MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 105 PM EDT THU APR 30 2015 VALID APR 30/1200 UTC THRU MAY 04/0000 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... THERE DID NOT APPEAR TO BE ANY INITIALIZATION ERRORS WITH THE 12Z NAM/GFS DEEMED SIGNIFICANT ENOUGH TO IMPACT THEIR FORECASTS. ...UPPER LOW SLIDING FROM THE OH VALLEY TO THE CAROLINAS THROUGH 01/1800Z... ...PAIR OF SURFACE LOWS OVER THE WESTERN ATLANTIC... PRELIMINARY PREFERENCE: GENERAL MODEL COMPROMISE FORECAST CONFIDENCE: SLIGHTLY ABOVE AVERAGE WATER VAPOR IMAGERY THIS MORNING SHOWED A CLOSED LOW DROPPING DOWN TOWARD THE UPPER OH VALLEY WHILE A SEPARATE FEATURE WAS BEGINNING TO EXIT THE CAROLINA COAST. THIS PAIR OF SYSTEMS HAVE HELPED SPREAD LIGHT TO MODERATE PRECIPITATION OVER THEIR RESPECTIVE REGIONS. OVERALL...THE EVENTUAL EVOLUTION OF THESE FEATURES HAS BECOME BETTER RESOLVED THAN RECENT DAYS. MOST OF THE DIFFERENCES APPEAR TO OCCUR ONCE THE SYSTEM IS OFFSHORE OF THE EASTERN U.S. REGARDING THE COASTAL LOW...THE PAST FEW RUNS HAVE BEGUN TO SHIFT THE TRACK FASTER EAST AND MORE AWAY FROM THE COAST. THE DETERMINISTIC MODELS HAVE APPEARED TO ADJUSTED THEIR FORECASTS ACCORDINGLY TO MATCH THESE TRENDS. OVERALL...GIVEN THE DIFFERENCES ARE PRIMARILY OFFSHORE...WILL LEAN ON A GENERAL MODEL COMPROMISE HERE. ...SHORTWAVE MOVING FROM CENTRAL MX TO SOUTH FL... PRELIMINARY PREFERENCE: BLEND OF THE 12Z GFS/00Z ECMWF FORECAST CONFIDENCE: AVERAGE A PROGRESSIVE SHORTWAVE WILL ADVANCE FROM CENTRAL MX ACROSS THE GULF OF MX EVENTUALLY REACHING SOUTH FL BY MIDDAY SATURDAY. LIKE YESTERDAY...THE 00Z ECMWF/ECMWF ENSEMBLE MEAN BECOME A BIT SLOWER WHILE TRACKING NORTH OF CUBA. GIVEN HOW INSISTENT IT IS ON THIS EVOLUTION...WILL SPLIT THE DIFFERENCE BETWEEN ITS SOLUTION AND ONE FARTHER DOWNSTREAM. THE RECOMMENDATION IS A BLEND OF THE 12Z GFS/00Z ECMWF. ...UPPER TROF GRAZING THE NORTHERN PLAINS THIS EVENING... ...REMNANT SHORTWAVES CROSSING THE OH VALLEY/CENTRAL APPALACHIANS... PRELIMINARY PREFERENCE: GENERAL MODEL COMPROMISE FORECAST CONFIDENCE: ABOVE AVERAGE AN AXIS OF HEIGHT FALLS WILL CONTINUE TO MARCH EASTWARD FROM THE NORTHERN ROCKIES TOWARD THE PLAINS THIS AFTERNOON. WHILE A MAJORITY OF THE SHORTWAVE ENERGY SHOULD MOVE UP INTO CENTRAL CANADA...SOME REMNANT VORTICITY IS FORECAST TO MOVE THROUGH THE LOWER GREAT LAKES/OH VALLEY REGION EARLY IN THE WEEKEND. THE MODEL SPREAD IS SURPRISINGLY LOW GIVEN THE COMPLEX EVOLUTION SO A GENERAL MODEL COMPROMISE IS SUPPORTED. ...BROAD LONGWAVE TROF EXTENDING ACROSS WESTERN/CENTRAL CANADA... ...COLD FRONT SWEEPING THROUGH THE GREAT PLAINS BY SUNDAY... PRELIMINARY PREFERENCE: THROUGH 03/0000Z: GENERAL MODEL COMPROMISE...THEREAFTER: BLEND OF THE 06Z GEFS/00Z ECMWF ENSEMBLE MEAN FORECAST CONFIDENCE: AVERAGE THROUGHOUT MUCH OF THE FORECAST PERIOD...A BROAD LONGWAVE TROF WILL EXTEND OVER WESTERN TO CENTRAL CANADA WHILE WEAKER FLOW EXISTS ACROSS THE CONUS. THE MOST PRONOUNCED SYSTEM SHOULD BEGIN TO CROSS SOUTHERN MANITOBA BY SUNDAY MORNING WITH A COLD FRONT SWEEPING THROUGH THE PLAINS. THE LARGEST OUTLIER APPEARS TO BE THE 00Z UKMET WHICH IS MUCH MORE PROGRESSIVE. THERE DOES SEEM TO BE ENOUGH DETERMINISTIC SPREAD TO SUPPORT MORE OF AN ENSEMBLE APPROACH FOR DAY 3. THROUGH DAY 2...A GENERAL MODEL COMPROMISE IS WARRANTED BEFORE SHIFTING TO THE MEANS FOR THE FOLLOWING DAY. ...ELONGATED UPPER LOW EXTENDING ALONG 30N IN THE SUBTROPICAL EAST PACIFIC... PRELIMINARY PREFERENCE: MODEL COMPROMISE BETWEEN THE 12Z NAM AND 00Z ECMWF/UKMET FORECAST CONFIDENCE: SLIGHTLY BELOW AVERAGE TOWARD THE END OF THE PERIOD MODELS ARE ADVERTISING A BROAD UPPER TROF ACROSS THE SUBTROPICAL EAST PACIFIC WITH THE GUIDANCE VARYING IN THE STRENGTH OF THIS SYSTEM. ON THE MORE AGGRESSIVE SIDE ARE THE 12Z GFS/00Z CMC WHICH IS SUPPORTED BY THE 06Z GEFS/00Z ECMWF ENSEMBLE MEANS. GIVEN IT IS A DAY 3 FORECAST AND THE SYSTEM EMERGES ONTO THE MAP DOMAIN LATE IN THE PERIOD...WILL PLAY IT MORE CONSERVATIVELY LEANING ON A MODEL COMPROMISE BETWEEN THE 12Z NAM AND 00Z ECMWF/UKMET. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... RUBIN-OSTER