012
FXUS63 KLOT 210651
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
151 AM CDT TUE JUL 21 2026
KEY MESSAGES
- THERE REMAINS A 30% CHANCE FOR A SEVERE STORM TO DEVELOP
BEFORE DAYBREAK WITH DAMAGING HAIL AS THE MAIN THREAT.
- A SERIES OF COLD FRONTS WILL MOVE THROUGH THE AREA TODAY
LEADING TO A PERIOD OF COOLER TEMPERATURES TONIGHT THROUGH
WEDNESDAY NIGHT.
- LARGE WAVES WILL MAKE SWIMMING DANGEROUS AT ALL SOUTHERN SHORE
LAKE MICHIGAN BEACHES THIS EVENING THROUGH EARLY THURSDAY
MORNING.
- A TREND TOWARD WARMER AND MORE SEASONABLE TEMPERATURES IS
EXPECTED TOWARD THE END OF THE WEEK.
DISCUSSION
ISSUED AT 151 AM CDT TUE JUL 21 2026
PER RECENT GOES-19 WATER VAPOR IMAGERY AND A HAND SURFACE
ANALYSIS, A 994 MB SURFACE LOW PRESSURE SYSTEM IS MOVING
EASTWARD THROUGH CENTRAL ONTARIO EARLY THIS MORNING. A COLD
FRONT EXTENDS SOUTHWARD AWAY FROM THE CENTER OF THE LOW PRESSURE
SYSTEM THROUGH CENTRAL WISCONSIN AND ACROSS NORTHERN IOWA. A
RIBBON OF LOWER 70 DEGREE DEW POINTS EXTENDS FROM THE MIDDLE
MISSISSIPPI RIVER VALLEY NORTHEASTWARD TOWARD THE DOOR COUNTY
PENINSULA AHEAD OF THE FRONT. REGIONAL RAOBS AND RECENT AMDAR
SOUNDINGS OUT OF ORD/MDW DEPICT AN INVERSION BETWEEN ABOUT 850
AND 750MB ABOVE WHICH LAPSE RATES EXCEED 7 K/KM. THE RESULTING
THERMODYNAMIC PROFILE IS SUPPORTING A BROAD REGION OF MUCAPE
>2000 J/KG AHEAD OF THE FRONT EARLY THIS MORNING. MEANWHILE, AN
UPPER-LEVEL SHORT-WAVELENGTH TROUGH IS RACING INTO THE GREAT
LAKES REGION ANCHORED BY A SEASONABLY STRONG UPPER-LEVEL JET
STREAK (105KT AT 300MB, PER THE 00Z MPX RAOB). PRESSURE FALLS ON
THE ORDER OF 6MB/6HR IN EASTERN ONTARIO AHEAD OF THE LOW
PRESSURE SYSTEM ARE SUPPORTING A 925-850MB LOW-LEVEL JET ACROSS
THE LOWER GREAT LAKES, AS CONFIRMED BY 45 TO 50KT OF FLOW AT 2KM
SAMPLED BY THE KLOT VWP.
OVER THE PAST FEW HOURS, HAVE BEEN WATCHING A ROUGHLY 50 MILE
WIDE AREA OF PERCOLATING RADAR RETURNS FIRST DEVELOP IN EASTERN
IOWA AND THEN CROSS INTO NORTHWESTERN ILLINOIS. THE RADAR
RETURNS APPEAR TO BE ASSOCIATED WITH ISENTROPIC ASCENT (VIA THE
LLJ) INTERACTING WITH THE BASE OF THE CAPPING INVERSION AND ARE
PROTOTYPICAL OF NOCTURNAL ATTEMPTS FOR THUNDERSTORMS TO INITIATE
WITHIN A STEEP MID-LEVEL LAPSE RATE ENVIRONMENT. THUS FAR,
ATTEMPTS FOR SUSTAINED CONVECTION HAVE BEEN SUBDUED LIKELY OWING
TO SYNOPTIC-SCALE SUBSIDENCE ALONG THE RIGHT EXIT REGION OF THE
AFOREMENTIONED UPPER-LEVEL JET STREAK CENTERED IN WISCONSIN AND
ON THE NORTHEASTERN FLANK OF AN EXPANSIVE MCS MOVING INTO
NORTHERN MISSOURI. HOWEVER, DO HAVE CONCERN THAT CONTINUED
MOISTENING AT THE BASE OF THE CAP WILL EVENTUALLY ALLOW ONE OR
TWO STORMS TO DEVELOP AT SOME POINT EARLY THIS MORNING,
PARTICULARLY IN THE 3 TO 6 AM WINDOW OVER THE CHICAGO
METROPOLITAN AREA AS THE UPPER-LEVEL SHORTWAVE MAKES ITS CLOSEST
APPROACH. PROVIDED A STORM DOES DEVELOP, STRONG EFFECTIVE SHEAR
(60KT IN THE CONVECTIVE LAYER ORIENTED DUE EASTERLY) WOULD
SUPPORT SPLITTING SUPERCELLS WITH A THREAT FOR DAMAGING HAIL.
WILL MAINTAIN A 30% CHANCE OF STORMS THROUGH THE MORNING FOR
THIS THREAT KEEPING IN MIND IT EQUATES TO A 70% CHANCE A STORM
DOESN'T DEVELOP.
THE THREAT FOR STORMS WILL END BY DAYBREAK AS THE LOW-LEVEL JET
PEELS EASTWARD AND SUBSIDENCE INCREASES IN THE WAKE OF THE
UPPER-LEVEL SHORT-WAVELENGTH TROUGH. WINDS WILL GRADUALLY TURN
NORTHWESTERLY THROUGH THE MORNING AS THE FRONTAL BOUNDARY WORKS
THROUGH THE AREA, THOUGH NOT BEFORE TEMPERATURES HAVE AN
OPPORTUNITY TO BOUNCE INTO THE LOW TO MID 80S. LATER THIS
AFTERNOON, A SECONDARY FRONT WILL RACE SOUTHWARD DOWN LAKE
MICHIGAN LEADING TO A NORTHERLY WIND SHIFT AND MORE NOTICEABLE
DROP IN TEMPERATURES. THE SOUTHWARD ADVECTION OF A SEASONABLY
COOL AIRMASS (850MB TEMPERATURES TUMBLING TOWARD +10C) ATOP THE
RELATIVELY WARM WATERS OF LAKE MICHIGAN (NEAR +23C) WILL SET THE
STAGE FOR A RARE JULY LAKE EFFECT RAIN SHOWER SET-UP ALONG AND
BEHIND THE FRONT ALONG THE NORTHEASTERN IL AND NORTHWESTERN IN
SHORELINE. IN FACT, WOULD NOT BE SURPRISED FOR A WATERSPOUT OR
TWO TO DEVELOP PARTICULARLY ALONG ANY FOCUSED CONVERGENCE BANDS
OVERNIGHT. WAVES WILL QUICKLY BUILD IN THE WAKE OF THE FRONT
LEADING TO A HIGH SWIM RISK AFTER SUNSET AT ALL SOUTHERN LAKE
MICHIGAN BEACHES.
ON WEDNESDAY, AN UPPER-LEVEL WAVE EMBEDDED IN RESIDENT BROAD
UPPER-LEVEL CYCLONIC FLOW WILL RIPPLE OVER THE LOWER GREAT
LAKES. ANOTHER BOUT OF LOW-LEVEL COLD AIR ADVECTION WILL KEEP
TEMPERATURES SEASONABLY COLD WITH HIGHS AROUND 70 DEGREES. CAN
EASILY ENVISION STRATOCUMULUS CLOUDS FILLING THE SKIES WITH
PERHAPS A FEW SPRINKLES BY LATE MORNING OWING TO STEEP LOW-LEVEL
LAPSE RATES OUTSIDE OF CONTINUED SCATTERED LAKE EFFECT RAIN
SHOWERS. CONTINUED NORTHERLY WINDS DOWN THE FETCH OF LAKE
MICHIGAN WILL MAINTAIN DANGEROUS SWIMMING CONDITIONS AT ALL LAKE
MICHIGAN BEACHES THROUGH THE ENTIRE DAY.
WEDNESDAY NIGHT, THE CYCLONIC SHEAR AXIS OF THE AGGREGATE
UPPER-LEVEL TROUGHING WILL SHIFT EASTWARD TOWARD LOWER
MICHIGAN, SETTING THE STAGE FOR CLOUDS TO CLEAR. OVERNIGHT LOWS
WILL BE COOL IF NOT LOCALLY CHILLY WITH A FEW UPPER 40S READINGS
NOT OUT OF QUESTION NEAR THE WISCONSIN STATE LINE.
OUR BRIEF TASTE OF FALL DURING THE MIDDLE OF SUMMER WILL BE
SHORT-LIVED AS LARGE-SCALE UPPER-LEVEL RIDGING BEGINS TO
REBUILD ACROSS THE CENTRAL UNITED STATES TOWARD THE END OF THE
WEEK. ACCORDINGLY, TEMPERATURES WILL INCREMENTALLY INCREASE
THURSDAY ONWARD WITH A RETURN OF HIGHS IN THE UPPER 80S TO LOWER
90S BY THIS WEEKEND. WE CONTINUE TO SEE A SIGNAL FOR A
CONVECTIVE EVENT IN THE GENERAL REGION SOMETIME IN THE JULY
27-29 TIMEFRAME.
BORCHARDT
AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/
ISSUED AT 1222 AM CDT TUE JUL 21 2026
KEY MESSAGES FOR THE 06Z TAF PERIOD:
* STILL SEEING A CHANCE FOR THUNDERSTORMS AFTER 06Z INTO THE
PREDAWN HOURS.
* GUSTY NORTHERLY WINDS TUESDAY EVENING INTO WEDNESDAY.
THE POTENTIAL IS STILL ALIVE AND WELL FOR A PERIOD OF SCATTERED
THUNDERSTORMS OVERNIGHT INTO THE PREDAWN HOURS. A NEWLY-
DEVELOPED CLUSTER OF CONVECTIVE SHOWERS IS ALREADY MOVING ACROSS
NW IL AND CLOSING IN ON RFD VCY. RFD MAY SEE SHOWERS AND MAYBE
A STORM NEARBY PRIOR TO 06Z, WITH CHANCES BUILDING ALL AROUND
AFTER 06Z. STORMS COULD LINGER INTO THE PREDAWN HOURS OF
TUESDAY, POSSIBLY EVEN STRETCHING INTO DAYBREAK AT MDW AND
ESPECIALLY GYY. SOME STORMS COULD BE ON THE STRONGER SIDE WITH
HIGH WINDS AND HEAVY RAIN LEADING TO REDUCED VSBYS. GIVEN
CURRENT EXPECTATIONS, FELT HANGING ONTO PROB30S FOR STORMS WAS
APPROPRIATE. LOOK FOR TACTICAL AMDS AS CONFIDENCE BUILDS.
W WINDS WILL BUILD IN THE WAKE OF THIS WAVE, GOING NW DURING
THE MORNING AND GUSTING TO 20 TO 25 KT FOR THE LATE MORNING AND
AFTERNOON. WINDS WILL SUBSIDE SOME BUT REMAIN BREEZY FOR THE
EVENING AND NIGHT AS DIRECTION TURNS NORTHERLY. WINDS MAY TOGGLE
ON EITHER SIDE OF NORTHERLY FROM THIS EVENING THROUGH THE
BETTER PART OF WEDNESDAY. NNW IS FAVORED THROUGH THE EARLY
EVENING BEFORE FLOPPING TO NNE FOR THE NIGHT, THEN LIKELY BACK
TO NNW BY DAWN ON WEDNESDAY.
DOOM
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY UNTIL 6 AM CDT EARLY THIS MORNING FOR THE
IL AND IN NEARSHORE WATERS.
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