MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 100 PM EDT SAT AUG 15 2015 VALID AUG 15/1200 UTC THRU AUG 19/0000 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... ...AN EVALUATION OF THE 12Z NAM/GFS ALONG WITH PRELIMINARY PREFERENCES/FORECAST CONFIDENCE... THERE DID NOT APPEAR TO BE ANY INITIALIZATION ERRORS WITH THE 12Z NAM/GFS DEEMED SIGNIFICANT ENOUGH TO IMPACT THEIR FORECASTS. ...MID-LEVEL SHORTWAVE LIFTING FROM THE NORTHERN ROCKIES TOWARD SOUTHERN MANITOBA... ...ASSOCIATED DEEP SURFACE LOW CROSSING CENTRAL CANADA... PRELIMINARY PREFERENCE: BLEND OF THE 12Z GFS/00Z ECMWF FORECAST CONFIDENCE: AVERAGE MID-LEVEL HEIGHT FALLS CURRENTLY SKIRTING THE MONTANA/ALBERTA BORDER WILL CONTINUE RACING EASTWARD TOWARD CENTRAL SECTIONS OF CANADA. A DEEPENING PROCESS AT THE SURFACE IS EXPECTED AND AGREED UPON BY ALL OF THE MODELS. AT A FIRST GLANCE...THE 00Z UKMET HAS THE SAME BIAS AS YESTERDAY BEING A TAD QUICKER WITH THE EJECTION OF THE UPPER TROUGH. AT THE SURFACE...THE 00Z CMC/UKMET CONTINUE TO BE QUICKER WITH THE INITIAL DEEPENING WHILE OTHER MODELS CATCH UP TO THIS STRENGTHENING PROCESS. ADDITIONALLY...THE 12Z NAM SEEMS TO BE TOO QUICK WITH THE SURFACE WAVE WHICH DOES NOT HAVE MUCH ENSEMBLE SUPPORT. WHILE THE 12Z GFS/00Z ECMWF EXHIBIT TIMING DIFFERENCES AS THE LOW LEAVES THE MAP DOMAIN...A COMBINATION OF THE TWO WILL HANDLE THE DEEPENING PROCESS MORE READILY THAN ANY OTHER SOLUTION. ...TRAILING IMPULSES MOVING THROUGH THE PACIFIC NORTHWEST AND INTO THE CENTRAL U.S. BY TUESDAY... ...DEEPENING SURFACE CYCLONE OVER THE CENTRAL PLAINS... PRELIMINARY PREFERENCE: BLEND OF THE 12Z GFS WITH THE 06Z GEFS/00Z ECMWF ENSEMBLE MEANS FORECAST CONFIDENCE: SLIGHTLY BELOW AVERAGE IN THE WAKE OF THE INITIAL HEIGHT FALLS MOVING TOWARD CENTRAL CANADA...NORTHWESTERLY FLOW WILL SET UP ALONG THE WESTERN BRITISH COLUMBIA COAST DRAWING A PAIR OF SYSTEMS TOWARD THE PACIFIC NORTHWEST. SIMILAR TO YESTERDAY...THE 00Z CMC FAVORS A SLOWER SOLUTION REGARDING THE INITIAL SHORTWAVE WHICH DEPARTS FROM THE CONSENSUS. THIS FEATURE EVENTUALLY SHEARS AS IT BECOMES ABSORBED IN THE PROGRESSIVE NORTHERN STREAM. LOOKING UPSTREAM...MODELS SHOW MORE VARIABILITY WITH THE NEXT SHORTWAVE DESCENDING UPON THE REGION. THERE HAS BEEN A PERSISTENT AND ABRUPT SHIFT IN THE PAST FOUR SPAGHETTI PLOTS WITH A MOVEMENT IN THE FASTER DIRECTION. WHILE THE 576-DM HEIGHT CONTOUR CLUSTERS WELL...THERE ARE QUITE A FEW 00Z ECMWF ENSEMBLE MEMBERS DEPICTING SOMETHING MORE AMPLIFIED AND SLOWER ACROSS ID BY 18/0000Z. TOWARD THE END OF THE PERIOD...THE 00Z UKMET DIVERGES SIGNIFICANTLY BEING QUICKER WITH THE FRONT EDGE OF THE SHORTWAVE WHILE BEING DECIDEDLY SLOWER ON THE BACK END. THIS LEADS TO A DEEPER/QUICKER SURFACE LOW TRACK WITH ENSEMBLE LOW PLOTS FAVORING A SOLUTION FARTHER WEST. OVERALL...THE ENSEMBLE MEANS...IN PARTICULAR THE 06Z GEFS/00Z ECMWF ENSEMBLE MEANS CLUSTER WELL AND SIT AT THE CENTER OF THE FORECAST SPREAD. THE 00Z ECMWF IS SLOWER/WESTWARD DISPLACED WHILE THE 12Z GFS SHOWS MORE AGREEMENT WITH THESE MEANS. WITH THAT SAID...WPC FAVORS A BLEND OF THE 12Z GFS WITH THE ENSEMBLE MEANS. ...LOW AMPLITUDE FEATURES CROSSING THE CENTRAL PLAINS/MIDDLE MS VALLEY BY MONDAY... PRELIMINARY PREFERENCE: GENERAL MODEL COMPROMISE FORECAST CONFIDENCE: AVERAGE BOUNDED BY THE AMPLIFIED/PROGRESSIVE NORTHERN STREAM AND BUILDING HEIGHTS ACROSS THE SOUTHWESTERN U.S./FOUR CORNERS REGION WILL BE A REGION OF LOW AMPLITUDE WAVES CROSSING THE CENTRAL PLAINS/MIDDLE MS VALLEY. NOT NEARLY AS ALIGNED AS YESTERDAY...BUT THE SOLUTIONS STILL SEEM TO PLACE WITHIN THE GENERAL ENVELOPE OF UNCERTAINTY. WILL FAVOR A GENERAL MODEL COMPROMISE TO ACCOUNT FOR THESE DIFFERENCES. ...MULTIPLE VORTICITY MAXIMA WITHIN WEAK FLOW REGIME OVER THE EASTERN HALF OF THE U.S... PRELIMINARY PREFERENCE: BLEND OF THE 06Z GEFS/00Z ECMWF ENSEMBLE MEANS FORECAST CONFIDENCE: BELOW AVERAGE WEAK TROPOSPHERIC FLOW ACROSS THE EASTERN HALF OF THE COUNTRY WILL BE THE NORM AS HEIGHTS BEGIN TO RISE ACROSS THE EASTERN CANADA. THE GENERAL THEME IS FOR A SERIES OF VORTICITY CENTERS TO SCATTER ABOUT FROM THE WESTERN/CENTRAL GULF COAST UP THROUGH THE TN/OH VALLEYS. THE LATTER ENERGETICS SHOULD MIGRATE EASTWARD TOWARD THE DELMARVA WHILE THE FORMER RETROGRADES OR REMAINS NEARLY QUASI-STATIONARY OVER THE SOUTHERN STATES. AS EXPECTED IN THIS FLOW REGIME...THE SOLUTION SPREAD INCREASES IN TIME WITH A GREAT DEAL OF UNCERTAINTY WITH THE PLACEMENT OF INDIVIDUAL FEATURES. THIS WOULD SUGGEST AN ENSEMBLE APPROACH HERE. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... RUBIN-OSTER