MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1150 AM EST SUN JAN 29 2017 VALID JAN 29/1200 UTC THRU FEB 02/0000 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... ...AN EVALUATION OF THE 12Z NAM/GFS ALONG WITH PRELIMINARY PREFERENCES/FORECAST CONFIDENCE... ...LONGWAVE TROUGH EXITING THE EASTERN SEABOARD EARLY NEXT WEEK... ...ASSOCIATED SURFACE WAVE LIFTING AWAY FROM THE DELMARVA PENINSULA... PRELIMINARY PREFERENCE: MODEL COMPROMISE BETWEEN THE 12Z NAM/GFS AND 00Z ECMWF FORECAST CONFIDENCE: ABOVE AVERAGE WATER VAPOR AND RADAR IMAGERY SHOWED A SERIES OF PERTURBATIONS WITHIN A LONGWAVE TROUGH AFFECTING THE EASTERN THIRD OF THE COUNTRY. EACH OF THESE IMPULSES HAS BEEN A CONDUIT FOR SNOW SHOWERS...PARTICULARLY IN REGIONS WHERE THE GREAT LAKES ARE AUGMENTING THE RADAR RETURNS. AS THE CORRIDOR OF LOWER 500-MB HEIGHTS SWEEPS THROUGH THE MID-ATLANTIC...A SURFACE WAVE IS FORECAST TO EMERGE OFF THE DELMARVA PENINSULA BY MONDAY MORNING. LIKE YESTERDAY...THE 00Z CMC/UKMET WERE DISPLACED EASTWARD OF THE STRONGER ENSEMBLE CONSENSUS AS NOTED IN THE SURFACE LOW PLOTS. CONSEQUENTLY...WILL LEAN IN THE DIRECTION OF A CONSENSUS FORMED BY THE 12Z NAM/GFS AND 00Z ECMWF. ...CLIPPER SYSTEM/SURFACE LOW RACING THROUGH THE GREAT LAKES ON TUESDAY... PRELIMINARY PREFERENCE: A NON-12Z NAM MODEL BLEND FORECAST CONFIDENCE: ABOVE AVERAGE ATOP A STOUT WESTERN U.S. RIDGE...A PROGRESSIVE SHORTWAVE SHOULD RACE ACROSS BRITISH COLUMBIA/ALBERTA TONIGHT BEFORE CROSSING THE U.S. INTERNATIONAL BORDER BY MONDAY MORNING. SIMILAR TO YESTERDAY...THE 12Z NAM IS WELL TO THE NORTH OF THE OTHER SOLUTIONS WHICH LEADS TO A MUCH DIFFERENT SURFACE LOW TRACK. RECENT ENSEMBLE LOW PLOTS SHOW A DECREASE IN THE SPREAD RELATIVE TO 24 HOURS AGO WHICH RAISES CONFIDENCE IN THE FORECAST. WITH THE 12Z NAM TO THE NORTH...ALL OTHER MODELS ARE GENERALLY IN LINE WITH ONE ANOTHER EXCEPT FOR SOME DIFFERENCES IN SPEED. THE 00Z CMC BECOMES A BIT QUICKER ACROSS THE LOWER GREAT LAKES BY TUESDAY. HOWEVER...IN THE GRAND SCHEME OF THINGS...DO NOT SEE ANY ISSUES IN USING ITS OUTPUT. WITH THE 12Z NAM BEING A NOTABLE OUTLIER...THE PREFERENCE HERE WILL BE A NON-12Z NAM MODEL BLEND. ...UPPER LOW STALLED OFFSHORE OF WESTERN MX THROUGHOUT MUCH OF THE PERIOD... PRELIMINARY PREFERENCE: BLEND OF THE 12Z GFS/00Z ECMWF FORECAST CONFIDENCE: AVERAGE DETACHED FROM THE PREVAILING WESTERLIES...A RETROGRADING SHORTWAVE ACROSS WESTERN MX WILL CONTINUE MOVING WESTWARD WHILE CLOSING OFF OVER THE SOUTHERN GULF OF CA/SEA OF CORTEZ. THE SYSTEM SHOULD GRADUALLY DRIFT SOUTHEASTWARD AS IT FEELS THE INFLUENCE OF AN UPSTREAM TROUGH. THROUGH 31/1200Z...MODELS ARE IN DECENT AGREEMENT BEFORE A FEW SOLUTIONS START TO ACCELERATE THE CLOSED LOW MORE QUICKLY THAN OTHERS. MOST NOTABLY...THE 12Z NAM AND EVENTUALLY THE 00Z CMC/UKMET ARE QUICKER TO OPEN UP THE WAVE AND TAKE IT TOWARD EASTERN MX. GIVEN THE SYSTEM WILL BE QUASI-STATIONARY AND CLOSED OFF...WOULD PREFER TO STAY ON THE SLOWER SIDE OF THE SPREAD WHICH FAVORS A BLEND OF THE 12Z GFS/00Z ECMWF. ...WESTERN FLANK OF LONGWAVE TROUGH ACROSS CANADA AFFECTING THE PACIFIC NORTHWEST... PRELIMINARY PREFERENCE: BLEND OF THE 12Z GFS/06Z GEFS MEAN FORECAST CONFIDENCE: AVERAGE ANOTHER CORRIDOR OF LOWER HEIGHTS WILL WORK THEIR WAY SOUTHWARD ACROSS BRITISH COLUMBIA/ALBERTA ON MONDAY/TUESDAY. THIS POSITIVELY-TILTED UPPER TROUGH MAY BE SLOW TO EXIT EASTWARD AS SOME OF THE SHORTWAVE ENERGY MAY SLIP UNDERNEATH A SPRAWLING RIDGE ACROSS THE NORTHEASTERN PACIFIC. SUCH A SCENARIO IS NOTED IN THE MANY OF THE ENSEMBLES CONSIDERING THE 546-DM SPAGHETTI PLOTS WEDNESDAY MORNING. MANY OF THE 00Z ECMWF ENSEMBLE MEMBERS ARE FARTHER TO THE EAST WHICH ALLOWS THE SYSTEM TO CARRY DOWNSTREAM MORE QUICKLY. THE DETERMINISTIC 00Z ECMWF IS EVEN EAST OF ITS OWN MEAN WHICH PLACES ITS SOLUTION IN QUESTION. GIVEN THE TENDENCY FOR SYSTEMS TO NEARLY CUT-OFF UNDERNEATH THE HIGHER HEIGHTS ACROSS THE GULF OF AK THE PAST COUPLE OF MONTHS...WILL FAVOR THE SLOWER CAMP HERE. THE PREFERENCE WILL BE LEAD BY THE 12Z GFS BUT COUPLED WITH THE 06Z GEFS MEAN TO ACCOUNT FOR SOME OF THE FORECAST SPREAD. ...SLOW-MOVING SYSTEM AFFECTING THE EASTERN PACIFIC BY MID-WEEK... PRELIMINARY PREFERENCE: BLEND OF THE 06Z GEFS/00Z ECMWF ENSEMBLE MEANS FORECAST CONFIDENCE: AVERAGE TOWARD THE END OF THE PERIOD...THE MEAN RIDGE ACROSS THE WESTERN U.S. STARTS TO FLATTEN IN ADVANCE OF AN IMPRESSIVE TROUGH APPROACHING 130W. ENSEMBLE SPAGHETTI PLOTS SHOW DECENT CLUSTERING SUGGESTING SOME EXTENT OF CONFIDENCE IN THE FORECAST. THERE ARE CONSIDERABLE DETAIL DIFFERENCES HOWEVER. GIVEN THE CONFLUENCE OF THE MID-LEVEL FLOW DOWNSTREAM...WILL STAY AWAY FROM THE 00Z ECMWF GIVEN ITS QUICKER PROGRESSION OF THE TROUGH ACROSS THE PACIFIC NORTHWEST. SUCH A SOLUTION LEADS TO MUCH LOWER QPF VALUES WHICH DOES NOT APPEAR REASONABLE GIVEN THE PATTERN SHAPING UP. FOR NOW WILL TAKE A BLEND OF THE ENSEMBLE MEANS BUT MAY ADJUSTMENT BASED ON FUTURE 12Z GUIDANCE. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... RUBIN-OSTER