EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1247 AM EST SAT DEC 19 2015 VALID 12Z TUE DEC 22 2015 - 12Z SAT DEC 26 2015 ...OVERVIEW... ANOTHER DEEP PACIFIC TROUGH IS EXPECTED TO SETTLE IN ACROSS A BROAD PORTION OF THE WESTERN STATES---AND GENERATE A SUCCESSION OF FOUR-CORNERS TYPE MID-LEVEL SHORTWAVES---AND SENSIBLE WEATHER CHANGES FOR THE CONTINENTAL DIVIDE AND PLAINS STATES. THE DOWNSTREAM PATTERN AND EASTERN THIRD OF THE LOWER 48---REACTS TO THE UPSTREAM AMPLIFICATION---WITH DEEP-LAYERED RIDGING...HOLDING FIRM ACROSS THE SOUTHEAST AND DIRECTING PACIFIC AND CANADIAN SYSTEM MIGRATIONS ACROSS THE GREAT LAKES AND ST LAWRENCE VALLEY. A STORM TRACK THAT PRODUCES LIMITED OPPORTUNITY FOR COLD ADVECTION ---EAST OF THE DIVIDE UNTIL LATE IN THE FORECAST PERIOD. BUT DOES ALLOW FOR APPRECIABLE MOISTURE TRANSPORT FROM THE GULF OF MEXICO. ...MODEL PREFERENCES... THE FORECAST PACKAGE FOLLOWS THE 18/12Z ECENS/NAEFS AND DETERMINISTIC ECMWF --- HEADING INTO DAY 6. THE GFS IS A LITTLE BETTER NOW (COMPARED TO THE ECMWF/CANADIAN SOLUTION) WITH RESPECT TO THE LEAD PACIFIC 'FRONT' RACING OUT OF THE NORTH CENTRAL DIVIDE AND INTO THE CENTRAL PLAINS (DAY 3-4). BUT SEEMS TO BE TOO 'INDECISIVE' WITH THE LEADING EDGE OF THE COLD ADVECTION MOVING SOUTHWARD ALONG THE PACIFIC NORTHWEST COAST AND THROUGH SOUTHEAST CANADA. ...UNCERTAINTY ASSESSMENT... STILL FAVOR THE 18/12Z CANADIAN/ECMWF SOLUTIONS CONCERNING THE LEAD PACIFIC SYSTEM TRACKING FROM BRITISH COLUMBIA TO NORTHERN MINNESOTA BETWEEN 22/12Z AND 24/12Z. IN THAT...THIS IS A LEAD SYSTEM WITH SOME ENERGY AND ARCTIC AIR --- ORIGINATING IN THE EASTERN GULF OF ALASKA --- BUT NOT THE REAL DEEP AND SIGNIFICANT CORE OF COLD ARCTIC AIR --- WHICH WILL SURGE DOWN ALONG 130W (THE WEST COAST) IN "SHEETS". THE GEFS/GFS SOLUTION HAS MAINTAINED A FLATTER...MORE PROGRESSIVE APPROACH ACROSS SOUTHWEST CANADA--- AND IT SEEMS TO BE ADJUSTING FOR IT --- WITH RATHER INCONSISTENT AND STRUCTURAL DIFFERENCES WITHIN ITS 500MB TROUGH SOLUTION OVER THE MIDWEST. SIMILARLY...THE LOW-LEVEL COLD ADVECTION MIGRATION OUT OF THE ROCKIES DAY 3-4 ---AND ACROSS THE EASTERN PORTION OF NORTH AMERICA AT DAY 5 AND BEYOND --- SEEMED TO BE PLAYING CATCH UP FROM ITS PREVIOUS TWO DAYS. WHAT THE PATTERN WILL LOOK LIKE AFTER DAY 5...IS THE CORE OF THE COLD AIR ALOFT (700MB AND ABOVE) WILL CONTINUE TO CARVE OUT ACROSS THE WEST COAST AND INTO THE SOUTHERN GREAT BASIN. THE LEAD PACIFIC SYSTEM TRYING TO 'SURVIVE' DOWNSTREAM ACROSS THE NORTHERN PLAINS WITH LITTLE SUPPORT ALOFT---CAUSING IT TO SHEAR OUT BUT NOT NECESSARILY WEAKEN---AND MIGRATE ACROSS THE GREAT LAKES. THIS SYSTEM MERGING DOWNSTREAM WITH COLDER FLOW PASSING ACROSS CENTRAL CANADA. THE END RESULT...A BLUSTERY WEST FLOW FOR THE ST LAWRENCE VALLEY AND PORTIONS OF NEW ENGLAND AND THE GREAT LAKES (WHERE MEANINGFUL COLD ADVECTION IS POSSIBLE). WHAT IS VERY IMPRESSIVE FOR THIS PATTERN --- THE DEPTH OF THE AIRMASS SEEPING INTO THE PACIFIC NORTHWEST...CALIFORNIA AND THE GREAT BASIN --- WILL COMPRISE THE NEXT UPPER-LEVEL TROUGH --- AND CONSOLIDATED 'FOUR-CORNERS' SYSTEM. A M12-15C TEMP FORECAST AT 700MB --- FOR A DAY 6 PROG --- SIGNALS LOW-ELEVATION SNOWFALL POTENTIAL. THE ONE UNANSWERED QUESTION---AND THERE IS TIME FOR ADJUSTMENT HERE--- TIMING THE EJECTION OF THE PRIMARY LOW AND BULK OF THIS ENERGETIC COLD CORE DOWNWIND OF THE SOUTHERN ROCKIES. UNTIL THEN...SIGNIFICANT AIRMASS INTRUSION AND POTENTIAL SENSIBLE WEATHER-MAKER FOR THE GREAT BASIN...SOUTHERN ROCKIES AND LOWER COLORADO BASIN. ...SENSIBLE WEATHER HIGHLIGHTS... THE WEST COAST WILL BE IN A VERY COLD AND UNSTABLE MARITIME POLAR AIRMASS FOR THE CHRISTMAS HOLIDAYS. TEMPERATURES WILL TREND MUCH COOLER THAN AVERAGE FOR MUCH OF THE REGION---WEST OF THE DIVIDE. PRECIPITATION WILL BE MOST PREVALENT ACROSS SOUTHWEST OREGON...NORTHERN CALIFORNIA AND THE SIERRA (WHERE IT IS NEEDED) ---WITH APPRECIABLE AMOUNTS IN THE COLORADO RIVER BASIN. THE SOUTH CENTRAL AND EASTERN STATES --- +10 TO +25F WHICH IS NEAR AND BEYOND DAILY RECORDS FOR DOZENS OF CITIES AND CLOSE TO MONTHLY EXTREMES. CLOUD COVER AND PRECIPITATION WILL FACTOR SOME IN REACHING MAXIMUM TEMPERATURE VALUES BUT HELP MAINTAIN 'MILD' OVERNIGHT LOW TEMPERATURES. A SECONDARY MAXIMUM OF PRECIPITATION IS ANTICIPATED ALONG THE GULF COAST AND SOUTHERN APPALACHIA. VOJTESAK