EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1059 AM EST THU NOV 13 2014 VALID 12Z SUN NOV 16 2014 - 12Z THU NOV 20 2014 ...COLD PATTERN WILL CONTINUE ACROSS THE LOWER 48... ...OVERVIEW... THE UPPER-LEVEL RIDGE/TROUGH PATTERN OVER CANADA AND ALASKA WILL SUSTAIN AND MAINTAIN THE NORTH AMERICAN ARCTIC AIRMASS THROUGH THIS MEDIUM RANGE PERIOD. THE PERSISTENCE OF THE PATTERN SPREADS THE MODIFIED-ARCTIC AIRMASS DEEP INTO SOUTHERN STATES...FLORIDA...THE RIO GRANDE VALLEY AND SOUTHERN GREAT BASIN. THE DE-AMPLIFICATION OF THE UPPER-LEVEL PATTERN ALONG THE WEST COAST PRODUCES AN ACTIVE PACIFIC JET AND A GRADUAL EQUATORWARD MIGRATION OF THE SENSIBLE WEATHER PATTERN INTO THE SOUTHWEST QUARTER OF THE NATION FOR DAYS 6-7. ...UNCERTAINTY ASSESSMENT... IT CANNOT BE OVERSTATED HOW IMPORTANT THE SUSTAINED PRESENCE OF ARCTIC AIR AND THE REPLENISHMENT OF THE AIRMASS DURING THIS MEDIUM RANGE PERIOD WILL BE---TO SETTING UP A WINTRY P-TYPE PATTERN ALONG ITS OUTER FRINGES---IN REGIONS OF THE DEEP SOUTH AND GREAT BASIN/LEE OF THE CASCADES. IN THE WEST...THE UNCERTAINTY AND FORECAST CHALLENGE IS THE ABILITY OF ORGANIZED ONSHORE FLOW---AT LOW LEVELS AND AT JET LEVEL---TO SCOUR OUT THE DRY/COLD CONTINENTAL AIRMASS DRAPE...EXPANDING OVER A GREATER SURFACE AREA OF THE WESTERN STATES. IN THE SOUTH...THE CHALLENGE WILL BE THE ABILITY OF THE JET-LEVEL WINDS EMBEDDED WITHIN THE BASE OF THE EASTERN NORTH AMERICAN TROUGH...TO PROMOTE LIFT AND ELEVATE THE PROSPECTS FOR A WINTER P-TYPE REGIME FROM THE SOUTHERN PLAINS TO THE GULF COAST. ...MODEL PREFERENCES... THE CANADIAN...ECMWF...GFS AND UKMET 13/00Z CYCLE AND THE ECENS...GEFS...NAEFS REASONABLY DEPICT THE MASS FIELDS THROUGH DAY 5. THE SUBTLE DIFFERENCES BETWEEN DETERMINISTIC RUNS VERSUS THE ENSEMBLE MEANS---FOCUS ON TIMING OF THE PRIMARY AND SECONDARY MID-LEVEL WAVE ENERGY TRANSPORT---ALONG AN ACTIVE UPPER-LEVEL JET AXIS. FOR THE DAY 5-7 TIME FRAME...A 50/30/20 BLEND OF THE 13/00Z ECENS-NAEFS-GEFS RESPECTIVELY SHOULD YIELD GOOD RESULTS FOR THE APPROXIMATE POSITION/ALIGNMENT OF THE POLAR JET (564DM HEIGHT LINE) AND THE ARCTIC JET (522DM HEIGHT CONTOUR) ACROSS THE NORTH AMERICAN CONTINENT. EARLY IN THE PERIOD---THE 500MB TROUGH AXIS AND ACCELERATED JET-LEVEL WINDS ADVANCING AHEAD OF THE TROUGH...MIGRATE EASTWARD AND NORTHEASTWARD ACROSS THE EASTERN STATES---DOWNWIND OF THE MISSISSIPPI VALLEY (ALONG 90W ON TUESDAY/DAY5). THE SURFACE FEATURE/WAVE TRACKING FROM THE MS DELTA TO THE OUTER BANKS SHOULD USHER IN THE 'NEW' ARCTIC BOUNDARY ACROSS THE GREAT LAKES AND INTO THE SOUTHEAST...WITH PRECIPITATION WHISKING RAPIDLY INTO THE WESTERN ATLANTIC BASIN BY DAY 5. THE DETAILS ASSOCIATED WITH THE DETERMINISTIC 13/00Z ECMWF MIGHT PROVIDE THE BETTER "MIDDLE GROUND" DETAILS WITH THE THE ENTIRE TROUGH AXIS CARVING OUT FROM THE NORTHERN HIGH PLAINS TO THE LOWER MS VALLEY---VERSUS A SLIGHTLY FASTER CANADIAN AND SLIGHTLY SLOWER GFS DETERMINISTIC SOLUTION BEYOND DAY 5. IN PARTICULARLY...THE TIMING OF THE EMBEDDED MID-LEVEL WAVE ENERGY IN COMBINATION WITH THE LOW-LEVEL THERMAL PROFILES WILL BE CRITICAL TO THE SENSIBLE WEATHER IMPACTS ACROSS KEY AREAS---INCLUDING THE GREAT LAKES (BIG LAKE-EFFECT SNOWS)...THE FLORIDA PENINSULA/THE NORTHERN GULF OF MEXICO (CONVECTION OVER OPEN WATERS MOVING ATOP A COLD AIRMASS) AND EASTERN MONTANA/WYOMING/COLORADO AND NORTHEAST TEXAS (STREAKS OF MID-LEVEL MOISTURE AND FRONTOGENESIS). NOT A PERFECT PROG...BUT HELPFUL WITH THE VERY SUBTLE...BUT WINTRY P-TYPE DETAILS ABOVE/WITHIN THE ARCTIC AIRMASS---AT THIS POINT IN TIME. IN THE WEST...FLOW PATTERN ALOFT AND PRESENCE OF AN ACTIVE NORTH CENTRAL PACIFIC JET UPWIND OF THE UPPER-LEVEL RIDGE ALONG WESTERN CANADA'S HIGH TERRAIN CREATES A VERY CHALLENGING 'SPLIT-FLOW/NEGATIVE-TILT' CONFIGURATION THAT INITIALLY PRODUCES THE THREAT OF OVER-RUNNING MOISTURE FOR WASHINGTON...OREGON...IDAHO. THEN ITS MIGRATION SOUTHWARD DOWN THE COAST...INTO CENTRAL-SOUTHERN CALIFORNIA...THE SIERRA AND DOWNSTREAM INFLUENCES ACROSS NEVADA CREATES SOME TIMING DIFFERENCES. SOMETHING MORE---DETERMINISTIC CANADIAN-ECMWF "LIKE" WOULD BE RECOMMENDED TO BETTER HANDLE THE TRICKY OFFSHORE-ONSHORE TRANSITION AT THE SURFACE AND ALOFT. THIS PRESUPPOSITION INCLUDES THE SOUTHWARD TRAJECTORY AND MIGRATION OF A MODERATELY-INTENSE MID-LEVEL SHORTWAVE ALONG THE CASCADE/SIERRA SPINE EARLY IN THE PERIOD. ITS PRESENCE REINFORCING THE CONTINENTAL AIRMASS INTO DAY 4-5...AND DELAYING THE EASTWARD-ADVANCING PACIFIC TROUGH AND ITS CHARACTERISTICS/CONFIGURATION---IE CUTOFF AND/OR OPEN-WAVE. ...SENSIBLE WEATHER HIGHLIGHTS... A VERY PROLONGED PERIOD OF LOCAL LAKE-EFFECT SNOWFALL EVENTS WILL PREVAIL AT VARIOUS TIMES ACROSS ALL 5 GREAT LAKES. WITH SIGNIFICANT SNOWS ALREADY PRESENT...HARD TO IMAGINE HOW MUCH MORE SNOWFALL IS POSSIBLE...PROLIFIC? NUMBERS ARE POSSIBLE. ANOMALOUS? CERTAINLY. THE PERSISTENCE OF THE COLD AIR...EMBEDDED SHORTWAVE ENERGY AND RELENTLESS CYCLONIC FLOW (THE SYNOPTIC-SCALE PARAMETERS IN PLACE) THROUGHOUT THE ENTIRE DAY 4-7 PERIOD TO GIVE EACH LAKE AND THEIR SHORELINES THE THREAT OF UNUSUALLY-HIGH AMOUNTS OF ACCUMULATION. OBVIOUSLY...THE SHORT-TERM WIND DIRECTION WILL PLAY A CRITICAL ROLE. THE VERY DRY AIRMASS WILL LIMIT THE PRECIPITATION EFFICIENCY AND INTENSITY OF LAKE-EFFECT DOWNSTREAM---WITH FAVORED WINDWARD SLOPES AND LOCAL OROGRAPHIC FEATURES PERPENDICULAR TO THE FLOW...BEARING THE BRUNT OF THE LAKE-EFFECT PROCESS. WELL-BELOW TEMPERATURES SHOULD CONTINUE FOR THE EASTERN HALF OF THE NATION... RANGING FROM 10-25F BELOW AVERAGE FOR THE MIDWEST AND PLAINS. THE MODIFIED...BUT STILL THE AIRMASS WILL PRODUCE VERY UNCOMFORTABLE AND CHILLY MORNINGS FOR THE GULF COAST AND FLORIDA. SECONDARY SURGE OF CANADIAN AIR---WEST OF THE DIVIDE AND INTO THE PACIFIC NORTHWEST/GREAT BASIN IN THE DAY 3-4 PERIOD 'SETS UP' AN ENVIRONMENT CONDUCIVE FOR LOW SNOW LEVELS AND THE THREAT OF LIGHT FREEZING DRIZZLE/RAIN. THE DEPTH OF AN ONSHORE FLOW ALONG THE WEST COAST...AND ACTIVE 'SCOURING PROCESS' OF THE MODIFIED-ARCTIC AIRMASS IN PLACE...WILL BE A LOCALLY-SENSITIVE SENSIBLE WEATHER FEATURE THROUGH DAY 6. VOJTESAK