MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 249 PM EDT MON MAY 01 2017 VALID MAY 01/1200 UTC THRU MAY 05/0000 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... 12Z MODEL EVALUATION...FINAL MODEL PREFERENCES/CONFIDENCE ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...DEEP CLOSED LOW LIFTING FROM THE UPPER MIDWEST ACROSS THE GREAT LAKES THROUGH TUE WITH TRAILING COLD FRONT CROSSING THE EASTERN SEABOARD TONIGHT/TUE... PREFERENCE: GENERAL MODEL BLEND CONFIDENCE: ABOVE AVERAGE THE 12Z SUITE OF MODEL SOLUTIONS REMAINS CONSISTENT WITH THE STRONG MODEL CONSENSUS SEEN OVER THE PAST COUPLE RUNS. THE 12Z RUN OF THE NAM - LIKE ITS PREVIOUS RUNS - MAINTAINS A SLIGHTLY TIGHTER CIRCULATION IN THE UPPER LOW LATE TUE AS IT MOVES NORTH OF THE GREAT LAKES...BUT IT IS A MINOR DIFFERENCE AND NEARLY BEYOND THE TIME THIS SYSTEM IS AFFECTING THE UNITED STATES. A GENERAL MODEL BLEND IS PREFERRED. ...PACIFIC FRONTAL SYSTEM CROSSING THE SOUTHERN CANADIAN ROCKIES...WITH 700 MB TROUGH/SURFACE REFLECTION THAT MOVES OUT OF ALBERTA/SASKATCHEWAN ACROSS NORTHERN PLAINS/UPPER MISSISSIPPI VALLEY THROUGH TUE... PREFERENCE: GENERAL MODEL BLEND CONFIDENCE: ABOVE AVERAGE THE 12Z MODELS REMAIN IN GOOD AGREEMENT WITH THIS FEATURE. A GENERAL MODEL BLEND IS AGAIN PREFERRED. ...WEAK PACIFIC WAVE ENTERING PACIFIC NORTHWEST CROSSING THE NORTHERN ROCKIES TONIGHT/TUE... ...DIGGING INTO THE CENTRAL/SOUTHERN PLAINS TUE NIGHT/WED WITH A FRONTAL SYSTEM DEVELOPING AND MOVING ACROSS THE MISSISSIPPI VALLEY THU... PREFERENCE: GENERAL MODEL BLEND CONFIDENCE: AVERAGE A WEAK SHORTWAVE TOPPING THE MEAN RIDGE AND MOVING THROUGH THE PACIFIC NORTHWEST TODAY WILL DIG ACROSS THE NORTHERN ROCKIES TONIGHT/TUE...THEN PROGRESS AND DEEPEN OVER THE CENTRAL AND SOUTHERN PLAINS TUE NIGHT/WED. THE MODELS ARE IN FAIRLY STRONG AGREEMENT THAT A CLOSED LOW WILL DEVELOP OVER THE LOWER MISSISSIPPI VALLEY WED NIGHT...AND THAT A WELL-DEFINED AREA OF LOW PRESSURE WILL GRADUALLY CROSS THE LOWER MS/TN VALLEY REGION. AS EXPECTED THERE IS MORE DISAGREEMENT ON FORECAST DETAILS...BUT EVEN THERE THEY AGREE GENERALLY ON SURFACE FRONTAL DEVELOPMENT AND THE IDEA THAT QPF IS CONCENTRATED IN ONE BAND ACROSS THE MIDDLE MISSISSIPPI VALLEY/WESTERN OHIO VALLEY AND ANOTHER AROUND THE LOWER MISSISSIPPI VALLEY/GULF COAST REGION. THE 12Z ECMWF SHOWS THE SAME TREND THAT WE SAW WITH THE 12Z NAM AND 12Z GFS SHOWING A MORE AMPLIFIED PATTERN...DEEPER TROUGH...WITH MORE ENERGY/VORTICITY FURTHER SOUTH AS IT SWINGS THROUGH THE BASE OF THE UPPER TROUGH ACROSS NORTHERN TX/LOWER MISSISSIPPI VALLEY. IT IS WORTH POINTING OUT THAT THE 12Z GFS OPERATIONAL RUN SHOWS THIS TENDENCY MORE THAN THE 12Z GFS ENSEMBLE MEAN...BUT THE TREND IS WELL SUPPORTED BY THE 12Z RUNS OF THE CMC AND UKMET AS WELL - AND IT LOOKS HARD TO DISCOUNT AT THIS POINT. IN ADDITION TO THE GENERAL MASS FIELDS...THIS MODEL TREND HAS AN IMPACT ON THE LIKELY MODEL QPF FIELDS. SPECIFICALLY THEY ARE REACHING A BROAD CONSENSUS THAT SOMEWHAT HEAVIER QPF WILL BE FOCUSED IN THE REGION OVER THE LOWER MISSISSIPPI VALLEY/GULF COAST WED NIGHT/THU. GIVEN THE BETTER AGREEMENT AMONG THE 12Z MODELS A GENERAL MODEL BLEND IS RECOMMENDED...BUT SINCE THE OVERALL MODEL SOLUTION IS IN A STATE OF CHANGE THE FORECAST CONFIDENCE REMAINS ONLY AVERAGE. ...NORTHERN STREAM SHORTWAVE DIGGING INTO ONTARIO/GREAT LAKES REGION THU... PREFERENCE: BLEND OF THE 12Z GFS...12Z NAM...AND 12Z ECMWF CONFIDENCE: AVERAGE MODELS AGREE THAT A SHORTWAVE EMBEDDED IN THE STRONG NORTHWEST JET ACROSS CENTRAL CANADA WILL DIG INTO ONTARIO/GREAT LAKES REGION THU...PROBABLY COMING INTO PHASE WITH THE LARGER SYSTEM DISCUSSED ABOVE. AS WITH THE SYSTEM DISCUSSED ABOVE...THE 12Z MODELS SHOW A SLIGHT TREND TOWARD A BIT MORE AMPLIFICATION AND MORE VORTICITY/ENERGY IN THE BASE OF THE SHORTWAVE BY THU. THE 12Z GFS...12Z NAM...AND 12Z ECMWF EXHIBIT THE TREND THE STRONGEST...WHILE THE 12Z CMC AND 12 UKMET SHOW IT LESS. AT THIS POINT A BLEND OF THE 12Z GFS...12Z NAM...AND 12Z ECMWF SHOULD GIVE A REPRESENTATIVE COMPROMISE FORECAST WITH AVERAGE CONFIDENCE. WWW.WPC.NCEP.NOAA.GOV/HTML/MODEL2.SHTML... /ALL LOWER CASE/ WWW.WPC.NCEP.NOAA.GOV/H5PREF/H5PREF.SHTML... /ALL LOWER CASE/ MCDONNAL