150
FXUS63 KLOT 170526
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
1226 AM CDT FRI APR 17 2026
KEY MESSAGES
- DENSE FOG IS POSSIBLE TONIGHT, ESPECIALLY CLOSER TO LAKE
MICHIGAN.
- ANOTHER ROUND OF SHOWERS AND THUNDERSTORMS IS EXPECTED LATER
FRIDAY AFTERNOON INTO FRIDAY NIGHT. SOME OF THE STORMS COULD
BE SEVERE. LOCALLY HEAVY RAINFALL COULD RESULT IN FLOODING
ISSUES.
- WINDY AND UNSEASONABLY WARM WEATHER FRIDAY 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.
UPDATE
ISSUED AT 824 PM CDT THU APR 16 2026
EVENING SURFACE ANALYSES DEPICT A SURFACE PRESSURE COL CENTERED
OVER LAKE MICHIGAN BETWEEN A DEPARTING SURFACE LOW PRESSURE
SYSTEM IN THE NORTHEASTERN UNITED STATES AND A NEW LOW PRESSURE
SYSTEM DEVELOPING IN THE CENTRAL PLAINS. THE RESULTING NEARLY
CALM PRESSURE GRADIENT HAS ALLOWED FOR A MARINE "BUBBLE" HIGH TO
DEVELOP, LEADING TO ONSHORE WINDS AROUND THE PERIMETER OF
SOUTHERN LAKE MICHIGAN. THE RESULTING INLAND BLEED OF RELATIVELY
COOL AIR INTO THE WARMER AND MORE MOIST CONTINENTAL AIRMASS IS
LEADING TO THE DEVELOPMENT OF DENSE FOG ALONG THE LAKE MICHIGAN
SHORELINE, AS CONFIRMED BY NUMEROUS WEBCAMS AND GOES-19
NIGHTTIME MICROPHYSICS IMAGERY. AT THIS POINT, FOG IS DENSEST
ACROSS PORTIONS OF LAKE (IL), SOUTHERN COOK, AND LAKE (IN)
COUNTIES, WHERE A DENSE FOG ADVISORY IS ALREADY IN EFFECT.
GOING FORWARD, THE RAPIDLY COOLING BOUNDARY LAYER SHOULD ALLOW
FOR THE MARINE AIRMASS TO ACCELERATE FURTHER INLAND. AS A
RESULT, THE EXPECTATION IS FOR DENSE FOG TO RAPIDLY EXPAND
INLAND THROUGH THE OVERNIGHT HOURS. AS IS TYPICAL IN THESE SORTS
OF REGIMES, DETERMINING EXACTLY HOW FAR INLAND FOG WILL SPREAD
IS AN ITEM OF LOW CONFIDENCE. NEVERTHELESS, GRIDDED VISIBILITY
GUIDANCE IS UBIQUITOUS IN SHOWING DENSE FOG MAKING IT AS FAR
SOUTH AS THE KANKAKEE RIVER VALLEY AND WEST AS THE FOX RIVER
VALLEY. WILL ADOPT THE USUAL APPROACH OF TACKING ON COUNTIES TO
THE DENSE FOG ADVISORY AS OBSERVATIONS DICTATE THROUGH THE
OVERNIGHT HOURS.
TAKING A QUICK LOOK AT THE FORECAST FOR TOMORROW, AM NOT SEEING
MUCH DIFFERENT THAN WHAT IS OUTLINED IN THE SHORT TERM
DISCUSSION BELOW. DO CONTINUE TO HIGHLIGHT THE HEIGHTENED
THREAT FOR FLASH FLOODING AND A WORSENING OF RIVER FLOODING NORTH
OF INTERSTATE 88 AND ESPECIALLY NEAR THE WISCONSIN STATE LINE
WHERE REPEATED ROUNDS OF THUNDERSTORMS HAVE LEFT SOILS SATURATED
AND MANY RIVERS ARE ALREADY IN MINOR TO MODERATE FLOOD STAGE
(ROCK, FOX, AND DES PLAINES RIVERS IN PARTICULAR).
UPDATED PRODUCTS WILL BE SENT SHORTLY.
BORCHARDT
DISCUSSION
ISSUED AT 251 PM CDT THU APR 16 2026
THROUGH FRIDAY NIGHT...
SURFACE RIDGE WILL MOVE ACROSS THE REGION TONIGHT. RELATIVELY
MOIST LOW LEVEL AIR MASS REMAINS IN PLACE, SO AS SKIES CLEAR
OUT AND WINDS DIMINISH, AREAS OF DENSE FOG COULD DEVELOP
TONIGHT. THE THREAT APPEARS GREATEST NEAR AND DOWNWIND FROM LAKE
MICHIGAN AS MARINE CHILLED AIR MASS BLEEDS INLAND INTO HIGHER
DEWPOINT AIR MASS OVER LAND. MAY VERY WELL NEED A DENSE FOG
ADVISORY TONIGHT, BUT WILL PUNT THAT DECISION TO THE EVENING
SHIFT WHO WILL BENEFIT FROM OBSERVATIONAL TRENDS AND BE ABLE TO
BETTER DEFINE THE AREAS OF GREATEST DENSE FOG THREAT.
FRIDAY WILL BE THE LAST SUMMER-LIKE WARM AND WINDY DAY OF THIS
STRETCH, OUT AHEAD OF AN ELONGATED SURFACE LOW LIFTING TOWARDS
WESTERN LAKE SUPERIOR. EXPECT HIGHS IN THE LOW TO LOCALLY MID
80S AWAY FROM THE FAR NORTHEAST IL SHORE (WHICH WILL REACH THE
70S TOWARDS EVENING), AND SOUTHERLY WINDS GUSTING UP TO 30-35
MPH. LINGERING LOWER DEW POINTS FROM UNDERNEATH THE DEPARTING
SURFACE HIGH, SOUTH-SOUTHEAST WINDS TO OFFSET SURFACE MOISTURE
RETURN WITH EASTWARD EXTENT, AND PARCHED MID-LEVELS WILL YIELD A
FAIRLY SHARP INSTABILITY GRADIENT FROM THE MS RIVER TO THE
LONGITUDE OF LAKE MICHIGAN BY MID FRIDAY AFTERNOON.
SIGNIFICANT UNCERTAINTIES REMAIN REGARDING SEVERE WEATHER THREAT
IN OUR CWA LATE FRIDAY AND FRIDAY NIGHT. A NUMBER OF HIGH
RESOLUTIONS MODELS ARE DEVELOPING OPEN WARM SECTOR CONVECTION
ACROSS WESTERN IL BY MID AFTERNOON FRIDAY. IT ISN'T CLEAR WHAT
IS FORCING THIS CONVECTION, WHICH GIVEN THE STOUT EML AND
POTENTIAL FOR A CAP, RAISES DOUBTS ABOUT THE VERACITY OF THE
MODEL SOLUTIONS DEPICTING THIS. ARE THESE MODELS TOO AGGRESSIVE
IN ERODING CONVECTIVE INHIBITION IN LIGHT OF THE STRONG EML? IT
REALLY ISN'T CLEAR AT THIS POINT, LEADING TO LOWER FORECAST
CONFIDENCE IN THE AFTERNOON.
SHOULD THIS CONVECTION DEVELOP IN THE OPEN WARM SECTOR DURING
THE AFTERNOON, THEN STORM MODE WOULD LIKELY BE SUPERCELLULAR
GIVEN THE LONG HODOGRAPHS. PRIMARY THREAT FROM THESE STORMS (IF
THEY MATERIALIZE) WOULD LIKELY BE LARGE TO VERY LARGE HAIL.
CANNOT RULE OUT A TORNADO THREAT, HOWEVER LOW LEVEL SHEAR LOOKS
FAIRLY MARGINAL. IN ADDITION, IT ISN'T CLEAR HOW MUCH DEWPOINTS
WILL MIX OUT AND EVEN MODEST MIXING OUT OF LOW LEVEL MOISTURE
WOULD LEAD TO LARGER T/TD SPREADS AND A MORE MUTED TORNADO RISK.
REGARDLESS OF WHETHER THIS OPEN WARM SECTOR CONVECTION
MATERIALIZES, AN EXTENSIVE FRONTAL SQUALL LINE IS LIKELY TO
DEVELOP WEST OF THE MISSISSIPPI RIVER ALONG THE APPROACHING COLD
FRONT. GIVEN THE PARAMETER SPACE THIS CONVECTION WILL DEVELOP
WITHIN, SEVERE WEATHER IS LIKELY, ESPECIALLY DAMAGING WINDS.
THIS QLCS SHOULD CROSS THE MS RIVER AND MOVE INTO OUR WESTERN
CWA EARLY-MID EVENING FRIDAY. THIS LINE WILL LIKELY STILL POSE A
THREAT OF DAMAGING WINDS AND POTENTIALLY LINE EMBEDDED
TORNADOES GIVEN THE STRENGTHENING LOW LEVEL SHEAR DURING THE
EVENING.
LOWER CONFIDENCE IN HOW QUICKLY THE CONVECTION WILL DECREASE IN
INTENSITY AS IT MOVES EAST ACROSS OUR CWA INTO LESS UNSTABLE AND
MORE CAPPED AIR MASS. THE LATEST SPC SWODY2 NICELY DEPICTS THE
LIKELY WEST TO EAST GRADIENT IN SEVERE WEATHER THREAT.
THERE IS A PRETTY STRONG SIGNAL IN GUIDANCE IN A SOUTHERN STREAM
SHORTWAVE SLOWING THE SOUTHWARD PROGRESSION OF THE EXTENSIVE
QLCS ACROSS CENTRAL IL FRIDAY NIGHT. IN FACT, THERE COULD BE A
REINVIGORATION OF CONVECTION CAPABLE OF PRODUCING HEAVY RAINFALL
IN ASSOCIATION WITH THIS WAVE. THE SOUTHERN PORTIONS OF OUR CWA
SEEM MOST SUSCEPTIBLE TO A MORE PROLONGED PERIOD OF HEAVY
RAINFALL FRIDAY NIGHT.
SOILS ARE SATURATED AND STREAMFLOW MUCH ABOVE AVERAGE OVER MUCH
OF THE REGION, LEAVING OUR CWA SUSCEPTIBLE TO BOTH FLASH
FLOODING AND RIVER FLOODING FROM HEAVY RAINFALL. OUR WI BORDER
COUNTIES AND THE IMMEDIATE CHICAGO AREA ARE MOST VULNERABLE TO
FLOODING, AND SHOULD HEAVY RAIN FALL IN THESE AREAS, THEN A
MORE SIGNIFICANT FLOOD THREAT WOULD EXIST. HAVE ISSUED AN ESF TO
HIGHLIGHT THIS THREAT FOR NOW, BUT ONCE WE ARE BETTER ABLE TO
DEFINE THE AREAS MOST LIKELY TO POTENTIALLY SEE HEAVY RAINFALL A
FLOOD WATCH WILL LIKELY BE NEEDED.
- IZZI
SATURDAY THROUGH WEDNESDAY:
THE MOST RECENT MODEL GUIDANCE FOR SATURDAY HAS PRETTY
UNANIMOUSLY SPED UP THE TIMING OF THE COLD FRONTAL PASSAGE TO
YIELD A MOSTLY DRY DAY FOR MUCH OF IF NOT THE ENTIRE AREA. BEST
CHANCE (30-50%) FOR ANY LINGERING SHOWERS PAST SUNRISE WILL BE
FOR AREAS SOUTHEAST OF I-55. SATURDAY AFTERNOON WILL TREND TO
PARTLY CLOUDY TO MOSTLY SUNNY, ALBEIT NOTABLY COOLER (50S) WITH
BLUSTERY WEST-NORTHWEST WINDS. COLD AIR ADVECTION AND DRY
ADVECTION THROUGH SATURDAY NIGHT-EARLY SUNDAY WILL SET THE STAGE
FOR LOWS IN THE 30S, POSSIBLY FLIRTING WITH FREEZING ACROSS
PARTS OF INTERIOR NORTHERN IL. FOLLOWING A COOL AND BREEZY
SUNDAY, ~1030 MB HIGH PRESSURE OVER THE REGION WILL LIKELY BE
CONDUCIVE TO AT LEAST AREAS OF FROST AS TEMPS DIP TO THE LOW-MID
30S OUTSIDE OF CHICAGO, IF NOT COLDER IN SPOTS. AGRICULTURAL
INTERESTS ARE ADVISED TO MONITOR FORECASTS THROUGH EARLY NEXT
WEEK.
COOLER CONDITIONS INTO MONDAY MORNING WILL BE SHORT-LIVED
HOWEVER, WITH EXTENDED GUIDANCE FAVORING A RETURN TO HIGH
AMPLITUDE RIDGING ACROSS THE CENTRAL CONUS AND UNSEASONABLY WARM
TEMPERATURES RETURNING TO THE REGION BY MIDWEEK NEXT WEEK.
DESPITE THE RETURNING WARMTH NEXT WEEK AWAY FROM LAKE MICHIGAN,
A BLOCKY MID-UPPER PATTERN WILL FAVOR MUCH LESS STORMY
CONDITIONS OVER THE LOCAL AREA, GIVING A LONGER PERIOD TO DRY
OUT FROM THIS WEEK'S RAIN.
CASTRO
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 ILZ005-
ILZ006-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.
VISIT US AT WEATHER.GOV/CHICAGO
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page