EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1157 AM EDT SUN MAR 26 2017 VALID 12Z WED MAR 29 2017 - 12Z SUN APR 02 2017 ...OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES... THE PATTERN ACROSS NORTH AMERICA THROUGH THE MEDIUM RANGE WILL PERSIST WITH A RELATIVELY ACTIVE FLOW REGIME FROM THE NORTH PACIFIC INTO BRITISH COLUMBIA AND THE U.S. NORTHWEST. THE FLOW THROUGHOUT THE PERIOD WILL SPLIT AS IT MOVE EAST, WITH A NUMBER OF RELATIVELY LOW AMPLITUDE WAVES PROPAGATING THROUGH THE NORTHERN STREAM ACROSS CANADA, AND A HIGHLY AMPLIFIED, LOW WAVENUMBER SOUTHERN STREAM FLOW ACROSS THE CONUS. PARTIAL PHASING OF THE TWO STREAMS WILL OCCUR PERIODICALLY ACROSS SOUTHEASTERN CANADA AND THE GREAT LAKES/NORTHEASTERN U.S, WITH THE POTENTIAL THE DEVELOPMENT OF A COUPLE LOW PRESSURE SYSTEMS ACROSS THE NORTH ATLANTIC WELL EAST OF NEW ENGLAND. THIS AREA OF PHASING RESULTS IN A MODERATE LEVEL OF SPREAD INITIALLY ON DAYS 3-4 (THU-FRI) AS A LOW DEVELOPS EAST OF NEW ENGLAND AND QUICKLY DEEPENS SOUTH OF THE CANADIAN MARITIMES. THE 06Z GFS AND 00Z ECMWF/UKMET AS WELL AS THE 00Z ECMWF ENS MEAN/06Z GEFS MEAN HAVE COME INTO SOMEWHAT BETTER AGREEMENT, HOWEVER, WITH A TREND TOWARD A QUICKER EASTWARD PROGRESSION RELATIVE TO WHAT WAS SHOWN BY THE 00Z GFS. ALSO ON DAYS 3-4, THE FIRST OF TWO DEEP UPPER LOWS DURING THE MEDIUM RANGE IS FORECAST TO MOVE FROM THE SOUTHERN ROCKIES INTO THE SOUTHERN/CENTRAL PLAINS. MODEL AGREEMENT WITH RESPECT TO THIS SYSTEM IS RELATIVELY GOOD INITIALLY, BUT SPREAD BEGINS TO INCREASE AS THE SYSTEM MOVES INTO THE MID-MS/OH VALLEYS ON DAY 5 (FRI), WITH THE LOW/TROUGH DEAMPLIFYING AS IT MOVES INTO A MID/UPPER RIDGE AXIS ACROSS THE EASTERN U.S. SPREAD HAS DECREASED COMPARED TO THIS TIME YESTERDAY, HOWEVER, WITH THE GFS AND ECMWF SOLUTIONS APPEARING TO MOVE TOWARD ONE ANOTHER. BY DAY 6 (SAT), A COASTAL LOW IS FORECAST TO DEVELOP AND QUICKLY MOVE EAST OFF THE MID-ATLANTIC COAST IN THE AFOREMENTIONED REGION OF POLAR/SUBTROPICAL STREAM PARTIAL PHASING. FOR DAY 6, SPREAD IS RELATIVELY LOW WITH RESPECT TO THIS COASTAL LOW, WITH THE 00Z EC/EC MEAN/06Z GFS/GEFS MEAN ALL WITHIN 100-200 MILES OF EACH OTHER WITH THEIR RESPECTIVE LOW POSITION AT 12Z SAT. A SECOND SIGNIFICANT SHORTWAVE IS FORECAST TO MOVE INTO THE PACIFIC NORTHWEST WED NIGHT/EARLY THU BEFORE QUICKLY DIVING SOUTHEAST AND AMPLIFYING IN THE SOUTHERN STREAM, WITH AN UPPER LOW FORECAST BY ALL GUIDANCE IN THE VICINITY OF THE FOUR CORNERS REGION BY 12Z FRI. THERE ARE SOME TIMING DIFFERENCES WITH RESPECT TO THE ARRIVAL OF THE SHORTWAVE IN THE NORTHWEST THU MORNING, WITH THE 06Z GFS SLIGHTLY ON THE FAST SIDE, BUT THESE DIFFERENCES DO NOT SEEM TO TRANSFER ONCE THE FEATURE CLOSES OFF ACROSS THE SOUTHWEST. THE 00Z ECMWF HAS TRENDED TOWARD THE GFS ON DAY 6, SHOWING THE UPPER LOW PERSISTING DEEPER AND FOR LONGER THAN ITS PREVIOUS RUNS. HOWEVER, THE EC CONTINUES TO WEAKEN THE FEATURE AND MOVE IT QUICKLY NORTHEAST INTO THE CENTRAL U.S. ON DAY 7 (SUN). THE GFS/GEFS HAVE BEEN HIGHLY CONSISTENT FROM RUN-TO-RUN IN KEEPING THE UPPER LOW STRONGER AND FARTHER WEST/SOUTH FOR LONGER. ADDITIONAL NORTHERN STREAM SHORTWAVE ENERGY IS FORECAST TO APPROACH THE NORTHWEST BY DAY 7, BUT SPREAD IS VERY HIGH WITH RESPECT TO THE TIMING OF THIS FEATURE DUE TO THE LOWER AMPLITUDE AND LOWER PREDICTABILITY OF THE MORE ACTIVE NORTHERN STREAM FLOW. AGAIN WITH THIS FEATURE, THE GFS/GEFS ARE SLOWER THAN THE EC/EC MEAN, BUT THE GFS/GEFS HAVE BEEN MORE CONSISTENT FROM RUN-TO-RUN. THE 00Z NAEFS ALSO SUPPORTS A SOLUTION SIMILAR TO THE GFS/GEFS. THE WPC FORECAST WAS HEAVILY WEIGHTED ON THE DETERMINISTIC SOLUTIONS, PRIMARILY THE 00Z ECMWF/06Z GFS ON DAYS 3-5, WITH MINORITY COMPONENTS OF THE 00Z EC MEAN/06Z GEFS MEAN INCLUDED. A SIGNIFICANT INCREASE IN ENSEMBLE MEANS WERE INCLUDED BY DAYS 6-7, WITH MORE WEIGHT PLACED TOWARD THE GEFS AND NAEFS, GIVEN THE BETTER RUN-TO-RUN CONSISTENCY DEMONSTRATED BY THE GFS/GEFS. A MINORITY COMPONENT OF THE 00Z ECMWF ENSEMBLE MEAN WERE ALSO INCLUDED IN DAYS 6-7. ...SENSIBLE WEATHER HIGHLIGHTS/POTENTIAL HAZARDS... GIVEN THE ANOMALOUS AND ENERGETIC NATURE OF THE SYSTEM ACROSS THE CENTRAL U.S. ON DAYS 3-5, MODELS SHOW THE POTENTIAL FOR SUBSTANTIAL LOW/MID-LEVEL MOISTURE FLUX ACROSS THE SOUTHERN/CENTRAL PLAINS AND LOWER/MID-MS VALLEY, AND THE POTENTIAL FOR DEEP CONVECTION ALONG WITH HEAVY/EXCESSIVE RAINFALL. THE STORM PREDICTION CENTER IS ALSO INDICATING THE POTENTIAL FOR SEVERE THUNDERSTORMS FROM THE SOUTHERN PLAINS TO THE LOWER MS VALLEY DURING THIS TIME. FARTHER WEST, THE SHORTWAVE ENTERING THE NORTHWEST WED/THU WILL BE ACCOMPANIED BY A POTENTIAL ATMOSPHERIC RIVER WITH CONNECTIONS TO THE TROPICAL PACIFIC, THUS WOULD EXPECT THE POTENTIAL FOR HEAVY RAIN AND HIGH ELEVATION SNOWS ACROSS THE PAC NW. STANDARDIZED ANOMALIES SHOW THE POTENTIAL FOR ANOMALOUS PWATS ASSOCIATED WITH THIS SYSTEM TO PENETRATE AS FAR INLAND AS THE GREAT BASIN/INTERMOUNTAIN WEST, THUS THE POTENTIAL IS THERE FOR SOME HEAVIER PRECIPITATION AMOUNTS IN THOSE REGIONS AS WELL, PARTICULARLY IN FAVORED TERRAIN. AS THE SECOND UPPER LOW BEGINS TO MOVE EAST TOWARD THE SOUTHERN PLAINS BY DAY 6-7, EXPECT ANOTHER ROUND OF POTENTIAL CONVECTION FOR AREAS FROM THE SOUTHERN PLAINS AND PERHAPS THE LOWER MS VALLEY. WITH RESPECT TO TEMPERATURES, THE SPLIT FLOW PATTERN AT THE MID/UPPER-LEVELS WILL KEEP MUCH OF THE CONUS GENERALLY MILD WITH NEAR OR ABOVE AVERAGE TEMPERATURES. THE NORTHEAST AND PORTIONS OF THE MID-ATLANTIC WILL SEE THE POTENTIAL FOR THE INCURSION OF COOLER MARITIME AIR BY LATE IN THE WEEK INTO NEXT WEEKEND AS HIGH PRESSURE BUILDS IN TO THE NORTH AND LOW PRESSURE POTENTIALLY DEVELOPS OFF THE COAST. RYAN