MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 144 AM EST SAT DEC 12 2015 VALID DEC 12/0000 UTC THRU DEC 15/1200 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... 00Z MODEL EVALUATION INCLUDING THE NAM/GFS/UKMET/CMC/ECMWF ...PACIFIC LOW WEAKENING ACROSS THE INTERIOR NORTHWEST EARLY SATURDAY MORNING... PREFERENCE: GENERAL MODEL COMPROMISE CONFIDENCE: ABOVE AVERAGE A PACIFIC SURFACE LOW WILL MOVE INLAND IN THE NORTHWEST BEFORE WEAKENING SATURDAY MORNING. MODELS SHOW GOOD AGREEMENT WITH RESPECT TO THIS FEATURE BEFORE IT DISSIPATES...AND NO ONE MODEL APPEARS TO HAVE A SUPERIOR SOLUTION. ...UPPER-LEVEL TROUGH CROSSING THE FOUR CORNERS REGION ON SATURDAY... ...CLOSED UPPER-LEVEL LOW AND SURFACE CYCLONE ACROSS THE CENTRAL U.S. SAT NIGHT/SUNDAY... ...SURFACE LOW DEVELOPING OFF THE NEW ENGLAND COAST EARLY TUESDAY... PREFERENCE: GFS/ECMWF BLEND CONFIDENCE: AVERAGE THE NAM AND GFS LINE UP WELL WITH THE MODEL CONSENSUS WITH RESPECT TO THIS FEATURE AS IT CROSSES THE FOUR CORNERS REGION ON SATURDAY AND EMERGES INTO THE SOUTHERN/CENTRAL PLAINS ON SUNDAY. AFTER 00Z SUNDAY...THE NAM APPEARS TO LIFT THE UPPER LOW NORTHEASTWARD A LITTLE TOO QUICKLY COMPARED TO THE CONSENSUS. THE SURFACE LOW SHOWN BY THE NAM ACROSS THE UPPER MIDWEST EARLY MONDAY IS ALSO ON THE NORTHEAST SIDE OF THE SPREAD. THE GFS FITS REASONABLY WELL WITHIN CONSENSUS WITH RESPECT TO BOTH THE SURFACE AND UPPER-LEVEL LOWS. THE ECMWF HAS SLOWED DOWN SLIGHTLY FROM ITS PREVIOUS RUN AND NOW LAGS A LITTLE BEHIND THE GFS. BOTH LIE WITHIN THE REALM OF CONSENSUS...THUS A BLEND OF THE TWO SHOULD SUFFICE. WITH RESPECT TO THE SURFACE COLD FRONT CROSSING THE MISSISSIPPI/OHIO/TENNESSEE VALLEYS...THE NAM MAY BE SLIGHTLY TOO FAST WITH THE EASTWARD PROGRESSION SUNDAY EVENING...BUT APPEARS CLOSER TO THE CONSENSUS AFTER THAT TIME. THE GFS ALSO APPEARS A SHADE QUICK WITH THE COLD FRONT OVER THE SAME AREA...WHEN COMPARED TO THE ECMWF AND GEFS MEAN. OVERALL...A BLEND OF THE GFS AND ECMWF SHOULD BE A REASONABLE SOLUTION. BY EARLY TUESDAY MODELS SHOW A SURFACE LOW DEVELOPING JUST OFF THE NEW ENGLAND COAST ALONG THE FRONT. THE NAM AND GFS ARE CLOSE TO THE COAST WITH THE LOW...WITH THE ECMWF A LITTLE FARTHER SOUTHEAST. THE CMC AND UKMET ARE IN BETWEEN. GIVEN THE FAVORED GFS/ECMWF BLEND WITH THE UPPER-LEVEL LOW OVER QUEBEC AT THIS TIME...WILL FAVOR A BLEND OF THESE SAME TWO SOLUTIONS WITH THIS SURFACE LOW. ...PACIFIC STORM SYSTEM AFFECTING THE NORTHWEST SATURDAY AFTERNOON THROUGH SUNDAY... PREFERENCE: GFS/ECMWF/UKMET BLEND CONFIDENCE: AVERAGE MODEL AGREEMENT REMAINS GOOD WITH RESPECT TO THE SURFACE LOW ARRIVING AT THE BRITISH COLUMBIA COAST SATURDAY AFTERNOON/EVENING. THE NAM CONTINUES TO SHOW A STRONG WAVE DEVELOPING ALONG THE FRONT OFF THE PACIFIC NORTHWEST COAST EARLY SUNDAY MORNING...WHICH ALSO RESULTS IN A LATER ARRIVAL OF THE FRONTAL BOUNDARY TO THE COAST. THE GFS SOLUTION IS REASONABLY CLOSE TO THAT SHOWN BY THE ECMWF/UKMET. AS THE SYSTEM MOVES INLAND ON SUNDAY...THE NAM REMAINS ON THE SLOW SIDE WHILE THE GFS/ECMWF/UKMET CONTINUE TO SHOW SIMILAR TIMING AND STRENGTH OF THE SYSTEM. A BLEND OF THE GFS ALONG WITH THE ECMWF/UKMET IS RECOMMENDED AT THIS TIME. ...UPPER-LEVEL TROUGH CROSSING THE GREAT BASIN SUNDAY NIGHT AND EVOLVING INTO A CLOSED UPPER LOW OVER THE ROCKIES ON MONDAY... ...SURFACE LOW DEVELOPING EAST OF THE ROCKIES ON MONDAY... PREFERENCE: GFS/UKMET CONFIDENCE: BELOW AVERAGE ENERGY ALONG THE SOUTHERN EDGE OF THE WESTERN U.S. TROUGH WILL CROSS THE GREAT BASIN SUNDAY NIGHT BEFORE CLOSING OFF INTO AN UPPER-LEVEL LOW ON MONDAY ACROSS THE CENTRAL ROCKIES. A SURFACE LOW IS FORECAST TO DEVELOP EAST OF THE ROCKIES LATE MONDAY BEFORE MOVING INTO THE CENTRAL PLAINS ON TUESDAY. THERE IS A RANGE OF SOLUTIONS WITH RESPECT TO HOW QUICKLY THE SURFACE LOW MOVES EAST INTO THE PLAINS. THE NAM/GFS/CMC DEPICT A DEEPER UPPER-LEVEL LOW BY 12Z TUESDAY ACROSS COLORADO...WHICH RESULTS IN A SLOWER EASTWARD PROGRESSION OF THE SURFACE LOW. THE UKMET AND ECMWF HAVE ALSO SLOWED THE EASTWARD PROGRESSION OF THE SURFACE LOW...BUT IT APPEARS TO BE DUE TO A SLOWER UPPER-LEVEL LOW...RATHER THAN A DEEPER ONE AS SHOWN BY THE GFS/NAM/CMC. IN FACT...THE ECMWF UPPER-LEVEL LOW AT 12Z TUESDAY IS NOW FARTHER WEST THAN MOST GUIDANCE. WHILE THIS DOES REQUIRE A SHIFT IN THE PREFERENCE TOWARD A SLOWER SOLUTION...THERE IS STILL QUITE A BIT OF UNCERTAINTY HERE TO TO RELATIVELY POOR RUN TO RUN CONSISTENCY AMONG THE ECMWF/UKMET. THE 12Z ECMWF ENSEMBLE MEAN NOW APPEARS TO BE AN OUTLIER. AT THIS POINT CONFIDENCE IS BELOW AVERAGE...BUT WILL RECOMMEND A BLEND OF THE GFS/UKMET AS THESE TWO SOLUTIONS SEEM TO LIE CLOSEST TO THE MIDDLE OF THE MODEL SPREAD. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... RYAN