EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1040 AM EST WED DEC 31 2014 VALID 12Z SAT JAN 03 2015 - 12Z WED JAN 07 2015 THE SPLIT FLOW REGIME PRESENTLY ESTABLISHED OVER NORTH AMERICA IS EXPECTED TO CONSOLIDATE BY THE END OF THIS WEEK...WITH GUIDANCE THEN IN FAIRLY STRONG AGREEMENT IN ALLOWING A MEAN LONGWAVE RIDGE TO PERSIST ALONG THE WEST COAST... WHILE A MEAN LONGWAVE TROUGH CONTINUES OVER CENTRAL AND EASTERN NORTH AMERICA. THIS PATTERN GENERALLY FAVORS WET CONDITIONS CLIPPING THE PACIFIC NORTHWEST...WARM AND DRY CONDITIONS ACROSS CALIFORNIA/INTERMOUNTAIN WEST/SOUTHWEST...COLD WITH PERIODIC SNOWS ACROSS THE NORTHERN PLAINS INTO THE GREAT LAKES...AND MOSTLY WET CONDITIONS ACROSS THE SOUTHEAST. REGARDING SPECIFIC MODEL PREFERENCES FOR THE MEDIUM RANGE GRIDS/PMSL FIELDS/500 MB PROGS...MODEL AGREEMENT CONTINUES TO INCREASE FOR THE INTERACTION OR POSSIBLE PHASING OF SEPARATE LOWS THAT REACH THE GREAT LAKES BY DAY 4/SUN...WITH THE EXISTING SOLUTION SPREAD AND TRENDS SUPPORTING A MODEL CONSENSUS FOR ITS OUTCOME ...INCLUDING ITS ACCOMPANYING COLD FRONTAL TIMING. THE CONSENSUS IS BEST DEPICTED BY AN EQUAL BLEND OF THE 06Z PARALLEL GFS WITH THE 00Z ECMWF. THE ONLY DETERMINISTIC SOLUTION THAT SEEMS UNLIKELY AT THIS TIME IS THE 00Z UKMET...WHICH REPRESENTS THE SLOW EDGE OF THE GUIDANCE...AND THUS WAS NOT USED. BY DAY 5/MON...THE PREFERENCE SHIFTS TOWARD A 3-WAY BLEND OF 00Z ECMWF/00Z ECMWF ENSEMBLE MEAN/06Z GEFS MEAN...TO HIGHLIGHT A WEAKER SHORTWAVE TROUGH INFLUENCING THE PACIFIC NORTHWEST INTO THE NORTHERN ROCKIES THAN DEPICTED BY THE LATEST OPERATIONAL AND PARALLEL RUNS OF THE GFS....WHICH EJECT THE MAJORITY OF THE LOW PORTION OF THE REX-BLOCK OVER THE GULF OF ALASKA SOUTHEASTWARD. WHILE CERTAINLY POSSIBLE...THIS SOLUTION HAS MINIMAL SUPPORT FROM EXISTING GUIDANCE. THESE PREFERENCES CONTINUE THROUGH DAYS 6-7/TUE-WED...WITH BROAD AND MODESTLY STRONG HIGH PRESSURE PREVAILING OVER THE PLAINS/MIDWEST VERSUS THE GFS THAT HAS A LOW PROGRESSING OVER THE REGION. FORECAST CONFIDENCE IS NEAR AVERAGE THROUGH DAY 5...THEN BELOW AVERAGE FOR DAYS 6/7...AS THE SOLUTION SPREAD INCREASES. JAMES