901
FXUS63 KLOT 161134
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
534 AM CST THU JAN 16 2025
KEY MESSAGES
- TODAY AND FRIDAY WILL FEATURE ABOVE-FREEZING TEMPERATURES WITH
BREEZY WINDS OVER 30 MPH (NORTHWESTERLY DIRECTION TODAY, AND
THEN A SOUTHWESTERLY DIRECTION TOMORROW).
- A STORM SYSTEM WILL BRING A WINTRY MIX TO THE REGION FRIDAY
NIGHT INTO SATURDAY MORNING, WITH SNOW ACCUMULATIONS EAST OF
I-55.
- ARCTIC AIR WILL RETURN SUNDAY THROUGH THE EARLY PART OF NEXT
WEEK RESULTING IN BITTERLY COLD TEMPERATURES AND SUB-ZERO
WIND CHILLS, ESPECIALLY SUNDAY AND MONDAY NIGHTS.
- TEMPERATURES FORECAST TO MODERATE TOWARDS THE END OF NEXT
WEEK.
DISCUSSION
ISSUED AT 232 AM CST THU JAN 16 2025
THROUGH FRIDAY:
RECENT WATER VAPOR IMAGERY DEPICTS AN UPPER-LEVEL SHORTWAVE
TROUGH TRAVERSING ACROSS THE GREAT LAKES. MEANWHILE, RECENT
SURFACE ANALYSES DEPICT A 1000MB SURFACE LOW PRESSURE SYSTEM IN
CENTRAL ONTARIO, WITH A NORTHWESTERLY WIND SHIFT EXTENDING
SOUTHWESTWARD THROUGH THE UPPER MISSISSIPPI RIVER VALLEY.
CURIOUSLY, TEMPERATURES BEHIND THE WIND SHIFT ARE ACTUALLY
WARMER THAN AHEAD OF IT (LOWER 30S IN NORTHERN MINNESOTA, LOWER
S IN NORTHERN ILLINOIS), LIKELY DUE TO THE GULF OF ALASKA
ORIGIN OF THE AIRMASS.
WHILE CLOUDS ARE CLEARING AT PRESS TIME, RECENT NIGHTTIME
MICROPHYSICS SATELLITE IMAGERY DEPICTS EXPANSIVE BANKS OF LOW-
LEVEL STRATUS ACROSS CENTRAL WISCONSIN AND NORTHERN MINNESOTA,
BOTH WITHIN THE INTERIOR OF THE CYCLONIC SHEAR AXIS OF AN UPPER-
LEVEL SHORTWAVE. SINCE THE BACK EDGE OF THE UPPER-LEVEL
SHORTWAVE WILL GRADUALLY SHIFT EASTWARD ACROSS THE AREA
THROUGHOUT THE DAY, THE INCOMING STRATUS BANKS SHOULD BE GUIDED
ACROSS FAR NORTHERN ILLINOIS AND NORTHWESTERN INDIANA WHILE
LARGELY MISSING AREAS GENERALLY SOUTH OF I-80 AND WEST OF I-55.
MEANWHILE, THE AFOREMENTIONED WIND SHIFT WILL SLIDE ACROSS THE
AREA THIS MORNING, LEADING TO BREEZY NORTHWEST WINDS (GUSTING
OVER 30 MPH AT TIMES) AND WARMING TEMPERATURES (TO THE LOW TO
MID 30S). TAKEN TOGETHER, TODAY IS LOOKING RELATIVELY WARM AND
QUIET.
TONIGHT, A STRONG UPPER-LEVEL SHORTWAVE AND ACCOMPANYING 985MB
SURFACE LOW WILL RACE ALONG THE US/CANADIAN BORDER. AT THE SAME
TIME, THE UPPER-LEVEL CUT-OFF LOW PRESSURE SYSTEM CURRENTLY
MEANDERING ALONG THE SOUTHERN CALIFORNIA COAST WILL GET SHEARED
EASTWARD. WHEN PAIRED WITH A 1020MB SURFACE HIGH SETTLING OVER
THE SOUTHEASTERN UNITED STATES, THE LOW-LEVEL PRESSURE GRADIENT
WILL BECOME INCREASINGLY TIGHTER OVER THE MISSISSIPPI RIVER
VALLEY CAUSING WINDS TO SHIFT FROM NORTHWESTERLY TO
SOUTHWESTERLY. INTERESTINGLY, SEVERAL CAMS DEPICT FOG AND LOW-
LEVEL STRATUS BLOSSOMING ACROSS MISSOURI AND SOUTHERN ILLINOIS
AFTER MIDNIGHT AND SPREADING NORTHWARD INTO OUR AREA TOWARD
DAYBREAK. CONSIDERING THE SOURCE REGION OF MOISTURE WOULD BE THE
SNOWPACK DOWNSTATE, AND DEW POINTS ARE FORECASTED TO BE BELOW
FREEZING (THAT IS, THE SNOWPACK IS UNLIKELY TO MELT TONIGHT),
WOULD HAVE TO THINK SUCH A SIGNAL FOR LOW CLOUDS AND FOG IS
ERRONEOUS.
TOMORROW, THE PARENT SURFACE LOW WILL MOVE OVER SOUTHERN
ONTARIO AS A SECONDARY LOW DEVELOPS IN THE SOUTHERN PLAINS. THE
INCREASINGLY PACKED LOW-LEVEL PRESSURE GRADIENT WILL FORCE THE
DEVELOPMENT OF A FAIRLY STRONG LOW-LEVEL JET, WITH FORECAST
SOUNDINGS DEPICTING SOME 50-60 MPH OF FLOW AS LOW AS 2000 FEET
ABOVE THE GROUND. NORMALLY THIS TIME OF YEAR, AGGRESSIVE WARM-
AIR ADVECTION WITHIN THE WARM SECTOR OF A CYCLONE IS TYPICALLY
ASSOCIATED WITH NORTHEASTWARD-RACING LOW CLOUDS AND STUNTING
MIXING HEIGHTS. HOWEVER, CONSIDERING LOW-LEVEL FLOW ALONG THE
GULF COAST WILL REMAIN OFFSHORE EVEN THROUGH EARLY FRIDAY
MORNING, AND UPPER-LEVEL CLOUDS WITH EACH INDIVIDUAL LOW
PRESSURE SYSTEM WILL BE NORTH AND SOUTHWEST OF OUR AREA, MUCH OF
OUR AREA MAY END UP MOSTLY SUNNY THROUGH AT LEAST FRIDAY
AFTERNOON. IN ADDITION, WARMING TEMPERATURES INTO THE MID TO
UPPER 30S (IF NOT LOWER 40S WHERE THERE IS NO SNOWPACK) MAY
CAUSE LOW-LEVEL THERMAL PROFILES TO TREND TOWARD NEUTRALLY
STABLE. AS A RESULT, MIXING HEIGHTS ON FRIDAY MAY END UP HIGHER
THAN ONE MAY TYPICALLY EXPECT WITHIN A COOL-SEASON WAA REGIME.
FOR THIS REASON, CAUTIOUSLY INCREASED WIND GUSTS ACROSS OUR AREA
ON FRIDAY TO THE 30 TO 40 MPH RANGE WITH THE HIGHEST VALUES
ACROSS NORTHWESTERN IL WHERE THERE IS LITTLE TO NO SNOW COVER.
IF MIXING PROFILES VERIFY EVEN DEEPER THAN CURRENTLY FORECASTED,
WIND GUSTS IN EXCESS OF 45 MPH WOULD BE IN PLAY (AGAIN, ACROSS
NORTHWESTERN IL WHERE THERE IS NO SNOW COVER). WE WILL BRIEF THE
INCOMING SHIFT TO TAKE A CLOSE LOOK AT SUCH A THREAT.
ONE FINAL THING NOT REALLY RELATED TO THE FORECAST FOR TODAY AND
TOMORROW... IT'S BEEN A BUSY PERIOD OF WEATHER ACROSS THE LOWER
GREAT LAKES. INCLUDING TODAY (AS IT WAS SNOWING AT MIDNIGHT),
THERE WAS AT LEAST A TRACE OF SNOW ON 5 OF THE PAST 7 DAYS, AND
MEASURABLE SNOW ON 4 OF THE PAST 7, IN AT LEAST PART OF OUR
FORECAST AREA. THANKFULLY, WE REMAIN IN A "NICKEL-AND-DIME" SNOW
REGIME RATHER THAN ONE SUPPORTIVE OF MOISTURE-LADEN WINTER
STORMS, BUT WE DID LOG JUST OVER 6 INCHES OF SNOW AT OUR OFFICE
OVER THE PAST 7 DAYS. OF COURSE, THIS ISN'T AT ALL UNUSUAL FOR
THIS PART OF THE COUNTRY THIS TIME OF YEAR. BUT, MULTIPLE DAYS
OF SNOW IN THE SPAN OF 7 DAYS IS SOMEWHAT NOTEWORTHY IN A WINTER
THAT HAS THUS FAR NOT BROUGHT A BIG SNOWFALL IN OUR LOCAL AREA.
BORCHARDT
FRIDAY NIGHT THROUGH WEDNESDAY:
LITTLE HAS CHANGED IN THE EXPECTATION FOR THE EXTEND FORECAST
PERIOD WITH OUR NEXT CHANCE FOR WINTER WEATHER FRIDAY NIGHT INTO
SATURDAY FOLLOWED BY BITTERLY COLD TEMPERATURES HEADING INTO
EARLY NEXT WEEK.
THE UPPER-LOW THAT IS EXPECTED TO BRING US SOME WINTRY WEATHER
CAN BE SEEN IN WATER VAPOR IMAGERY THIS MORNING OFF THE SOUTHERN
COAST OF CALIFORNIA. THIS LOW IS EXPECTED TO MOVE ONSHORE LATER
TODAY AND BROADEN INTO A SHORTWAVE TROUGH AS IT TRAVERSES
ACROSS THE SOUTHWEST CONUS AND EVENTUALLY SOUTHERN PLAINS
TONIGHT INTO THE DAY ON FRIDAY. AS THIS OCCURS, THE NORTHERN
STREAM TROUGH ASSOCIATED WITH THE POLAR JET (ALSO NOTED IN WATER
VAPOR IMAGERY OVER THE PACIFIC NORTHWEST AND WESTERN CANADA)
WILL BE DIVING SOUTHWARD ACROSS THE NORTHERN PLAINS LATER TODAY
AND THROUGH THE DAY ON FRIDAY. THESE TWO FEATURES ARE EXPECTED
TO PHASE AS THEY ENTER THE MISSISSIPPI VALLEY FRIDAY EVENING
WHICH WILL ALLOW FOR THE DEVELOPMENT OF A WEAK SURFACE LOW IN
SOUTHERN MO. THE SURFACE LOW IS THEN EXPECTED TO TRACK EAST-
NORTHEAST ACROSS THE OHIO RIVER VALLEY FRIDAY NIGHT WHICH SHOULD
ALLOW THE SYSTEM'S PRECIPITATION SHIELD TO EXTEND INTO
NORTHEAST IL AND NORTHWEST IN. WHILE GUIDANCE REMAINS LOCKED IN
ON THIS SOLUTION, THERE CONTINUES TO BE SOME SUBTLETIES IN THE
NORTHERN EXTENT OF THE PRECIPITATION SHIELD. REGARDLESS,
CONFIDENCE IS FAIRLY HIGH THAT AT LEAST THE SOUTHEASTERN HALF OF
OUR AREA (AREAS ALONG AND SOUTHEAST OF I-55) WILL SOME
PRECIPITATION WITH AREAS TO THE NORTH OF I-55 SEEING A SHARP
GRADIENT IN PRECIPITATION.
AS THE PRECIPITATION BEGINS LATE FRIDAY EVENING, IT SHOULD BE
IN THE FORM OF RAIN GIVEN THAT SURFACE TEMPERATURES ARE FORECAST
TO BE IN THE MID TO UPPER 30S AND MID-LEVELS WILL BE ABOVE
FREEZING. HOWEVER, A COLD FRONT ASSOCIATED WITH THE NORTHERN
STREAM TROUGH WILL BE ADVANCING ACROSS NORTHERN IL AS
PRECIPITATION BEGINS AND IS EXPECTED TO QUICKLY COOL
TEMPERATURES ALOFT BELOW FREEZING BEFORE PRECIPITATION ENDS.
THAT SAID, THE RAIN IS EXPECTED TO TRANSITION OVER TO A RAIN-
SNOW MIX AND EVENTUALLY ALL SNOW OVERNIGHT FRIDAY NIGHT INTO
SATURDAY MORNING. WHILE SURFACE TEMPERATURES WILL ALSO BE
COOLING DURING THIS TIME, IT REMAINS UNCLEAR HOW QUICKLY THEY
WILL FALL BELOW FREEZING BEFORE PRECIPITATION ENDS WHICH MAY
LIMIT THE AMOUNT OF SNOW THAT IS ABLE TO ACCUMULATE. THOUGH,
ENSEMBLE GUIDANCE DOES SHOW A 30-50% CHANCE OF SEEING AT LEAST
AN INCH OF SNOW SOUTHEAST OF I-55 WITH THOSE PROBABILITIES
DROPPING TO VIRTUALLY ZERO FOR SNOW AMOUNTS OF AT LEAST 3 INCHES
OR MORE. GIVEN THE AFOREMENTIONED UNCERTAINTY IN SURFACE
TEMPERATURE RESPONSE, HAVE GONE LOWER WITH OUR OFFICIAL SNOW
FORECAST AT THIS TIME (AMOUNTS MORE IN THE 0.5 TO 1.0 INCH
RANGE) BUT WILL OBVIOUSLY CONTINUE TO MONITOR TRENDS AND MAKE
ADJUSTMENTS AS NEED IN FUTURE FORECASTS. IN THE MEANTIME, AVOID
FOCUSING TOO MUCH ON THE AMOUNTS AND MORE SO ON THE ANTICIPATED
IMPACTS WHICH AT THIS POINT LOOK TO BE MORE WET ROADS WITH
ISOLATED SLICK SPOTS WHERE SNOW ACTUALLY ACCUMULATES.
PRECIPITATION ASSOCIATED WITH THIS SYSTEM WILL TAPER BY
SATURDAY AFTERNOON AS THE SURFACE LOW MOVES INTO THE MID-
ATLANTIC REGION. THEREFORE, SATURDAY LOOKS TO BE FAIRLY TRANQUIL
WEATHER WISE WITH TEMPERATURES HOVERING IN THE LOWER 20S DURING
THE DAY BEFORE DIPPING INTO THE SINGLE DIGITS SATURDAY NIGHT.
HOWEVER, A CONVERGENCE AXIS IS FORECAST TO DEVELOP IN THE
EASTERN PART OF LAKE MICHIGAN WHICH IS EXPECTED TO LEAD TO THE
DEVELOPMENT OF LAKE EFFECT SNOW SHOWERS SATURDAY AFTERNOON
THROUGH SUNDAY. WHILE GUIDANCE REMAINS IN GOOD AGREEMENT THAT
THE LAKE EFFECT SHOULD FORM OVER NORTHWEST IN (MAINLY EASTERN
LAKE COUNTY, IN AND PORTER COUNTY), WINDS ARE FORECAST TO BECOME
MORE WESTERLY THROUGH THE DAY ON SUNDAY WHICH SHOULD FOCUS THE
LAKE EFFECT SNOW MORE INTO NORTHERN IN AND LOWER MI. HOWEVER,
THE CONVECTIVE NATURE OF LAKE EFFECT SNOW MAKES IT DIFFICULT TO
PINPOINT SPECIFICS EVEN AT THIS RANGE; SO CHANGES TO THIS PART
OF THE FORECAST MAY OCCUR AS WE GET CLOSER. NEVERTHELESS, HAVE
MAINTAINED SOME 20-30% POPS FOR OUR EASTERN NORTHWEST IN
COUNTIES THROUGH THE DAY ON SUNDAY (AND EVEN INTO THE DAY ON
MONDAY) TO ACCOUNT FOR THIS POTENTIAL AND ANY UNCERTAINTY.
HEADING INTO NEXT WEEK, THE PHASED UPPER TROUGH IS EXPECTED TO
REMAIN LOCKED IN PLACE ACROSS MUCH OF THE CONUS WHICH WILL ALLOW
PLENTIFUL AMOUNTS OF ARCTIC AIR TO FILTER ACROSS THE COUNTRY
AND ESPECIALLY THE MIDWEST AND GREAT LAKES. AS A RESULT,
BITTERLY COLD TEMPERATURES (HIGHS IN THE SINGLE DIGITS AND LOWS
BELOW ZERO) ARE EXPECTED FROM SUNDAY THROUGH AT LEAST TUESDAY
NIGHT. COUPLE THESE TEMPERATURES WITH THE EXPECTATION FOR SOME
BREEZY NORTHWEST WINDS AT TIMES AND WIND CHILLS DURING THIS
PERIOD ARE EXPECTED TO BE WELL BELOW ZERO, ESPECIALLY SUNDAY AND
MONDAY NIGHTS WHERE WIND CHILLS IN THE -15F TO -25F CONTINUE TO
LOOK LIKELY. WHILE IT REMAINS TOO SOON FOR THE ISSUANCE OF ANY
COLD WEATHER HEADLINES AT THIS POINT, IF THESE TRENDS CONTINUE
WE MAY NEED TO CONSIDER HEADLINES BEING ISSUED IN THE COMING
DAYS SO STAY TUNED. ON A SOMEWHAT MORE POSITIVE NOTE, THE DRY
AIR ASSOCIATED WITH THE BITTER COLD SHOULD KEEP THE THREAT FOR
ANY PRECIPITATION LOW ASIDE FROM THE AFOREMENTIONED LAKE EFFECT
SNOW IN THE EASTERN PORTIONS OF NORTHWEST IN.
TEMPERATURES DO LOOK TO MODERATE TOWARDS THE END OF NEXT WEEK
AS THE BROAD TROUGHING OVER THE CONUS SHIFTS EAST AND ALLOWS
SUBTLE RIDGING TO BUILD IN. WHILE THERE IS STILL SOME WIGGLE
ROOM IN EXACTLY HOW SOON THE MODERATION OCCURS, IT APPEARS THE
HIGHS SHOULD TREND TOWARDS MORE SEASONABLE MID-JANUARY VALUES BY
WEEKS END WITH HIGHS IN THE 20S TO AROUND 30 AND LOWS IN THE
TEENS. HOWEVER, WITH THE MODERATING THERE ALSO LOOKS TO BE A
SIGNAL FOR A RETURN TO MORE ACTIVE WEATHER AS WELL WHICH WE WILL
PROVIDE MORE DETAILS ON AS WE GET CLOSER.
YACK
AVIATION /12Z TAFS THROUGH 18Z FRIDAY/
ISSUED AT 534 AM CST THU JAN 16 2025
FORECAST CONCERNS FOR THE TERMINALS...
- MVFR CEILINGS THIS MORNING ATTEMPTING TO LIFT TOWARD VFR THIS
AFTERNOON
- BREEZY NORTHWEST WINDS GUSTING 25-30 KTS THIS AFTERNOON
- THREAT FOR LOW-LEVEL WIND SHEAR FRIDAY AFTERNOON WITH GUSTY
SOUTHWEST WINDS IN 30+ KT POSSIBLE
THE UPPER-LEVEL DISTURBANCE THAT BROUGHT US SNOW SHOWERS LAST
EVENING CONTINUES TO PIVOT OVERHEAD THIS MORNING RESULTING IN
WIDESPREAD MVFR CEILINGS FROM NORTHERN IL THROUGH EASTERN MN.
THESE MVFR CEILINGS WILL CONTINUE TO SPREAD SOUTHWARD ACROSS THE
TERMINALS THROUGH THE MORNING AND GIVEN THEIR UPSTREAM COVERAGE
WILL LIKELY PERSIST THROUGH MUCH OF THE DAY. HOWEVER, THERE IS
THE POTENTIAL FOR CEILINGS TO RISE TOWARDS THE 3000-3500 FT
RANGE THIS AFTERNOON AS DEEPER MIXING ENSUES. EXACTLY WHEN OR
EVEN IF CEILINGS RETURN TO VFR IS SOMEWHAT LOWER CONFIDENCE AT
THIS TIME, BUT WILL CONTINUE TO MONITOR OBSERVATIONS AND MAKE
ADJUSTMENTS AS NEEDED. REGARDLESS, CEILINGS SHOULD SCATTER OUT
THIS EVENING AS A SURFACE HIGH BUILDS OVERHEAD WITH VFR
CONDITIONS EXPECTED FOR THE CONCLUSION OF THE PERIOD.
WINDS TODAY WILL ALSO BECOME BREEZY OUT OF THE NORTHWEST IN THE
WAKE OF A FRONTAL BOUNDARY AND THE AFOREMENTIONED DEEPER MIXING.
GUSTS THIS AFTERNOON ARE EXPECTED TO TOP OUT IN THE 25-30 KT
RANGE BEFORE SUBSIDING THIS EVENING AS THE HIGH MOVES IN.
THOUGH, WIND DIRECTIONS WILL GRADUALLY BECOME MORE SOUTHWESTERLY
OVERNIGHT. WINDS WILL ONCE AGAIN PICK UP IN INTENSITY ON FRIDAY
AS A STRONG (50-55 KT) LOW-LEVEL JET MOVES OVERHEAD. DEPENDING
ON HOW DEEPLY THE ATMOSPHERE MIXES ON FRIDAY, GUSTS COULD
CERTAINLY EXCEED 30-35 KTS BUT FOR NOW HAVE GONE WITH A MORE
CONSERVATIVE 25-30 KT MENTION. FURTHERMORE, THE JET WILL ALSO
RESULT IN A THREAT FOR LOW-LEVEL WIND SHEAR BEGINNING AROUND
15Z AND PERSISTING THROUGH FRIDAY AFTERNOON WHEN THE JET IS
EXPECTED TO PUSH EAST.
YACK
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY UNTIL 3 PM CST THIS AFTERNOON FOR
WINTHROP HARBOR IL TO GARY IN.
SMALL CRAFT ADVISORY UNTIL 9 PM CST THIS EVENING FOR GARY TO
BURNS HARBOR IN-BURNS HARBOR TO MICHIGAN CITY IN.
VISIT US AT WEATHER.GOV/CHICAGO
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