100
FXUS63 KLOT 171536
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
1036 AM CDT SUN MAY 17 2026
KEY MESSAGES
- SCATTERED SHOWERS AND EMBEDDED THUNDERSTORMS THIS MORNING,
MAINLY FOR AREAS ALONG AND NORTHWEST OF A PERU, IL TO
WAUKEGAN, IL LINE.
- ADDITIONAL ROUNDS OF SHOWERS AND THUNDERSTORMS MONDAY
THROUGH TUESDAY. SOME OF WHICH COULD BE SEVERE.
- SUMMER-LIKE WARMTH AND BREEZY SOUTH-SOUTHWEST WINDS WILL
PREVAIL THROUGH TUESDAY BEFORE COOLER TEMPERATURES RETURN FOR
THE MIDDLE OF THE WEEK.
UPDATE
ISSUED AT 1035 AM CDT SUN MAY 17 2026
A ROBUST, CONVECTIVELY-AUGMENTED MCV IS IN THE PROCESS OF
SHIFTING INTO SOUTHWEST WISCONSIN. LATEST OBJECTIVE ANALYSES
REVEAL A SIGNIFICANT LOW AND MID-LEVEL MASS RESPONSE ASSOCIATED
WITH THIS FEATURE, WITH 700 MB SOUTHWESTERLY FLOW NEAR 50-60
KNOTS IN A VERY NARROW CORRIDOR IMMEDIATELY SOUTHEAST OF THE
CORE OF THE VORT MAX. GIVEN THE MID-LATE MORNING ARRIVAL OF THIS
FEATURE, COUPLED WITH EXPANSIVE CLOUD COVER, IT APPEARS THE
THREAT FOR SEVERE CONVECTION, AT LEAST IN OUR IMMEDIATE AREA,
REMAINS LOW AT THIS TIME. IF THIS FEATURE WERE SEVERAL HOURS
SLOWER AND PERHAPS A BIT FARTHER SOUTHEAST, THIS WOULD LIKELY
BE A MUCH DIFFERENT STORY, WITH A NOTABLY HIGHER RISK FOR SEVERE
WEATHER, SUPERCELLS, AND AN ATTENDANT TORNADO THREAT GIVEN A
FEW MORE HOURS OF HEATING AND THE STRONG DEGREE OF LOW- LEVEL
CLOCKWISE HODOGRAPH CURVATURE. STILL WILL BE KEEPING A VERY
CLOSE EYE ON ANY DEEPER CORES THAT MANAGE TO SNEAK TOWARDS THE
WI STATE LINE, HOWEVER. THE PRIMARY THREAT AT THIS TIME APPEARS
TO BE FROM SMALL HAIL IF ANY MORE SUSTAINED STORMS MANAGE TO
DEVELOP ACROSS FAR NORTHERN ILLINOIS.
BY LATER THIS AFTERNOON, THE MCV AND ASSOCIATED BULK OF FORCING
WILL BE MOVING NORTH OF THE REGION. SUPPOSE SOME LINGERING
SHOWERS COULD STILL BE POSSIBLE ACROSS OUR WEST WITHIN THE
LINGERING MID-LEVEL MOIST AXIS.
CARLAW
DISCUSSION
ISSUED AT 330 AM CDT SUN MAY 17 2026
A PAIR OF MCVS ARE TRAVERSING SOUTHERN IA AND NORTHERN MO THIS
MORNING AHEAD OF A MID-LEVEL SHORTWAVE TROUGH THAT IS LIFTING
TOWARDS THE UPPER MISSISSIPPI VALLEY. GIVEN THE TRAJECTORY OF
THE SHORTWAVE, THE TWO MCVS SHOULD GET PUSHED NORTHEAST ACROSS
EASTERN IA INTO SOUTHWEST WI THROUGH THE MORNING WHICH IN TURN
SHOULD KEEP THE BULK OF ANY ASSOCIATED SHOWER AND THUNDERSTORM
ACTIVITY IN NORTHWEST IL, EASTERN IA, AND SOUTHERN WI. HOWEVER,
WITH HOW BROAD THE RAIN SHIELD IS IN CURRENT RADAR IMAGERY
SUSPECT THAT SOME SCATTERED SHOWERS WILL IMPACT PORTIONS OF OUR
CWA (MAINLY AREAS ALONG AND NORTHWEST OF A PERU, IL TO WAUKEGAN,
IL LINE) AROUND DAYBREAK THROUGH LATE MORNING. WITH INSTABILITY
LOOKING RATHER LIMITED IN RECENT FORECAST SOUNDINGS THE THREAT
FOR THUNDER IN OUR AREA IS LOW, BUT CANNOT BE FULLY RULED OUT SO
HAVE MAINTAINED A 20% CHANCE FOR SUCH IN THE FORECAST.
ONCE THESE SHOWERS/ISOLATED STORMS LIFT INTO WI LATER THIS
MORNING, DRY CONDITIONS ARE EXPECTED FOR THE REST OF OUR SUNDAY.
WITH SKIES EXPECTED TO BECOME PARTLY TO MOSTLY SUNNY THIS
AFTERNOON AND THE PERSISTENT SOUTH-SOUTHWEST FLOW, TEMPERATURES
WILL TOP OUT IN THE LOWER TO MID-80S. HOWEVER, A WEAK FRONT AND
OUTFLOW BOUNDARY FROM THE AFOREMENTIONED SHOWERS/STORMS IS
FORECAST TO SETTLE NEAR THE IL-WI LINE WHICH WILL KEEP WINDS
MORE SOUTHEASTERLY ALONG THE NORTHERN IL SHORE AND THUS KEEP
TEMPERATURES COOLER IN THE 70S IN THESE LOCATIONS. ADDITIONALLY,
WINDS TODAY WILL BE ON THE BREEZY SIDE WITH GUSTS AROUND 25-30
MPH. IN FACT IF DEEPER MIXING CAN BE ACHIEVED THIS AFTERNOON,
LOCALLY HIGHER GUSTS UPWARDS OF 35+ MPH MAY ALSO BE SEEN.
HEADING INTO TONIGHT, WIND GUSTS WILL EASE AS THE ATMOSPHERE
ATTEMPTS TO DECOUPLE, THOUGH WITH THE CONTINUED WARM ADVECTION
AND THE TIGHT PRESSURE GRADIENT DUE TO THE DEVELOPING SURFACE
LOW IN THE PLAINS A FEW 20-25 MPH GUSTS MAY LINGER THROUGH THE
NIGHT. OTHERWISE, EXPECT A MOSTLY DRY NIGHT WITH MILD
TEMPERATURES IN THE MID TO UPPER 60S (LOWER 70S CLOSER TO
CHICAGO).
WHILE WE ENJOY A GENERALLY QUIET NIGHT, THE BROAD UPPER TROUGH
OVER THE WESTERN CONUS WILL BE EJECTING EASTWARD WITH A COUPLE
OF SHORTWAVES TRAVERSING THE PATTERN IN THE PLAINS AND MID-
MISSISSIPPI VALLEY. THESE SHORTWAVES ARE EXPECTED TO LEAD TO THE
DEVELOPMENT OF SHOWERS AND THUNDERSTORMS ACROSS THE CENTRAL
PLAINS AND IA ALONG A FRONTAL BOUNDARY WHICH WILL IN TURN MOVE
TOWARDS NORTHERN IL AND NORTHWEST IN ON MONDAY. AS A RESULT, THE
SHOWERS AND STORMS THAT DEVELOP IN THE PLAINS TONIGHT ARE
FORECAST TO MOVE INTO OUR AREA EARLY MONDAY MORNING (LIKELY
AFTER DAYBREAK). GIVEN THAT THESE SHOWERS/STORMS WILL ARRIVE
DURING ON NON-DIURNALLY FAVORABLE TIME, INSTABILITY SHOULD BE
SOMEWHAT LIMITED AND THEREFORE ALLOW THE SHOWERS/STORMS TO ENTER
IN A DECAYING STATE. HOWEVER, IF THE STORMS ARRIVE IN A MORE
ORGANIZED FASHION THE STEEP LAPSE RATES (AROUND 7-8 C/KM) AND
30-35 KTS OF SHEAR COULD SUPPORT A COUPLE OF STRONGER STORMS
CAPABLE OF GUSTY WINDS AND HAIL.
DEPENDING ON HOW WIDESPREAD AND INTENSE SHOWERS/STORMS ARE
MONDAY MORNING WILL HAVE AN IMPACT ON WHETHER OR NOT ADDITIONAL
STORMS DEVELOP IN OUR AREA MONDAY AFTERNOON. WHILE GUIDANCE IS
IN DECENT AGREEMENT ON THE MORNING SHOWERS/STORMS BEING IN A
DECAYING STATE (I.E. MORE SCATTERED COVERAGE), FORECAST
SOUNDINGS DO SHOW THE ATMOSPHERE TO BE WEAKLY CAPPED MONDAY
AFTERNOON. COUPLE THIS WITH THE FACT THAT THE BETTER FORCING IS
FORECAST TO RESIDE IN THE CENTRAL PLAINS AND WESTERN IA (CLOSER
TO THE UPPER TROUGH) AND MOST GUIDANCE IS SUGGESTING THAT
NORTHERN IL AND NORTHWEST IN COULD STAY DRY MONDAY AFTERNOON.
THAT SAID, IF ANY THUNDERSTORMS DO DEVELOP MONDAY AFTERNOON THE
WARM AND WEAKLY CAPPED AIRMASS IN COMBINATION WITH 40-45 KTS OF
SHEAR WILL SUPPORT A THREAT FOR SEVERE THUNDERSTORMS. WITH WIND
PROFILES LOOKING MOSTLY UNIDIRECTIONAL, THE MAIN THREAT WITH
ANY SEVERE STORMS SHOULD BE DAMAGING WINDS AND LARGE HAIL.
THOUGH, IF STORMS DEVELOP AND/OR LINGER INTO THE EVENING WHERE
THE LOW-LEVEL JET INCREASES THE LOW-LEVEL SHEAR THEN A TORNADO
THREAT COULD MATERIALIZE. BECAUSE OF THIS POTENTIAL THE STORM
PREDICTION CENTER HAS KEPT ALL OF NORTHERN IL AND NORTHWEST IN
IN A LEVEL 2 OF 5 RISK FOR SEVERE WEATHER ON MONDAY.
REGARDLESS OF HOW MONDAY AFTERNOON GOES, THE AFOREMENTIONED
FRONTAL BOUNDARY WILL BEGIN TO MOVE INTO THE AREA LATE MONDAY
NIGHT INTO TUESDAY WHICH WILL BRING ADDITIONAL PERIODS OF
SHOWERS AND THUNDERSTORMS. CONCEPTUALLY SPEAKING, THE MORNING
ROUND OF STORMS ON TUESDAY SHOULD PUSH THE EFFECTIVE FRONTAL
BOUNDARY MORE INTO OUR EASTERN CWA (NAMELY AREAS EAST OF I-55)
AND THAT SHOULD BE WHERE THE BULK OF ANY SHOWERS/STORMS THAT
REDEVELOP TUESDAY AFTERNOON RESIDE. NEVERTHELESS, ANY STORMS
THAT DEVELOP ON TUESDAY WILL ONCE AGAIN HAVE THE POTENTIAL TO
BECOME SEVERE DUE TO THE 35-40 KTS OF SHEAR AND DECENT
INSTABILITY. THE MAIN THREAT WITH ANY SEVERE STORMS ON TUESDAY
LOOKS TO BE MAINLY DAMAGING WINDS AND HAIL, HOWEVER, AS WIND
PROFILES SHOULD BE UNIDIRECTIONAL BY THAT POINT.
OUTSIDE OF THE SHOWER AND STORM CHANCES MONDAY AND TUESDAY,
BOTH DAYS WILL ALSO FEATURE SUMMER-LIKE WARMTH. HIGHS BOTH DAYS
ARE EXPECTED TO BE IN THE LOWER TO MID-80S, THOUGH THE PRESENCE
OF STORMS COULD LIMIT WARMTH IN SOME LOCATIONS. WINDS BOTH DAYS
WILL ALSO BE BREEZY WITH SOUTHWESTERLY GUSTS AROUND 25-35 MPH
EXPECTED.
THE FRONTAL BOUNDARY WILL MOVE THROUGH OUR AREA TUESDAY NIGHT
BRINING AN END TO THE SHOWERS AND STORMS AS HIGH PRESSURE BUILDS
INTO THE GREAT LAKES. THEREFORE, THE MIDDLE PART OF THE WEEK
LOOKS TO BE RAIN FREE. TEMPERATURES THOUGH WILL BE NOTABLY
COOLER WITH HIGHS ONLY IN THE 60S FOR WEDNESDAY AND THURSDAY
INLAND AND IN THE 50S NEAR THE LAKE DUE TO ONSHORE WINDS. AS WE
HEAD TOWARDS MEMORIAL DAY WEEKEND ANOTHER BROAD UPPER TROUGH IS
FORECAST TO DEVELOP OVER THE WESTERN CONUS. WHILE THIS SHOULD
ALLOW MORE SOUTHWEST FLOW TO RETURN AND WARM TEMPERATURES BACK
TOWARDS MORE SEASONAL VALUES, IT WILL ALSO BRING CHANCES FOR
SHOWERS AND POSSIBLE THUNDERSTORMS LATE WEEK INTO THE WEEKEND.
OBVIOUSLY CONFIDENCE ON COVERAGE AND TIMING OF ANY
SHOWERS/STORMS IS LOW AT THIS RANGE SO STAY TUNED.
YACK
AVIATION /12Z TAFS THROUGH 18Z MONDAY/
ISSUED AT 647 AM CDT SUN MAY 17 2026
AVIATION FORECAST CONCERNS FOR THE 12Z TAFS:
- LOW CHANCE (25-30%) FOR SCATTERED SHRA MAINLY NORTHWEST OF
CHICAGO THIS MORNING.
- LIGHT SOUTHEAST WINDS BECOME GUSTY (NEAR 30 KTS) THIS
AFTERNOON, AND REMAIN BREEZY WITH GUSTS 20-25 KTS TONIGHT.
- STRONG LOW LEVEL JET DEVELOPS OVERNIGHT (45-50 KTS) WHICH MAY
PRODUCE LLWS CONDITIONS IF SURFACE GUSTS WERE TO SUBSIDE.
- WEAKENING LINE OF SHRA/TSRA SHOULD APPROACH THE TERMINALS
AROUND/JUST AFTER DAYBREAK MONDAY MORNING.
A WEAK, LAKE-ASSISTED COLD FRONT HAS PUSHED INTO NORTHEAST IL
EARLY THIS MORNING, BUT HAS SLOWED APPRECIABLY OVER THE PAST 1-2
HOURS AND IS EXPECTED TO WASH OUT ACROSS THE CHICAGO TERMINALS
THIS MORNING. WHILE A BRIEF LIGHT NORTHEAST OR EAST WINDS IS
POSSIBLE EARLY ESPECIALLY AT ORD, WINDS SHOULD GENERALLY REMAINS
SOUTH-SOUTHEASTERLY THIS MORNING. WINDS ARE EXPECTED TO BECOME
SOUTHERLY AND STRENGTHEN BY MIDDAY, WITH GUSTS 25-30 KTS THIS
AFTERNOON WITH A FEW HIGHER GUSTS POSSIBLE LATE. WARM AND BREEZY
CONDITIONS WILL PERSIST TONIGHT, WITH WINDS LIKELY REMAINING
GUSTY IN THE 20-25 KT RANGE. A STRONG (45-50 KT) LOW-LEVEL JET
IS EXPECTED TO DEVELOP OVERNIGHT WHICH MAY PRODUCE LLWS
CONDITIONS, THOUGH WITH SURFACE WINDS EXPECTED TO REMAIN
ELEVATED AND GUSTY HAVE NOT INCLUDED A LLWS MENTION IN TAFS AT
THIS TIME, BUT COULD NEED TO BE ADDED WITH LATER UPDATES IF
SURFACE WINDS DECREASE MORE THAN EXPECTED.
AN AREA OF RAIN/EMBEDDED THUNDERSTORMS WAS OVER EASTERN IA EARLY
THIS MORNING, TRACKING TO THE NORTH-NORTHEAST IN ASSOCIATION
WITH A MID-LEVEL CIRCULATION (MCV). VARIOUS CAM GUIDANCE HAS
GENERALLY BEEN TO GENEROUS WITH THE FORECAST EASTWARD EXTENT OF
SHRA/TSRA EARLY THIS MORNING, THUS CONFIDENCE IN IMPACTS TO THE
TERMINALS HAVE DECREASED. RFD APPEARS TO HAVE THE GREATEST
POTENTIAL FOR SEEING SHRA DEVELOP THIS MORNING AND PERSIST
THROUGH MIDDAY. WITH SOME GUIDANCE CONTINUING TO DEPICT SOME
SHRA AS FAR EAST AS CHICAGO, HAVE MAINTAINED PROB30 SHRA
MENTIONS FOR ALL BUT GYY AT THIS TIME - THOUGH IT'S POSSIBLE
ESPECIALLY THE CHICAGO TERMINALS REMAIN DRY.
LATE TONIGHT, A LINE OF THUNDERSTORMS IS FORECAST TO PUSH EAST
FROM IA, AND APPROACH THE TERMINALS TOWARD/JUST AFTER DAYBREAK
IN A DECAYING MODE. SOME DIFFERENCES IN STRENGTH AND TIMING OF
THIS LINE EXISTS WITH MODELS, THOUGH DID AT A PROB30 FOR TSRA
AFTER 10Z AT RFD AND 12-15Z FOR ORD/MDW FOR NOW.
RATZER
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY UNTIL 10 PM CDT THIS EVENING FOR WILMETTE
HARBOR IL TO MICHIGAN CITY IN.
VISIT US AT WEATHER.GOV/CHICAGO
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