255
FXUS63 KLOT 190845
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
345 AM CDT TUE MAY 19 2026
KEY MESSAGES
- SCATTERED THUNDERSTORMS DIMINISH EARLY THIS MORNING ACROSS
NORTHERN IL. THUNDERSTORM CHANCES THEN INCREASE AGAIN THIS
AFTERNOON, MAINLY SOUTHEAST OF A PERU IL TO VALPARAISO IN
LINE. A FEW STRONG STORMS ARE POSSIBLE.
- SUMMER-LIKE WARMTH AND BREEZY WINDS TODAY WILL GIVE WAY TO
COOLER AND DRY CONDITIONS FOR THE MIDDLE OF THE WEEK.
- INTERMITTENT SHOWER/THUNDERSTORM CHANCES RETURN LATE IN THE
WEEK AND INTO THE WEEKEND, THOUGH WITH A RETURN OF WARMER
WEATHER BY MEMORIAL DAY.
DISCUSSION
ISSUED AT 345 AM CDT TUE MAY 19 2026
EARLY MORNING SURFACE ANALYSIS PLACES 1005 MB LOW OVER
NORTHWEST WISCONSIN, WITH A COLD FRONT TRAILING THROUGH CENTRAL
IOWA INTO SOUTHEAST KANSAS. STRONG/SEVERE THUNDERSTORMS THAT HAD
DEVELOPED MONDAY AFTERNOON AHEAD OF THE FRONT ACROSS THE MID-
MISSOURI VALLEY AND ELSEWHERE AHEAD OF THE BOUNDARY, HAVE SINCE
EVOLVED INTO ANOTHER LINEAR MCS, WHICH CURRENTLY STRETCHES ALL
THE WAY FROM THE CENTRAL UPPER PENINSULA OF MICHIGAN TO
NORTHWEST/WESTERN ILLINOIS, THEN SOUTHWEST TO OKLAHOMA. ALOFT,
GOES VAPOR IMAGERY INDICATES A DEEPENING MID-LEVEL SHORT WAVE
TROUGH PROPAGATING NORTHEASTWARD FROM THE NORTHERN/CENTRAL
PLAINS TOWARD THE NORTHERN GREAT LAKES. STRONG LARGE-SCALE
ASCENT, DEPICTED BY MID-LEVEL HEIGHT FALLS OF 60-80 METERS
ACROSS THE UPPER MISSISSIPPI VALLEY, IS AIDING IN MAINTAINING A
STRONG SOUTHWESTERLY LOW-LEVEL JET ACROSS OUR REGION, WHICH
LATEST RAP SOUNDINGS/MESOANALYSIS DEPICT AT 60 KTS ACROSS
WESTERN IL.
WHILE LOW-LEVELS HAVE DIURNALLY STABILIZED, THE STRONG LOW
LEVEL JET AND ASSOCIATED WARM/MOIST ADVECTION HAS MAINTAINED AN
ELEVATED MUCAPE AXIS OF 1500-2000 J/KG PER RAP SOUNDINGS. THIS,
COMBINED WITH LOW-LEVEL FORCING PROVIDED BY THE EASTWARD-
PROPAGATING OUTFLOW BOUNDARY ALONG THE LEADING EDGE OF THE
AFOREMENTIONED MCS HAS ALLOWED SCATTERED CONVECTIVE CELLS TO
CONTINUE TO INITIATE ATOP THE OUTFLOW BOUNDARY ACROSS WESTERN
AND NORTHWEST ILLINOIS. FORECAST SOUNDINGS DO INDICATE A
WEAKENING AND VEERING OF THE LOW-LEVEL JET OVER THE NEXT FEW
HOURS HOWEVER, ALONG WITH A DECREASE IN ELEVATED INSTABILITY.
COMBINED WITH THE STABLE BOUNDARY LAYER CONDITIONS, THIS SHOULD
CONTINUE TO RESULT IN WEAKENING TREND TO THE CONVECTION AS THE
OUTFLOW CONTINUES TO SHIFT EAST ACROSS NORTHERN IL THIS MORNING,
WHICH IS ALREADY EVIDENT IN RECENT RADAR AND LIGHTNING DATA.
STABLE OUTFLOW AND LINGERING SCATTERED MORNING SHOWERS, COMBINED
WITH A STREAM OF HIGH-LEVEL CLOUD COVER OFF OF STRONGER
CONVECTION WELL TO OUR SOUTHWEST SHOULD LIMIT LOCAL CONVECTIVE
REDEVELOPMENT FOR THE BALANCE OF THE MORNING.
THE SURFACE COLD FRONT IS PROGGED MOVE INTO THE FORECAST AREA
EARLY THIS AFTERNOON, PUSHING SOUTHEAST ACROSS THE AREA INTO
THIS EVENING. FORECAST SOUNDINGS ARE GENERALLY CAPPED ACROSS THE
AREA INITIALLY, THOUGH BECOME CONDITIONALLY UNSTABLE WITH
DIURNAL WARMING LATER THIS AFTERNOON AHEAD OF THE FRONT MAINLY
ACROSS THE SOUTHEAST HALF OF THE FORECAST AREA (AND ESPECIALLY
SOUTHEAST OF A PONTIAC-VALPARAISO LINE). GUIDANCE HAS GENERALLY
BACKED-OFF WITH COVERAGE OF AFTERNOON CONVECTION ACROSS OUR
AREA, THOUGH FRONTAL FORCING AND FAVORABLE DIURNAL TIMING
ESPECIALLY IN OUR FAR SOUTHEAST SHOULD SUPPORT AN INCREASING
THUNDERSTORM THREAT MID-LATE AFTERNOON. FORECAST 1000-1500 J/KG
MLCAPES AND UP TO 35 KTS OF DEEP-LAYER SHEAR (GENERALLY
PARALLEL TO THE FRONTAL ZONE) WOULD SUPPORT SOME STORM
ORGANIZATION WITH SOME WIND/HAIL POTENTIAL. NOTE SPC HAS MOVED
EARLIER SLIGHT RISK OUT OF THE WFO CHICAGO AREA WITH THE NEW
DAY 1 OUTLOOK, LEAVING A MARGINAL (LEVEL 1 OF 5) RISK IN OUR FAR
SOUTHEAST.
A FEW SHOWERS MAY LINGER ACROSS OUR SOUTHEASTERN TIER OF
COUNTIES THIS EVENING UNTIL THE FRONT EXITS THE REGION, WITH A
PERIOD OF DRY AND COOLER WEATHER THEN EXPECTED WEDNESDAY THROUGH
THURSDAY. AFTER SEVERAL RECENT DAYS OF HIGHS IN THE 70S AND
80S, DAYTIME TEMPERATURES IN THE 60S ARE EXPECTED MID-LATE WEEK
WITH 50S CLOSER TO LAKE MICHIGAN WHERE BREEZY NORTHEASTERLY
WINDS WILL PERSIST AROUND AN AREA OF HIGH PRESSURE BUILDING
ACROSS THE GREAT LAKES REGION. NIGHTTIME TEMPS BOTH WEDNESDAY
AND THURSDAY NIGHT WILL DIP INTO THE 40S IN MOST AREAS.
RAIN CHANCES THEN RETURN THURSDAY NIGHT INTO FRIDAY, AS
GUIDANCE DEPICTS AN AREA OF LOW PRESSURE TRACKING FROM MISSOURI
INTO ILLINOIS AND INDIANA IN ASSOCIATION WITH AN UPPER LEVEL
TROUGH TRANSITING THE REGION INTO THE WEEKEND. SOME DETAIL AND
TIMING DIFFERENCES REMAIN WITHIN THE GLOBAL GUIDANCE SUITE,
THOUGH RAIN CHANCES APPEAR TO BE HIGHEST ON FRIDAY, THEN MORE
SPOTTY FROM SATURDAY INTO MONDAY. IT DOES APPEAR THAT WARMER
TEMPERATURES WILL RETURN LATER IN THE WEEKEND, BUT LIKELY WITH
COOLER CONDITIONS NEAR LAKE MICHIGAN.
RATZER
AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/
ISSUED AT 103 AM CDT TUE MAY 19 2026
FORECAST CONCERNS FOR THE TERMINALS...
- A LINE OF SHOWERS AND EMBEDDED THUNDERSTORMS WILL MOVE ACROSS
THE TERMINALS LATE TONIGHT INTO EARLY TUESDAY MORNING.
- BREEZY SOUTHWEST WINDS TO DEVELOP TUESDAY MORNING BEHIND THE
RAIN AND PERSIST THROUGH THE AFTERNOON.
- PERIOD OF MVFR CEILINGS IS EXPECTED TUESDAY AFTERNOON BEHIND A
COLD FRONT, BUT SKIES SHOULD SCATTER BACK TO VFR AROUND
SUNSET.
A LINE OF SHOWERS AND EMBEDDED THUNDERSTORMS IS PROGRESSING
EASTWARD ACROSS THE MISSISSIPPI RIVER AND WILL BE ENCROACHING
UPON THE TERMINALS OVER THE NEXT 1-3 HOURS (EARLIEST AT RFD,
LATEST CHICAGO AREA SITES). WHILE THE LINE IS EXPECTED TO BE
GRADUALLY WEAKENING AS IT MOVES THROUGH, RECENT AMDAR SOUNDINGS
OUT OF MDW DO SHOW SOME DEVELOPING MID-LEVEL INSTABILITY WHICH
MAY ALLOW FOR A COUPLE OF LIGHTNING STRIKES TO OCCUR. DESPITE
CONFIDENCE IN THUNDER ACTUALLY REACHING THE CHICAGO TERMINALS
BEING SOMEWHAT LOW (~20% CHANCE) HAVE OPTED TO INTRODUCE A
PROB30 FOR TSRA AT DPA, ORD, AND MDW AS A PRECAUTION GIVE THE
AFOREMENTIONED INSTABILITY AND LACK OF BROAD WEAKENING IN THE
LINE SO FAR. THAT SAID, STILL THINK THAT THE MOST LIKELY
SCENARIO AT THE CHICAGO SITES (ESPECIALLY GYY) IS FOR SOME
ISOLATED TO WIDELY SCATTERED SHOWERS TO OCCUR.
THE LINE OF SHOWERS/STORMS SHOULD DIMINISH BY DAYBREAK AND
RESULT IN A MOSTLY DRY MORNING. WINDS WILL BE INCREASING WITH
SOUTHWESTERLY GUSTS PEAKING AROUND 25-30 KTS THROUGH EARLY
TUESDAY AFTERNOON, BUT LOCALLY HIGHER GUSTS COULD MATERIALIZE
IF DEEPER MIXING IS ACHIEVED. AS THE MAIN COLD FRONT APPROACHES
TUESDAY AFTERNOON IT WILL BRING WITH IT A PERIOD OF MVFR
CEILINGS AND PERHAPS AN ISOLATED SHOWER OR TWO THOUGH THE
GREATEST SHOWER/STORM COVERAGE SHOULD ESTABLISH MORE INTO
NORTHERN AND CENTRAL IN. WITH THE INCREASING CLOUD COVER, GUSTS
SHOULD EASE A BIT INTO THE LOWER 20 KT RANGE THROUGH THE REST OF
THE AFTERNOON. DIRECTIONS DURING THIS TIME WILL ALSO TRANSITION
FROM SOUTHWEST TO WEST-NORTHWEST BEFORE TURNING NORTHWESTERLY
BEHIND THE COLD FRONT EARLY TUESDAY EVENING. WHILE A FEW UPPER
TEEN TO LOWER 20 KT GUSTS CANNOT BE RULED OUT TUESDAY NIGHT, IT
SEEMS THEY FREQUENCY OF GUSTS SHOULD BE MORE SPORADIC SO HAVE
OPTED TO REMOVE GUSTS FROM THE TAFS FROM THIS POINT ONWARD.
IN THE WAKE OF THE COLD FRONT, CEILINGS SHOULD RISE BACK TO VFR
TUESDAY EVENING AND REMAIN AS SUCH FOR THE REST OF THE FORECAST
PERIOD. LASTLY WINDS WILL FINALLY SETTLE INTO A NORTHEAST
DIRECTION EARLY WEDNESDAY MORNING.
YACK
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY UNTIL 4 PM CDT THIS AFTERNOON FOR THE IL
AND IN NEARSHORE WATERS.
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