058
FXUS63 KLOT 220109
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
709 PM CST SAT DEC 21 2024
KEY MESSAGES
- 30% CHANCE OF LIGHT FREEZING DRIZZLE/DRIZZLE LATE SUNDAY
NIGHT THROUGH EARLY MONDAY MORNING.
- TURNING MILD WITH ON-AND-OFF CHANCES FOR RAIN DURING THE
HOLIDAY WEEK.
UPDATE
ISSUED AT 709 PM CST SAT DEC 21 2024
HAVE MADE SOME ADJUSTMENTS TO THE FORECAST THIS EVENING,
PRIMARILY TO LOWER OVERNIGHT (EVENING) MIN TEMPERATURES AND
TWEAK SKY COVER PER SATELLITE TRENDS. IN SHORT, EXPECT SINGLE
DIGIT MINS THIS EVENING ACROSS PARTS OF NORTHERN IL WEST/NORTH
OF CHICAGO URBAN HEAT ISLAND BEFORE MID-LEVEL CLOUD COVER
INCREASES LATER THIS EVENING AND OVERNIGHT.
EARLY EVENING SURFACE ANALYSIS PLACES 1034 MB HIGH PRESSURE
CENTERED OVER EAST-CENTRAL WI, WITH A RIDGE AXIS EXTENDING
SOUTHWARD INTO NORTHERN IL. A COMBINATION OF CALM OR
LIGHT/VARIABLE WINDS WITHIN THE RIDGE AXIS, OUR RECENT SNOW
COVER AND MOSTLY CLEAR TO PARTLY CLOUDY SKIES ACROSS NORTHERN IL
EARLY THIS EVENING, HAS ALREADY ALLOWED TEMPS TO DIP INTO THE
SINGLE DIGITS IN PARTS OF THE FOX RIVER VALLEY WEST/NORTHWEST OF
CHICAGO. MID-LEVEL WARM AIR ADVECTION WAS ALREADY DEVELOPING
WEST OF THE RIDGE ACROSS THE UPPER MIDWEST HOWEVER, PRODUCING
FAIRLY EXTENSIVE MID-LEVEL CLOUD COVER JUST TO OUR WEST AND
NORTHWEST WHICH IS EXPECTED TO DEVELOP SOUTHEASTWARD ACROSS THE
FORECAST AREA LATER THIS EVENING AND OVERNIGHT. THIS WILL
LIKELY CAUSE TEMPS TO RISE A FEW DEGREES FROM EVENING LOWS,
ESPECIALLY FOR AREAS WEST/NORTHWEST OF CHICAGO.
HAVE THEREFORE ADJUSTED FORECAST TO INCLUDE SINGLE DIGIT LOWS
NEAR AND WEST OF THE FOX VALLEY, WITH TEMPS THEN RISING SLIGHTLY
LATE. ALSO ADJUSTED SKY COVER DOWN ACROSS THESE AREAS THROUGH
MID-EVENING BASED ON SATELLITE TRENDS, THEN INCREASED COVERAGE
LATE EVENING AND AFTER MIDNIGHT ACROSS THE AREA AS THE EXPANSIVE
MID-DECK SPREADS IN ALOFT.
FORECAST OTHERWISE LOOKS TO BE IN GOOD SHAPE INTO SUNDAY, SO NO
OTHER SIGNIFICANT CHANGES WERE MADE AT THIS TIME. UPDATED
DIGITAL AND TEXT FORECAST PRODUCTS AVAILABLE.
RATZER
DISCUSSION
ISSUED AT 257 PM CST SAT DEC 21 2024
THROUGH MONDAY NIGHT:
LES CONTINUES TO STRUGGLE ACROSS NORTHWEST INDIANA DUE TO A
COMBINATION OF LOWERING INVERSION HEIGHTS AND CONTINUED DRY AIR
ADVECTION. ADDITIONALLY, AUGMENTATION OF THE BACKGROUND FLOW BY
A LAND BREEZE EARLY THIS MORNING HAS KEPT THE MAIN CONVERGENT
AXIS NORTH AND EAST OF THE FORECAST AREA. AS A SURFACE RIDGE
SHIFTS EASTWARD ACROSS THE AREA TONIGHT, REMAINING LES OVER
SOUTHERN LAKE MICHIGAN WILL QUICKLY SHIFT FARTHER NORTH AND EAST
OF THE FORECAST AREA. WITH THE CURRENT LOCATION OF ANY LES
REMOVED TO THE NORTHEAST AND THE EXPECTED DEVELOPMENT OF ANOTHER
LAND BREEZE TONIGHT, ANY REMAINING LES SHOULD BE LIMITED TO
JUST SOME FLURRIES NEAR THE INDIANA SHORE THROUGH THIS EVENING.
THOUGH WINDS WILL BE PRIMARILY CALM OVER SNOW COVER FOR MUCH OF
THE AREA TONIGHT, TEMPS SHOULD ONLY SLOWLY FALL THROUGH THE
NIGHT AS A DISJOINTED POCKET OF MID TO UPPER-LEVEL MOISTURE
WITHIN STRONGER NW FLOW ALOFT LIKELY MAINTAINS A DECENT AMOUNT
OF CIRRUS AND/OR ALTOSTRATUS OVER THE AREA. IF CLOUDS DO CLEAR
MORE THAN EXPECTED, MIN TEMPS MAY PLUMMET WELL INTO THE SINGLE
DIGITS ABOVE ZERO ACROSS INTERIOR NORTHERN ILLINOIS.
SOME SEMBLANCE OF MID TO UPPER-LEVEL CLOUD COVER SHOULD PERSIST
THROUGH THE DAY ON SUNDAY, LEADING TO FILTERED SUNSHINE ON
INCREASING SOUTHERLY WINDS. AFTERNOON TEMPS WILL NEAR THE
FREEZING MARK WHILE WINDS GUST UP TO 25 MPH. PRECIP WITH A SMALL
MID-LEVEL WAVE SHIFTING ESE ACROSS SOUTHERN WISCONSIN DURING
THE AFTERNOON SHOULD BE LIMITED TO VIRGA GIVEN DRY AIR BELOW
10KFT CLOUD BASES.
A STRONG PACIFIC-BASED TROUGH CROSSING THE WESTERN GREAT LAKES
REGION ON MONDAY WILL BRING AN ASSOCIATED COLD FRONT ACROSS THE
FORECAST AREA MONDAY AFTERNOON. BROAD LOW-LEVEL ISENTROPIC
ASCENT WITHIN A SHALLOW BUT MOISTENING PRE-FRONTAL AIRMASS
SHOULD SPREAD LIGHT PRECIP ACROSS AT LEAST NORTHERN PORTIONS OF
THE FORECAST AREA LATE SUNDAY NIGHT INTO MONDAY MORNING. A 1-2C
WARM NOSE ADVECTING INTO THE AREA ON 850-900HPA 50 KNOT FLOW
WILL YIELD LIQUID PRECIP, THOUGH IT IS BECOMING MORE LIKELY THAT
TEMPS WILL NOT RISE ABOVE FREEZING BEFORE THE PRECIP BEGINS.
HAVE CONTINUED TO INCREASE THE MENTION OF FREEZING DRIZZLE
ACROSS MUCH OF THE AREA THROUGH AT LEAST MID-MORNING, WITH SOME
IMPACTS FROM SLIPPERY SURFACES POSSIBLE FOR THE MORNING COMMUTE.
TEMPS WILL RISE ABOVE FREEZING BY THE AFTERNOON WITH RESIDUAL
POST-FRONTAL STRATUS AND PATCHY DRIZZLE INTO THE EARLY EVENING.
KLUBER
TUESDAY THROUGH SATURDAY:
THERE REMAINS A STRONG SIGNAL FOR WELL ABOVE NORMAL
TEMPERATURES NEXT WEEK (TYPICALLY LOW-MID 30S) AS THE NORTHERN
STREAM UPPER JET LIFTS INTO CANADA, PARTICULARLY DURING THE
LATTER HALF OF THE HOLIDAY WEEK WHEN HIGHS IN THE MID 40S TO
LOWER 50S ARE FORECAST.
THE UPPER PATTERN ALSO LOOKS RATHER MESSY OVERALL AS A SERIES
OF UPPER LEVEL WAVES AND ASSOCIATED DISTURBANCES EJECT OFF THE
ROCKIES BENEATH THE AFOREMENTIONED NORTHERN STREAM JET/LONGWAVE
RIDGE. THIS WILL BRING SEVERAL CHANCES FOR RAIN TO THE REGION
TUESDAY NIGHT (CHRISTMAS EVE) THROUGH THE END OF THE WEEK WITH
THE WARMER TEMPERATURES LIMITING THE POTENTIAL FOR ANY WINTRY
PRECIPITATION. WHILE THE OFFICIAL FORECAST HAS RAIN CHANCES
NEARLY EVERY PERIOD BEGINNING CHRISTMAS EVE NIGHT THROUGH THE
END OF THE WEEK, THERE WILL BE DRY PERIODS IN BETWEEN SYSTEMS.
PINPOINTING WHEN THESE OCCUR REMAINS LOWER CONFIDENCE AT THIS
RANGE GIVEN CONTINUED MODEL DIFFERENCES WITH THE TIMING AND
TRACK OF EACH SYSTEM.
LOOKING BEYOND INTO EARLY JANUARY, LONG-RANGE ENSEMBLE GUIDANCE
IS POINTING TOWARD A PATTERN SHIFT OCCURRING TOWARD THE NEW
YEAR WITH UPPER TROUGHING SETTING UP ACROSS THE EASTERN HALF OF
THE CONUS. THIS MATCHES UP WELL WITH THE LATEST CPC GUIDANCE
WHICH ALREADY HAS A 55-60% CHANCE OF BELOW NORMAL TEMPERATURES
OCCURRING DURING THE FIRST HALF OF JANUARY ACROSS NORTHEAST
ILLINOIS AND NORTHWEST INDIANA.
PETR
AVIATION /00Z TAFS THROUGH 06Z SUNDAY/
ISSUED AT 523 PM CST SAT DEC 21 2024
AVIATION FORECAST CONCERN FOR 00Z TAFS:
- FEW MVFR LAKE EFFECT CLOUDS MAY BRUSH GYY THIS EVENING.
- BREEZY SOUTH-SOUTHEAST WINDS SUNDAY AFTERNOON WITH 20-25 KT
GUSTS.
LATE AFTERNOON SURFACE ANALYSIS DEPICTS WEAK SURFACE RIDGE AXIS
DRIFTING EAST TOWARD THE TERMINALS ACROSS WI/IL. EAST OF THE
RIDGE, NORTH-NORTHWEST WINDS CONTINUE TO PRODUCE SOME MVFR/LOW
VFR (2000-3000 FT) LAKE-EFFECT STRATOCU INTO PARTS OF NORTHERN
IN, AND SOME OF THIS MAY SLIP BRIEFLY BACK WEST INTO GYY EARLY
THIS EVENING AS THE SURFACE RIDGE MOVES IN. OTHERWISE, MAINLY
EXPECTING ONLY MID-LEVEL VFR CLOUDS IN THE 8000-10000 FOOT
RANGE ACROSS THE TERMINALS THROUGH THE REST OF THE FORECAST
PERIOD.
WINDS WILL BECOME LIGHT/VARIABLE BY MID-EVENING OR SO AS THE
SURFACE RIDGE MOVES ACROSS THE AREA, THEN SOUTH-SOUTHEAST 5-10
KTS EARLY SUNDAY MORNING AS THE RIDGE DRIFTS OFF TO THE EAST.
WINDS WILL THEN INCREASE MIDDAY/AFTERNOON SUNDAY AS THE GRADIENT
TIGHTENS AHEAD OF SURFACE LOW PRESSURE TRACKING FROM THE DAKOTAS
TO THE UPPER MISSISSIPPI VALLEY THROUGH SUNDAY NIGHT. SOUTH-
SOUTHEAST WINDS WILL BECOME GUSTY IN THE 20-25 KT RANGE EARLY
SUNDAY AFTERNOON AND EVENING. STRONGEST WINDS MAY COME TOWARD OR
DURING EARLY EVENING, WITH A FEW GUSTS ABOVE 25 KT POSSIBLE.
RATZER
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...NONE.
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