MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1250 PM EDT WED APR 12 2017 VALID APR 12/1200 UTC THRU APR 16/0000 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... 12Z MODEL EVALUATION INCLUDING INITIAL PREFERENCES AND CONFIDENCE ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...UPPER TROUGH AND ITS WEAK SURFACE LOW OVER LOWER GREAT LAKES BECOMING NEGATIVELY TILTED AS IT MOVES THROUGH THE NORTHEAST LATE TODAY AND TONIGHT... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: GENERAL MODEL BLEND CONFIDENCE: ABOVE AVERAGE GIVEN ONLY MINOR DIFFERENCES BETWEEN THE LATEST MODEL GUIDANCE...INCLUDING THE 12Z NAM AND GFS RUNS...A GENERAL MODEL BLEND IS RECOMMENDED TO IRON OUT SMALLER SCALE DIFFERENCES. ...MID-UPPER LEVEL TROUGH CROSSING THE SOUTHEAST TODAY AND TONIGHT... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: GENERAL MODEL BLEND CONFIDENCE: ABOVE AVERAGE THE 12Z NAM AND GFS SOLUTIONS ARE SIMILAR TO THE MODEL CONSENSUS...WHICH HAS SHOWN GOOD AGREEMENT AND RUN TO RUN CONSISTENCY FOR THE PAST SEVERAL MODEL CYCLES. THEREFORE A GENERAL MODEL BLEND IS STILL RECOMMENDED. ...MID-UPPER LEVEL SHORTWAVE MOVING ACROSS THE UPPER MIDWEST LATE TODAY AND TONIGHT AND REACHING THE EAST COAST ON THU... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: GENERAL MODEL BLEND CONFIDENCE: ABOVE AVERAGE ALL OF THE MOST RECENT MODEL RUNS...INCLUDING THE 12Z NAM AND GFS...ARE IN GOOD AGREEMENT. THE UKMET...WHICH WAS THE NOTABLE OUTLIER IN PREVIOUS RUNS...HAS TRENDED TOWARD THE MODEL CONSENSUS WITH THE 12Z RUN. A GENERAL MODEL BLEND SHOULD WORK WELL. ...UPPER LEVEL SHORTWAVE MOVING INTO THE SOUTHERN HIGH PLAINS TODAY...CENTRAL PLAINS ON THU...AND UPPER MISSISSIPPI VALLEY AND GREAT LAKES FRI THROUGH SAT... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: NON-12Z GFS BLEND CONFIDENCE: AVERAGE THE MODELS AGREE WELL AT UPPER LEVELS AS THE SHORTWAVE PROGRESSES NORTHEAST FROM THE SOUTHERN HIGH PLAINS INTO THE UPPER MISSISSIPPI VALLEY AND GREAT LAKES ON FRI...THEN WEAKENS ON SAT. UNLIKE THE OTHER MODELS THOUGH...THE 12Z GFS SOLUTION DEEPENS AN ASSOCIATED SURFACE LOW THU NIGHT AND FRI AND THEN MAINTAINS IT THROUGH SAT. THIS IS APPARENTLY A RESULT OF CONVECTIVE FEEDBACK...AND IT LOOKS WELL OVERDONE IN THE QPF FIELDS AS WELL. PLEASE SEE QPFPFD FOR MORE DETAIL ON THAT. ASIDE FROM THAT CONSIDERATION...A GENERAL BLEND OF MODELS SHOULD WORK WELL. ...SURFACE LOW MOVING NORTH OVER PACIFIC NORTHWEST OFFSHORE WATERS TODAY... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: GENERAL MODEL BLEND CONFIDENCE: ABOVE AVERAGE THERE IS NO SIGNIFICANT DIFFERENCE IN MODELS...INCLUDING THE 12Z NAM AND GFS. A GENERAL MODEL BLEND IS RECOMMENDED. ...DEEP CLOSED LOW MOVING SLOWLY NORTH OVER PACIFIC NORTHWEST OFFSHORE WATERS TODAY THROUGH FRI AND WEAKENING ON SAT... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: 2/3 12Z GFS...1/3 00Z ECMWF BLEND CONFIDENCE: AVERAGE THE MODELS ARE IN GOOD AGREEMENT THROUGH THU NIGHT...BUT SEPARATION IN MODEL SOLUTIONS COMES INTO PLAY ON FRI AS SHORTWAVE ENERGY CAUSES A SECONDARY UPPER LOW TO DEVELOP NEAR THE BORDER OF ALBERTA AND SASKATCHEWAN. (THAT FEATURE IS DISCUSSED BELOW.) THE 12Z NAM LOOKS LIKE THE OUTLIER ON FRI...AS IT GIVES UP ENERGY FROM THE OFFSHORE UPPER LOW TO THE NEW LOW THAT IS FORMING INLAND OVER CANADA. THE MODEL SOLUTIONS SEPARATE MORE DRAMATICALLY FRI NIGHT AND SAT AS THE CIRCULATION WEAKENS IN A COMPLEX UPPER PATTERN...SHEARING BETWEEN THE LOWS TO ITS EAST AND WEST AND THE SOUTHWARD EXTENSION OF A STRONG BLOCKING RIDGE NORTH OF ALASKA. A BLEND OF THE 00Z ECMWF AND 12Z GFS...WEIGHTED TOWARD THE 12Z GFS...IS PROBABLY BEST IN THIS SITUATION. ...UPPER LOW FORMING NEAR BORDER OF ALBERTA AND SASKATCHEWAN FRI AND MOVING SLOWLY EAST THROUGH SAT... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: GENERAL MODEL BLEND CONFIDENCE: AVERAGE THIS FEATURE DEVELOPS AS A RESULT OF SHORTWAVE ENERGY ROUNDING THE BASE OF THE UPPER LOW OVER THE OFFSHORE WATERS OF THE PACIFIC NORTHWEST DISCUSSED ABOVE. THERE ARE MINOR DIFFERENCES WITH REGARD TO THE POSITION OF THE LOW. IN PARTICULAR THE 12Z NAM POSITIONS THE LOW A BIT NORTHWEST OF THE CLUSTER OF OTHER MODEL SOLUTIONS. GIVEN THAT THIS IS IN THE 72+ HOUR FORECAST RANGE...IT DOES NOT LOOK UNREALISTIC. AT THIS POINT A GENERAL MODEL BLEND IS PROBABLY BEST WITH THE UNCERTAINTY ONE WOULD NORMALLY EXPECT. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... MCDONNAL/ORRISON