MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1257 AM EDT WED AUG 19 2015 VALID AUG 19/0000 UTC THRU AUG 22/1200 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... 00Z NAM/GFS EVALUATION WITH MODEL PREFERENCES AND FORECAST CONFIDENCE ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ NAM/GFS INITIALIZATION ERRORS APPEAR MINIMAL AND SHOULD NOT DEGRADE SHORT RANGE FORECASTS. DEEP UPPER TROUGH AFFECTING THE NORTHERN PLAINS/MIDWEST INTENSE SURFACE LOW LIFTING UP THROUGH THE UPPER GREAT LAKES ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: NON-GFS BLEND CONFIDENCE: SLIGHTLY ABOVE AVERAGE DETERMINISTIC AND ENSEMBLE CLUSTERS SHOW VERY STRONG AGREEMENT NOW WITH THE SURFACE LOW EXITING THE NORTHWEST GREAT LAKES...AND MOST SIGNIFICANT DIFFERENCES EXIST WITH THE TIMING OF THE SURFACE FRONT ACROSS THE WESTERN MID-ATLANTIC AND DEVELOPMENT OF COASTAL LOW OVER NORTH CAROLINA...THOUGH DUE TO RISING HEIGHTS...THE FRONT SLOWS OVER THE HUDSON RIVER VALLEY FOR BETTER AGREEMENT ON LATE FRIDAY INTO SATURDAY. THE 00Z GFS REMAINS THE FASTEST DETERMINISTIC INCLUDING THE FASTER/MORE EASTERN SURFACE DEVELOPMENT. THE 12Z ECMWF IS THE SLOWEST BUT NOT DRAMATICALLY SLOWER. THE 00Z NAM HAS BECOME A BIT SLOWER BECOMING MORE CENTRALIZED. STILL DIFFERENCES ARE NOT TOO DRAMATIC COMPARED TO EARLIER THIS WEEK...SO CONFIDENCE IS A BIT HIGHER. AT THIS TIME...KEEPING WITH CONTINUITY SHOULD BE GOOD...BUT WILL INCLUDE THE 00Z NAM TO THE BLEND. SO A NON-00Z GFS SOLUTION IS PREFERRED AT THIS TIME AT SLIGHTLY ABOVE AVERAGE CONFIDENCE. GLANCING PASSAGE OF SHORTWAVE WITH ATTENDANT SURFACE LOW/COLD FRONT ALONG U.S/CANADA BORDER THURSDAY TO FRIDAY ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: BLEND OF OPERATIONAL MODELS CONFIDENCE: NEAR TO ABOVE AVERAGE PRIOR TO THE MAIN WAVE COMING INTO PACIFIC NORTHWEST AND NORTHERN ROCKIES FRIDAY INTO SATURDAY...A WELL DEFINED UPPER LOW SWINGING THROUGH THE CANADIAN TERRITORIES WED/THURSDAY HAS A SHORTWAVE THAT SKIRTS THE BASE ACROSS ALBERTA/SASKATCHEWAN. THIS DEVELOPS A WEAK SURFACE LOW OVER SOUTHERN ALBERTA WITH A WEAK FRONTAL ZONE AFFECTING NORTHERN MONTANA/NW NORTH DAKOTA. MODELS ARE IN GOOD AGREEMENT WITH THE HANDLING OF THE POSITIONING/TIMING OF THE SURFACE LOW AND FRONT...THOUGH IF ONE WANTS TO BE REALLY NIT-PICKY... THE 00Z NAM AND 21Z SREF ARE THE DEEPEST MEMBERS INCLUDING MUCH OF THE 18Z GEFS AND 12Z ECMWF ENSEMBLE MEMBERS. AS SUCH A GENERAL OPERATIONAL MODEL BLEND SHOULD BE FINE HERE AT ABOVE AVERAGE CONFIDENCE. SOUTHWEST ALASKA SHORTWAVE DEEPENING AS IT DROPS INTO THE NORTHERN ROCKIES BY FRIDAY ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: BLEND OF 00Z NAM/GFS AND 12Z ECMWF CONFIDENCE: AVERAGE GOOD MODEL CONSENSUS EXISTS WITH THE GENERAL TIMING AS THE SYSTEM BEGINS TO DEEPEN ON THE SW SIDE OF LARGER SCALE LOW OVER NORTHERN CANADA... AS TYPICAL WITH SYSTEMS JUST PRIOR TO ENTERING THE HIGHER OBSERVATION NETWORK CURRENTLY OVER SW ALASKA...SPREAD EXISTS WITH SHAPE AND DEPTH OF THE SYSTEM. THE 12Z CMC IS MOST LIKELY THE MOST DRAMATIC OUTLIER BEING QUITE BROAD AND WEAK WITH THE WAVE MAKING IT AN OUTLIER. THE 12Z UKMET IS MUCH CLOSER TO THE CONSENSUS BUT SEEMS A BIT STRONGER IN THE NORTHERN PORTION OF THE WAVE/INTERACTION WITH THE PRECEDING WAVE MAKING A BIT STRONGER ACROSS THE NORTHERN PORTION OF THE NORTH-SOUTH BROAD SURFACE LOW IN THE NORTHERN PLAINS. IT IS ALSO A BIT FAST TOO...SO ONE CANNOT FULLY DISMISS IT BUT GIVEN OTHER STRONGER CONSENSUS WILL NOT BLEND THE 12Z UKMET INTO THE PREFERENCE AT THIS TIME. THE 00Z NAM...TRUE TO ITS BIAS IS CONSOLIDATED AND DEEPER THAN ITS COUNTERPARTS ALMOST 60 DM BY SAT 22/00Z THOUGH ITS LOW LEVEL AND SURFACE MASS FIELDS SEEM IN LINE WITH THE OTHER GUIDANCE/ENSEMBLE MEMBERS...RESERVING SOME UTILITY. THE 00Z GFS HAS BEEN CONSISTENT WITH THE SPEED AND TIMING WITH ITS PRIOR RUNS THE 18Z GEFS AND THE 12Z ECMWF. THIS IS ALSO TRUE WITH THE DEVELOPMENT OF THE SURFACE LOW WITH GOOD AGREEMENT THOUGH MAYBE A SHADE FASTER THAN THE 12Z ECMWF. STILL WITH GOOD CLUSTERING OF THE UPPER LEVEL WAVE AND AGREEMENT OF THE POSITION OF THE LOW WITHIN THE LARGE SURFACE DEPRESSION AND ORIENTATION ESPECIALLY TO HIGHLY AGREEABLE SREF/GEFS AND ECMWF ENSEMBLE MEANS... A BLEND OF THE 00Z NAM/GFS AND 12Z ECMWF IS PREFERRED AT AVERAGE CONFIDENCE. UPPER SHEAR LINE NEAR THE TX/MX BORDER ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: NON-CANADIAN COMPROMISE CONFIDENCE: ABOVE AVERAGE MOST OF THE GUIDANCE WEAKENS THIS SYSTEM AS IT RETROGRADES WESTWARD ACROSS NORTHERN MEXICO DUE TO A STRENGTHENING SUBTROPICAL RIDGE NEAR NORTHERN FL. THE EXCEPTION IS THE 12Z CMC WHICH SHEARS IT INTO THE MAIN STREAM AND EAST RETAINING LITTLE TO NO ENERGY ACROSS TX/MX BY SAT WHICH SEEMS OUT OF PHASE WITH OTHER GUIDANCE. OTHERWISE MASS FIELD-WISE THE REST OF THE DETERMINISTIC LOOK FINE. QPF DIFFERENCES APPEAR A BIT LARGER THOUGH WITH THE 00Z GFS MUCH LESS PARTICULARLY FRIDAY COMPARED TO THE ECMWF...PLEASE SEE WPC QPFPFD FOR FURTHER DETAILS. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... GALLINA