MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 257 AM EDT SUN MAY 18 2014 VALID MAY 18/0000 UTC THRU MAY 21/1200 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... ============================================== 00Z MODEL EVALUATION AND PREFERENCES ============================================== NAM/GFS INITIALIZATION ERRORS DO NOT APPEAR TO SIGNIFICANTLY AFFECT THEIR LARGE SCALE OUTPUT. ============================================== ...DEEP SYSTEM FORMING IN THE WEST... PREFERENCE: BLEND OF 00Z NAM/GFS/ECMWF/CANADIAN CONFIDENCE: SLIGHTLY ABOVE AVERAGE THE OPERATIONAL MODELS SHOW STRONG AGREEMENT HERE...AND RUN TO RUN VARIABILITY IS QUITE SMALL. WE DO NOTE...HOWEVER...THAT WHILE THE GFS TRENDED SOMEWHAT QUICKER...THE ECMWF/UKMET TRENDED SOMEWHAT SLOWER AND DEEPER...DIGGING THE 500 MB LOW TOWARD LAS VEGAS BY 21/12Z. SPREAD IS MINIMAL ENOUGH THAT WE CAN STILL RECOMMEND A FAIRLY GENERAL BLEND...BUT WE DID OMIT THE 00Z UKMET. IT WAS NOT AN OUTLIER IN THE STATISTICAL SENSE...BUT IS A LITTLE MORE SIGNIFICANTLY REMOVED FROM THE CONSENSUS CLUSTER. WE ALSO NOTE THAT THE NAM FORECAST IS BETTER THAN IT TYPICALLY IS ON DAY 3. THIS RUN HAS LARGELY AVOIDED ITS BIAS OF COOLER MID LEVEL THERMAL FIELDS. ...PROGRESSIVE SYSTEM TRACKING FROM THE NORTHERN ROCKIES TO GREAT LAKES... PREFERENCE: BLEND OF 00Z ECMWF/UKMET/CANADIAN CONFIDENCE: AVERAGE JUST AS THE NAM HAD TRENDED INTO AGREEMENT WITH THE GLOBAL MODEL CONSENSUS...THE ECMWF/UKMET/CANADIAN CAME IN WITH SLOWER SOLUTIONS. THE DIFFERENCE IS MORE NOTICEABLE AT 500 MB...WHEREAS THE NAM SURFACE TO 700 MB FORECAST IS STILL IN DECENT AGREEMENT WITH THE GLOBAL MODELS. TRENDS BROUGHT THE EC/UK/CMC CLOSER TO THE 00Z GFS FOR TIMING...BUT THE GFS IS STILL MORE WEAKLY DEFINED IN THE MID LEVEL HEIGHT AND WIND FIELDS...AND THEREFORE IS NOT PREFERRED. ...EASTERN U.S TROUGH... PREFERENCE: BLEND OF 00Z ECMWF/CANADIAN CONFIDENCE: AVERAGE A SHORTWAVE AT THE BASE OF THE TROUGH WILL DRIVE A PRECIPITATION EVENT ACROSS THE SOUTHERN STATES TODAY...WHILE THE LARGE SCALE TROUGH PROGRESSES EASTWARD. BY DAYS 2 AND 3 A CLOSED LOW IS FORECAST OFF THE NEW ENGLAND COAST AS THIS TROUGH SETTLES INTO ITS ROLE AS THE WESTERN MEMBER OF AN OMEGA BLOCK. MODELS ARE VERY SIMILAR THROUGH DAY 2...MONDAY MORNING. GREATER DIFFERENCES APPEAR ON DAY 3. GIVEN THE WELL DEFINED BLOCK WE FAVOR THE SOLUTIONS THAT REMAIN CLOSED AND MORE STAGNANT...SUCH AS THE 12Z/00Z ECMWF...12Z UKMET...AND 00Z CANADIAN. AS EXPECTED...THE ECMWF DID TREND NORTHWARD WITH ITS CLOSED LOW...BUT NOT EASTWARD. THE GFS IS NOT PREFERRED ON DAY 3 AS IT IS MORE PROGRESSIVE AND FAILS AT CAPTURING THE UPSTREAM SHORTWAVE RIDGE THAT WOULD KEEP MORE CYCLONIC FLOW IN PLACE OFF THE CAROLINA COASTLINE. THE 00Z NAM AMPLIFIES THE INITIAL SHORTWAVE AT THE BASE OF THE TROUGH SO MUCH THAT IT BECOMES SEPARATED FROM INTERACTING WITH THE DEVELOPING CLOSED LOW FARTHER NORTH. THE GLOBAL MODELS USE THIS SHORTWAVE INTERACTION TO TUG THE CLOSED LOW SOUTHWARD...WHICH LEAVES THE NAM CIRCULATION LOOKING SLIGHTLY TOO FAR NORTH WITH POTENTIAL EFFECTS ALONG THE NEW ENGLAND COAST. IT IS A SUBTLE DIFFERENCE...BUT THE ECMWF/CANADIAN ARE PREFERRED FOR THESE DETAILS. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... BURKE