EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 247 AM EDT MON MAR 27 2017 VALID 12Z THU MAR 30 2017 - 12Z MON APR 03 2017 ...OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES... EXPECT A CONTINUATION OF THE SPLIT FLOW PATTERN FEATURING A TENDENCY TOWARD AN AMPLIFIED MEAN TROUGH OVER THE WEST AND A MORE PROGRESSIVE NRN STREAM THAT WILL CONTAIN SHRTWVS WHOSE CHARACTER, AND DEGREE OF INTERACTION WITH SRN STREAM FLOW, WILL BE MORE DIFFICULT TO RESOLVE SEVERAL DAYS IN ADVANCE. A LEADING SYSTEM EJECTING FROM THE WRN MEAN TROUGH IN THE SHORT RANGE WILL CONTINUE ONWARD FROM THE PLAINS DURING THE LATTER HALF OF THE WEEK WHILE SHRTWV ENERGY REACHING THE WEST COAST ON THU WILL QUICKLY AMPLIFY INTO A FAIRLY DEEP CLOSED LOW DROPPING TOWARD AZ/NM. AS GUIDANCE SPREAD HAS DECREASED FOR THE LEADING SYSTEM, AN INCREASINGLY SIGNIFICANT FOCUS OF FCST UNCERTAINTY WILL BE ON PACIFIC INTO WRN U.S. FLOW AND ITS EFFECT ON THE WRN UPR LOW. FOR THE LEADING SYSTEM CROSSING THE EAST THU ONWARD, GFS/ECMWF RUNS AND THEIR ENSEMBLE MEANS APPEAR TO BE CONVERGING TOWARD A COMMON SOLN BTWN PRIOR DAYS' CLUSTERS THAT HAD GFS-BASED SOLNS TRACKING FARTHER SWD AND ECMWF SOLNS WELL NWD. IN NEW 00Z GUIDANCE THUS FAR THE UKMET IS THE ONE HOLDOUT FOR A MORE SRN TRACK WHILE THE CMC IS HANGING ONTO MORE GRTLKS SFC LOW EMPHASIS VS MOST OTHER GUIDANCE AFTER FRI. OVERALL TIMING HAS TRENDED A TAD FASTER OVER THE PAST 12-24 HRS WITH SOMEWHAT STRONGER/EWD DEVELOPMENT JUST OFF THE NRN MID ATLC COAST BY SAT. FOR THE UPR TROUGH/CLOSED LOW SETTLING INTO THE WEST THERE ARE STILL SOME DETAIL/TRACK DIFFS AROUND WHAT HAS BEEN A FAIRLY STABLE CONSENSUS SOLN INTO DAY 5 SAT. EARLY IN THE EXTENDED PERIOD MODELS/ENSEMBLES BEGIN TO DIVERGE REGARDING SHRTWV EVOLUTION WITHIN INITIALLY LOW AMPLITUDE FLOW ACROSS THE WRN 2/3 OF THE PACIFIC. AS A RESULT SPAGHETTI PLOTS SHOW FEATURES BECOMING 180 DEGS OUT OF PHASE OVER THE CNTRL PAC AND THEN TOWARD THE WEST COAST AS EARLY AS DAYS 4-5 FRI-SAT. BROADLY SPEAKING 12Z ECMWF/EC ENSEMBLES WANT TO BRING A SHRTWV TROUGH INTO THE NORTHWEST BY LATE SAT WITH GFS-BASED SOLNS SHOWING A RIDGE AT THE SAME TIME. THEN BY SUN THE GFS/GEFS FINALLY BRING A SHRTWV TOWARD THE WEST COAST WHILE THE EC/ECENS START TO BUILD IN A RIDGE. 12Z CMC ENSEMBLES HAVE VARIATIONS OF EACH POSSIBILITY. BY SUN-MON THE SPECIFICS OF THIS PACIFIC FLOW WILL AFFECT THE ULTIMATE PATH/TIMING OF THE CLOSED LOW FCST TO BE NEAR AZ/NM AS OF SAT, WITH THE 12Z ECMWF SCENARIO RESULTING IN FASTER EJECTION OF THE UPR LOW VS THE GFS/CMC AND THEIR MEANS. CONFIDENCE IS VERY LOW IN ANY PARTICULAR SOLN OVER THE PAC GIVEN THE INITIALLY LOW AMPLITUDE OF FLOW. HOWEVER MULTI-DAY CONTINUITY FOR THE WRN UPR LOW THUS FAR HAS SHOWN ECMWF RUNS TO BE QUITE UNSTABLE BTWN SLOWER AND PROGRESSIVE SOLNS WHILE THE GFS/GEFS AND CMC HAVE BEEN MORE CONSISTENT WITH SLOWER EJECTION. SO EVEN WITH THE MINIMAL CONFIDENCE IN DETAILS OF PAC FLOW REACHING THE WEST, THERE STILL SEEMS TO BE A BETTER SIGNAL TOWARD MAINTAINING PRIOR PREFERENCE ABOUT 2/3 TOWARD SLOWER EJECTION OF THE WRN UPR LOW AS PER GFS/GEFS RUNS. LOOKING AT THE NEW 00Z GFS/UKMET/CMC THERE ARE STILL SIGNIFICANT DIFFS WITH PAC TO WEST COAST FLOW. HOWEVER ALL STILL ARGUE AGAINST THE 12Z ECMWF/ECMWF MEAN DETAILS AND CONTINUE TO SUGGEST SLOWER EJECTION OF THE WRN UPR LOW. BASED ON THE ABOVE CONSIDERATIONS THE UPDATED FCST REFLECTED A CONSENSUS BLEND OF THE 18Z GFS/12Z ECMWF AND THEIR MEANS WITH MORE OPERATIONAL MODEL WEIGHTING, QUICKLY TRANSITIONING AROUND MID-PERIOD TOWARD ABOUT 2/3 TOTAL WEIGHTING OF THE 18Z GEFS/12Z-18Z GFS/12Z NAEFS VERSUS 1/3 ECMWF MEAN. THIS BLEND YIELDED REASONABLY GOOD CONTINUITY FOR A FCST THAT BECOMES INCREASINGLY UNCERTAIN DURING THE LATTER HALF OF THE PERIOD. ...SENSIBLE WEATHER HIGHLIGHTS/POTENTIAL HAZARDS... SYSTEM DEPARTING FROM THE PLAINS AND TRACKING THROUGH THE EAST THU-SAT WILL BE SUFFICIENTLY ENERGETIC TO PRODUCE AREAS OF HVY RAIN AND POTENTIALLY STRONG TO SEVERE CONVECTION, THE LATTER ACROSS SRN AREAS. CHECK SPC OUTLOOKS FOR LATEST INFO ON SEVERE THREATS. HIGHEST RNFL TOTALS SHOULD EXTEND FROM THE MID-LWR MS VLY INTO THE MID ATLC/CAROLINAS. MEANWHILE A BAND OF ENHANCED MSTR ASSOC WITH THE SHRTWV INITIALLY REACHING THE WEST COAST AND THEN FORMING A CLOSED LOW TRACKING TOWARD AZ/NM SHOULD SUPPORT SOME AREAS OF SIGNIFICANT RAIN/HIGH ELEV SNOW ACROSS THE NRN 2/3 TO 3/4 OF THE WEST. OVER THE PAST COUPLE DAYS GUIDANCE HAS STABILIZED IN SUGGESTING THAT THE COMBINATION OF THE UPR LOW AND A PERIOD OF EASTERLY LOW LVL UPSLOPE FLOW MAY PRODUCE A CONCENTRATED AREA OF HVY SNOW AND PSBLY LOWER ELEV RAIN FOCUSED PRIMARILY OVER COLORADO AROUND FRI NIGHT-SAT. IN THE SAT-MON TIME FRAME THERE MAY BE ANOTHER EPISODE OF HVY RAIN/CONVECTION EXTENDING FROM THE SRN PLAINS AT LEAST INTO LWR MS VLY. THIS PART OF THE FCST WILL BE VERY SENSITIVE TO TIMING OF THE UPSTREAM UPR LOW. AT LEAST ONE PERIOD OF LGT-MDT PCPN IS PSBL OVER THE NORTHWEST BUT WITH LOW CONFIDENCE GIVEN THE SPREAD FOR SHRTWV DETAILS ALOFT. THE ACTIVE WEATHER PATTERN WILL TEND TO KEEP MIN TEMPS ABOVE AVERAGE OVER MOST OF THE LOWER 48 DURING THE PERIOD. MAX TEMPS WILL VARY RELATIVE TO INDIVIDUAL SYSTEMS/FLOW PATTERN. THE NRN PLAINS AND SOUTHEAST SHOULD BE MOST CONSISTENTLY ABOVE NORMAL WITH THE NRN PLAINS SEEING THE HIGHEST MAX TEMP ANOMALIES OF PLUS 10-15F. THE SYSTEM DROPPING INTO THE WEST WILL SUPPORT ONE OR MORE COOL DAYS AND A LOCALIZED AREA OF MINUS 10-20F ANOMALIES FOR HIGHS ARE PSBL AROUND SAT WITH THE POTENTIAL CNTRL HIGH PLAINS UPSLOPE EVENT. BEHIND THIS SYSTEM EXPECT MODERATELY WARM AIR TO MOVE INTO THE WRN STATES. PORTIONS OF THE EAST COAST MAY SEE ONE OR MORE DAYS OF ONSHORE FLOW THAT WOULD KEEP HIGHS ON THE COOL SIDE. RAUSCH