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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0409Z Dec 16, 2014)
 
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1109 PM EST MON DEC 15 2014

VALID 12Z FRI DEC 19 2014 - 12Z TUE DEC 23 2014

...SNOW POSSIBLE FOR PORTIONS OF THE MID-SOUTH EASTWARD INTO THE
CENTRAL/SOUTHERN APPALACHIANS...


...OVERVIEW...

ACTIVE PATTERN TO CONTINUE INTO NEXT WEEK AS UPPER RIDGING SLOWLY
BUILDS JUST OFFSHORE CALIFORNIA. PATTERN SHIFT IS FORECAST TO MOVE
THE MAIN RIDGE/TROUGH CENTERS ABOUT A HALF WAVELENGTH DOWNSTREAM
IN THE MEDIUM RANGE... AS THE RIDGE IN CENTRAL CANADA MOVES TO
EASTERN CANADA AND THE RIDGE TO THE NW OF HAWAII MOVES CLOSER TO
CALIFORNIA. THIS SHOULD FAVOR TROUGHING IN THE SW CONUS MOVING
EASTWARD PAST 100W BY NEXT TUESDAY. A LEAD SYSTEM IN THE SOUTHEAST
QUARTER OF THE CONUS WILL PUSH AWAY FROM THE
SOUTHEAST/MID-ATLANTIC LATE THIS WEEKEND BUT ITS LATITUDINAL
EXODUS IS UNCERTAIN.


...GUIDANCE EVALUATION AND PREFERENCES...

IN THE EAST... THE MODELS/ENSEMBLES HAVE OFFERED A VARIETY OF
SOLUTIONS OVER THE PAST FEW DAYS FROM A VERY LOW AMPLITUDE AND
FLAT WAVE MOVING THROUGH THE SOUTHEAST TO A WRAPPED UP SFC LOW
HUGGING THE COAST INTO NEW ENGLAND. THE ECMWF ENSEMBLE MEAN HAS
OFFERED THE BEST CONSISTENCY AND HAVE CONTINUED TO RELY ON ITS
ROBUSTNESS. THE DETERMINISTIC ECMWF HAS BEEN LESS STABLE IN ITS
DETAILS BUT BY NO MEANS UNUSABLE... AND IN FACT USED A 50/50
WEIGHTING BETWEEN IT AND ITS ENSEMBLE MEAN FOR THE FIRST PART OF
THE FORECAST -- FRIDAY THROUGH SUNDAY -- AS THE LOW PUSHES THROUGH
THE SOUTHEAST AND THEN TURNS NORTHEASTWARD AND HEADS INTO THE
ATLANTIC... LIKELY PASSING JUST SOUTHEAST OF THE 40N/70W
BENCHMARK. AGAIN... ENSEMBLES STILL SPAN AN AREA ROUGHLY 500 BY
500 MILES JUST OFF THE DELMARVA EARLY SUNDAY.

FOR THE WEST... GOOD AGREEMENT EXISTS THROUGH ABOUT THE WEEKEND AS
THE TROUGH OFF THE COAST SPLITS AND THE SOUTHERN PORTION DROPS
THROUGH THE GREAT BASIN AND INTO THE FOUR CORNERS REGION. FROM
THERE... THE GEFS MEMBERS ARE QUICKER TO MOVE THIS ENERGY EASTWARD
TOWARD 90W BY NEXT TUESDAY WHILE THE ECMWF ENSEMBLE MEMBERS LAG
BEHIND BY ABOUT 10 DEGREES LONGITUDE. THE CANADIAN ENSEMBLES LIE
EVEN FARTHER WEST INTO NW MEXICO. ONE FACTOR OF INFLUENCE IS HOW
STRONG AND EAST UPPER RIDGING BECOMES OFFSHORE CALIFORNIA... ALSO
RELATED TO UPSTREAM CENTRAL PACIFIC TROUGHING DOWNSTREAM OF A
STRONG JET OFF JAPAN EARLY NEXT WEEK. WITH THE TENDENCY TOWARD
MORE AMPLIFICATION OVER THE PACIFIC... WOULD TEND TO FAVOR THE
SLOWER SOLUTIONS -- ECMWF ENSEMBLES -- OVER THE QUICKER GEFS
MEMBERS. THE NAEFS MEAN CORRELATES WELL WITH THE ECMWF ENSEMBLE
MEAN GIVEN ITS BALANCING QUICKER VS MUCH SLOWER MEMBERS... AND
TRENDED THE FORECAST TO A 50/50 WEIGHTING BY NEXT TUESDAY.


...SENSIBLE WEATHER HIGHLIGHTS...

SOUTHERN/EASTERN SYSTEM LATE THIS WEEK INTO THE WEEKEND SHOULD AT
LEAST BRING WET AND SOME WHITE CONDITIONS TO THE
MID-SOUTH/SOUTHEAST INTO THE CENTRAL/SOUTHERN APPALACHIANS AND
MID-ATLANTIC. LACK OF ARCTIC AIR OVER THE CONUS SPELLS A MARGINAL
WINTER P-TYPE EVENT BUT POTENTIAL EXISTS FOR AT LEAST A FEW INCHES
ON THE N/NW SIDE OF THE PRECIP SHIELD AS IT ADVANCES EASTWARD.
TEMPERATURES IN THE WAKE OF THE SYSTEM SHOULD BE BELOW AVERAGE BY
5-10 DEG F.

WESTERN TROUGH AND SFC BOUNDARY LATE THIS WEEK WILL BRING A GOOD
1-3 INCHES OF QPF TO THE COASTAL RANGE AND THEN A SECOND SYSTEM
EARLY NEXT WEEK SHOULD PUSH THE PRECIP EASTWARD PAST THE DIVIDE
WITH A MORE ZONAL FLOW. DEPENDING ON HOW QUICKLY THE SOUTHERN
TROUGH MOVES EASTWARD OUT OF THE SW/TX RAIN SHOULD EXPAND IN AREAL
COVERAGE IN S TEXAS AND THE LOWER MS VALLEY NEXT TUESDAY.


FRACASSO