753
FXUS63 KLOT 190051
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
751 PM CDT FRI APR 18 2025
KEY MESSAGES
- SCATTERED SHOWERS/STORMS THIS EVENING, SOME POSSIBLY SEVERE
WITH HAIL AND STRONG/GUSTY WINDS THE MAIN THREATS.
- MUCH COOLER SATURDAY WITH SHOWER CHANCES LINGERING NEAR/SOUTH
OF HWY 24, ESPECIALLY EARLY.
- PERIODS OF SHOWERS AND POSSIBLY EMBEDDED THUNDERSTORMS ON
SUNDAY BECOMING MORE WIDESPREAD SUNDAY NIGHT. THERE REMAINS A
CONDITIONAL THREAT FOR STRONG TO SEVERE STORMS SUNDAY
AFTERNOON AND EVENING MAINLY FOR THE SOUTHERN PORTIONS OF OUR
AREA.
UPDATE
ISSUED AT 750 PM CDT FRI APR 18 2025
A SURFACE COLD FRONT, TIED TO A ~1004 MB SURFACE LOW JUST NORTH
OF MILWAUKEE IS SHIFTING EAST ACROSS THE MISSISSIPPI RIVER.
AHEAD OF THIS BOUNDARY, WIDESPREAD CLOUD COVER DURING THE
AFTERNOON GREATLY LIMITED INSOLATION, WITH TEMPERATURES
GENERALLY IN THE LOW 70S AT THIS TIME. WHILE DEWPOINTS HAVE
MEANDERED THEIR WAY INTO THE LOWER 60S, THE EXPANSIVE NATURE OF
THE LOW-LEVEL STRATUS DECK SUGGESTS THAT INHIBITION FOR SURFACE
PARCELS REMAINS IN PLACE, AS EVIDENCED BY BILLOW CLOUDS ACROSS
NORTHEAST ILLINOIS (ON EARLIER VISIBLE SATELLITE IMAGERY). THIS
ALSO APPEARS TO BE TRUE EVEN ACROSS OUR FAR WESTERN LOCALES
WHERE THE RAP IS ANALYZING A LOCAL MINIMA IN MLCIN, WHICH IS
PROBABLY A RESULT OF ITS DEWPOINT FIELDS WHICH REMAIN A BIT
HIGHER THAN OBSERVATIONS SUGGEST. REGARDLESS, A RESERVOIR OF
ELEVATED INSTABILITY REMAINS IN PLACE, WITH MUCAPE AROUND 1500
J/KG PER RECENT OBJECTIVE ANALYSIS (SIMILAR VALUES WERE DEPICTED
ON A 19Z SOUNDING FROM NIU).
A LOW-AMPLITUDE DISTURBANCE APPEARS TO BE TRANSLATING UP THROUGH
THE MIDDLE MISSISSIPPI RIVER VICINITY, WITH A MODEST 500 MB JET
ENHANCEMENT (70+ KT) PUSHING INTO SOUTHERN WISCONSIN. CONTINUED
BROAD ASCENT ASSOCIATED WITH THIS PERTURBATION HAS RESULTED IN
A SLOW LIFTING/MOISTENING AT THE BASE OF AN ELEVATED MIXED
LAYER. THIS HAS SUPPORTED A **VERY** GRADUAL INCREASE IN
CONVECTION IN AN AXIS STRETCHING FROM JANESVILLE, WI,
SOUTHWESTWARD TOWARDS MACOMB, IL.
WHILE WE'LL STEADILY BE LOSING THE STEEPEST LAPSE RATES
ASSOCIATED WITH THE PRIMARY CORE OF THE EML PLUME (WHICH
ADVECTED OVERHEAD LATE LAST NIGHT AND EARLY THIS MORNING),
INSTABILITY WILL REMAIN SUFFICIENT IN CONCERT WITH 40-60 KTS OF
EFFECTIVE DEEP LAYER SHEAR TO SUPPORT SLOWLY ORGANIZING
UPDRAFTS AND AT LEAST SOME SEVERE WEATHER THREAT. GIVEN THE
OVERALL MODEST NATURE OF ASCENT AND ONGOING SATELLITE/RADAR
TRENDS, ANY INTENSIFICATION PROCESS WILL LIKELY REMAIN SLOW,
WITH THE SEVERE WEATHER THREAT EXPECTED TO REMAIN SOMEWHAT
ISOLATED. BASED ON THE LATEST TRENDS WITH ACTIVITY STILL
STRUGGLING TO MEANINGFULLY INTENSIFY ACROSS NORTHWEST ILLINOIS,
WONDERING IF THE ACTIVITY PUSHING OUT OF WEST-CENTRAL ILLINOIS
WILL END UP POSING THE MAIN STRONG-SEVERE RISK AS IT PUSHES
NORTHEAST TOWARDS OUR REGION AFTER ABOUT 9 PM, WITH THE MAIN
SEVERE THREAT AREA LOOKING LIKE IT'LL SET UP ACROSS THE
SOUTHEAST HALF OF THE FORECAST AREA (AND EVEN THIS MAY REMAIN
SOMEWHAT LIMITED IN SCOPE/COVERAGE). MAIN THREATS WILL BE FROM
HAIL AND 50-60 MPH WIND GUSTS. GIVEN THE LOSS OF THE STEEPEST
LAPSE RATES, SUSPECT THE THREAT FOR MORE SIGNIFICANT HAIL WILL
REMAIN IN CHECK.
CARLAW
DISCUSSION
ISSUED AT 309 PM CDT FRI APR 18 2025
THROUGH TONIGHT:
EXTENSIVE CLOUD COVER PERSISTS ACROSS THE AREA THIS AFTERNOON
WITHIN THE WARM SECTOR OF A SURFACE LOW CENTERED NEAR DUBUQUE, IA.
IN SPITE OF THE CLOUD COVER, BREEZY SOUTH WINDS CONTINUE TO AID
IN THE ADVECTION OF WARMER TEMPERATURES. IN FACT, AS OF THIS
WRITING TEMPERATURES HAVE WARMED INTO THE MID TO UPPER 70S, SAVE
AREAS ALONG THE LAKE COUNTY IL LAKE MICHIGAN SHORE. COOLER
TEMPERATURES (IN THE 60S) HAVE PERSISTED HERE DUE TO THE
CONTINUED ONSHORE FLOW FOLLOWING THIS MORNINGS OUTFLOW BOUNDARY.
EXPECT THESE BREEZY AND WARM CONDITIONS TO PERSIST INTO THIS
EVENING AS A COLD FRONT GRADUALLY SHIFTS EASTWARD INTO THE AREA.
THE PRIMARY WEATHER CONCERN CONTINUES TO CENTER AROUND THE
THREAT FOR STRONG TO SEVERE THUNDERSTORMS, PARTICULARLY THIS
EVENING (ROUGHLY 6 PM THROUGH 11PM) AS THE COLD FRONT SHIFTS
EASTWARD ACROSS THE AREA. A PERSISTENT AND EXCEPTIONALLY STRONG
EML AND ASSOCIATED CAPPING INVERSION CONTINUES TO BE SAMPLED IN
RECENT AMDAR SOUNDINGS OUT OF MDW AND RFD. THIS IS EXPECTED TO
CURTAIL NEAR SURFACE BASED STORM DEVELOPMENT, AT LEAST FOR THE
NEXT FEW HOURS MID TO LATE THIS AFTERNOON. IN SPITE OF THIS,
ELEVATED STORMS, BASED ABOVE THE EML INVERSION, WILL CONTINUE TO
FESTER ACROSS FAR NORTHERN SECTIONS OF IL FOR THE NEXT FEW
HOURS AS A WEAK IMPULSE TRACKS NORTHEAST INTO SOUTHERN WI. ANY
SEVERE THREAT WITH THESE INITIAL STORMS SHOULD BE CONFINED TO
SOME INSTANCES OF MARGINALLY SEVERE HAIL NEAR AND NORTH OF THE
WI STATE LINE.
AS WE HEAD INTO EARLY THIS EVENING (AFTER 6 PM), THE SURFACE COLD
FRONT, WHICH AS OF THIS WRITING RESIDES WEST OF THE AREA ACROSS
EASTERN IA, WILL SLIDE EASTWARD ACROSS NORTHERN IL. IT IS NEAR
THIS BOUNDARY IN WHICH ADDITIONAL (POTENTIALLY NEAR SURFACE
BASED) STORMS LOOK TO DEVELOP. AS MENTIONED IN PREVIOUS
DISCUSSIONS, QUESTIONS REMAIN WITH THE EXACT TIMING AND OVERALL
COVERAGE OF STORMS NEAR THE FRONTAL BOUNDARY THIS EVENING. MUCH OF
THIS CONTINUED UNCERTAINTY REVOLVES AROUND HOW EFFECTIVELY THE
EML CAPPING INVERSION WILL ERODE INTO THIS EVENING, ESPECIALLY
CONSIDERING THAT MOST OF THE FORCED ASCENT AND COOLING LOOKS TO
COME AS A RESULT OF FRONTAL SCALE FORCING.
NEVERTHELESS, ANY STORMS THAT DO DEVELOP NEAR THE FRONT THIS
EVENING WILL DO SO IN AN ENVIRONMENT QUITE FAVORABLE FOR SEVERE
STORMS. THE COMBINATION OF STRONG DEEP LAYER SHEAR AND VERY
STEEP MID-LEVEL LAPSE RATES WOULD BE SUPPORTIVE OF SOME
SUPERCELLS CAPABLE OF PRODUCING INSTANCES OF VERY LARGE HAIL
(1.5"+). LARGE HAIL LOOKS TO BE THE PRIMARY SEVERE THREAT THIS
EVENING WITH THESE STORMS, THOUGH CANNOT RULE OUT THE POTENTIAL
FOR SOME DAMAGING WIND GUSTS AND POSSIBLE A BRIEF TORNADO,
PARTICULARLY WITH ANY STORMS THAT ARE ABLE TO BECOME NEAR
SURFACE BASED. THE HIGHEST THREAT AREA FOR THESE SEVERE STORMS
WILL LARGELY BE ALONG AND SOUTH OF A LINE FROM NEAR LASALLE-PERU
TO CHICAGO, WITH OVERALL LOWER POTENTIAL THE FARTHER NORTHWEST
OF THIS LINE YOU ARE. EXPECT THE THREAT OF STORMS TO END NORTH
OF I-80 LATER THIS EVENING. HOWEVER, STORM ACTIVITY IS LIKELY TO
LINGER OVERNIGHT WELL SOUTH OF I-80, THOUGH THE SEVERE
POTENTIAL IS EXPECTED TO GRADUALLY EASE HERE AFTER MIDNIGHT.
KJB
SATURDAY THROUGH FRIDAY:
SOME SHOWERS FROM DECAYING OVERNIGHT CONVECTION MOVING ACROSS
OUR SOUTHERN CWA SHOULD LINGER THROUGH THE 7-10AM CDT TIMEFRAME
ON SATURDAY ACROSS OUR FAR SOUTHERN CWA, PRIMARILY SOUTH OF
US-24 (60-70% POPS). THE REST OF THE DAY AND THROUGH SATURDAY
NIGHT/EARLY SUNDAY, ADDITIONAL IMPULSES INTERACTING WITH THE
LOW-LEVEL BAROCLINIC ZONE FOCUSED TO OUR SOUTH MAY BRING
OCCASIONAL (PRIMARILY LIGHT INTENSITY) SHOWERS AT TIMES TO
PORTIONS OF CENTRAL ILLINOIS AND SOUTH OF THE KANKAKEE RIVER IN
INDIANA (20-50% POPS/HIGHEST SOUTH OF US-24). OTHERWISE, IT WILL
BE NOTABLY COOLER ON SATURDAY WITH CLOUDY SKIES AND PERHAPS
SOME SUNNY BREAKS IN THE AFTERNOON. HIGHS WILL ONLY REACH THE
MID 50S TO AROUND 60F INLAND AND LIKELY BE LIMITED TO THE UPPER
40S ALONG THE IMMEDIATE LAKESHORE. THIS WILL BE FOLLOWED BY LOWS
IN THE LOWER TO MID 40S ON SATURDAY NIGHT.
ON SUNDAY, THE NEGATIVELY TILTED UPPER TROUGH EJECTING FROM THE
SOUTHERN PLAINS AND ASSOCIATED SURFACE CYCLOGENESIS WILL RESULT IN
A SURFACE LOW TRACKING TO THE MID MS VALLEY LATE SUNDAY NIGHT TO
CENTRAL LAKE MICHIGAN EARLY MONDAY AFTERNOON. THIS IS EXPECTED TO
RESULT IN A BREEZY (EASTERLY GUSTS TO 30 MPH), COOL, AND SHOWERY
AT TIMES EASTER SUNDAY, UNFORTUNATELY. IT DOESN'T LOOK TO BE AN
ALL-DAY WASHOUT, HOWEVER, WITH A FEW ROUNDS OF FORCING INTERACTING
WITH THE WARM/MOIST ADVECTION OVER-TOPPING THE WARM FRONTAL ZONE.
THE FORECAST SURFACE LOW PATH AND TIMING REPRESENTS A FURTHER
SLOWING TREND VS. A FEW MODEL CYCLES AGO. THIS IS IMPORTANT WITH
RESPECT TO THE NORTHWARD PROGRESSION OF A WARM FRONT LIFTING NORTH
AND THE POTENTIAL FOR A CONDITIONAL SURFACE BASED SEVERE THREAT
LATE DAY SUNDAY INTO THE EVENING. GIVEN THE SYSTEM'S SLOWING
TREND, IT APPEARS THE WARM FRONT WILL BE HUNG UP JUST SOUTH OF OUR
SOUTHERN CWA, WITH PERHAPS A CHANCE FOR ISOLATED THUNDERSTORMS
SOUTH OF THE KANKAKEE RIVER SUNDAY AFTERNOON.
SUNDAY NIGHT IS WHEN THE MOST WIDESPREAD SHOWERS AND ISOLATED TO
SCATTERED EMBEDDED THUNDERSTORMS ARE ANTICIPATED, ALONG WITH GUSTY
SOUTHEAST WINDS. RAINFALL TOTALS MAY END UP NEAR OR JUST UPWARDS
OF AN INCH NEAR AND WEST OF I-39 IN NORTHWEST TO NORTH CENTRAL
ILLINOIS, WITH AMOUNTS TAPERING GRADUALLY OFF TO THE EAST. CAN'T
RULE OUT SOME PONDING ON ROADS WITH ANY HEAVIER CONVECTIVE RAIN
RATES SUNDAY NIGHT, THOUGH THE OVERALL FLOODING THREAT APPEARS LOW.
THE SURFACE LOW WILL TRACK JUST TO OUR NORTHWEST LATE SUNDAY
NIGHT AND THEN TO OUR NORTH ON MONDAY, WITH A FEW LINGERING
SHOWERS POSSIBLE UNTIL LATE MORNING/MIDDAY IN THE COLD ADVECTION
REGIME BEHIND THE COLD FRONT. THE AFTERNOON WILL FEATURE BREEZY
WEST- SOUTHWEST WINDS GUSTING TO 35 MPH, AND SOME UPSIDE POTENTIAL
REMAINS WITH THIS.
CASTRO
FOR TUESDAY ONWARD, THE UPPER-LEVEL PATTERN IS FORECAST TO
BECOME MORE ZONAL (WEST-EAST ORIENTED) AND PERSISTENTLY MILDER.
BOTH DETERMINISTIC AND ENSEMBLE GUIDANCE CONTINUES TO INDICATE
THAT SUBTLE SHORTWAVES MAY PIVOT THROUGH THE PATTERN AND RESULT
IN ADDITIONAL PERIODS OF SHOWERS AND PERHAPS THUNDERSTORMS (FIRST
WINDOW WOULD BE LATE TUESDAY-TUESDAY EVENING). GIVEN THE
UNCERTAINTY IN TIMING AND COVERAGE OF SUCH ACTIVITY, SEE NO REASON
TO ALTER THE PERIODIC 20-50% POPS OFFERED BY THE NBM THROUGH THE
END OF NEXT WEEK, BUT SUSPECT THAT MANY DRY HOURS WILL OCCUR AS
WELL. ON A MORE POSITIVE NOTE, CONTINUED MID-LEVEL WARM ADVECTION
(850MB TEMPS FORECAST IN THE 10-12C RANGE) IS EXPECTED TO KEEP
TEMPERATURES IN THE ABOVE NORMAL CATEGORY FOR MID-APRIL WITH HIGHS
IN THE UPPER 60S TO 70S AND OVERNIGHT LOWS IN THE 40S TO AROUND
50F. THE EXCEPTION HOWEVER, WILL BE FOR THOSE NEAR THE LAKE WHERE
THE POTENTIAL FOR DAILY LAKE BREEZES MAY KEEP READINGS NOTABLY
COOLER.
YACK/CASTRO
AVIATION /00Z TAFS THROUGH 06Z SUNDAY/
ISSUED AT 639 PM CDT FRI APR 18 2025
AVIATION FORECAST CONCERNS FOR THE 00A TAFS:
- SCATTERED THUNDERSTORM DEVELOPMENT EXPECTED THIS EVENING, WITH
CHICAGO TERMINALS MOST LIKELY TO BE IMPACTED IN THE 01-05Z
PERIOD.
- MVFR CEILINGS LIKELY BEHIND COLD FRONT OVERNIGHT THROUGH
SATURDAY MORNING, WITH A BRIEF PERIOD OF IFR CEILINGS POSSIBLE
SATURDAY MORNING.
- WIND SHIFT TO WEST-NORTHWEST BEHIND COLD FRONT LATE
EVENING/OVERNIGHT. WINDS LIKELY SHIFT NORTHEAST FOR CHICAGO
SITES PREDAWN, THEN BACK NORTHWEST BY MIDDAY BEFORE LAKE
BREEZE SHIFTS WINDS EASTERLY AGAIN LATE SATURDAY AFTERNOON.
EARLY EVENING SURFACE MAP DEPICTS LOW PRESSURE OVER SOUTHERN WI,
WITH A COLD FRONT TRAILING SOUTHWEST ALONG THE MISSISSIPPI
RIVER. CONTINUE TO SEE A GRADUAL INCREASE IN CONVECTIVE UPDRAFT
ATTEMPTS AHEAD OF THE FRONT FROM WESTERN IL INTO SOUTHERN WI,
AND EXPECTATION IS THAT SCATTERED THUNDERSTORMS WILL DEVELOP
ACROSS NORTHERN IL BY MID-EVENING. KRFD MAY BE ON THE WESTERN
FRINGE OF THIS (DEPENDING ON HOW QUICKLY THINGS DEVELOP) SO HAVE
EASED OFF TO A VCTS MENTION THERE, THOUGH CHICAGO AREA
TERMINALS CONTINUE TO SEE A THREAT OF TSRA FROM ROUGHLY 01-05Z.
THE COLD FRONT THEN MOVES SLOWLY EAST ACROSS THE AREA LATE
EVENING/OVERNIGHT, WITH MVFR CEILINGS LIKELY THROUGH SATURDAY
MORNING BEFORE IMPROVING AND SCATTERING TO VFR IN THE AFTERNOON.
COULD BE A PERIOD OF IFR CEILINGS FOR THE CHICAGO TERMINALS
SATURDAY MORNING AS FLOW TURNS OFF LAKE MICHIGAN.
BREEZY SOUTHWEST WINDS CONTINUE TO GUST 25-30 KTS IN THE WARM
SECTOR AHEAD OF THE COLD FRONT EARLY THIS EVENING. WINDS WILL
SHIFT WEST-NORTHWEST AND EVENTUALLY NORTHWEST BEHIND THE FRONT
LATE EVENING/OVERNIGHT, WITH DECREASING GUSTS. SEVERAL MODELS
NOW DEPICT A WIND SHIFT TO NORTHEAST OFF THE LAKE FOR THE
CHICAGO TERMINALS PREDAWN SATURDAY, WITH WINDS REMAINING
NORTHEAST THROUGH THE MORNING BEFORE BRIEFLY TURNING LIGHT
NORTHWEST AGAIN BY EARLY AFTERNOON...ONLY TO GO EASTERLY AGAIN
AS A LAKE BREEZE MOVES THROUGH LATE IN THE AFTERNOON.
RATZER
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY UNTIL 9 PM CDT THIS EVENING FOR WINTHROP
HARBOR TO WILMETTE HARBOR IL.
GALE WARNING UNTIL 9 PM CDT THIS EVENING FOR WILMETTE HARBOR
IL TO MICHIGAN CITY IN.
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