EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1157 AM EDT WED APR 08 2015 VALID 12Z SAT APR 11 2015 - 12Z WED APR 15 2015 ...OVERVIEW... AHEAD OF AMPLIFYING WRN CONUS TROUGHING EXPECTED TO DEVELOP IN RESPONSE TO AMPLIFICATION/SHARPENING OF UPSTREAM PACIFIC FLOW... THE FCST HAS BEEN CHALLENGING FOR THE COMBINATION OF AMPLITUDE/TIMING FOR NRN STREAM FLOW AND TIMING/TRACK OF A SRN STREAM ERN PAC TROUGH/UPR LOW THAT SHOULD REACH THE PLAINS BY EARLY NEXT WEEK. SOME RECENT TRENDS SEEM TO BE EMERGING WITH RESPECT TO BOTH THE LATE PERIOD WRN TROUGH AS WELL AS FLOW FARTHER EWD BUT GUIDANCE SPREAD AND VARIABILITY UP TO THIS POINT KEEP CONFIDENCE FAIRLY MODEST. ...GUIDANCE EVALUATION AND PREFERENCES... OVER THE PAST 12-24 HRS THERE HAS BEEN A NOTICEABLE TREND TOWARD GREATER SEPARATION OF INCOMING NRN STREAM PACIFIC FLOW AND THE COMPACT SRN STREAM ERN PAC TROUGH/UPR LOW HEADING TOWARD/INTO THE PLAINS. THROUGH THE 00Z-06Z CYCLE THE 00Z ECMWF MEAN HOLDS ONTO THE SLOWEST NRN STREAM TROUGH/MOST PHASED DEPICTION ALOFT OVER THE PLAINS BY DAY 5 MON. AMONG OPERATIONAL MODELS THERE IS STILL GREAT SPREAD FOR NRN STREAM TIMING/AMPLITUDE. HOWEVER THE 00Z-06Z GFS AND 00Z ECMWF/UKMET/CMC/NAVGEM ALL SHARE THE COMMON THEME OF HAVING THE LEADING NRN STREAM ENERGY BE FAST/FLAT ENOUGH TO BYPASS THE SRN STREAM SHRTWV... LEAVING THE LATTER TO BE CARRIED NEWD BY SRN STREAM FLOW WITH PERHAPS EVENTUAL REMOTE INFLUENCE FROM LATE PERIOD TROUGHING OVER THE WEST. THIS SCENARIO CONTRASTS WITH SOME EARLIER GUIDANCE RUNS THAT SHOWED MORE POTENTIAL FOR THE LEADING NRN STREAM SHRTWV TO INTERACT WITH THE SRN STREAM FEATURE. GUIDANCE SPREAD AND INCONSISTENCY SO FAR KEEP CONFIDENCE LOWER THAN DESIRED BUT THERE IS NOW ENOUGH AGREEMENT IN PRINCIPLE TO ADJUST CONTINUITY TOWARD THE IDEA OF GREATER SEPARATION ALOFT. THIS RESULTS IN AN ASSOC COLD FRONT THAT SHOWS GREATER EWD PROGRESSION BUT LESS SWD EXTENT. THE ECMWF MODEL REMAINS ON THE FASTER SIDE OF THE SPREAD WITH THE SRN STREAM FEATURE ALOFT WITH CONSENSUS AND RECENT TRENDS FAVORING TIMING NO FASTER THAN THE MIDDLE OF THE ENVELOPE. WHAT TRENDS EXIST FOR THE DEVELOPING WRN TROUGH BY THE EARLY/MIDDLE PART OF NEXT WEEK ARE IN THE MORE AMPLIFIED DIRECTION. AS HAS APPEARED TO BE THE CASE DOWNSTREAM... ECMWF MEANS LOOK TO BE CATCHING UP TO GFS/GEFS-BASED SOLNS WITH EVEN THE PAST 2-3 OPERATIONAL ECMWF RUNS BEING MORE AMPLIFIED/CLOSED. GIVEN TYPICAL MODEL ERRORS 6-7 DAYS OUT IN TIME AND TELECONNECTIONS RELATIVE TO THE RIDGE IMMEDIATELY UPSTREAM SUGGESTING A POSITIVELY TILTED AND OPEN MEAN TROUGH... KEEPING ANY EMBEDDED CLOSURE ON THE BRIEF/PROGRESSIVE SIDE... PREFER TO LEAN MORE TOWARD THE ENSEMBLE MEANS AND SPECIFICALLY 2/3 TOWARD GEFS/NAEFS SOLNS BY TUE-WED. FROM DAY 3 SAT INTO DAY 5 MON A SOLN CONSISTING OF THE 06Z GFS/00Z ECMWF AND SLIGHTLY LESSER WEIGHTING OF THE LATEST ENSEMBLE MEANS CAPTURES THE HIGHER LIKELIHOOD OF SEPARATE STREAMS ALOFT WHILE ACCOUNTING FOR DIFFS WITHIN THAT THEME... AND YIELDS INTERMEDIATE TIMING FOR THE SRN STREAM FEATURE. BY DAYS 6-7 TUE-WED A 40/30/30 BLEND OF THE 00Z NAEFS/06Z GEFS/00Z ECMWF MEANS REFLECTS RECENT TRENDS SUGGESTING SOMEWHAT LOWER PROBABILITY OF THE ECMWF MEAN SOLN FOR THE WRN TROUGH AND FLOW DOWNSTREAM. ...SENSIBLE WEATHER/THREATS HIGHLIGHTS... BEST POTENTIAL FOR HIGHEST RNFL TOTALS DURING THE FULL FCST PERIOD SHOULD EXTEND FROM AROUND TEXAS EWD AND/OR NEWD TOWARD THE APLCHNS. EARLY IN THE PERIOD A STNRY SFC FRONT NEAR THE GULF COAST MAY HELP TO FOCUS ACTIVITY... AND THEN THE SRN STREAM FEATURE TRACKING OUT OF THE ERN PAC AS WELL AS MULTIPLE DAYS OF LOW LVL FLOW FROM THE GULF SHOULD BE SUPPORTIVE OF LOCALLY HVY RNFL. DEPENDING ON TRACK/TIMING OF THE SRN STREAM FEATURE THERE COULD ALSO BE ENHANCED PCPN AS FAR NWWD AS THE SRN-CNTRL ROCKIES INTO CNTRL PLAINS. THE UPR TROUGH FCST TO DEEPEN OVER THE WEST BY NEXT TUE-WED SHOULD SPREAD PCPN ACROSS NRN-CNTRL AREAS IN THE WEST FROM MON ONWARD WITH SOME LOCALLY ENHANCED AMTS AND DECREASING SNOW LVLS. SOME OF THIS MSTR COULD EXTEND INTO THE NRN PLAINS BY THE MIDDLE OF NEXT WEEK. CONSULT LATEST SPC OUTLOOKS FOR FURTHER INFO REGARDING SEVERE POTENTIAL OVER CNTRL/ERN AREAS... WITH MANY DETAILS IN THE EXTENDED TIME FRAME SUFFICIENTLY UNCERTAIN THUS FAR TO ALLOW FOR CONFIDENT PLACEMENT OF THREAT AREAS. ASIDE FROM SLIGHTLY BELOW NORMAL READINGS DURING THE WEEKEND IN THE EAST... MOST OF THE ERN 2/3 OF THE CONUS SHOULD SEE ABOVE NORMAL TEMPS FOR THE OVERALL PERIOD WITH CLOUD COVER PARTICULARLY FAVORING WARM MINS. ON THE OTHER HAND UPR TROUGHING SHOULD LEAD TO A COOLING TREND OVER THE WEST BY NEXT TUE-WED. RAUSCH