EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1105 AM EST SAT DEC 30 2017 VALID 12Z TUE JAN 02 2018 - 12Z SAT JAN 06 2018 ...ARCTIC AIR SPREADING EAST OF THE ROCKIES WILL MAINTAIN WELL BELOW NORMAL TEMPERATURES... ...PATTERN OVERVIEW... AN UPPER TROUGH ANCHORED OVER THE EASTERN HALF OF THE COUNTRY SHOULD MAINTAIN ITS PRESENCE INTO THE FIRST WEEK OF THE NEW YEAR. 500-MB HEIGHT DEPARTURES AT TIMES WILL DIP BELOW 2 STANDARD DEVIATIONS BELOW AVERAGE FOR EARLY JANUARY. INITIAL SPLIT FLOW OVER THE CENTRAL/EASTERN U.S. SHOULD LEAD TO EVENTUAL PHASING OVER THE WESTERN ATLANTIC SPAWNING A POWERFUL LOW TAKING AIM AT NOVA SCOTIA AND NEW BRUNSWICK LATE THURSDAY. CONTINUED COLD ADVECTION IN THE WAKE WILL ASSURE A COLD PERIOD FOR THE NEXT WORK WEEK. MEANWHILE...A PAIR OF SYSTEMS ARE EXPECTED TO AFFECT THE WEST COAST WITH THE INITIAL ONE EXPECTED TO SHEAR WHILE PUSHING INTO NORTHERN CA ON WEDNESDAY NIGHT. IN ITS WAKE...ADDITIONAL LONGWAVE TROUGHING TAKES SHAPE WITHIN A POTENTIAL SPLIT FLOW REGIME ACROSS THE EASTERN PACIFIC. THIS NEGATIVE HEIGHT ANOMALY IS FORECAST TO REACH THE WESTERN U.S. BY DAY 6/JANUARY 5 AS THE ENTIRE SYNOPTIC-SCALE PATTERN SHIFTS EASTWARD. CONSEQUENTLY...A MEAN RIDGE WILL LIKELY BE POSITIONED OVER THE INTERMOUNTAIN WEST/ROCKIES BY DAY 7/JANUARY 6. ...GUIDANCE/PREDICTABILITY ASSESSMENT AND PREFERENCES... COMPARED TO PREVIOUS DAYS...THE VOLATILITY OF THE FORECAST HAS DIMINISHED CONSIDERABLY AS THE GUIDANCE HAVE COME INTO MUCH BETTER AGREEMENT. REGARDING THE SPLIT FLOW ACROSS THE CENTRAL/EASTERN U.S. ON TUESDAY...THE 00Z UKMET CONTINUES TO BE A WESTWARD OUTLIER WITH THIS FEATURE AS WELL AS OTHER UPSTREAM SYNOPTIC SYSTEMS. THE SOUTHERN STREAM SHORTWAVE IS EXPECTED TO CROSS THE FL PANHANDLE ON WEDNESDAY MORNING WHILE THE NORTHERN BRANCH OF THE JET FAVORS A TROUGH PUSHING INTO THE GREAT LAKES. EVENTUAL PHASING OF THESE TWO STREAMS IS LIKELY OVER THE WESTERN ATLANTIC WITH THE EASTWARD TREND OF THIS SCENARIO HOLDING UP FROM YESTERDAY. RAPID CYCLOGENESIS SHOULD UNFOLD WEST OF BERMUDA LATE WEDNESDAY/EARLY THURSDAY LEADING TO A CYCLONE POSSIBLY NEARING 960-MB PRESSURES BY THURSDAY NIGHT IN THE VICINITY OF NOVA SCOTIA/NEW BRUNSWICK. THE 06Z GFS DID TREND A BIT WEST OF THE 00Z GFS WHICH PLACES IT MUCH CLOSER TO THE 00Z ECMWF SOLUTION. MEANWHILE...THE 00Z CMC IS EAST OF THIS CONSENSUS BEFORE DEVELOPING A SECOND WAVE UNDER A STRENGTHENING SHORTWAVE. WHILE THERE IS STILL A LOT OF ENSEMBLE SPREAD AS SEEN IN THE SURFACE LOW PLOTS...MOST PASS EAST OF THE INFAMOUS 40N/70W BENCHMARK AND INTO NOVA SCOTIA. IN ITS WAKE...A REINFORCEMENT OF THE MEAN TROUGH WILL BE ESTABLISHED AS ADDITIONAL ARCTIC AIR DIGS TOWARD THE SOUTHERN U.S. A MEAN UPPER TROUGH WILL INITIALLY BE POSITIONED OVER THE WEST COAST WHILE A SLOW MOVING UPPER LOW APPROACHES UPSTREAM. AS MENTIONED EARLIER...THE 00Z UKMET IS A WESTERN OUTLIER HERE WITH NO DISCERNIBLE TRENDS AMONG THE GLOBAL GUIDANCE. RECENT MODELS SUPPORT A SHEARING SHORTWAVE REACHING NORTHERN CA BY LATE WEDNESDAY BEFORE FLATTENING OUT COMPLETELY UPON IMPACT WITH THE UPPER RIDGE. FARTHER UPSTREAM...MODEL SPREAD IS DECIDEDLY HIGHER AS THE GUIDANCE ATTEMPTS TO RESOLVE SPLIT FLOW EVOLVING OVER THE NORTHEASTERN/EASTERN PACIFIC. THERE HAS BEEN AN EASTWARD TREND SEEN IN THE OPERATIONAL ECMWF AS WELL AS MANY OF ITS CORRESPONDING ENSEMBLES. THIS PLACES ITS SOLUTIONS AHEAD OF THE 06Z/00Z GEFS AND 00Z NAEFS MEANS. ULTIMATELY THIS WILL AFFECT HOW QUICKLY PRECIPITATION WILL MOVE BACK TOWARD THE WEST COAST AS WELL AS THE POSITION OF THE DOWNSTREAM RIDGE. GIVEN THE REMARKABLY GOOD AGREEMENT SEEN THROUGH DAY 5/THURSDAY...WAS ABLE TO AFFORD HEAVY USE OF THE OPERATIONAL MODELS...IN PARTICULAR THE 06Z GFS/00Z ECMWF. BEYOND THIS PERIOD...ENSEMBLE MEANS STARTED TO WEIGH INTO THE BLEND MORE HEAVILY GIVEN THE GROWING SYNOPTIC-SCALE SPREAD. UTILIZED A COMBINATION OF THE 06Z GEFS/00Z NAEFS MEANS WITH THE 00Z ECMWF ENSEMBLE MEAN WITH 30 PERCENT OF THE BLEND CONSISTING OF THE 06Z GFS/00Z ECMWF OUT TO DAY 7/JANUARY 6. ...WEATHER HIGHLIGHTS/THREATS... MULTIPLE ARCTIC SURGES WILL CARRY TEMPERATURES ON THE ORDER TO 20 TO 30 DEGREES BELOW CLIMATOLOGY TO MANY LOCATIONS EAST OF THE ROCKIES. THROUGHOUT THE PERIOD...THE COLDEST AIR WILL SETTLE FROM NORTHERN PLAINS EASTWARD ACROSS THE UPPER MIDWEST/MIDDLE MS VALLEY INTO THE GREAT LAKES AND NEW ENGLAND. HIGH TEMPERATURES MAY STRUGGLE TO GO ABOVE 0 DEGREES OVER AREAS OF MN AND THE INTERIOR NORTHEAST WHILE OVERNIGHT LOWS PLUNGE INTO THE NEGATIVE TEENS AND PERHAPS A FEW -20S IN SOME LOCATIONS. OF COURSE AREAS ALONG THE GULF COAST WILL NOT BE SPARED EITHER AS LOW TEMPERATURES SURGE BELOW FREEZING AS FAR SOUTH AS SOUTH TX AND ALONG THE ENTIRE GULF COAST INTO CENTRAL FL. ON TUESDAY...MANY LOCATIONS WITHIN SOME OF THESE MENTIONED AREAS MAY SEE DAILY TEMPERATURE RECORDS BROKEN FOR BOTH OVERNIGHT LOWS AND CHILLY AFTERNOON HIGHS. IN ADDITION TO THE ARCTIC AIR SPREADING THROUGH...GUSTY WINDS AT TIMES WILL LOWER WIND CHILLS TOWARD SOME VERY DANGEROUS LEVELS. WHILE FRIGID CONDITIONS PERSIST EAST OF THE ROCKIES...READINGS AROUND 10 TO 15 DEGREES ABOVE AVERAGE WILL BE COMMONPLACE OVER MANY AREAS OF THE WESTERN U.S...PARTICULARLY ACROSS THE CENTRAL GREAT BASIN INTO THE DESERT SOUTHWEST. AS CYCLONIC FLOW CONTINUES OVER THE GREAT LAKES...LAKE EFFECT SNOWS WILL LIKELY CONTINUE ALTHOUGH VERY LOW PRECIPITABLE WATER VALUES COULD BE A LIMITING FACTOR AT TIMES. HOWEVER...700-MB TEMPERATURES DROPPING DOWN INTO THE -20C TO -30C RANGE WILL SIGNIFICANTLY STEEPEN LOW-LEVEL LAPSE RATES LEADING TO VERY STRONG UPDRAFTS AIDING IN HEAVY SNOW PRODUCTION. FARTHER SOUTH AND EAST...MODERATE RAINFALL WILL BE POSSIBLE ALONG THE ADVANCING FRONT ACROSS FL WHILE WAVE DEVELOPMENT OVER THE WESTERN ATLANTIC SHOULD AFFORD AN INCREASE IN WINTRY PRECIPITATION CHANCES ACROSS AREAS OF NEW ENGLAND. ANY WESTWARD TREND IN THIS CYCLONE TRACK WOULD SPREAD SNOW FURTHER INTO THE NORTHEASTERN U.S. ACROSS THE WEST COAST...A RETURN TO A WET PATTERN IS ANTICIPATED GIVEN THE SERIES OF SYSTEMS APPROACHING FROM THE WEST. SNOW WILL BE LIKELY ACROSS THE HIGHER ELEVATIONS WHILE SOME WELL NEEDED RAIN SHOULD PUSH INTO SOUTHERN/CENTRAL CA WHICH HAS NOT SEEN MUCH PRECIPITATION DURING THEIR RAINY SEASON. RUBIN-OSTER