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475
FXUS02 KWNH 220700
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
159 AM EST MON DEC 22 2014

VALID 12Z THU DEC 25 2014 - 12Z MON DEC 29 2014

...PATTERN/WEATHER HIGHLIGHTS AND GUIDANCE/UNCERTAINTY
ASSESSMENT...

AN ACTIVE PATTERN PERSISTS OVER THE LOWER 48 STATES FOR THE NEXT
WEEK. DESPITE A REASONABLY SIMILAR LARGER SCALE FLOW
REGIME...DETERMINISTIC MODELS HAVE BECOME INCREASINGLY VARIED WITH
THE TIMING/DEPICTION OF MANY OF THE EMBEDDED SYSTEMS. ACCORDINGLY
PREFER TO INCREASINGLY PUNT MEDIUM RANGE SPECIIFCS OVER TIME AND
USE AN ENSEMBLE MEAN APPROACH WITH A BLEND OF THE GFS/ECMWF
ENSEMBLE MEANS...LEANING MORE WEIGHT ON THE ECMWF MEAN TO MAINTAIN
CLOSER WPC CONTINUITY.

IN THIS SCENARIO IT REMAINS FAIRLY EVIDENT THAT AT LEAST THREE
DEFINED MID-UPPER LEVEL TROUGHS AND ASSOCIATED/UNSETTLING LEAD
SURFACE LOW/FRONTAL SYSTEMS AND PCPN SWATHS PROGRESS STEADILY
ACROSS THE LOWER 48. THE MOST AMPLIFIED/DEEP LEAD SYSTEM EXITS
FROM THE ERN/NERN US INTO ERN CANADA ON CHRISTMAS ALONG WITH A
LINGERING SNOW THREAT FROM THE GREAT LAKES TO THE NRN INTERIOR
NERN US.

MEANWHILE...A SECOND SYSTEM DIGS SEWD INTO THE N-CENTRAL GREAT
BASIN/ROCKIES ON CHRISTMAS. SUBSEQUENT ENERGY TRANSFER AND WAVY
SYSTEM GENESIS OCCURS OVER THE CENTRAL TO ERN US FRI/SAT THAT MAY
SUPPORT A MODEST SWATH OF OVERRUNNING SNOW FROM THE N-CENTRAL US
TO UPPER MIDWEST/GREAT LAKES AND NRN NEW ENG.

A THIRD SYSTEM OFFERS MUCH GREATER FORECAST SPREAD/UNCERTAINTY BUT
BASED ON OUR ENSEMBLE BLEND DIGS SEWD OVER THE NW US AND N-CENTRAL
INTERMOUNTAIN WEST/ROCKIES THIS WEEKEND. THIS WOULD PROLONG AN
ACTIVE FLOW PATTERN INCLUDING TERRAIN FOCUSING SNOW THREATS. SOME
FORM OF THIS SYSTEM MAY REACH THE S-CENTRAL US BY EARLY NEXT WEEK
TO FORCE A RETURN FLOW AND PCPN RESPONSE BUT EMPHASIS IS TO BE
DETERMINED.

SCHICHTEL

$$





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