195
FXUS63 KLOT 241711
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
1211 PM CDT TUE MAR 24 2026
KEY MESSAGES
- WARMING TREND EXPECTED THROUGH THURSDAY, HIGH TEMPS EXPECTED
TO WARM AROUND 10F EACH DAY.
- STRONG COLD FRONT WILL USHER IN DRAMATICALLY COLDER
TEMPERATURES LATE THURSDAY THROUGH FRIDAY.
- SHOWERS & THUNDERSTORMS ARE LIKELY TO DEVELOP NEAR AND JUST
BEHIND COLD FRONT THURSDAY AFTERNOON, HIGHEST RAIN CHANCES
FAVORING AREAS SOUTH OF I-80.
DISCUSSION
ISSUED AT 256 AM CDT TUE MAR 24 2026
SHORTWAVE TROUGH IS MOVING ACROSS THE REGION EARLY THIS MORNING,
THOUGH THE TOTAL DEARTH OF LOW LEVEL MOISTURE IS RESULTING IN
NOTHING MORE THAN MID-LEVEL DECK WITH SOME VIRGA. THIS SHORTWAVE
WILL MOVE EAST OF THE AREA THIS MORNING WITH SOME PARTIAL
CLEARING IN ITS WAKE. THAT PARTIAL CLEARING WILL LIKELY BE
SHORT-LIVED AS HIGH LEVEL CLOUDINESS ASSOCIATED WITH AN
APPROACHING 140-160KT UPPER LEVEL JET STREAK SHOULD RESULT IN
EXTENSIVE HIGH LEVEL OVERCAST BY EARLY AFTERNOON. THE CLOUD
COVER COULD HELP LIMIT MIXING SOME, KEEPING TEMPS A COUPLE
DEGREES COOLER AND DEWPOINTS SEVERAL DEGREES HIGHER THAN THEY
WOULD OTHERWISE BE. LOW CONFIDENCE ON HOW DEWPOINTS WILL BEHAVE
TODAY, SO MADE NO CHANGES TO THE INHERITED DEWPOINT FORECAST
WHICH LOOKED VERY REASONABLE. LIGHT ENOUGH SOUTHERLY SHORE
PARALLEL FLOW SHOULD ALLOW FOR AN AFTERNOON LAKE BREEZE FOR
ESPECIALLY THE ILLINOIS SIDE OF THE LAKE THIS AFTERNOON.
CONTINUED LOW LEVEL WARM AIR ADVECTION, A MORE STOUT SOUTHERLY
FLOW, AND LIKELY MORE MIXED SUNSHINE AMIDST HIGH CLOUDINESS
SHOULD ALLOW FOR HIGHS TO CLIMB INTO THE UPPER 60S TO LOWER 70S
WEDNESDAY. WITH THE PROSPECT FOR A BIT MORE SUNSHINE ON
WEDNESDAY, THERE IS A BETTER CHANCE THAT DEWPOINTS WILL MIX OUT
IN THE AFTERNOON AND HAVE UNDERCUT GUIDANCE DEWPOINTS
SUBSTANTIALLY WEDNESDAY AFTERNOON.
THERE IS CONSIDERABLE SPREAD, EVEN AMONG THE SHORT RANGE
GUIDANCE, WEDNESDAY NIGHT REGARDING SEVERAL KEY ELEMENTS THAT
WILL DICTATE OUR THREAT OF SOME ELEVATED CONVECTION. MOST
GUIDANCE DOES DEPICT A 40-50KT+ WESTERLY LOW LEVEL JET NOSING
INTO THE ILLIANA REGION WEDNESDAY NIGHT. THERE'S SOME SIZABLE
SPREAD IN WHAT LATITUDE THIS LOW LEVEL JET WILL SET UP WHICH
MAKES A BIG DIFFERENCE IN WHERE THE STRONG LOW LEVEL MOISTURE
CONVERGENCE WILL SET UP. FINALLY, THE NAM IS MUCH MORE
AGGRESSIVE WITH THE MAGNITUDE OF LOW LEVEL MOISTENING WITH
12-13C 850MB DEWPOINTS INTO ILLINOIS. NAM IS OFTEN TOO
AGGRESSIVE WITH LOW LEVEL MOISTURE, SO THE MORE MUTED 8-10C
850MB DEWPOINTS IN THE GFS AND ECMWF LOOK MORE REASONABLE. MOST
GUIDANCE DOES HAVE SOME CONVECTION BREAKING OUT NEAR THE
TERMINUS OF THE LOW LEVEL JET JUST AHEAD OF AN IMPRESSIVE
ELEVATED MIXED LAYER WHICH IS PROGGED TO OVERSPREAD THE AREA.
GIVEN THE UNCERTAINTIES REGARDING WHERE, AND EVEN IF, ELEVATED
CONVECTION WILL DEVELOP, OPTED NOT TO INCREASE THE SLIGHT CHANCE
POPS OFFERED UP BY THE NBM. THE BEST CHANCE OF THUNDERSTORMS IN
OUR AREA LOOKS TO BE EAST OF I-55 AND INTO NW IN. THE
ENCROACHING EML AND ASSOCIATED STEEP MID LEVEL LAPSE RATES ALONG
WITH THE LONG STRAIGHT EFFECTIVE HODOGRAPHS WOULD SUGGEST THAT
THERE COULD BE A POTENTIAL FOR AT LEAST SMALL HAIL. SHOULD THE
MORE AGGRESSIVE MOIST NAM VERIFY, THEN ELEVATED INSTABILITY
WOULD EVEN BE SUFFICIENT TO SUPPORT A THREAT OF SOME ISOLATED
INCIDENTS OF LARGE HAIL (1"+ DIAMETER) WHERE EVER CONVECTION
DOES DEVELOP.
NOCTURNAL LOW LEVEL JET IS EXPECTED TO WEAKEN AND MOVE SOUTH
AND EAST OF THE AREA BY THURSDAY MORNING WITH ASSOCIATED WEAK
MID-LEVEL SHORTWAVE ALSO PROGGED TO BE EAST OF THE AREA. THIS
SHOULD ALLOW FOR SOME SUNSHINE THURSDAY MORNING WHICH COMBINED
WITH STRONG WARM AIR ADVECTION SHOULD ALLOW TEMPS TO RAPIDLY
WARM UP THURSDAY MORNING.
ON SYNOPTIC SCALE, AN AMPLIFYING NORTHERN STREAM TROUGH IS
PROGGED TO DIG SOUTHEASTWARD INTO THE UPPER GREAT LAKES.
ASSOCIATED WEAK SFC LOW IS EXPECTED TO MOVE EASTWARD ACROSS
LOWER GREAT LAKES THURSDAY WITH A FORMIDABLE COLD FRONT
DROPPING SOUTHWARD ACROSS OUR CWA IN THE WAKE OF THE SURFACE
LOW. UNUSUALLY STRONG EML FOR SO EARLY IN THE YEAR WILL RESULT
IN VERY STEEP MID-LEVEL (700-500MB) LAPSE RATES IN EXCESS OF >8
C/KM ALONG WITH A STRONG CAPPING INVERSION AT THE BASE OF THE
EML. MOST GUIDANCE IS MUCH MORE PROMINENT WITH A SHORTWAVE
TROUGH EMBEDDED WITHIN THE SUBTROPICAL JET, CRESTING THE RIDGE
AND ARRIVING IN MID-MISSISSIPPI VALLEY JUST IN TIME FOR PEAK
HEATING THURSDAY AFTERNOON. DESPITE THE LOW AMPLITUDE NATURE OF
THIS SHORTWAVE, GFS, ECMWF, AND NAM ALL HAVE THIS FEATURE
APPROACHING NORTHERN ILLINOIS LATER THURSDAY AFTERNOON. PRETTY
GOOD MODEL AGREEMENT THAT THE ASCENT FROM THAT SHORTWAVE
COMBINED WITH FRONTAL ASCENT WILL BE ENOUGH TO ERODE THE CAP AND
ALLOW FOR ROBUST CONVECTION TO DEVELOP LATER THURSDAY AFTERNOON
INTO THE EVENING.
THE NAM AND RDPS MOVE A MORE SHALLOW COLD FRONT MORE QUICKLY
SOUTHWARD THURSDAY AFTERNOON, RESULTING IN MOST (IF NOT ALL) OF
THE CONVECTION DEVELOPING POST FRONTAL. GIVEN THE STEEP LAPSE
RATES AND STRONG EFFECTIVE SHEAR, THOSE STORM COULD STILL POSE A
THREAT FOR LARGE HAIL, PARTICULARLY EARLIER IN THEIR EVOLUTION.
THE GFS AND ECMWF ARE SLOWER WITH THE DEEPER COLD FRONTAL
BOUNDARY AND WOULD FAVOR SURFACE BASED STORMS DEVELOPING MAINLY
ALONG AND JUST AHEAD OF THE BOUNDARY. SURFACE BASED CONVECTION
WOULD HAVE STRONGER INSTABILITY AND SHEAR TO WORK WITH AND COULD
POSE A THREAT FOR LARGE TO VERY LARGE HAIL AS WELL AS DAMAGING
WINDS. GIVEN THE TIME OF YEAR, VERY COLD LAKE, AND TENDENCY FOR
THE NAM TO HANDLE SHALLOW COLD FRONTS BETTER, IT WOULD SEEM THE
FORMER SCENARIO WITH MOSTLY POST FRONTAL CONVECTION WOULD BE
SLIGHTLY FAVORED, BUT NEITHER SCENARIO CAN BE RULED OUT AT THIS
DISTANCE.
COLD FRONT LATE THURSDAY IS EXPECTED TO BE LAKE ENHANCED COULD
POTENTIALLY BORDER ON BECOMING A PNEUMONIA FRONT NEAR THAT LAKE
DEPENDING ON HOW WARM IT CAN PRIOR TO FRONTAL PASSAGE.
REGARDLESS, IT WILL BE A DRAMATIC TEMPERATURE CHANGE FROM NEAR
RECORD WARMTH AHEAD OF THE FRONT TO WINDY AND COLD CONDITIONS IN
THE WAKE OF THE FRONT. MUCH LIKE OUR LAST STRONG COLD FRONT,
SOME AREAS COULD SEE 24 HOUR TEMP CHANGES OF 40F BY FRIDAY
AFTERNOON, WITH WIND CHILLS MAKING IT FEEL EVEN COLDER.
TEMPERATURES DO LOOK TO GRADUALLY MODERATE OVER THE WEEKEND WITH
MAINLY DRY CONDITIONS EXPECTED.
- IZZI
AVIATION /18Z TAFS THROUGH 00Z THURSDAY/
ISSUED AT 1211 PM CDT TUE MAR 24 2026
VFR CONDITIONS WILL PREVAIL THROUGH THE FORECAST PERIOD. SOUTHERLY
WINDS WILL BECOME EAST-SOUTHEAST TO SOUTHEASTERLY THIS EVENING
AS THE BOUNDARY LAYER DECOUPLES, WITH SPEEDS UNDER 10 KNOTS.
INITIALLY SOUTHEASTERLY WINDS WILL BECOME SOUTHERLY AND GUSTY
THROUGH WEDNESDAY MORNING AND PARTICULARLY DURING THE AFTERNOON
WHEN SOME INTERMITTENT GUSTS INTO THE MID TO EVEN UPPER 20 KNOT
RANGE.
CARLAW
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...NONE.
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