MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 144 AM EST SAT JAN 31 2015 VALID JAN 31/0000 UTC THRU FEB 03/1200 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 00Z NAM/GFS DEEMED SIGNIFICANT ENOUGH TO IMPACT THEIR FORECASTS. ...NEGATIVELY-TILTED TROF OVER NEW ENGLAND EARLY IN THE PERIOD... ...ASSOCIATED COASTAL LOW... FINAL PREFERENCE: GENERAL MODEL COMPROMISE FORECAST CONFIDENCE: ABOVE AVERAGE A RATHER POTENT AXIS OF HEIGHT FALLS MOVING OFFSHORE OF THE MID-ATLANTIC HAS SPREAD LIGHT TO MODERATE SNOW ACROSS UPPER NEW ENGLAND TONIGHT. THE SYSTEM BECOMES FURTHER WRAPPED UP AS A CLOSED MID-LEVEL CENTER DEVELOPS EAST OF MA BY 31/1200Z. MODELS ARE IN AGREEMENT THAT THE NEGATIVE ANOMALY WILL CONTINUE LIFTING NORTHWARD TOWARD NOVA SCOTIA. IN PARTICULAR...THE PAST FEW ENSEMBLE LOW PLOTS HAVE SHOWN A DEFINITIVE TREND IN THE QUICKER DIRECTION TO ACCELERATE THE AREA OF LOW PRESSURE TOWARD THE NORTH. OVERALL DIFFERENCES WHILE THE SYSTEM AFFECTS NEW ENGLAND ARE MINIMAL WHICH SUGGESTS A GENERAL MODEL COMPROMISE HERE. ...CLOSED LOW MOVING FROM THE DESERT SOUTHWEST TOWARD WESTERN/CENTRAL MX... FINAL PREFERENCE: BLEND OF THE 00Z GFS/ECWMF FORECAST CONFIDENCE: BELOW AVERAGE AN EXPANSIVE UPPER LOW SPINNING OVER THE DESERT SOUTHWEST HAS BEEN ACCOMPANIED BY AN ANOMALOUS TONGUE OF MOISTURE LIFTING INTO THE SOUTHWESTERN CONUS. AGREEMENT EXISTS THAT A BROAD AXIS OF VORTICITY WILL SINK SOUTHWARD TOWARD BAJA CA/FAR WESTERN MX. HOW LONG THIS SYSTEM LINGERS OVER THE REGION BEFORE IT SLIDES EASTWARD ACROSS MX IS A BIT MORE UNCERTAIN. THE MULTI-MODEL TRENDS BASED ON SPAGHETTI PLOTS SUGGEST A QUICKER TREND IN THE ENSEMBLES. ON THE FASTER SIDE OF THE ENVELOPE ARE THE 00Z NAM AND 21Z SREF/00Z GEFS MEANS WHILE THE SLOWEST MODELS ARE THE 00Z ECMWF/UKMET. BY 03/1200Z...THE 21Z SREF/00Z GEFS MEANS ARE ON THE FAR EASTERN SIDE OF THE SPREAD. INTERESTINGLY...WHILE THE MULTI-MODEL TRENDS HAD SUGGESTED A MOVE IN THE FASTER DIRECTION...THE 00Z GUIDANCE HAS TAKEN A SHIFT TOWARD THE SLOWER SIDE. THE 00Z GFS/ECMWF/CMC ALL EXHIBITED SUCH BEHAVIOR. THE WAFFLING OF THE MODELS IS NOT TERRIBLY SURPRISING GIVEN THE DIFFICULTY OF FORECASTING THE SPEED OF A CLOSED UPPER LOW. PLAN ON STICKING WITH THE PREVIOUS PREFERENCE WHILE INCORPORATING THE LATEST ECMWF. THIS WILL ACCOUNT FOR THE SLOWING TREND GIVEN THE 00Z GFS/ECMWF BOTH SLOWED DOWN RELATIVE TO THEIR PREVIOUS CYCLES. CONFIDENCE WILL BE SHIFTED TO BELOW AVERAGE GIVEN THE GROWING UNCERTAINTY. ...DEEPENING TROF MIGRATING FROM THE MIDDLE MS VALLEY TO THE MID-ATLANTIC ON SUNDAY/MONDAY... ...STRENGTHENING SURFACE WAVE CROSSING THE MID-ATLANTIC... FINAL PREFERENCE: BLEND OF THE 00Z GFS/ECMWF FORECAST CONFIDENCE: SLIGHTLY BELOW AVERAGE HEIGHT FALLS CURRENTLY MOVING THROUGH BRITISH COLUMBIA WILL EVENTUALLY CARVE OUT A MORE DEFINED TROF ACROSS THE CENTER OF THE COUNTRY BY EARLY SUNDAY. THE MOST OUTLYING MODEL AT THIS POINT IN TIME IS THE 00Z UKMET WHICH SHOWS MORE DEFINITION IN THE SHORTWAVE ACROSS THE MO VALLEY. LOOKING AT THE SURFACE...A PRONOUNCED WAVE OF LOW PRESSURE WILL SLIDE EASTWARD FROM THE LOWER OH VALLEY TOWARD THE MID-ATLANTIC HEADING INTO LATE SUNDAY/EARLY MONDAY. HAVE SEEN MODELS CONVERGING ON A SOLUTION CARRYING THE LOW ALONG THE VICINITY OF 40N WITH VARIABLE TIMING SUGGESTED. THE SLOWER TREND IN THE 00Z ECMWF INCREASES THE SPREAD BETWEEN THE 00Z GFS/ECMWF ON DAY 3. HOWEVER...WILL PLAN ON CONTINUING TO SPLIT THE DIFFERENCE BETWEEN THESE TWO SOLUTIONS. ...LOW-AMPLITUDE WAVES AFFECTING THE PACIFIC NORTHWEST EARLY NEXT WEEK... FINAL PREFERENCE: BLEND OF THE 00Z NAM/ECMWF FORECAST CONFIDENCE: ABOVE AVERAGE AN UPPER RIDGE ANCHORING THE WESTERN U.S. BETWEEN 30 AND 40 N LATITUDE WILL GRADUALLY BREAK DOWN AS A SERIES OF LOW-AMPLITUDE IMPULSES TRACK ACROSS THE PACIFIC NORTHWEST LATE SUNDAY ONWARD. OVERALL...THE MODELS ARE REASONABLY CLOSE TO ONE ANOTHER WITH THE LARGER SCALE PATTERN WITH MORE VARIANCE NOTED WITH THE INDIVIDUAL SHORTWAVES. GIVEN THE BROADER SIMILARITIES...WPC WILL RECOMMEND A COMBINATION OF HIGHER-RESOLUTION DETERMINISTIC GUIDANCE TO ACCOUNT FOR THE BETTER HANDLING OF THE COMPLEX TERRAIN OUT WEST. THAT IS...A BLEND OF THE 00Z NAM/ECMWF. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... RUBIN-OSTER