337  
FXUS10 KWNH 311623  
PMDHMD  
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1122 AM EST SUN DEC 31 2017  
 
VALID DEC 31/1200 UTC THRU JAN 04/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z MODEL EVALUATION INCLUDING PREFERENCES AND FORECAST CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
FAST MOVING S/W OVER GREAT LAKES TODAY INTO NEW ENGLAND EARLY  
TOMORROW AND ARCTIC S/W OVER N ONTARIO TODAY REINFORCE THE LARGER  
SCALE TROF OVER NEW ENGLAND MON/TUES  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: ABOVE AVERAGE  
 
FAST MOVING CLIPPER S/W IS STREAKING ACROSS THE SOUTHERN GREAT  
LAKES TOWARD NEW ENGLAND TODAY WITH A STRONGER ARCTIC S/W NOTED IN  
NW ONTARIO ATTM. THE COMBINATION OF THESE TWO FEATURES (AND COLD  
AIR) WILL REINFORCE THE LARGER SCALE TROF AXIS OVER NEW ENGLAND  
INTO MON/TUES. 12Z NAM/GFS CONTINUE TO DEPICT A SOLID CONSISTENCY  
IN TIMING/STRENGTH AND SHAPE TO THE PATTERN AS WELL AS COLD DEPTH  
TO SUPPORT A GENERAL MODEL BLEND AT ABOVE AVERAGE CONFIDENCE.  
 
SHORTWAVE SLIDING ALONG EASTERN SIDE OF ROCKIES FROM CANADA TO CO  
TODAY INTO TUES THEN ENTERS GULF/SOUTH WED SUPPORTING COASTAL LOW  
DEVELOPMENT IN N BAHAMAS  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z GFS/06Z GEFS AND 00Z ECMWF/ECENS MEAN BLEND  
CONFIDENCE: AVERAGE  
 
COMPACT S/W CURRENTLY SEEN IN GOES-WV MOSAIC OVER THE CENTRAL  
CANADIAN ROCKIES CURRENTLY. THIS WAVE WILL REMAIN FAIRLY COMPACT  
AS IT ROLLS DOWN THE EASTERN SIDE OF THE RIDGE COINCIDENT ON THE  
LEE OF THE US ROCKIES BEFORE BREAKING OFF THE TERRAIN TUES ACROSS  
THE SOUTHERN PLAINS TO THE LOWER MS VALLEY. AT THIS TIME GREATER  
AMPLIFICATION AND INTERACTION WITH OTHER S/W UPSTREAM WILL LEAD TO  
A FULL LATITUDE TROF ACROSS THE EASTERN THIRD OF THE US BY WED AS  
IT FILLS OUT THE BASE INTO THE NORTHERN GULF. SPREAD IN GUIDANCE  
IS FAIRLY TIGHT THROUGH EARLY WED BEFORE THE DIFFERENCES IN  
MAGNITUDE/INTERACTION WITH THE UPSTREAM WAVE LEAD TO SMALL  
TIMING/SHAPE DIFFERENCES THAT COMPOUND TOWARD THURS. THE NAM  
WHICH HAD BE SLOW TO THE ENSEMBLE MEAN DEPICTED A BIT LESS  
AMPLIFICATION AND MORE N-S STRETCHING THROUGH THE VORT ON WED TO  
INCREASE TIMING...HOWEVER IS STILL A TAD SLOWER AND TYPICAL OF THE  
NAM A BIT STRONGER OVERALL; WHICH CAN BE SEEN IN THE SURFACE  
PATTERN AS WELL MUCH DEEPER AND NW OF THE CLUSTER. THE 00Z CMC IS  
ALSO A BIT SLOW BUT THIS IS MORE RELATED TO A DEEPER/SOUTHWARD  
SHIFT WITH THE CORE OF THE VORT ENTERING THE GULF HOWEVER DOES NOT  
SEEM TO NEGATIVELY AFFECT THE LOCATION/DEPTH OF THE DEVELOPING  
SURFACE WAVE NORTH OF THE BAHAMAS BY 00Z THURS. THE 00Z UKMET 5H  
PATTERN ESPECIALLY IN THE UPSTREAM INTERACTION SHOWS UNREALISTIC  
VORTICITY PATTERN AND FEEDBACK WITHIN THE STREAM TO FEEL CONFIDENT  
IN ITS INCLUSION IN THE BLEND AS THE TWO FEATURES PHASE INTO THE  
LARGER SCALE TROF OVER THE EAST AND THEREFORE WILL NOT INCLUDE IT  
IN PREFERENCE (THOUGH MAY BE USEFUL UPSTREAM OVER THE  
PLAINS/ROCKIES). THE GFS HAD BEEN A SHADE FASTER THAN ITS GEFS  
MEAN AS WELL AS THE ECMWF/ECENS THAT HAS ANCHORED THE MEAN  
ENSEMBLE SUITE; WHILE THERE IS A CONDUCIVE CONVECTIVE UPSCALE  
ENHANCEMENT ASSOCIATED WITH SURFACE CYCLOGENESIS NORTH OF THE  
BAHAMAS...THE MAIN SHORTWAVE IS A BIT SLOWER HAVING GREATER  
UPSTREAM INTERACTION MAKING THE 12Z GFS TIMED WITH THE GEFS/ECMWF.  
ITS DIFFERENCE THOUGH DUE TO THE UPSTREAM INTERACTION/SLOWING IS  
A STRONGER/DEEPER TROF REPRESENTATION THROUGH GA/FL BY 00Z THURS.  
A BLEND WITH THE 00Z ECMWF/GEFS AND ECENS MEAN SHOULD SUFFICE FOR  
MASS FIELDS. CONFIDENCE REMAINS QUITE HIGH EARLY IN THE BLEND BUT  
REDUCES TO AVERAGE BY WED NEARLY BECOMING SLIGHTLY BELOW AVERAGE  
AT 00Z THURS AS THERE IS GREATER RUN TO RUN VARIATION IN THE  
MAGNITUDE OF THE UPSTREAM INTERACTION AS WELL AS MAGNITUDE OF  
UPSCALE CONVECTIVE FEEDBACK WITH THE CYCLOGENESIS.  
 
FAST MOVING GULF OF AK SHORTWAVE BECOMES NORTHERN PORTION OF  
GLOBAL TROF OVER EASTERN THIRD OF US BY WED WITH SURFACE  
WAVE/ARCTIC COLD BLAST THROUGH NORTHERN PLAINS/GREAT LAKES LATE  
TUES INTO WED  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: NON-UKMET BLEND  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
SHORTWAVE ENERGY EMERGING FROM THE GULF OF AK OVERTOPS THE MEAN  
RIDGE IN NW CANADA AND RAPIDLY ROLLS DOWN THE EASTERN SIDE OF THE  
RIDGE/BACK SIDE OF THE MEAN DOWNSTREAM MEAN TROF...OPENING UP  
TIGHT NW FLOW PACKING OF HEIGHTS ALLOWING FOR CAA OF DEEP ARCTIC  
AIR. THIS ALSO SPURS A FAST MOVING SFC WAVE THROUGH THE NORTHERN  
TIER INTO THE GREAT LAKES WHERE IS TAKES UP RESIDENCE AS THE  
AXIS/FULCRUM OF ROTATION FOR THE LATER WEEK. IN THIS FAST  
FLOW...THERE RESIDES MODEST SPREAD IN MAGNITUDE OF SOUTHWARD  
SHEARING OF THE S/W WHICH IN TURN INTERACTS WITH THE S/W MOVING  
THROUGH THE GULF AND BUILDS A FULL LATITUDE TROF OVER THE EASTERN  
THIRD OF THE US BY 00Z THURS. THE 00Z UKMET SHOWS SOME TYPICAL  
BIAS IN MAINTAINING A SMALL COMPACT VORT CENTER THAT SHOULD HAVE  
STRETCHED IN THIS SHEARING MID-LEVEL ENVIRONMENT... BUT IT DOES  
NOT AND SO IS A BIT LESS RELIABLE TO USE IN A BLEND. THE REMAINING  
GUIDANCE IS IN FAIRLY GOOD AGREEMENT ESPECIALLY IN TIMING GIVEN  
THE ROADBLOCKS/INTERACTIONS IN THE WAY THROUGH THE FORECAST PERIOD  
THAT WOULD NORMAL LEAD TO COMPOUNDING SPREAD. AS SUCH A FAIRLY  
HIGH CONFIDENCE IS GIVEN TO A NON-UKMET BLEND AS WPC PREFERENCE.  
 
CLOSED LOW WELL SW OF CA LIFTS AND FILLS TOWARD PACIFIC NORTHWEST  
TOWARD THURS... WITH SURFACE LOW IN TOW  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: NON-UKMET BLEND  
CONFIDENCE: AVERAGE  
 
COMPACT CLOSED LOW SOUTH OF 30N AND WEST OF 130W BEGINS TO FEEL  
SOME UPSTREAM INFLUENCE FROM THE APPROACHING TROF TO BEGIN TO LIFT  
NORTHEAST BY WED. THE 00Z UKMET IS SLOWEST WITH THIS TROF AND  
THEREFORE MAINTAINS A MORE STATIC/DEEPER UPPER LOW TOWARD THURS.  
THE 12Z NAM/GFS BOTH ARE A BIT FURTHER SOUTHEAST OF THE MAIN  
CLUSTER IN THE ENSEMBLE AS WELL AS SHIFTING NORTHEAST FASTER (NAM  
LEADING GFS SLIGHTLY). LIKEWISE THE ECMWF/CMC ARE CLUSTERED AS  
WELL BUT ARE A BIT FURTHER NW AND SLOWER TO FILL WHICH IS TYPICAL.  
THIS ENVIRONMENT OF A WEAKENING CLOSED LOW SW OF CA IS MORE  
CLASSIC OF LARGE SPREAD AND LOWER PREDICTABILITY YET...THIS  
CLUSTERING IS NOT BAD FOR THREE DAYS OUT. AS SUCH AVERAGE  
CONFIDENCE IS GIVEN TO A COMPROMISE OF THE ECMWF/CMC AND GFS/NAM  
CLUSTERS WITHIN A NON-UKMET BLEND.  
 
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