624
FXUS63 KLOT 172007
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
307 PM CDT THU APR 17 2025
KEY MESSAGES
- STRONG THUNDERSTORMS MAY TRACK NEAR AND NORTH OF I-88 AFTER
MIDNIGHT TONIGHT WITH A THREAT FOR LARGE HAIL AND LOCALLY
GUSTY WINDS (30 TO 60% CHANCE).
- FRIDAY WILL OFFER AN EARLY TASTE OF SUMMER-LIKE WARMTH WITH
HIGHS IN THE LOWER 80S, ALBEIT WITH GUSTY SOUTH WINDS (GUSTS 35 TO
40 MPH).
- SHOWERS AND STORMS MAY DEVELOP FRIDAY EVENING, ESPECIALLY
ALONG AND SOUTH OF I-55. A FEW STORMS MAY BE SEVERE WITH
DAMAGING WINDS AND HAIL.
- PERIODS OF SHOWERS AND PERHAPS SOME THUNDERSTORMS ARE FORECAST
ON SUNDAY WITH WIDESPREAD RAIN AND SOME STORMS SUNDAY NIGHT. A
HIGHLY CONDITIONAL SEVERE THUNDERSTORM THREAT EXISTS FOR
MAINLY FAR SOUTHERN PORTIONS OF THE FORECAST LATE SUNDAY
AFTERNOON AND EVENING.
DISCUSSION
ISSUED AT 307 PM CDT THU APR 17 2025
THROUGH FRIDAY NIGHT:
THE PRIMARY WEATHER CONCERNS CONTINUE TO FOCUS ON THE THREAT OF
STRONG TO SEVERE CONVECTION, PARTICULARLY LATE TONIGHT AND AGAIN
FRIDAY EVENING.
IN THE NEAR TERM (LATE THIS AFTERNOON THROUGH EARLY EVENING), A
BATCH OF SHOWERS WILL SHIFT EAST-NORTHEASTWARD ACROSS THE AREA.
SEVERE WEATHER IS NOT EXPECTED WITH THIS ACTIVITY, THOUGH SOME
INSTANCES OF SMALL HAIL AND POSSIBLY A FEW LIGHTNING STRIKES
CANNOT BE RULED OUT GIVEN SOME RATHER STEEP LAPSE RATES ALOFT.
THIS ACTIVITY IS EXPECTED TO MOVE OUT OF THE AREA SHORTLY AFTER
SUNSET THIS EVENING. A FEW ISOLATED TO WIDELY SCATTERED SHOWERS
COULD PERSIST FOR A COUPLE HOURS THIS EVENING, BUT THE COVERAGE OF
SHOWERS IS EXPECTED TO DECREASE THROUGH THE EVENING AS WE AWAIT
OUR NEXT PERIOD OF SHOWERS AND THUNDERSTORMS NORTH OF I-80 AFTER
MIDNIGHT TONIGHT.
THE THUNDERSTORM THREAT OVERNIGHT HAS INCREASED A BIT OVER THE
PREVIOUS FORECAST, THOUGH THE BEST CHANCES CONTINUE TO BE FAVORED
ACROSS SOUTHERN WI AND FAR NORTHERN IL. THIS THUNDERSTORM ACTIVITY
OVERNIGHT IS EXPECTED TO DEVELOP ACROSS EASTERN NE AND FAR
WESTERN IA EARLY THIS EVENING. AS THIS ACTIVITY DEVELOPS IN
ADVANCE OF A COLD FRONT WELL TO OUR WEST, THE THERMODYNAMIC AND
KINEMATIC ENVIONRMENT WILL BE SUPPORTIVE OF EASTWARD MOVING
SUPERCELLS, WITH AN ATTENDANT VERY LARGE HAIL THREAT. WE WILL BE
WATCHING THE EVOLUTION OF THESE STORMS CLOSELY THROUGH THE
EVENING, AS IT APPEARS THAT THIS ACTIVITY WILL SHIFT EASTWARD
ACROSS SOUTHERN WI AND FAR NORTHERN IL AFTER MIDNIGHT TONIGHT.
THERE DOES CONTINUE TO BE SOME UNCERTAINTY AS TO HOW FAR SOUTH
INTO IL THIS STORM THREAT WILL REACH, OWING LARGELY TO THE
PRESENCE OF A STOUT EML CAPPING INVERSION, WHICH IS EXPECTED TO
STRENGTHEN WITH SOUTHWARD EXTENT INTO IL. AT THE PRESENT TIME,
AREAS ROUGHLY ALONG AND NORTH OF I-88 HAVE THE BEST SHOT AT SEEING
A FEW OF THESE STRONG STORMS OVERNIGHT. THIS IS THE GENERAL AREA
THAT WILL REMAIN NEAR THE NORTHERN PERIPHERY OF THIS EML CAPPING
INVERSION. ACCORDINGLY, THERE WILL BE A CORRIDOR ACROSS SOUTHERN
WI AND FAR NORTHERN IL IN WHICH AN EASTWARD FOCUS WILL EXIST TO
STEER THIS EVENINGS SEVERE CONVECTION OVER IA EASTWARD INTO PARTS
OF OUR AREA OVERNIGHT. THE PRIMARILY THREAT WITH THESE STORMS
WILL BE LARGE HAIL (1-1.5" DIAMETER), GIVEN THE PRESENCE OF VERY
STEEP MID-LEVEL LAPSE RATES. THIS THREAT WILL ESPECIALLY BE
MAXIMIZED WITH ANY LINGERING ELEVATED SUPERCELL STRUCTURES.
THESE OVERNIGHT STORMS ARE EXPECTED TO COME TO AN END BY, OR
SHORTLY AFTER, DAYBREAK FRIDAY MORNING. IN THEIR WAKE, THE SPINE
OF THE ELONGATED EML CAPPING INVERSION WILL OVERSPREAD THE WHOLE
AREA AGAIN FOR THE DAYLIGHT HOURS OF FRIDAY. THIS WILL THUS RESULT
IN ROBUST CAPPING OVERHEAD, AND RELATIVE DEARTH OF SHOWERS AND
STORMS FOR MUCH OF THE DAYLIGHT HOURS OF FRIDAY. IN THE ABSENCE OF
SHOWERS AND STORMS, DEEP ATMOSPHERIC MIXING INTO INCREASING
SOUTHERLY FLOW WILL RESULT IN GUSTY SOUTH WINDS (35-40 MPH)
THROUGH THE DAY. TEMPERATURES WILL TURN MUCH WARMER ON THESE WARM
SOUTHERLY WINDS, WITH READINGS LIKELY PEAKING IN THE LOWER 80S IN
MOST LOCATIONS.
OUR SECOND WINDOW OF POTENTIAL SEVERE THUNDERSTORMS LOOKS TO COME
AS A COLD FRONT SHIFTS INTO NORTHERN IL INTO FRIDAY EVENING,
LIKELY AFTER 6 PM. GRADUALLY INCREASING LARGE-SCALE ASCENT IS
EXPECTED AS THIS FRONT MOVES INTO THE AREA IN THE EVENING, AND
SHOULD THUS FOSTER SUFFICIENT LOW-LEVEL MOISTENING AND WEAKENING
OF THE RESIDUAL CAPPING INVERSION TO SUPPORT DEVELOPING STORMS
ALONG THE FRONTAL BOUNDARY AFTER 6 PM FRIDAY EVENING. STILL THINK
THAT THE HIGHEST STORM COVERAGE INTO FRIDAY NIGHT SHOULD BE NEAR
AND EAST OF I-55, WHERE OUR FORECAST POPS ARE IN THE CATEGORICAL
RANGE.
THERMODYNAMIC AND KINEMATIC PROFILES WILL BE SUPPORTIVE OF
STRONG TO SEVERE THUNDERSTORMS DEVELOPING ACROSS THE FRONTAL
ZONE, THOUGH IT APPEARS THAT MOST CONVECTION THAT DEVELOPS WILL
EITHER FORM ATOP THE EML'S BASE OR QUICKLY BECOME UNDERCUT BY
THE COLD FRONT. THE ELEVATED NATURE OF THE CONVECTION AND
PROGGED FRONT-PARALLEL STORM MOTIONS, PLUS UNCERTAINTIES
REGARDING LINGERING INHIBITION AND HOW QUICKLY STORMS WILL BE
ABLE TO INTENSIFY DO RAISE QUESTIONS ABOUT THE EXTENT TO WHICH
STORMS WILL BE ABLE TO FULLY HARNESS THE PARAMETER SPACE THAT
WILL BE IN PLACE. THAT SAID, ANY ROBUST CONVECTION THAT MANAGES
TO DEVELOP SHOULD POSSESS A THREAT FOR LARGE HAIL, WHILE ANY
CONVECTION THAT MANAGES TO STAY ALONG OR AHEAD OF THE COLD FRONT
WILL ALSO FEATURE A THREAT FOR DAMAGING WINDS. SOME POTENTIAL
FOR FLASH FLOODING ALSO APPEARS EVIDENT IN OUR SOUTHEASTERN CWA
GIVEN THE AFOREMENTIONED STORM MOTIONS AND POTENTIAL FOR
TRAINING CONVECTION.
KJB
SATURDAY THROUGH THURSDAY:
ON SATURDAY, THE COLD FRONT MOVING ACROSS THE AREA LATE FRIDAY
THROUGH FRIDAY NIGHT WILL STALL OUT SOUTH OF THE CWA AS SURFACE
HIGH PRESSURE SETS UP FROM THE NORTHERN PLAINS TO THE GREAT LAKES
REGION. THE LOW-LEVEL BAROCLINIC ZONE WILL ALSO BE NUDGED SOUTH,
BUT REMAIN CLOSE ENOUGH BY TO PRESENT A CHANCE FOR AT LEAST
SCATTERED SHOWERS INTO AREAS WELL SOUTH OF I-80. CAN'T RULE OUT
SHOWER ACTIVITY AT TIMES DRIFTING NORTHWARD TOWARD THE I-80
CORRIDOR, BUT BY AND LARGE AREAS NEAR AND ESPECIALLY NORTH OF
I-80 SHOULD REMAIN DRY (ALBEIT MOSTLY CLOUDY) SATURDAY THROUGH
EARLY SUNDAY. HIGH TEMPERATURES SATURDAY AFTERNOON ARE EXPECTED TO
BE JUST SHY OF NORMAL FOR THE DATE INLAND, RANGING FROM THE UPPER
50S TO AROUND 60F/LOW 60S, BUT ONLY NEAR/AROUND 50F ALONG THE
LAKEFRONT DUE TO ONSHORE FLOW THROUGH THE DAY.
SUNDAY, FOR THOSE CELEBRATING THE EASTER HOLIDAY, IT UNFORTUNATELY
WILL BE A RATHER INCLEMENT (BREEZY, SHOWERY AT TIMES, COOL) DAY.
THE MAIN TROUGH RESPONSIBLE FOR THE APPROACHING SURFACE LOW ON
FRIDAY WILL FINALLY EJECT NORTHEASTWARD WHILE BECOMING NEGATIVELY
TILTED OVER THE UPPER MISSISSIPPI RIVER VALLEY LATE SUNDAY. THE
VARIOUS ENSEMBLE MODEL SUITES HAVE REMAINED IN BETTER AGREEMENT
WITH THE EVOLUTION OF THE SURFACE LOW, WITH THE FAVORED MEAN TRACK
ACROSS EASTERN IOWA OR FAR NORTHWEST ILLINOIS SUNDAY NIGHT. THIS
TRACK WILL KEEP MOST OF THE AREA ON THE COOL SIDE OF A WARM FRONT
SLOWLY LIFTING NORTHWARD IN ADVANCE OF THE SURFACE LOW, RESULTING
IN HIGHS IN THE 50S (MID-UPPER 40S IL SHORE) AND EASTERLY WINDS
GUSTING TO 30 MPH.
MASS RESPONSE AHEAD OF THE AFOREMENTIONED TROUGH/LOW WILL QUICKLY
ADVECT THE MOISTURE-RICH AIRMASS SOUTH OF THE FRONT NORTHWARD
ACROSS THE AREA ON SUNDAY. THIS WILL RESULT IN AN AXIS OF SHOWERS
LIFTING NORTHWARD THROUGH EARLY AFTERNOON AND THEN PERHAPS
ADDITIONAL WAVES OF SHOWERS IN THE MID TO LATE AFTERNOON. WHILE
THERE'S TOO MUCH UNCERTAIN IN EXACT PRECIP TIMING, THERE IS A
SIGNAL FOR THE GUIDANCE IN SOME DRY PERIODS, SO IT DOESN'T
CURRENTLY LOOK LIKE AN ALL DAY WASHOUT. AS THE EFFECTIVE WARM
FRONT SLOWLY LIFTS BACK NORTH TOWARD AND INTO THE FAR SOUTHERN CWA
DURING THE AFTERNOON, ENOUGH MUCAPE MAY BUILD TO SUPPORT A CHANCE
FOR ISOLATED TO SCATTERED THUNDERSTORMS.
AS IMPRESSIVE DEEP LAYER FORCING NEARS FROM THE SOUTHWEST LATE IN
THE DAY, ARCS OF CONVECTION RACING NORTHEAST SHOULD EXPAND INTO
THE CWA SUNDAY EVENING. REGARDING THE EFFECTIVE WARM FRONTAL
POSITION, THERE MAY BE A WINDOW OF OVERLAP OF MODEST UNCAPPED
MLCAPE IN THE LATE AFTERNOON AND EARLY EVENING IN PARTS OF THE
SOUTHERN CWA (GENERALLY NEAR/SOUTH OF US-24). WE'LL NEED TO KEEP A
CLOSE EYE ON TRENDS FOR A CONDITIONAL SEVERE STORM THREAT IN THESE
AREAS WITHIN A HIGHLY SHEARED ENVIRONMENT FROM THE LATE AFTERNOON
THROUGH THE EARLY TO MID EVENING. RIGHT MOVING BUNKERS MOTIONS
COULD RESULT IN ANY SUPERCELLS QUICKLY CROSSING THE FRONT, WHICH
MAY LIMIT THE TORNADO THREAT IN THE CWA, EVEN IF A SEVERE STORM
RISK DOES MATERIALIZE. CAN'T RULE OUT A THREAT FOR STRONG STORMS
EXPANDING NORTHWARD WITH THE FRONT IN THE EVENING, THOUGH THE
DIURNALLY UNFAVORABLE TIMING AND ALREADY MINIMAL INSTABILITY MAY
LIMIT THE THREAT FARTHER NORTH TO LOCALIZED WIND GUSTS STRONGER
THAN THE ALREADY BREEZY BACKGROUND WIND FIELD.
REGARDLESS OF HOW THE POTENTIAL CONDITIONAL SEVERE THREAT PANS OUT
OR NOT, THE DYNAMIC SYSTEM WILL STILL BRING WIDESPREAD SHOWERS AND
SCATTERED EMBEDDED STORMS OVER THE AREA THROUGH PART OF THE
OVERNIGHT, WITH THUNDERSTORM COVERAGE DIMINISHING AS INSTABILITY
FURTHER WANES INTO THE PRE-DAWN HOURS OF MONDAY.
AFTER SUNDAY'S SYSTEM DEPARTS ACROSS THE GREAT LAKES REGION
EARLY MONDAY, A MOSTLY ZONAL REGIME WILL PREVAIL ACROSS MUCH OF
NORTH AMERICA FOR SEVERAL DAYS. THIS WILL BRING A PERIOD OF
SEASONABLE (MONDAY) TO SEASONABLY WARM (TUESDAY-THURSDAY) WEATHER
TO THE REGION, WITH LOCALLY COOLER CONDITIONS FROM DAILY LAKE
BREEZES AROUND THE LAKE MICHIGAN SHORE. MONDAY'S MAIN WEATHER OF
NOTE WILL LIKELY BE GUSTY WESTERLY WINDS, WITH GUSTS UP TO 35 MPH
IN THE OFFICIAL FORECAST AND ADVISORY (45+ MPH GUSTS) CRITERIA
UPSIDE PER THE ECMWF/EPS ENSEMBLE. CAN'T RULE OUT A FEW LIGHT
SHOWERS IN THE MORNING, THOUGH SUSPECT MOST AREAS WILL BE DRY.
THE ONE NOTABLE PERTURBATION IN THE ZONAL PATTERN NEXT WORK WEEK
WILL BE A LOW-AMPLITUDE WAVE TRACKING ALONG THE U.S./CANADIAN
BORDER TUESDAY INTO WEDNESDAY. AN ASSOCIATED WEAK COLD FRONT MAY
SLOW OR STALL ACROSS THE AREA TUESDAY NIGHT INTO WEDNESDAY, WITH
MARGINAL MOISTURE PROFILES RESULTING IN PERHAPS ISOLATED TO
SCATTERED SHOWERS NEAR THE FRONT.
CASTRO/KLUBER
AVIATION /18Z TAFS THROUGH 00Z SATURDAY/
ISSUED AT 1221 PM CDT THU APR 17 2025
ONGOING SHOWERS WITH EMBEDDED ISOLATED THUNDERSTORMS NEAR AND
EAST OF THE MISSISSIPPI RIVER SHOULD CONTINUE SHIFTING EASTWARD
THROUGHOUT THE AFTERNOON. WILL SHIFT THE INHERITED TEMPO GROUPS
TO MATCH THE CURRENT EXPECTED PEAK COVERAGE OF ACTIVITY AT EACH
INDIVIDUAL TERMINAL, GENERALLY PEAKING IN THE 21 TO 01Z
TIMEFRAME. WITH MID-LEVEL LAPSE RATES DECREASING WITH EASTWARD
EXTENT, STILL THINK THE THREAT FOR THUNDER THIS AFTERNOON IS 20%
OR LESS AT ANY GIVEN TERMINAL. WITH THAT SAID, IF THUNDER WERE
TO OCCUR, IT WOULD FAVOR THE LATTER 1 OR 2 HOURS OF EACH TEMPO
(FAVORING THE 23 TO 01Z WINDOW) AS THE LOW-LEVEL JET
REINTENSIFIES DIRECTLY OVERHEAD. WINDS THROUGH THE REMAINDER OF
THE AFTERNOON SHOULD BE EAST OF SOUTH (FAVORING A 170 DEG
DIRECTION) WITH GUSTS OF 25 TO LOCALLY 30KT.
TONIGHT, THE LOW-LEVEL JET WILL RAPIDLY INTENSIFY OVERHEAD (FLOW
AT 2KM OF NEARLY 60KT), WARRANTING LLWS IN SPITE OF CONTINUED
SURFACE GUSTS OF 20 TO 25KT. ATTENTION WILL THEN TURN TOWARD
THE EXPECTED DEVELOPMENT OF CONVECTION ACROSS EASTERN NE/WESTERN
IA. WITH DEEP SOUTHWESTERLY FLOW ADVECTING THE EDGE OF THE
EML/STEEP MID-LEVEL LAPSE RATES NORTHEASTWARD TOWARD LAKE
MICHIGAN, DO HAVE CONCERN THAT THE SOUTHERN EDGE OF CONVECTION
ACROSS IOWA OVERNIGHT WILL TEND TO BUILD FURTHER SOUTH THAN
DEPICTED IN MODEL GUIDANCE. FOR WHAT IT IS WORTH, THE OFTEN
TRUSTWORTHY HRDPS HAS CONSISTENTLY SHOWN CONVECTION IMPACTING
THE TERMINALS FOR SEVERAL CONSECUTIVE RUNS. FOR THESE REASONS,
WILL INTRODUCE PROB30 GROUPS AT ALL BUT GYY DURING THE PRE-DAWN
HOURS OF FRIDAY (GENERALLY 08-12Z). NOTE THAT ANY STORM
OVERNIGHT MAY BRING HAIL AS WELL AS A WEST-SOUTHWESTERLY WIND
SHIFT.
BARRING ANY UNEXPECTED REGENERATING OF CONVECTION AFTER
DAYBREAK, THE CURRENT EXPECTATION IS FOR DAYLIGHT HOURS OF
FRIDAY TO REMAIN DRY AS LOW-LEVEL CAPPING BECOMES FORMIDABLE. A
STRENGTHENING LOW-LEVEL PRESSURE GRADIENT AHEAD OF AN
APPROACHING SURFACE PRESSURE TROUGH WILL LEAD TO GUSTY SOUTHWEST
WINDS NEAR OR ABOVE 30KT. CHANCES FOR THUNDERSTORMS WILL
INCREASE AFTER 21Z ALONG AND JUST BEHIND A COLD FRONT. FOR NOW,
WILL WITHHOLD A FORMAL MENTION AT ORD/MDW IN FAVOR OF FINE-
TUNING TIMING IN THE UPCOMING 00Z TAF PACKAGE.
BORCHARDT
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY UNTIL 10 PM CDT FRIDAY FOR THE IL AND IN
NEARSHORE WATERS.
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