687
FXUS63 KIND 201128
AFDIND
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE INDIANAPOLIS IN
628 AM EST FRI DEC 20 2024
KEY MESSAGES
- MINOR SNOW ACCUMULATIONS UP TO AN INCH EXPECTED THIS MORNING;
LOCALLY 2 INCHES NEAR MUNCIE AND WINCHESTER
- MINOR TRAVEL IMPACTS THIS MORNING
- WIND GUSTS 25-35 MPH DEVELOPING THIS AFTERNOON/EVENING
- PERIODS OF RAIN AND A WARMING TREND NEXT WEEK
MESOSCALE UPDATE
ISSUED AT 529 AM EST FRI DEC 20 2024
LATEST SURFACE OBSERVATIONS INDICATE THE SURFACE LOW IS POSITIONED
JUST SOUTH OF CHICAGO AND THE COLD FRONT IS FROM NEAR DELPHI TO
DANVILLE TO BLOOMINGTON. PRECEDING THIS IS A BAND OF RAIN WITHIN THE
SYSTEM'S MODEST WARM CONVEYOR BELT. TEMPERATURES HAVE BEEN SLIGHTLY
WARMER THAN MODEL GUIDANCE INDICATED SO THE DOMINANT PRECIPITATION
TYPE HAS BEEN RAIN. INFERRING UPSTREAM OBSERVATIONS THIS MAY
CONTINUE TO BE THE CASE INTO MID-MORNING ACROSS EAST-CENTRAL
INDIANA, WHERE >32F WET BULB DEPTH MAY BE GREAT ENOUGH FOR
COMPLETE MELTING.
WE DO THINK TWO FACTORS WILL EVENTUALLY RESULT IN A TRANSITION TO
SNOW WITHIN THIS WARM CONVEYOR PRECIPITATION BAND AS IT MOVES INTO
EASTERN INDIANA: 1. INCREASING OCCLUSION AND MIXING OF COLDER LOWER
TROPOSPHERIC ADVECTION WITH THIS BAND, AND 2. MORE INTENSE DIABATIC
COOLING FROM HEAVIER PRECIPITATION RATES DRIVEN BY MESOSCALE FORCING
FOR ASCENT. DEPENDING ON HOW MUCH OF THE HRRR AND NAM 0.20-0.25" QPF
AMOUNTS FALL BEFORE THE TRANSITION, THERE IS A LOW CHANCE OF LOCALLY
2-3 INCHES IN A NARROW SWATH NEAR MUNCIE AND WINCHESTER. THIS IS A
REASONABLE WORST CASE SCENARIO, AND MODEL 10:1 SLR PLOTS SHOW THIS,
ALTHOUGH WE MUST CONSIDER THE LOW LEVEL THERMAL LIMITATIONS MAY BE
SUCH THAT IT KEEPS US FROM REALIZING SLRS THAT HIGH. HOWEVER, THE
DENDRITIC OMEGA AND MOISTURE WOULD SUPPORT IT IF NOT FOR INITIAL
RAIN PRECIPITATION TYPE AND SOME MELTING. A MORE REALISTIC SCENARIO
AT THIS TIME SEEMS TO BE AROUND OR JUST OVER AN INCH, THERE.
WEST OF THIS BAND, DEFORMATION AREA AND A COLDER THERMAL ENVIRONMENT
WILL RESULT IN AROUND ONE HALF INCH TAPERING TO MINIMAL AMOUNTS
ACROSS THE SOUTHWEST THIRD OF CENTRAL INDIANA THROUGH EARLY
AFTERNOON.
ROAD SURFACES ARE ABOVE FREEZING AT THE MOMENT AND THE BIGGEST
TROUBLE SPOTS FOR TRAVEL WILL BE WITHIN HEAVIER SNOWFALL RATES
PRIMARILY ACROSS NORTHEAST PORTIONS OF CENTRAL INDIANA, AND SOME
BRIDGES AND OVERPASSES.
SHORT TERM (TODAY AND TONIGHT)
ISSUED AT 248 AM EST FRI DEC 20 2024
THE MAIN FOCUS IN THE SHORT TERM PERIOD IS THE SNOW EXPECTED LATE
TONIGHT THROUGH TOMORROW MORNING FOR PORTIONS OF CENTRAL AND NORTH
CENTRAL INDIANA.
THE SET UP INCLUDES A DIGGING TROUGH OVER THE NORTHERN PLAINS WITH
THE TROUGH AXIS PASSING OVER THE STATE WITHIN THE 09-12Z TIMEFRAME
TOMORROW MORNING. AREA OF LOW PRESSURE STACKED FROM THE SURFACE TO
AROUND 850MB TRACKS FROM SW MINNESOTA INTO NORTH CENTRAL INDIANA BY
SUNRISE FRIDAY PLACING PORTIONS OF CENTRAL INDIANA WITHIN AN AREA OF
WEAK LOW LEVEL "WARM" AIR ADVECTION TONIGHT. BEST FORCING FOR ASCENT
ARRIVES AS THE WIND SHIFT/COLD FRONT APPROACH FROM THE NORTHWEST
DURING THE EARLY MORNING HOURS. CURRENT ACARS SOUNDINGS SHOW QUITE A
BIT OF DRY AIR THROUGH THE COLUMN WHICH WILL LIKELY TAKE MUCH OF THE
NIGHT TO SATURATE ENOUGH TO SUPPORT PRECIPITATION REACHING THE
GROUND. AND WITH THE BEST LIFT AND SATURATION NOT ARRIVING UNTIL
AFTER 08Z, THINKING MOST AREAS WILL REMAIN DRY FOR THE FIRST 12
HOURS OF THE FORECAST.
THE AFOREMENTIONED "WARM" AIR ADVECTION AHEAD OF THE SYSTEM AND
FRONT WILL KEEP TEMPERATURES RELATIVELY STEADY THIS EVENING AND
TONIGHT IN THE LOWER TO MID 30S WITH INCREASING CLOUD COVER AIDING
IN INSULATING THE BOUNDARY LAYER. DEW POINTS CURRENTLY ARE RUNNING A
DEGREE OR TWO BELOW GUIDANCE, WHICH WILL BE SOMETHING TO WATCH AS
THE LOWER LEVELS SATURATE TONIGHT. THE WET BULB TEMPERATURE (TW)
WILL BE CRITICAL FOR INITIAL SNOWFALL ACCUMULATIONS IN THE 3AM-6AM
TIMEFRAME BEFORE MUCH COLDER AIR RUSHES IN. LATEST FORECAST CALLS
FOR WET BULB TEMPERATURES TO RISE TO THE FREEZING MARK IN NORTH
CENTRAL INDIANA AND IN THE 33-34 DEGREE RANGE ALONG AND SOUTH OF I-
70. THIS MEANS SNOW INITIALLY FALLING LIKELY WILL STRUGGLE TO
ACCUMULATE WITHIN THE FIRST COUPLE HOURS AS GROUND TEMPERATURES WILL
ALSO BE AT OR JUST ABOVE FREEZING. IF DEW POINTS STRUGGLE TO RISE
TONIGHT AND TW ENDS UP BEING JUST A DEGREE COOLER, ACCUMULATIONS MAY
OCCUR SOONER WITH A HIGHER CHANCE OF SLICK SPOTS DEVELOPING. IT IS
LIKELY THAT AREAS ALONG AND SOUTH OF THE I-74 CORRIDOR COULD BEGIN
AS AN LIGHT RAIN DUE TO TEMPERATURES JUST ABOVE FREEZING AND THE DGZ
NOT FULLY SATURATED. PRECIPITATION LIKELY WILL TRANSITION OVER TO
ALL SNOW FOR MOST PLACES THROUGH THE MID MORNING HOURS.
CURRENT FORECAST CALLS FOR THE COLD FRONT TO PUSH EAST THROUGH
INDIANA WITHIN THE 4AM-7AM TIMEFRAME, WHICH IS WHEN SURFACE
TEMPERATURES WILL LIKELY FALL TO OR BELOW FREEZING AND SNOWFALL
RATES WILL BE THE HEAVIEST. CROSS SECTIONAL PROFILES FROM HI-RES
GUIDANCE SHOWING THIS BEING THE PERIOD THE TROUGH AXIS PUSHES
THROUGH WITH THE GREATEST LIFT EXTENDING UP INTO A SATURATED DGZ.
LAPSE RATES ARE ONLY MARGINAL FOR BANDING TO OCCUR, SO MOST OF THE
LIFT WILL BE SYNOPTICALLY DRIVEN FROM THE FRONT PUSHING THROUGH.
STILL, A FEW HOUR PERIOD OF MODERATE SNOW, HEAVY ENOUGH TO
ACCUMULATE, IS EXPECTED TO PUSH WEST TO EAST ALONG AND NORTH OF THE
I-74 CORRIDOR DURING THE EARLY TO MID MORNING HOURS.
GREATEST SNOWFALL ACCUMULATIONS ARE EXPECTED TO BE IN PORTIONS OF
EASTERN AND NORTH CENTRAL INDIANA WHERE UP TO 1 TO 2 INCHES OF SNOW
COULD FALL... THIS INCLUDES AREAS LIKE RUSH COUNTY, MUNCIE,
ANDERSON, AND KOKOMO. A FEW FACTORS GOING AGAINST HIGHER SNOWFALL
TOTALS ARE WARMER GROUNDS, MARGINAL LAPSE RATES, AND NOT ENOUGH
SATURATION WITHIN THE DGZ LEADING TO INEFFICIENT SNOW PRODUCTION AND
LOWER SNOW TO LIQUID RATIOS. THE 1 TO 2 INCHES MAY NOT BE VERY
WIDESPREAD AS MOST LOCATIONS LIKELY WILL RECEIVE A HALF INCH TO AN
INCH. NEVERTHELESS, JUST A HALF INCH OF SNOW IS MORE THAN ENOUGH TO
CAUSE TRAVEL IMPACTS, ESPECIALLY DURING THE FRIDAY MORNING RUSH
HOUR; THEREFORE A SPECIAL WEATHER STATEMENT HAS BEEN ISSUED FOR
AREAS ALONG AND NORTH OF THE I-74 CORRIDOR FOR EXPECTED IMPACTS DUE
TO SNOW.
AS THE LOW PASSES BY AND THE FRONT EXITS TO THE EAST, CENTRAL
INDIANA WILL BE LEFT WITHIN A CYCLONIC FLOW PATTERN AROUND THE LOW
WITH ENOUGH MOISTURE PRESENT FOR ISOLATED SNOW SHOWERS TO PERSIST
THROUGH THE AFTERNOON HOURS WITH ONLY MINOR ADDITIONAL ACCUMULATIONS
EXPECTED.
THE FOCUS SHIFTS TO THE STRONGER WINDS DEVELOPING DURING THE DAY
FRIDAY AS COLDER AIR RUSHES IN ALOFT AND WINDS WITHIN THE LOW LEVEL
JET MIX DOWN TO THE SURFACE. WINDS SHIFT TO THE WEST NORTHWEST IN
THE MORNING AND INCREASE TO 10 TO 15 MPH HOWEVER STRONGER GUSTS OF
25 TO 35+ MPH ARRIVE MID AFTERNOON AND EVENING. SNOW SHOWERS WILL
LIKELY BE TAPERING OFF BY THIS TIME, SO BLOWING SNOW IS NOT MUCH OF
AN ISSUE. FEELS LIKE TEMPERATURES WILL BE ONLY IN THE TEENS AND 20S
THOUGH, MAKING IT FEEL NOTICEABLY COLDER.
LONG TERM (SATURDAY THROUGH THURSDAY)
ISSUED AT 248 AM EST FRI DEC 20 2024
LOW-LEVEL CYCLONIC FLOW EASES SOME BY EARLY SATURDAY, POSSIBLY
OFFERING THE OPPORTUNITY FOR SOME BREAKS IN THE STRATUS. NOT VERY
CONVINCED THOUGH GIVEN STEEP LOW-LEVEL LAPSE RATES, RESIDUAL
MOISTURE (POTENTIALLY AUGMENTED BY TRAJECTORIES OFF LAKE MICHIGAN),
AND PRONOUNCED SUBSIDENCE INVERSION ATOP THE PBL. THUS, TRENDED SKY
COVER MORE PESSIMISTIC FOR SATURDAY. POST-FRONTAL COLD ADVECTION
WILL HOLD TEMPERATURES IN THE UPPER 20S TO MID 30S, AND THIS SHOULD
BE THE DOMINANT DRIVER FOR TEMPERATURES REGARDLESS OF STRATUS GIVEN
LOW SUN ANGLE AT WINTER SOLSTICE. THUS, MODELS ARE CLUSTERED FAIRLY
WELL ON TEMPERATURES.
MEAN HEIGHTS BUILD QUICKLY BY SUNDAY, BUT RIDGE/TROUGH PLACEMENT
HOLDS BACK STRONGER RETURN FLOW UNTIL LATER IN THE DAY, WITH LITTLE
CHANCE FOR MORE THAN MODEST WARMING AND TEMPERATURES AROUND 3-5
DEGREES BELOW MID-LATE DECEMBER CLIMATOLOGY. REVERSAL OF LOW-LEVEL
FLOW SHOULD BRING AN END TO ANY RESIDUAL STRATUS.
PERIODIC PRECIPITATION CHANCES ARE EXPECTED NEXT WEEK WITH MULTI-
MODEL ENSEMBLE QPF TOTALS IN THE 0.50-0.75 INCH RANGE BY THE END OF
THE WEEK. ROUND #1 WILL COME MONDAY WITH A TRANSIENT SHORTWAVE
TROUGH INTERACTING WITH MODEST MOISTURE/PWAT ANOMALIES. MODELS
DIFFER IN COVERAGE/MAGNITUDE OF RAIN NEAR STATIONARY FRONT TUESDAY
AND WARM ADVECTION-DRIVEN RAIN PRECEDING THE NEXT SHORTWAVE TROUGH
APPROACHING WEDNESDAY, WHICH WOULD BE ROUND #2. THERE ARE SOME
ENSEMBLE MEMBERS SHOWING OVER 1.00 INCH QPF TOTAL FOR NEXT WEEK, AND
THIS IS PLAUSIBLE IF THE MINORITY SCENARIO OF A FURTHER POLEWARD
POSITIONED IVT PLUME OCCURS FOR ROUND #3 FRIDAY (RE: ECMWF/EPS
CAMP).
PREDICTABILITY WITH REGARDS TO SPECIFIC TIMING OF MORE CONTIGUOUS
RAIN AREAS AND BREAKS BETWEEN IS LIMITED AT THIS TIME, ESPECIALLY
THE LATTER HALF OF THE WEEK. THIS IS DUE TO SUBSTANTIAL HIGH
LATITUDE HEIGHT ANOMALIES, SPLIT FLOW, AND GENERALLY WEAK U-WIND
ANOMALIES. SO, SOME REFINEMENT TO PRECIPITATION PROBABILITIES WILL
CAN BE EXPECTED IN SUBSEQUENT FORECASTS. THIS IS A PATTERN WHERE
WARM ADVECTION SHOULD BE ENOUGH FOR POSITIVE 2-M TEMPERATURE
ANOMALIES (POTENTIALLY +15 BY THURSDAY-FRIDAY) AND GENERALLY ALL
RAIN, WITH LITTLE CONCERN FOR WINTRY PRECIPITATION AND ASSOCIATED
IMPACTS.
THIS PATTERN MAY FAVOR SOME FOG POTENTIAL, PARTICULARLY TUESDAY
NORTH OF STATIONARY BOUNDARY WITH A FAVORABLE MSLP PATTERN, IF
STRATUS BREAKS CAN OCCUR. ONCE THIS BECOMES MORE CLEAR WE MAY NEED
TO ADD IT TO THE FORECAST.
DAY 8-14: MEDIUM-RANGE MODELS ARE OUT OF PHASE WITH CONSIDERABLE
CHAOS IN THIS PERIOD, BUT HAVE A GENERAL SIGNAL FOR HIGH-LATITUDE
POSITIVE HEIGHT ANOMALIES AND PERIODIC FORCING AMID WAVES OF
ANOMALOUS MOISTURE. THUS, ABOVE NORMAL TEMPERATURES (ALBEIT PROBABLY
NOT SUBSTANTIALLY) AND ABOVE NORMAL PRECIPITATION ARE INDICATED.
AVIATION (12Z TAF ISSUANCE)
ISSUED AT 628 AM EST FRI DEC 20 2024
IMPACTS:
- ONE BAND OF RAIN/SNOW WILL EXIT EARLY THIS MORNING AND A SECOND
BAND OF SNOW WITH LOW VISIBILITY WILL ARRIVE EARLY-MID MORNING
- CEILINGS WILL LOWER EARLY THIS MORNING TO IFR AND IMPROVE TO
MVFR LATE IN THE TAF PERIOD
- WINDS WILL GRADUALLY VEER TO NORTHWESTERLY AND BECOMING GUSTY
DISCUSSION:
MINIMAL CHANGES EXCEPT FOR REFINING TIMING MORE, WITH A BREAK EARLY
THIS MORNING, FOLLOWED BY LOW VISIBILITIES AND SNOW LATER THIS
MORNING WITH THE SECOND BAND. CEILINGS WILL BE IFR AND NOT INCREASE
TO MVFR UNTIL LATE TODAY/TONIGHT. OVERALL FORECAST CONFIDENCE IS
MODERATE TO HIGH.
IND WATCHES/WARNINGS/ADVISORIES
NONE.
SHORT TERM...BRB
LONG TERM...BRB
AVIATION...BRB
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