977
FXUS63 KLOT 201635
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
1035 AM CST WED NOV 20 2024
KEY MESSAGES
- COLDEST AIR MASS OF THE AUTUMN THUS FAR ARRIVES TODAY/TONIGHT.
- SNOW SHOWERS DEVELOP LATE THIS AFTERNOON THROUGH TONIGHT AND
MAY BRING THE SEASON'S FIRST MEASURABLE ACCUMULATION IN SOME
LOCATIONS.
- MORE SUBSTANTIAL WET SNOW EXPECTED THURSDAY, WITH SLUSHY
ACCUMULATION LIKELY EVEN ON ROADS ESPECIALLY DURING THE
MORNING.
- LIMITED RISK FOR MINOR LAKESHORE FLOODING AND EROSION ALONG
PRIMARILY THE PORTER COUNTY SHORELINE THURSDAY NIGHT INTO
FRIDAY MORNING.
UPDATE
ISSUED AT 1035 AM CST WED NOV 20 2024
THERE ARE NO MAJOR CHANGES IN FORECAST THINKING REGARDING THE
CONVECTIVE SHOWERS (RAIN/GRAUPEL TO SNOW) LATER TODAY INTO
TONIGHT AND THEN THE EXPECTED BURST OF ACCUMULATING WET SNOW
THURSDAY MORNING INTO THE EARLY AFTERNOON.
AN EXPANSIVE MOSTLY CLOUD FREE DRY SLOT IN THE WAKE OF THE
NARROW BAND OF RAIN EARLY THIS MORNING WILL WORK TO SOMEWHAT
OFFSET LOW-LEVEL COLD AIR ADVECTION (CAA) TO ENABLE TEMPS TO
RISE INTO THE MID 40S TO NEAR/AROUND 50F. THIS IS SLIGHTLY
WARMER THAN PREVIOUS FORECASTS. BREEZY WESTERLY WINDS WILL
FURTHER INCREASE THIS AFTERNOON AS CAA STRENGTHENS, WITH GUSTS
UP TO 35 MPH.
LATER THIS AFTERNOON, THE SHOWER ACTIVITY UPSTREAM OVER IOWA AND
THE UPPER MS VALLEY, ASSOCIATED WITH ANOTHER STOUT VORT MAX
PIVOTING AROUND THE UPPER LOW, WILL OVERSPREAD THE AREA FROM THE
WEST. PRECIP MAY START AS RAIN, THOUGH GIVEN THE LOW WET-BULB
ZERO HEIGHTS AND STEEPENING LOW-LEVEL LAPSE RATES FROM ROBUST
CAA, PERIODS OF GRAUPEL APPEAR LIKELY. ONCE TEMPERATURES FURTHER
COOL THIS EVENING, THAT WILL PRESENT THE OPPORTUNITY FOR SPOTTY
MINOR SLUSHY ACCUMS DETAILED IN THE FULL SHORT TERM DISCUSSION.
REGARDING THURSDAY MORNING INTO THE AFTERNOON, IT APPEARS TO BE
ALL SYSTEMS GO FOR A COUPLE HOUR "THUMP" OF WIND-WHIPPED WET
SNOW. STRONG SYNOPTIC ASCENT AND TRANSIENT STRONG LOW-LEVEL
FRONTOGENESIS AMIDST A DEEP ISOTHERMAL LAYER AND RELATIVELY DEEP
DGZ TOPPED BY STEEP MID- TO UPPER-LEVEL LAPSE RATES WILL LIKELY
YIELD MODERATE TO HEAVY SNOW AND SHARPLY REDUCED VISIBILITY FOR
A TIME. RATES COULD APPROACH OR EVEN EXCEED 1"/HOUR IN THE ~2-3
HOUR WINDOW OF PEAK RATES PIVOTING NORTH TO SOUTH ACROSS THE
AREA FROM ABOUT 8AM FAR NORTH TO ~2-3PM FAR SOUTH. DESPITE THE
ANTECEDENT MILD GROUND CONDITIONS, THESE HEAVY RATES SHOULD BE
ENOUGH FOR SOME SLUSHY ROAD ACCUMS, WITH THE POSSIBLE EXCEPTION
OF HEAVILY TRAVELED ROADS IN/NEAR CHICAGO. UNUSUAL WARM AIR
ADVECTION FROM THE NORTH AND EVENTUAL LOSS OF CLOUD ICE SHOULD
TRANSITION SNOW OVER TO RAIN AND DRIZZLE, WITH MOST OF THE SNOW
THAT FALLS EARLIER LIKELY TO MELT BY THE MID AFTERNOON.
CASTRO
DISCUSSION
ISSUED AT 350 AM CST WED NOV 20 2024
THROUGH THURSDAY:
THE COMPLEX EVOLUTION OF A DEEP UPPER TROUGH/LOW OVER THE
REGION WILL BRING ABOUT AN ACTIVE PERIOD OF WEATHER LOCALLY OVER
THE NEXT COUPLE OF DAYS, INCLUDING OUR FIRST ACCUMULATING SNOW
OF THE SEASON ALONG WITH BLUSTERY/WINDY AND COLDER CONDITIONS.
INITIALLY THIS MORNING, A NORTH-SOUTH BAND OF RAIN WAS
DEVELOPING INTO WESTERN IL IN ASSOCIATION WITH STRONG MID-LEVEL
FRONTOGENESIS BENEATH THE DIVERGENT/DIFFLUENT LEFT EXIT REGION
OF A STRONG UPPER LEVEL JET STREAK ROUNDING THE BASE OF THE DEEP
UPPER TROUGH OVER THE UPPER MIDWEST. THIS AREA OF RAIN IS
EXPECTED TO CONTINUE TO DEVELOP EAST ACROSS THE WFO LOT FORECAST
AREA EARLY THIS MORNING AS THE JET STREAK/MID-LEVEL SHORT WAVE
WRAP CYCLONICALLY ACROSS THE AREA, THOUGH IT SHOULD LARGELY MOVE
EAST OF THE CWA BY MID-MORNING AS THE MID-LEVEL FRONT PASSES. A
PERIOD OF CLEARING IS EXPECTED FROM LATER MORNING THROUGH
MIDDAY IN THE WAKE OF THIS WAVE, THOUGH BLUSTERY WEST WINDS WILL
BE USHERING COLDER AIR INTO THE REGION INTO THIS AFTERNOON.
THE CORE OF THE UPPER LOW (CURRENTLY OVER NORTHWEST MN) IS
PROGGED TO DEVELOP SOUTHEASTWARD TOWARD THE WESTERN LAKES THIS
AFTERNOON AND TONIGHT, WITH ITS MID-LEVEL COLD POOL (700 HPA
TEMPS AROUND -15C) AND PERSISTENT LOW-LEVEL COLD ADVECTION
EXPECTED TO RESULT IN CONVECTIVE SHOWER DEVELOPMENT INTO THE CWA
FROM THE NORTHWEST THIS AFTERNOON. WHILE THESE MAY BEGIN
BRIEFLY AS RAIN, THE COOLING OF THE COLUMN SHOULD ALLOW THESE TO
BECOME MIXED WITH AND CHANGE TO SNOW SHOWERS FAIRLY QUICKLY BY
THIS EVENING. WHILE INTERMITTENT AT ANY ONE LOCATION, COVERAGE
IS EXPECTED TO BE SUFFICIENT TO PRODUCE SOME MINOR ACCUMULATIONS
(A FEW TENTHS OF AN INCH) IN MANY AREAS OVERNIGHT AS SURFACE
TEMPERATURES FALL INTO THE LOW-MID 30S. FORECAST SOUNDINGS
ACTUALLY DEPICT A FAIRLY DEEP (10-12 KFT) MOISTURE LAYER WITHIN
THE DENDRITIC GROWTH ZONE WHICH ALSO LENDS SUPPORT FOR SOME
ACCUMS.
THE CENTER OF THE UPPER LOW IS PROGGED TO MOVE EAST ACROSS
MI/IN/OH BY THURSDAY MORNING. HOWEVER, THE STRONG MID-LEVEL VORT
CURRENTLY MOVING INTO OUR AREA EARLY THIS MORNING IS PROGGED TO
WRAP CYCLONICALLY AROUND THE UPPER LOW AND DROP SOUTHWARD WITH
A A SUB- 1000 HPA SURFACE LOW ACROSS LAKE MICHIGAN AND VICINITY
THURSDAY. INCREASING FORCING IS EXPECTED TO RESULT WITH
INCREASING PRECIPITATION COVERAGE DEVELOPING THURSDAY MORNING,
AND IS EXPECTED TO INITIALLY BE MAINLY SNOW GIVEN THE SUB-
FREEZING THERMODYNAMIC PROFILES. THE MAIN UPPER LEVEL COLD POOL
IS FORECAST TO SHIFT SOUTHEAST OF THE AREA DURING THE
MIDDAY/AFTERNOON HOURS, WHICH ALONG WITH LOW-LEVEL WINDS TURNING
NORTH-NORTHEASTERLY OFF OF LAKE MICHIGAN WILL LIKELY ALLOW
PRECIPITATION TO TRANSITION BACK TO A RAIN/SNOW MIX INTO THE
AFTERNOON. SNOW ACCUMULATION APPEARS LIKELY (PERHAPS A COUPLE OF
INCHES) DURING THE MORNING HOWEVER, INCLUDING THE POTENTIAL FOR
SLUSHY ACCUMS ON ROADS WITH TEMPERATURES IN THE LOW-30S.
IN ADDITION TO OUR WINTRY PRECIPITATION, STRONG NORTH-NORTHWEST
WINDS ARE EXPECTED TO AFFECT THE AREA THURSDAY AS THE DEEP,
COMPACT SURFACE LOW TRACKS SOUTH-SOUTHEAST DOWN LAKE MICHIGAN.
SUSTAINED WINDS OF 25+ MPH APPEAR POSSIBLE WITH GUSTS UP TO 45
MPH. THERE DOES REMAIN SOME MODEL SPREAD IN THE EXACT
PATH/INTENSITY OF THE LOW, AND THIS MAY AFFECT JUST WHERE THE
STRONGEST WINDS (AND THEIR MAGNITUDE) OCCUR. IT'S POSSIBLE WE'LL
NEED TO CONSIDER THE POTENTIAL FOR A WIND ADVISORY HEADLINE FOR
PART OF THE CWA WITH LATER FORECASTS.
RATZER
THURSDAY NIGHT THROUGH TUESDAY:
THURSDAY'S SYSTEM WILL BE QUICKLY DEPARTING TO THE SOUTHEAST
THURSDAY EVENING AS THERMAL PROFILES CONTINUE TO WARM. PRECIP
TYPE LOOKS TO BE PRIMARILY RAIN ACROSS MUCH OF THE AREA AS
COVERAGE OF PRECIP DIMINISHES WITH TIME. HOWEVER, SOME LAKE
ENHANCEMENT AND A SLIGHTLY EAST OF NORTH WIND SHIFT MAY BRING A
MORE FOCUSED RAIN BAND INTO CHICAGO AND THE SOUTH SUBURBS BEFORE
SHIFTING EASTWARD INTO NORTHWEST INDIANA BY THE OVERNIGHT
HOURS. ALSO OF NOTE WILL BE STRONG NORTH WINDS THURSDAY INTO
THURSDAY EVENING THAT MAY PRODUCE WAVES HIGH ENOUGH TO GENERATE
MINOR LAKESHORE EROSION AND FLOODING INTO PRIMARILY PORTER
COUNTY THURSDAY NIGHT INTO FRIDAY.
THOUGH WIDESPREAD PRECIP IS EXPECTED TO END BY FRIDAY MORNING,
PERSISTENT STRATUS UNDER A WEAK SUBSIDENCE INVERSION MAY BE DEEP
ENOUGH TO SUPPORT RESIDUAL PATCHES OF DRIZZLE OR SPRINKLES
THROUGH THE DAY. OTHERWISE, BLUSTERY NORTHWEST WINDS WILL
CONTINUE AS TEMPS RISE INTO THE LOW TO MID 40S. SCATTERED LAKE
EFFECT RAIN SHOWERS WILL ALSO CONTINUE IN THE NORTHWEST WIND
LAKE EFFECT BELTS THROUGH FRIDAY NIGHT, THOUGH INTENSITY WILL BE
WANING WITH TIME AS THE INVERSION LOWERS.
RIDGING SHOULD THEN BRING QUIET CONDITIONS ON SUNDAY BEFORE A
LONGWAVE TROUGH OVER THE NORTHERN GREAT PLAINS INTERACTS WITH A
POCKET OF PACIFIC MOISTURE TO PRODUCE AN AREA OF RAIN SOMEWHERE
OVER THE WESTERN GREAT LAKES SUNDAY NIGHT INTO MONDAY, WITH
ENSEMBLE GUIDANCE FAVORING WISCONSIN AND MICHIGAN. GENERAL
PACIFIC FLOW WITH SMALL CHANCES OF RAIN IS THEN EXPECTED TUESDAY
INTO WEDNESDAY BEFORE THE CONUS-SCALE PATTERN AMPLIFIES WITH
DEEPER TROUGHING ACROSS THE GREAT LAKES INTO THE NORTHEAST
AROUND THANKSGIVING.
KLUBER
AVIATION /12Z TAFS THROUGH 18Z THURSDAY/
ISSUED AT 557 AM CST WED NOV 20 2024
KEY AVIATION MESSAGES:
- RAIN AND SNOW SHOWERS DEVELOPING LATE THIS AFTERNOON BEFORE
TRANSITIONING TO ALL SNOW THIS EVENING AND OVERNIGHT.
- AREA OF WET SNOW WITH SLUSHY ACCUMULATIONS STARTING MID-
MORNING THURSDAY AS NORTHWEST WINDS BEGIN TO GUST UP TO 35
KNOTS.
A NARROW BAND OF RAIN WITH WEST WINDS GUSTING OVER 25 KNOTS IS
CURRENTLY CROSSING ORD/MDW AND WILL QUICKLY EXIT TO THE EAST
OVER THE NEXT HOUR. OUTSIDE OF THE BAND OF RAIN, WEST WINDS TO
AROUND 10 KNOTS WILL INCREASE THROUGH THE MORNING AND GUST TO
AROUND 25 KNOTS LATE THIS MORNING AND AFTERNOON.
SCATTERED RAIN SHOWERS DEVELOPING LATE THIS AFTERNOON WILL
BEGIN TO MIX WITH SNOW THROUGH EARLY EVENING BEFORE
TRANSITIONING TO ALL SNOW BY MID-EVENING. MVFR CEILINGS AND
VISIBILITY ARE INITIALLY EXPECTED WITH THESE SHOWERS, THOUGH
SOME OF THE SNOW SHOWERS WILL BE CAPABLE OF PRODUCING IFR
VISIBILITY AND BOUTS OF GUSTY WINDS. COVERAGE OF THE SNOW
SHOWERS SHOULD THEN GREATLY DIMINISH FOR A PERIOD LATE TONIGHT
THROUGH SUNRISE THURSDAY.
A COMPACT LOW PRESSURE SYSTEM WILL THEN SHIFT SOUTHWARD ACROSS
LAKE MICHIGAN THROUGH THE DAY THURSDAY, PRODUCING A SHIELD OF
WET SNOW ACROSS THE CHICAGO TERMINALS MID TO LATE MORNING BEFORE
PRECIP SLOWLY TRANSITIONS TO RAIN DURING THE AFTERNOON. SLUSHY
ACCUMULATIONS OF 1-2" AND POTENTIALLY UP TO 3" ARE EXPECTED.
ADDITIONALLY, NW WINDS WILL QUICKLY INCREASE THROUGH THE
MORNING, WITH GUSTS UP TO 35 KNOTS BY THE END OF THE FORECAST
PERIOD 18Z THURSDAY.
KLUBER
MARINE
ISSUED AT 958 PM CST TUE NOV 19 2024
- GALE WATCH ISSUED FOR THE ILLINOIS AND NORTHWEST INDIANA
NEARSHORE WATERS THURSDAY MORNING THROUGH THURSDAY EVENING.
STRONG WINDS ARE EXPECTED TO DEVELOP OVER LAKE MICHIGAN ON
THURSDAY IN RESPONSE TO A DEEPENING SURFACE LOW RETROGRADING
OVER THE WESTERN GREAT LAKES REGION. WEST-NORTHWEST GUSTS IN
EXCESS OF 40 KT WILL BE POSSIBLE THURSDAY MORNING BEFORE
STEADILY VEERING NORTH-NORTHWEST OR NORTH BY EARLY EVENING.
WINDS WILL THEN DIMINISH THROUGH THE NIGHT.
A SMALL CRAFT ADVISORY IS IN EFFECT THIS AFTERNOON THROUGH THE
START OF THE GALE WATCH PERIOD WHEN WINDS WILL BE GUSTING TO
AROUND 25 KT AND WAVES WILL BE BUILDING ALONG THE NORTHWEST
INDIANA SHORE.
PETR
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY UNTIL 7 AM CST THURSDAY FOR THE IL
NEARSHORE WATERS.
GALE WATCH FROM THURSDAY MORNING THROUGH THURSDAY EVENING FOR
THE IL NEARSHORE WATERS.
SMALL CRAFT ADVISORY UNTIL 10 AM CST THURSDAY FOR THE IN
NEARSHORE WATERS.
GALE WATCH FROM THURSDAY MORNING THROUGH THURSDAY EVENING FOR
THE IN NEARSHORE WATERS.
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