779
FXUS63 KLOT 170834
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
334 AM CDT FRI APR 17 2026
KEY MESSAGES
- DENSE FOG AROUND PARTS OF THE AREA THIS MORNING, ESPECIALLY
AROUND LAKE MICHIGAN.
- 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.
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 TOMORROW 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 /06Z TAFS THROUGH 12Z SATURDAY/
ISSUED AT 1226 AM CDT FRI APR 17 2026
THE KEY AVIATION WEATHER MESSAGES ARE:
- LIFR/VLIFR CIGS AND VSBYS THROUGH EARLY MORNING, MAINLY AT
ORD, MDW, GYY, AND POSSIBLY DPA. CONDITIONS WILL IMPROVE
RAPIDLY AFTER 12-13Z.
- SE WINDS TURNING S TO SSW THIS AFTERNOON, WITH SOME OCCASIONAL
GUSTS GREATER THAN 30 KNOTS POSSIBLE.
- LINE OF THUNDERSTORMS--SOME SEVERE--EXPECTED TO IMPACT ALL
TERMINALS THIS EVENING.
- COLD FRONT ARRIVES OVERNIGHT WITH A BREEZY NORTHWEST WIND
SHIFT AND MVFR/BRIEF IFR CIGS.
AN AREA OF LOW STRATUS AND FOG CONTINUES TO OOZE WESTWARD OFF
THE LAKE. OVER THE PAST HOUR, CIGS AND VSBYS HAVE DETERIORATED
AT ORD/MDW, AND EXPECT THAT TREND TO CONTINUE OVERNIGHT, WITH
200-300 FOOT CIGS/VVS PREVAILING. WITH RVRS DROPPING, FELT
COMFORTABLE WITH CONTINUED TEMPO GROUPS FOR 1/4SM FG THROUGH
THE NIGHT. TRENDS AT DPA ARE MORE UNCERTAIN, WITH MOIST EASTERLY
TRAJECTORIES EVIDENTLY BEING BLOCKED BY HIGH-RISE STRUCTURES IN
CHICAGO. IT'S UNCLEAR HOW CIG/VSBY TRENDS WILL EVOLVE AS A
RESULT, BUT OVERALL EXPECTATION IS FOR EVENTUAL LIFR/VLIFR CIGS
AND VSBYS TO DEVELOP OVERNIGHT. HAVE LEFT RFD VFR AT THIS TIME
GIVEN INCREASING LOW-LEVEL FLOW OVERNIGHT AND MOIST TRAJECTORIES
TURNING MORE NORTHWESTERLY WITH TIME (INTO WISCONSIN).
CONDITIONS WILL RAPIDLY IMPROVE IN THE 12-14Z TIMEFRAME AS SE
BREEZES DEVELOP. WINDS WILL EVENTUALLY TURN TO A 180-190
DIRECTION DURING THE AFTERNOON WITH INCREASING GUSTS. THERE WILL
BE A POTENTIAL FOR SOME INTERMITTENT 35 KT GUSTS TOWARDS MID
AFTERNOON, PARTICULARLY INVOF RFD. THEREAFTER, A LINE OF
THUNDERSTORMS IS EXPECTED TO ROLL EAST ACROSS THE REGION.
CONFIDENCE IN OVERALL STORM TIMING IS MODERATE-HIGH. SOME STORMS
WILL BE SEVERE.
AFTER STORMS CLEAR, LINGERING SHOWERS AND EMBEDDED TS WILL BE
POSSIBLE THROUGH THE LATE EVENING. A COLD FRONT WILL EVENTUALLY
SHUT ANY PRECIPITATION CHANCES OFF WITH A GUSTY NW WIND SHIFT
AND DEVELOPING MVFR CIGS OVERNIGHT INTO SATURDAY MORNING.
CARLAW
LOT WATCHES/WARNINGS/ADVISORIES
IL...DENSE FOG ADVISORY UNTIL 10 AM CDT THIS MORNING FOR ILZ004-
ILZ005-ILZ006-ILZ012-ILZ013-ILZ020-ILZ103-ILZ104-ILZ105-
ILZ106-ILZ108.
IN...DENSE FOG ADVISORY UNTIL 10 AM CDT THIS MORNING FOR INZ001-
INZ002.
LM...DENSE FOG ADVISORY UNTIL 7 AM CDT THIS MORNING FOR THE IL AND
IN NEARSHORE WATERS.
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