708  
FXUS02 KWBC 300651  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
151 AM EST SAT DEC 30 2017  
 
VALID 12Z TUE JAN 02 2018 - 12Z SAT JAN 06 2018  
   
..OVERVIEW
 
 
RIDGING ALOFT OVER WESTERN NORTH AMERICA WILL SUPPORT  
RE-AMPLIFICATION OF TROUGHING TO THE EAST, KEEPING VERY COLD  
TEMPERATURES OVER THE CENTRAL/EASTERN U.S. THOUGH EVENTUALLY WITH  
A HINT OF MODERATION OVER PARTS OF THE PLAINS. AS THE EASTERN  
TROUGH DEEPENS THERE IS STILL A GOOD SIGNAL FOR SIGNIFICANT  
WESTERN ATLANTIC SURFACE DEVELOPMENT AROUND WED-THU WITH PRECISE  
EFFECTS ALONG THE EAST COAST STILL UNCERTAIN. FARTHER WEST THE  
GUIDANCE IS HAVING A DIFFICULT TIME RESOLVING SOME OF THE DETAILS  
REGARDING MULTIPLE PIECES OF ENERGY WITHIN A GENERAL AREA OF SPLIT  
FLOW/TROUGHING OVER THE EASTERN PACIFIC AND POSSIBLY HEADING INTO  
THE WESTERN U.S. MEAN RIDGE, WITH AN INCREASE OF MOISTURE INTO THE  
WEST BEING THE OVERALL THEME.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT AND PREFERENCES
 
 
OVER THE EASTERN PACIFIC, ALREADY AT THE START OF THE PERIOD DAY 3  
TUE INDIVIDUAL ENSEMBLE MEMBERS SHOW DRAMATIC SPREAD FOR TIMING OF  
A LEADING TROUGH/EMBEDDED UPPER LOW OFFSHORE CALIFORNIA AND  
EXPECTED TO WEAKEN AS IT HEADS TOWARD/INTO THE DOWNSTREAM RIDGE.  
A GREATER PERCENTAGE OF 12Z ECMWF MEMBERS LEAN TO THE FASTER SIDE  
OF THE SPREAD THAN IN OTHER ENSEMBLE SYSTEMS, LEADING THE ECMWF  
MEAN TO HAVE THE FASTEST DEPICTION OF THIS FEATURE AMONG  
MODELS/MEANS. SLOWER TREND OF THE 18Z GFS VERSUS THE 12Z RUN  
PROVIDED SOME ADDED SUPPORT FOR LEANING AWAY FROM THE FASTER SIDE  
OF THE SPREAD, WITH AN OPERATIONAL MODEL CONSENSUS LOOKING MOST  
REASONABLE. ARRIVAL OF THE 00Z GFS/GEFS MEAN/UKMET/CMC FURTHER  
REINFORCES THE IDEA THAT THE 12Z ECMWF MEAN IS LIKELY TOO FAST.  
 
WHILE OPERATIONAL MODEL CLUSTERING OVER THE PAST COUPLE CYCLES IS  
HELPING TO BOOST CONFIDENCE SOMEWHAT FOR THIS LEADING FEATURE,  
ARRIVING AT A SINGLE MOST LIKELY SCENARIO FOR UPSTREAM TROUGH  
ENERGY REMAINS QUITE DIFFICULT. INDIVIDUAL MODELS/ENSEMBLES SHOW  
A WIDE VARIETY OF SOLUTIONS FOR HOW THE ENERGY SPLITS AND HOW FAST  
IT WILL PROGRESS EASTWARD. BROADLY SPEAKING THERE ARE TWO  
CLUSTERS FOR TROUGH ENERGY THAT WILL APPROACH AND POSSIBLY REACH  
THE WEST, WITH THE 12Z ECMWF/CMC AND THEIR ENSEMBLE MEANS BECOMING  
INCREASINGLY PROGRESSIVE RELATIVE TO GFS/GEFS MEAN RUNS. THERE  
ARE OFFSETTING CONSIDERATIONS THAT PRECLUDE FULLY DISCOUNTING  
EITHER OPTION. STRENGTH/PERSISTENCE OF WESTERN U.S. MEAN RIDGING  
FAVOR SLOWER PROGRESSION OF THE TROUGH, BUT MULTI-DAY MEANS BY D+8  
SHOWING A CORE OF POSITIVE HEIGHT ANOMALIES SOMEWHERE BETWEEN  
ALASKA AND HAWAI'I TELECONNECT TO SOME MEAN TROUGHING REACHING THE  
WEST. THERE CAN BE A DELAY BETWEEN ESTABLISHMENT OF A WELL  
DEFINED ANOMALY CENTER AND TELECONNECTION-FAVORED FLOW SO WOULD  
ULTIMATELY PREFER TO MAINTAIN A SOLUTION THAT HEDGES SOMEWHAT  
TOWARD THE SLOWER GFS/GEFS. THE 00Z CMC HAS SLOWED SOME FROM ITS  
PRIOR RUN AND THE UKMET THROUGH 00Z FRI IS QUITE SLOW.  
 
GUIDANCE SPREAD AND VARIABILITY OVER THE PAST COUPLE DAYS SUGGEST  
THERE MAY BE MORE ADJUSTMENTS IN STORE FOR THE MIDWEEK WESTERN  
ATLANTIC STORM WHOSE DYNAMIC SUPPORT WILL COME FROM A CENTRAL  
ROCKIES/PLAINS SHORTWAVE AND STRONGLY AMPLIFYING CANADIAN FLOW  
REACHING THE EASTERN STATES. THE 12Z/18Z CYCLES OF GUIDANCE  
GENERALLY MAINTAINED THE IDEA OF THE PRIOR 12 HOURS IN SUGGESTING  
THAT THE SURFACE LOW TRACK WOULD BE SUFFICIENTLY FAR OFFSHORE TO  
LIMIT PRECIP EFFECTS OVER THE MID ATLANTIC/NEW ENGLAND, AS  
INDICATED BY FEWER 12Z ENSEMBLE MEMBERS BEING NEAR THE EAST COAST  
THAN IN THE PRIOR DAY. HOWEVER TYPICAL ERROR RANGE FOR DAYS 4-6  
FORECASTS IS STILL ENOUGH TO ALLOW FOR DETAIL CHANGES THAT WOULD  
INCREASE POTENTIAL FOR EAST COAST IMPACTS. THE NEW 00Z UKMET  
PROVIDES AN ILLUSTRATION OF THAT, EVEN IF IT MAY SEEM UNLIKELY  
GIVEN THAT ITS SPECIFICS FOR THE LEADING SHORTWAVE AND THE FARTHER  
WEST AXIS OF UPSTREAM HEIGHT FALLS OVER THE EAST DO NOT COMPARE  
FAVORABLY TO OTHER GUIDANCE. THE 00Z GEFS MEAN ALSO MAINTAINS  
SOME CONTINUITY FROM ITS PREVIOUS RUN WHICH WAS A BIT WEST OF MOST  
OPERATIONAL 12Z/18Z RUNS. PREFERENCE GOES TO A COMPROMISE AMONG  
THE MODELS/ENSEMBLE MEANS AT THIS TIME.  
 
REGARDING THE PROGRESSION OF CLIPPER SYSTEMS ALONG THE NORTHERN  
TIER, THE LEADING SYSTEM ASSOCIATED WITH FLOW AMPLIFYING FROM  
CANADA HAS HAD DECENT CLUSTERING/CONTINUITY IN THE GUIDANCE FOR  
THE PAST DAY OR SO. SOME MODELS ARE HINTING AT A WAVE OVER THE  
NORTHERN PLAINS/GREAT LAKES LATE IN THE WEEK, WITH ENOUGH OF A  
SIGNAL TO HAVE A WEAK REFLECTION IN THE MANUAL FORECAST GIVEN SOME  
SIMILARITIES IN THE SHORTWAVE ENERGY ALOFT. SOME WAVINESS OR A  
FRONT MAY EMERGE AROUND OR SHORTLY AFTER DAY 7 SAT WITH THE  
ARRIVAL OF EASTERN PACIFIC SHORTWAVE ENERGY.  
 
IN ORDER TO ACCOMMODATE THE ABOVE CONSIDERATIONS, THE UPDATED  
FORECAST STARTED WITH AN OPERATIONAL MODEL BLEND (MORE 18Z GFS/12Z  
ECMWF THAN 12Z UKMET/CMC) EARLY IN THE PERIOD AND THEN INCREASED  
ENSEMBLE WEIGHTING TOWARD DAYS 6-7 FRI-SAT WITH MORE GOING TO THE  
18Z GEFS MEAN VERSUS THE 12Z ECMWF MEAN TO REFLECT EAST  
PACIFIC/WESTERN U.S. PREFERENCES. SOME DETAILS NEAR THE WEST  
COAST MAY APPEAR TO HAVE A WEAK ECMWF/CMC FLAVOR IN SOME RESPECTS  
BUT ULTIMATELY THE BLEND REPRESENTS THE DESIRED INTERMEDIATE  
TIMING OF HEIGHT FALLS ALOFT.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
VERY COLD TEMPS OVER THE EASTERN HALF OR SO OF THE LOWER 48 WILL  
CONTINUE TO BE A PROMINENT STORY DURING THE PERIOD, WITH MANY  
AREAS 10-25F BELOW NORMAL FOR MINS AND/OR DAYTIME HIGHS AND  
OCCASIONAL PERIODS OF WIND THAT WILL PRODUCE DANGEROUS WIND  
CHILLS. WHILE THE COLD REMAINS ENTRENCHED OVER THE EAST, MUCH OF  
THE PLAINS SHOULD SEE A MODERATING TREND WITH TEMPS REACHING TO  
WITHIN A FEW DEGREES EITHER SIDE OF NORMAL BY FRI OR SAT. THE  
MIDWEEK SYSTEM OVER THE WESTERN ATLANTIC SHOULD AT LEAST BRING A  
PERIOD OF STRONGER WINDS TO THE NORTHEAST AS IT TRACKS TOWARD/INTO  
THE CANADIAN MARITIMES, AND THERE IS STILL ENOUGH UNCERTAINTY IN  
THE TRACK TO REQUIRE CLOSE MONITORING FOR POTENTIAL PRECIP IMPACTS  
ALONG THE EAST COAST. EPISODES OF LAKE EFFECT SNOWFALL WILL  
CONTINUE THROUGH MUCH OF THE PERIOD.  
 
EASTERN PACIFIC FEATURES ALOFT WILL HELP TO BRING INCREASED  
MOISTURE TO THE WEST COAST STATES DURING THE PERIOD. SOME OF THIS  
MOISTURE MAY EXTEND INTO THE ROCKIES. LOCATIONS FROM CENTRAL  
CALIFORNIA INTO THE PACIFIC NORTHWEST SHOULD SEE THE HIGHEST  
TOTALS FOR THE 5-DAY PERIOD. EXPECT THE MAJORITY OF RAINFALL AND  
HIGH ELEVATION SNOW TO BE IN THE LIGHT TO MODERATE RANGE FOR  
INTENSITY BUT THERE IS POTENTIAL FOR GREATER LOCALIZED ENHANCEMENT  
DEPENDING ON SPECIFICS OF FLOW ALOFT. MOST OF THE SOUTHERN HALF  
OF THE WEST SHOULD SEE TEMPS 5-15F ABOVE NORMAL WHILE NORTHERN  
AREAS WILL BE CLOSER TO NORMAL.  
 
RAUSCH  
 

 
 
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