797
FXUS63 KLOT 171816
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
116 PM CDT FRI APR 17 2026
KEY MESSAGES
- ANOTHER ROUND OF SHOWERS AND THUNDERSTORMS IS EXPECTED LATER
THIS EVENING INTO TONIGHT. SOME OF THE STORMS COULD BE
SEVERE. LOCALLY HEAVY RAINFALL COULD RESULT IN FLOODING
ISSUES.
- WINDY AND UNSEASONABLY WARM WEATHER TODAY WILL BE REPLACED BY
MUCH COLDER WEATHER OVER THE WEEKEND INTO EARLY NEXT WEEK.
- FROST/FREEZE CONDITIONS ARE EXPECTED MANY AREAS SUNDAY AND
ESPECIALLY MONDAY MORNINGS.
MESOSCALE
ISSUED AT 1219 PM CDT FRI APR 17 2026
THE OPEN WARM SECTOR HAS REMAINED STABLE BASED ON RECENT
SATELLITE TRENDS, WITH LITTLE CUMULUS SO FAR ACROSS ILLINOIS,
AND MAINLY BILLOWS STREETS OVER NORTHERN MISSOURI AND EASTERN
IOWA. HOWEVER, THERE HAVE BEEN RECENT HINTS THAT THE REMAINING
INHIBITION IS QUICKLY ERODING WEST OF THE MISSISSIPPI RIVER WITH
POCKETS OR OPEN CELLED CUMULUS CLUSTERS BEGINNING TO APPEAR.
ATTEMPTS AT CONVECTIVE INITIATION HAVE ALREADY BEEN OBSERVED,
WITH MORE IMMINENT ORGANIZED DEEP CONVECTION LIKELY WITHIN THE
NEXT HOUR OVER FAR NORTH-CENTRAL MISSOURI INTO SOUTHEAST IOWA.
INITIAL DISCRETE SUPERCELLS ARE EXPECTED IN THIS AREA, WITH AN
ULTIMATE TRANSITION INTO A LINEAR QLCS SEGMENT WITH NORTHEAST
EXTENT. WHAT REMAINS LESS CLEAR IS HOW QUICK THIS MODE
TRANSITION OCCURS, WHICH FURTHER DICTATES CONVECTIVE EVOLUTION
FARTHER EAST. THERE APPEARS TO BE TWO MAIN SCENARIOS AT PLAY:
1) CONVECTION OVER IOWA REMAINS MORE DISCRETE OR SEMI-DSICRETE
ACROSS THE MISSISSIPPI RIVER, GREATLY INCREASING THE SEVERE RISK
FOR ALL HAZARDS, INCLUDING TORNADOES AND VERY LARGE HAIL,
AROUND AND WEST OF THE I-39 CORRIDOR IN THE 3-5PM WINDOW. IN THE
ABSENCE OF A MORE FORMIDABLE COLD POOL AND WITH RESIDUAL LOW-
LEVEL CAPPING AND/OR MIXING REMAINING FARTHER EAST, CONVECTION
WOULD LIKELY STRUGGLE TO REMAIN ORGANIZED INTO THE CHICAGO METRO
THROUGH THIS AFTERNOON.
2) CONVECTION CONGEALS INTO A MORE ORGANIZED LINE WITH A
DEVELOPING COLD POOL ACROSS NORTHWEST ILLINOIS, PROSPECTS OF
CONVECTION EXTENDING WELL EASTWARD ACROSS NORTHERN ILLINOIS
WOULD THEN INCREASE SUBSTANTIALLY THIS AFTERNOON. IN THIS CASE,
A GROWING DCAPE RESERVOIR FROM AFOREMENTIONED MIXING FARTHER TO
THE EAST FAVOR AN OUTFLOW DOMINANT LINE OF CONVECTION WITH
PRIMARILY DAMAGING WIND SHIFTING EASTWARD AROUND AND NORTH OF
THE I-80 CORRIDOR THROUGH LATE AFTERNOON.
BEYOND THIS INITIAL ROUND, A SECOND ROUND OF FRONTAL-FORCED
CONVECTION IS EXPECTED ACROSS THE AREA THIS EVENING. COVERAGE OF
THE INITIAL ROUND WILL GREATLY AFFECT THE AREAL EXTENT OF
POTENTIAL SEVERE CONVECTION OVER OUR AREA, BUT A QLCS MODE WITH
DAMAGING WINDS AND EMBEDDED TORNADOES REMAINS A FOCUS WHEREVER
THE WARM SECTOR REMAINS UNCONTAMINATED.
KLUBER
DISCUSSION
ISSUED AT 334 AM CDT FRI APR 17 2026
MESOSCALE HIGH PRESSURE CENTERED OVER SOUTHERN LAKE MICHIGAN IS
PUSHING A BANK OF FOG AND LOW STRATUS INLAND OFF THE LAKE EARLY
THIS MORNING. HALF MILE OR LESS VISIBILITIES ARE SPREADING
AROUND THE INTERIOR CHICAGO METRO AREA AND FARTHER WEST UP NEAR
THE WI STATE LINE. A DENSE FOG ADVISORY IS IN EFFECT THROUGH
10AM AROUND THE NORTHEASTERN QUADRANT OR SO OF OUR CWA.
SOUTHERLY WINDS WILL RAMP UP QUICKLY BEYOND SUNRISE AS THE HIGH
MOVES AWAY AND PRESSURE FALLS BEGIN AHEAD OF AN ADVANCING LOW.
STRONG WARM ADVECTION AHEAD OF THE WAVE WILL ALLOW FOR HIGHS IN
THE LOWER 80S AROUND MOST OF THE AREA THIS AFTERNOON. RELATIVELY
DRY BL AIR TODAY WILL PROMOTE DEEP MIXING AND ENCOURAGE STRONG
SOUTHERLY WINDS WITH GUSTS TO 30 TO 35 MPH, OCCASIONALLY NEAR 40
MPH, DURING THE AFTERNOON.
MAIN FOCUS FOR THE FORECAST IS THE POTENTIAL FOR SEVERE
THUNDERSTORMS THIS EVENING WITH THE PASSAGE OF A STRONGLY-FORCED
COLD FRONT. AS A LARGE UPPER TROUGH SCOOTS EASTWARD FROM THE
ROCKIES INTO THE CENTRAL PLAINS, AN EML WILL GET ADVECTED INTO
THE REGION. THE HEART OF THE EML WILL FOCUS JUST WEST OF THE
MISSISSIPPI, BUT FORECAST SOUNDINGS STILL DEPICT A DEEP COLUMN
OF LOW TO MID LEVEL LAPSE RATES GREATER THAN 8 K/KM ALLOWING FOR
ANYWHERE FROM 1,500 TO 3,000 JOULES OF AFTERNOON MUCAPE. IT'S
NOT ENTIRELY CLEAR HOW MUCH CAPPING WE'LL MAINTAIN INTO THE
AFTERNOON WITH SOME CAMPS SUGGESTING WE MAY BE LARGELY UNCAPPED.
HOWEVER, DRY LOW LEVEL AIR AND WEAK AMBIENT FORCING IN THE OPEN
WARM SECTOR SHOULD DO WELL TO INHIBIT FREELY CONVECTING STORMS
EVEN IN A WEAKLY CAPPED ENVIRONMENT. ONLY A COUPLE OF CAMS
RESOLVE ISOLATED CONVECTION IMMEDIATELY AHEAD OF THE STORM'S
COLD FRONT LATER IN THE DAY, BUT CHANCES FOR FREE CONVECTION IN
THE WARM SECTOR THROUGHOUT THE AFTERNOON ARE SEEMINGLY LOW.
SHOULD ANY AFTERNOON FREE CONVECTION DEVELOP, THEY WOULD HAVE
THE INGREDIENTS NECESSARY TO QUICKLY BECOME SEVERE AND LARGE
HAIL AND DAMAGING WINDS WOULD BE THE PRIMARY THREAT. CAN'T RULE
OUT A TORNADO THREAT WITH ANYTHING THAT MIGHT SPAWN DURING THE
AFTERNOON EITHER GIVEN ELONGATING, LOOPING LOW LEVEL HODOGRAPHS,
BUT HIGH LCLS AND LOW INSTABILITY THROUGH THE LOW LEVELS ARE
NOT FAVORABLE FOR TORNADOGENESIS.
THE PRIMARY WINDOW OF CONCERN FOR SEVERE CONVECTION IS THIS
EVENING AS THE AFOREMENTIONED LOW'S COLD FRONT GETS DRAGGED
ACROSS THE AREA. A CLUSTER OF SUPERCELLS IS FAVORED TO FORM
ALONG THE FRONT ACROSS EASTERN IA DURING THE AFTERNOON BEFORE
CONGEALING INTO A STRONGLY- FORCED LINE OF CONVECTION ACROSS
NORTHERN IL DURING THE EVENING. STORMS LOOK TO APPROACH THE I-39
CORRIDOR DURING THE EARLY EVENING AND EXIT TO THE EAST EARLY IN
THE OVERNIGHT, POSSIBLY WITH ADDITIONAL SHOWERS LINGERING LATE
INTO THE NIGHT. GUIDANCE GENERALLY DEPICTS STORMS ON A WEAKENING
TREND AS THEY MOVE INTO A THERMODYNAMICALLY LESS FAVORABLE
ENVIRONMENT ACROSS NORTHERN IL (AND WORSENING WITH TIME VIA
DIURNAL COOLING). HOWEVER, THE ENVIRONMENT WILL REMAIN PLENTY
SUPPORTIVE OF STRONG TO SEVERE CONVECTION ACROSS OUR CWA,
ESPECIALLY GIVEN THE STRONG DYNAMIC FORCING AT PLAY WITH THE
FRONT. THE BIGGEST CONCERN WITH STORMS IS THE DAMAGING WIND
THREAT WHICH RECENTLY PROMPTED THE STORM PREDICTION CENTER TO
DRAG THE DAY 1 ENHANCED RISK (LEVEL 3 OF 5) EAST TO THE INDIANA
STATE LINE, INCLUDING THE CITY OF THE CHICAGO. BL CIN WILL BE
INCREASING THROUGH THE EVENING, BUT CAN'T RULE OUT A SPIN UP
TORNADO THREAT ALONG THIS LINE EITHER GIVEN THE STRONG FORCED
ASCENT AND IMPRESSIVE LOW LEVEL WIND FIELD.
ANOTHER BIG CONCERN WITH RAIN AND STORMS LATER TODAY IS THE
HEAVY RAIN AND FLOODING THREAT. AREA SOILS ARE ESPECIALLY MOIST
FROM RECENT RAINS MAKING THEM MORE SUSCEPTIBLE TO FLOODING AND
MANY LOCAL RIVERS AND CREEKS, ESPECIALLY THE ROCK, FOX, AND DES
PLAINES, ARE ALREADY WELL INTO FLOOD STAGE. NEAR OR OVER 1.5"
OF PWAT AMID DEEP DYNAMIC FORCING AND INSTABILITY FAVORS PERIODS
OF HEAVY RAINFALL WITH THE FRONTAL PASSAGE. THERE IS ALSO A
GROWING SIGNAL IN MODEL GUIDANCE FOR A SHORTWAVE IMPULSE TO
TRACK ACROSS THE AREA AND PROVIDE AN ADDITIONAL PERIOD OF
MODERATE TO HEAVY RAIN INTO THE OVERNIGHT IN THE WAKE OF LINE OF
STORMS. CAN'T SAY FOR CERTAIN WHERE THIS WAVE WILL TRACK, BUT
GUIDANCE LIKES NEAR THE I-80 CORRIDOR AND NEAR AND SOUTH OF I-80
IS WHERE THE HIGHEST QPF IS GENERALLY FAVORED. PROBS FOR 0.5"
OF RAIN FROM LAST EVENING'S HREF RANGE FROM 60 TO NEARLY 100%
AROUND THE ENTIRE CWA AND YOU WON'T FIND A PIECE OF RECENT
DETERMINISTIC GUIDANCE WITHOUT A SWATH OF 1-2" ACROSS AT LEAST A
SMALL PORTION OF THE AREA. AS LITTLE AS 0.5" OF RAIN COULD
EASILY SPELL TROUBLE FOR PARTS OF THE CWA FROM A FLASH FLOODING
PERSPECTIVE, ESPECIALLY THOSE NORTH OF I-80. GIVEN THE
UNCERTAINTIES AND THE FACT THAT THE HIGHEST AMOUNTS ARE
GENERALLY FAVORED OUTSIDE OF THE MORE FLOOD-SENSITIVE AREAS,
DECIDED TO HOLD OFF ON ISSUING A FLASH FLOOD WATCH ON THIS
SHIFT, BUT THIS IS SOMETHING THE DAY SHIFT WILL HAVE TO
RECONSIDER. A HYDROLOGIC OUTLOOK HAS BEEN ISSUED FOR THESE
FLOODING CONCERNS.
BEHIND THE COLD FRONT, A MUCH COOLER AIRMASS WILL SETTLE INTO
THE REGION FOR THIS WEEKEND. IN FACT, WE'LL LIKELY SEE
SATURDAY'S HIGH TEMPERATURES OCCUR AT MIDNIGHT WITH COOLER
CONDITIONS EXPECTED DURING THE DAY. DAYTIME HIGHS ARE FORECAST
IN THE MIDDLE 40S TO LOWER 50S, A WHOLE 30+ DEGREES COOLER THAN
TODAY'S HIGH TEMPERATURE FORECAST. SIMILAR HIGHS ARE EXPECTED
FOR SUNDAY. LOW TEMPERATURES ARE FORECAST IN THE 30S SATURDAY
NIGHT INTO SUNDAY MORNING WITH SOME SPOTS SLATED TO GET DOWN TO
FREEZING. A MODEST WESTERLY BREEZE MAY HELP MITIGATE
FROST/FREEZE IMPACTS SOME, BUT WE MAY BE HEADED FOR OUR FIRST
FROST/FREEZE HEADLINES OF THE GROWING SEASON THIS SATURDAY
NIGHT. BETTER FROST/FREEZE CONDITIONS WILL SET UP SUNDAY
NIGHT/MONDAY MORNING WITH LOW TEMPERATURES FORECAST IN THE LOWER
AND MIDDLE 30S WITH CALM WINDS AND CLEAR SKIES AS SURFACE HIGH
PRESSURE CENTERS OVER THE REGION. BEYOND TONIGHT'S RAIN CHANCES
WHICH COULD STRETCH INTO EARLY SATURDAY MORNING, NO PRECIP IS
ANTICIPATED OVER THE WEEKEND. IN FACT, CONDITIONS LOOKS TO
REMAIN LARGELY QUIET THROUGH EARLY NEXT WEEK AS WE TREND BACK
TOWARD UNSEASONABLY MILD CONDITIONS.
DOOM
AVIATION /18Z TAFS THROUGH 00Z SUNDAY/
ISSUED AT 116 PM CDT FRI APR 17 2026
KEY MESSAGES:
- HIGH CONFIDENCE IN A LINE OF THUNDERSTORMS IMPACTING THE
TERMINALS LATE THIS AFTERNOON AND EVENING. LOCALIZED WIND
GUSTS IN EXCESS OF 50 KTS MAY OCCUR IN SOME PORTIONS OF THE
LINE OF STORMS.
- SOUTHERLY WINDS THIS AFTERNOON WILL GUST TO AROUND 30 KTS AT
TIMES BEFORE A GUSTY WESTERLY TO NORTHWESTERLY WIND SHIFT
OCCURS WITH BOTH THE LINE OF STORMS AND AGAIN WITH A TRAILING
COLD FRONT LATER TONIGHT.
- POST-FRONTAL MVFR CEILINGS ARE EXPECTED TONIGHT.
CONFIDENCE REMAINS HIGH IN ALL TERMINALS BEING AFFECTED BY
THUNDERSTORMS TODAY. UNCERTAINTY REMAINS WITH THE PRECISE TIMING
OF THE STORMS, BUT THIS EVENING STILL APPEARS TO THE MOST LIKELY
TIME FRAME FOR STORMS AT THE CHICAGO METRO TERMINALS, WHILE RFD
COULD SEE STORMS AS EARLY AS THE MID-LATE AFTERNOON. THE STORMS
WILL LIKELY AFFECT THE TERMINALS AS A SOLID LINE, THOUGH SOME
POTENTIAL EXISTS FOR RFD TO BE AFFECTED BY MORE CELLULAR OR
SUPERCELLULAR CONVECTION BEFORE STORMS GROW UPSCALE INTO A LINE
WITH TIME. A GUSTY WESTERLY TO NORTHWESTERLY WIND SHIFT AND
LOCALIZED WIND GUSTS IN EXCESS OF 50 KTS MAY OCCUR IN SOME
PORTIONS OF THE THUNDERSTORM LINE, AND SOME HAIL AND TORNADO
THREAT EXISTS TOO -- PRIMARILY AT RFD. THE STORMS SHOULD LAST
FOR ABOUT 2-3 HOURS AT ANY ONE LOCATION (POSSIBLY LONGER AT RFD
IF MULTIPLE ROUNDS OF STORMS MATERIALIZE THERE) WITH RAIN THEN
CONTINUING FOR A BIT LONGER UNTIL A COLD FRONT PUSHES ACROSS
THE AREA.
PRIOR TO THE ARRIVAL OF THE STORMS, SOUTHERLY WINDS WILL GUST
TO AROUND 30 KTS AT TIMES THIS AFTERNOON. ANOTHER GUSTY
NORTHWESTERLY WIND SHIFT IS EXPECTED BEHIND THE AFOREMENTIONED
COLD FRONT WITH BLUSTERY POST-FRONTAL WEST-NORTHWESTERLY WINDS
THEN CONTINUING THROUGH THE REMAINDER OF THE TAF PERIOD.
LASTLY, MVFR (AND POTENTIALLY IFR) CEILINGS ARE LIKELY TO OCCUR
DURING THE THUNDERSTORMS AND AGAIN BEHIND THE COLD FRONTAL
PASSAGE. A RETURN TO VFR CONDITIONS IS THEN LIKELY TO OCCUR
SOMETIME TOMORROW MORNING.
OGOREK
LOT WATCHES/WARNINGS/ADVISORIES
IL...FLOOD WATCH THROUGH LATE TONIGHT FOR ILZ003-ILZ004-ILZ005-
ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019-ILZ020-
ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ103-ILZ104-ILZ105-
ILZ106-ILZ107-ILZ108.
IN...FLOOD WATCH THROUGH LATE TONIGHT FOR INZ001-INZ002-INZ010-
INZ011-INZ019.
LM...SMALL CRAFT ADVISORY UNTIL 10 PM CDT SATURDAY FOR THE IL AND
IN NEARSHORE WATERS.
VISIT US AT WEATHER.GOV/CHICAGO
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