215
FXUS63 KLOT 190146
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
846 PM CDT MON MAY 18 2026
KEY MESSAGES
- A PERIOD OF SHOWERS AND EMBEDDED THUNDERSTORMS IS EXPECTED
AFTER MIDNIGHT TONIGHT THROUGH EARLY TUESDAY MORNING, SOME OF
WHICH COULD BE LOCALLY STRONG, MAINLY WEST OF I-55.
- ADDITIONAL THUNDERSTORMS MAY REDEVELOP TUESDAY AFTERNOON,
MAINLY EAST OF I-55, SOME OF WHICH COULD BE STRONG TO SEVERE.
- SUMMER-LIKE WARMTH AND BREEZY WINDS WILL PREVAIL THROUGH
TUESDAY, THEN COOLER TEMPERATURES AND DRY CONDITIONS WILL
RETURN FOR THE MIDDLE OF THE WEEK.
- MILDER WEATHER RETURNS HEADING INTO THE WEEKEND WITH
INTERMITTENT SHOWER/THUNDERSTORM POTENTIAL.
UPDATE
ISSUED AT 846 PM CDT MON MAY 18 2026
A ROBUST LINE OF CONVECTION FROM WICHITA, KANSAS TO DES MOINES,
IOWA, WHICH CREATED QUITE THE SEVERE EVENT IN THE CENTRAL
PLAINES THIS EVENING, IS SLOWLY MOVING EAST. A STRONG
INSTABILITY GRADIENT FROM NORTHWEST MISSOURI TOWARD ST. LOUIS
SHOULD FOCUS MOST OF THE STRONGEST PARTS OF THE LINE'S MOVEMENT
THROUGH THE OVERNIGHT SOUTH OF THE FORECAST AREA TOWARD CENTRAL
ILLINOIS. HOWEVER, A PLUME OF BETTER LAPSE RATES INTO CENTRAL
IOWA WILL SLOWLY MEANDER EAST AHEAD OF THE LINE OF STORMS.
OUTFLOW FROM THE STORMS SHOULD PROVIDE SUFFICIENT FORCING, BUT
BEING IN A DIURNALLY UNFAVORABLE TIME OF DAY, THE LAPSE RATES
SHOULD START TO WEAKEN BY THE TIME IT APPROACHES THE ROCKFORD
METRO AFTER MIDNIGHT, BUT CONFIDENCE LOWERS IN COVERAGE TOWARD
THE CITY OF CHICAGO AS IT STARTS TO FALL APART AWAY FROM THE
BETTER FORCING.
MOST OF THE FORECAST AREA WILL BE DRY TOMORROW MORNING, BUT
LINGERING SHOWERS MAY CROP UP. THE COLD FRONT WILL FINALLY MAKE
IT TO THE FORECAST AREA AND TURN WINDS FROM THE SOUTHWEST TO
THE WEST AND EVENTUALLY NORTHWEST BEHIND THE FRONT. ADDITIONAL
SHOWERS AND STORMS ARE POSSIBLE ALONG THE FRONT, WITH THE BEST
TIME BEING IN THE AFTERNOON. BUT BY THAT TIME, THE FRONT COULD
ALREADY BE AROUND AND EAST OF INTERSTATE 55. STORMS ALONG THE
FRONT IN THE AFTERNOON COULD BE STRONG TO EVEN SEVERE WITH
DAMAGING WIND AND HAIL THE PRIMARY SEVERE THREATS.
FORECAST UPDATES THIS EVENING WERE FOCUSED ON THE NEAR-TERM
TIMING OF SHOWER AND THUNDER AS WELL AS FOR TOMORROW AFTERNOON.
DK
DISCUSSION
ISSUED AT 244 PM CDT MON MAY 18 2026
THROUGH TONIGHT...
CONVECTION AND ASSOCIATED MCV RESPONSIBLE FOR THIS MORNING'S
SEVERE WEATHER HAS MOVED INTO LOWER MICHIGAN. AIR MASS IN THE WAKE
OF THIS WAVE IS SUBSTANTIALLY MORE STABLE AND COMBINED WITH MODEST
HEIGHT RISES IN ITS WAKE SHOULD PREVENT ANY RENEWED CONVECTIVE
DEVELOPMENT THIS AFTERNOON OR EVENING.
SEVERE CONVECTION CURRENTLY DEVELOPING OVER THE CENTRAL PLAINS
IS LIKELY TO CONGEAL INTO A LARGE QLCS THAT WILL MOVE TOWARD THE
MID-MISSISSIPPI VALLEY TONIGHT. MOST INTENSE CONVECTION IS LIKELY
TO FOCUS FARTHER SOUTHWEST ACROSS MISSOURI, LIKELY INTERCEPTING
THE LOW LEVEL JET AND ORPHANING THE CONVECTION FARTHER NORTH
ACROSS IOWA. THIS SHOULD RESULT IN CONVECTION OVER IOWA
DIMINISHING AS IT MOVES EASTWARD INTO NORTHERN IL WITH SOME
GUIDANCE SUGGESTING VERY LITTLE SHOWER ACTIVITY WOULD ACTUALLY
MAKE IT INTO OUR CWA. HAVE REFLECTED THIS TREND IN POP GRIDS, BUT
OPTED TO MAINTAIN AT LEAST CHANCE POPS GIVEN THE INHERENT
UNCERTAINTIES IN FORECASTING CONVECTIVE EVOLUTION.
PRESSURE FIELD IS RECOVERING IN THE WAKE OF THE MORNING CONVECTION
WHICH HAS ALLOWED WINDS TO PICK AND BECOME RATHER GUSTY FROM THE
SOUTHWEST. GUSTINESS OF THE WINDS WILL GENERALLY TEND TO ABATE BY
SUNSET, EXCEPT PERHAPS REMAINING SOMEWHAT GUSTY TONIGHT IN THE
URBAN HEAT ISLAND OF CHICAGO.
-IZZI
TUESDAY...
CONVECTIVELY INDUCED VORTICITY MAXIMUM WILL SLIDE THROUGH NORTHEAST
ILLINOIS INTO EARLY TUESDAY, WELL OUTPACING THE COLD FRONT, AND
THEREFORE WE MAY HAVE SOME SHOWERS HANGING ON INTO THE AREA, BUT
CONFIDENCE IS MEDIUM-HIGH THAT THUNDERSTORM COVERAGE WILL BE ON THE
LOWER SIDE. ANY LINGERING MCS COLD POOL FOOTPRINT AND RESIDUAL
CLOUDINESS WILL LIKELY DELAY DESTABILIZATION INTO MIDDAY TUESDAY,
THOUGH ADDITIONAL STORM DEVELOPMENT APPEARS SOMEWHAT PROBABLE ACROSS
THE SOUTHEAST HALF OF THE FORECAST AREA TUESDAY AFTERNOON/EVENING AS
THE COLD FRONT FINALLY PUSHES THROUGH THE FORECAST AREA.
SOUTHWESTERLY FLOW AHEAD OF THE FRONT WILL MAINTAIN MID TO POSSIBLY
UPPER 60S DEWPOINTS AND SUPPORT MLCAPE APPROACHING 1500 J/KG.
STRONGER MID-LEVEL FLOW ON THE SOUTHEASTERN PERIPHERY OF THE MODEL
GUIDANCE WOULD SUPPORT SOME ADDITIONAL VORTICITY MAXIMUM RIDING
ALONG THE FRONT, AND FLOW TO THE SOUTHEAST OF THE MID LEVEL TROUGH
ACROSS THE NORTHERN GREAT LAKES WILL LIKELY SUPPORT ENOUGH STORM
DEVELOPMENT AND SOME ORGANIZATION FOR A HAIL/WIND THREAT DURING
THIS TIME. THIS WOULD BE FAVORED ALONG AND SOUTHEAST OF I-55,
WITH SHORT TERM ENSEMBLE GUIDANCE SUGGESTING THE WINDOW FOR MUCH
OF NE IL AND NW IN BEING FAIRLY LIMITED IN THE MID TO LATE
AFTERNOON. FOR THE CHICAGO METRO AREA THE PICTURE IS ALSO A BIT
MUDDLED FOR PRECIPITATION CHANCES. HIGH RES GUIDANCE IS MOSTLY
DRY FOR THE AREA, WHEREAS GLOBAL GUIDANCE AND NAM/RAP DOES BRING
SOME SHOWER ACTIVITY ACROSS CHICAGOLAND ALONG THE FRONT IN THE
EARLY AFTERNOON.
KMD
WEDNESDAY ONWARD...
FOLLOWING THE COLD FRONT, SURFACE HIGH PRESSURE IS PROGGED TO
BUILD EAST INTO THE WESTERN GREAT LAKES REGION WEDNESDAY AND
THURSDAY. DRY, BUT BREEZY AND COOLER WEATHER IS EXPECTED BOTH
DAYS, WITH TEMPS MAINLY IN THE 60S, BUT ONLY IN THE 50S NEAR
LAKE MICHIGAN WITH NORTHEAST-EAST FLOW OFF THE LAKE. NIGHTTIME
LOWS IN THE 40S ARE EXPECTED. THE HIGH THEN DRIFTS OFF TO THE
EAST THURSDAY NIGHT, WITH THE UPPER LEVEL PATTERN FAVORING A
SERIES OF MID-LEVEL DISTURBANCES TRACKING THROUGH THE AREA
FRIDAY INTO THE WEEKEND. THIS WILL SUPPORT MILDER TEMPERATURES,
BUT ALSO SOME OCCASIONAL SHOWER/THUNDERSTORM CHANCES INTO THE
HOLIDAY WEEKEND.
RATZER
AVIATION /00Z TAFS THROUGH 06Z WEDNESDAY/
ISSUED AT 656 PM CDT MON MAY 18 2026
KEY AVIATION FORECAST MESSAGES:
- DECAYING LINE OF SHRA/TSRA MAY MOVE THROUGH OVERNIGHT INTO
EARLY TUESDAY AM (>30% CHANCE).
- BREEZY SSW WINDS TURN W TUESDAY AM THEN NW BY THE EVENING.
MADE SOME TIMING ADJUSTMENTS TO THE OVERNIGHT SHOWERS AND
THUNDERSTORM POTENTIAL BASED ON LATEST MODEL GUIDANCE AND
UPSTREAM STORM EVOLUTION. MAIN CHANGE WAS TO SPEED UP THE PROB30
START TIME BY A COUPLE HOURS FOR ALL SITES, NOW 6Z RFD, 7Z DPA,
8Z ORD/MDW, AND 9Z GYY. THIS IS MAINLY TIED TO THE LINE OF
STORMS CURRENTLY RACING EAST ACROSS SOUTHERN IOWA TOWARD THE
AREA. THERE REMAINS A WEAK SIGNAL FOR ISOLATED THUNDERSTORMS
DEVELOPING AHEAD OF THIS FEATURE BETWEEN ROUGHLY ~4-8Z, HOWEVER,
CONFIDENCE IN THIS OCCURRING REMAINS LOW OWING TO ONLY VERY
SUBTLE FORCING MECHANISMS FOR LIFT WITHIN THE INCREASING
ELEVATED INSTABILITY (<30% CHANCE).
THE POTENTIAL FOR THUNDER IS LOCALLY HIGHEST TOWARD RFD WHERE
TSRA HAS BEEN MAINTAINED IN THE PROB30. CAN'T RULE OUT TS AS
FAR EAST AS THE CHICAGO TERMINALS BUT THE LINE WILL LIKELY BE
DECAYING EVEN FURTHER WITH EASTWARD EXTENT.
A SECOND ROUND OF SHOWERS AND THUNDERSTORMS IS EXPECTED DURING
THE AFTERNOON ON TUESDAY BUT THIS ACTIVITY SHOULD REMAIN
SOUTHEAST OF THE TERMINALS IN AREAS MAINLY EAST OF I-55 AND
SOUTH OF I-80.
WIND GUSTS THIS EVENING MAY SUBSIDE BRIEFLY WITH SUNSET BUT ARE
EXPECTED TO REMAIN AT LEAST SPORADICALLY GUSTY THROUGH THE REST
OF THE OVERNIGHT HOURS AROUND 20-25 KT OUT OF THE SSW. WINDS
THEN TREND WESTERLY BEHIND THE EARLY AM SHOWERS AND THEN
NORTHWEST LATE AFTERNOON/EARLY EVENING.
A PERIOD OF MVFR CIGS ARE POSSIBLE IN THE WAKE OF THE AM
SHOWERS/STORMS AND ASSOCIATED COLD FRONT INTO THE AFTERNOON.
HIGHEST COVERAGE IS FAVORED OUT TOWARD RFD AND THEY MAY EXPAND
EAST INTO THE METRO TOWARD MIDDAY, THOUGH CONFIDENCE IS NOT HIGH
ENOUGH TO INCLUDE IN THE CHICAGO TAFS WITH THIS UPDATE.
PETR
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY FROM 6 AM TO 4 PM CDT TUESDAY FOR THE IL
AND IN NEARSHORE WATERS.
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