MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1148 PM EST MON DEC 29 2014 VALID DEC 30/0000 UTC THRU JAN 02/1200 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... ...AN EVALUATION OF THE 00Z NAM/GFS ALONG WITH PRELIMINARY PREFERENCES/FORECAST CONFIDENCE... THERE DID NOT APPEAR TO BE ANY INITIALIZATION ERRORS WITH THE 00Z NAM/GFS DEEMED SIGNIFICANT ENOUGH TO IMPACT THEIR FORECASTS. ...COLD FRONT EXITING THE SOUTHEASTERN U.S. ON TUESDAY... PRELIMINARY PREFERENCE: GENERAL MODEL COMPROMISE FORECAST CONFIDENCE: HIGH THE 03Z SURFACE ANALYSIS DEPICTED A BAROCLINIC ZONE STRETCHING FROM JUST OFFSHORE OF THE CAROLINAS ACROSS THE FL PANHANDLE AND INTO THE GULF OF MX. ANY MEANINGFUL DIFFERENCES REGARDING THE PROGRESSION OF THIS BOUNDARY APPEAR TO BE CONFINED TO THE OFFSHORE WATERS. THUS...A GENERAL MODEL COMPROMISE IS RECOMMENDED. ...SHORTWAVE CROSSING THE MID-ATLANTIC ON TUESDAY AFTERNOON... PRELIMINARY PREFERENCE: GENERAL MODEL COMPROMISE FORECAST CONFIDENCE: HIGH A SHORTWAVE NOTED IN THE WATER VAPOR IMAGERY ACROSS THE CO ROCKIES HAS HELPED TRIGGER A SWATH OF LIGHT SNOW ACROSS EASTERN CO AND INTO WESTERN NE AND KS. THE IMPULSE IS FORECAST TO QUICKLY MARCH OFF TOWARD THE EAST BECOMING MORE ELONGATE IN NATURE IN TIME. THE CORE OF THE SHORTWAVE SHOULD EXIT THE DELMARVA BY 31/0000Z WITH NO SIGNIFICANT DIFFERENCES NOTED IN THE GUIDANCE. WPC SUGGESTS A GENERAL MODEL COMPROMISE HERE. ...STRONG CLOSED LOW AFFECTING THE SOUTHWESTERN U.S. TUESDAY THROUGH THURSDAY... ...MODIFIED ARCTIC AIR INVADING CA... PRELIMINARY PREFERENCE: BLEND OF THE 00Z GFS/12Z ECMWF FORECAST CONFIDENCE: AVERAGE ALONG THE WESTERN EXTENT OF A LONGWAVE TROF EXTENDING FROM THE WESTERN U.S. INTO THE CENTRAL PLAINS AND GREAT LAKES REGION WILL BE A STRONG LOBE OF VORTICITY FORECAST TO SLIDE DOWN ALONG THE WEST COAST. THIS UPPER LOW AND ITS ASSOCIATED SHOT OF MODIFIED ARCTIC AIR SHOULD REACH THE DESERT SOUTHWEST BY EARLY WEDNESDAY. AT THIS POINT...THE 12Z ECMWF/ECMWF ENSEMBLE MEAN ARE SLIGHTLY WEST OF THE CONSENSUS REGARDING THE POSITION OF THE CLOSED LOW. AS THE SYSTEM EJECTS EASTWARD TOWARD THE FOUR CORNERS REGION...THE 00Z NAM/21Z SREF MEAN DIVERGE THE MOST FROM THE PACK WITH THE FORMER BEING QUICKER WHILE THE LATTER IS DISPLACED TO THE NORTH. MULTI-DAY SPAGHETTI PLOT TRENDS SHOW A SHIFT IN THE FASTER DIRECTION WHILE BEING WEAKER ALOFT GIVEN THE DIMINISHING NUMBER OF MEMBERS INDICATING A 540-DM HEIGHT. TOWARD THE END OF THE PERIOD...THE 12Z UKMET BECOMES DECIDEDLY SLOW WHILE THE 00Z NAM CONTINUES TO BE A FAST OUTLIER. CHOOSE TO SIT IN THE MIDDLE OF THE BUILDING SPREAD FAVORING A COMBINATION OF THE 00Z GFS/12Z ECMWF. AS A SIDE...THE 00Z GFS-PARALLEL APPEARED REASONABLE AT TIMES BUT GOT A BIT SLOW BY DAY 3 TO ADD TO THE PREFERENCE. ...SERIES OF SHORTWAVES SWINGING THROUGH THE GREAT LAKES... PRELIMINARY PREFERENCE: BLEND OF THE 18Z GEFS/12Z ECMWF ENSEMBLE MEANS FORECAST CONFIDENCE: AVERAGE AN ACTIVE NORTHERN BRANCH OF THE JET WILL DRAW A MULTITUDE OF SHORTWAVES ACROSS THE GREAT LAKES AND INTO THE NORTHEASTERN U.S. THROUGHOUT THE PERIOD. HEIGHT FALLS REACHING THE INTERIOR NORTHEAST BY 31/1200Z SEEMS TO BE RESOLVED WELL EXCEPT FOR THE 12Z UKMET WHICH BECOMES SLOW AT TIMES. REGARDING THE NEXT BATCH OF ENERGY DRAGGING THROUGH THE GREAT LAKES ON 01/0000Z...MODEL SPREAD IS NOTED BUT IT DOES NOT SEEM TO BE TOO SIGNIFICANT. THE UPPER FLOW GRADUALLY RELAXES AS THE CORE OF LOWER HEIGHTS LIFTS A BIT TO THE NORTH. ALL AND ALL...THERE ARE ENOUGH SMALL-SCALE DIFFERENCES THROUGHOUT THE PERIOD THAT A BLEND OF THE ENSEMBLE MEANS SHOULD HELP RESOLVE THESE ISSUES. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... RUBIN-OSTER