FXUS63 KIWX 270004
AFDIWX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE NORTHERN INDIANA
703 PM EST SAT DEC 26 2009

.AVIATION...
MOIST BNDRY LYR REMAINS ENTRENCHED WITH LARGER SCALE DEEP OCCLUDED 
CYCLONE CNTRD OVR CNTRL WI. WKNG SW TROUGH ROTATING THROUGH SRN 
PERIPHERY OF PARENT CIRCULATION YIELDING EXPANSIVE AREA OF LT/MOD 
SNOW ACRS IL/IN/MI THIS EVENING BUT XPC A GRADUAL DIMINISHING TREND 
OVERNIGHT YET MVFR CIGS/VSBYS WILL CONT.

PARENT MID LVL CIRC WILL DUMBBELL SEWD ON SUN W/WHAT SHLD BE ANOTHER 
INTENSE ROUND OF SNOWFALL. WILL LIKELY NEED TO ADD A PD OF IFR VSBY 
RESTRICTION IN MOD SNOWFALL FM ABT MID MORNING THROUGH MID AFTN IN 
NXT FCST ISSUANCE. FOR NOW HAVE KEPT W/MVFR CONDS AT BEST.

&&

.UPDATE...
COMPACT BUT ENERGETIC SW TROUGH OVR WRN IL SPURNING INTENSE LL WAA
DOWNSTREAM ACRS IL/IN LT THIS AFTN AND YIELDING INTENSE...BANDED
SNOWFALL. MANY REPORTS OUT OF CNTRL/NE IL OF INCH/HR SNOW RATES
W/SOME LOCALES ACRS NE IL REPORTING 10 INCHES SO FAR.

12/18Z GUIDANCE CERTAINLY NOT CAPTURING INTENSITY OF MID LVL SW
NOR DEGREE OF LL MSTR ENTRAINMENT. HWVR 280K SFC SHOWS INTENSE
ISENTROPIC LIFT TIL ABT 06Z AND W/MIXR NR 2G/KG...WOULD SPRT A
GENERAL 2-4 INCH BURST OF SNOW THIS EVENING. AS SUCH...UPDATED
GRIDS/ZONES AND ISSUED A WW.Y AS ROADS WILL QUICKLY BECOME ICY.

&&

.PREV DISCUSSION... /ISSUED 358 PM EST SAT DEC 26 2009/ 

SHORT TERM /TONIGHT THROUGH SUNDAY NIGHT/...

QUASI-STNRY DEEP UPR LOW OVER WI EXPECTED TO MOVE EAST ON SUNDAY IN 
RESPONSE TO SHRTWV/JET MAX TOPPING RIDGE OVER NWRN CANADA AND 
DIGGING SE TOWARD THE GRTLKS. THESE TWO SYSTEMS WILL MERGE TO FORM 
LONGWAVE TROF OVER ERN CANADA/U.S. BY MONDAY MORNING. 

MID LEVEL TROF ROTATING AROUND CLOSED LOW HAS RESULTED IN AN AREA OF 
SNOWFALL REQUIRING ADVISORIES/WARNINGS TO OUR WEST TODAY. THIS TROF 
SHOULD SWING THROUGH OUR CWA BTWN 00Z-06Z WITH ANOTHER TROF MOVG 
THROUGH THE AREA SUNDAY AS UPR LOW BEGINS TO MOVE EAST. MODEST 
FORCING IN MOIST/DEEP DGZ SUGGEST GOOD POTENTIAL FOR ACCUMULATING 
SNOW WITH BOTH SHRTWV'S. EXPECT SOME DECENT SNOWFALL RATES BUT BANDS 
SHOULD HAVE BETTER MOVEMENT ACROSS OUR AREA THAN WHAT OCCURRED 
UPSTREAM TODAY SO KEPT ACCUMS BLO ADVISORY CRITERIA WITH 1-3" FCST 
TO ACCOMPANY FIRST WAVE ACROSS NW PORTION OF CWA THIS AFTN/EVE, AND 
1-2" ON SUNDAY. 

BEHIND SUNDAY'S SHRTWV, LOW LEVEL WINDS WILL VEER WNW. LAKE INDUCED 
EQUILIBRIUM LEVEL FCST TO BE RESPECTABLE IN THE AFTN/EVE AROUND 10K 
FT WITH LAYER BENEATH INVERSION FAIRLY MOIST. WAA AHEAD OF APCHG 
ARCTIC SHRTWV/CDFNT WILL LOWER INVERSION BLO 3K FT LATE SUNDAY 
NIGHT. OTHER NEGATIVES ARE LIMITED INSTABILITY WITH LAKE INDUCED 
CAPES ONLY MAXING OUT AROUND 350-400J/KG AND SHORT CROSS LAKE FETCH 
FOR MUCH OF THE EVENT. BUMPED UP SNOW ACCUMS ABOUT AN INCH EAST OF 
THE LAKE SUNDAY AFTN TO ACCOUNT FOR THE LAKE EFFECT WITH ANOTHER 
1-2" EXPECTED SUNDAY NGT IN NORMAL WNW FLOW SNOWBELT AREAS OF SW 
MI/NC INDIANA...GENERALLY NORTH OF INDIANA TOLL ROAD WITH ONLY A 
SHORT WINDOW OF NW FLOW TO BRING BANDS FARTHER SOUTH. 

MOS TEMPS LOOK REASONABLE THROUGH THE PERIOD. WITH THERMAL TROF MOVG 
THROUGH THE AREA SUNDAY, SHOULD GENERALLY BE A FEW DEGREES COLDER 
THAN TODAY. CLOUD COVER AND CONTD DECENT MIXING SHOULD LIMIT SFC 
TEMP DROPS OVERNIGHT DESPITE RATHER COLD 850MB TEMPS.
  

LONG TERM...

MONDAY THROUGH SATURDAY

THE PERIOD WILL BEGIN WITH CONTINUED INFLUENCE OF THE PERSISTENT BROAD 
VERTICALLY STACKED CYCLONE AS IT WILL BE IN THE PROCESS OF LIFTING 
NE OUT OF THE REGION. FORECAST MODELS CONTINUE TO HIGHLIGHT THE 
ADVECTION OF A FAST MOVING WAVE ON THE HEELS OF THE EJECTING 
VORTEX...SUPPORTING ANOTHER ROUND OF POSSIBLE LIGHT SNOW AND A 
REJUVENATED LAKE RESPONSE...ALTHOUGH INTENSITY LOOKS MARGINAL AT 
THIS POINT PER LAKE SNOW. ADVECTING/DIGGING PAC UPPER JET OVER THE 
EASTERN PAC ASSOCIATED WITH DISTURBED FAST FLOW WILL SUPPORT A 
SPLIT FLOW REGIME BY EARLY NEXT WEEK OVER MUCH OF THE CENTRAL AND 
WESTERN CONUS WITH RELOADING OF THE SW CONUS UPPER TROUGH. AS STATED 
YESTERDAY...EXPANSIVE NEGATIVE THERMAL ANOMALY OVER SIBERIA WITH 
TEMPS DOWN TO -40C AT H85 WILL BEGIN TO ADVECT NORTHWARD WITH A 
PERIOD OFF CROSS POLAR FLOW...SUPPORTING A LOBE OF ARCTIC AIR INTO 
NORTHERN CANADA BY EARLY NEXT WEEK...SETTING THE STAGE FOR ANOTHER 
ARCTIC-CP AIRMASS INTRUSION INTO THE MIDWEST/GREAT LAKES LATE NEXT 
WEEK. MAIN FORECAST FEATURE/CONCERN WILL BE THE EXPECTED SOUTHERN 
STREAM TROUGH EJECTION AND THE EVOLUTION/INFLUENCE OF AN EXPECTED 
NORTHERN STREAM WAVE. CONCERNS FOR BLOCKED ATLANTIC FLOW PER LONG 
WAVE RIDGING TRENDING NORTH INTO GREENLAND WILL TREND SYNOPTIC 
SOLUTION PREFERENCE TOWARD A LESS PROGRESSIVE UPPER 
TROUGH...DEPICTED HEAVILY IN THE ECMWF BY DAY 5-6. FORECAST MODELS 
AND MEAN ENSEMBLE SOLUTIONS CONTINUE TO DEMONSTRATE DECENT CONTINUITY 
AND AGREEMENT THROUGH THE PERIOD. PREFERENCE IS MAINLY TOWARD THE 
ECMWF AT THIS JUNCTURE.

MON-TUE...LAKE EFFECT SNOW SHOWERS WILL BE POSSIBLE AT THE START OF 
THE MONDAY PERIOD. HOWEVER...NAM/GFS BUFR DATA CONTINUES TO PROG 
INVERSION HEIGHTS FALLING DOWN TO 2KFT BY LATE MONDAY MORNING 
COINCIDENT WITH BACKSIDE WAVE SUBSIDENT FLOW. SECONDARY NORTHERN 
STREAM WAVE ON THE HEALS OF THE EXITING VORTEX WILL LEAD TO 
INCREASE LARGE SCALE ASCENT AND COOLING BY LATER IN THE 
DAY...SUPPORTING RENEWED SYNOPTIC SNOW CHANCES...WITH LAKE 
ENHANCEMENT POSSIBLE LATE MONDAY AFTERNOON. IN ADDITION...BUFR DATA 
MAY BE TOO QUICK TO SHUTDOWN ANY LAKE RESPONSE...A TYPICAL MODEL 
BIAS. HENCE...HAVE GENERALLY LEFT POPS IN THE NW AS IS...WITH A 
SLIGHT RAISE IN THE NE HALF OF THE FA PER THE ADVECTING MID LEVEL 
IMPULSE. ALSO RETAINED LAKE EFFECT SNOW MENTION THROUGH MONDAY NIGHT 
WITH A PERIOD OF CAA AND AN ELEVATED LK SFC-H85 DELTA T/S. SFC HIGH 
PRESSURE WILL DOMINATE INTO WED WITH WEAK MIXING AND LIGHT FLOW 
SUPPORTING COOLER SFC THERMAL FIELDS. 

WED NIGHT-SAT...SW CONUS UPPER IMPULSE WILL BEGIN TO EJECT EAST WITH 
POSSIBLE WAVE PHASING OF AN EXPECTED NORTHERN STREAM IMPULSE. 
POSITION OF THE PROGGED LL BAROCLINIC ZONE/FINAL JET STRUCTURE/AND 
LL ADVECTION FIELDS WILL BE PARAMOUNT IS DETERMINING LOW TRACK AND 
PRECIP CHANCES LOCALLY. CURRENT PROGS CERTAINLY OPT FOR A SFC LOW 
TRACK THROUGH THE DEEP SOUTH...A TYPICAL EL NINO TRACK. 
HOWEVER...THE SW UPPER TROUGH OFFERED IN MODEL SOLUTIONS MAY BE TOO 
WEAK PER POORLY HANDLED JET DYNAMICS AND LL THERMAL FIELDS GIVEN 
MORE THAN WEEK OUT. A MORE POTENT SYSTEM...WITH NORTHERN STREAM WAVE 
PHASING WOULD LIKELY SHIFT THE TRACK FARTHER NORTH AND SUPPORT A 
MORE ENCOMPASSING PRECIP EVENT FOR THE EASTERN HALF OF THE CONUS AND 
WILL NEED TO BE WATCHED. HAVE OPTED FOR A POP MENTION WED NIGHT 
THROUGH SAT GIVEN THE EXPECTED ADVECTION OF THE LONG WAVE TROUGH AND 
EXPECTED EVOLUTION TO ANOTHER STALLED CUT OFF VORTEX. HIGHEST POP 
VALUES IN THE WED NIGHT AND THUR PERIOD GIVEN GUIDANCE TRENDS. IN 
ADDITION...IF A STRONG EPISODE OF CAA IS REALIZED IN MAJOR AXIS 
FLOW...A HEAVY LAKE SNOW EPISODE WOULD BE POSSIBLE.

AVIATION...
SHRTWV'S ROTATING AROUND UPR LOW OVER WI EXPECTED TO BRING PERIODS
OF SNOW AND IFR CONDITIONS TO TERMINALS THIS PERIOD. FIRST SHRTWV
WILL MOVE THROUGH THE AREA THIS EVE CAUSING SNOW TO SPREAD ACROSS
NRN INDIANA THIS AFTN/EVE. SHOULD BE A BREAK IN PRECIP LATE
TONIGHT AND EARLY SUNDAY BEFORE ANOTHER TROF APPROACHES AND
SPREADS SNOW BACK INTO THE AREA BY MIDDAY. EXPECT IFR/MVFR
CONDITIONS DURING SNOWFALL PERIODS, WITH VFR OR PSBLY MVFR DUE TO
CIGS DURING DRY PORTION OF THE FCST EARLY THIS AFTN AND LATE TONIGHT. 

&&

.IWX WATCHES/WARNINGS/ADVISORIES...
IN...WINTER WEATHER ADVISORY UNTIL 1 AM EST /MIDNIGHT CST/ SUNDAY 
     FOR INZ003>006-008-012>016-020-022>024.
MI...WINTER WEATHER ADVISORY UNTIL 1 AM EST SUNDAY FOR MIZ077>079.
OH...NONE.
LM...NONE.

&&

$$

SHORT TERM...JT
LONG TERM...JC
AVIATION...T
UPDATE...T