318
FXUS63 KLOT 160605
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
105 AM CDT SAT MAY 16 2026
KEY MESSAGES
- LOCALIZED DAMAGING WIND GUSTS POSSIBLE WITH A WEAKENING LINE
OF STORMS EARLY THIS MORNING NEAR THE WI-IL STATE LINE.
- SOME ISOLATED STORM CHANCES EXIST TODAY THROUGH SUNDAY
MORNING, THOUGH MUCH OF THE TIME COULD END UP DRY.
- EXPECT MORE WIDESPREAD SHOWERS AND AT LEAST SCATTERED STORMS
AT TIMES MONDAY-TUESDAY, SOME OF WHICH MAY BE SEVERE.
- SUMMER-LIKE WARMTH AND HUMIDITY IS IN STORE THIS WEEKEND
THROUGH EARLY NEXT WORKWEEK.
MESOSCALE
ISSUED AT 105 AM CDT SAT MAY 16 2026
WE ARE CLOSELY MONITORING A LINE OF THUNDERSTORMS WHICH CURRENTLY
EXTENDS FROM SOUTHWEST IOWA NORTHEAST INTO SOUTHERN WISCONSIN. THE
APEX OF THIS LINE IS PUSHING EAST ACROSS SOUTHERN WISCONSIN IN AN
ENVIRONMENT FEATURING STRONG DEEP LAYER SHEAR (50+ KTS) WITH A POOL
OF ELEVATED INSTABILITY IN PLACE (MUCAPE OF 1500-2000 J/KG). THE
FOCUS FOR THE STRONGEST WINDS WILL FAVOR NORTH TO SOUTH ORIENTED
PORTIONS OF THE LINE OF STORMS (PERPENDICULAR TO THE SHEAR VECTOR)
WHICH COULD IMPACT PORTIONS OF FAR NORTHERN ILLINOIS ALONG THE
WISCONSIN-ILLINOIS STATE LINE WHERE A SEVERE THUNDERSTORM WATCH
REMAINS IN EFFECT. HOWEVER, IT APPEARS THAT THE SEVERE WIND
GUSTS COULD END UP REMAINING *JUST* NORTH OF THE STATE LINE
WITH THE LINE ORIENTATION NOT AS FAVORABLE INTO OUR AREA.
ADDITIONAL STORMS HAVE DEVELOPED AHEAD OF THIS LINE ACROSS NORTHERN
ILLINOIS ASSOCIATED WITH A REMNANT MCV THAT RECENTLY MOVED OUT OF
IOWA. CAN'T RULE OUT SMALL HAIL WITH THIS ACTIVITY OVER THE NEXT
HOUR OR SO BEFORE IT FULLY LIFTS NORTH INTO WISCONSIN.
AS THE STORMS PROGRESS FARTHER SOUTHEAST ACROSS MORE OF
ILLINOIS THEY WILL ENCOUNTER A LESS FAVORABLE ENVIRONMENT FOR
STRONG TO SEVERE STORM MAINTENANCE AND STEADILY WEAKEN WITH TIME
(AFTER AN INITIAL GUSTY WIND SHIFT) WITH ADDITIONAL WAVES OF
SHOWERS AND STORMS POSSIBLE ACROSS THE AREA THROUGH THE REST OF
THE PRE-DAWN HOURS.
PETR
DISCUSSION
ISSUED AT 251 PM CDT FRI MAY 15 2026
THROUGH TONIGHT:
INTO THIS EVENING, IT WILL BE BREEZY AND PLEASANT WITH
TEMPERATURES GRADUALLY FALLING FROM THE 70S INTO THE 60S.
OUR MAIN FOCUS TONIGHT IS THE LIKELIHOOD OF A WEAKENING MCS
(SHOWERS AND EMBEDDED STORMS) TO MOVE ACROSS THE AREA OVERNIGHT
TONIGHT, WITH GUSTY OUTFLOW ON ITS LEADING EDGE. THUNDERSTORMS
INITIATING BY EVENING FROM THE OMAHA AREA TO WESTERN WISCONSIN
WILL LIKELY EVOLVE INTO ONE OR MORE LINEAR CLUSTERS. GIVEN THE
DRY AND MIXED PROFILES TODAY, NOT EXPECTING ANY SURFACE BASED
INSTABILITY OVERNIGHT. ON THE OTHER HAND, A MUCAPE RESERVOIR OF
UP TO 1500-2000 J/KG WILL EXPAND ACROSS THE MS RIVER IN TANDEM
WITH A STOUT EML, WHICH MAY MAINTAIN SUFFICIENT ORGANIZATION IN
THE MCS (ALBEIT STILL ON AN OVERALL WEAKENING TREND DUE TO
UNFAVORABLE DIURNAL TIMING) TO YIELD SOME STRONG TO LOCALLY
DAMAGING WIND GUSTS ON THE LEADING EDGE OUTFLOW BOUNDARY. THE
BEST CHANCE FOR THIS APPEARS TO BE IN THE NORTHWEST 1/3 OR SO OF
THE CWA (IE. NEAR AND WEST OF A WAUKEGAN TO PERU IL LINE). A
RELATIVELY GREATER WIND THREAT MAY PRESENT ITSELF JUST NORTH OF
THE IL/WI STATE LINE DUE TO THE WEST-SOUTHWESTERLY FLOW
ORIENTATION ALOFT.
SINCE THE COMPLEX WILL BE ROOTED ABOVE THE BOUNDARY LAYER,
EFFECTIVE BULK SHEAR WILL BE FAIRLY MODEST. CAN'T COMPLETELY
RULE OUT ISOLATED LARGE HAIL IN ANY MORE ROBUST CORES BEHIND THE
OUTFLOW IF EFFECTIVE SHEAR ENDS UP A BIT STRONGER THAN
ANTICIPATED.
SATURDAY THROUGH FRIDAY:
BY SATURDAY MORNING, THE PRIMARY MCS FROM TONIGHT IS EXPECTED
TO MOVE EAST OF THE AREA, THOUGH A COMBINED OUTFLOW/COLD FRONTAL
BOUNDARY IS FORECAST TO BECOME NEARLY STATIONARY ACROSS PROBABLY
SOUTHERN PORTIONS OF THE FORECAST AREA (THOUGH THE EXACT
POSITION IS OF LOW CONFIDENCE FROM CURRENT GUIDANCE RUNS). A
TRAILING WEAKER CONVECTIVELY MODULATED SHORT-WAVE AND LINGERING
WARM AIR ADVECTION/ASCENT MAY KEEP SCATTERED SHOWERS AND
ISOLATED EMBEDDED THUNDERSTORMS GOING PAST DAYBREAK INTO OUR
SOUTHEAST 1/3 TO 1/2 OF THE CWA OR SO.
INTO SATURDAY AFTERNOON, SUBSIDENCE IN THE WAKE OF THE MORNING
SHORT-WAVE(S) ENTAIL NEBULOUS AT BEST FORCING MECHANISMS. HAVE
MAINTAINED SOME CHANCE POPS (30-50%) MAINLY SOUTH OF I-80,
WHERE A BIT BETTER POTENTIAL OF ADDITIONAL DIURNAL
SHOWERS/ISOLATED STORMS WOULD MORE LIKELY BE FOCUSED NEAR/SOUTH
OF THE STALLED BOUNDARY. POTENTIAL FOR LINGERING CLOUD COVER
AND OUTFLOW-DISTURBED WIND FIELD COULD LIMIT HIGH TEMPERATURES
SOMEWHAT, THOUGH CURRENT MODEL LOW-LEVEL THERMAL FIELDS SUGGEST
AFTERNOON HIGHS FROM NEAR/AROUND 80F EAST/FAR SOUTH TO THE LOW-
MID 80S FARTHER NORTH/WEST WOULD BE ATTAINABLE WITH SOME
AFTERNOON SUNSHINE. AS AN ADDITIONAL MINOR NOTE ON TEMPERATURES,
A WEAK COLD FRONT MAY SLIP INTO FAR NORTHERN ILLINOIS AND ENABLE
AN ONSHORE WIND SHIFT AND MORE QUICKLY COOLING TEMPS TOWARDS
SUNSET NEAR THE FAR NORTHEAST ILLINOIS SHORE.
FORCING MECHANISMS FOR ANY ADDITIONAL CONVECTION REMAIN
NEBULOUS SATURDAY NIGHT, AS FORECAST MID-LEVEL HEIGHT RISES CAST
UNCERTAINTY ON ANY AREAS OF FOCUS. A NORTHEASTWARD PROPAGATING
SHORT-WAVE MAY CONCEIVABLY SUPPORT ISOLATED TO PERHAPS WIDELY
SCATTERED SHOWERS AND STORMS (20-40% POPS/HIGHEST NEAR AND WEST
OF I-39) AS THE STALLED FRONT/OUTFLOW BOUNDARY LIFTS BACK NORTH
AS A WARM FRONT. A BROAD LEVEL 1 OF 5 SEVERE THREAT IN THE
CURRENT OUTLOOK MAY BE A BIT AGGRESSIVE FOR THE PROGGED SETUP,
(DECREASING MID-LEVEL LAPSE RATES, SUB MARGINAL EFFECTIVE BULK
SHEAR, AND AN AGAIN UNFAVORABLE DIURNAL TIMING).
MOST RECENT GUIDANCE CONTINUES TO INDICATE BREEZY (30-35 MPH
GUSTS) SOUTH WINDS WILL DEVELOP SUNDAY AFTERNOON, AFTER THE WARM
FRONT LIFTS NORTH OF THE AREA. WITH ANY MORNING SHOWERS/STORMS
(FAVORING NEAR/NORTH OF I-88) DEPARTING AND DECREASING CLOUD
COVER IN THE WARM SECTOR, HIGH TEMPERATURES IN THE MID-UPPER 80S
APPEAR LIKELY AT THIS DISTANCE. THE EXCEPTION MAY BE NEAR THE
LAKE COUNTY IL SHORE, AS A JUST EAST OF SOUTH COMPONENT TO THE
WINDS COULD CAP HIGHS IN THE 70S LATER IN THE DAY. BREEZY AND
MILD CONDITIONS LOOK TO PERSIST SUNDAY NIGHT, WITH LOWS
GENERALLY IN THE MID TO UPPER 60S.
THUNDERSTORM CHANCES THEN RETURN MONDAY AND TUESDAY, AS A
CENTRAL PLAINS SURFACE LOW PRESSURE SLOWLY LIFTS NORTHEAST
THROUGH THE NORTHERN GREAT LAKES AND EVENTUALLY TRAILS A COLD
FRONT THROUGH THE FORECAST AREA. AHEAD OF THE COLD FRONT, THE
FORECAST AREA WILL RESIDE IN BREEZY WARM SOUTHERLY FLOW,
CHARACTERIZED BY DAYTIME TEMPERATURES IN THE 80S AND INCREASING
LOW-LEVEL MOISTURE RETURN (SURFACE DEW POINTS POTENTIALLY UPPER
60S-TO NEAR 70F) FROM THE WESTERN GULF. RESULTING INSTABILITY,
COMBINED WITH MID-LEVEL FLOW NEAR 40 KTS WOULD POTENTIALLY
SUPPORT STRONG/SEVERE THUNDERSTORMS (AS WELL AS SOME THREAT FOR
FLOODING GIVEN HIGH COLUMN MOISTURE), THOUGH FORCING MECHANISMS
REMAIN UNCLEAR WITHIN THE BROAD WARM SECTOR MAKING CONVECTIVE
TIMING/EVOLUTION OF LOW CONFIDENCE AT THIS DISTANCE. MONDAY'S
OVERALL MESSY LOOK ALSO LENDS UNCERTAINTY TO HOW MUCH
INSTABILITY CAN BE REALIZED AT PEAK HEATING, PARTICULARLY IF
MORNING ACTIVITY ENDS UP FAIRLY WIDESPREAD.
ON TUESDAY, IN THE WAKE OF PROBABLE CONTINUED OVERNIGHT/EARLY
DAY CONVECTION, THE AFOREMENTIONED COLD FRONT WILL APPROACH.
IT'S NOT UNCOMMON IN THESE SETUPS FOR RENEWED STORMS AND ANY
ASSOCIATED STRONG TO SEVERE THREAT AHEAD OF THE FRONT TO FOCUS
PRIMARILY NEAR/SOUTHEAST OF I-55, THOUGH TIME WILL TELL IN THIS
REGARD. FOR THE MONDAY-TUESDAY PERIOD, SPC DAYS 4-5 OUTLOOKS
CONTINUE TO EXTEND SEVERE PROBABILITIES INTO OUR AREA, WHICH
APPEAR REASONABLE. QUIET AND COOLER CONDITIONS ARE THEN
EXPECTED BY MID-WEEK, AS SURFACE HIGH PRESSURE BUILDS ACROSS
THE REGION IN THE WAKE OF THE TUESDAY COLD FRONTAL PASSAGE.
CASTRO/RATZER
AVIATION /06Z TAFS THROUGH 12Z SUNDAY/
ISSUED AT 1240 AM CDT SAT MAY 16 2026
AVIATION FORECAST CONCERNS FOR THE 06Z TAFS:
- PERIODS OF TSRA/SHRA THROUGH THE PRE-DAWN HOURS.
- SOMEWHAT LOW CONFIDENCE IN LATE AFTERNOON WIND DIRECTIONS,
THOUGH A TREND FROM SOUTH-SOUTHWEST TO THE SOUTHEAST IS
LIKELY BY EARLY EVENING.
- LOW CHANCE (<20%) FOR ISOLATED AFTERNOON SHRA/TSRA.
- GREATER (30-40%) CHANCE FOR TSRA LATER SATURDAY EVENING/NIGHT.
AN EASTWARD-MOVING LINE OF TSRA WAS ONGOING FROM SOUTH-CENTRAL
WI INTO CENTRAL IA AT ISSUANCE TIME, WITH A COUPLE OF EMBEDDED
CIRCULATIONS (MCVS) NOTED ACROSS NORTHWEST IL/SOUTHWEST WI.
TIMING OF THE PRIMARY LINE OF STORMS LOOKS TO BE AROUND 06Z AT
RFD, 0740-0800Z FOR DPA/ORD/MDW AND SHORTLY AFTER FOR GYY. THIS
LINE MAY BE ACCOMPANIED BY A PERIOD OF WEST-NORTHWEST OUTFLOW
WINDS IN EXCESS OF 30-35 KTS. COMPLICATING MATTERS IS THE
DEVELOPMENT OF NORTHEASTWARD-MOVING SCATTERED TSRA OVER NORTH
CENTRAL IL (RFD-DKB VICINITY) AHEAD OF THE AFOREMENTIONED MCV
OVER NORTHWEST IL. THIS ACTIVITY (WHILE VISIBLE FROM ORD PER
WEBCAMS DUE TO RELATIVELY HIGH CLOUD BASES) IS EXPECTED TO LIFT
NORTH/NORTHEAST OF DPA/ORD PRIOR TO THE MAIN LINE ARRIVAL,
THOUGH CAN'T COMPLETELY RULE OUT TSRA ARRIVING EARLIER THAN THE
CURRENT 07-08Z TAF TIMING. FARTHER WEST, ADDITIONAL TSRA WAS
OCCURRING OVER CENTRAL IA, GENERALLY TRACKING EAST-SOUTHEAST.
HIGH-RES CAM GUIDANCE SUGGESTS THUNDER THREAT WILL DIMINISH WITH
THIS ACTIVITY AS IT PROPAGATES INTO CENTRAL IL (MAINLY SOUTH OF
THE TERMINALS, BUT PERHAPS NOT TOO FAR SOUTH). CURRENTLY HAVE
MAINTAINED VCSH/PROB30 SHRA FOR THIS BUT IT IS POSSIBLE WE MAY
NEED TO ADD A TSRA MENTION WITH THIS IF TS PERSISTS (NEWLY
ARRIVING 04Z HRRR RUN IS MORE BULLISH WITH STRONGER CONVECTIVE
CORES). ALL OF THIS SHOULD MOVE OFF TO THE EAST BY 12Z FOR RFD
AND 13-14Z FOR CHICAGO TERMINALS.
FORECAST SOUNDINGS DEPICT STRONG DRYING OF THE COLUMN BEHIND
DEPARTING MORNING MCS. BY LATE AFTERNOON HOWEVER, SOUNDINGS DO
BECOME CONDITIONALLY UNSTABLE - BUT WITH LITTLE IN THE WAY OF
ORGANIZED SUPPORT FOR CONVECTIVE INITIATION. SOME CAMS INDICATE
THE POTENTIAL FOR ISOLATED TS DEVELOPMENT BY LATE AFTERNOON
ACROSS NORTHERN IL, THOUGH THE SIGNAL IS NOT CONSISTENT IN THE
GUIDANCE. A GREATER POTENTIAL FOR ANOTHER PERIOD OF TSRA LOOKS
TO ARRIVE LATE IN THE EVENING FOR RFD AND AFTER MIDNIGHT/PRE-
DAWN SUNDAY, AS A MID-LEVEL DISTURBANCE LIFTS NORTHEAST ACROSS
THE REGION.
WIND-WISE, BREEZY SOUTH WINDS WITH GUSTS AROUND 20 KTS SHOULD
GIVE WAY TO GUSTY WEST WINDS BRIEFLY WITH TS PREDAWN, THEN
SETTLE LIGHT SOUTHWEST DURING THE DAY. IT APPEARS THAT AN
OUTFLOW BOUNDARY ASSOCIATED WITH MORNING STORMS WILL LAY OUT
SOUTH OF THE TERMINALS DURING THE DAY, WITH A WEAKER GRADIENT TO
THE NORTH MAKING FOR LOWER CONFIDENCE IN DIRECTION OF LIGHT
WINDS THIS AFTERNOON. CAN'T RULE OUT A LAKE BREEZE GETTING AS
FAR WEST AS ORD/MDW LATE AFTERNOON, THOUGH CONFIDENCE IS LOW.
IT DOES APPEAR THAT WINDS WILL BECOME SYNOPTICALLY SOUTHEAST BY
EARLY EVENING HOWEVER, AS THE OUTFLOW BOUNDARY BEGINS TO LIFT
BACK NORTH IN RESPONSE TO THE APPROACHING MID-LEVEL WAVE.
RATZER
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...NONE.
VISIT US AT WEATHER.GOV/CHICAGO
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page