MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 148 AM EST MON DEC 29 2014 VALID DEC 29/0000 UTC THRU JAN 01/1200 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... ...AN EVALUATION OF THE 00Z MODEL GUIDANCE ALONG WITH FINAL PREFERENCES/FORECAST CONFIDENCE... THERE DID NOT APPEAR TO BE ANY INITIALIZATION ERRORS WITH THE 00Z NAM/GFS DEEMED SIGNIFICANT ENOUGH TO IMPACT THEIR FORECASTS. ...PROGRESSIVE SHORTWAVE MOVING FROM THE ARKLATEX TOWARD THE MID-ATLANTIC... FINAL PREFERENCE: GENERAL MODEL COMPROMISE FORECAST CONFIDENCE: ABOVE AVERAGE EMBEDDED WITHIN SOUTHWESTERLY ALOFT...AN IMPULSE ORIGINATING OVER THE ARKLATEX WILL GRADUALLY SHEAR WHILE LIFTING INTO THE MID-ATLANTIC BY 29/1200Z. MODELS ARE IN SUFFICIENT AGREEMENT WITH THIS WAVE ALONG WITH THE POSITION OF THE BOUNDARY MOVING INTO THE OFFSHORE WATERS. A GENERAL MODEL COMPROMISE WILL BE RECOMMENDED. ...MID-LEVEL ENERGY TRACKING THROUGH THE CENTRAL PLAINS ON 30/0000Z... FINAL PREFERENCE: NON-21Z SREF MEAN MODEL CONSENSUS FORECAST CONFIDENCE: ABOVE AVERAGE HEIGHT FALLS CURRENTLY MOVING THROUGH THE CENTRAL GREAT BASIN WILL HELP CARVE OUT AN ELONGATED AXIS OF VORTICITY INTO THE CENTRAL HIGH PLAINS BY MONDAY EVENING. THE SYSTEM SHOULD BECOME RATHER PROGRESSIVE AS IT MOVES EAST OF THE GREAT PLAINS REACHING THE CENTRAL APPALACHIANS/MID-ATLANTIC BY 30/1800Z. THE GUIDANCE IS IN MUCH BETTER AGREEMENT THAN PREVIOUS DAYS AS THE SPREAD HAS DIMINISHED MARKEDLY. THE 21Z SREF MEAN IS PERHAPS A BIT FARTHER NORTH WITH THE SHORTWAVE ENERGY SO WILL GO WITH A NON-21Z SREF MEAN MODEL CONSENSUS. ...CLOSED LOW ANCHORING THE SOUTHWESTERN U.S. TUESDAY THROUGH THURSDAY... ...ACCOMPANYING SURGE OF ARCTIC AIR... FINAL PREFERENCE: BLEND OF THE 00Z GFS/ECMWF FORECAST CONFIDENCE: SLIGHTLY ABOVE AVERAGE A HIGH AMPLITUDE PATTERN IS IN PLACE TO START THE PERIOD WITH A STOUT UPPER RIDGE EXTENDING ALONG THE EAST PACIFIC UP TO THE YUKON TERRITORY. STRONG HEIGHT FALLS ASSOCIATED WITH A VIGOROUS SHORTWAVE WILL DIVE DOWN ACROSS THE WESTERN U.S. BEFORE SETTLING INTO THE DESERT SOUTHWEST BY TUESDAY. THE FORECAST SPREAD IS NOT TOO SIGNIFICANT THROUGH EARLY WEDNESDAY WITH SOLUTIONS IN THE BALLPARK OF ONE ANOTHER. TIMING DIFFERENCES EMERGE BY MID-WEEK ONWARD AS THE UPPER LOW SPINS OVER SOUTHERN CA BEFORE SHIFTING EASTWARD. THE 12Z ECMWF ENSEMBLE MEAN IS SLOWER THAN THE 00Z GEFS MEAN WHILE THE 21Z SREF IS DISPLACED TO THE NORTH. REGARDING MODEL TRENDS...THE 00Z CMC/ECMWF MOVED IN THE SOMEWHAT FASTER DIRECTION WHILE THE 00Z UKMET HAS GENERALLY MAINTAINED ITS CONTINUITY. THIS SHIFT IN THE ECMWF DOES KEEP IT WELL WITHIN THE SPREAD IN THE 540-DM SPAGHETTI PLOTS. WPC WILL KEEP THE PREVIOUS PREFERENCE AS IS WHICH FEATURES A BLEND OF THE 00Z GFS/ECMWF. ...SERIES OF NORTHERN STREAM WAVES CROSSING THE GREAT LAKES REGION... FINAL PREFERENCE: MODEL COMPROMISE BETWEEN THE 00Z GFS/ECMWF/UKMET THROUGH DAY 2...THEREAFTER A BLEND OF THE 00Z GEFS/12Z ECMWF ENSEMBLE MEANS FORECAST CONFIDENCE: AVERAGE AN ACTIVE NORTHERN STREAM WILL DRAW A SERIES OF IMPULSES FROM THE UPPER MIDWEST ACROSS THE GREAT LAKES FROM TUESDAY THROUGH THURSDAY. REGARDING THE FIRST SYSTEM CROSSING THE UPPER GREAT LAKES ON 31/0000Z...THE 21Z SREF MEAN IS SLOWER/MORE NORTHWARD. THERE IS MORE UNCERTAINTY WITH THE NEXT DISTURBANCE SLIDING INTO MN BY WEDNESDAY. THE BIGGEST OUTLIERS ARE THE 00Z NAM/CMC WHICH FEATURE NOTHING CLOSE TO WHAT THE ENSEMBLE MEANS AND OTHER DETERMINISTIC GUIDANCE SHOW. THEREAFTER...A THIRD IMPULSE WILL BEGIN TO CROSS INTO CENTRAL CANADA WITH A SLEW OF DIFFERENCES NOTED WITH THAT FEATURE. WILL FOLLOW A BLEND OF THE ENSEMBLE MEANS FOR DAY 3 IN THAT CASE. BEFORE THAT...A MODEL COMPROMISE BETWEEN THE 00Z GFS/ECMWF/UKMET IS FEASIBLE. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... RUBIN-OSTER