EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 254 AM EDT THU APR 24 2014 VALID 12Z SUN APR 27 2014 - 12Z THU MAY 01 2014 ...A STORMY AND AMPLIFIED/BLOCKY LATE APRIL PATTERN... ...PATTERN OVERVIEW AND WEATHER HIGHLIGHTS/THREATS... A MAJOR COOLING/UNSETTLING MID-UPPER LEVEL S-CENTRAL ROCKIES/PLAINS TROUGH AND DEEP LEAD CYCLOGENESIS SUN/MON SETS THE STAGE FOR A SPRING N-CENTRAL ROCKIES/HIGH PLAINS SNOW THREAT IN A COOLED AIRMASS WRAPPED BACK FROM THE NRN HALF OF THE CENTRAL AND ERN US. SPC ALSO SHOWS MULTI-DAY SEVERE WEATHER POTENTIAL IN A WELL DEFINED WARM SECTOR OVER THE S-CENTRAL PLAINS/MS VALLEY INTO SUN/MON INCLUDING A STRONG TORNADO THREAT. THE CLOSED TROUGH/LOW AND WELL ORGANIZED SURFACE SYSTEM SHOULD SUBSEQUENTLY BE FORCED GRADUALLY EASTWARD ACROSS THE EAST-CENTRAL US TUE-THU DAYS 5-7 IN A DEVELOPING REX BLOCK SETUP AS WARMING RIDGES BUILD STRONGLY UPSTREAM OVER THE WRN US AND OVERTOP OVER S-CENTRAL CANADA. THIS OCCURS AS A LEAD NERN US/CANADIAN MARITIME CLOSED LOW LOSES INFLUENCE. SLOW/WRAPPED SYSTEM TRANSLATION AND INCREASINGLY DEEP MOISTURE INFLUX FROM THE GULF OF MEXICO THEN WRN ATLANTIC WILL FUEL A WIDESPREAD CONVECTIVE AND OVERRUNNING HEAVY TO EXCESSIVE RAINFALL THREAT SPREADING SLOWLY ACROSS THE EAST-CENTRAL TO ERN US AS DEPICTED ON RECENT/UPCOMING WPC MEDIUM RANGE DAYS 4-7 QPF PROGS. ...GUIDANCE/UNCERTAINTY ASSESSMENT... THE WPC MEDIUM RANGE GUIDANCE SUITE HAS BEEN PRIMARILY DERIVED FROM A COMPOSITE BLEND OF THE GFS/ECMWF/CANADIAN AND GFS/ECMWF ENSEMBLE MEANS ALONG WITH WPC/NDFD CONTINUITY DAYS 3-5 SUN-TUE. A FORECAST PLAN BASED CLOSEST TO THE 12 UTC ECMWF ENSEMBLE MEAN AND TO LESSER DEGREE THE 12 UTC ECMWF WAS INVOKED DAYS 6/7 TO MAINTAIN MAX PATTERN AMPLITUDE/IDENTITY THAT SEEMS WELL SUPPORTED BY LEAD-IN WATER VAPOR LOOP IMAGERY. CONSIDERING THE CLOSED/BLOCKY NATURE OF THE MAIN WEATHER FEATURES AND THREATS AS DESCRIBED ABOVE...GUIDANCE OVERALL REMAINS WELL CLUSTERED WITH FORECAST CONFIDENCE ABOVE NORMAL. A NOTABLE EXCEPTION WAS EVIDENT FROM THE ERN PACIFIC INTO WRN NOAM DAYS 6/7 AS THE 12 UTC GFS AND GEFS/CANADIAN/NAEFS ENSEMBLE MEANS WERE SIGNIFICANTLY FASTER TO BRING AMPLE PACIFIC MID-UPPER LEVEL TROUGH ENERGY INLAND THROUGH THE AMBIENT WRN US RIDGE THAN THE 12 UTC ECMWF/CANADIAN/ECMWF ENSEMBLE MEAN. WE SEE MINIMAL COMPELLING REASON TO CRASH THE AMPLIFIED RIDGE IN A BLOCKY PATTERN AND MORE RECENT 18/00 UTC GFS AND 00 UTC ECMWF TRENDS BLEND WELL INTO THE WPC BLENDED SOLUTION. SCHICHTEL