217  
FXUS02 KWBC 300654  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
153 AM EST THU NOV 30 2017  
 
VALID 12Z SUN DEC 03 2017 - 12Z THU DEC 07 2017  
   
..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  
 

 
 
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