MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 153 PM EST MON JAN 26 2015 VALID JAN 26/1200 UTC THRU JAN 30/0000 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... FINAL 12Z MODEL EVALUATION WITH PREFERENCES ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ NAM/GFS INITIALIZATION ERRORS DO NOT SEEM TO DEGRADE THEIR SHORT RANGE FORECASTS. EVOLVING COASTAL STORM ALONG THE NORTHERN MID ATLANTIC/NORTHEAST MON-WED ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: NAM/00Z AND 12Z ECMWF COMPROMISE CONFIDENCE: AVERAGE TO BELOW AVERAGE ONE OF THE MORE NOTABLE RUN-TO-RUN CHANGES BETWEEN THE 00/12Z NAM OCCURS EARLY IN THE PERIOD. THE 12Z RUN IS A LITTLE DEEPER WITH THE SHORTWAVE LIFTING ALONG THE MID ATLANTIC COAST AND IS A FEW HOURS FASTER TO CLOSE OFF A 500MB CENTER EARLY TUE. THIS IS REFLECTED AT THE SURFACE WITH A LOW THAT TRACKS A LITTLE CLOSER TO THE COAST ON TUE. THIS PLACES THE NAM SURFACE LOW TRACK ALONG THE WESTERN EDGE OF THE DETERMINISTIC GUIDANCE FOR ABOUT A 12-HR PERIOD FROM 06-18Z TUE. FROM EARLY WED ONWARD...THE 12Z NAM IS A LITTLE SLOWER AND LESS AMPLIFIED THAN ITS 00Z RUN AS THE SYSTEM MOVES FROM OFF OF THE NEW ENGLAND COAST INTO THE CANADIAN MARITIMES. BUT OVERALL...THE NAM HAS REMAINED FAIRLY CONSISTENT. IN FURTHER COMPARISON TO THE LATEST MODEL RUNS...FROM 12Z TUE ONWARD...THAN NAM BEARS MORE IN COMMON WITH THE LAST TWO RUNS OF THE ECMWF THAN THE MORE PROGRESSIVE 12Z UKMET/CMC GLOBAL/GEFS MEAN AND THE 09Z SREF MEAN. THE 12Z GFS IS A LITTLE DEEPER AND FASTER THAN ITS 00Z RUN AT 500 MB AS THE LOW CLOSES OFF AND THEN MOVES FROM THE NORTHERN MID ATLANTIC TO THE SOUTHERN NEW ENGLAND COAST ON TUE. OVERALL HOWEVER...RUN-TO-RUN DIFFERENCES BETWEEN THE 00/06/12Z RUNS ARE MINOR. IN COMPARISON TO THE LATEST DETERMINISTIC RUNS...THE GFS REMAINS BETTER CLUSTERED WITH THE UKMET/CANADIAN GLOBAL ALONG THE EASTERN/PROGRESSIVE EDGE OF THE GUIDANCE. CONFIDENCE IS TEMPERED BY THE LINGERING MODEL SPREAD...HOWEVER GIVEN THE ORIENTATION OF THE SYSTEM ALOFT (NEGATIVE TILT) AND A SIGNIFICANT UPSTREAM RIDGE NOSING INTO THE CANADIAN MARITIMES...A SLOWER AND MORE AMPLIFIED SOLUTION SIMILAR TO THE LATEST NAM/ECMWF SEEMS MORE PLAUSIBLE THAN THE FASTER CAMP. AS IS ALWAYS THE CASE WITH THESE BIG STORMS...EVEN SMALL DETAIL DIFFERENCES RESULT IN LARGE IMPACT DIFFERENCES...ESPECIALLY WITH PRECIPITATION. PREFERENCES LISTED HERE REFLECT MODEL POSITION AND MASS FIELDS...BUT REFER TO WPC WINTER WEATHER AND QPF DESK PRODUCTS FOR INFORMATION ON LIQUID EQUIVALENTS/SNOW. WEAK CLIPPER PASSING THROUGH THE UPPER MISS VALLEY MON/TUE ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: GENERAL MODEL COMPROMISE CONFIDENCE: AVERAGE THE GUIDANCE IS IN GOOD AGREEMENT HERE. WEAKENING CLOSED LOW MOVING INTO CALIFORNIA EARLY TUESDAY REMNANT SHORTWAVE PASSING THROUGH THE CENTRAL PLAINS INTO THE MID MS AND OH VALLEYS WED NIGHT-THU ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: GFS/UKMET/ECMWF CONFIDENCE: AVERAGE THE 00/12Z NAM SHOW GOOD RUN-TO-RUN CONTINUITY UNTIL AROUND 18Z WED WHEN THE 12Z NAM MAINTAINS A MORE AMPLIFIED SHORTWAVE MOVING FROM WY/CO INTO THE PLAINS. THIS IS WHEN THE NAM ALSO STARTS TO BECOME A SLOW/AMPLIFIED OUTLIER IN COMPARISON TO THE LATEST DETERMINISTIC RUNS AND THE 09Z SREF MEAN. THE 12Z GFS IS A LITTLE SLOWER THAN ITS 00Z RUN AS THE SYSTEM MOVES FROM THE MID MS INTO THE OH VALLEY THU MORNING...BUT IT IS BETTER CLUSTERED WITH THE MORE PROGRESSIVE CAMP. WHILE THE ECMWF/UKMET SHOW THIS SYSTEM SHEARING OUT MORE TO THE NORTHEAST AHEAD OF A SHORTWAVE TROUGH DIGGING INTO THE GREAT LAKES...THE GFS SHOWS A BETTER DEFINED SYSTEM MOVING INTO THE UPPER OH VALLEY/NORTHERN MID ATLANTIC ON THU...BUT IT REMAINS FASTER AND LESS AMPLIFIED THAN THE NAM. MID-LEVEL SHORTWAVE MOVING INTO BRITISH COLUMBIA MON NIGHT SUBSEQUENT MOVEMENT INTO THE N-CENTRAL U.S./GREAT LAKES THU ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: GFS/ECMWF/UKMET CONFIDENCE: AVERAGE TO BELOW AVERAGE IN COMPARISON TO THEIR 00Z RUNS...BOTH THE 12Z NAM/GFS HAVE SLOWED TO SOME DEGREE AS THE SYSTEM DIPS INTO THE NORTHERN PLAINS/UPPER MIDWEST WED NIGHT/EARLY THU. OVERALL...THE MODELS ARE IN FAIRLY GOOD AGREEMENT WITH ONE ANOTHER INTO WED EVENING WHEN THE GFS BECOMES A LITTLE FLATTER THAN THE MAJORITY OF THE GUIDANCE. GIVEN THE AFOREMENTIONED SHIFT IN THE GUIDANCE...THIS SOLUTION MAY BE LESS PREFERRED. BY EARLY THU...THE ECMWF BECOMES MORE OF AN OUTLIER...BECOMING MORE AMPLIFIED THAN THE OTHER DETERMINISTIC RUNS AND ITS OWN ENSEMBLE MEAN AS THIS SYSTEM MOVES FROM THE UPPER MS VALLEY INTO THE UPPER GREAT LAKES REGION. AT THIS POINT...A COMPROMISE BETWEEN THE GFS..ECMWF AND UKMET MAY BE PREFERRED. UPPER TROUGH MOVING INTO CALIFORNIA THU ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: GFS/ECMWF/UKMET CONFIDENCE: AVERAGE TO VARYING DEGREES...THE GFS..ECMWF..UKMET..CMC GLOBAL HAVE ALL SHIFTED TOWARD A SLOWER/MORE AMPLIFIED TROUGH AS THE SYSTEM DRIFTS ONSHORE LATE WED-EARLY THU. THE NAM/CMC GLOBAL FALL ALONG THE LESS AMPLIFIED SIDE OF GUIDANCE. GIVEN THE TRENDS...WILL RECOMMEND LEANING TOWARD THE MORE AMPLIFIED SET OF SOLUTIONS. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... PEREIRA