MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1150 AM EST SAT NOV 29 2014 VALID NOV 29/1200 UTC THRU DEC 03/0000 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... ================================================== 12Z NAM/GFS EVALUATION AND PRELIMINARY PREFERENCES ================================================== NAM/GFS INITIALIZATION ERRORS DO NOT APPEAR TO SIGNIFICANTLY AFFECT THEIR LARGE SCALE OUTPUT. ================================================== DAY 3/TUESDAY IN CALIFORNIA ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: BLEND OF 12Z NAM WITH 00Z ECMWF/CANADIAN THE MODELS AND ENSEMBLES ARE LARGELY IN AGREEMENT AS TO THE DEPTH AND LATITUDE THAT A CLOSED SOUTHERN STREAM LOW WILL ACHIEVE...AND ALSO IN THE SLOW PACE IT WILL SET WHILE MOVING INTO CALIFORNIA. THE 12Z NAM AND 00Z ECMWF/CANADIAN CLUSTER WELL WITH EACH OTHER AND WITH THE ENSEMBLES. OUTLYING SOLUTIONS THAT HAD BEEN MUCH SLOWER ARE NO LONGER PRESENT IN THE ENSEMBLES...GIVING US INCREASED CONFIDENCE IN THE TIMING. THE 12Z GFS IMPROVED RELATIVE TO 00Z AND 06Z RUNS. IN FACT...PRECIPITATION OUTPUT FROM THE 12Z GFS IS SIMILAR TO THE MODEL CONSENSUS OVER CALIFORNIA. WE WITHHELD THE GFS FROM OUR PREFERENCE...HOWEVER...AS IT IS STILL A LITTLE FAST TO BRING THIS SYSTEM TOWARD SHORE. THE 00Z UKMET WAS ALSO ON THE FAST SIDE...BUT IT WAS MORE GREATLY OUT OF PHASE WITH CONSENSUS REGARDING THE DOWNSTREAM SHORTWAVE RIDGE OVER THE WESTERN U.S. SOUTHERN STREAM IMPULSE INTO CALIFORNIA SUNDAY AND CONTINUING EAST ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: BLEND OF 12Z NAM/GFS AFTER DELIVERING A DAY 1 PRECIPITATION EVENT TO THE NORTHERN HALF OF CALIFORNIA...THIS LOW AMPLITUDE WAVE WILL RACE ACROSS THE U.S...AGAIN RESULTING IN PRECIPITATION NEAR A SINKING FRONTAL ZONE FROM THE MISSISSIPPI VALLEY TO THE MID ATLANTIC. BEING OF SUCH LOW AMPLITUDE...THE MODEL DIFFERENCES ARE SUBTLE. BUT THERE IS A NOTICEABLE TREND TOWARD SLOWER MOVEMENT IN THE MORE RECENTLY INITIALIZED MODELS. THE 12Z NAM/GFS AND 09Z SREF SHOWING SIMILAR TRENDS AND SOLUTIONS GIVES US SOME CONFIDENCE THAT THE TRENDS ARE TOWARD A BETTER FORECAST. TROUGH PROGRESSING FROM THE PAC NW TO THE NORTHERN PLAINS MON/TUE ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: BLEND OF THE OPERATIONAL MODELS...LESS WEIGHT ON THE NAM THIS WAVE DROPS INTO THE NORTHWEST AND NORTH-CENTRAL U.S...REPLENISHING ARCTIC AIR OVER THOSE AREAS. MODELS ARE SIMILAR IN THE LOWER AND MIDDLE LEVELS. THE ONLY LARGE SCALE ODDITY IS THE NAM BEING SLOWER THAN CONSENSUS IN MOVING THE FRONT THROUGH THE NORTHERN PLAINS ON DAY 3. UPPER TROUGH AND SURFACE COLD FRONT SWEEPING THE CENTRAL/EASTERN U.S. SUN/MON ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~ PREFERENCE: BLEND OF 12Z NAM/GFS WITH 00Z ECMWF IN THE NORTHERN STREAM THERE HAS BEEN A DICHOTOMY...WITH THE GEFS MEAN SUPPORTING THE FASTER GFS RUNS...AND ECMWF ENSEMBLE MEAN SUPPORTING THE SLOWER ECMWF RUNS. THESE DIFFERENCES ARE MOST PRONOUNCED AS THE FRONT PASSES THROUGH NEW ENGLAND ON DAY 2. THE GFS CERTAINLY HAS A KNOWN FAST BIAS...BUT THE ECMWF IS SOMETIMES TOO AMPLIFIED AND SLOW WHEN A WAVE IS DIGGING ATOP ARCTIC AIR...UNFAVORABLE FOR AMPLIFICATION/SLOWING. RECENT NAM RUNS HAVE REPRESENTED A MIDDLE GROUND...AND THE 12Z GFS DID SLOW DOWN A TAD...BRINGING IT CLOSER TO THE NAM. WE RECOMMEND BLENDING THESE SOLUTIONS WITH THE 00Z ECMWF TO FIND A CONSENSUS TIMING FOR THE ARRIVAL OF COLDER AIR ACROSS NEW ENGLAND...AND ACROSS THE MID ATLANTIC...ALTHOUGH MODEL SPREAD IS LOWER THERE. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... BURKE