EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1000 AM EST THU NOV 30 2017 VALID 12Z SUN DEC 03 2017 - 12Z THU DEC 07 2017 15Z UPDATE... THE SPECIFICS OF THE FLOW PATTERN EVOLUTION ACROSS THE CONUS BY THE MIDDLE OF NEXT WEEK HAVE NOT BECOME APPRECIABLY CLEARER RELATIVE TO OVERNIGHT OR EVEN TO THIS TIME YESTERDAY. WHILE MODEL SPREAD REMAINS RELATIVELY LOW (AND CONFIDENCE ABOVE AVERAGE) EARLY IN THE MEDIUM RANGE, SPREAD CONTINUES TO INCREASE SUBSTANTIALLY FROM DAY 5 (TUE) ONWARD. DIFFERENCES AMONG THE GUIDANCE CONTINUE TO REVOLVE AROUND WHETHER ENERGY SEPARATES FROM THE AMPLIFYING TROUGH ACROSS THE ROCKIES/PLAINS INTO A CUTOFF UPPER LOW (AS SHOWN BY THE 00Z CMC AND TO A LESSER DEGREE BY THE GFS), OR WHETHER THE ENERGY REMAINS MORE CONSOLIDATED RESULTING IN A DEEPER CENTRAL/EASTERN U.S. TROUGH BY WED-THU (AS SHOWN BY THE 00Z ECMWF). WHAT DOES REMAIN CLEAR IS THAT THE FLOW PATTERN ACROSS NORTH AMERICA WILL BECOME HIGHLY AMPLIFIED, WITH AN ANOMALOUS 500 HPA RIDGE BUILDING ALONG THE WEST COAST (+2 TO +3 STANDARD DEVIATIONS) AND A TROUGH ACROSS THE EAST. ENSEMBLE SPREAD HAS NOT IMPROVED SIGNIFICANTLY COMPARED TO YESTERDAY DURING THE MON-TUE TIME FRAME, WITH A SUBSTANTIAL NUMBER OF CMC ENSEMBLE MEMBERS AS WELL AS A NUMBER OF ECMWF AND GEFS ENSEMBLE MEMBERS DEVELOPING THE CUTOFF LOW. AS A RESULT, THE FORECAST REMAINED A RELATIVE COMPROMISE BETWEEN THE TWO SCENARIOS BY LATE IN THE PERIOD, PERHAPS SHOWING A BRIEF DEVELOPMENT OF A CUTOFF LOW BUT THEN QUICKLY REABSORBING IT INTO AN AMPLIFYING EASTERN TROUGH. A BLEND OF THE 00Z ECMWF/UKMET AND 06Z GFS SERVED AS A STARTING POINT FOR THE FORECAST DURING DAYS 3-4, WITH A TREND TOWARD MAJORITY 00Z ECENS/NAEFS WEIGHTING DURING DAYS 5-7. THIS RESULTED IN REASONABLY GOOD CONTINUITY WITH THE PREVIOUS FORECAST FROM OVERNIGHT. RYAN PREVIOUS DISCUSSION (ISSUED AT 0653 UTC)... ...OVERVIEW... LATEST GUIDANCE CONTINUES TO SHOW GOOD AGREEMENT WITH THE GENERAL IDEA OF FLOW AMPLIFICATION, FEATURING A STRONG RIDGE THAT BUILDS NEAR THE WEST COAST AND DOWNSTREAM MEAN TROUGH WHICH BY THE END OF THE PERIOD SHOULD EXTEND SOUTH-SOUTHWEST FROM AN UPPER LOW NEAR HUDSON BAY. D+8 MULTI-DAY MEANS ARE REMARKABLY SIMILAR IN DEPICTING THE RIDGE'S POSITIVE HEIGHT ANOMALY CORE OVER THE VICINITY OF THE BC COAST/SOUTHERN ALASKA PANHANDLE. THE EAST-CENTRAL U.S. TROUGH AT THAT TIME-- WITH GREATEST NEGATIVE HEIGHT ANOMALIES OVER THE UPPER MIDWEST/WESTERN GREAT LAKES AND SOME DEGREE OF ELONGATION TO THE SOUTHWEST-- COMPARES QUITE WELL TO TELECONNECTION-FAVORED FLOW. IN CONTRAST TO THE WELL ADVERTISED PATTERN CHANGE, THE GUIDANCE HAS HAD A VERY DIFFICULT TIME IN RESOLVING THE DETAILS EN ROUTE TO THE AMPLIFIED REGIME. ...GUIDANCE EVALUATION/PREFERENCES... FOR MULTIPLE DAYS THE MODELS AND ENSEMBLES HAVE SHOWN QUITE VARIED IDEAS (AND SOMETIMES ALTERNATING BETWEEN TWO EXTREMES IN CONSECUTIVE RUNS) FOR THE CHARACTER OF THE UPPER TROUGH REACHING THE WEST COAST AROUND THE START OF THE PERIOD EARLY SUN, AND THEN WHAT PROPORTION CONTINUES EASTWARD ALONG THE NORTHERN STREAM VERSUS SEPARATING OVER THE SOUTHWEST OR EVEN OFF THE CA COAST ALONG WITH ASSOCIATED TIMING DIFFERENCES. FROM THE BIG PICTURE PERSPECTIVE RECENT ENSEMBLE MEANS ON AVERAGE HAVE COMPARED BEST TO TELECONNECTION FAVORED FLOW WITH LESS ERRATIC BEHAVIOR THAN THE OPERATIONAL RUNS, SHOWING A MODERATE AMOUNT OF ENERGY LINGERING OVER THE SOUTHWEST. THE MEANS STILL VARY SOMEWHAT ON HOW DEEP THE SOUTHWESTERN ENERGY WILL BE, RANGING FROM THE DEEPER 12Z CMC MEAN TO INTERMEDIATE GEFS MEAN AND WEAKER ECMWF MEAN. A BLEND OF ENSEMBLE MEANS (18Z GEFS AND 12Z ECMWF/NAEFS) APPEARED REASONABLE AS AN INITIAL FRAMEWORK FOR THE FORECAST GIVEN THE VOLATILE HISTORY OF GUIDANCE SO FAR. WITH SUPPORT FROM THE MEANS, DAYS 3-4 SUN-MON WERE ABLE TO INCORPORATE MORE OPERATIONAL GUIDANCE. THE BLEND REMOVED THE 18Z GFS AFTER EARLY MON DUE TO ITS LOW CONFIDENCE EVOLUTION NEAR THE WEST COAST AND FASTER THAN CONSENSUS PROGRESSION OF HEIGHT FALLS/SURFACE FRONT CROSSING THE EAST, AND THE CMC ABOUT A DAY LATER AS IT HELD LITTLE ENERGY OVER SOUTHWEST. THE 12Z ECMWF STAYED CLOSER TO THE ENSEMBLE MEAN CLUSTER THROUGH THE PERIOD BUT WEIGHTING HAD TO BE KEPT FAIRLY LOW BY MIDWEEK DUE TO LOW PREDICTABILITY OF ENERGY THAT MAY REACH THE PLAINS FROM CANADA. THE UKMET WAS FASTER THAN OTHER SOLUTIONS WITH THE UPPER TROUGH. THIS UPDATE IS SIMILAR TO CONTINUITY IN PRINCIPLE BUT REFLECTS SLIGHTLY SLOWER TIMING. OPERATIONAL MODELS ARE GETTING GRADUALLY CLOSER WITH WHAT MAY BE A FAIRLY STRONG STORM SYSTEM BY THE TIME NORTHERN TIER LOW PRESSURE TRACKS FROM THE UPPER MIDWEST INTO ONTARIO. 12Z/18Z GFS RUNS WERE ON THE FAST SIDE OF THE SPREAD VERSUS THE 12Z ECMWF/UKMET/CMC. NEW 00Z GUIDANCE SO FAR IS PROVIDING SOME INITIAL HOPE FOR BETTER AGREEMENT/STABILIZATION. THE GFS NOW COMPARES MORE FAVORABLY TO THE ENSEMBLE MEANS OVER THE SOUTHWEST AND HAS SLOWED PROGRESSION OF THE EASTERN U.S. FRONT TO CONSENSUS. WHILE THE CMC MAY BE SOMEWHAT DEEP WITH ITS SOUTHWEST UPPER LOW THE OVERALL PATTERN IS ALSO MUCH CLOSER TO THE MEANS THAN ITS PRIOR RUN. THE UKMET STILL SHOWS A FAST TENDENCY BUT HAS PULLED BACK SOME FROM THE 12Z RUN. SOLUTIONS HAVE SHUFFLED A BIT BUT THERE IS SIMILAR SPREAD AS BEFORE FOR THE UPPER MIDWEST/ONTARIO SYSTEM. ...WEATHER HIGHLIGHTS/THREATS... AS THE UPPER TROUGH INITIALLY REACHING THE WEST COAST CONTINUES EASTWARD/SOUTHEASTWARD, EXPECT MOSTLY LIGHT TO MODERATE RAIN AND HIGHER ELEVATION SNOW TO SPREAD ACROSS FAVORED TERRAIN PRIMARILY OVER THE NORTHERN HALF OF THE WEST. A LITTLE ACTIVITY MAY EXTEND INTO THE FOUR CORNERS STATES DEPENDING ON EVOLUTION OF FLOW ALOFT OVER THE SOUTHWEST. THE WEST COAST STATES/INTERIOR WEST WILL SEE DRIER CONDITIONS FROM MON ONWARD. AS DEEPENING NORTHERN TIER LOW PRESSURE TRACKS NEAR THE UPPER GREAT LAKES AND INTO CANADA AND PUSHES ITS TRAILING FRONT ACROSS THE EASTERN HALF OF THE LOWER 48, ANTICIPATE RAINFALL TO DEVELOP OVER THE EASTERN PLAINS/MS VALLEY AROUND THE START OF THE WEEK AND PROGRESS EAST/SOUTHEAST THEREAFTER. AFTER FRONTAL PASSAGE SOME MOISTURE MAY LINGER OVER PARTS OF THE SOUTHERN TIER, WITH SOME ENSEMBLES SHOWING MORE OF SUCH A SIGNAL THAN OPERATIONAL RUNS. MODEL/ENSEMBLE SOLUTIONS AS A WHOLE ARE SUGGESTING THE BEST POTENTIAL FOR HIGHEST RAINFALL TOTALS WILL BE WITHIN AN AREA FROM EASTERN TX INTO THE LOWER OH VALLEY/TN VALLEY/SOUTHERN APPALACHIANS. SNOW WILL BE POSSIBLE IN THE COLD SECTOR OF THE NORTHERN TIER SYSTEM AND COLD CYCLONIC FLOW BEHIND THE LOW/FRONT WILL START AN EPISODE OF LAKE EFFECT SNOW BY THE LATTER HALF OF THE PERIOD. AHEAD OF THE COLD FRONT, WELL ABOVE NORMAL TEMPS WILL PREVAIL FROM THE ROCKIES/HIGH PLAINS INTO THE EAST-CENTRAL U.S. EARLY IN THE WEEK. MANY PLACES SHOULD SEE PLUS 10-20F ANOMALIES WITH LOCALLY HIGHER READINGS POSSIBLE ESPECIALLY FOR MINS. INITIALLY THE PACIFIC ORIGIN OF THE SYSTEM WILL MEAN FAIRLY MODEST BELOW NORMAL DEPARTURES BEHIND THE FRONT ASIDE FROM SOME MINUS 10-15F ANOMALIES OVER THE SOUTHERN HIGH PLAINS FOR HIGHS AROUND MIDWEEK. ESTABLISHMENT OF FLOW ALOFT FROM CANADA SHOULD PRODUCE A GRADUAL COOLING TREND OVER THE EASTERN HALF OF THE COUNTRY LATE IN THE PERIOD WITH A BROAD AREA OF MINUS 5-10F ANOMALIES BY NEXT THU. ON THE OTHER HAND TEMPS ALONG THE WEST COAST WILL TREND WARMER AS UPPER RIDGING BUILDS IN. RAUSCH