Area Forecast Discussion
Issued by NWS Indianapolis, IN

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000
FXUS63 KIND 241059
AFDIND

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE INDIANAPOLIS IN
559 AM EST WED DEC 24 2014

.UPDATE...
THE AVIATION SECTION HAS BEEN UPDATED BELOW.

&&

.SYNOPSIS...
ISSUED AT 357 AM EST WED DEC 24 2014

RAIN WILL CHANGE TO SNOW THIS AFTERNOON AND TONIGHT AS COLDER AIR
WRAPS IN BEHIND A STRENGTHENING LOW PRESSURE SYSTEM. SNOW SHOWERS
AND FLURRIES WILL END CHRISTMAS MORNING WITH DRY AND MILDER AIR
RETURNING WITH HIGH PRESSURE THROUGH FRIDAY. RAIN WILL IMPACT THE
OHIO VALLEY FRIDAY NIGHT AND SATURDAY AS A COLD FRONT PASSES THROUGH
THE REGION. AS COLDER AIR BUILDS IN FOR THE END OF THE WEEKEND INTO
EARLY NEXT WEEK...POTENTIAL FOR LIGHT SNOW WILL EXIST AS WELL WITH A
SERIES OF UPPER DISTURBANCES IMPACTING THE OHIO VALLEY.

&&

.NEAR TERM /TODAY AND TONIGHT/...
ISSUED AT 357 AM EST WED DEC 24 2014

LIGHT RAIN CONTINUING ACROSS THE REGION EARLY THIS MORNING AS LOW
PRESSURE CONTINUES TO GET ORGANIZED OVER THE TENNESSEE VALLEY.
INVERTED TROUGH EXTENDING NORTH-NORTHEAST FROM THE LOW INTO
SOUTHEAST INDIANA. TEMPS HAD FALLEN INTO THE LOWER AND MIDDLE 40S
WEST OF THE BOUNDARY WITH UPPER 40S/LOWER 50S ACROSS OUR FAR EASTERN
COUNTIES AT 08Z.

FOCUS REMAINS ON THE APPROACHING LOW PRESSURE SYSTEM AND THE IMPACTS
EXPECTED AS IT PASSES THROUGH CENTRAL INDIANA LATER TODAY. MODEL
PERFORMANCE WITH THIS SYSTEM SINCE LATE LAST WEEK HAS BEEN
FASCINATING AND FRANKLY FRUSTRATING FROM A FORECASTING STANDPOINT.
00Z MODEL SUITE HAS TOSSED ANOTHER CURVEBALL IF YOU WILL...WITH
MAJORITY OF THE GUIDANCE NOW SUBTLY SHIFTING THE TRACK OF THE LOW
FURTHER EAST. EXPECT THE LOW TO TRACK ALONG THE LOCATION OF THE
INVERTED TROUGH EXTENDING FROM SOUTHEAST INDIANA INTO NORTHWEST
OHIO. MODEL GUIDANCE TIGHTLY CLUSTERED ALONG THIS ARCING PATH WITH
SUBTLE DIFFERENCE CONFINED TO SPEED AND HOW QUICKLY THE SYSTEM
INTENSIFIES. THE BIGGEST TAKEAWAY IS THAT THIS SUBTLE SHIFT EASTWARD
HAS BROUGHT SOME POTENTIAL FOR HIGHER SNOWFALL AMOUNTS BACK INTO
PLAY OVER THE NORTHWEST COUNTIES OF CENTRAL INDIANA. MODELS ARE
DETERMINED TO MAKE THIS FORECAST DIFFICULT RIGHT DOWN TO THE END
WITH MESOSCALE FEATURES BECOMING CRITICAL IN A INCREASINGLY APPARENT
NOWCAST SCENARIO.

RAIN WILL EXPAND AGAIN ACROSS THE FORECAST AREA THROUGH THE MORNING
WITH THE APPROACH OF THE LOW. ELEVATED INSTABILITY AXIS WILL RIDE UP
THE EAST SIDE OF THE INVERTED TROUGH WITH SOME LOW TOPPED CONVECTION
AND ISOLATED THUNDER LIKELY TO CLIP OR PASS JUST EAST OF THE
FORECAST AREA BY EARLY AFTERNOON. HAVE ADDED ISOLATED THUNDER TO FAR
EASTERN COUNTIES FROM 15-20Z TO ACCOUNT AS THE LOW PASSES BY. WINDS
WILL BE GRADUALLY INCREASING AS THE INTENSIFYING LOW ARRIVES...AND
COULD SEE SOME STRONGER GUSTS ACCOMPANYING THE LOW TOPPED
CONVECTION. CERTAINLY SOMETHING TO WATCH THROUGH THE DAY.

THE FOCUS WILL BEGIN SHIFTING TO POTENTIAL SNOW IMPACTS AS EARLY AS
18-19Z OVER THE NORTHERN WABASH VALLEY AS SHARP MID LEVEL
DEFORMATION BAND ALIGNS ACROSS THE NORTHWEST FORECAST AREA. FORCING
ALOFT IS IMPRESSIVE AS THE UPPER LOW STRENGTHENS DIRECTLY OVER THE
AREA...WITH THE DEFORMATION BAND LINING UP WITH AN IMPRESSIVE AREA
OF EPV AND Q VECTOR DIVERGENCE. A DYNAMIC SYSTEM THAT SUPPORTS
STRONG TENDENCIES TOWARDS BANDING OF HEAVIER SNOW RATES...WITH
HIGHEST POTENTIAL EXISTING UNDER THE DEFORMATION AXIS FROM THE MID
AFTERNOON INTO THE EARLY EVENING.

SOME CAVEATS EXIST HOWEVER...LOW LEVEL THERMAL PROFILES REMAIN
MARGINAL WITH SURFACE TEMPS HOVERING IN THE MID TO POSSIBLY UPPER
30S DURING THE 18-00Z TIME PERIOD. SNOW TO LIQUID RATIOS ARE LIKELY
TO BE IN THE 8-10/1 RANGE DURING THE AFTERNOON AND EVENING WITH
POTENTIAL FOR BRIEF PERIODS OF HIGHER RATIOS IN LOCALIZED SPOTS.
BELIEVE THE DYNAMIC NATURE OF THE SYSTEM AND AVAILABLE FORCING ALOFT
MENTIONED ABOVE WILL PRODUCE PERIODS WHERE HIGHER SNOWFALL RATES
WILL OVERWHELM THE MARGINAL LOW LEVEL THERMALS AND ENABLE SOME
ACCUMS FOCUSED ON GRASSY AREAS AND SOME ELEVATED SURFACES. WHILE
BANDING POTENTIAL IS GREATEST UNDERNEATH THE DEFORMATION AXIS IN OUR
FAR NORTHWEST...COULD SEE LOCALLY HIGHER POCKETS OF SNOW LATE THIS
AFTERNOON AND EVENING FURTHER SOUTHEAST.

AT THIS POINT...CONFIDENCE IS HIGHEST IN IMPACTS THIS AFTERNOON AND
EVENING IN THE NORTHERN WABASH VALLEY. AFTER MUCH DELIBERATION WITH
IWX/LOT/ILX OFFICES...HAVE CHOSEN TO INTRODUCE A WINTER WEATHER
ADVISORY FOR THE FOUR NORTHWEST COUNTIES (VERMILLION/FOUNTAIN/
WARREN/TIPPECANOE). AMOUNTS ALONE ARE SUB-ADVISORY...BUT MY CONCERNS
LIE IN RAISING AWARENESS WITH POTENTIAL IMPACTS ON A BUSY TRAVEL
DAY...SPECIFICALLY WITH REDUCED VISIBILITIES WHERE SNOWFALL RATES
ARE HIGHER AND POTENTIAL FOR SOME SLICK SPOTS ON ELEVATED SURFACES AS
TEMPS FALL THROUGH THE 30S. IN THE ADVISORY AREA...THINK 2-3 INCHES
ARE POSSIBLE BY EARLY THURSDAY. EXPECT SLIGHTLY HIGHER AMOUNTS
FURTHER SOUTH AND EAST ACROSS THE FORECAST AREA WITH A SLOPPY INCH
OR SO BY EARLY THURSDAY ON GRASSY SURFACES GENERALLY ALONG AND
NORTHWEST OF A TERRE HAUTE-INDY METRO-MUNCIE LINE. SNOW SHOWERS WILL
CONTINUE TONIGHT WITH RATES DIMINISHED AS THE DEEPER FORCING LIFTS
AWAY TO THE NORTHEAST.

TO SUM UP...WINTER WEATHER ADVISORY FOR THE NORTHERN WABASH VALLEY
WITH A SPECIAL WEATHER STATEMENT SURROUNDING THE ADVISORY. WE ARE
ESSENTIALLY ENTERING A NOWCAST SITUATION OVER THE NEXT 12-24 HOURS.
MESOSCALE PROCESSES AND ANY ADDITIONAL SUBTLE SHIFTS IN STORM TRACK
COULD LEAD TO ADDITIONAL ADJUSTMENTS LATER TODAY. WITH THAT BEING
SAID...SOME WILL WAKE UP TO A WHITE COATING CHRISTMAS MORNING.

TEMPS...UTILIZED THE RAP/NAM TEMP PROFILES TO BUILD THE HOURLY CURVE
TODAY. HIGHS WILL BE THIS MORNING WITH FALLING TEMPS THIS AFTERNOON.
MOS GUIDANCE LOOKED REASONABLE TONIGHT WITH LOWS FALLING INTO THE
LOWER 30S.

&&

.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
ISSUED AT 357 AM EST WED DEC 24 2014

FOCUS IS ON LINGERING SNOW SHOWERS/FLURRIES CHRISTMAS MORNING AND
PRECIP CHANCES FRIDAY NIGHT AS A COLD FRONT TRACKS INTO THE REGION.

UPPER TROUGH AXIS TRAILING THE DEPARTING LOW PRESSURE WILL BE MOVING
AWAY TO THE EAST AT DAYBREAK THURSDAY...WITH A RIDGE ALOFT POISED TO
QUICKLY BUILD INTO CENTRAL INDIANA DURING THE COURSE OF THE DAY. THE
UPPER TROUGH WILL REMAIN IN CLOSE ENOUGH PROXIMITY TO JUSTIFY A
MENTION OF ISOLATED SNOW SHOWERS AND SCATTERED FLURRIES THURSDAY
MORNING WITH DRY CONDITIONS THEREAFTER AS THE RIDGE AND ACCOMPANYING
SURFACE HIGH PRESSURE TAKE OVER. MODEL SOUNDINGS SUGGESTIVE THAT LOW
STRATUS MAY TAKE SOME TIME TO FULLY MIX OUT ON THURSDAY WITH A
LINGERING LOW LEVEL INVERSION AS DRIER AIR ALOFT ADVECTS IN. EXPECT
SKIES TO GRADUALLY CLEAR BY THURSDAY AFTERNOON INTO THE EVENING AS
THE RIDGE AXIS QUICKLY PASSES AND SOUTHERLY FLOW REESTABLISHES.

REMAINDER OF THE SHORT TERM IS FAIRLY QUIET AS THE UPPER LEVEL
PATTERN TRANSITIONS TO A SOUTHWEST FLOW ALOFT AS AN UPPER LOW DIVES
INTO THE SOUTHERN ROCKIES. DRY WEATHER IS EXPECTED THURSDAY NIGHT
AND FRIDAY WITH BRISK SOUTHWEST FLOW DEVELOPING BETWEEN STRONG HIGH
PRESSURE OVER THE CAROLINAS AND AN APPROACHING COLD FRONT FROM THE
WEST. CLOUDS WILL INCREASE IN ADVANCE OF THE FRONT...WITH SCATTERED
SHOWERS DEVELOPING PRIMARILY AFTER MIDNIGHT AS DEEPER MOISTURE IS
PULLED NORTH INTO THE REGION. PRECIPITATION LATE FRIDAY NIGHT WILL
FALL AS RAIN WITH WARM ADVECTION AND 850MB TEMPS HOVERING AROUND 5C.

TEMPS...UNDERCUT MOS FOR HIGHS THURSDAY AS MODELS APPEAR TOO
AGGRESSIVE WITH WARMING CONSIDERING THE COLD ADVECTION IN THE
MORNING AND POTENTIAL FOR CLOUDS TO LINGER WELL INTO THE AFTERNOON.
GENERALLY LEANED TOWARDS THE METMOS THE REST OF THE WAY WITH TEMPS
RECOVERING INTO THE 40S ON FRIDAY.

&&

.LONG TERM /SATURDAY THROUGH TUESDAY/...
ISSUED AT 152 AM EST WED DEC 24 2014

THE INITIALIZATION FROM THE REGIONAL MODEL WILL BE ACCEPTED...BUT
THIS IS A LOW CONFIDENCE FORECAST.

THE EUROPEAN MODEL AND THE GFS DIVERGE RAPIDLY. 12Z SUNDAY THE
EUROPEAN HAS LOW PRESSURE FROM THE LOWER MISSISSIPPI VALLEY TO
OHIO. AT THE SAME TIME THE GFS HAS LITTLE OR NO SIGN OF THAT
SYSTEM. MONDAY AND TUESDAY THEY ALSO HANDLE LOWS THAT WILL AFFECT
OUR AREA VERY DIFFERENTLY.

LAST WEEK NEITHER OF THE MODELS DID WELL FORECASTING FOR THIS WEEK.
GIVEN THAT TRACK RECORD...AND THE SIGNIFICANT DIFFERENCES THEY HAVE
FOR THIS FORECAST...THERE CANT BE MUCH CONFIDENCE IN IT.

&&

.AVIATION /DISCUSSION FOR THE 241200Z TAF ISSUANCE/...

ISSUED AT 600 AM EST WED DEC 24 2014

LOW PRESSURE WILL PASS THE AREA TO THE EAST TODAY AND WITH COLDER
AIR ARRIVING AS IT PULLS AWAY. CONDITIONS WILL BE VARIABLE BUT
MOSTLY LIFR OCCASIONALLY VLIFR IN RAIN FOR THE FIRST PART OF THE
PERIOD AND THEN SNOW FROM THIS AFTERNOON INTO THE EVENING.

CONDITIONS WILL IMPROVE TO MVFR CEILINGS AND UNRESTRICTED
VISIBILITIES OVERNIGHT AS THE LOW CONTINUES TO MOVE AWAY.
HOWEVER..SCATTERED FLURRIES MAY CONTINUE UNTIL ALMOST THE END OF THE
PERIOD.

EARLY IN THE PERIOD WINDS WILL BE MOSTLY FROM THE NORTH. THEY SHOULD
GRADUALLY BACK AND BE SOUTHWEST BY THURSDAY MORNING. SPEEDS
SHOULD BE MOSTLY 10-15 KNOTS. GUSTS AS HIGH AS 25 KNOTS COULD OCCUR
THIS AFTERNOON.

&&

.IND WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY FROM 1 PM THIS AFTERNOON TO MIDNIGHT EST
TONIGHT FOR INZ028-029-035-043.

&&

$$

SYNOPSIS...RYAN
NEAR TERM...RYAN
SHORT TERM...RYAN
LONG TERM....JK
AVIATION...JK

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