EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1148 AM EDT TUE MAY 12 2015 VALID 12Z FRI MAY 15 2015 - 12Z TUE MAY 19 2015 ...OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES... AN ACTIVE WEATHER PATTERN SHOULD CONTINUE ACROSS A SIGNIFICANT PORTION OF THE LOWER 48 WITH PERSISTENCE OF A MEAN PATTERN INVOLVING ERN PAC ENERGY SETTLING INTO AND EJECTING FROM A WEST COAST MEAN TROUGH. AT THE SAME TIME FAIRLY STRONG RIDGES SHOULD PRESIDE OVER WRN CANADA AS WELL AS OVER FLORIDA AND VICINITY. LATEST MODEL AND ENSEMBLE GUIDANCE SHOWS BETTER THAN AVERAGE AGREEMENT WITH MOST ASPECTS OF THE FCST THROUGH THE PERIOD. THE DOMINANT FEATURE OF INTEREST WILL BE AN UPR LOW THAT SHOULD EJECT FROM CA/NV AND TRACK THROUGH THE N-CNTRL ROCKIES/NRN PLAINS BEFORE OPENING UP AS REMAINING ENERGY IS PICKED UP BY AN AMPLIFYING CANADIAN TROUGH. UPSTREAM ERN PAC ENERGY SHOULD THEN SETTLE OVER THE WEST COAST IN THE FORM OF ANOTHER CLOSED LOW... WEAKER THAN ITS PREDECESSOR... BY NEXT MON-TUE. THE ONLY DISCERNIBLE TREND OVER THE PAST 1-2 DAYS APPEARS EARLY IN THE PERIOD WITH A SLIGHTLY FASTER ADJUSTMENT FOR THE LEADING UPR LOW EJECTING FROM THE WEST. OVERALL THE 06Z GFS/GEFS MEAN AND 00Z ECMWF/ECMWF MEAN FORM A GOOD CLUSTER SFC/ALOFT AND A BLEND OF THEIR SOLNS WILL RESOLVE MINOR TIMING AND DETAIL DIFFS. THE BLEND LEANS A TAD TOWARD THE ECMWF-BASED SOLNS WHICH ARE SLIGHTLY FASTER ACRS THE UPR MS VLY/GRTLKS AND CLOSER TO CONTINUITY. THE 00Z GFS IS THE ONE PIECE OF GUIDANCE AT ODDS WITH CONSENSUS BY THE LATTER HALF OF THE FCST... WITH A FASTER/FLATTER CANADIAN TROUGH ALOFT AND SLOWER NRN TIER SYSTEM. AHEAD OF THE TWO MOST PROMINENT FEATURES... EXPECT A FRONTAL WAVE NEAR THE UPR MS VLY TO TRACK INTO THE ATLC DURING THE FIRST HALF OF THE PERIOD. AN AVG OF LATEST GUIDANCE APPEARS REASONABLE WITH SOME WAVE/FRONTAL DETAILS LIKELY NOT TO BE RESOLVED UNTIL THE VERY SHORT RANGE GIVEN PSBL CONVECTIVE INFLUENCES. ...SENSIBLE WEATHER HIGHLIGHTS... ONE AXIS OF ENHANCED TO HVY PCPN SHOULD EXTEND FROM OR/NV AND EXTREME NRN CA ACROSS THE N-CNTRL ROCKIES AND NRN PLAINS. THE WRN PART OF THIS AXIS SHOULD CONSIST OF PCPN FROM A COMBINATION OF THE LEAD EJECTING SYSTEM AND TROUGH/UPR LOW NEAR THE WEST COAST LATER IN THE PERIOD. A PERIOD OF LOW LVL UPSLOPE FLOW TO THE NW OF SFC LOW PRES CROSSING THE PLAINS MAY PROVIDE AN ADDED FOCUS OVER EWD/NWD-FACING TERRAIN OVER AND JUST SE OF MT. SOME PCPN MAY FALL AS SNOW IN HIGHER ELEVS. AHEAD OF THIS SYSTEM THERE WILL BE A SEVERE WEATHER THREAT GRADUALLY PUSHING EWD THROUGH THE PLAINS INTO THE MID-UPR MS VLY. CONSULT LATEST SPC OUTLOOKS FOR FURTHER DETAILS. AT THE SAME TIME MULTIPLE DAYS OF LOW LVL FLOW FROM THE GULF WILL CONTRIBUTE TO LOCALLY HVY RNFL FROM THE SRN PLAINS EWD/NEWD PERHAPS INTO THE APLCHNS. THIS ACTIVITY MAY BE PROBLEMATIC FOR SOME AREAS OVER THE SRN PLAINS WHERE SIGNIFICANT RNFL HAS RECENTLY OCCURRED. THERE IS SOMEWHAT MORE UNCERTAINTY OVER RNFL AMTS ACRS THE GRTLKS/NORTHEAST/MID ATLC. MEAN FLOW ALOFT/SYSTEM EVOLUTION SHOULD SUPPORT A BROAD AREA OF UNSEASONABLY COOL TEMPS FROM A MAJORITY OF THE WEST THROUGH THE NRN TIER STATES... ESPECIALLY FOR MAX TEMPS. THE ONE EXCEPTION SHOULD BE THE EXTREME NRN INTERIOR WEST WHICH MAY BE INFLUENCED BY THE VERY SRN PERIPHERY OF THE STRONG WRN CANADA RIDGE ALOFT. MOST OF THE ERN CONUS SHOULD SEE ABOVE NORMAL READINGS WITH ANOMALIES FOR MINS PSBLY GREAT ENOUGH TO SET DAILY RECORDS FOR WARM LOWS AT SOME LOCATIONS. RAUSCH