EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 200 AM EST MON DEC 25 2017 VALID 12Z THU DEC 28 2017 - 12Z MON JAN 01 2018 ...WIDESPREAD ICE/SNOW THREAT WITHIN ARCTIC AIRMASS OVER THE LOWER 48 THIS WEEK... OVERALL PATTERN AND GUIDANCE/PREDICTABILITY ASSESSMENT ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ IT REMAINS THE CASE THAT SYSTEMS MOVING IN FROM THE MID-LATITUDES OF THE PACIFIC ALONG WITH SOME 500 HPA RIDGE STRENGTHENING AND RETROGRESSION IN THE SUBTROPICS ARE EXPECTED TO BROADEN/WEAKEN OTHERWISE PERSISTENT RIDGING NEAR WESTERN NORTH AMERICA...THOUGH THE RIDGE'S EXISTENCE IS SUPPORTED BY THE UPSTREAM GYRE ALOFT WHICH STRENGTHENS/RETROGRADES INTO THE BERING SEA WITH TIME. DEEP CYCLONIC FLOW ALOFT DOMINATES THE PATTERN FROM THE INTERMOUNTAIN WEST TO THE ERN SEABOARD...WITH LITTLE CHANGE EXPECTED AS ADDITIONAL SHORTWAVES HELP ASSERT REINFORCING SHOTS OF ARCTIC AIR TO THE REGION. DETERMINISTIC MODEL RUNS CONTINUE TO OFFER POOR RUN-TO-RUN AND RANDOM DAY-TO-DAY CONTINUITY WITH THE OVERALL FLOW IN THE 4-7 DAY TIME FRAME...PARTICULARLY WITH SMALL SCALE SHORTWAVES...THOUGH ENSEMBLE MEAN SOLUTIONS REMAIN CONSISTENT. DESPITE THE JUMPINESS OF THE DETERMINISTIC GUIDANCE...FORECAST SPREAD HAS BEEN GRADUALLY DECREASINGING OVER THE LAST COUPLE OF DAYS. THE BIGGEST ISSUES CONTINUE TO LIE WITH A LOW EXPECTED TO FORM OFF THE EAST COAST HEADING INTO THE NEW YEARS WEEKEND WHICH TRACKS INTO/NEAR ATLANTIC CANADA BY AROUND NEXT SUNDAY AND A SYSTEM MOVING TOWARDS BRITISH COLUMBIA AND THE PACIFIC NW FRIDAY INTO SATURDAY THROUGH A LEAD MID-UPPER LEVEL RIDGE POSITION. UPDATE...THE 00 UTC ECMWF HAS TRENDED MORE IN LINE WITH RECENT GFS RUNS WITH THE ERN PACIFIC FEATURE MOVING INTO BC AND THE PAC NW AND DOWNSTREAM ALSO TRENDS TOWARD A MORE SUPPRESSED LOW/QPF/WINTER WEATHER THREAT AS WELL. THE EARLIER RELEASED WPC MEDIUM RANGE PRODUCT SUITE WAS PRIMARILY DERIVED FROM A COMPOSITE BLEND OF THE GEFS/NAEFS/ECMWF ENSEMBLES ALONG WITH CONTINUITY. THAT SOLUTION OFFERED LESS AMPLITUDE/MORE PROGRESSION THAN THE 12 UTC ECMWF...BUT MAY HAVE TO REVISITED IF THE UPCOMING MODELS AND ENSEMBLES CONTINUE THE TRENDS OF THE 00 UTC ECMWF. THE UPCOMING QPF AND WINTER WEATHER OUTLOOK PROGS WILL SHOW THIS TREND. WEATHER/THREATS HIGHLIGHTS ~~~~~~~~~~~~~~~~~~~~~~~~~~ THE SPECIFICS AND LONGER TERM RAMIFICATIONS/TRANSITIONS WITHIN THIS WINTRY PATTERN INCLUDING ICE/SNOW THREATS REMAIN A QUESTION MARK. THE WPC MEDIUM RANGE WINTER WEATHER OUTLOOK AND WEATHER GRIDS SHOW A FOCUS FOR HEAVIER SNOW PROBABILITIES THROUGH MID-LATER WEEK OVER THE NORTHWEST/NORTHERN ROCKIES AND TO THE LEE OF THE GREAT LAKES ALONG WITH AN EMERGING WIDESPREAD THREAT THAT DEVELOPS ACROSS THE SOUTHEAST AND EAST. THE POTENTIAL FOR FRIGID HIGH PRESSURE AND POTENTIALLY ACCUMULATING FREEZING RAIN OVER AR, GA, AND THE CAROLINAS THURSDAY INTO FRIDAY MORNING WITH SNOW FARTHER INTO THE COLD AIR FROM THE MID-SOUTH AND MID-ATLANTIC STATES NORTHEASTWARD THROUGH NEW ENGLAND ASSOCIATED WITH SHORTWAVE PASSAGES AND ANY POSSIBLE TROWAL/NORLUN TROUGH-LIKE FEATURES TO THE NORTHWEST OF THE LOW DEVELOPMENT OFFSHORE THE EAST COAST. ROTH/SCHICHTEL