MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1241 PM EDT SAT MAR 18 2017 VALID MAR 18/1200 UTC THRU MAR 22/0000 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... 12Z MODEL EVALUATION INCLUDING PREFERENCES AND CONFIDENCE INTERVALS ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ SHORTWAVE SWINGING THROUGH THE MID-ATLANTIC TODAY WITH SURFACE LOW TRACKING UP THE NORTHEAST US COAST THROUGH MON ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: NON-UKMET BLEND (MAINLY AFTER 20/12Z) CONFIDENCE: SLIGHTLY ABOVE AVERAGE THE CYCLE TO CYCLE ENSEMBLE ACCORDION HAS ONCE AGAIN SQUEEZED WITH THE MOST RECENT 00Z SUITE COMING TO A MORE COMMON CONSENSUS BETWEEN SOLUTION CAMPS SEEN OVER THE PAST FEW DAYS/CYCLES. BOTH THE GFS/GEFS AND ECMWF/ECENS SHIFTED TOWARD THE CENTER THOUGH WHILE THE BULK OF THE ENSEMBLE TRENDED WEAKER WITH THE 00Z CYCLE THE 00Z ECMWF IS NOW DEEPER AND FLIPPED TO BEING THE NORTHERN MOST MEMBER NEARING S NOVA SCOTIA LATE MONDAY LEADING TO A BIT OF CONCERN IN FULLY SUPPORTING IT IN THE BLEND. THE ONLY CLEARLY DEPARTING MEMBER OF THE DETERMINISTIC SUITE APPEARS TO BE THE 00Z UKMET THOUGH ONLY AFTER 20/12Z MONDAY WHEN IT HAS KEPT WITH PRIOR SOLUTIONS BEING FAST AND ACCELERATING UP THE CANADIAN MARITIMES. THE 12Z NAM APPEARS TO HAVE A SLIGHT NORTHWARD BIAS TO THE CONSENSUS INITIALLY BRINGING THE WOBBLE OF THE OCCLUDED LOW VERY NEAR CAPE COD AND TRANSPORTING MOISTURE THROUGH THE COLD CONVEYOR BELT...STILL IT EVOLVES THE REPLACEMENT TRIPLE POINT LOW QUITE WELL AND REMAINS FAVORABLE. THE 12Z GFS SHOWS SIMILAR EVOLUTION TO THE 00Z RUN THOUGH PERHAPS A BIT DEEPER AND FURTHER OFFSHORE/SOUTH. AS SUCH A NON-UKMET BLEND IS SUPPORTED AND WITH MILD SPREAD IN SMALLER SCALES YET TO BE RESOLVED THE CONFIDENCE IS ONLY SLIGHTLY ABOVE AVERAGE. DEEP UPPER LOW AND SURFACE CYCLONE TRACKING THROUGH CANADA WITH DRAPED COLD FRONT ACROSS THE PLAINS SUN/GREAT LAKES MON AND DRAPED THROUGH THE OH/TN VALLEY TO THE SOUTHERN PLAINS THRU TUES ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: NON-UKMET BLEND UP TO 21/12Z 12Z GFS/NAM AND 00Z CMC BLEND AFTERWARD CONFIDENCE: SLIGHTLY ABOVE AVERAGE SHORTWAVE ENERGY FROM THE SOUTHERN STREAM LIFTS NORTH TO CENTRAL CANADA AND SUPPORTS A VIGOROUS SURFACE RESPONSE AND IN COMBINATION WITH CONTINUED WESTERLY FLOW SUPPORTS A FRONTAL ZONE TO TRACK ACROSS THE NORTHERN TIER INTO THE GREAT LAKES SUNDAY INTO MON EXTENDING BACK TOWARD A WEAK BUT WELL AGREED UPON LEE SURFACE LOW IN THE CENTRAL PLAINS. THE ONLY SIGNIFICANT MODEL/ENSEMBLE GUIDANCE DISCREPANCIES ARE RELATED WITH RESPECT TO SHAPE/TIMING/STRENGTH OF THE REMNANTS OF THE DEEP UPPER LOW THAT IS CURRENTLY OFFSHORE HAIDA GWAII AND HOW IT IS DRAWN SOUTHWARD TO BECOME THE TRAILING SHORTWAVE OF THE DEEPENING UPPER LOW OVER HUDSON BAY. THIS WAVE WILL SKIRT THE US/CANADIAN BORDER BROADENING THE OVERALL BASE OF THE GROWING GLOBAL SCALE TROF TUESDAY INTO WED. THE 00Z UKMET RESOLVES THIS FEATURE AS QUITE CONSOLIDATED/STRONG/DEEP NEGATIVELY AFFECTING THE LOWER LEVEL MASS PATTERN AS WELL INCLUDING DEVELOPING A WEAK CLIPPER SURFACE REFLECTION BY LATE TUES; WHILE OTHER GUIDANCE SUCH AS THE 12Z NAM/00Z ECMWF SUPPORT THIS TIMING/PLACEMENT THE MAGNITUDE IS MUCH MORE TEMPERED AND REASONABLE GIVEN THE FLOW PATTERN/SHEARING REGIME THAT SHOULD RESULT IN A FLATTER WAVE. THIS SUGGESTS A NON-UKMET BLEND ALONG THE US/CANADIAN BORDER...WHILE A GENERAL MODEL BLEND MAY SUFFICE FOR THE FRONTAL ZONE (THOUGH PLEASE REFER TO QPFPFD FOR ADDITIONAL QPF INFORMATION AND PREFERENCES). DUE TO A SLOWER TO LIFT AND DEEPER NORTHWARD REPRESENTATION OF THE WEAKENING UPPER LOW NEAR NOVA SCOTIA (SEE SECTION ABOVE FOR MORE DETAILS) PRESENTED IN THE 00Z ECMWF...THE LEAD SHORTWAVE/HEIGHT FALLS WITH THIS TROF BEGIN TO ACCELERATE IN COMPARISON TO THE REST OF THE GUIDANCE (INCLUDING MANY ECENS MEMBERS) AS A RESULT. THIS LEADS TO A LOWER FAVORABILITY OF THE 00Z ECMWF WITHIN THE WPC PREFERRED BLEND AFTER 21/12Z TUES WHEN THE EVOLUTION ARE MOST PRONOUNCED. CONFIDENCE OVERALL IS SLIGHTLY ABOVE AVERAGE IN THE BLENDS. NEXT TROF APPROACHING THE PACIFIC NORTHWEST WITH PRECURSORY WEAK SURFACE WAVE LIFTING NORTH PARALLEL TO CA COAST MON BEFORE MAIN HEIGHT FALLS/SURFACE LOW NEAR PAC NW LATE TUES ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: 12Z NAM/GFS AND 00Z ECMWF BLEND CONFIDENCE: AVERAGE IN A LARGER SCALE SENSE THE EVOLUTION OF THE DEEP UPPER WAVE APPROACHING THE WEST COAST BY MON INTO TUES IS FAIRLY WELL AGREED UPON EVEN IN THE ENSEMBLE SUITE. GIVEN THIS IS AN UPSCALE GROWTH/COMBINATION OF SYNOPTIC LEVEL FEATURES FROM THE NORTHERN STREAM AND SOUTHERN STREAM...SMALL TIMING DIFFERENCES AND SHAPE OF THE MASS FIELDS RESULT LEADING TO IMPACT DIFFERENCES. LEAD SHORTWAVE/HEIGHT FALLS REACH THE CENTRAL/N CA COAST OUT OF THE SOUTHERN STREAM EARLY MON...WITH THE 00Z ECMWF/CMC SLIGHTLY OUTPACING THE SLOWER 12Z NAM/GFS. THE 00Z UKMET FAVORS THE TIMING OF THE ECMWF/CMC BUT IS FURTHER EAST WITH A FLATTER ORIENTATION. WHILE THIS MAY NOT COME WITH MUCH SENSIBLE WEATHER/QPF...ACCOMPANYING LARGER SCALE HEIGHT FALLS FROM NORTHERN STREAM ENERGY IS PRESSING EASTWARD RESULTING IN WEAK SURFACE REFLECTION THAT WILL LIFT NORTH VERY NEAR THE 130W MERIDIAN WITH THE ECMWF OUTPACING THE GFS/NAM...THOUGH THE 00Z ECMWF IS ALSO ON THE SOUTHERN SIDE OF THE ECENS CLUSTER AS WELL AS BEING STRONGER THAN THAN THE GFS/NAM. AS THIS WAVE MELDS TO THE LARGER DEEPER UPPER LOW BY TUESDAY THERE IS GROWING CONSENSUS PLACEMENT/STRENGTH OF THE MEAN WAVE AND ASSOCIATED SURFACE LOW. THE UKMET SHOWS SIMILARITY TO THE EVOLUTION BUT IS A BIT STRONGER/DEEPER IN THE BASE OF THE TROF SLOWING ITS PROGRESS. WHILE THE 00Z CMC SHOWS SOME TYPICAL BIAS OF SHOWING BROADER TROF EVOLUTION. ALL CONSIDERED A BLEND OF THE 00Z ECMWF AND 12Z NAM/GFS SHOULD ENCOMPASS THE GREATEST AMOUNT OF POSSIBILITIES IN THE SPREAD THOUGH MAY WISH TO FAVOR THE ECMWF A BIT GREATER IN THE BLEND AT LEAST INITIALLY WITH THE LEAD WAVE GIVEN A BIT MORE CONSISTENCY/CONTINUITY. CONFIDENCE IS AVERAGE. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... GALLINA