EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 244 AM EDT SAT APR 25 2015 VALID 12Z TUE APR 28 2015 - 12Z SAT MAY 02 2015 ...OVERVIEW... WITHIN A MEAN PATTERN THAT SHOULD EVOLVE TOWARD TROUGHS NEAR EACH COAST AND A RIDGE OVER THE CNTRL CONUS... GREATEST UNCERTAINTIES CONTINUE TO BE WITH ERN CONUS/WRN ATLC EVOLUTION WHILE ISSUES OF SHRTWV AMPLITUDE/TIMING PERSIST FOR THE MEAN TROUGH SETTLING TOWARD/OVER THE WEST COAST. ...GUIDANCE EVALUATION AND PREFERENCES... FOR ERN CONUS/WRN ATLC FCST DETAILS THE CONFIDENCE LEVEL IN ANY PARTICULAR SOLN HAS IMPROVED LITTLE OVER THE PAST DAY. THE ONE AGREEABLE PRINCIPLE IS THAT NRN GULF LOW PRES INITIALLY ASSOC WITH A MID LVL SYSTEM TRACKING ACROSS THE SRN HALF OF THE PLAINS SHOULD PROGRESS INTO THE ATLC BY WED-THU. INDIVIDUAL MODEL RUNS AND ENSEMBLE MEMBERS STILL DISPLAY A WIDE RANGE OF POSSIBILITIES IN STRENGTH/TRACK/TIMING FOR THE SFC LOW DUE TO A SIMILARLY LARGE ARRAY OF PSBL DETAILS ALOFT. ALREADY EARLY IN THE PERIOD DIFFS EMERGE WITH THE EVOLUTION OF NRN STREAM ENERGY OVER THE UPR MIDWEST WITH CORRESPONDING DIFFS IN HOW THIS ENERGY AFFECTS THE SRN PLAINS FEATURE. AMONG THE OPERATIONAL MDLS THE PAST COUPLE ECMWF RUNS AND NEW 00Z UKMET SEEM TO FORM ONE CLUSTER THAT IS QUITE AGGRESSIVE WITH THE DEPTH/CLOSURE OF ENERGY FROM THE GRTLKS INTO MID ATLC... AND STILL WITH VERY DIFFERENT SFC SOLNS... WHILE SOME RECENT GFS RUNS AND THE 00Z/24 ECMWF WERE MOST PRONOUNCED IN BRINGING TRAILING NERN PAC AND/OR CANADIAN ENERGY ACROSS THE NORTHEAST AS A CONCENTRATED UPR LOW LATE IN THE PERIOD. THE LATTER FEATURE WOULD BRING DEEP SFC LOW PRES CLOSE TO NEW ENGLAND OR AT LEAST THE CANADIAN MARITIMES AS IN THE THE NEW 00Z GFS. THE FACT THAT MODELS TEND TO HAVE GREATER DIFFICULTY WITH HIGHER LATITUDE FLOW FURTHER RESTRAINS ANY FCST CONFIDENCE. PERHAPS IT IS REFLECTIVE OF THE DISJOINTED EVOLUTION ALOFT INVOLVING MULTIPLE FEATURES EACH WITH THEIR OWN UNCERTAINTIES THAT THE GEFS AND ECMWF MEANS ARE ACTUALLY A LITTLE WEAKER WITH THEIR SFC SYSTEMS THAN IN SOME PRIOR RUNS. THE RELATIVELY HIGH UNCERTAINTY IN THIS OVERALL EVOLUTION FAVORS MAINTAINING AN ENSEMBLE MEAN/CONTINUITY THEME. FOR THIS CYCLE A 70/30 BLEND OF THE 12Z ECMWF MEAN/12Z ECMWF OFFERED THE BEST BALANCE OF CONTINUITY AND SOME DETAIL THROUGH DAY 5 THU. DAY 6 FRI REDUCES 00Z ECMWF WEIGHTING WHILE STARTING TO INCORPORATE SOME OF THE 18Z GEFS MEAN WITH DAY 7 SAT GOING EXCLUSIVELY WITH THE 18Z GEFS/12Z ECMWF MEANS. WITHIN THE MEAN TROUGH NEARING THE WEST COAST... MULTIPLE GFS RUNS HAD BEEN ON THE SLOW/SEPARATED SIDE OF GUIDANCE WITH A LEADING SHRTWV NEAR THE START OF THE PERIOD BUT THE 00Z RUN NOW COMPARES MUCH BETTER TO ESTABLISHED CONSENSUS. TRAILING SHRTWVS STILL EXHIBIT SOME SPREAD IN AMPLITUDE AND TIMING. CURRENTLY MOST ENSEMBLE MEANS SUGGEST A WEST COAST TROUGH THAT IS SOMEWHAT WEAKER AND/OR SLOWER THAN THE 18Z/00Z GFS AND 12Z CMC. THE PAST TWO ECMWF RUNS ARE FAVORABLY SLOWER WITH ENERGY IN THIS STREAM RELATIVE TO PREVIOUS RUNS. ECMWF/ECMWF MEAN TIMING RESPECTS THE DOWNSTREAM RIDGE ALOFT THROUGH MID-PERIOD WHILE THE GEFS/ECMWF MEANS BECOME SIMILAR ENOUGH TO INCLUDE BOTH BY FRI-SAT. ...SENSIBLE WEATHER HIGHLIGHTS... THERE IS CONTINUED POTENTIAL FOR AREAS OF HVY RNFL FROM THE LOWER MS VLY/CNTRL GULF COAST THROUGH THE SOUTHEAST/FLORIDA IN ASSOC WITH THE SYSTEM TRACKING E/NE FROM THE GULF OF MEXICO... BUT ISSUES WITH DETAILS ALOFT FROM EARLY IN THE PERIOD BEGIN TO CAUSE DIFFICULTY IN PINPOINTING LOCATION/TIMING. CONFIDENCE REMAINS LOW REGARDING PCPN COVERAGE/INTENSITY OVER THE REST OF THE EAST. AT THE VERY LEAST ONE OR MORE MID LVL FEATURES MAY GENERATE SCT PCPN WITH SOMEWHAT MORE ACTIVITY DURING DIURNALLY FAVORED HOURS. WHILE A LOW PROBABILITY SCENARIO AT THE MOMENT... EXISTENCE OF A SUFFICIENTLY DEEP CLOSED LOW COULD SUPPORT HIGH ELEV SNOW FOR A TIME. MEANWHILE THE MEAN TROUGH SETTLING NEAR THE WEST COAST SHOULD LEAD TO PERIODS OF MOSTLY LGT-MDT PCPN OVER THE NORTHWEST. SOME OF THIS MSTR MAY EXTEND INTO THE NRN PLAINS AND SCT ACTIVITY COULD INCREASE FARTHER SWD OVER/NEAR THE ROCKIES BY NEXT WEEKEND. COOLEST TEMPS RELATIVE TO NORMAL SHOULD EXTEND FROM THE SRN HALF OF THE PLAINS THROUGH THE SOUTHEAST TUE-WED. EXPECT THE GREATEST COVERAGE OF MINUS 10-20F ANOMALIES FOR HIGHS OVER THE SRN PLAINS/LWR MS VLY ON TUE. ON THE OTHER HAND WRN CONUS WARMTH WITH SOME MAX TEMPS OF 10-20F ABOVE NORMAL WILL SPREAD INTO THE NRN TIER STATES FAIRLY QUICKLY AND PERSIST. THE AREA FROM THE NRN GRTBASIN INTO NRN HIGH PLAINS SHOULD SEE THE HIGHEST ANOMALIES FOR THE OVERALL TUE-SAT PERIOD. RAUSCH