EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1154 AM EDT MON MAY 16 2016 VALID 12Z THU MAY 19 2016 - 12Z MON MAY 23 2016 OF THE CHOICES OFFERED BY THE 16/00Z MODEL GUIDANCE CYCLE --- THE ECENS/NAEFS MEANS AND DETERMINISTIC ECMWF APPEARED TO MAINTAIN THE OVERALL PROGRESSION OF THE WPC MEDIUM RANGE FORECAST AND LOWER 48'S FLOW PATTERN. EAST OF THE DIVIDE... A DAY 4 SHORTWAVE EXITING THE SOUTHERN HIGH PLAINS AND ITS MIGRATION DOWNSTREAM THROUGH DAY 6 OFFERED THE GREATER UNCERTAINTY --- ESPECIALLY AT THE SURFACE --- WHERE RATHER STRONG CANADIAN HIGH PRESSURE PRECEDES IT. PREFER SOMETHING ALONG THE LINES OF THE 16/00Z DETERMINISTIC ECMWF WOULD PROVIDE SOME SURFACE REFLECTION DETAIL AT THIS JUNCTURE AND 'TRACK-ABLE' SPOT LOW PROGRESSION THAT EVENTUALLY PUSHES OFF THE EAST COAST (INVOF OUTER BANKS DAY 6-ISH). WEST OF THE DIVIDE... ALTHOUGH THE 16/00Z DETERMINISTIC GUIDANCE VARIES ON THE 500MB VORTICITY PATTERN EMBEDDED WITHIN THE UPPER-LEVEL TROUGH --- THE AMPLITUDE OF THE SYNOPTIC-SCALE MIGRATORY TROUGH IS IN VERY REASONABLE 'ORDER' HEADING INTO DAY 7. THOUGHT THE DETERMINISTIC ECMWF ACTUALLY BALANCES OUT THE SURFACE DIFFERENCES --- GENERATED BY THE EXITING SHORTWAVE ENERGY --- AND THEIR IMPACTS ON THE SHAPE OF THE DOWNSTREAM RIDGE. THE POINT BEING... THE RIDGE IS MIGRATORY IN NATURE ... AND WILL LIKELY AMPLIFY THROUGH TIME ALONG THE MISSISSIPPI RIVER ... AND GRADUALLY SLOW DOWN THE DAY 6-7 PACIFIC 'FRONT' OVER THE PLAINS. THE GENERAL WEATHER PATTERN FOR THE LOWER 48 INCLUDES A WETTER THAN NORMAL ACROSS THE SOUTHEAST/GULF COAST --- AND MORE RAIN FOR THE MID-ATLANTIC NEXT WEEKEND. COOLER AND WETTER ACROSS THE PACIFIC NORTHWEST AND NORTHERN HALF OF THE CONTINENTAL DIVIDE. VOJTESAK