MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 303 PM EDT MON APR 20 2015 VALID APR 20/1200 UTC THRU APR 24/0000 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... ...AN EVALUATION OF THE 12Z MODEL GUIDANCE ALONG WITH FINAL PREFERENCES/FORECAST CONFIDENCE... THERE DID NOT APPEAR TO BE ANY INITIALIZATION ERRORS WITH THE 12Z NAM/GFS DEEMED SIGNIFICANT ENOUGH TO IMPACT THEIR FORECASTS. ...ANOMALOUS CLOSED LOW AFFECTING THE GREAT LAKES... ...ASSOCIATED SURFACE LOW/ADVANCING COLD FRONT... FINAL PREFERENCE: MODEL COMPROMISE OF THE 12Z GFS/ECMWF/UKMET FORECAST CONFIDENCE: SLIGHTLY ABOVE AVERAGE A DEEP UPPER LOW WILL CONTINUE TO REMAIN A DOMINANT FORCE ACROSS THE GREAT LAKES REGION AS A SERIES OF VORTICITY MAXIMA ROTATE AROUND THE CYCLONE. MODELS VARY GREATLY REGARDING EACH INDIVIDUAL IMPULSE WHICH IS TO BE EXPECTED GIVEN THE COMPLEX NATURE OF THESE BROAD CIRCULATIONS. LOOKING AT THE SURFACE BOUNDARIES...THE GUIDANCE HAS A GOOD HANDLE ON THE INITIAL FRONTAL SYSTEM EXITING THE EASTERN U.S. WITH SLIGHTLY MORE UNCERTAINTY WITH THE SECONDARY COLD FRONT. THE 12Z NAM APPEARS OUTLYING AS IT DEVELOPS A STRONG WAVE OF LOW PRESSURE ACROSS THE OH VALLEY ON WEDNESDAY MORNING WHICH IS NOT SEEN IN ANY OTHER MODEL. ADDITIONALLY...THE 00Z CMC APPEARED MUCH TOO QUICK RELATIVE TO THE MORE STABLE LOOKING MEANS. THE 12Z GFS/ECMWF/UKMET SEEM TO BEST SUPPORT THESE MEANS AND A MODEL COMPROMISE OF THESE SOLUTIONS IS RECOMMENDED. ...INITIAL CUT-OFF LOW OFFSHORE OF BAJA CA... ...MORE ELONGATED UPPER LOW SETTING UP ACROSS THE SOUTHWESTERN U.S. BY LATE WEDNESDAY... FINAL PREFERENCE: BLEND OF THE 12Z UKMET WITH THE 12Z GEFS/00Z ECMWF ENSEMBLE MEANS FORECAST CONFIDENCE: SLIGHTLY BELOW AVERAGE THE INITIAL WEAK CIRCULATION OFF OF SOUTHERN CA SHOULD SLOWLY STRENGTHEN IN TIME AS IT MOVES TOWARD INLAND LOCALES ON TUESDAY EVENING. MODELS SEEM TO AGREE WELL WITH THIS EVOLUTION BUT THINGS BECOME MORE COMPLICATED AS ADDITIONAL UPSTREAM ENERGY APPROACHES THE REGION. BY MID-WEEK...THE SPREAD GROWS CONSIDERABLY WITH THE 12Z NAM BEING ON THE FAR WESTERN SIDE OF THE SOLUTION ENVELOPE. MEANWHILE...THE 00Z CMC IS QUICKER TO SLIDE HEIGHT FALLS TOWARD THE DESERT SOUTHWEST. AT THE CENTER OF THE BUILDING SPREAD ARE THE 12Z UKMET/00Z ECMWF AND 12Z GEFS/00Z ECMWF ENSEMBLE MEANS. IT APPEARS THE 12Z ECMWF TRENDED SIGNIFICANTLY SLOWER AND POTENTIALLY AWAY FROM THE STRONGER ENSEMBLE SUPPORT. PLAN ON INCORPORATING THE 12Z UKMET IN PLACE OF THE 00Z ECMWF FROM THE PREVIOUS PREFERENCE. ...STRONG HEIGHT FALLS REACHING THE PACIFIC NORTHWEST BY TUESDAY NIGHT... FINAL PREFERENCE: BLEND OF THE 12Z GEFS/00Z ECMWF ENSEMBLE MEANS FORECAST CONFIDENCE: BELOW AVERAGE A PRONOUNCED AREA OF HEIGHT FALLS WILL WORK THEIR WAY FROM THE NORTHEAST PACIFIC TOWARD THE PACIFIC NORTHWEST ON TUESDAY. THE MODELS BEGIN TO DIVERGE FROM ONE ANOTHER PRETTY QUICKLY ONCE THE SYSTEM PUSHES INLAND WITH MANY SOLUTIONS SUGGESTING THE SYSTEM WILL BECOME CUT-OFF IN NATURE. THE TWO SOLUTIONS PRESENTING OPPOSING IDEAS ARE THE 12Z GFS WHICH IS MUCH SLOWER AND AT TIMES ALMOST STATIONARY WHILE THE 00Z UKMET ACCELERATES THE SYSTEM OFF TOWARD THE NORTH AND EAST. UNFORTUNATELY THE DETERMINISTIC MODELS ARE NOT REALLY SUPPORTING THEIR OWN ENSEMBLE MEANS AT THIS TIME WHICH SIGNIFICANTLY LOWERS THE CONFIDENCE IN THIS FORECAST. WHILE A VAST MAJORITY OF THE DETERMINISTIC MODELS SUGGEST THE SYSTEM WILL BE CLOSED OFF...IT IS DIFFICULT TO PINPOINT HOW QUICKLY THIS SYSTEM WILL SLIDE EASTWARD. ALTHOUGH ENSEMBLE MEANS SHOW MORE OF AN OPEN WAVE IN THEIR MASS FIELDS...THIS IS THE BEST WAY TO HANDLE THE SITUATION. ...TRAILING CLOSED LOW NEARING THE WA COAST BY 24/0000Z... FINAL PREFERENCE: BLEND OF THE 12Z GEFS/00Z ECMWF ENSEMBLE MEANS FORECAST CONFIDENCE: BELOW AVERAGE IN THE WAKE OF THE INITIAL CLOSED LOW MOVING TOWARD THE NORTHERN ROCKIES WILL BE A MORE ROBUST CIRCULATION APPROACHING VANCOUVER ISLAND BY 24/0000Z. THERE APPEARS TO BE A LARGE ARRAY OF SOLUTIONS AT HAND WHICH IS SIGNIFIED IN THE MESSY LOOKING 540/552-DM SPAGHETTI PLOTS. THE 00Z ECMWF ENSEMBLE MEMBERS SEEM TO SHOW MORE AMPLIFICATION...PARTICULARLY IN THE PAST TWO MODEL CYCLES. IT LOOKS LIKE THE 12Z ECMWF HAS MOVED AWAY FROM ITS ENSEMBLE MEAN SHIFTING WELL TO THE NORTH. THERE DOES NOT SEEM TO BE GREAT RUN-TO-RUN CONTINUITY IN THE MODEL SOLUTIONS WHICH MAKES IT MORE REASONABLE TO STICK WITH AN ENSEMBLE APPROACH AS STATED IN THE PREVIOUS PREFERENCE. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... RUBIN-OSTER