EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1100 AM EST SUN DEC 18 2016 VALID 12Z WED DEC 21 2016 - 12Z SUN DEC 25 2016 ...OVERVIEW/GUIDANCE EVALUATION/PREFERENCES... DURING THE PERIOD COVERING WEDNESDAY TO NEXT SUNDAY/CHRISTMAS DAY...THE MID-LATITUDE PATTERN WILL FEATURE A SERIES OF TRANSIENT SHORTWAVES CROSSING THE COUNTRY. THIS PROGRESSIVE FLOW REGIME EXHIBITS DECENT MODEL AGREEMENT EARLY ON BUT SHIFTS TOWARD A MORE UNCERTAIN FOREACAST MOVING INTO THE DAY 5-7 REALM. THE INITIAL SYSTEM OF INTEREST IS CURRENTLY OUT OVER THE NORTH-CENTRAL PACIFIC WITHIN A FLAT/PROGRESSIVE SYNOPTIC PATTERN. THIS SHORTWAVE SHOULD GRADUALLY AMPLIFY AS IT CROSSES THE NORTHERN PLAINS ON WEDNESDAY MORNING WITH THESE HEIGHT FALLS SPREADING EASTWARD INTO THE GREAT LAKES REGION BY THE FOLLOWING DAY. THERE IS SOMEWHAT DECENT PREDICTABILITY WITH THIS FEATURE ALTHOUGH MODELS ARE ALREADY STARTING TO SCATTER SOMEWHAT ON 22/1200Z. THE ECMWF SUITE HAS BEEN TRENDING DEEPER WHILE THE 06Z GFS TOOK A JOG TO THE NORTH AND EAST RELATIVE TO ITS PREVIOUS RUNS. MEANWHILE...THE FLATTER 00Z CMC/UKMET ARE MUCH WEAKER WITH THE SURFACE LOW TRACKING THROUGH THE LOWER GREAT LAKES/OH VALLEY. DIFFERENCES CONTINUE TO INCREASE AS COASTAL CYCLOGENESIS EVENTUALLY TAKES PLACE OFF THE NEW ENGLAND COAST. ENSEMBLE LOW PLOTS DEPICT QUITE A BIT OF SCATTER FROM JUST OFFSHORE OF MAINE UP TOWARD NEWFOUNDLAND ON FRIDAY MORNING. SOME OF THIS MAY BE DUE TO THE PRESENCE OF THE PRIMARY/TRIPLE POINT LOWS BOTH BEING ACCOUNTED FOR. HOWEVER...THE DETAILS ARE FUZZY IN WHERE AND HOW THIS PROCESS WILL OCCUR. ENSEMBLE SPAGHETTI PLOTS SHOW MOST SOLUTIONS FAVOR A NEGATIVE TILT TO THE TROUGH WHICH MAKES SENSE GIVEN THE POWERFUL UPPER RIDGE ANCHORING THE WESTERN ATLANTIC. THUS...A DEEPENING COASTAL LOW IS LIKELY BUT TIMING/PLACEMENT ARE STILL UP IN THE AIR. LOOKING UPSTREAM...MODEL UNCERTAINTY PLAGUES THE FORECAST EVEN MORE AS SHORTWAVE ENERGY SWINGS THROUGH THE WESTERN U.S. ON FRIDAY MORNING. HOW MUCH THIS PACIFIC SYSTEM DIGS WILL PLAY A KEY ROLE IN THE FUTURE EVOLUTION OF THE PATTERN INTO THE WEEKEND. CONSIDERING RECENT ENSEMBLE SPAGHETTI PLOTS...THE 00Z UKMET LOOKS MOST OUT OF PLACE WITH THE ORIENTATION OF ITS UPPER TROUGH BEING MORE AMPLIFIED AND DISPLACED WESTWARD. HOWEVER...EACH DETERMINISTIC SOLUTION DIFFERS FROM THE OTHER ADDING TO THE NEBULOUS NATURE OF THE FORECAST AHEAD. A SERIES OF SCENARIOS ARE EVIDENT RANGING FROM SPLIT FLOW NOTED IN THE 00Z GFS/ECMWF...ALBEIT WITH FEATURES 500 TO 1000 MILES APART...TO A SINGLE FULL-LATITUDE TROUGH DEPICTED IN THE 06Z GFS AND THE PREVIOUS 12Z ECMWF. ENSEMBLE MEANS ARE EVEN DIFFERING FROM ONE ANOTHER...PARTICULARLY THE 00Z ECMWF ENSEMBLE MEAN AS IT FAVORS MORE PROGRESSION IN THE PATTERN. ALL AND ALL...THE FORECAST MOVING INTO THE CHRISTMAS WEEKEND REMAINS UNCERTAIN WITH FUTURE MODEL RUNS HOPEFULLY ADDING FURTHER INSIGHT TO THE SITUATION. FOR DAYS 3 AND 4...FELT A COMBINATION OF THE 00Z GFS/ECMWF AND THE OVERNIGHT CONTINUITY WOULD HANDLE THE GREAT LAKES SYSTEM WELL BEFORE DIFFERENCES CLOUDED UP THE FORECAST. THEREAFTER...RESIGNED TO UTILIZING ENSEMBLE MEANS...A SPLIT BETWEEN THE 00Z NAEFS MEAN AS WELL AS THE PREVIOUS 12Z ECMWF ENSEMBLE MEAN WITH THE LATTER BEING PREFERRED FROM THE PREVIOUS SHIFT. AS MENTIONED...CONFIDENCE REMAINS BELOW AVERAGE FOR DAYS 6/7...DECEMBER 24/25. ...WEATHER HIGHLIGHTS... GIVEN THE LACK OF ANY DISCERNIBLE ARCTIC INTRUSIONS DURING THE MEDIUM RANGE...EXPECT FORECAST TEMPERATURE ANOMALIES TO BE WITHIN 5 TO 10 DEGREES OF CLIMATOLOGY. OF COURSE THIS DOES FLUCTUATE BASED ON THE MODELS...ESPECIALLY LATE IN THE PERIOD. HOWEVER...THERE IS NOT ENOUGH CONFIDENCE TO SUGGEST ANYTHING BEYOND THE FORECAST ABOVE. LOOKING AT PRECIPITATION...A THREAT FOR SNOW IS POSSIBLE FROM THE UPPER MIDWEST EASTWARD THROUGH THE GREAT LAKES AND INTO THE INTERIOR NORTHEAST IN RESPONSE TO THE MODEST SURFACE WAVE. AS COASTAL CYCLOGENESIS UNFOLDS OFF OF NEW ENGLAND AND INTO NOVA SCOTIA...AN UPTICK IN PRECIPITATION IS POSSIBLE OVER THE REGION. DOWN ACROSS THE SOUTHEASTERN U.S...MODERATE TO LOCALLY HEAVY RAINFALL MAY OCCUR IN THE VICINITY OF THE ADVANCING COLD FRONT. AND FINALLY ACROSS THE NORTHWESTERN STATES...LOWER ELEVATION RAIN/MOUNTAIN SNOW IS LIKELY EARLY IN THE PERIOD BEFORE CONDITIONS BECOME A BIT QUIETER TO CONCLUDE THE WEEK. RUBIN-OSTER