EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 150 AM EST MON JAN 01 2018 VALID 12Z THU JAN 04 2018 - 12Z MON JAN 08 2018 ...OVERVIEW... A DEEP/AMPLIFIED UPPER TROUGH OVER THE EAST WILL PROMOTE VERY COLD TEMPERATURES FROM LATE WEEK INTO THE WEEKEND. A RAPIDLY DEEPENING WESTERN ATLANTIC SYSTEM NEAR THE START OF THE PERIOD SHOULD BRING MEANINGFUL SNOW AT LEAST TO NEW ENGLAND AND LIKELY GENERATE A PERIOD OF STRONG WINDS OVER THE NORTHEAST. EXPECT EVENTUAL PROGRESSION OF EASTERN PACIFIC TROUGH ENERGY TO BRING SOME MOISTURE INTO THE WEST DURING THE PERIOD AND RESULT IN A FLATTENING OF MEAN FLOW ACROSS THE LOWER 48 BY SUN-MON, LEADING TO A PRONOUNCED WARMING TREND OVER THE CENTRAL U.S. AND THEN FARTHER EASTWARD. THERE IS STILL CONSIDERABLE UNCERTAINTY OVER THE SPECIFICS OF THIS PACIFIC ENERGY THOUGH. ...GUIDANCE/PREDICTABILITY ASSESSMENT AND PREFERENCES... LEADING SHORTWAVE ENERGY THAT IS WEAKENING AS IT REACHES THE WEST COAST BY EARLY THU HAS ATTAINED REASONABLE AGREEMENT IN THE GUIDANCE. HOWEVER MODELS/ENSEMBLES CONTINUE TO SHOW SIGNIFICANT SPREAD AND RUN TO RUN VARIABILITY FOR DETAILS OF THE UPSTREAM TROUGH. INDIVIDUAL SOLUTIONS HAVE BEEN INDICATING A SPLITTING OF FLOW IN DIFFERENT PLACES LEADING TO DIFFERENT TIMING OF HEIGHT FALLS DEPENDING ON LOCATION, AND THEN DIFFERENCES IN HOW MUCH PHASING WITH CANADIAN FLOW MAY OCCUR AS THE OVERALL TROUGH REACHES THE CENTRAL U.S.. LOOKING AT THE OVERALL EASTWARD PROGRESSION OF HEIGHT FALLS, ENSEMBLE MEAN TRENDS OVER THE PAST 1-2 DAYS VALID AROUND FRI-SAT HAVE BEEN TOWARD A COMPROMISE CLOSER TO THE GEFS MEAN WHILE THE ECMWF MEAN HAS BEEN TRENDING SLOWER FROM SOME EARLY RUNS THAT WERE QUITE PROGRESSIVE. FROM THAT POINT ONWARD THERE HAS BEEN A FAIR DEGREE OF INCONSISTENCY OVER HOW MUCH TROUGH ENERGY CROSSES THE SOUTHWEST. LATEST CYCLE GENERALLY LEANS TOWARD KEEPING SOMEWHAT HIGHER HEIGHTS OVER THAT REGION. EVEN WITH THESE DIFFERENCES, BY DAY 7 MON THE MEANS END UP BEING MORE SIMILAR/CONSISTENT THAN MIGHT BE EXPECTED WITH AN UPPER TROUGH OVER THE PLAINS/MS VALLEY AND ASSOCIATED SURFACE LOW PRESSURE OVER THE MS VALLEY/GREAT LAKES. BEHIND THIS TROUGH THE MEANS ADVERTISE A RIDGE BUILDING INTO AT LEAST THE NORTHERN HALF OF THE WEST BUT WITH GEFS MEANS WAFFLING ON TIMING/PHASE OF SOUTHERN STREAM FLOW. INCONSISTENCY/DIFFERENCES AMONG RECENT OPERATIONAL MODEL RUNS REFLECT THE WIDE ENSEMBLE SPREAD. FOR EXAMPLE THE 12Z ECMWF TRENDED MUCH SLOWER THAN ITS PREVIOUS RUNS OR CURRENT MAJORITY TIMING. MOST RECENT GFS RUNS HAVE BEEN A LITTLE LESS ERRATIC BUT STILL VARY WITH SOME SIGNIFICANT DETAILS. THROUGH THE 12Z/18Z CYCLE, THE 12Z-18Z GFS AND 12Z CMC WERE THE CLOSEST SOLUTIONS IN PRINCIPLE TO THE ENSEMBLE MEANS ALBEIT WITH QUESTIONABLE DETAILS AT SOME PARTICULAR VALID TIMES. NOT SURPRISINGLY SOME DETAILS HAVE CHANGED IN THE 00Z GFS/UKMET/CMC BUT THEIR SOLUTIONS STILL LOOSELY COMPARE WITH THE MEANS AND REINFORCE THE LOW PROBABILITY OF THE 12Z ECMWF SCENARIO MID-LATE PERIOD. RECENT D+8 MULTI-DAY MEANS HAVE BEEN VARYING WITH THE LATITUDE OF STRONGEST POSITIVE HEIGHT ANOMALIES WITHIN AN AXIS OF ABOVE NORMAL VALUES OVER THE EAST-CENTRAL PACIFIC INTO ALASKA. LATITUDE DIFFERENCES ARE ENOUGH FOR TELECONNECTIONS TO SUGGEST A FAIRLY SENSITIVE REGIME WITH RESPECT TO HOW MUCH SOUTHERN STREAM/UNDERCUTTING FLOW ULTIMATELY REACHES THE WEST COAST, AND EVENTUALLY SIGNIFICANT IMPLICATIONS FOR TEMPS/PRECIP ACROSS THE LOWER 48. THIS BIG-PICTURE ISSUE REINFORCES THE FORECAST UNCERTAINTY AS EXHIBITED BY THE FULL ARRAY OF ENSEMBLE MEMBERS/MODEL RUNS. OVER THE WESTERN ATLANTIC/EAST COAST, AT LEAST THROUGH THE 12Z/18Z CYCLES THE PAST DAY OF GUIDANCE APPEARED TO HAVE STABILIZED SOMEWHAT FOR THE RAPIDLY DEEPENING WESTERN ATLANTIC STORM FORECAST TO TRACK INTO THE CANADIAN MARITIMES BY EARLY FRI WITH A POSSIBLE DEPTH IN THE 950'S-960'S MB AT THAT TIME. THERE IS STILL UNCERTAINTY OVER THE FINER DETAILS OF HOW LEADING SHORTWAVE ENERGY AND VIGOROUS UPSTREAM DYNAMICS WILL INTERACT BUT DEEPER ENSEMBLE MEAN AND UKMET/CMC TRENDS SEEM TO SUGGEST INCREASING POTENTIAL FOR A VERY INTENSE SYSTEM WITH SOME MARGIN FOR ERROR IN EXACT DETAILS OF EVOLUTION. ENSEMBLE CLUSTERING FOR TRACK HAS BEEN MORE STABLE OVER THE PAST 24 HOURS THAN IN PREVIOUS DAYS BUT TRACK DIFFERENCES ARE STILL MEANINGFUL FOR THE EAST COAST/NEW ENGLAND-- ESPECIALLY WITH THE NEWLY ARRIVED 00Z UKMET/CMC SHOWING A MORE WESTWARD WOBBLE CLOSE TO NEW ENGLAND BY LATE THU COMPARED TO THE GFS/GEFS MEAN. 12Z/18Z SOLUTIONS APPEARED MORE TIGHTLY CLUSTERED. BASED ON RELATIVE AGREEMENT FOR SIGNIFICANT FEATURES, THE LATEST FORECAST INCORPORATED OPERATIONAL GUIDANCE DAYS 3-4 THU-FRI WITH THE 12Z-18Z GFS AND 12Z ECMWF GIVEN EQUAL WEIGHT AND LESSER INPUT FROM THE 12Z CMC/UKMET. DAY 5 SAT SERVED AS A TRANSITION WITH INCREASING 18Z GEFS/12Z ECMWF MEAN INCLUSION AND TAPERING OF THE 12Z ECMWF, FOLLOWED BY DAYS 6-7 SUN-MON THAT FOLLOWED A GFS/CMC AND ENSEMBLE MEAN BLEND. ...WEATHER HIGHLIGHTS/THREATS... CONFIDENCE IS INCREASING THAT PORTIONS OF NEW ENGLAND SHOULD RECEIVE A SIGNIFICANT SNOWFALL FROM THE POWERFUL STORM SYSTEM EXPECTED TO TRACK FROM THE WESTERN ATLANTIC INTO THE CANADIAN MARITIMES THU-FRI, WITH MUCH OF THE NORTHEAST ALSO LIKELY TO SEE A PERIOD OF STRONG WINDS THAT WILL CAUSE BLOWING OF SNOW AND DANGEROUSLY COLD WIND CHILLS. THERE IS POTENTIAL FOR SOME SNOW ALONG THE MID ATLANTIC COAST AS WELL WITH AMOUNTS VERY SENSITIVE TO EXACT TRACK OF THE LOW. BEHIND THIS SYSTEM, MUCH OF THE EASTERN HALF OF THE COUNTRY WILL SEE VERY COLD TEMPS THU INTO SAT WITH ONE OR MORE DAYS OF READINGS 20-30F BELOW NORMAL OVER NORTHERN/CENTRAL AREAS. PARTS OF THE NORTHEAST MAY BE MORE THAN 30F BELOW NORMAL UNDER THE CORE OF UPPER TROUGHING. DAILY RECORDS SHOULD BE MOST PLENTIFUL FOR LOW MAX VALUES ON FRI AND SAT WITH SOME SCATTERED RECORD LOWS AS WELL. A FEW COLD RECORDS ARE POSSIBLE THU AND TEMPS MAY STILL BE COLD ENOUGH TO PRODUCE A FEW RECORDS OVER NEW ENGLAND ON SUN. TEMPS WILL MODERATE FROM WEST TO EAST WITH TIME AS PACIFIC FLOW EVENTUALLY PUSHES ACROSS THE WEST/CENTRAL U.S.. THIS PROGRESSION WILL ALSO SUPPORT A GRADUAL DECREASE OF LAKE EFFECT SNOW THAT WILL BE IN PROGRESS LATE THIS WEEK INTO THE WEEKEND. THE WEST WILL SEE A MORE UNSETTLED PERIOD COMPARED TO RECENT DAYS, WITH WEAKENING SHORTWAVE ENERGY SUPPORTING ONE AREA OF RAIN/HIGH ELEVATION SNOW TO THE CENTRAL WEST COAST AT THE START OF THE FORECAST PERIOD AND UPSTREAM TROUGH ENERGY BRINGING FURTHER PRECIP INTO AND ACROSS THE WEST. DIFFICULTY THAT MODELS ARE HAVING IN RESOLVING DETAILS OF FLOW WITHIN THE OVERALL TROUGH LOWERS CONFIDENCE IN DETERMINING WHERE BEST ENHANCEMENT MAY OCCUR. CURRENTLY MODEL/ENSEMBLE GUIDANCE SHOWS NORTHERN CALIFORNIA AND PACIFIC NORTHWEST AS HAVING THE BEST PROBABILITY OF SEEING THE MOST PRECIP FOR THE 5-DAY PERIOD. SOME OF THIS MOISTURE WILL EXTEND INLAND THROUGH THE ROCKIES. PROGRESSION OF FLOW ALOFT SHOULD BRING A LOW/FRONTAL SYSTEM INTO THE EAST-CENTRAL U.S. BY THE START OF NEXT WEEK, WITH ACCOMPANYING INCREASE IN PRECIP POTENTIAL FROM THE EASTERN PLAINS/MS VALLEY EASTWARD. EXPECT SNOW OVER NORTHERN AREAS AND RAIN TO THE SOUTH WITH A TRANSITION OVER CENTRAL LATITUDES. RAIN COULD BE ON THE HEAVY SIDE FROM THE LOWER HALF OF THE MS VALLEY EASTWARD. MUCH OF THE WEST WILL LIKELY SEE ABOVE NORMAL TEMPS WITH SOME LOCATIONS SEEING MULTIPLE DAYS OF PLUS 10-20F ANOMALIES FOR MIN AND/OR MAX READINGS. WITH LESSER ANOMALIES, SOME OF THIS WARMTH WILL EXTEND INTO THE PLAINS AND BEYOND BY THE LATTER HALF OF THE PERIOD. RAUSCH