940
FXUS63 KLOT 150836
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
336 AM CDT FRI MAY 15 2026
KEY MESSAGES
- CHANCE OF SHOWERS AND A FEW THUNDERSTORMS MAINLY ACROSS
CENTRAL IL THIS MORNING. SCATTERED STRONG STORM CHANCES THEN
EXIST LATE TONIGHT INTO EARLY SATURDAY AND SATURDAY NIGHT INTO
EARLY SUNDAY.
- BETTER POTENTIAL FOR WIDESPREAD SHOWERS AND STORMS, SOME
POSSIBLY SEVERE, MONDAY INTO TUESDAY.
- WARMER SUMMER-LIKE TEMPERATURES THROUGH THE WEEKEND AND INTO
EARLY NEXT WEEK.
DISCUSSION
ISSUED AT 336 AM CDT FRI MAY 15 2026
EARLY MORNING GOES VAPOR IMAGERY DEPICTS AN UPPER LEVEL TROUGH
EXTENDING FROM A CLOSED LOW OVER MANITOBA SOUTHWARD ACROSS THE
UPPER MIDWEST. DEEP (SUB-990 MB) SURFACE LOW PRESSURE WAS
STACKED BENEATH THE MID-LEVEL CIRCULATION, WITH A COLD FRONT
TRAILING SOUTH-SOUTHWEST ACROSS THE PLAINS TO ANOTHER AREA OF
LOW PRESSURE OVER SOUTHEASTERN CO. BROAD SOUTHERLY LOW-LEVEL
FLOW WAS BEING INDUCED DOWNSTREAM OF THE UPPER TROUGH AND
ENHANCED MID-UPPER WESTERLY FLOW ALOFT, WITH A 35-40 KT 850 MB
LOW LEVEL JET ORIENTED FROM OK AND EASTERN KS INTO SOUTHEAST IA.
ASSOCIATED ISENTROPIC ASCENT INTO NORTHERN IL HAS RESULTED IN
SATURATION ON THE 305-310K SURFACES NEAR THE BASE OF 6.5-7 C/KM
MID-LEVEL LAPSE RATES, ALLOWING DEVELOPMENT OF ELEVATED
CONVECTIVE SHOWERS/THUNDERSTORMS WITHIN WEAK (<200 J/KG) OF
MUCAPE WHICH DECREASES WITH EASTWARD EXTENT PER RAP SOUNDINGS
AND MESOANALYSIS FIELDS. MORE FAVORABLE CONDITIONS FOR ELEVATED
CONVECTION WERE LOCATED FARTHER SOUTHWEST OF THE FORECAST AREA
ACROSS NORTHEASTERN MO, WHERE THE COMBINATION OF A SUBTLE,
COMPACT MID-LEVEL SHORT WAVE, GREATER ELEVATED INSTABILITY AND
STRONGER CONVERGENCE ON THE NOSE OF THE 850 MB JET WAS EVIDENT
IN THE DEVELOPMENT OF SCATTERED THUNDERSTORMS. HIGH-RES CAMS
INDICATE THESE STORMS WILL SPREAD EAST ACROSS CENTRAL IL THROUGH
DAYBREAK. GRADUAL VEERING OF THE LOW LEVEL JET LOOKS TO KEEP
THE BULK OF THIS ACTIVITY SOUTH OF THE WFO CHICAGO CWA, THOUGH
AREAS SOUTH OF THE ILLINOIS/KANKAKEE RIVER VALLEYS WILL LIKELY
SEE SOME SHOWERS AND POTENTIALLY ISOLATED THUNDER ALONG THE
NORTHERN PERIPHERY THIS MORNING.
AFTER THE PASSAGE OF THESE MORNING SHORT WAVE TROUGH AXES,
RELATIVELY QUIET WEATHER CONDITIONS APPEAR IN STORE FOR THE
FORECAST AREA THE REMAINDER OF THE DAY THROUGH THIS EVENING.
INCREASING SOUTH-SOUTHWEST FLOW AHEAD OF THE PLAINS COLD
FRONTAL TROUGH AND SURFACE LOW WILL SUPPORT MODEST BUT
PERSISTENT WARM ADVECTION, WHICH COMBINED WITH DECREASING CLOUD
COVER BY THIS AFTERNOON SHOULD SUPPORT HIGHS FROM THE LOW 70S IN
THE EAST TO THE MID-UPPER 70S WEST. SOUTH WINDS GUSTING 20-25
MPH SHOULD PREVENT SIGNIFICANT LAKE COOLING, EXCEPT ALONG THE
IMMEDIATE IL SHORE NORTH OF CHICAGO WHERE A SLIGHT SOUTHEAST
WIND COMPONENT IS EXPECTED. BREEZY WINDS WILL PERSIST INTO THE
EVENING, WITH TEMPS SLOWLY FALLING IN THE 60S.
THUNDERSTORM CHANCES WILL INCREASE WEST OF THE MISSISSIPPI RIVER THIS
AFTERNOON AND EVENING WHERE DAYTIME HEATING, RETURNING LOW
LEVEL MOISTURE AND CONVERGENCE ALONG THE COLD FRONTAL BOUNDARY
WILL BE ENHANCED BY COOLING ALOFT AND INCREASING MID-LEVEL
WINDS ASSOCIATED WITH ANOTHER MID-LEVEL SHORT WAVE TRACKING EAST
ACROSS THE REGION. THUNDERSTORM WHICH DEVELOP ACROSS PARTS OF
IA AND SOUTHEAST MN LATE THIS AFTERNOON AND EARLY EVENING MAY
EVOLVE INTO ONE OR MORE LINEAR CLUSTERS, WHICH WOULD EVENTUALLY
MOVE INTO NORTHERN IL TOWARD/AFTER MIDNIGHT TONIGHT. WHILE
SURFACE-BASED INSTABILITY WILL BE WEAKER WITH EASTERN EXTENT AND
LOSS OF DIURNAL HEATING OVERNIGHT, GUIDANCE SUGGESTS SUFFICIENT
MUCAPE (~2000 J/KG) AND DEEP-LAYER SHEAR (~35 KTS) TO MAINTAIN
SUFFICIENT STORM ORGANIZATION TO PRESENT A WIND/MARGINAL HAIL
THREAT ESPECIALLY INTO OUR NORTHERN IL COUNTIES BEFORE
DECREASING WITH FURTHER EASTWARD EXTENT. SPC'S NEW DAY 1 OUTLOOK
APPROPRIATELY HAS BROUGHT THE MARGINAL (LEVEL 1 OF 5) RISK INTO
THE CHICAGO METRO AREA AND THE SLIGHT (LEVEL 2 OF 5) NEAR THE
I-39 CORRIDOR FOR OVERNIGHT TONIGHT.
BY SATURDAY MORNING THE PRIMARY MCS FROM TONIGHT IS EXPECTED TO
MOVE EAST OF THE AREA, THOUGH A COMBINED OUTFLOW/COLD FRONTAL
BOUNDARY IS FORECAST TO BECOME NEARLY STATIONARY ACROSS A
PORTION OF THE FORECAST AREA (THOUGH THE EXACT POSITION IS OF
LOW CONFIDENCE FROM CURRENT GUIDANCE RUNS). LINGERING WARM
ADVECTION/ASCENT MAY BE ONGOING ATOP THE LOW-LEVEL BOUNDARY
SATURDAY MORNING, WHICH MAY KEEP THE THREAT OF SHOWERS AND A FEW
THUNDERSTORMS AROUND ESPECIALLY ACROSS THE SOUTHEASTERN CWA.
HAVE LIMITED NBM BLENDED POPS BUT MAINTAINED SOME CHANCE POPS
(30-50%) MAINLY SOUTH OF I-80 SATURDAY AFTERNOON, WHERE BETTER
POTENTIAL OF ADDITIONAL DIURNAL SHOWERS/STORMS WOULD MORE LIKELY
BE FOCUSED NEAR/SOUTH OF THE STALLED BOUNDARY. POTENTIAL FOR
LINGERING CLOUD COVER AND OUTFLOW-DISTURBED WIND FIELD COULD
LIMIT HIGH TEMPERATURES SOMEWHAT, THOUGH CURRENT MODEL LOW-LEVEL
THERMAL FIELDS SUGGEST AFTERNOON HIGHS FROM NEAR 80 EAST/FAR
SOUTH TO THE LOW-MID 80S FARTHER NORTH/WEST WOULD BE ATTAINABLE
WITH SOME AFTERNOON SUNSHINE.
GUIDANCE IS IN GOOD OVERALL AGREEMENT IN RISING HEIGHTS ACROSS
THE WESTERN GREAT LAKES REGION LATE SATURDAY/SATURDAY EVENING AS
THE UPPER PATTERN BEGINS TO EVOLVE INTO A WESTERN-CONUS LONG
WAVE UPPER TROUGH AND A DOWNSTREAM UPPER RIDGE DEVELOPING
DOWNSTREAM TO THE EAST. THIS INDUCES RENEWED SURFACE LOW
DEVELOPMENT OVER THE HIGH PLAINS, WITH A CORRESPONDING INCREASE
IN SOUTHERLY RETURN FLOW INTO THE MISSISSIPPI VALLEY AND GREAT
LAKES REGION. ALSO DEPICTED IS A NORTHEASTWARD-PROPAGATING SHORT
WAVE LATER SATURDAY NIGHT INTO EARLY SUNDAY WHICH MAY SUPPORT
ADDITIONAL SHOWER/THUNDERSTORM DEVELOPMENT WITH THE STALLED
FRONT/OUTFLOW BOUNDARY LIFTING BACK NORTH AS A WARM FRONT.
AGAIN, DIURNALLY LESS-FAVORABLE TIMING WOULD TEND TO LIMIT
SURFACE-BASED INSTABILITY, THOUGH GUIDANCE FORECASTS OF
1500-2000 J/KG OF MUCAPE AND 30-35 KTS OF DEEP-LAYER SHEAR COULD
SUPPORT STORM ORGANIZATION AND A MARGINAL WIND THREAT WITH
STRONGER STORMS. SPC DAY 2 OUTLOOK INCLUDES OUR AREA IN A LEVEL
1 OF 5 (MARGINAL) RISK FOR SATURDAY NIGHT/EARLY SUNDAY.
CURRENT GUIDANCE INDICATES BREEZY (30-35 MPH GUSTS) SOUTH WINDS
WILL DEVELOP SUNDAY AFTERNOON, AFTER THE WARM FRONT LIFTS NORTH
OF THE AREA. WITH ANY MORNING SHOWERS/STORMS DEPARTING AND
DECREASING CLOUD COVER IN THE WARM SECTOR, HIGH TEMPERATURES IN
THE MID-UPPER 80S APPEAR LIKELY AT THIS DISTANCE. BREEZY AND
MILD CONDITIONS LOOK TO PERSIST SUNDAY NIGHT, WITH LOWS
GENERALLY IN THE MID-60S.
THUNDERSTORM CHANCES THEN RETURN MONDAY AND TUESDAY, AS THE
CENTRAL PLAINS SURFACE LOW SLOWLY LIFTS NORTHEAST THROUGH THE
NORTHERN GREAT LAKES AND EVENTUALLY TRAILS A COLD FRONT THROUGH
THE FORECAST AREA. AHEAD OF THE COLD FRONT, THE FORECAST AREA
WILL RESIDE IN BREEZY WARM SOUTH FLOW, CHARACTERIZED BY DAYTIME
TEMPERATURES IN THE 80S AND INCREASING LOW-LEVEL MOISTURE RETURN
(SURFACE DEW POINTS POTENTIALLY UPPER 60S-TO NEAR 70F) FROM THE
WESTERN GULF. RESULTING INSTABILITY, COMBINED WITH MID-LEVEL
FLOW NEAR 40 KTS WOULD POTENTIALLY SUPPORT STRONG/SEVERE
THUNDERSTORMS, THOUGH FORCING MECHANISMS REMAIN UNCLEAR WITHIN
THE BROAD WARM SECTOR MAKING CONVECTIVE TIMING/EVOLUTION OF LOW
CONFIDENCE AT THIS DISTANCE. SPC DAYS 4-5 OUTLOOKS CONTINUE TO
EXTEND SEVERE PROBABILITIES INTO OUR AREA. QUIET AND COOLER
CONDITIONS ARE THEN EXPECTED BY MID-WEEK, AS SURFACE HIGH
PRESSURE BUILDS ACROSS THE REGION IN THE WAKE OF THE TUESDAY
COLD FRONTAL PASSAGE.
RATZER
AVIATION /06Z TAFS THROUGH 12Z SATURDAY/
ISSUED AT 1244 AM CDT FRI MAY 15 2026
FORECAST CONCERNS FOR THE TERMINALS...
- PERIOD OF LIGHT SHOWERS/SPRINKLES OVERNIGHT ENDING BY
DAYBREAK.
- BREEZY SOUTH-SOUTHWEST WINDS FRIDAY WITH GUSTS UPWARDS OF 25
KTS.
- ADDITIONAL ROUND OF SHOWERS AND THUNDERSTORMS FRIDAY NIGHT
INTO SATURDAY MORNING.
A SHORTWAVE DISTURBANCE CONTINUES TO TRAVERSE ACROSS IA AND
NORTHERN MO THIS EVENING WHICH HAS ALLOWED FOR SOME SHOWERS TO
DEVELOP. WHILE THERE WERE A COUPLE OF LIGHTNING STRIKES WITHIN
THE SHOWERS EARLIER, RECENT RADAR TRENDS CONTINUE TO SHOW THE
CORES STRUGGLING TO MAINTAIN STRENGTH. THEREFORE, THE THREAT FOR
THUNDER AT THE TERMINALS TONIGHT REMAINS VIRTUALLY ZERO. THAT
SAID, THE NARROW BAND OF LIGHT SHOWERS/SPRINKLES THAT IS
DRIFTING OVER NORTHERN IL DOES LOOK TO PIVOT OVER THE TERMINALS
LATER TONIGHT AND MAY RESULT IN A BRIEF (1-2 HOUR) PERIOD OF
RAIN. HOWEVER, GIVEN THE VERY DRY SUB-CLOUD LAYER NOTED IN AMDAR
SOUNDINGS CONFIDENCE IS LOW AS TO HOW FAR EAST THE
SHOWERS/SPRINKLES WILL GET SO HAVE OPTED TO JUST MAINTAIN THE
PROB30S AT THE CHICAGO AREA SITES FOR NOW.
ANY SHOWERS THAT DO MAKE IT INTO THE CHICAGO SITES SHOULD TAPER
BY DAYBREAK RESULTING IN A MOSTLY DRY FRIDAY FOR US. THOUGH
THOSE SOUTH OF I-80 COULD SEE SOME LINGERING RAIN THROUGH MID-
MORNING AS ANOTHER CLUSTER OF SHOWERS DRIFTS THROUGH. OTHERWISE,
EXPECT WINDS TO BECOME SOUTH-SOUTHWEST AND INCREASE IN SPEED
FRIDAY MORNING. GUSTS LOOK TO GENERALLY TOP OUT IN THE 20-25 KT
RANGE FRIDAY, BUT IF DEEPER MIXING CAN BE ACHIEVED THEN SOME
LOCALLY HIGHER GUSTS MAY BE REALIZED. WIND GUSTS ARE EXPECTED TO
EASE FRIDAY EVENING AS THE ATMOSPHERE DECOUPLES WITH WINDS
BECOMING MORE SOUTHERLY. HOWEVER, THERE IS A CHANCE FOR
DIRECTIONS TO VARY FROM A 170-190 DIRECTION DURING THE EVENING
HOURS.
HEADING INTO FRIDAY EVENING, ANOTHER SHORTWAVE DISTURBANCE WILL
BE PIVOTING ACROSS THE MIDWEST AND SHOULD RESULT IN ANOTHER
ROUND OF SHOWERS AND THUNDERSTORMS DEVELOPING IN IA. WHILE
GUIDANCE REMAINS IN GOOD AGREEMENT THAT THESE SHOWERS/STORMS
WILL PERSIST INTO NORTHERN IL AND NORTHWEST IN FRIDAY NIGHT,
CONFIDENCE ON THEIR OVERALL INTENSITY AND COVERAGE IS SOMEWHAT
LOW. THE REASON IS BECAUSE THE CLUSTER OF SHOWERS AND STORMS MAY
BE OUTRUNNING SOME OF THE BETTER INSTABILITY AND MOISTURE WHICH
COULD ALLOW THEM TO WEAK WITH EASTWARD EXTENT. REGARDLESS,
CONFIDENCE IS HIGH ENOUGH TO WARRANT THE INTRODUCTION OF SHRA
WITH A PROB30 FOR TSRA IN THE 30-HOUR TAFS. NOTE THAT A FORMAL
MENTION WAS LEFT OUT OF THE 24-HOUR TAFS SINCE RAIN LOOKS TO
HOLD OFF UNTIL RIGHT AT THE TAIL END OF THE PERIOD, BUT WILL
NEED TO ADD IT WITH THE 12Z TAF ISSUANCE.
VFR CONDITIONS ARE EXPECTED TO PREVAIL THROUGH THE ENTIRE TAF
PERIOD.
YACK
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY FROM 8 AM THIS MORNING TO 7 PM CDT THIS
EVENING FOR THE IL AND IN NEARSHORE WATERS.
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