697
FXUS63 KLOT 150427
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
1027 PM CST SAT DEC 14 2024
KEY MESSAGES
- AREAS OF DENSE FOG ARE POSSIBLE LATE TONIGHT INTO SUNDAY
MORNING AS WIDESPREAD RAIN TAPERS OFF TO PATCHY LIGHT RAIN AND
DRIZZLE. FOG MAY BE SLOW TO IMPROVE SUNDAY MORNING/MIDDAY.
UPDATE
ISSUED AT 1027 PM CST SAT DEC 14 2024
EARLIER FREEZING RAIN/ICING HAS ENDED, AS SURFACE TEMPERATURES
AND DEW POINTS HAVE CLIMBED ABOVE FREEZING AS OF MID-LATE
EVENING. MAIN CONCERN OVERNIGHT TURNS TO THE POTENTIAL FOR DENSE
FOG DEVELOPMENT AND POSSIBLE DENSE FOG ADVISORY ISSUANCE,
ESPECIALLY NORTHWEST OF I-55 AFTER MIDNIGHT INTO SUNDAY MORNING.
EVENING GOES VAPOR IMAGERY SHOWS CLOSED MID-LEVEL LOW CENTERED
NORTH OF ST. LOUIS, STACKED ABOVE AN ASSOCIATED OCCLUDING
SURFACE LOW WITH A WARM FRONT EXTENDING EAST-SOUTHEAST INTO THE
LOWER OHIO VALLEY. THE MID-LEVEL LOW IS PROGGED TO TRACK
NORTHEAST ACROSS THE FORECAST AREA TO SOUTHERN LAKE MICHIGAN BY
MID-MORNING SUNDAY, WHILE THE OCCLUDING SURFACE LOW SLOWS AND
FILLS ACROSS NORTHWEST IL. WARM/MOIST ADVECTION HAS ALLOWED
SURFACE TEMPERATURES TO SLOWLY BUT STEADILY RISE INTO THE MID-
UPPER 30S ACROSS THE FORECAST AREA, WITH DEW POINTS AND SURFACE
WET-BULBS ABOVE FREEZING AREA-WIDE ENDING THE FREEZING RAIN
THREAT.
MID-LEVEL DRY AIR WAS ALREADY WRAPPING INTO THE CIRCULATION, AS
INDICATED BY THE DECREASE IN RAIN INTENSITY ACROSS PARTS OF
WESTERN AND CENTRAL IL. RAIN OVER EASTERN PARTS OF THE CWA WILL
CONTINUE TO LIFT NORTHEAST OVER THE NEXT SEVERAL HOURS, WITH
PRECIPITATION TAPERING TO SCATTERED LIGHTER SHOWERS AND AREAS OF
DRIZZLE OVERNIGHT. AS PRECIPITATION DIMINISHES AND WINDS
LIGHTEN UP NEAR THE FILLING SURFACE LOW, FOG DEVELOPMENT IS
EXPECTED WITH AREAS OF DENSE FOG LIKELY. ALREADY SEEING
VISIBILITIES LOWERING TO LESS THAN A MILE ACROSS SOUTHEAST IA AT
THIS HOUR, AND EXPECT THAT THESE CONDITIONS WILL LIKELY SPREAD
INTO NORTHERN IL AFTER MIDNIGHT TONIGHT. AREAS NORTHWEST OF I-55
APPEAR TO BE MOST AT RISK FOR DENSE FOG WHERE LIGHTER WINDS AND
SLIGHTLY COOLER TEMPERATURES WILL BE. IT'S VERY POSSIBLE WE'LL
NEED A DENSE FOG ADVISORY FOR A PORTION OF OUR IL CWA LATER
TONIGHT INTO SUNDAY MORNING. IMPROVEMENT MAY BE FAIRLY SLOW,
WITH SOME FOG POTENTIALLY LINGERING INTO MIDDAY/EARLY AFTERNOON.
RATZER
DISCUSSION
ISSUED AT 231 PM CST SAT DEC 14 2024
THROUGH SUNDAY:
THE LEADING EDGE OF RAIN/FREEZING RAIN CONTINUES TO GRADUALLY
SLIDE EASTWARD ACROSS NORTHERN IL EARLY THIS AFTERNOON.
TEMPERATURES HAVE WARMED ABOVE FREEZING IN MOST LOCATIONS, BUT
WETBULB TEMPERATURES REMAIN NEAR (OR EVEN A BIT BELOW FREEZING),
ACROSS INTERIOR SECTIONS OF FAR NORTHERN IL. ACCORDINGLY, THE
THREAT FOR A PERIOD OF FREEZING RAIN AND SLICK TRAVEL CONTINUES
ACROSS THE ADVISORY AREA THIS AFTERNOON. SOME SLEET AND/OR SNOW
MAY ALSO BRIEFLY ACCOMPANY THE FREEZING RAIN AS IT ONSETS THIS
AFTERNOON.
ICE ACCUMULATIONS IN THE ADVISORY AREA SHOULD GENERALLY REMAIN
UNDER A TENTH OF AN INCH, AND LIKELY ONLY END UP BEING A LIGHT
GLAZE ACROSS EASTERN SECTIONS OF THE ADVISORY AREA BEFORE WETBULB
TEMPERATURES WARM ABOVE FREEZING EARLY THIS EVENING. NEVERTHELESS,
UNTREATED ROADS AND SIDEWALKS ARE LIKELY TO BECOME SLICK. OUTSIDE
OF THE CURRENT ADVISORY, CONDITIONS HAVE WARMED ENOUGH THAT
FREEZING PRECIPITATION IS NOT EXPECTED TO BECOME A PROBLEM.
PERIODS OF RAIN WILL CONTINUE ACROSS THE AREA THROUGH THE EVENING
BEFORE GRADUALLY TRANSITIONING TO DRIZZLE FROM WEST-TO-EAST
OVERNIGHT TONIGHT AS THE CENTER OF THE APPROACHING STORM SYSTEM
SHIFTS OVERHEAD. LOW STRATUS AND AREAS OF FOG ARE ALSO LIKELY TO
ACCOMPANY THE DRIZZLE OVERNIGHT, AND THESE CONDITIONS WILL
PERSIST DURING THE DAY ON SUNDAY. TEMPERATURES IN THIS PATTERN
WILL MOVE LITTLE, SO DON'T EXPECT ANY REAL DIURNAL SWINGS IN
TEMPERATURES THROUGH SUNDAY NIGHT. THROUGH THIS PERIOD,
READINGS ARE EXPECTED TO HOLD NEARLY STEADY IN THE 30S TO AROUND
40.
KJB
SUNDAY NIGHT THROUGH SATURDAY:
FORECAST THINKING DURING THE EXTENDED PERIOD HAS NOT CHANGED
FROM THE PREVIOUS THINKING HIGHLIGHTED BELOW.
SUNDAY NIGHT, ATTENTION WILL TURN TO THE AFOREMENTIONED SYSTEM
INCOMING FROM THE PLAINS. STRENGTHENING WARM AIR ADVECTION AND
LARGE SCALE ASCENT IN ADVANCE OF THE APPROACHING UPPER TROUGH
SHOULD LEAD TO A BLOSSOMING AREA OF SHOWERS TO EXPAND/MOVE
NORTHEASTWARD ACROSS THE AREA VERY LATE THIS EVENING AND MORESO
OVERNIGHT. TEMPS SUNDAY NIGHT SHOULD START OUT NEARLY STEADY,
THEN BEGIN RISING OVERNIGHT AS WAA RAMPS UP.
THERE HAS BEEN A DECIDED SHIFT IN GUIDANCE TOWARD A QUICKER
PROGRESSION WITH THIS UPPER TROUGH, LIKELY RESULTING IN RAIN
ENDING EARLIER MONDAY. NOT ONLY WILL THE EARLIER PASSAGE OF THE
UPPER TROUGH/ASSOCIATED SFC FRONTAL RESULT IN AN EARLIER END IN
SHOWERS, IT WILL PROBABLY KEEP TEMPS A COUPLE OF DEGREES COOLER
THAN EARLIER RUNS, BUT STILL WELL ABOVE AVERAGE (MID 40S
NORTHWEST CWA TO MID 50S NW IN/EC IL).
COOLER TEMPERATURES ARE EXPECTED IN THE WAKE OF THIS SYSTEM, BUT
AIR MASS WILL BE OF PACIFIC ORIGIN, SO TEMPS SHOULD SETTLE BACK
TO NEAR AVERAGE FOR MID DECEMBER MIDWEEK. ANOTHER FAIRLY LOW
AMPLITUDE/PROGRESSIVE SHORTWAVE TROUGH LOOKS TO MOVE ACROSS THE
REGION TUESDAY NIGHT/WEDNESDAY TIME FRAME WITH AN ASSOCIATED
CHANCE OF SOME LIGHT RAIN/LIGHT SNOW.
LATE IN THE WEEK, FORECAST CONFIDENCE IS ON THE LOWER END OF THE
SPECTRUM. MEDIUM RANGE ENSEMBLES HAVE GENERALLY BEEN ADVERTISING
AN AMPLIFICATION OF A LONG WAVE TROUGH OVER EASTERN NORTH
AMERICA. HOWEVER, THE LONGITUDE AND AMPLITUDE OF THE TROUGH HAS
SHOWN CONSIDERABLE VARIABILITY FROM RUN TO RUN AND AMONG
VARIOUS ENSEMBLE SUITES. GIVEN THE SPREAD IN GUIDANCE, THERE'S
NO REASON TO MAKE ANY CHANGES MADE TO THE NBM BLENDED GUIDANCE,
BUT DEPENDING ON HOW GUIDANCE TRENDS WITH THE LATE WEEK LARGE
SCALE PATTERN, CHANGES TO THE LATE WEEK FORECAST ARE CERTAINLY
POSSIBLE IN COMING DAYS.
- IZZI/KJB
AVIATION /00Z TAFS THROUGH 06Z MONDAY/
ISSUED AT 536 PM CST SAT DEC 14 2024
FORECAST CONCERNS INCLUDE...
LIGHT RAIN WITH MVFR/IFR CIGS THIS EVENING.
DRIZZLE WITH LIFR/VLIFR CIGS OVERNIGHT/SUNDAY MORNING.
DENSE FOG POSSIBLE SUNDAY MORNING.
LIGHT RAIN WILL CONTINUE THIS EVENING TRANSITIONING TO DRIZZLE
FROM WEST TO EAST LATE THIS EVENING INTO THE EARLY OVERNIGHT
HOURS. CIGS WILL LOWER THROUGH MVFR EARLY THIS EVENING AND THEN
INTO IFR DURING THE MID/LATE EVENING, THOUGH STILL SOME
UNCERTAINTY FOR TIMING. CIGS WILL LOWER INTO LIFR OVERNIGHT WITH
AREAS OF DENSE FOG POSSIBLE SUNDAY MORNING. GUIDANCE APPEARS TO
BE FOCUSED ALONG AND NORTH OF DPA/ORD AND THEN ACROSS NORTHWEST
AND WESTERN IL FOR THE BEST POTENTIAL FOR DENSE FOG. HAVE
MAINTAINED 1/2SM MENTION FOR ORD/DPA/RFD BUT 1/4SM IS CERTAINLY
POSSIBLE FOR RFD/DPA BY SUNRISE AND IT MAY PERSIST FOR MOST OF
THE MORNING BEFORE IMPROVING. CIGS WILL LIKELY RISE BACK INTO
IFR SUNDAY AFTERNOON BUT CONFIDENCE REMAINS LOW FOR TIMING AND
TRENDS.
SOUTHEAST WINDS ARE EXPECTED TO INCREASE THIS EVENING WITH GUSTS
INTO THE LOWER 20KT RANGE AND THEN SPEEDS/GUSTS WILL DIMINISH
OVERNIGHT AS DIRECTIONS TURN MORE SOUTH/SOUTHEAST AFTER SUNRISE.
EVENTUALLY WINDS WILL TURN TO THE SOUTH/SOUTHWEST SUNDAY, WITH
SPEEDS EXPECTED TO BE UNDER 10KTS. CMS
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY UNTIL 9 AM CST SUNDAY FOR THE IL
NEARSHORE WATERS.
SMALL CRAFT ADVISORY UNTIL 4 AM CST SUNDAY FOR THE IN
NEARSHORE WATERS.
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