EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1158 AM EDT THU APR 28 2016 VALID 12Z SUN MAY 01 2016 - 12Z THU MAY 05 2016 ...OVERVIEW... THE 00Z/06Z CYCLE OF GUIDANCE MAINTAINS THE SAME THEME FROM RECENT RUNS... NAMELY A FAIRLY RAPID DISSIPATION OF A BRIEF WRN NOAM REX BLOCK CONFIGURATION FOLLOWED BY INCREASINGLY AMPLIFIED ERN NOAM TROUGHING IN RESPONSE TO A STRONG ERN PAC/WEST COAST TROUGH AND W-CNTRL NOAM RIDGE. THERE IS STILL GOOD AGREEMENT FOR THE LARGE SCALE EVOLUTION IN PRINCIPLE BUT PERSISTENT UNCERTAINTY IN SOME EMBEDDED DETAILS THAT WILL HAVE A MEANINGFUL IMPACT ON SENSIBLE WEATHER OVER SOME AREAS. ...GUIDANCE EVALUATION/PREFERENCES... DIFFS OVER THE ERN PAC/WRN U.S. ARE GENERALLY MODEST. INITIAL GRTBASIN/SOUTHWEST UPR LOW ENERGY SHOULD OPEN UP BY DAY 4 MON AND THEN BE REPLACED BY A STRENGTHENING RIDGE WHOSE AXIS SHOULD REACH THE ROCKIES HIGH PLAINS BY DAY 7 THU. WITH THE ERN PAC TROUGH NEARING THE WEST COAST THERE HAS BEEN A SLIGHTLY FASTER TREND OVER THE PAST 1-2 DAYS. THIS MIGHT FAVOR LEANING AWAY FROM THE 00Z NAEFS MEAN THAT IS A TAD SLOWER THAN THE GUIDANCE AVG. HOWEVER IT REMAINS TO BE SEEN HOW MUCH LONGER THIS FASTER TREND CAN CONTINUE GIVEN THE STRONGLY AMPLIFIED NATURE OF THE PATTERN BY LATE IN THE PERIOD... SO WOULD ALSO TEND TO LEAN AWAY FROM THE 06Z GFS THAT IS A BIT FASTER THAN MOST OTHER SOLNS WITH LEADING HGT FALLS. AT THE MOMENT THERE IS A DECENT SIGNAL THAT AN EMBEDDED UPR LOW MAY CLOSE OFF NEAR THE CA COAST BY NEXT THU. SOMEWHAT GREATER SPREAD/UNCERTAINTY EXIST FOR SOME DETAILS OVER THE ERN HALF OF THE LOWER 48. IN THE PAST COUPLE DAYS THERE HAS BEEN A PRONOUNCED FASTER TREND WITH THE SYSTEM FCST TO TRACK FROM THE MS VLY/OH VLY REGION INTO THE CANADIAN MARITIMES SUN-TUE. MODEL/ENSEMBLE CLUSTERING IS BETTER THAN IN PREVIOUS DAYS BUT LINGERING ISSUES WITH STREAM INTERACTION CONTINUE TO TEMPER CONFIDENCE. COMPONENTS ALOFT AFFECTING SFC EVOLUTION WILL INCLUDE INITIAL ENERGY EJECTING FROM THE PLAINS... GRTLKS/ERN CANADA FLOW... AND MORE INDIRECTLY THE UPSTREAM SHRTWV ENERGY TRACKING SEWD FROM CNTRL CANADA. PREFER TO ADHERE TO THE MAJORITY CLUSTER THAT INCLUDES THE 00Z GFS/ECMWF-EC MEAN/UKMET AND 00Z/06Z GEFS MEANS. IN SPITE OF RECENT TRENDS THE COMPLEXITY OF THE FCST FAVORS WAITING FOR MORE CONFIRMATION BEFORE TRENDING AS FAST AS THE 06Z GFS. BEHIND THIS SYSTEM GUIDANCE IS MIXED AS TO WHAT DEGREE THE COMBINATION OF EJECTING WRN UPR LOW ENERGY AND ERN U.S. TROUGH ENERGY WILL SUPPORT FRONTAL WAVINESS OVER THE SERN STATES BY DAY 5 TUE. BASED ON THE FULL ARRAY OF CURRENT AND RECENT GUIDANCE THE BEST SOLN APPEARS TO LIE BTWN THE SUPPRESSED GFS RUNS AND WELL NWD/NWWD ECMWF RUNS. TOWARD WED-THU THE AMPLIFYING UPR TROUGH SUPPORTS AN AXIS OF LOW PRESSURE ALONG THE EAST COAST... WITH DETAILS EVENTUALLY COMING INTO CLEARER FOCUS AS SHRTWV EVOLUTION ALOFT BECOMES BETTER RESOLVED. UPSTREAM THE GUIDANCE HAS BEEN CONSISTENT WITH THE IDEA OF A COLD FRONT DROPPING SEWD THROUGH THE UPR MS VLY/GRTLKS AND BEYOND BUT WITH SOME RUN TO RUN TIMING DIFFS. TOWARD THE END OF THE PERIOD THE 06Z GFS IS THE ONE PARTICULARLY QUESTIONABLE PIECE OF GUIDANCE AS IT BECOMES WEAKER THAN CONSENSUS WITH THE CORE OF THE ERN TROUGH ALOFT AND TRACKS S OF CONSENSUS WITH A DEEP COMPACT UPR LOW OVER CANADA. BASED ON DECENT CLUSTERING THROUGH MID-PERIOD THE LATEST WPC FCST REFLECTS A CONSENSUS BLEND THAT EMPHASIZES THE 00Z GFS/ECMWF/UKMET MORE THAN THE GEFS/ECMWF MEANS THROUGH DAY 5 TUE. DECREASING CONFIDENCE IN SPECIFICS WITH TIME LEADS TO INCREASING WEIGHT OF THE ENSEMBLE MEANS RELATIVE TO THE 00Z GFS/ECMWF BY DAYS 6-7 WED-THU. ...WEATHER HIGHLIGHTS/THREATS... RNFL OF VARYING INTENSITY WILL ACCOMPANY THE SYSTEM TRACKING THROUGH THE EAST SUN ONWARD. THOUGH NOT TO THE EXTENT SEEN IN PREVIOUS DAYS THERE IS STILL A FAIR AMOUNT OF SPREAD REGARDING FOCUS OF HEAVIEST ACTIVITY. MEANWHILE THE UPR LOW INITIALLY OVER THE WEST WILL GENERATE AREAS OF ENHANCED PCPN OVER PORTIONS OF THE SOUTHWEST/FOUR CORNERS/SRN ROCKIES FROM SUN INTO EARLY NEXT WEEK. SOME SNOW IS PSBL AT HIGHER ELEVS. OPENING UP/EJECTION OF THIS ENERGY SHOULD SUPPORT SOME POCKETS OF LOCALLY HVY RNFL ACROSS THE SRN PLAINS AND WRN-CNTRL GULF COAST. CONFIDENCE IS BELOW AVG FOR COVERAGE/AMTS OF RNFL THAT MAY SPREAD EWD/NEWD FROM THE SRN PLAINS AFTER SUN. TOWARD THE LATTER HALF OF THE PERIOD THE WEST COAST STATES SHOULD SEE A GRADUAL INCREASE IN MSTR AHEAD OF THE APPROACHING UPR TROUGH. EXPECT THE WRN-CNTRL STATES TO SEE THE MOST EXTREME ANOMALIES FOR TEMPS DURING THE PERIOD. ON THE COLD SIDE A CHILLY SFC RIDGE OVER THE HIGH PLAINS WILL LIKELY SUPPORT HIGHS AS MUCH AS 20-30F BELOW NORMAL AT SOME LOCATIONS ON SUN WITH MINUS 10-25F ANOMALIES PERSISTING INTO MON. ON THE OTHER HAND THE NWRN STATES SHOULD SEE MULTIPLE DAYS WITH HIGHS 10-20F ABOVE NORMAL... WITH SOME OF THIS WARMTH EVENTUALLY EXTENDING INTO THE NRN HIGH PLAINS. RAUSCH