EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1101 AM EST SUN DEC 31 2017 VALID 12Z WED JAN 03 2018 - 12Z SUN JAN 07 2018 ...HEAVY SNOW LIKELY OVER NEW ENGLAND ON THURSDAY WITH CONTINUED FRIGID CONDITIONS EAST OF THE ROCKIES... ...PATTERN OVERVIEW... PERSISTENT UPPER TROUGHING WILL REMAIN A FIXTURE IN THE FORECAST ACROSS THE EASTERN U.S. WITHIN THIS NEGATIVE HEIGHT ANOMALY...A STRENGTHENING SOUTHERN STREAM FEATURE WILL IGNITE A ROBUST AREA OF LOW PRESSURE OVER THE WESTERN ATLANTIC WITH A TRACK REACHING NOVA SCOTIA BY LATE THURSDAY. IN ITS WAKE...STRONG COLD ADVECTION WILL BRING REINFORCING SHOTS OF ARCTIC TO AREAS EAST OF THE ROCKIES. THE MID/UPPER-LEVEL PATTERN MAY RELAX INTO THE END OF NEXT WEEKEND AS LOWER AMPLITUDE FLOW TRAVERSES TOWARD THE GREAT PLAINS. ACROSS THE WEST...AN UPPER RIDGE INITIALLY PARKED OVER THE REGION WILL GRADUALLY MOVE EASTWARD REACHING THE ROCKIES BY DAY 6/JANUARY 6. THIS IS IN RESPONSE TO LOWERING HEIGHTS ACROSS THE EASTERN PACIFIC AS MULTIPLE SYSTEMS TAKE AIM AT THE WEST COAST. THE INITIAL SHORTWAVE WILL SHEAR WHILE PUSHING INTO NORTHERN CA ON THURSDAY MORNING WHILE A MORE PRONOUNCED TROUGH LIKELY MOVES THROUGH AROUND FRIDAY/SATURDAY. THIS PACIFIC ENERGY IS FORECAST TO TRANSLATE EASTWARD WHICH ULTIMATELY HELPS DECREASE THE AMPLITUDE OF THE MID-LEVEL FLOW ACROSS THE U.S. ...GUIDANCE/PREDICTABILITY ASSESSMENT AND PREFERENCES... AMPLIFIED FLOW ACROSS THE EASTERN U.S. WILL CONTINUE THROUGHOUT MUCH OF THE MEDIUM RANGE PERIOD. A POTENT SHORTWAVE WITHIN THE SOUTHERN STREAM IS EXPECTED TO MOVE THROUGH THE FL PANHANDLE MIDDAY WEDNESDAY WITH EVENTUAL PHASING IN THE OFFSHORE WATERS. MULTI-DAY MODEL COMPARISONS AS WELL AS ANALYSIS OF THE ENSEMBLE LOW PLOTS SUGGEST A FAIRLY CONSISTENT FORECAST IN PLACE. THE HIGHEST PROBABILITY WOULD BE A CYCLONE TRACK INTO THE VICINITY OF NOVA SCOTIA BY THURSDAY EVENING/NIGHT. THERE IS ENOUGH SCATTER AMONG THE 90 ENSEMBLE MEMBERS THAT A PUSH TOWARD THE WEST OR EAST IS STILL POSSIBLE WHICH WOULD SIGNIFICANTLY CHANGE IMPACTS ACROSS THE NORTHEASTERN U.S. AS WAS THE ISSUE THE PAST FEW DAYS...THE 00Z UKMET CAME IN MUCH TOO SLOW WHILE THE 00Z CMC EVENTUALLY DEPICTS A PERSISTENT DUAL-LOW STRUCTURE BEGINNING THURSDAY EVENING. IN ITS WAKE...STRONG COLD ADVECTION WILL ENSUE AS THE NORTHERN STREAM TROUGH CROSSES THE GREAT LAKES/NEW ENGLAND. THIS SHOULD ULTIMATELY ENSURE A COLD END TO THE WEEK FOR LOCATIONS EAST OF THE ROCKIES. TOWARD THE END OF NEXT WEEKEND...A MIXED SIGNAL EXISTS IN THE GUIDANCE BUT MODELS DO SEEM TO AGREE ON A RELAXATION OF THE FLOW AS PACIFIC AIR MASSES MOVE INTO THE PICTURE. FOR DAYS 3/4...WEDNESDAY/THURSDAY...MODELS AGREE ON PUSHING THE WESTERN U.S. RIDGE FARTHER DOWNSTREAM AS HEIGHTS BEGIN TO LOWER ACROSS THE EASTERN PACIFIC. THE INITIAL SHORTWAVE WILL LIKELY LIFT NORTHEASTWARD INTO NORTHERN CA ON THURSDAY MORNING BEFORE QUICKLY SHEARING. THE GUIDANCE BEGINS TO STRUGGLE BEYOND THIS AS SPLIT FLOW SETS UP OVER THE NORTHEASTERN/EASTERN PACIFIC. BY 05/1200Z...THE 00Z UKMET SHOWS A POWERFUL TROUGH ACROSS NORTHERN ALBERTA WHICH ERODES MUCH OF THE INTERMOUNTAIN WEST RIDGE. ADDITIONALLY...THE 00Z/12Z ECMWF HAVE BEEN SEVERAL MILLIBARS STRONGER WITH THE WAVE MOVING TOWARD BRITISH COLUMBIA WHICH DID NOT HAVE A LOT OF ENSEMBLE SUPPORT AT THIS TIME. REGARDING THE LONGWAVE TROUGH PROGRESSION...THE ECMWF ENSEMBLE MEAN HAS BEEN THE MOST UNSTABLE AS IT CONTINUES TO SLOW RELATIVE TO PRIOR RUNS. HOWEVER...IT STILL SITS WAY AHEAD OF THE GLOBAL MEAN CONSENSUS. THIS DIFFERENT PROJECTS FORWARD INTO THE WEEKEND AS THE 00Z ECMWF ENSEMBLE MEAN IS OVER THE FOUR CORNERS REGION ON SUNDAY WHILE THE 06Z GEFS/00Z NAEFS ENSEMBLE MEANS FAVOR ZONAL FLOW. ENSEMBLE SPAGHETTI PLOTS CONSIST OF A LOT OF SPREAD WHICH DIMINISHES CONFIDENCE IN THIS PATTERN CHANGE. THROUGH DAY 4/THURSDAY...WAS ABLE TO UTILIZE A COMBINATION OF THE 06Z/00Z GFS WITH THE 00Z ECMWF/CMC WITH MORE WEIGHT TOWARD THE GFS/ECMWF SOLUTIONS. THEREAFTER...REMOVED THE 00Z CMC DUE TO THE DUAL-LOW STRUCTURE OVER THE WESTERN ATLANTIC WITH DECREASING CONTRIBUTIONS FROM THE 00Z ECMWF AS WELL. DURING THE DAY 5-7...JANUARY 5-7 PERIOD...PRIMARILY WENT WITH A COMBINATION OF THE 06Z/00Z GFS AND 06Z GEFS/00Z ECMWF/NAEFS ENSEMBLE MEANS. DID WEIGHT THE GEFS/NAEFS MORE HEAVILY TO ESTABLISH LOWER AMPLITUDE FLOW ACROSS THE SOUTHWESTERN U.S. LATE IN THE PERIOD. ...WEATHER HIGHLIGHTS/THREATS... MULTIPLE ARCTIC INTRUSIONS WILL REINFORCE THE FRIGID TEMPERATURES EAST OF THE ROCKIES. THE GREAT PLAINS COULD SEE SOME RELIEF BY NEXT WEEKEND AS THE UPSTREAM RIDGES SLIDES OVERHEAD. UNFORTUNATELY...BRUTALLY COLD TEMPERATURES SHOULD BE THE NORM ACROSS THE EASTERN U.S. ADDITIONALLY...GUSTY WINDS SHOULD LOWER WIND CHILL VALUES INTO THE UNCOMFORTABLE AND LIKELY DANGEROUS RANGES FOR ANY LONG-TERM EXPOSURES. THROUGHOUT THE FORECAST PERIOD. GENERALLY SPEAKING...EXPECTS HIGHS/LOWS WILL BE AROUND 20 TO 25 DEGREES BELOW CLIMATOLOGY WITH POSSIBLY SOME 30 DEGREE DEPARTURES MIXED IN. A NUMBER OF DAILY TEMPERATURE RECORDS WILL LIKELY BE BROKEN FOR BOTH OVERNIGHT LOWS AS WELL AS THE COLD MAXIMA. COMPARED TO FORECAST TEMPERATURES...THE DAY WHERE MANY RECORDS MAY BE BROKEN IS ON FRIDAY AS HIGHS STRUGGLE TO GET OUT OF THE SINGLE DIGITS AND TEENS ALONG THE I-95 CORRIDOR OVER NEW ENGLAND/MID-ATLANTIC. THIS ARCTIC BLAST SHOULD ALSO AFFECT THE SOUTHERN TIER OF THE U.S. AS OVERNIGHT READINGS DROP BELOW FREEZING DOWN ALONG THE ENTIRE GULF COAST INTO CENTRAL FL ON FRIDAY MORNING. ACROSS THE WESTERN HALF OF THE COUNTRY...MILD CONDITIONS ARE FORECAST TO PERSIST WITH ANOMALIES IN THE 10 TO 15 DEGREE RANGE. ABOVE NORMAL READINGS SHOULD REACH THE GREAT PLAINS BY NEXT WEEKEND. ACROSS THE EASTERN U.S...A POTENTIAL WINTER STORM IS ON THE HORIZON FOR NEW ENGLAND GIVEN THE FAVORABLE WESTERN ATLANTIC LOW TRACK. BOUNDARY LAYER TEMPERATURES SHOULD BE PLENTY COLD TO KEEP THIS AN ALL SNOW EVENT OVER ALL OF NEW ENGLAND AND POSSIBLY DOWN INTO THE COASTAL UPPER MID-ATLANTIC. ANY WESTWARD MOVEMENT OF FUTURE TRACKS WOULD LEAD TO POTENTIAL IMPACTS FARTHER WEST INTO THE NORTHEASTERN U.S. OTHERWISE...EXPECT LINGERING RAINFALL ACROSS FL AS THE FRONTAL ZONE EXITS THE REGION. OVER THE GREAT LAKES...PERSISTENT CYCLONIC WILL FAVOR CONTINUED LAKE EFFECT SNOW GENERATION. VERY COLD TEMPERATURES ALOFT SHOULD SIGNIFICANTLY STEEPEN LAPSE RATES LEADING TO FAIRLY IMPRESSIVE LOCAL UPDRAFTS. OUT WEST...THE MULTITUDE OF SYSTEMS MOVING INLAND WILL FAVOR ACTIVE PRECIPITATION CHANCES ALONG THE WEST COAST AND INTO THE INTERIOR DURING MUCH OF THE PERIOD. THIS WILL BRING WELL NEEDED RAINFALL TO SOUTHERN/CENTRAL WHICH HAS BEEN DRY MUCH OF ITS RAINY SEASON. EVENTUALLY SOME WINTRY PRECIPITATION WILL MOVE INTO THE HIGHER ELEVATIONS OF THE SIERRA AS WELL AS THE INTERIOR MOUNTAIN RANGES. RUBIN-OSTER