Area Forecast Discussion
Issued by NWS Northern Indiana

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000
FXUS63 KIWX 271922
AFDIWX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE NORTHERN INDIANA
222 PM EST SAT DEC 27 2014

.SYNOPSIS...
ISSUED AT 201 PM EST SAT DEC 27 2014

A COLD FRONT ASSOCIATED WITH A LOW PRESSURE AREA MOVING ACROSS
THE WESTERN GREAT LAKES REGION WILL BE ACCOMPANIED BY RAIN SHOWERS
AND AREAS OF DRIZZLE. AFTERNOON HIGHS IN THE MID 40S TO LOWER 50S
CAN BE EXPECTED. A BRIEF MIX OF SNOW IS POSSIBLE TONIGHT AS COLDER
AIR FILTERS INTO THE REGION BUT NO ACCUMULATION IS EXPECTED. LOWS
TONIGHT SHOULD BE IN THE UPPER 20S TO MIDDLE 30S.

&&

.SHORT TERM...(TODAY AND TONIGHT)
ISSUED AT 334 AM EST SAT DEC 27 2014

PIECE OF OLD SOUTHWEST CONUS TROUGH IS LIFTING QUICKLY NORTHEAST
THIS MORNING. STRONG SUBTROPICAL RIDGE OVER FLORIDA IS SUPPORTING
HIGHLY CONFLUENT FLOW OVER THE OHIO VALLEY AND GREAT LAKES. THIS
HAS CAUSED THE WAVE TO BECOME HIGHLY SHEARED AS IT ENTERS OUR AREA
WITH AN OVERALL WEAK SURFACE LOW MOVING FROM THE UPPER MIDWEST TO
NORTHERN MICHIGAN BY LATER TODAY. LIGHT RAIN IS FINALLY BEGINNING
TO MOVE INTO OUR CWA AT PRESS TIME...THANKS TO ELEVATED WARM FRONT
SEEN NICELY IN 290K ISENTROPIC FIELDS. MOISTURE ADVECTION AND
FORCING FOR ASCENT IN THIS LEAD WAA WING REMAIN CONFINED TO THE
LOWER LEVELS THOUGH...PREVENTING A MORE WIDESPREAD/MODERATE
RAINFALL. QPF AMOUNTS UPSTREAM HAVE BEEN GENERALLY LIGHT AND
EXPECT SIMILAR CONDITIONS LOCALLY. SURFACE COLD FRONT WILL THEN
SLOWLY CROSS THE AREA LATER TODAY. THIS PROCESS WILL TAKE SOME
TIME GIVEN LACK OF STRONG HEIGHT FALLS ALOFT AND A LARGE COMPONENT
OF THE MEAN WIND PARALLEL TO THE FRONT. BEST CHANCES FOR RAIN WILL
BE IN OUR SOUTHEAST HALF THIS AFTERNOON AND EVENING. SEVERAL
MODELS INDICATING A DECENT DEFORMATION/FGEN BAND LIGHTING UP ALONG
THIS CONVERGENT BOUNDARY AROUND 600-700MB AS HIGHLY SHEARED
VORTICITY FILAMENT APPROACHES. CONFIDENCE IN THIS SCENARIO IS NOT
QUITE HIGH ENOUGH FOR CATEGORICAL POPS YET THOUGH. FGEN FORCING IS
FAIRLY ELEVATED AND THERE ARE MOISTURE CONCERNS BY THE TIME THIS
FORCING ARRIVES. SEVERAL NWP MEMBERS ARE ALSO MUCH LESS BULLISH
WITH THIS FGEN AND/OR CONFINE IT SOUTHEAST OF OUR CWA. WILL
THEREFORE HOLD WITH HIGH END LIKELY POPS AND RELATIVELY LOW QPF
AMOUNTS. DAY SHIFT CAN ADJUST WITH THE BENEFIT OF MORE
OBSERVATIONAL DATA AND 12Z MODEL RUNS. STILL EXPECT A PRIMARILY
LIQUID EVENT. COLDER AIR DOES FILTER INTO THE AREA OVERNIGHT AND
COULD LEAD TO SOME LIGHT SNOW ON THE BACK EDGE OF EXITING
PRECIP...ESPECIALLY IF LOW-MIDLEVEL FGEN BAND IS MORE AGGRESSIVE
THAN CURRENTLY EXPECTED. VERY LATE ARRIVAL OF SUB-FREEZING SURFACE
WET BULB TEMPS AND WARM/WET GROUND CONDITIONS WILL LIKELY NEGATE
ANY ACCUMULATIONS THOUGH.

TEMPS TODAY A BIT TRICKY WITH INCREASING SOUTHWEST FLOW/WAA AHEAD OF
THE FRONT BEING AT LEAST PARTIALLY OFFSET BY ABUNDANT CLOUD COVER.
WILL THEREFORE HOLD WITH HIGH TEMPS SIMILAR TO YESTERDAY...GENERALLY
AROUND 50F. OVERNIGHT LOWS WILL LIKELY JUST BARELY REACH THE
FREEZING MARK BY EARLY SUNDAY MORNING AS COOL WNW FLOW RAMPS UP.

&&

.LONG TERM...(SUNDAY THROUGH FRIDAY)
ISSUED AT 334 AM EST SAT DEC 27 2014

QUIET WEATHER TO BEGIN THE LONG TERM WITH MAIN STORY FOR THIS
FORECAST CYCLE CONTINUING TO BE THE TREND TO COLDER TEMPERATURES
EARLY TO MIDDLE OF NEXT WEEK...FOLLOWED BY POTENTIAL OF INCREASING
PRECIP CHANCES TOWARD THE END OF THE PERIOD.

AXIS OF WEAK MID LEVEL FRONTOGENESIS FORCING WILL LIKELY BE SPRAWLED
ACROSS SOUTHEAST PORTIONS OF THE AREA EARLY SUNDAY...ALTHOUGH DRY
LOW LEVEL AIR ADVECTION SHOULD PRODUCE ENOUGH OF A DRY LAYER ABOVE
FRONTAL INVERSION TO LIMIT ANY ADDITIONAL PRECIP. HAVE KEPT
SOUTHEAST PORTIONS OF THE AREA DRY EARLY SUNDAY...BUT NOT COMPLETELY
OUT OF THE QUESTION A STRAY SPRINKLE OR FLURRY COULD LINGER EARLY
MORNING. MID AND HIGH CLOUDS MAY PERSIST INTO EARLY AFTERNOON ACROSS
THE SOUTHEAST...WHILE MODEL LOW LEVEL RH PROGS HINTING AT THE
POSSIBILITY OF FRONTAL INVERSION ALLOWING FOR SOME LINGERING STRATUS
ACROSS THE NORTH IN CLOSER PROXIMITY TO SHEARED VORT MAX. MOST
LOCATIONS ARE EXPECTED TO BECOME PARTLY CLOUDY BY THE MID TO LATE
AFTERNOON HOWEVER. FRONTAL INVERSION/WEAK MIXING SHOULD LIMIT HIGHS
TO THE MID 30S MOST LOCATIONS...AND POSSIBLY UPPER 30S ACROSS THE
EAST.

FORECASTED CHARACTERISTICS OF LARGE SCALE FLOW PATTERN HAVE REMAIN
UNCHANGED FOR THE EARLY TO MIDDLE PORTIONS OF NEXT WEEK WITH UPPER
LEVEL TROUGH CUTTING OFF ACROSS SOUTHWEST CONUS AS EASTERN PACIFIC
UPPER RIDGING AMPLIFIES. THIS UPSTREAM AMPLIFICATION WILL ALLOW FOR
INTRUSION OF COLDER AIR EARLY TO MIDDLE OF NEXT WEEK. A LOWER
AMPLITUDE NORTHERN STREAM TROUGH WORKING ACROSS THE NORTHERN PLAINS
MAY INDUCE SOME WEAK DOWNSTREAM FGEN FORCING ACROSS WESTERN GREAT
LAKES/UPPER MS VALLEY...BUT HARD TO LATCH ONTO ANY PERIODS OF
STRONGER SYNOPTIC FORCING FOR LOCAL AREA TO INCLUDE POPS.

MAIN CHALLENGE FOR MON-WED WILL BE RESOLVING LAKE EFFECT SNOW SHOWER
POTENTIAL. STRENGTHENING LOW LEVEL ANTICYCLONE ASSOCIATED WITH
UPSTREAM FLOW AMPLIFICATION SHOULD YIELD A NORTHEAST LOW LEVEL FLOW
COMPONENT THAT SHOULD TAKE MAIN LAKE EFFECT SNOW SHOWER POTENTIAL
ACROSS NORTHWEST INDIANA/NORTHEAST ILLINOIS BEFORE FLOW BACKS MORE
NORTHERLY FOR TUESDAY WITH PASSAGE OF THIS TROUGH. SOUTHWARD
PROGRESSION OF STRONG PLAINS ANTICYCLONE WILL ALLOW FOR MORE
NORTH-NORTHWEST FETCH TO BE ESTABLISHED LATER TUESDAY/WEDNESDAY
ALTHOUGH DRYING MID LEVEL PROFILES IN THIS REGIME...AND EVENTUAL
TRANSITION TO MORE WESTERLY FETCH BY MID WEEK MAY TEND TO LIMIT
POTENTIAL. OVERALL LAKE EFFECT SETUP LOOKS TO BE SUPPORTIVE OF ONLY
LIGHT ACCUMULATIONS AT THIS TIME BUT UNCERTAINTIES REGARDING SHORT
WAVE TIMING/FETCH WILL NEED TO BE RESOLVED. IT DOES APPEAR
HOWEVER...THAT REINFORCING UPPER TROUGH SPILLING OVER THE RIDGE WILL
PROVIDE THE COLDEST CONDITIONS OF THE PERIOD FOR WEDNESDAY...WITH
LOW LEVEL THERMAL PROGS SUPPORTING HIGHS NOT MUCH ABOVE THE 20
DEGREE MARK.

CONCERN FOR THE REMAINDER OF THE PERIOD WILL SHIFT TO EVOLUTION OF
CUT-OFF PV ANOMALY ACROSS SOUTHWEST CONUS...WHICH WILL EVENTUALLY
GET INGESTED BACK INTO MAIN BELT OF WESTERLIES TOWARD THE END OF THE
PERIOD. WILDCARD IN THIS FORECAST IS TRYING TO RESOLVE NATURE OF
NORTHWEST UPPER FLOW DOWNSTREAM OF EASTERN PAC RIDGE...WHICH WILL
HAVE BIG IMPLICATIONS ON TIMING/TRACK/STRENGTH OF THE UPPER PV
ANOMALY AS IT TRACKS ACROSS CENTRAL CONUS TOWARD END OF PERIOD. 00Z
EC/GEFS MEAN/GEM ENSEMBLES WOULD SUGGEST 00Z GFS IS A BIT TOO FAR
NORTH AND PROGRESSIVE KICKING OUT THE CUT-OFF UPPER TROUGH...WITH
00Z EC ALSO TRENDING TOWARD A SLOWER AND SLIGHTLY MORE SUPPRESSED
SOLUTION FOR NEXT WEEKEND. MINOR DETAILS REGARDING AFOREMENTIONED
WAVES DROPPING DOWN IN NORTHWEST FLOW WILL ALSO BE IMPORTANT
REGARDING EXTENT OF ISENTROPIC UPGLIDE OVER COLD DOME FRIDAY/FRIDAY
NIGHT DOWNSTREAM OF SOUTHWEST CONUS CUT OFF TROUGH. GIVEN LARGE
DEGREE OF UNCERTAINTY WILL MAINTAIN ONLY SLIGHT CHANCE POPS FOR
FRIDAY...ALTHOUGH PRECIP CHANCES MAY INCREASE SHARPLY LATE
FRIDAY/SATURDAY WITH BETTER CHANCE OF ISENTROPIC/WAA FORCING. STILL
A GOOD DEAL OF SPREAD IN DETERMINISTIC/ENSEMBLE SOLUTIONS FOR TRACK
OF THIS CUT-OFF SYSTEM AND WILL NEED TO BE MONITORED FOR NEXT
WEEKEND. DEPENDING ON TRACK...RETREATING COLD DOME COULD ALSO CAUSE
SOME CONCERN FOR VARIETY OF PRECIP TYPES ACROSS THE GREAT LAKES/OHIO
VALLEY REGION. TEMPS SHOULD MODERATE TOWARD END OF THE PERIOD
ALTHOUGH DID GO SLIGHTLY LOWER THAN CONSENSUS BLEND MAX TEMPS FOR
FRIDAY GIVEN PREFERENCE AWAY FROM THE MORE PROGRESSIVE 00Z GFS.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z SUNDAY AFTERNOON)
ISSUED AT 203 PM EST SAT DEC 27 2014

AREAS OF DRIZZLE AND LOWS CLOUDS WILL PERSIST AHEAD OF A COLD
FRONT UNTIL THE FRONT PASSES LATE THIS AFTERNOON/EARLY THIS
EVENING. IFR CIGS EARLY SHOULD BECOME VFR BY LATE IN THE PERIOD AS
DRIER AIR SPREADS INTO NORTHERN INDIANA. THE FRONT PASSED GARY
ABOUT 17Z AND SHOULD BE ACCOMPANIED BY BRIEFLY HIGHER SUSTAINED
WINDS AND WIND GUSTS.

&&

.IWX WATCHES/WARNINGS/ADVISORIES...
IN...NONE.
MI...NONE.
OH...NONE.
LM...SMALL CRAFT ADVISORY UNTIL 11 AM EST SUNDAY FOR LMZ043-046.

&&

$$

SYNOPSIS...AGD
SHORT TERM...AGD
LONG TERM...MARSILI
AVIATION...SKIPPER


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