MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 257 AM EDT FRI SEP 18 2015 VALID SEP 18/0000 UTC THRU SEP 21/1200 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... AN EVALUATION OF THE 00Z 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. BROAD MID-LEVEL TROF ALONG THE EASTERN SEABOARD INTO THE EASTERN GULF OF MEXICO ALONG WITH ASSOCIATED SURFACE TROF/SURFACE LOWS ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: BLEND OF THE 00Z ECMWF/21Z SREF AND 12Z ECMWF ENSEMBLE MEAN CONFIDENCE: BELOW AVERAGE OVERALL SYNOPTIC PATTERN APPEARS WELL INITIALIZED WITH THE BROAD TROF ALONG THE EAST COAST...WITH A DEVELOPING WEAK CYCLONE OFF NJ/LONG ISLAND ALREADY DEPICTED WELL IN WV...AND DEEP POSITIVELY TILTED TROF ACROSS THE SOUTHEAST AND THE EASTERN GULF WITH HIGHLY AMPLIFIED RIDGE OVER THE CENTRAL BAHAMAS AND NORTHEAST INTO THE SARGASSO SEA AS WELL AS OVER TX INTO THE MID-MISSISSIPPI VALLEY. WEAK SURFACE INFLECTIONS EXIST EITHER SIDE OF THE FLORIDA PENINSULA...EVOLUTION OF THE LOWS INTO A BROADER CYCLONE OVER THE GULF STREAM OFF THE GA/CAROLINA COASTLINE... REMAIN IN GOOD AGREEMENT THROUGH SATURDAY. INCREASING SPREAD IN OPERATIONAL AND ENSEMBLE MEMBERS EMPHASIZE THE TEETERING POINT BETWEEN THE CYCLONE INFLUENCED NORTHWARD BY APPROACHING NORTHERN STREAM TROF OR CYCLONE REMAINING RELATIVELY STATIONARY NOT BEING INFLUENCED NORTH. THE 12Z ECMWF IS INFLUENCED LEAST REMAINING NEARLY STATIONARY...WITH SUPPORT OF NUMEROUS 12Z ECMWF ENSEMBLE MEMBERS (AND THEREFORE THE ENSEMBLE MEAN) AND AT ONE POINT THE 12Z GFS. THE 00Z ECMWF CONTINUED THE TREND THOUGH HAS DEEPENED THE LOW A BIT MORE AS IT WOBBLES AROUND 30-32N. LIKELY DUE TO THIS UNSTABLE TEETERING POINT THE GFS HAS WAFFLED BACK AND FORTH AND ITS OPERATIONAL RUNS HAVE NOT MATCHED THE GEFS MEANS MUCH UNTIL THIS 00Z RUN. THE 00Z GFS ALONG WITH THE 00Z CMC IS THE FASTEST WITH A SURFACE LOW MOVING UP THE COAST OVER GEORGES BANK BY MONDAY...BUT DUE TO THE ELONGATION OF THE TROF ALSO PRODUCES A STRING OF PEARLS SOUTH AND EAST INCLUDING A WEAK WAVE EAST OF THE 00Z ECMWF RUN. THE 00Z CMC ALSO RETAINS ENERGY NEARER THE SOUTHERN LOW ALSO PROVIDING SOME SUPPORT FOR THE ECMWF. THE 00Z UKMET IS THE WEAKEST WITH THE TROF AND SURFACE LOW EVEN MORE THAN ITS 12Z RUN...HOWEVER IT KEEPS WHAT LIMITED ENERGY IT DOES RESOLVE NEARER THE SLOWER/ECMWF SOLUTION. THE 00Z NAM HAS BEEN CONSISTENT RUN TO RUN AND HAS SUPPORT FROM THE 12Z AND 18Z GEFS...BUT TYPICAL TO ITS BIAS APPEARS A BIT DEEPER IN THE HEIGHT FIELDS AND STRENGTHENS A MIDDLE GROUND LOW WELL OFF THE MID-ATLANTIC COAST NEAR 70W. THE 21Z SREF LAGS THE 18Z AND 00Z NAM RUNS AND SPLITS THE DIFFERENCE IN THE 12Z ECMWF AND 00Z NAM SOLUTION...BUT LOOKS LIKE A GOOD MATCH TO THE 12Z ECMWF ENSEMBLE MEAN WHICH INCREASES ITS SOLUTION AS A DECENT COMPROMISE...AND IS FURTHER STRENGTHENED BY STRONGER AGREEMENT WITH THE 00Z ECMWF. GIVEN THIS LARGE SPREAD IN SURFACE SOLUTIONS AND HIGH VARIABILITY...VERY LOW CONFIDENCE IS GIVEN TO THE PREFERENCE: A BLEND THAT FAVORS THE 00Z ECMWF ENSEMBLE MEAN WHICH CAN BE REPRESENTED BY THE 21Z SREF AND 12Z ECMWF RUN. INITIAL SURFACE CYCLONE LIFTING INTO WESTERN ONTARIO...MIGRATORY SHORTWAVE ADVANCING FROM THE CENTRAL GREAT BASIN TO THE GREAT LAKES BY 19/12Z WITH ATTENDANT SURFACE LOW/COLD FRONT ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: NON-NAM BLEND CONFIDENCE: ABOVE AVERAGE THE 12Z ENSEMBLE SURFACE LOW CLUSTER WAS REMARKABLY TIGHT WITH THE INITIAL SURFACE LOW IN ONTARIO AS WELL AS THE POSITION/TRACK OF THE LEE-CYCLONE LEAVING THE TX/OK PANHANDLES THURSDAY NIGHT THROUGH THE MISSOURI VALLEY INTO THE LP OF MICHIGAN SATURDAY MORNING. AS SUCH ONLY SMALL DIFFERENCES EXIST WITHIN THE MASS FIELDS. THE 00Z NAM IS SLOWER AND DEEPENED BELOW 1000 MB IN MI...MAKING IT THE MOST SIGNIFICANT OUTLIER...THOUGH ONLY SUBTLY SO THAT IF THERE WERE LESS AGREEMENT WITH THE OTHER MODELS AND ENSEMBLE MEANS...ONE WOULD LIKELY BLEND IT IN AS WELL. HOWEVER...GIVEN THE STRENGTH OF PACKING IN POSITIONING/STRENGTH/TIMING A NON-NAM BLEND IS PREFERRED AT ABOVE AVERAGE CONFIDENCE. IMPULSE CROSSING THE NORTH CENTRAL U.S. THROUGH THE WEEKEND ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: BLEND OF THE 00Z GFS/ECMWF CONFIDENCE: SLIGHTLY ABOVE AVERAGE IN THE WAKE OF THE MAIN TROF A LESS DEFINED AND BROADER IMPULSE MOVES THROUGH THE NORTHERN PLAINS BY SUNDAY AND OVER THE GREAT LAKES AT THE END OF THE PERIOD. THIS LOW AMPLITUDE TROUGH IS CHARACTERIZED BY THE ENSEMBLE SPREAD BASED ON THE 576-DM SPAGHETTI PLOT WITH A MULTI-DAY TREND IN THE SLIGHTLY FASTER DIRECTION WITH MUCH BETTER AGREEMENT. AS WITH KNOWN BIAS...THE 00Z NAM IS THE DEEPEST ALONG WITH THE 21Z SREF. SMALL CHANGES IN THE 00Z ECMWF DO NOT AFFECT ITS CONTINUED INCLUSION IN THE PREFERRED BLEND....AS IT IS REMAINS A GOOD REPRESENTATION OF THE ENSEMBLE MEANS. THE 00Z CMC WAS A BIT FAST AND THE 00Z UKMET THOUGH DEEPER THAN ITS 12Z RUN HAD A NARROW WAVELENGTH MAKING IT APPEAR OUT OF PLACE WITH THE OTHER GUIDANCE. STILL ORIENTATION AND POSITION IS GOOD ENOUGH FOR SLIGHTLY ABOVE AVERAGE CONFIDENCE IN BLENDING THE MIDDLE SOLUTIONS OF THE 00Z GFS/12Z ECMWF BEING GOOD REPRESENTATIVES OF THE STRENGTHENING ENSEMBLE SUPPORT FROM THE GEFS AND ECMWF ENSEMBLE MEMBERS. STRONG ZONAL ONSHORE FLOW SLOWLY DROPPING SOUTH ALONG THE BC COAST UNTIL AFFECTING THE NORTHWEST U.S CLIPPING FAR NORTHWEST U.S. WITH TRAILING SHORTWAVE ON MONDAY MORNING ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: NON-UKMET BLEND CONFIDENCE: AVERAGE A PROLONGED STRONG ZONAL FLOW IS CURRENTLY POINTED AT SOUTHERN ALASKA AND NORTHERN BRITISH COLUMBIA COAST WITH STRONG ASCENT AND MOISTURE FLUX FOR HIGH QPF...THIS FLOW REMAINS STRONG AND SLOWLY SHIFTS DOWN THE COAST TO IMPACT NORTHERN VANCOUVER ISLAND SATURDAY. A STRONG SHORTWAVE INCREASES QPF/IMPACTS SUNDAY MORNING WITH SOME PERIPHERAL AFFECTS BEGINNING TO AFFECT THE OLYMPICS AND NORTHERN CASCADES AND HITTING IN EARNEST SUNDAY NIGHT INTO MONDAY MORNING A WEAK KICKER WHICH SHOULD BRING THE CLOSE TO THE HIGHEST MOISTURE FLUX/QPF SIGNAL. THE 00Z UKMET REMAINS ESSENTIALLY UNCHANGED BRINGING THE SHORTWAVE AND ASSOCIATED SURFACE LOW ASHORE FIRST FOLLOWED BY THE 00Z GFS...00Z NAM/00Z ECMWF WITH THE 00Z NOW FOLLOWING UP THE REAR...THOUGH THE TIMING IS NOT SUBSTANTIALLY DIFFERENT GIVEN THE STRENGTH/SPEED OF THE FLOW. THE GEFS AND SREF TIMING IS IN LINE WITH THE 00Z GFS BUT ARE FURTHER NORTH...LIKELY LAGGING THE TREND SOUTHWARD OF THE AXIS THE PACIFIC PLUME. WITH THE 00Z ECMWF SPEEDING UP...IT IS ESSENTIALLY IN PHASE WITH THE 12Z ECMWF ENSEMBLE MEAN AND MATCHES THE 00Z NAM QUITE WELL...BLENDING THE 00Z CMC AND 00Z GFS WOULD ALSO MATCH WELL WITH THE 00Z ECMWF/NAM LOCATION/SPEED. SO ALL IN ALL WITH GOOD IMPACT/QPF AGREEMENT AND DECENT MASS FIELD AGREEMENT IN TIMING/STRENGTH OF THE LOW/SHORTWAVE...A NON-UKMET BLEND IS PREFERRED WITH SUPPORT OF THE 12Z ECMWF ENSEMBLE MEAN AT AVERAGE CONFIDENCE....THOUGH WEIGHTING SHOULD FAVOR THE ECMWF AND NAM. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... GALLINA