EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1059 AM EST SUN DEC 21 2014 VALID 12Z WED DEC 24 2014 - 12Z SUN DEC 28 2014 ...ACTIVE CHRISTMAS HOLIDAY WEATHER PATTERN... ...OVERVIEW... A LARGE-SCALE AND RATHER INTENSE CYCLONE CARVES OUT ACROSS SOUTHEAST CANADA DURING THE 2-DAY CHRISTMAS PERIOD---AFFECTING A BROAD AREA OF EASTERN NORTH AMERICA---WITH A WET AND WINDY WARM SIDE...AND A SHORTER-DURATION POST-FRONTAL COOL SIDE---INCLUDING A WINTRY MIX ACROSS THE GREAT LAKES. ACROSS THE PACIFIC NORTHWEST AND INTERMOUNTAIN WEST...A TROUGH MIGRATES MOISTURE AND COOLER TEMPERATURES DURING THE DAY 3-4 PERIOD...AND EJECTS ACROSS THE PLAINS AND CENTRAL MIDWEST/LOWER MISSOURI VALLEY ON DAYS 4-5. THIS SYSTEM SHOULD GENERATE A SWATH OF WINTER WEATHER ALONG ITS NORTHERN PERIPHERY AND USHER IN A COLDER AIRMASS BEHIND ITS FRONTAL PASSAGE---EXTENDING FROM THE SOUTHERN HIGH PLAINS TO THE CENTRAL OHIO VALLEY BY DAY 7. ...MODEL UNCERTAINTY... THE LAKE HURON/WEST CENTRAL ONTARIO CYCLONE DETAILS EARLY IN THE PERIOD REMAIN AN ENERGY-TRANSFER FORECAST CHALLENGE...INVOF THE WESTERN OHIO VALLEY NORTHWARD INTO SOUTHWEST ONTARIO---AND WILL EXTEND TO THE MESOSCALE---IN THE COMING DAYS. THE LOUISVILLE-DETROIT CORRIDOR CONTINUES TO BE THE REGION WHERE THERE IS A HIGH DEGREE OF VOLATILITY. WITH NEGATIVE-TILT TROUGH SCENARIOS...THE EXACT LOCATION OF THE TRIPLE-POINT SEEMS TO BE THE ROOT CAUSE OF LOCALLY-ACTIVE WEATHER CHANGES AND CHARACTERISTICALLY VARIES FROM RUN-TO-RUN. OVERALL...THE SYNOPTIC-SCALE FEATURES AND MASS FIELDS REMAIN ON A RATHER CONSISTENT COURSE---SWEEPING SOUTHWEST TO NORTHEAST ACROSS THE EASTERN THIRD OF THE NATION---DAY 3-4. A GREAT DEAL OF HIGH-LATITUDE NORTH AMERICA 'RESHUFFLING' TAKES PLACE THIS MEDIUM RANGE PERIOD---WITH THE DEEP CYCLONE IN ONTARIO WRAPPING AND FILLING THROUGH TIME. ENSEMBLE SPREAD BEGINS TO INCREASE DRAMATICALLY---AT THE SURFACE AND ALOFT--- OVER LABRADOR/THE DAVIS STRAIT AND IN THE NORTHERN ROCKIES...IN RESPONSE TO THE DEGREE OF AMPLIFICATION AND TRAJECTORY OF THE DAY3-4 CYCLONE MIGRATING INTO NORTHEAST CANADA. IN THE WAKE OF THE DEPARTING SYSTEM...THE FLOW ACROSS THE LOWER 48 AND SPREADS OVER THE ROCKIES PERTAINS TO THE MID-LEVEL TROUGH AND HEIGHT FALLS DIGGING AND MIGRATING THROUGH THE GREAT BASIN. FOR THE PACIFIC MID-LEVEL TROUGH MIGRATION INTO THE PLAINS--- THE 21/00Z GEFS/GFS SOLUTION WOULD SERVE AS THE NORTHERN ENVELOPE SOLUTION ACROSS WYOMING/SRN MONTANA WITH THE INITIAL HEIGHT FALLS EJECTING EAST OF THE CONTINENTAL DIVIDE AND THE UPPER HALF OF THE MISSOURI VALLEY. THE ENERGY ALOFT APPARENTLY PHASING WITH ENERGY OVER SOUTH CENTRAL CANADA. THE EC/CMC AND UKMET SOLUTIONS MAINTAINING SEPARATION---WITH THE UKMET THE SOUTHERN---MOST INTENSE SOLUTION WITH ITS ENERGY HANGING BACK OVER THE FOUR CORNERS BETWEEN 26/00Z AND 27/00Z. THIS CAMP WOULD SUGGEST THE POTENTIAL FOR SOME ADDITIONAL OVER-RUNNING PRECIPITATION EVENTS ALONG THE I-40 CORRIDOR BEYOND DAY5---IN THE POST-FRONTAL 'WAKE' E EC/CMC SOLUTION IS PREFERRED /U ...MODEL PREFERENCES... MAINTAIN THE ECENS/ECMWF AND ITS 21/00Z CYCLE SOLUTION THROUGH FRIDAY (27/00Z) FROM THE PLAINS EASTWARD INTO ATLANTIC CANADA. THIS WILL ALLOW MID-LEVEL ENERGY TO STREAK ACROSS THE COLORADO/WYOMING BORDER AND CONSOLIDATE A SURFACE WAVE ALONG THE MID-MS VALLEY/OZARKS ON FRIDAY. THE SYSTEM ORIGINATES AS A LEE SIDE LOW RATHER THAN A CLOSED VERTICALLY-STACKED SYSTEM---SO THE FRIDAY/SATURDAY FORECAST HAS THIS CONSOLIDATED ENERGY STREAKING NORTHEASTWARD ALONG/NORTH OF THE OHIO VALLEY AND ST LAWRENCE VALLEY...RATHER THAN CENTRAL UPPER GREAT LAKES---WHICH IS THE 21/00Z GEFS/GFS SOLUTION. I DO NOT WANT TO MINIMIZE THE SENSIBLE WEATHER IMPACTS---BUT RATHER WANT TO IMPLY---THAT THERE IS 'ROOM' FOR ADDITIONAL IMPULSES TO MIGRATE ATOP THE COLD FRONT---AFTER THE INITIAL SYSTEM EXITS THE ROCKIES. TO ACCOUNT FOR THIS--- THE CURRENT MEDIUM RANGE PACKAGE UTILIZED A 21/00Z ECENS/NAEFS WEIGHTED BLEND BEYOND THE 27/00Z PERIOD. THIS HAS THE EFFECT OF---INCREASING THE POPS ALONG THE PACIFIC NORTHWEST AND PIEDMONT/OUTER BANKS FOR DAYS 6-7. THIS IS A PURELY A FUNCTION OF THE 'WET' MEMBERS---A MINORITY AT THIS TIME SCALE. THE BROAD-BRUSH APPROACH AFTER 27/00Z ACCOUNTS FOR THE PROJECTED WEST-NORTHWEST FLOW ALOFT FROM THE CASCADES TO THE OZARKS...AND THE FILLING OF THE DEEP SYSTEM IN EASTERN CANADA. ...SENSIBLE WEATHER HIGHLIGHTS... THE DEEP SURFACE LOW OVER CENTRAL ONTARIO/INVOF EASTERN SHORELINE OF LAKE HURON AND INTENSE 'WRAP-AROUND' SYNOPTIC-SCALE FLOW ACROSS NEW ENGLAND ON CHRISTMAS EVE ALLOWS FOR MILD WARM-SECTOR AIR TEMPERATURES TO SPREAD NORTHWARD ACROSS THE MID-ATLANTIC STATES---WHERE HIGHS/LOWS ARE FORECAST TO BE WELL ABOVE NORMAL. THERE SHOULD BE A WELL-DEFINED 'TROWAL' FEATURE ACCOMPANYING THE BROAD CYCLONE...BUT GENERALLY THIS PERTAINS TO THE UPPER GREAT LAKES WITH RESPECT TO SENSIBLE WEATHER. THE EXITING PACIFIC TROUGH AND HEIGHT FALLS WILL LIKELY 'STREAK OUT' AN ORGANIZED PRECIPITATION SHIELD ACROSS THE PLAINS AND MID-MISSOURI VALLEY---WHICH MIGRATES ACROSS THE REGION---IN THE DAY 4-5 TIME FRAME. VOJTESAK