MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 103 AM EDT SUN SEP 20 2015 VALID SEP 20/0000 UTC THRU SEP 23/1200 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... 00Z NAM/GFS MODEL EVALUATION WITH FORECAST PREFERENCE AND CONFIDENCE ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ THERE DID NOT APPEAR TO BE ANY INITIALIZATION ERRORS WITH THE NAM/GFS DEEMED SIGNIFICANT ENOUGH TO IMPACT THEIR FORECASTS. UPPER TROUGH AND SURFACE LOW(S) OFF EAST COAST OF U.S. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: BLEND OF 21Z SREF/12Z ECMWF ENSEMBLE MEAN CONFIDENCE: BELOW AVERAGE WITH LIMITED SHEARED ENERGY THROUGH SUCH A LARGE ELONGATED TROF...THE MODELS HAVE ONCE AGAIN BEGUN TO INCREASE SPREAD OF WEAK SURFACE LOWS ALONG THE SURFACE TROF. MUCH OF THE WEATHER IS WELL OFF SHORE EXCEPT FOR THE 00Z GFS/18Z GEFS TRYING TO PULL A WEAK SURFACE LOW OFF THE CAROLINAS BACK TOWARD THE COAST TUESDAY AND EVENTUALLY AFFECT THE CAPE/NANTUCKET/MARTHA'S VINEYARD ON WED...THERE IS VERY LITTLE MODEL SUPPORT FOR THIS SOLUTION....AS SUCH MUCH OF THE SENSIBLE WEATHER REMAINS OFF SHORE WITHIN THE SHEAR AXIS. GIVEN THE VARIABILITY A PREFERENCE TOWARD THE MORE CONSISTENT ENSEMBLE MEANS...12Z ECENS MEAN AND 21Z SREF ARE PREFERRED BUT AT BELOW AVERAGE CONFIDENCE. COLD FRONT CROSSING NEW ENGLAND THROUGH SUNDAY ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: NON-GFS CONFIDENCE: ABOVE AVERAGE STRONG CONTINUITY CONTINUES WITH THE STEADY PROGRESSION OF THE COLD FRONT ACROSS NEW ENGLAND THROUGH SUNDAY EVENING...ONLY THE GFS AND ITS ENSEMBLE MEMBERS IN THE GEFS REMAIN ABOUT 2-4 HRS FASTER THAN THE REMAINING OPERATIONAL MODELS AND ENSEMBLES. SO WILL CONTINUE WITH CONTINUITY AT ABOVE AVERAGE CONFIDENCE OF A NON-GFS PREFERENCE. CLOSED 7H LOW IN THE OHIO VALLEY SUNDAY NIGHT THROUGH TUES ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: 00Z GFS CONFIDENCE: AVERAGE STRONG AGREEMENT OF THE DETERMINISTIC AND ENSEMBLE SUITE OF THE DEVELOPMENT OF A SMALL CLOSED 7H LOW BY 00Z MON ACROSS OH/KY BOARDER WHICH EVENTUALLY SHEARS OUT A DEEP CYCLONIC SHEAR AXIS THROUGH TUE/WED ACROSS THE EAST COAST. BOTH THE 12Z ECMWF AND 00Z NAM DEVELOP THE DEEPEST LOW INCLUDING A CUT-OFF DROPPING INTO THE GULF COAST BY THE 12Z EC. THIS SOLUTION IS NOT SUPPORTED EVEN BUT A FEW ECMWF ENSEMBLE MEMBERS. THE 12Z CMC/UKMET ARE FASTEST TO LIFT THE SHEAR AXIS NORTHEAST AND WEAKENING IT. WHILE THE 00Z GFS IS MOST MIDDLE GROUND BOTH DETERMINISTICALLY BUT ALSO IS A GOOD COMPROMISE TO THE 18Z GEFS (A BIT STRONGER) AND THE 12Z ECENS MEAN. AS SUCH THE 00Z GFS IS PREFERRED BUT WITH LARGE SPREAD IN A WEAK SHEAR AXIS AT AVERAGE CONFIDENCE. ZONAL ONSHORE FLOW INTO PACIFIC NORTHWEST WITH EMBEDDED SHORTWAVE CROSSING BRITISH COLUMBIA LATE SUN/EARLY MON...SURFACE LOW TRAVERSING CANADA WITH ATTENDANT COLD FRONT INTO THE NORTHERN TIER THROUGH TUES/WED ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: BLEND OF 00Z GFS/NAM AND 12Z ECMWF CONFIDENCE: AVERAGE ORIENTATION AND DURATION REMAIN IN FAIRLY GOOD AGREEMENT WITH STRONG ZONAL ONSHORE FLOW INTO VANCOUVER ISLAND THAT CLIPS THE OLYMPICS AND NORTHER CASCADES AS THE EVENT ENDS AND THE SHORTWAVE DRIVES THROUGH SUNDAY NIGHT INTO MONDAY MORNING. SMALL DIFFERENCES IN TIMING EXIST AS SOME MODELS (12Z ECMWF/UKMET RETAIN A BIT OF EXTRA ENERGY. THIS DIFFERENCE COMPOUNDS AS ENERGY CROSSES THE SOUTHERN CANADIAN ROCKIES IN THE FORMATION OF A DEEP SURFACE CYCLONE...HERE MODEL DIFFERENCES ARE GREATER BUT IMPACTS APPEAR MINIMAL ACROSS THE GREAT PLAINS WITH THE TIMING/ORIENTATION OF THE COLD FRONT. THERE ARE SOME TIMING DIFFERENCES WITH THE 00Z GFS/NAM BEING A BIT FORWARD COMPARED TO THE SLOWER 12Z ECMWF/CMC. THE 12Z UKMET LOOKS LIKE A GOOD MASS FIELD BLEND OF THE FAVORED GFS/ECMWF BUT SOME CONCERN IN QPF ACROSS THE GREAT PLAINS CAN BE SEEN AS INFLUENCES OF A FASTER TROPICAL CONNECTION OUT OF THE SOUTHWEST APPEAR UNSOUND. SIMILARLY THE 12Z CMC IS FASTER IN EJECTING ENERGY NORTH FROM THE TROPICS. AS SUCH A BLEND OF THE 00Z GFS/NAM AND 12Z ECMWF ARE PREFERRED AT AVERAGE CONFIDENCE. MID LEVEL CIRCULATION AND POSSIBLE SURFACE LOW MOVING NORTH NEAR BAJA CA INTO THE NEARBY SOUTHWEST US EARLY NEXT WEEK ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: BLEND OF 00Z NAM/GFS CONFIDENCE: SLIGHTLY ABOVE AVERAGE DECENT SPREAD EXISTS PARTICULARLY ALOFT WITH PLACEMENT OF THE UPPER LOW SHEARING OUT TO THE NORTHEAST. THE 12Z CMC/UKMET BOTH ARE PROGRESSIVE AND EASTWARD LIKELY HAVING THE TROPICAL SURFACE LOW AND ASSOCIATED MOISTURE DRIVE NORTH FASTER THAN OTHER DETERMINISTIC MODELS...THIS FASTER SOLUTION IS SUPPORTED BY THE MOST RECENT SREF (21Z) BUT THIS IS A SHIFT FOR THE SREF...PARTICULARLY GIVEN THE 00Z NAM IS NOW SLOWER THAN THE 00Z GFS AND 18Z GEFS MEAN. THERE APPEARS GOOD AGREEMENT WITH THE MOISTURE/TROPICAL LOW/QPF IN THE 00Z GFS/NAM AND 12Z ECMWF. THE 12Z ECMWF IS A BIT SLOWER LIKELY DUE TO THE SLOWER UPPER LOW TO THE WEST (WHICH HAS REMAINED WEST OF THE OTHER GUIDANCE FOR NUMEROUS MODEL CYCLES). AS SUCH THE THE PREFERRED BLEND IS THE 00Z NAM/GFS FOR MASS FIELDS ABOVE THE SURFACE BUT INCLUSION OF THE 12Z ECMWF WITH THE SURFACE LOW AND QPF AT SLIGHTLY ABOVE AVERAGE CONFIDENCE. INITIAL S/W OVER PACIFIC NORTHWEST BY TUESDAY WITH DEEPENING LARGE SCALE TROF BY WED ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: BLEND OF 21Z SREF/12Z ECMWF AND 00Z GFS/NAM CONFIDENCE: AVERAGE HEIGHTS BEGIN TO LOWER OVER THE GULF OF ALASKA INTO THE PAC NORTHWEST AS A DEEP LAYER TROF DEVELOPS IN THE EARLY PORTION OF NEXT WEEK. AN INITIAL SHORTWAVE SWINGS THROUGH BC BY TUESDAY WITH THE 12Z UKMET LEANING FORWARD AND DEEPER...THIS TREND CONTINUES WITH THE NEXT WAVE DIGGING THE LARGER SCALE TROF. THE 12Z CMC IS STRONGER/MORE CONSOLIDATED AND FURTHER EAST WITH THE INITIAL WAVE ALLOWING THE NEXT WAVE PACKETS TO BROADEN WHILE DEEPENING THE LOW...PRODUCING A TROF AXIS JUST OFF SHORE MIDDAY WED...THAT IS NOT WELL SUPPORTED THROUGH THE ENSEMBLE SUITE/SPAGHETTI PLOTS...INCLUDING ITS OWN CMC ENSEMBLE...WHERE THERE IS FAIRLY STRONG PACKING FOR DAY 3. SO THE 12Z ECENS MEAN AND 18Z GEFS ARE BEST REPRESENTED BY THE 21Z SREF AND 12Z ECMWF. THE 00Z GFS/NAM ARE IN MUCH BETTER AGREEMENT...PARTICULARLY WITH EACH OTHER...THEY CAN BE ADDED TO THE SREF/ECMWF BLEND THOUGH AT LOWER WEIGHTING. WITH SMALL DIFFERENCES IN VORT CENTERS AROUND THEIR PERIPHERY OF THE DEVELOPING LARGE SCALE LOW BUT GOOD OVERALL PLACEMENT/SIZE/STRENGTH CONFIDENCE IS AVERAGE IN SUCH A LARGE BLEND. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... GALLINA