MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 149 AM EDT MON SEP 28 2015 VALID SEP 28/0000 UTC THRU OCT 01/1200 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. ...STATIONARY UPPER LOW LIFTING OUT OF THE WESTERN GULF COAST... ...EVENTUAL PHASING WITH THE NORTHERN BRANCH OF THE JET... PRELIMINARY PREFERENCE: BLEND OF THE 00Z NAM/GFS WITH THE 12Z ECMWF FORECAST CONFIDENCE: SLIGHTLY BELOW AVERAGE A PERSISTENT UPPER LOW CIRCULATING OVER THE UPPER TX COAST CONTINUES TO LIFT ABUNDANT TROPICAL MOISTURE AND EMBEDDED VORTICITY MAXIMA TOWARD THE CENTRAL/EASTERN GULF COAST. HOW QUICKLY THIS PROCESS OCCURS WILL DICTATE THE AMOUNT OF PHASING WHICH OCCURS BETWEEN IT AND THE PROGRESSIVE NORTHERN STREAM. THE TREND IN THE 582-DM SPAGHETTI PLOTS SHOW THE ECMWF ENSEMBLE MEMBERS HAVE BEEN THE MOST STABLE FAVORING A SLOWER SOLUTION EJECTING OUT OF THE GULF OF MX. THE PREVIOUS GEFS SOLUTIONS WERE A TAD QUICKER BUT THE 00Z GFS APPEARS TO HAVE TRENDED TOWARD THE SLOWER ECMWF SUITE. THROUGH 30/0000Z...THE 00Z NAM/GFS AND 12Z ECMWF WERE IN CLOSE AGREEMENT WITH ONE ANOTHER AND THE 12Z ECMWF ENSEMBLE MEAN. THE 12Z CMC AND 21Z SREF MEAN REMAIN FAST OUTLIERS AND WILL NOT BE CONSIDERED HERE. WILL FAVOR THIS SLOWER SCENARIO FAVORING SOME COMBINATION OF THE 00Z NAM/GFS WITH THE 12Z ECMWF. ...CONVECTIVE LOWS MIGRATING TOWARD THE GULF COAST... ...WAVE QUICKLY EJECTING UP THE EASTERN SEABOARD/ADJACENT COASTAL ATLANTIC... PRELIMINARY PREFERENCE: BLEND OF THE 18Z GEFS/12Z ECMWF ENSEMBLE MEANS FORECAST CONFIDENCE: BELOW AVERAGE THE 05Z SURFACE OBSERVATIONS SHOW A PAIR OF SURFACE CYCLONES...ONE JUST SOUTH OF THE TX/LA BORDER WHILE ANOTHER IS OVER THE SOUTHEASTERN GULF OF MX. THE WESTERN WAVE SHOULD REMAIN NEARLY QUASI-STATIONARY WHILE THE LATTER RACES TOWARD THE NORTH. THIS PARTICULAR LOW CENTER CURRENTLY AS A 40 PERCENT CHANCE FOR TROPICAL DEVELOPMENT WITHIN THE NEXT 48 HOURS PER THE NHC OUTLOOK. AS THE SYSTEM EJECTS TOWARD THE NORTH...ADDITIONAL AREAS OF LOW PRESSURE MAY DEVELOP AND IMPINGE ON THE ADVANCING BAROCLINIC ZONE. IT IS A PRETTY LOW CONFIDENCE FORECAST GIVEN THE SURFACE LOWS IN THE GULF OF MX ARE QUITE CONVECTIVE AT THE MOMENT AND IN A GENERAL VOID OF SAMPLING. WILL TAKE A SPLIT BETWEEN THE TWO ENSEMBLE SYSTEMS...THE 18Z GEFS MEAN AND THE 12Z ECMWF ENSEMBLE MEAN. ...NORTHERN STREAM TROUGH ADVANCING ALONG THE INTERNATIONAL BORDER... ...ASSOCIATED COLD FRONT INTERCEPTING ABUNDANT TROPICAL MOISTURE... PRELIMINARY PREFERENCE: BLEND OF THE 00Z NAM/GFS WITH THE 12Z ECMWF FORECAST CONFIDENCE: AVERAGE WATER VAPOR IMAGERY TONIGHT SHOWED A STRONG 500-MB HEIGHT GRADIENT ACCOMPANYING SWIFT HEIGHT FALLS MOVING ALONG THE INTERNATIONAL BORDER. DIFFERENCES ARE SMALL UNTIL AROUND TUESDAY EVENING WHERE SOME TIMING/AMPLITUDE UNCERTAINTIES ARE EVIDENT. SUCH SOLUTIONS DIVERGE FROM THE MORE CLUSTERED ENSEMBLE MEANS AT THIS TIME. THE 12Z CMC IS ON THE SLOWER END WHILE THE 12Z UKMET CONTAINS A LOWER AMPLITUDE TROUGH. WILL STICK WITH THE CAMP OF THE 00Z NAM/GFS AND 12Z ECMWF. THIS IS CONSISTENT WITH THE PREFERENCE ACROSS THE SOUTHERN U.S. WHERE PHASING BETWEEN THE TWO SYSTEMS WILL OCCUR LEADING TO A POTENTIAL FOR A HEAVY RAINFALL EVENT. ...POTENTIAL REX BLOCK SET UP OVER THE NORTHERN PLAINS BY MID-WEEK... PRELIMINARY PREFERENCE: BLEND OF THE 18Z GEFS/12Z ECMWF ENSEMBLE MEANS FORECAST CONFIDENCE: BELOW AVERAGE TOWARD THE END OF THE PERIOD...THE MODELS SUGGEST A REX BLOCK SET UP AS AN UPPER RIDGE DEVELOPS ACROSS CENTRAL CANADA BOUNDED BY A WEAKNESS TO THE SOUTH. GIVEN THE COMPLEXITIES OF THIS EVOLUTION...THE MODELS ARE ALL OVER THE PLACE. THUS...WILL SUPPORT AN EVEN BLEND OF THE 18Z GEFS/12Z ECMWF ENSEMBLE MEANS. ...UPPER TROUGH APPROACHING THE WEST COAST TUESDAY THROUGH THURSDAY... PRELIMINARY PREFERENCE: BLEND OF THE 18Z GEFS/12Z ECMWF ENSEMBLE MEANS...WEIGHTED 3/4 TOWARD THE FORMER FORECAST CONFIDENCE: LOW AN UPPER TROUGH ADVANCING TOWARD THE WESTERN U.S. HAS QUITE A VARIETY OF FORECAST SCENARIOS AT HAND. WHAT INITIALLY EXISTS AS A CUT-OFF LOW IN THE VICINITY OF 40N LATITUDE IS EVENTUALLY SHUNTED EASTWARD IN RESPONSE TO AN UPSTREAM SHORTWAVE PASSAGE. ENSEMBLE SPAGHETTI PLOTS ARE QUITE MESSY SUGGESTING A LOW CONFIDENCE FORECAST AT BEST. THE GENERAL TREND IN THE PAST COUPLE OF DAYS IS TO BE SLOWER WHICH MAKES SENSE GIVEN THE FORECAST DEALS WITH A SLOW-MOVING/NEARLY QUASI-STATIONARY UPPER CYCLONE. EVEN WITHIN THEIR OWN ENSEMBLE SYSTEMS...INDIVIDUAL SOLUTIONS VARY QUITE MARKEDLY. THE GEFS SUITE IS SLOWER THAN THE ECMWF ENSEMBLES SO WILL GIVE MORE WEIGHTING TO THE FORMER. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... RUBIN-OSTER