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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0426Z Dec 18, 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
1126 PM EST WED DEC 17 2014

VALID 12Z SUN DEC 21 2014 - 12Z THU DEC 25 2014

...STRONG SYSTEM POSSIBLE IN THE EAST FOR CHRISTMAS EVE/DAY...


..OVERVIEW AND GUIDANCE PREFERENCES...

AMPLIFIED PATTERN SET TO RETURN TO THE US AS THE STRONG W PACIFIC
JET TRANSLATES DOWNSTREAM THIS WEEKEND INTO THE PAC NW. BUILDING
HEIGHTS OVER SE CANADA SHOULD ALSO HELP DIG THE TROUGH INTO THE
OHIO VALLEY MIDWEEK AND LIKELY CLOSE OFF AS IT SWINGS INTO
SOUTHERN CANADA ON CHRISTMAS DAY. MODELS/ENSEMBLES HAVE BEEN
FAIRLY CONSISTENT IN RECENT RUNS ON THIS AMPLIFIED PATTERN AND THE
MOST RECENT GEFS/ECMWF ENSEMBLE GUIDANCE SHOW GOOD CORRELATION TO
NEXT THURSDAY. ONGOING DIFFERENCE BETWEEN THE TWO IS A TYPICAL
ONE... WITH THE GEFS MEMBERS A BIT QUICKER WITH THE PATTERN THAN
THE ECMWF MEMBERS. AMPLIFIED PATTERN WOULD TYPICALLY FAVOR THE
SLOWER /ECMWF-LIKE/ SOLUTIONS BUT UPSTREAM PACIFIC ENERGY MAY BE
ENOUGH OF A KICKER TO KEEP THINGS MOVING. BLEND OF THE GUIDANCE...
GFS/ECMWF/GFS PARALLEL AND THEIR MEANS... SHOULD PROVE GOOD ENOUGH
BUT OPTED TO USE THE BETTER CLUSTERED 12Z ECMWF AND 18Z PARALLEL
GFS WITH THE ECMWF ENSEMBLE MEAN WHICH OFFERED A BIT MORE DETAIL.
HOWEVER... DESPITE THE GOOD AGREEMENT THE MODELS ALWAYS FIND A WAY
TO CHANGE THE DETAILS... WHICH WAS ONE REASON FOR TRENDING TOWARD
THE ECMWF ENSEMBLE MEAN BY THURSDAY. MONTHLY RECORD LOW SEA LEVEL
PRESSURE RECORDS COULD BE CHALLENGED IN SOME LOCATIONS OF THE
CENTRAL GREAT LAKES -- WHICH ARE NEAR 970 IN THE UP OF MICHIGAN TO
THE LOW 980S IN N OHIO. DEEPEST ENSEMBLE MEMBERS ARE IN THE 950S
MB RANGE WHICH IS IMPRESSIVE.

ALSO BY LATE IN THE PERIOD IS UPSTREAM KICKER ENERGY THAT MAY DIVE
DOWN INTO THE WEST. 12Z ECMWF REPRESENTS ABOUT THE STRONGEST SIDE
OF THE ENSEMBLE SPREAD. GIVEN THE FACT THAT THE SPAGHETTI CHARTS
OVER THE PACIFIC LOOK LIKE ABSTRACT ART... A TREND TOWARD THE
MEANS /ECMWF ENSEMBLE MEAN IN THIS CASE/ WORKED WELL AT BOTH ENDS
OF THE CONUS.


...SENSIBLE WEATHER HIGHLIGHTS...

OFF THE EAST COAST... THE PESKY SYSTEM THAT REFUSES TO BE
WELL-FORECAST MAY VERY WELL TAKE ITS TIME EXITING INTO THE
ATLANTIC. MODELS HAVE TRENDED TOWARD THIS WEAKER SOLUTION OVER THE
PAST SEVERAL DAYS AND THE LOW LEVEL BAROCLINIC ZONE MAY NOT BUDGE
PAST THE GULF STREAM AS THE FLOW TURNS SOUTHWESTERLY/PARALLEL TO
THE FRONT. SOME LIGHT RAIN/SNOW IS STILL POSSIBLE BACK INTO THE
MID-ATLANTIC/NORTHEAST UNTIL IT PUSHES NORTHEASTWARD AND
DISSIPATES MIDWEEK.

DEVELOPING SYSTEM IN THE CENTRAL/EASTERN CONUS DEC 23-24 MAY STEAL
THE HEADLINES GIVEN ITS TIMING. MOST PRECIP WILL BE LIQUID WITH A
DEEP SUBTROPICAL CONNECTION BACK ACROSS MEXICO... PW ANOMALIES
FORECAST TO RISE TO BETWEEN +1 TO +3 /OR HIGHER/... AND
TEMPERATURES WELL ABOVE AVERAGE. AS THE SYSTEM WRAPS UP INTO
CANADA... COLD AIR ADVECTION SHOULD CIRCULATE AROUND THROUGH THE
LAKES AND OHIO VALLEY EASTWARD ON WEST TO SOUTHWEST FLOW... FIRING
UP THE LAKE MACHINE.


FRACASSO