MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1142 AM EST THU JAN 16 2014 VALID JAN 16/1200 UTC THRU JAN 20/0000 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... ================================================== 12Z NAM/GFS EVALUATION AND PRELIMINARY PREFERENCES ================================================== NAM/GFS INITIALIZATION ERRORS DO NOT APPEAR TO SIGNIFICANTLY AFFECT THEIR LARGE SCALE OUTPUT. ================================================== ...SYSTEM IN THE MIDWEST SWINGING OFF THE ATLANTIC COAST BY SATURDAY... PREFERENCE: BLEND OF OPERATIONAL MODELS...EXCLUDING THE 00Z UKMET A SHORTWAVE WILL DIVE TO THE BASE OF THE LONGWAVE TROUGH OVER THE EASTERN U.S. AND THEN LIFT ALONG THE MID ATLANTIC/NEW ENGLAND COAST ON SATURDAY. A PRIMARY SURFACE LOW IS EXPECTED OFFSHORE...BUT A SECONDARY LOW BECOMES BETTER DEFINED BRIEFLY OVER THE MID ATLANTIC AND SOUTHERN NEW ENGLAND COASTS. IN THEIR 12Z RUNS THE NAM AND GFS PRODUCED SLOWER SOLUTIONS WITH MORE SHARPLY DEFINED SURFACE FEATURES...WHICH BRINGS THEM IN PHASE WITH THE 00Z ECMWF AND CANADIAN. THE 00Z UKMET WAS CERTAINLY SLOWER...AND IS THE ONLY ONE OF THE GLOBAL MODELS THAT DOES NOT CLUSTER VERY WELL WITH THE OTHER SOLUTIONS. ...SYSTEM DIVING INTO THE MIDWEST SATURDAY... PREFERENCE: BLEND OF 12Z NAM WITH 00Z ECMWF/UKMET THE NEXT RIPPLE IN NORTHWEST FLOW WILL PUSH A SURFACE LOW THROUGH THE MIDWEST INTO ILLINOIS BY 19/00Z. THERE IS VERY LITTLE MODEL SPREAD THROUGH THAT TIME...WITH NO CLEAR PREFERENCE EXCEPT THAT THE GFS IS SLIGHTLY FASTER THAN THE REST. ON DAY 3 THE SPREAD INCREASES AS THE MID LEVEL WAVE SWINGS INTO THE WESTERN ATLANTIC...WITH THE GFS CONTINUING FASTER AND LESS AMPLIFIED...AND THE CANADIAN FALLING TO THE SLOW SIDE OF GUIDANCE. THE MAJORITY OF GUIDANCE...INCLUDING THE 06Z GEFS MEAN...SUPPORTS THE SLOWER/SHARPER SOLUTIONS OFFERED BY THE 12Z NAM AND 00Z ECMWF/UKMET. ...MID LEVEL DISTURBANCE STRENGTHENING OVER THE SOUTHERN ROCKIES SUNDAY... PREFERENCE: BLEND OF 12Z NAM WITH 00Z ECMWF/CANADIAN AS THE SYSTEM ABOVE TOPS THE RIDGE IN WESTERN NORTH AMERICA ON FRIDAY...A SEPARATE...SMALLER DISTURBANCE IS FORECAST TO COME THROUGH THE RIDGE POSITION AT A LOWER LATITUDE. RELATIVE VORTICITY WILL INCREASE AS THE FEATURE DIPS TO MORE SOUTHERLY LATITUDES...CAUSING THE SYSTEM TO GAIN CHARACTER BY THE TIME IT REACHES THE SOUTHERN ROCKIES ON SUNDAY. FOR SUCH A SMALL AND INITIALLY SHALLOW FEATURE THE MODELS SHOW FAIRLY GOOD AGREEMENT. WE PREFER THE NAM/ECMWF/CANADIAN WHICH CLUSTER WELL AND ARE DOING A GOOD JOB HANDLING THE REST OF THE FLOW PATTERN. THE 12Z GFS AND 00Z UKMET ARE JUST SLIGHTLY FASTER...HOWEVER...AND THEY MAY STILL HAVE SOME UTILITY FOR THIS SYSTEM. ...PACIFIC SYSTEM BRUSHING THE NORTHWEST AND NORTHERN ROCKIES... PREFERENCE: BLEND OF 00Z GFS/ECMWF/UKMET ANOTHER LOW AMPLITUDE MID LEVEL WAVE AND ASSOCIATED SURFACE WAVE WILL DIVE TOWARD THE NORTHERN PLAINS AND MIDWEST BY SUNDAY. THE 00Z GFS/ECMWF/UKMET CLUSTER WITH EACH OTHER AND WERE WELL SUPPORTED BY THE 00Z ENSEMBLE RUNS. AT THE SURFACE THE 06Z AND 12Z GFS RUNS TRENDED QUICKER WITH FRONTAL PASSAGE THROUGH THE MIDWEST...WITH LESS EMPHASIS ON LOW DEVELOPMENT THERE. THERE IS A CORRESPONDING ABSENCE OF MID LEVEL SHORTWAVE RIDGING IN THE NEW GFS RUNS. THE NAM DID NOT FOLLOW THE GFS TREND AT THE SURFACE...BUT IT IS ALSO QUESTIONABLE ALOFT WHERE ITS SOLUTION BECOMES RATHER FLAT. WE WILL KEEP OUR PREFERENCE TIED TO THE 00Z CYCLE UNTIL SEEING THE FULL 12Z SUITE. www.wpc.ncep.noaa.gov/html/model2.shtml... BURKE