884
FXUS63 KLOT 210040
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
640 PM CST WED NOV 20 2024
KEY MESSAGES
- SNOW SHOWERS THROUGH TONIGHT AND MAY BRING THE SEASON'S
FIRST MEASURABLE ACCUMULATION IN SOME LOCATIONS.
- A 2-3 HOUR PERIOD OF MODERATE TO HEAVY WIND-WHIPPED WET SNOW
EXPECTED THURSDAY, WITH SLUSHY ACCUMULATIONS LIKELY EVEN ON
ROADS, ESPECIALLY DURING THE LATE MORNING AND EARLY AFTERNOON.
- STRONG NORTHWEST WINDS GUSTING UP TO 40-45 MPH MAY PRESENT A
LIMITED RISK FOR MINOR LAKESHORE FLOODING AND EROSION ALONG
THE PORTER COUNTY SHORELINE THURSDAY NIGHT INTO FRIDAY
MORNING.
DISCUSSION
ISSUED AT 234 PM CST WED NOV 20 2024
THROUGH THURSDAY NIGHT:
A FASCINATING (AND HIGHLY UNUSUAL) SYNOPTIC EVOLUTION WILL
RESULT IN AN ACTIVE PERIOD OF WINTRY WEATHER THROUGH THURSDAY
NIGHT, INCLUDING TEMPS AROUND FREEZING, THE FIRST MEASURABLE
SNOW OF THE SEASON FOR MANY AREAS, AND STRONG WEST-NORTHWEST
WINDS. COMING RIGHT ON THE HEELS OF YET ANOTHER MILD STRETCH IN
OUR EXCEPTIONALLY WARM FALL TO DATE, THIS SHORT STRETCH OF
WINTRY CONDITIONS WILL FEEL MORE HARSH THAN IT TYPICALLY WOULD
BY THIS LATE INTO NOVEMBER.
A SPECIAL WEATHER STATEMENT WILL BE ISSUED ALONG WITH MORE
AGGRESSIVE GRAPHICAL MESSAGING OF THE LIKELY HEAVY AT TIMES
WIND-WHIPPED SNOW FOR THURSDAY MORNING.
METEOROLOGICAL SYNOPSIS:
THE CORE OF THE EXTREMELY WOUND UP AND WELL DEFINED UPPER LOW
(ON GOES-EAST WATER VAPOR IMAGERY) CENTERED OVER NORTHERN
MINNESOTA AS OF THIS AFTERNOON WILL TRANSLATE SOUTHEASTWARD TO
THE WESTERN GREAT LAKES THROUGH TONIGHT. ITS MID-LEVEL COLD POOL
WILL FEATURE 700 MB TEMPS DOWN AS LOW AS -15C AMIDST PERSISTENT,
ROBUST LOW-LEVEL COLD AIR ADVECTION (CAA), BRINGING 850 MB
TEMPS DOWN TO AROUND -10C BY PRE-DAWN THURSDAY. THE CENTER OF
THE UPPER LOW WILL MOVE EAST ACROSS MI/IN/OH THURSDAY MORNING.
MEANWHILE, A STRONG MID-LEVEL VORT THAT BROUGHT THIS MORNING'S
BRIEF PERIOD OF RAIN WILL WRAP CYCLONICALLY ALL THE WAY BACK
AROUND THE UPPER LOW WITH AN ACCOMPANYING LOW-MID 990S MB
SURFACE LOW PRESSURE CENTER DROPPING SOUTHWARD ALONG THE LENGTH
OF LAKE MICHIGAN. THIS EVOLUTION WILL ACTUALLY BRING MODEST WARM
AIR ADVECTION FROM THE NORTH (!) THURSDAY AFTERNOON, PERTINENT
TO THE PRECIPITATION TYPE FORECAST DISCUSSED IN THE RUNDOWN
BELOW.
SNOW/GRAUPEL SHOWERS THROUGH TONIGHT THROUGH EARLY THURSDAY:
CONVECTIVE SHOWER DEVELOPMENT OCCURRED A BIT EARLIER THAN
EXPECTED WITH THE APPROACH OF THE MID-LEVEL COLD POOL. AS IS
COMMON WITH INITIALLY WELL ABOVE FREEZING SURFACE TEMPS, BUT
DECREASING WET-BULB TEMPS, LOWERING WET-BULB ZERO HEIGHTS, AND
STEEP LOW-LEVEL LAPSE RATES, THE MORE ROBUST CORES WILL LIKELY
PRODUCE GRAUPEL (SOFT HAIL/SNOW PELLETS) THROUGH SUNSET. THE
CONTINUED COOLING OF THE COLUMN WILL THEN FORCE THE CONVECTIVE
SHOWERS OVER TO PRIMARILY SNOW (AND GRAUPEL AT TIMES) FAIRLY
QUICKLY. WHILE TRANSIENT HEAVIER RATES WILL BE INTERMITTENT AT
ANY ONE LOCATION, SNOW SHOWER COVERAGE WILL PEAK LATE THIS
EVENING THROUGH THE PRE-DAWN HOURS WITH ANOTHER VORT MAX. AIDED
BY A FAIRLY DEEP DENDRITIC GROWTH ZONE (DGZ) NOTED ON FORECAST
SOUNDINGS, THIS HIGHER COVERAGE SHOULD BE SUFFICIENT TO PRODUCE
SOME MINOR ACCUMULATIONS (UP TO A FEW TENTHS OF AN INCH) AS
TEMPS FALL INTO THE 29-33F RANGE.
THURSDAY'S BURST OF MODERATE TO HEAVY WIND-WHIPPED WET SNOW:
AFTER RESULTING IN HEAVIER PRECIP RATES OVER CENTRAL AND
EASTERN WISCONSIN, IMPRESSIVE LARGE SCALE FORCING AND TRANSIENT
STRONG LOW-LEVEL FRONTOGENESIS WITH THE NEXT VORT MAX WILL PIVOT
STEADILY SOUTHWARD ACROSS ESPECIALLY THE EASTERN 2/3RDS OF OUR
CWA ON THURSDAY. THERMAL PROFILES FAVOR A PRECIP TYPE OF ALL
SNOW DURING THE HEAVIEST PRECIPITATION RATES.
STRONG ASCENT THROUGH DEPTH OF THE TROPOSPHERE, WITH ASCENT
MAXIMIZED NEAR THE DGZ, AND MAX CLOUD LAYER TEMPS > -5C WOULD
FAVOR THE DEVELOPMENT OF WET AGGREGATE SNOWFLAKES. GIVEN THIS
AND THE PRESENCE OF STEEP MID- TO UPPER-LEVEL LAPSE RATES, WE
WILL LIKELY SEE MODERATE TO HEAVY SNOW AT TIMES WITH SHARPLY
REDUCED VISIBILITY (BRIEFLY DOWN AS LOW AS 1/2 MILE OR SO).
ADDING TO THE VISIBILITY ISSUES WILL BE THE WEST-NORTHWEST WINDS
GUSTING UP TO 35-45 MPH, STRONGEST WITH WESTWARD EXTENT. BASED
ON HREF PROBABILITIES, RATES COULD APPROACH OR EVEN EXCEED
1"/HOUR IN THE ~2-3 HOUR WINDOW OF PEAK RATES FROM ABOUT 8AM FAR
NORTH TO ~2-3PM CST FAR SOUTH.
DESPITE THE ANTECEDENT VERY MILD GROUND CONDITIONS, IT'S
IMPORTANT TO KEEP IN MIND THAT LATE NOVEMBER SUN ANGLES ARE THE
EQUIVALENT OF LATE JANUARY. THEREFORE, GIVEN THE INCREASINGLY
HIGH CONFIDENCE IN THE DETAILS ABOVE, WE REMAIN CONCERNED FOR
ACCUMULATIONS ON AT LEAST SOME ROADS GIVEN THE EXPECTED HEAVY
SNOWFALL RATES. ELEVATED AND/OR LESS HEAVILY TRAVELED ROADS WILL
LIKELY BE MOST SUSCEPTIBLE TO ACCUMULATING SNOWFALL. MILDER
ROAD PAVEMENT TEMP STARTING POINTS COULD LARGELY MITIGATE
IMPACTS ON THE MORE HEAVILY TRAVELED ROADS IN AND NEAR THE CITY
OF CHICAGO, THOUGH IT REMAINS POSSIBLE THAT THE HEAVY SNOWFALL
RATES MAY ULTIMATELY END UP OVERWHELMING THE MILDER PAVEMENT
TEMPERATURES AND CAUSE SOME SLICK SPOTS TO DEVELOP ON THESE
MORE TRAVELED THOROUGHFARES ANYWAYS. TOTAL SLUSHY ACCUMULATIONS
SHOULD LARGELY END UP IN THE 1-3" RANGE, WITH 2-3" AMOUNTS ON A
MORE LOCALIZED BASIS AWAY FROM THE HEART OF THE METRO IN
ILLINOIS AND ACROSS PORTIONS OF INTERIOR NORTHWEST INDIANA.
UNUSUAL WARM AIR ADVECTION FROM THE NORTH AND EVENTUAL LOSS OF
CLOUD ICE SHOULD TRANSITION SNOW OVER TO RAIN AND DRIZZLE WITH
TIME DURING THE EARLY TO MID AFTERNOON (EARLIEST FAR NORTH AND
LATEST FAR SOUTH/SOUTHEAST). WITH TEMPERATURES FORECAST TO RISE
INTO THE 35-40F RANGE BY ABOUT 5PM CST THURSDAY, MOST OF THE
SNOW THAT FALLS EARLIER IN THE DAY IS LIKELY TO QUICKLY MELT
OFF. TEMPERATURES WILL THEN REMAIN NEARLY STEADY THROUGH
THURSDAY NIGHT, PREVENTING ANY RE-FREEZE ISSUES.
STRONG WEST-NORTHWEST WINDS:
PRESSURE FALLS WILL MAXIMIZE IN THE EARLY TO MID AFTERNOON ON
THURSDAY, WITH STEEPEST LOW-LEVEL LAPSE RATES AND DEEPER MIXING
CONFINED TO AREAS WEST OF THE FOX RIVER VALLEY (ESPECIALLY
NEAR/WEST OF I-39) WHERE PRECIP RATES WILL BE LOWER. IN THESE
AREAS, WEST-NORTHWEST WIND GUSTS SHOULD APPROACH OR EVEN BRIEFLY
EXCEED THE 45 MPH WIND ADVISORY CRITERIA.
THURSDAY EVENING, WINDS WILL SHIFT TO NORTH-NORTHWEST AS THE
PRESSURE FALL-RISE COUPLET TRANSLATES THE AREA, WITH STRENGTHENING
FLOW OFF THE FRICTIONLESS LAKE INTO FAR NORTHWEST INDIANA,
PARTICULARLY THE PORTER COUNTY SHORE. GUSTS COULD AGAIN APPROACH
OR EXCEED 45 MPH. NO WIND ADVISORY HEADLINES WERE HOISTED WITH
THIS AFTERNOON'S FORECAST PACKAGE, THOUGH THEY MAY NEED TO BE
CONSIDERED WITH SUBSEQUENT UPDATES. FINALLY, THE PERSISTENCE OF
STRONG NORTHERLY WINDS INTO PORTER COUNTY THROUGH THURSDAY NIGHT
COULD LEAD TO SOME MINOR LAKESHORE FLOODING AND BEACH EROSION
ISSUES INTO FRIDAY MORNING.
CASTRO/OGOREK
FRIDAY THROUGH WEDNESDAY:
LONG WAVE TROUGH AND LARGE UPPER LOW IS PROGGED TO MOVE EASTWARD
AND EVENTUALLY OFF THE NEW ENGLAND COAST BY LATER THIS WEEKEND
AS THE HIGHLY AMPLIFIED PATTERN ACROSS THE U.S. IS REPLACED BY A
MORE ZONAL FLOW. GENERALLY DRY CONDITIONS WITH MORE TYPICAL
TEMPS FOR LATE NOVEMBER ARE EXPECTED FRIDAY INTO SATURDAY. BY
LATE IN THE WEEKEND INTO EARLY NEXT WEEK, MEDIUM RANGE GUIDANCE
IS IN PRETTY GOOD AGREEMENT IN BRINGING A FAIRLY LOW AMPLITUDE
SHORTWAVE RIPPLING EASTWARD THROUGH THE LARGELY ZONAL FLOW
ACROSS THE MIDWEST AND GREAT LAKES. WARM AIR ADVECTION AND
SOUTHERLY WINDS IN ADVANCE OF THIS WAVE SHOULD ALLOW HIGH TEMPS
TO REBOUND BACK INTO THE 50S, THOUGH ONLY FOR A DAY...
AS THE AFOREMENTIONED SHORTWAVE CONTINUES EASTWARD, LOOK FOR
ASSOCIATED SFC LOW AND TRAILING COLD FRONT TO MOVE ACROSS THE
AREA SUNDAY NIGHT/EARLY MONDAY MORNING WITH COLDER AIR
FILTERING INTO THE AREA IN THE WAKE OF THE FRONT. MAINTAINED
THE LOW CHANCE POPS OFFERED UP BY THE NBM FOR LATE SUNDAY NIGHT
INTO MONDAY, BUT WITH GULF OF MEXICO CLOSED FOR BUSINESS IN AS
FAR AS MOISTURE RETURN GOES, AND THE LOW AMPLITUDE NATURE OF THE
WAVE, IT IS QUITE PLAUSIBLE THAT IT COULD MOVE THROUGH WITH
LITTLE/NO PRECIP. COLDER TEMPERATURES, EVEN SLIGHTLY BELOW
AVERAGE FOR A CHANGE, ARE EXPECTED TUESDAY INTO WEDNESDAY NEXT
WEEK.
- IZZI
AVIATION /00Z TAFS THROUGH 06Z THURSDAY/
ISSUED AT 535 PM CST WED NOV 20 2024
KEY AVIATION IMPACTS FOR THE CURRENT TAF PERIOD:
- SCATTERED SNOW SHOWERS THROUGH THE OVERNIGHT
- AFTER A BRIEF LULL AFTER DAYBREAK, A SECOND SHOT OF SNOW
MID TO LATE MORNING WHICH COULD HAVE A THUMPING OF WET
SNOW WITH IFR/LIFR VISIBILITIES
- SNOW TRANSITIONS TO RAIN/DRIZZLE IN THE AFTERNOON, BUT
CONFIDENCE IS LOW ON THE EXACT TIMING
- GUSTY WEST TO NORTHWEST WINDS EXPECTED THROUGH THE TAF PERIOD
WITH STRONGEST GUSTS 30 TO 35 KNOTS MID TO LATE MORNING
AS TEMPERATURES HAVE DROPPED, MANY LOCAL AREA WEBCAMS ARE
SNOWING SNOWFLAKES COMING DOWN. TAFS WERE UPDATED TO PREVAIL
SNOW SHOWERS INSTEAD OF RAIN OUT THE GATE. INTERMITTENT SNOW
SHOWERS CAN BE EXPECTED THROUGH THE OVERNIGHT HOURS. MOST
SHOWERS ARE EXPECTED TO HAVE VFR VISIBILITIES, BUT OCCASIONAL
STRONGER SHOWERS MAY HAVE VISIBILITIES DOWN TO AROUND 3SM FOR A
SHORT WINDOW.
MODELS ARE CONSISTENTLY SHOWING A BRIEF LULL IN THE SNOW SHOWERS
JUST AFTER DAYBREAK TOMORROW, BEFORE THE NEXT WAVE MOVES
SOUTHWARD OUT OF WISCONSIN. CONFIDENCE IS GRADUALLY INCREASING
FOR A PERIOD OF FAIRLY ROBUST SNOW DURING THE MID TO
LATE MORNING. SNOW RATES COULD BE SUCH THAT VISIBILITIES DROP
DOWN TO IFR/LIFR CONDITIONS. IT WAS DECIDED TO LEAVE THE TIME
PERIOD IN QUESTION AS A PROB30 FOR NOW, BUT AS THE 00Z MODEL
GUIDANCE COMES IN, IF TRENDS REMAIN, THEN IT IS LIKELY THAT WILL BE
CONVERTED TO A TEMPO. SOME SLUSHY ACCUMULATIONS ARE POSSIBLE,
DURING THE HIGHEST SNOW RATES.
GRADUALLY WARMER TEMPERATURES WILL TRANSITION THE SNOW TO RAIN
AND DRIZZLE IN THE AFTERNOON, THOUGH CONFIDENCE IS LOWER ON THE
EXACT TIMING OF TRANSITION. SHOWERS MAY LINGER THROUGH THE
AFTERNOON BEFORE DRIER CONDITIONS MOVE IN AFTER 00Z.
LASTLY, WINDS ARE CURRENTLY OUT OF THE WEST WITH GUSTS TO 30
KNOTS AND INTERMITTENT GUSTS TO 35 KNOTS. THOSE GUSTS ARE EXPECT
TO GRADUALLY DIMINISH DOWN TO 20 TO 25 KNOTS OVERNIGHT AND
BECOME OUT OF THE NORTHWEST. HOWEVER, AS THE NEXT WAVE MOVES
DOWN WITH THE SNOW, WIND GUSTS ARE EXPECTED TO INCREASE ONCE
AGAIN WITH PREVAILING GUSTS 30-35 KNOTS EXPECTED. STRONG WINDS
ARE EXPECTED TO CONTINUE INTO THE AFTERNOON BEFORE GRADUALLY
DIMINISHING INTO THE EVENING AND PIVOTING MORE TO THE NORTH-
NORTHWEST.
DK
MARINE
ISSUED AT 640 PM CST WED NOV 20 2024
- GALE WATCH UPGRADED TO A GALE WARNING THURSDAY MORNING INTO
THURSDAY EVENING. SMALL CRAFT ADVISORY REMAINS IN EFFECT
TONIGHT.
STRONG AND GUSTY WESTERLY WINDS WILL CONTINUE THIS EVENING AND
OVERNIGHT. WHILE SOME INTERMITTENT GALE FORCE GUSTS TO 35 KNOTS
ARE LIKELY TONIGHT (MAINLY ON ELEVATED PLATFORMS), A STRONGER
PUSH OF NORTHWESTERLY WINDS WILL ARRIVE THROUGH THURSDAY
MORNING. FREQUENT 40 TO 45 KNOT GALES ARE EXPECTED, ESPECIALLY
THURSDAY AFTERNOON. WINDS WILL TURN NORTH AND EVENTUALLY
NORTHEASTERLY THURSDAY EVENING.
ONCE GALES SUBSIDE THURSDAY NIGHT, SMALL CRAFT ADVISORIES WILL
BE NEEDED INTO FRIDAY EVENING FOR THE ILLINOIS NEARSHORE AND
INTO SATURDAY MORNING FOR THE INDIANA NEARSHORE WATERS.
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY UNTIL 7 AM CST THURSDAY FOR THE IL
NEARSHORE WATERS.
GALE WARNING FROM 7 AM THURSDAY TO MIDNIGHT CST THURSDAY NIGHT
FOR THE IL NEARSHORE WATERS.
SMALL CRAFT ADVISORY UNTIL 10 AM CST THURSDAY FOR THE IN
NEARSHORE WATERS.
GALE WARNING FROM 10 AM THURSDAY TO MIDNIGHT CST THURSDAY
NIGHT FOR THE IN NEARSHORE WATERS.
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