EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1159 AM EDT SAT MAY 03 2014 VALID 12Z TUE MAY 06 2014 - 12Z SAT MAY 10 2014 ...OVERVIEW AND PREFERENCES... OVER THE PAST DAY THE GUIDANCE SPREAD HAS TIGHTENED SOMEWHAT FOR THE DETAILS OF A VIGOROUS UPR SYSTEM CROSSING THE WEST TUE-WED AND THEN EJECTING NEWD TO SUPPORT LOW PRESSURE THAT SHOULD TRACK NEWD INTO SERN CANADA. THERE IS ALSO SOME CONVERGENCE OF SOLNS WITH RESPECT TO UPR TROUGHING THAT SHOULD PERSIST FOR A TIME OVER THE SWRN CONUS/NWRN MEXICO... LIKELY CAUSING THE TRAILING PART OF THE FRONT EXTENDING FROM THE AFOREMENTIONED SFC LOW TO STALL OVER THE SRN HALF OF THE PLAINS. HOWEVER DOWNSTREAM EFFECTS OF DIVERGING SOLNS WITH SOME DETAILS OF PACIFIC/NWRN NOAM FLOW BEGIN TO LOWER CONFIDENCE WITH THE CONUS/SRN CANADA FCST BY LATE IN THE PERIOD. A 06Z GFS AND 00Z ECMWF/UKMET CONSENSUS IS REASONABLE FOR DEPICTING SIGNIFICANT FEATURES DAYS 3-4 TUE-WED. THE 00Z GEFS/ECMWF MEANS ARE GIVEN PARTIAL WEIGHTING IN A DAY 5 TRANSITION TO AN EVEN BLEND OF THE 00Z MEANS BY FRI-SAT... AS 06Z GUIDANCE IS STRONGER THAN DESIRED WITH WEST COAST RIDGING LATE IN THE PERIOD. ...GUIDANCE EVALUATION... FROM THE LARGER SCALE PERSPECTIVE THE MOST SIGNIFICANT UNCERTAINTY ARISES IN THE LAST COUPLE DAYS OF THE PERIOD AS MODELS/ENSEMBLES DISPLAY INCREASING SPREAD WITH THE SHAPE OF WRN NOAM FLOW IN RESPONSE TO DETAIL DIFFS THAT DEVELOP EARLIER IN THE PERIOD UPSTREAM. BY NEXT FRI-SAT THE 00Z GEFS/ECMWF/CMC ENSEMBLES DISPLAY A FULL RANGE OF POSSIBILITIES FROM A RIDGE TO BROAD TROUGHING OVER OR JUST INLAND FROM THE WEST COAST. SOME OF THE ASPECTS OF THE UPSTREAM FLOW IN QUESTION ARE RATHER SENSITIVE AND MAY TAKE SOME TIME TO BE RESOLVED SO CONFIDENCE IS NOT PARTICULARLY HIGH IN A SPECIFIC SCENARIO. LOOKING AT D+7/D+8 MULTI-DAY MEAN CHARTS AND TELECONNECTIONS... POSITIVE HGT ANOMALY CENTERS FCST OVER THE ERN ALEUTIANS/AK PENINSULA AS WELL AS NRN CANADA APPEAR TO FAVOR LOW AMPLITUDE FLOW NEAR THE WEST COAST. THEREFORE WOULD AT LEAST FAVOR LEANING AWAY FROM THE 00Z GFS AND 06Z GFS/GEFS MEAN WHICH ARE STRONGEST WITH WEST COAST RIDGING BY NEXT SAT... AND THE 06Z SOLNS THAT ARE SLOWEST/STRONGEST WITH NWRN MEXICO TROUGHING AS A RESULT. ON THE OTHER SIDE OF THE SPECTRUM... WOULD ALSO WAIT FOR FURTHER SUPPORT BEFORE LEANING TOO HEAVILY ON THE PAST COUPLE ECMWF RUNS THAT BRING CONCENTRATED MID LVL ENERGY INTO THE NORTHWEST BY SAT. WHILE SOLNS AGREE RATHER WELL NOW WITH A TROUGH/EMBEDDED COMPACT UPR LOW CROSSING THE WEST EARLY IN THE PERIOD... THIS ENERGY IS THEN EXPECTED TO BECOME RAPIDLY SHEARED NEWD AS IT IS SQUEEZED BETWEEN AN UPR LOW OVER SRN CANADA AND A RIDGE BUILDING INTO E-CNTRL NOAM. THIS EVOLUTION ALOFT HAS LED TO A WEAKENING OF SOLNS THAT HAD BEEN ON THE STRONGER SIDE OF THE ENVELOPE FOR THE SFC LOW REACHING THE UPR MS VLY AROUND DAY 5 THU. MEANWHILE SOME PREVIOUS ECMWF/ECMWF MEAN GUIDANCE IN PARTICULAR THAT HAD FLATTENED DOWNSTREAM RIDGING HAS REVERTED TO MAINTAINING A BETTER RIDGE ALOFT... THUS LEADING TO A SLOWER AND/OR LESS SUPPRESSED SFC SYSTEM FROM THE UPR MS VLY EWD/NEWD. UNRESOLVED SMALLER SCALE SHRTWV DETAIL DIFFS WITHIN THE MEAN TROUGH ALOFT OVER ERN CANADA AND THE NERN CONUS/WRN ATLC LEAD TO SOME QUESTION MARKS ON EXACTLY HOW QUICKLY THE SFC FRONT INITIALLY OVER THE LWR OH VLY/SRN MID ATLC WILL LIFT NEWD AS A WARM FRONT AHEAD OF THE PLAINS SYSTEM. AT THE VERY LEAST... BASED ON THE FULL RANGE OF GUIDANCE THE 00Z/06Z GFS MAY BE TOO STG WITH SHRTWV ENERGY BRUSHING EXTREME NRN NEW ENGLAND DAY 6 FRI SO THOSE RUNS MAY BE A LITTLE TOO SUPPRESSED WITH THE FRONT AT THAT TIME. ...SENSIBLE WEATHER HIGHLIGHTS... THE INITIAL WRN SYSTEM WILL GENERATE A BROAD AREA OF PCPN AND COOL DAYTIME TEMPS FROM THE NRN ROCKIES/GRTBASIN NEWD DURING THE FIRST HALF OF THE PERIOD WITH HEAVIEST TOTALS PSBL OVER THE NRN ROCKIES/NRN PLAINS WITH SOME LOW LVL UPSLOPE FLOW CONTRIBUTING TO ENHANCED PCPN DURING PORTIONS OF THIS EVENT. PCPN MAY FALL AS SNOW IN THE HIGHER ELEVS OF THE ROCKIES. AS LOW PRESSURE TRACKS NEWD FROM THE PLAINS MIDWEEK ONWARD... THE LEADING WARM FRONT WILL LIKELY GENERATE SOME RNFL WITH ANY HEAVIER AMTS TENDING TO BE MORE LOCALIZED. BY THU-SAT GUIDANCE IS CONTINUING TO SHOW THE POTENTIAL FOR THE TRAILING FRONT TO BECOME STNRY OVER THE SRN HALF OF THE PLAINS INTO MID MS VLY DUE TO PERSISTENCE OF TROUGHING ALOFT OVER THE SWRN CONUS/NWRN MEXICO. A FEED OF MSTR FROM THE WRN GULF MAY HELP TO SUPPORT MULTIPLE INCHES OF RNFL AT SOME LOCATIONS WITHIN AN AREA FROM TX/OK NEWD INTO THE MID MS VLY WITH SOME OF THIS MSTR PSBLY REACHING THE OH VLY AS WELL. ALSO MONITOR SPC OUTLOOKS FOR INFO REGARDING SEVERE POTENTIAL. HIGH TEMPS REACHING 15-20F OR SO ABOVE NORMAL ARE PSBL OVER THE CNTRL/S-CNTRL PLAINS TUE-WED. THE PLAINS WARMTH WILL EXPAND EWD/NEWD THOUGH WITH LESSER ANOMALIES. BEHIND THE DOMINANT SYSTEM OF INTEREST... PAC FLOW MAY BRING ONE OR MORE PERIODS OF PCPN TO THE NORTHWEST FRI INTO THE WEEKEND. RAUSCH