931
FXUS63 KLOT 272320
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
620 PM CDT MON APR 27 2026
KEY MESSAGES
- SCATTERED STRONG TO SEVERE THUNDERSTORMS WILL BE POSSIBLE
LATER THIS AFTERNOON INTO THIS EVENING (THROUGH ~10 PM CDT),
WITH THE MAIN THREATS DAMAGING WINDS AND LARGE HAIL.
- A COOLER AND GENERALLY DRIER PATTERN IS IN STORE THROUGH THIS
WEEKEND.
MESOSCALE
ISSUED AT 620 PM CDT MON APR 27 2026
AIR MASS ACROSS OUR CWA IN THE WAKE OF THE EARLY CONVECTION AND
WAKE LOW IS THERMODYNAMICALLY NOT VERY CONDUCIVE FOR INTENSE
CONVECTION OVER MOST OF OUR CWA. RECENT ACARS SOUNDINGS FROM MDW
SHOW VERY LITTLE BOUNDARY LAYER BASED INSTABILITY AND STRONG
INHIBITION FOR ANY BOUNDARY LAYER BASED PARCELS, WHICH CONFIRMS
OBJECTIVE ANALYSIS FROM SPC MESOANALYSIS PAGE. RECENT
PROGRESSION OF ACARS SOUNDINGS FROM MDW OVER THE PAST HOUR OR
TWO DO SHOW EVIDENCE OF LIFT IN THE 800-600MB LAYER, LIKELY A
RESULT OF THE STRENGTHENING ASCENT ASSOCIATED WITH THE
APPROACHING >80KT 500MB JET STREAK. IT IS LIKELY THIS ASCENT
THAT IS DRIVING THE RECENT NORTHEASTWARD EXPANSION OF SHOWERS
AND THUNDERSTORMS IN OUR CWA. THIS ACTIVITY IS ROOTED ABOVE THE
BOUNDARY LAYER AND OBJECTIVE ANALYSIS WOULD SUGGEST ONLY A
COUPLE HUNDRED J/KG OF MUCAPE, SO DESPITE THE STRONG FORCING AND
SHEAR, CONVECTION SO FAR ACROSS NORTHERN IL HAS BEEN PRETTY TAME
AND NON-SEVERE.
FARTHER SOUTHWEST, ACROSS CENTRAL IL CONVECTION HAS SHOWN SIGNS
OF A DEVELOPING COLD POOL. RECENT HRRR AND WOFS OUTPUT DOES HINT
AT THIS CONVECTION EVOLVING INTO A FORWARD PROPAGATING BOWING
SEGMENT, WHICH COULD GET INTO OUR SOUTHEASTERN CWA. KILX WSR-88D
DOES SHOW A FAIRLY STRONG REAR INFLOW JET DEVELOPING WITH THE
CONVECTIVE CLUSTER, WHICH LENDS SOME SUPPORT TO RECENT CAM
GUIDANCE WHICH HINTS AT A BOWING CONVECTIVE CLUSTER SURGING
EAST ACROSS CENTRAL IL, INCLUDING NEAR/SOUTH OF THE HWY 24
CORRIDOR INTO OUR SOUTHERN CWA. SFC OBSERVATIONS AND SATELLITE
IMAGERY SUGGESTS THAT THERE WILL BE VERY LITTLE OPPORTUNITY FOR
BOUNDARY LAYER BASED DESTABILIZATION IN ADVANCE OF THIS LINE, SO
THIS LINE MAY STRUGGLE TO PRODUCE DAMAGING WINDS. OBSERVATIONS
AROUND THIS LINE NOW HAVEN'T SHOWN SIGNS OF STRONG WINDS
PENETRATING TO THE SFC AND WITH THE STORMS LIKELY TO PROGRESS
INTO AN ENVIRONMENT THAT IS A BIT MORE HOSTILE, IF ANYTHING,
THE SEVERE THREAT ISN'T LOOKING TERRIBLY HIGH. WE WILL CONTINUE
TO MONITOR TRENDS, BUT IN THE NEXT COUPLE OF HOURS THE SEVERE
THREAT APPEARS LOW, THOUGH CERTAINLY COULDN'T RULE OUT SOME
SPOTTY DAMAGING WIND GUSTS PENETRATING THROUGH THE STABLE LAYER
AND MAKING IT TO THE SFC.
PERHAPS OF GREATER CONCERN OVER NEXT FEW HOURS OVER OUR SOUTHERN
CWA WILL BE THE HEAVY RAINFALL THREAT. MRMS ESTIMATES SUGGEST
HOURLY RAINFALL RATES OF 1-2" PER HOURS WITH THE MORE INTENSE
CONVECTION. HOURLY FLASH FLOOD GUIDANCE ACROSS OUR SOUTHERN CWA
IS GENERALLY AROUND AN INCH AND HALF, SO THERE IS SOME THREAT OF
FLASH FLOODING DEVELOPING EARLY THIS EVENING, MAINLY SOUTH OF
I-80.
FARTHER NORTHWEST, RECENT ACARS SOUNDINGS FROM RFD DOES SHOW A
SOMEWHAT MORE HOSPITABLE ENVIRONMENT FOR CONVECTION WITH SOME
MODEST BL BASED INSTABILITY WITH VERY LITTLE INHIBITION. SPC
MESOANALYSIS DOES SHOW AN AXIS OF AROUND 1000 J/KG OF MLCAPE
ACROSS NORTHWEST IL AND EASTERN IA. THERE HAS BEEN SOME
CONVECTION DEVELOPING WITHIN A PREFRONTAL CONVERGENCE AXIS NEAR
THE MISSISSIPPI RIVER THAT IS LIKELY ROOTED IN OR VERY NEAR THE
BOUNDARY LAYER. THUS FAR, THIS CONVECTION HAS REMAINED SUB-
SEVERE AND HASN'T YET SHOWN INDICATIONS OF BECOMING SEVERE,
THOUGH GIVEN THE MORE FAVORABLE INSTABILITY AND STRONG SHEAR,
THIS ACTIVITY WILL BEAR CLOSE WATCHING. THE MAGNITUDE OF BL
INSTABILITY DECREASES WITH EASTWARD EXTENT INTO OUR WESTERN CWA
AND IT DOESN'T SEEM LIKELY THAT THERE WILL BE SIGNIFICANT
DESTABILIZATION OVER THE NEXT COUPLE HOURS. EVEN SO, VERY STRONG
SHEAR AND SUFFICIENT INSTABILITY SUGGESTS THAT THIS ACTIVITY
WILL NEED TO BE MONITORED CLOSELY FOR SIGNS OF INTENSIFICATION.
- IZZI
DISCUSSION
ISSUED AT 300 PM CDT MON APR 27 2026
THROUGH TONIGHT:
ON THE BACKSIDE OF THE MIDDAY MCS AND IMPRESSIVE WAKE LOW DRIVEN
NON-CONVECTIVE WINDS (AND LOCALIZED DAMAGE), IT WILL TAKE SOME
TIME FOR ANY MEANINGFUL DESTABILIZATION. AS OF THIS WRITING,
MODEST DESTABILIZATION WAS CONFINED TO PRIMARILY NEAR AND WEST
OF THE MISSISSIPPI RIVER. OUT AHEAD OF THE COLD FRONT
PROGRESSING ACROSS THE REGION THROUGH THIS EVENING, EXPECT
SUFFICIENT LARGE SCALE FORCING FOR CLUSTERS OF SHOWERS AND
THUNDERSTORMS TO FILL IN TOWARDS EVENING, WHEN OUR FORECAST
INDICATES POPS BACK IN THE 60-90% RANGE.
THE RECENT TENDENCY ON HIGH-RES GUIDANCE HAS BEEN FOR HIGHEST
COVERAGE FAVORED NEAR AND SOUTH OF I-80. GIVEN THE MUCH MORE
MUTED INSTABILITY, IT'S UNCERTAIN IF ANYTHING MORE THAN
SCATTERED STORMS CAN BE REALIZED. A FEW SEVERE STORMS (WINDS AND
HAIL PRIMARY THREATS) REMAIN POSSIBLE, AGAIN LIKELY WITH A
BETTER CHANCE OF OCCURRENCE FOR LOCALES NEAR AND SOUTH OF I-80.
EXPECT MESOSCALE DISCUSSIONS TO PROVIDE UPDATES ON OUR LATEST
THINKING PER OBSERVATIONAL TRENDS INTO THIS EVENING.
THE LIKELY INABILITY FOR THE PROMINENT SURFACE-BASED
INSTABILITY AXIS TO MAKE IT INTO OUR AREA SHOULD CONTINUE TO
RELEGATE THE (WHAT WILL BE A HIGHER-END) TORNADO THREAT SOUTH
OF OUR AREA ACROSS CENTRAL AND SOUTHERN ILLINOIS. WE'LL NEED TO
KEEP AN EYE ON ANY CORRIDORS OF HEAVY RAINFALL TONIGHT AND AN
ASSOCIATED FLOODING THREAT, THOUGH SPATIAL FOOTPRINT SHOULD BE
FAIRLY LIMITED.
AFTER THE COLD FRONTAL PASSAGE TONIGHT, EXPECT A SHIFT TO BREEZY
WESTERLY WINDS, WHICH WILL USHER IN A COOLER AND DRIER AIR MASS.
DEW POINTS, AFTER BEING NEAR TO ABOVE 60F THIS EVENING, WILL BE
IN THE 40S AREAWIDE BY DAYBREAK, WITH AIR TEMPS IN THE UPPER 40S
TO LOWER 50S.
CASTRO/BORCHARDT
TUESDAY ONWARD:
THE UPPER-LEVEL PATTERN WILL UNDERGO A SUBSTANTIAL PATTERN
SHIFT THROUGH THE REST OF THE WEEK WITH PREDOMINANT AGGREGATE
TROUGHING ALONG THE US/CANADIAN BORDER. AS A RESULT, THE
EXPECTATION IS FOR COOLER AND DRIER CONDITIONS (ONLY A FEW
OPPORTUNITIES FOR LIGHT PRECIP AT TIMES) TO PREVAIL WITH HIGHS
IN THE 50S AND 60S AND OVERNIGHT LOWS IN THE UPPER 30S TO LOWER
40S. WE CAN EASILY ENVISION A THREAT FOR FROST BY THE WEEKEND.
THEREAFTER, LONG-RANGE ENSEMBLES ADVERTISE WARMING TREND DURING
THE FIRST WEEK OF MAY.
BORCHARDT/CASTRO
AVIATION /00Z TAFS THROUGH 06Z WEDNESDAY/
ISSUED AT 620 PM CDT MON APR 27 2026
SHOWERS AND THUNDERSTORMS WILL EXPAND IN COVERAGE ACROSS THE REGION
THIS EVENING. IN GENERAL, NO SIGNIFICANT CHANGES TO TIMING EXCEPT TO
PUSH BACK THE END OF THE TEMPO GROUPS AT THE CHICAGO-AREA TERMINALS
TO 02/03Z (LATEST AT GYY) AND TO ADD A TS MENTION AT RFD. THERE IS
SOME UNCERTAINTY WHETHER THUNDERSTORMS WILL LINGER A BIT BEYOND 02Z,
BUT THE POTENTIAL CURRENTLY IS A BIT TOO LOW TO CARRY THUNDER DEEPER
INTO THE EVENING, BUT WILL CONTINUE TO MONITOR TRENDS. SURFACE WINDS
OUTSIDE OF THUNDERSTORMS WILL BE SOUTHWESTERLY THIS EVENING.
EVENTUALLY, A COLD FRONT WILL SHIFT WINDS OUT OF THE W/WNW. LOW-MVFR
CIGS AND GUSTY WINDS WILL OVERSPREAD ALL OFF THE TERMINALS, LIKELY
LINGERING THROUGH THE TUESDAY MORNING. CIGS SHOULD THEN GRADUALLY
LIFT THROUGH THE AFTERNOON AND THEN SCATTER TO VFR. TIMING THE
RETURN OF VFR REMAINS A BIT UNCLEAR AT THIS TIME, BUT LIKELY AROUND
OR PRIOR TO 21Z.
CARLAW
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY UNTIL 10 AM CDT TUESDAY FOR THE IL AND IN
NEARSHORE WATERS.
VISIT US AT WEATHER.GOV/CHICAGO
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