605
FXUS63 KLOT 260000
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
700 PM CDT FRI JUL 25 2025
KEY MESSAGES
- TORRENTIAL RAIN PRODUCING THUNDERSTORMS (WITH RATES GREATER
THAN 2 INCHES PER HOUR) ARE LIKELY LATE THIS EVENING AND
OVERNIGHT AND MAY LEAD TO FLASH FLOODING.
- THE FLASH FLOOD WATCH HAS BEEN EXPANDED TO INCLUDE ALL AREAS
ALONG AND NORTH OF I-80 OVERNIGHT THROUGH SATURDAY AFTERNOON
TO ACCOUNT FOR THIS POTENTIAL.
- STRONG TO SEVERE THUNDERSTORMS MAY DEVELOP SATURDAY AFTERNOON,
MAINLY NEAR AND SOUTH OF I-80 (SEVERE THREAT LEVEL 1 OF 5).
- DANGEROUS HEAT AND HUMIDITY RETURNS MONDAY (POSSIBLY AS EARLY
AS SUNDAY) FOLLOWED BY PERIODS OF THUNDERSTORMS TUESDAY INTO
WEDNESDAY.
MESOSCALE
ISSUED AT 612 PM CDT FRI JUL 25 2025
RECENT RADAR AND SATELLITE IMAGERY DEPICT A COMPACT MCV AND
ASSOCIATED BATCH OF SHOWERS AND STORMS CROSSING THE MISSISSIPPI
RIVER NEAR THE QUAD CITIES AT PRESS TIME. MEANWHILE, ANOTHER,
LARGER, MCV IS PRESENT IN NORTHEASTERN KANSAS. BOTH MCVS ARE
MOVING ALONG/WITHIN A BROAD FRONTAL ZONE DRAPED ACROSS THE
MIDDLE MISSISSIPPI RIVER VALLEY.
OVER THE NEXT 3 TO 6 HOURS, THE FLOW FROM 850 TO 700 MB IS
EXPECTED TO INCREASE MARKEDLY ACROSS CENTRAL AND NORTHERN
ILLINOIS IN RESPONSE TO THE COMBINED EFFECTS OF BOTH
APPROACHING MCVS. SUCH INCREASING FLOW ATOP THE STALLED FRONTAL
BOUNDARY (E.G., ISENTROPIC ASCENT) SHOULD LEAD TO A
CORRESPONDING INCREASE IN COVERAGE OF SHOWERS AND STORMS THIS
EVENING AND ESPECIALLY OVERNIGHT, PRIMARILY NEAR AND PERHAPS
NORTH OF I-80 (NOSE OF THE LOW-LEVEL JET). WITH PWATS OF 2.25 TO
LOCALLY 2.4" IN PLACE ALONG THE FRONTAL BOUNDARY, HOURLY RAIN
RATES OF 2 TO LOCALLY 4 INCHES PER HOUR ARE EXPECTED WHERE
THUNDERSTORMS ARE MOST PERSISTENT TONIGHT SUPPORTING A THREAT
FOR FLASH FLOODING.
OF PARTICULAR CONCERN IS A SCENARIO WHERE STORMS MATERIALIZE IN
A TRAINING FASHION OVER THE I-55 CORRIDOR WHERE 3 TO LOCALLY 6
INCHES OF RAIN HAS ALREADY FALLEN IN THE PAST 24 HOURS. IN SUCH
A SCENARIO, SIGNIFICANT FLASH FLOODING WOULD BE A DISTINCT
THREAT, INCLUDING SUBSTANTIAL FLOODING OF BASEMENTS, ROADWAYS,
AND FIELDS, AS WELL AS RAPID INCREASES IN RIVER LEVELS
(PARTICULARLY THE DUPAGE AND DES PLAINES RIVERS AND ASSOCIATED
TRIBUTARIES IN DUPAGE, COOK, AND WILL COUNTIES). THIS IS A VERY
CONDITIONAL SCENARIO, BUT ONE NOT TO IGNORE.
ALL THINGS CONSIDERED, OPTED TO EXPAND THE FLOOD WATCH FOR ALL
AREAS ALONG AND NORTH OF I-80. WITH ADDITIONAL ROUNDS OF STORMS
EXPECTED AFTER DAYBREAK TOMORROW, FELT THE COURSE OF LEAST
REGRET WAS TO ALSO EXTEND THE EXPIRATION TIME OF THE FLOOD WATCH
THROUGH TOMORROW EVENING. FOR ANYONE LIVING IN A FLOOD PRONE
AREA, ENSURE YOU HAVE MULTIPLE WAYS TO GET WARNING INFORMATION
INCLUDING AT LEAST ONE WAY TO WAKE YOU UP.
BORCHARDT
DISCUSSION
ISSUED AT 238 PM CDT FRI JUL 25 2025
THROUGH SATURDAY:
COMPACT AREA OF SURFACE CONVERGENCE--LIKELY WITH SOME MID-LEVEL
FLOW ENHANCEMENT--CONTINUES ACROSS PARTS OF THE CHICAGO AREA
EARLY THIS AFTERNOON, EXTENDING WEST ROUGHLY MIDWAY BETWEEN I-88
AND I-80. GOING THROUGH THE REST OF THE AFTERNOON, IT SEEMS
LIKE THIS MAY END UP BEING THE PRIMARY FOCUS FOR ADDITIONAL
DEVELOPMENT, WITH PERSISTENT DEBRIS CLOUD COVER FARTHER TO THE
SOUTH HAVING STUNTED INSOLATION AND INSTABILITY. ADDITIONAL
SHOWERS/EMBEDDED STORMS COULD FIRE AND FESTER ALONG ANOTHER
ZONE OF ENHANCED SURFACE CONVERGENCE TIED TO AN INLAND-OOZING
FRONT/PSEUDO LAKE BREEZE IN THE VICINITY OF I-90. INDIVIDUAL
STORM MOTIONS WILL REMAIN VERY SLOW, LIKELY AROUND 10 MPH OR
LESS, AND PROPAGATIVE EFFECTS WILL ADDITIONALLY LEAD TO NEAR
NET-0 STORM MOTIONS AT TIMES. SIMILAR TO WHAT WE SAW EARLIER
TODAY ACROSS PARTS OF CHICAGO, HIGHLY LOCALIZED RAINFALL AMOUNTS
OVER 2-3 INCHES WILL BE POSSIBLE.
ANOTHER COMPACT MCV, CURRENTLY INCHING NORTHEAST OUT OF THE
OTTUMWA AREA IN SOUTHEAST IOWA LOOKS TO BE THE MAIN FORCING
MECHANISM FOR ADDITIONAL STORMS THIS EVENING AND OVERNIGHT. NOT
SEEING A TON OF FLOW ENHANCEMENT ON REGIONAL VWPS, BUT SUSPECT
THIS IS BECAUSE THIS FEATURE IS CURRENTLY IN BETWEEN RADAR
SITES. HIRES GUIDANCE WHICH TYPICALLY HANDLES THESE TYPES OF
FEATURES FAIRLY WELL SHOWS A SIGNIFICANT INCREASE IN THE LOW-
LEVEL MASS RESPONSE (IN THE 850 TO 700 MB LAYER MOST NOTABLY)
THIS EVENING AND OVERNIGHT AS THIS FEATURE CONTINUES NORTHEAST.
CURRENT MOTION OF THIS FEATURE SEEMS TO HAVE IT TRACKING MOSTLY
WEST OF OUR FORECAST AREA, BUT THE GUIDANCE WHICH HAS RESOLVED
IT INSISTS ON AN EASTERLY COMPONENT DEVELOPING THROUGH THE
EVENING, AND PERHAPS WE'RE SEEING SOME OF THAT IN RECENT
SATELLITE LOOPS. SHOULD THIS OCCUR, CONCERN EXISTS FOR ANOTHER
BATCH OF SHOWERS AND STORMS WITH TORRENTIAL RAINFALL RATES WHICH
WOULD CARRY WITH THEM A LOCALIZED FLASH FLOOD THREAT. SEEMS
LIKE THIS FEATURE WOULD HAVE SOME FORWARD/NORTHEAST MOTION TO
IT, SO IT REMAINS UNCLEAR HOW WIDESPREAD AND PROLONGED ANY
CONVECTIVE TRAINING WOULD BE. FOR THE TIME BEING, HAVE DECIDED
TO HOLD OFF ON AN EXPANSION/ISSUANCE OF A NEW FLOOD WATCH FOR
OUR WEST GIVEN THE AFOREMENTIONED UNCERTAINTIES IN FAVOR OF
HANDLING THESE HEAVY RAIN EPISODES SIMILAR TO TYPICAL CONVECTIVE
SEVERE WATCHES. HOWEVER, IF TRENDS CONTINUE, FLOOD WATCHES MAY
BECOME NECESSARY LATER THIS AFTERNOON/EARLY EVENING, INITIALLY
WEST OF THE FOX VALLEY, AND THEN PERHAPS FARTHER SOUTH IF WARM
ADVECTION-DRIVEN ACTIVITY DEVELOPS OVERNIGHT.
ON SATURDAY, SHOWERS WITH EMBEDDED THUNDERSTORMS WILL LIKELY BE
ONGOING DURING THE MORNING WITH SOME GRADUAL DOWNTICK IN
COVERAGE THROUGH THE MORNING. CONTINUED WAVES OF EMBEDDED
STORMS COULD LEAD TO ISOLATED STREAKS OF HEAVIER RAINFALL
AMOUNTS.
FORECAST EVOLUTION INTO THE AFTERNOON REMAINS SOMEWHAT UNCLEAR.
IT SEEMS LIKELY THAT PERSISTENT CLOUD COVER WILL HOLD
TEMPERATURES DOWN NORTH OF I-80, BUT THERE IS A SIGNAL FOR
CLEARING TO TAKE HOLD FARTHER TO THE SOUTH. WITH DEWPOINTS IN
THE LOW TO MID 70S, ANY ISOLATION WILL LEAD TO A QUICK INCREASE
IN INSTABILITY. GUIDANCE INDICATES MODERATE TO PERHAPS EVEN
STRONG INSTABILITY MATERIALIZING DURING THE AFTERNOON SOUTH OF
I-80 (AND INCREASING WITH SOUTHWARD EXTENT). WHILE SHEAR DOESN'T
LOOK PARTICULARLY HIGH, A VERY MODEST MID-LEVEL FLOW
ENHANCEMENT IS FORECAST TO YIELD PERHAPS 30-35 KTS OF DEEP LAYER
SHEAR, WITH SOME DEGREE OF HODOGRAPH CURVATURE IN THE LOW-
LEVELS. GIVEN THE MAGNITUDE OF INSTABILITY PROGGED TO BE IN
PLACE, SEMI-ORGANIZED MULTICELL/TRANSIENT SUPERCELLS WOULD BE
THE PRIMARY STORM MODE. CURRENT SPC LEVEL 1/5 SEVERE RISK
APPEARS REPRESENTATIVE OF THE CURRENT UNCERTAINTY/COVERAGE
COMBO, BUT WILL NEED TO KEEP AN EYE ON THIS GOING FORWARD.
CARLAW
SATURDAY NIGHT THROUGH FRIDAY:
TWO PRIMARY CONCERNS ARE HIGH TEMPS/HEAT INDICES SUNDAY AND
MONDAY AND THEN THUNDERSTORM CHANCES THROUGH MIDWEEK.
THUNDERSTORMS MAY BE ONGOING SATURDAY EVENING WITH A LINE MOVING
SOUTHEAST THROUGH THE CWA. SOME OF THESE WOULD HAVE THE
POTENTIAL TO BE STRONG TO LOCALLY SEVERE ALONG WITH TORRENTIAL
RAINFALL. AS TRENDS LOOK NOW, THIS ACTIVITY SHOULD BE MOVING
INTO THE SOUTHERN CWA BY THE OVERNIGHT HOURS AND POSSIBLY ENDING
OVERNIGHT/BY DAYBREAK SUNDAY. WHILE THERE REMAINS A CHANCE OF
THUNDERSTORMS SUNDAY, ESPECIALLY ACROSS CENTRAL IL AND CENTRAL
IN, SUNDAY MAY END UP MAINLY DRY FOR MOST OF THE AREA AND IF
THAT MATERIALIZES, HIGH TEMPERATURES WILL LIKELY REACH THE UPPER
80S TO AROUND 90 FOR MOST AREAS. COMBINED WITH DEWPOINTS IN THE
MID/UPPER 70S, PEAK HEAT INDICES IN THE 100-105 RANGE WOULD BE
POSSIBLE. IT APPEARS THERE WILL BE A LAKE BREEZE KEEPING THE
AREAS CLOSE TO LAKE MICHIGAN COOLER SUNDAY AFTERNOON.
CONFIDENCE FOR THUNDERSTORM TIMING/LOCATION COVERAGE DECREASES
SUNDAY NIGHT THROUGH MONDAY NIGHT. THERE WILL LIKELY BE
THUNDERSTORM COMPLEXES ROAMING ACROSS THE UPPER MIDWEST OR
WESTERN GREAT LAKES, WITH SOME POTENTIAL FOR THESE TO REACH AT
LEAST THE NORTHERN CWA. HOWEVER, THERE CURRENTLY IS NOT MUCH
GUIDANCE/ENSEMBLE SUPPORT FOR PRECIP INTO THE LOCAL AREA AND THE
BLENDED POPS ARE MAINLY DRY. IF THIS DRY FORECAST PANS OUT, THEN
HIGH TEMPS ON MONDAY WILL LIKELY BE IN THE LOWER/MID 90S WITH
DEWPOINTS BACK INTO THE UPPER 70S MAYBE AROUND 80. THIS WOULD
PUSH PEAK HEAT INDICES INTO THE 105-110 RANGE. IF THESE
CONDITIONS APPEAR MORE LIKELY AS THIS TIME PERIOD APPROACHES,
SOME HEAT HEADLINE MAY BE NEEDED.
THUNDERSTORM CHANCES WILL INCREASE TUESDAY INTO WEDNESDAY AS A
COLD FRONT MOVES SOUTH ACROSS THE GREAT LAKES REGION. GIVEN THE
EXPECTED STRENGTHEN OF THIS FRONT, A FASTER ARRIVAL INTO
NORTHERN IL TUESDAY AFTERNOON/EVENING SEEMS PLAUSIBLE. WHETHER
THE FRONT SLOWS/STALLS ACROSS THE AREA TUESDAY NIGHT/WEDNESDAY
IS UNCLEAR. THUNDERSTORMS WILL BE POSSIBLE DURING THIS TIME,
POSSIBLY A FEW ROUNDS WITH PERHAPS THE MAIN ROUND ON WEDNESDAY.
STRONG/SEVERE STORMS AND TORRENTIAL RAIN/LOCALIZED FLOODING
WOULD BE POSSIBLE.
NORTHERLY WINDS THEN LOOK TO INCREASE INTO THE 15-25 MPH RANGE
OVER THE LAKE WEDNESDAY INTO WEDNESDAY NIGHT, WHICH WOULD LIKELY
LEAD TO HIGH WAVES AND DANGEROUS SWIMMING CONDITIONS FOR LAKE
MICHIGAN BEACHES.
MUCH COOLER AND LESS HUMID CONDITIONS ARE THEN EXPECTED FOR LATE
WEEK. CMS
AVIATION /00Z TAFS THROUGH 06Z SUNDAY/
ISSUED AT 700 PM CDT FRI JUL 25 2025
KEY MESSAGES:
- PERIODS OF SHOWERS AND THUNDERSTORMS ARE LIKELY LATE THIS
EVENING THROUGH SATURDAY MORNING WITH ASSOCIATED VSBY/CIG
REDUCTIONS IN TORRENTIAL RAIN.
- ADDITIONAL THUNDERSTORMS ARE POSSIBLE SATURDAY AFTERNOON (30%
CHANCE), SOME COULD BE STRONG TO SEVERE.
ATTENTION CONTINUES TO BE ON THE THREAT FOR TORRENTIAL RAIN IN
SHOWERS AND THUNDERSTORMS DEVELOPING LATE THIS EVENING THROUGH
THE OVERNIGHT HOURS INTO SATURDAY MORNING. AN AXIS OF STORMS IS
ALREADY SETTING UP OVER SOUTHEAST IOWA INTO WEST CENTRAL
ILLINOIS THAT IS POISED TO DRIFT NORTHEAST INTO AND ALONG THE
I-80/88 CORRIDORS. WHILE THIS AXIS MAY ULTIMATELY END UP MORE
FOCUSED ALONG A NARROWER AREA, AT THIS RANGE, FELT THE BEST WAY
OF HANDLING IT WAS TO INTRODUCE PREVAILING SHOWERS WITH MVFR
VSBYS OVERNIGHT THROUGH THE MORNING ON SATURDAY AT ALL CHICAGO
AREA TERMINALS. ADDITIONALLY, TRENDED LOWER WITH THE VSBYS IN
THE TEMPO TSRA GROUP TO LOW-END IFR (1SM) DUE TO THE EXPECTED
HIGH RAINFALL RATES POTENTIALLY IN EXCESS OF 2 INCHES PER HOUR.
SUSPECT EVEN LOWER VSBYS TO LIFR MAY END UP BEING WARRANTED
ONCE THE EXACT POSITION OF THIS AXIS OF STORMS BEGINS TO SET UP
LATER TONIGHT. THERE IS A DISTINCT POSSIBILITY THAT TSRA WILL
END UP NEEDING TO BE INTRODUCED FOR THE MORNING HOURS ON
SATURDAY AS WELL, PARTICULARLY BETWEEN 12-15Z IF CONFIDENCE
INCREASES.
FURTHER NORTHWEST TOWARD ROCKFORD CONFIDENCE IN SHOWER AND STORM
TRENDS IS A BIT LOWER, BUT THERE REMAINS A SIGNAL FOR A
SECONDARY AXIS OF STORMS POTENTIALLY SETTING UP NEARBY SO OPTED
TO INCLUDE SIMILAR CONDITIONS AS THE CHICAGO METRO BY
CONVERTING THE PROB30 TSRA GROUP TO A TEMPO FOR OVERNIGHT WITH
PREVAILING SHRA.
THE ACTIVE WEATHER PATTERN CONTINUES LATER INTO THE DAY ON
SATURDAY WHEN ADDITIONAL SHOWERS AND THUNDERSTORMS MAY
REDEVELOP OVER THE CHICAGO METRO. HAVE ACCORDINGLY MAINTAINED
THE INHERITED PROB30 GROUPS FOR TSRA FROM 20-24Z. ANY STORMS
THAT DO DEVELOP WOULD HAVE THE POTENTIAL TO PRODUCE GUSTY TO
LOCALLY DAMAGING WINDS AND HAIL IN ADDITION TO HEAVY DOWNPOURS.
WINDS WILL REMAIN GENERAL EAST TO SOUTHEAST THROUGH THE EVENING
AND EARLY OVERNIGHT HOURS. THERE IS A SIGNAL THAT THE INITIAL
THUNDERSTORM ACTIVITY COULD COME WITH A GUSTY WEST WIND SHIFT SO
OPTED TO INCLUDE THAT IN THE TEMPO TSRA GROUPS. WINDS THEN
EVENTUALLY SETTLE INTO A LIGHT SOUTH TO SOUTHWEST DIRECTION
DURING THE DAY ON SATURDAY.
PETR
LOT WATCHES/WARNINGS/ADVISORIES
IL...FLOOD WATCH THROUGH SATURDAY EVENING FOR ILZ003-ILZ004-ILZ005-
ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019-ILZ020-
ILZ021-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.
IN...FLOOD WATCH THROUGH SATURDAY EVENING FOR INZ001.
LM...NONE.
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