EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1159 AM EDT WED MAY 21 2014 VALID 12Z SAT MAY 24 2014 - 12Z WED MAY 28 2014 ...OVERVIEW... PERSISTENT BLOCK DOWNSTREAM OVER NORTHEAST NORTH AMERICA GETS A SUBTLE SHOVE EASTWARD FROM TWO MIGRATORY UPPER-LEVEL EAST PACIFIC WAVES. THE 'LEAD' SYSTEM EJECTS A SERIES OF MID-LEVEL IMPULSE OUT OF THE HIGH PLAINS...FOR NEARLY TWO DAYS...BEFORE THE BULK OF THIS 500-MB TROUGH MIGRATES OUT OF THE ROCKIES AND INTO THE CENTRAL HIGH PLAINS (AROUND MONDAY MORNING). THIS LEAD SYSTEM AND ITS AMPLIFYING RIDGE SERVE TO SET THE TRANSITION IN MOTION DOWNSTREAM AND EAST OF THE MISSISSIPPI VALLEY. A SECOND PACIFIC SYSTEM IS POISED TO MOVE SOUTHEASTWARD FROM THE ALASKA PANHANDLE INTO THE PACIFIC NORTHWEST COAST BETWEEN DAYS 4-6....ANCHORING OVER SOUTHWEST CANADA BY THE END OF DAY 7. ...MODEL PREFERENCES/UNCERTAINTY ASSESSMENT... THE 21/00Z ENSEMBLES WERE IN RATHER GOOD AGREEMENT WITH A COMPLEX SYNOPTIC-SCALE FLOW PATTERN THAT TRANSITIONS AWAY FROM A QUASI-STATIONARY DOWNSTREAM BLOCK ACROSS THE NORTHEAST PORTION OF NORTH AMERICA. BUT THERE ARE INHERENT...SUBTLE AND SIGNIFICANT DIFFERENCES IN THE 21/00Z DETERMINISTIC GUIDANCE. AND PRIMARILY WITH THE OUTCOME/FEATURES EAST OF THE MISSISSIPPI RIVER BEYOND DAY 4---WILL BE CHALLENGING WITH RESPECT TO SENSIBLE WEATHER CHANGES (PRECIPITATION AND TEMPERATURE). MUCH OF THE DETAIL IS THE RESULT OF CONVECTIVE INITIATION AND THE UNCERTAIN DURATION/STRENGTH AND MIGRATION OF CONVECTIVE CLUSTERS EXPECTED TO EMERGE OVER THE PLAINS STATES IN THIS PATTERN. FOR DAY 3... THE 21/00Z UKMET-CANADIAN-ECMWF AND GFS ALL FALL NICELY WITHIN THE ENSEMBLE SPREAD...WITH THE 500MB TROUGH AXIS INVOF THE NM/AZ BORDER. OUT AHEAD OF THE SYSTEM...A MID-LEVEL SHORTWAVE WILL BE FANNING OUT TO THE NORTHEAST ACROSS NORTHERN COLORADO...SOUTHERN WYOMING AND MIGRATING INTO THE WESTERN DAKOTAS. A BUILDING SHORTWAVE RIDGE IS A PROMINENT...DISTINCTIVE FEATURE ALONG 95W---AND A KEY TO THE SUBTLE PATTERN TRANSITION FROM TROUGH TO RIDGE IN THE NORTHEAST/MID ATLANTIC REGIONS. FROM THIS VANTAGE POINT...A BLEND OF THE 21/00Z ECENS/NAEFS AND GEFS THROUGH DAY 7 WILL PROBABLY PROVIDE THE BEST DEPICTION OF THE UPPER-LEVEL FLOW BY THE END OF DAY 7...WITH UPPER-LEVEL RIDGING EAST OF THE MISSISSIPPI AND APPRECIABLE HEIGHT FALLS (POSITIVE-TILT TROUGHING) BETWEEN 120W-125W FROM OREGON NORTHWARD INTO BC/AB. DAY 4 AND BEYOND... WITH RESPECT TO THE 'LEAD' SYSTEM...THE DETERMINISTIC GUIDANCE IS TRENDING SLOWER AND IT APPEARS THAT THE SHORTWAVE RIDGE THAT IS BUILDING AHEAD OF THE TROUGH AXIS IS BEGINNING TO IMPACT THE SOLUTIONS AFTER DAY 5. FOR THE MOST PART...THE DIFFICULTY LIES AT THE SURFACE...WITH POTENTIAL FOR MCS DEVELOPMENT MUDDLING THE SURFACE LOW PRESSURE PATTERN. AS THESE NOCTURNAL PRECIPITATION EVENTS PROPAGATE ALONG THE WESTERN FLANKS OF THE AMPLIFYING RIDGE...THE SYNOPTIC-SCALE PATTERN SUGGESTS A MORE NORTHEASTWARD MOVEMENT INTO THE GREAT LAKES REGION WITH CLOUD DEBRIS DISPERSING INTO AND BEING ABSORBED BY THE RIDGE ITSELF. A MORE INTENSE SCENARIO...IS UNFORTUNATELY...LIKELY TO GO UNDETECTED IN MY MEDIUM RANGE SURFACE GRAPHICS. MY THOUGHT WAS THE 21-00Z/06Z GFS SUPPORTED TOO MUCH TROUGH ENERGY AND/OR A CLOSED LOW SOLUTION IN THE CANADIAN MARITIMES AFTER DAY 4...TO BE OF MUCH USE FOR THE LOWER 48. PARTICULARLY IN THE EAST...THIS TROUGH SUPPRESSES ANY TRUE AMPLIFICATION OF A RIDGE FROM 95W EASTWARD TO 85W...WHICH THE ENSEMBLES (GEFS/NAEFS AND ECENS) ALL AGREE ON WILL BE PRESENT BY THE END OF DAY 5 (27/00Z). IN TURN...THIS LEAVES THE 21/06Z GFS SOLUTION ON ITS OWN WITH A VERY SLOW CUTOFF/H5 SYSTEM TRACKING THROUGH THE PLAINS...DAYS 6-7. OF NOTE THOUGH---IN THE PACIFIC NORTHWEST...THERE IS ENOUGH SEPARATION/SPACING BETWEEN SYSTEMS TO ALLOW THE 21/00Z GEFS/GFS SOLUTIONS TO HAVE UTILITY THROUGH DAY 7...IF ONE WAS SO INCLINED TO FOLLOW THE GFS ONLY. ...SENSIBLE WEATHER... MULTI-DAY CONVECTIVE SCENARIO FOR THE HIGH PLAINS...WITH DOWNSTREAM PROPAGATION BEING THE CHALLENGE FOR QPF AND TEMPERATURES. WITH THE GULF OF MEXICO NOT IN A CLASSIC---'OPEN'---SCENARIO...EXPECT THE LOW-LEVEL JET AND MESOSCALE ENVIRONMENT TO DRIVE THIS PATTERN DURING THE AFTERNOON AND EVENING HOURS. THROUGH THIS FORECAST PERIOD...ENVISION A GRADUALLY-SOUTHWARD SAGGING BOUNDARY WITH CONVECTION INITIATION DEVELOPING INVOF THE BLACK HILLS ON DAY 3...THEN THE UPPER PLATTE VALLEY DAY 4...THEN BETWEEN I-80 AND I-40 CORRIDORS (IN SOUTHWARD FASHION) DAY 5 THROUGH 7. WOULD NOT BE SURPRISED FOR AT LEAST ONE OR TWO MCS-TYPE SYSTEMS TO HOLD TOGETHER AND REACH THE MS/OH VALLEYS (ALONG AND GENERALLY NORTH OF I-70). ACROSS THE SOUTHEAST AND EAST COAST...RIDGING WILL BE THE PREVAILING SYNOPTIC-SCALE PATTERN WITH THE PERSISTENT ROUGH GRADUALLY SHIFTING NORTHEASTWARD...CAN SEE A WEAKENING BACKDOOR FRONT ATTEMPTING TO WORK THROUGH THE MID-ATLANTIC REGION DAYS 4-5. VOJTESAK