171
FXUS63 KLOT 200950
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
350 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.
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 /06Z TAFS THROUGH 12Z THURSDAY/
ISSUED AT 1101 PM CST TUE NOV 19 2024
FORECAST CONCERNS FOR THE TERMINALS...
- PERIOD OF NEAR MVFR CEILINGS TONIGHT, MAINLY NEAR THE IL-WI
LINE
- BRIEF PERIOD OF RAIN SHOWERS AROUND DAYBREAK WEDNESDAY MORNING
- BREEZY WESTERLY WINDS WEDNESDAY WITH GUSTS IN THE 20-30 KT
RANGE
- SECOND ROUND OF RAIN AND SNOW SHOWERS WEDNESDAY AFTERNOON
TRANSITIONING TO SNOW SHOWERS WEDNESDAY EVENING/NIGHT
A BROAD AREA OF 2500-3500 FT CEILINGS CONTINUE TO PIVOT ACROSS
NORTHERN IL THIS EVENING AHEAD OF AN ORGANIZING UPPER LOW IN THE
NORTHERN PLAINS. WHILE PERIODS OF MVFR CONDITIONS HAVE BEEN
OBSERVED AT THE TERMINALS, THEIR LONGEVITY HAS BEEN RELATIVELY
SHORT SO WILL CONTINUE TO HANDLE THIS POTENTIAL WITH A SCT025
MENTION THROUGH TONIGHT. AS WE HEAD INTO WEDNESDAY MORNING, THE
SHORTWAVE DISTURBANCE ALONG THE MO-KS LINE WILL BE LIFTING OVER
THE AREA WITH ITS ASSOCIATED ELEVATED FRONT. ALONG THE FRONT A
BAND OF SHOWERS IS ALREADY DEVELOPING IN IA AND SHOULD MOVE
THROUGH THE TERMINALS BETWEEN 10Z AND 15Z. GIVEN THAT THESE
SHOWERS ARE EXPECTED TO BE QUICK MOVING HAVE DECIDED TO MAINTAIN
THE TEMPO GROUPS SO EXPECT ANY SITE TO HAVE SHOWERS FOR ABOUT AN
HOUR OR TWO AT MOST.
ASIDE FROM THESE SHOWERS, WINDS WILL BEGIN TO INCREASE WEDNESDAY
MORNING AS THE UPPER LOW DRIFTS INTO THE GREAT LAKES REGION.
INITIALLY WINDS ARE EXPECTED TO GUST IN THE 20-25 KT RANGE, BUT
WILL INCREASE BY THE AFTERNOON WITH GUSTS OF 25-30 KTS.
REGARDLESS, WIND DIRECTIONS ARE EXPECTED TO PREVAIL OUT OF THE
WEST. WHILE THE STRONGEST GUSTS ARE FORECAST TO DIMINISH DURING
THE EVENING ON WEDNESDAY, A WELL MIXED ATMOSPHERE OVERNIGHT
SHOULD SUPPORT AT LEAST SOME OCCASIONAL 17-20 KT GUSTS THROUGH
THE END OF THE TAF PERIOD.
FINALLY, THE APPROACHING UPPER LOW WILL ALSO GENERATE A SECOND
AREA OF SHOWERS WEDNESDAY AFTERNOON THROUGH WEDNESDAY NIGHT. AT
FIRST THESE SHOWERS WILL START OFF AS RAIN OR EVEN A RAIN/SNOW
MIX AS THE SNOW GROWTH REGION QUICKLY SATURATES, BUT THE SHOWERS
WILL TRANSITION OVER TO ALL SNOW BETWEEN 02Z AND 04Z WEDNESDAY.
HOWEVER, SURFACE TEMPERATURES DURING THE TRANSITION ARE FORECAST
TO BE AT OR JUST ABOVE FREEZING WHICH SHOULD LIMIT THE AMOUNT OF
ACCUMULATION TO MAINLY GRASSY AND ELEVATED SURFACES. THAT SAID,
MOST GUIDANCE IS IN DECENT AGREEMENT THAT SNOW AMOUNTS SHOULD
GENERALLY BE UNDER A QUARTER INCH BUT THERE REMAINS THE
POTENTIAL FOR SOME LOCALIZED POCKETS OF HIGHER TOTALS IN EXCESS
OF HALF AN INCH WITH THE MOST ROBUST SHOWERS. FURTHERMORE, THE
SNOW SHOWERS WILL ALSO COME WITH A THREAT OF REDUCED
VISIBILITIES WHICH COULD DIP INTO THE IFR CATEGORY. DUE TO
UNCERTAINTY AS TO WHERE THE STRONGEST SHOWERS WILL TRACK, HAVE
DECIDED TO MAINTAIN A VAGUE 5SM -SHSN IN THE TAFS FOR NOW BUT
MAY NEED TO CONSIDER LOWER VISIBILITIES WITH FUTURE UPDATES.
NEVERTHELESS, CEILINGS ARE EXPECTED TO BECOME MVFR WITH THE SNOW
SHOWERS AND PREVAIL AS SUCH THROUGH THE END OF THE PERIOD.
YACK
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 FROM NOON TODAY TO 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 FROM 9 AM THIS MORNING TO 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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