CONFIDENCE...MEDIUM/HIGH.

BEACHLER

&&

.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...
345 AM CDT

LOW PRESSURE WILL REMAIN NEARLY STATIONARY OVER THE EASTERN
DAKOTAS AND WEST CENTRAL MINNESOTA TODAY WHILE A WARM FRONT
CONTINUES NORTHWARD FROM CENTRAL LAKE MICHIGAN TO NORTHERN LAKE
MICHIGAN WHERE IT WILL BECOME STATIONARY BY THIS EVENING. THIS
WILL MAINTAIN EAST TO SOUTHEAST WINDS ON THE FAR NORTHERN PORTION
OF THE LAKE WITH SOUTHERLY WINDS ON THE CENTRAL AND SOUTHERN
PORTIONS. THE LOW WILL MOVE SLOWLY EAST AND GRADUALLY WEAKEN AS IT
CROSSES THE UPPER MIDWEST TUESDAY AND TUESDAY NIGHT...AND LAKE
MICHIGAN WEDNESDAY MORNING. WINDS TURN TO OUT OF THE NORTHEAST AND
NORTH BEHIND THE DEPARTING LOW. AS A LARGE AREA OF HIGH PRESSURE
GRADUALLY BUILDS SOUTH-SOUTHEAST ACROSS SOUTH CENTRAL CANADA...
THE UPPER MIDWEST AND THE MISSISSIPPI VALLEY DURING WEDNESDAY
THROUGH THURSDAY THE NORTH WINDS WILL FRESHEN SIGNIFICANTLY. WINDS
GRADUALLY DIMINISH THURSDAY NIGHT AND FRIDAY AS THE HIGH PRESSURE
RIDGE DRIFTS EAST TO WESTERN ONTARIO THE UPPER MIDWEST AND THE
MISSISSIPPI VALLEY.

TRS

&&

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

&&

$$

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