EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1159 AM EDT TUE MAR 22 2016 VALID 12Z FRI MAR 25 2016 - 12Z TUE MAR 29 2016 ...OVERVIEW AND GUIDANCE PREFERENCES... THE LARGE SCALE PATTERN IS FCST TO TRANSITION FROM A BROAD NORTH AMERICAN MEAN TROUGH TO A DEEP WRN TROUGH/EMBEDDED UPR LOW IN RESPONSE TO STRONG RIDGING THAT BUILDS OVER THE NERN PACIFIC. THIS EVOLUTION SHOULD LEAD TO FLATTER MEAN FLOW OVER THE ERN STATES BY THE END OF THE PERIOD BUT WITH SOME UNCERTAINTY OVER HOW QUICKLY THIS FLATTENING TREND WILL OCCUR. FROM THE START OF THE FCST THERE ARE PERSISTENT TIMING DIFFS FOR SIGNIFICANT SYSTEMS... MOST FALLING INTO THE HISTORICALLY TYPICAL FASTER GFS/GEFS AND SLOWER ECMWF BIASES. ACCOUNTING FOR THESE BIASES... CONTINUITY... AND TELECONNECTION FAVORED FLOW BY THE END OF THE PERIOD... LEADS TO AN UPDATED FCST THAT LEANS AT LEAST 2/3 TOWARD THE 00Z ECMWF MEAN VS THE 06Z GEFS MEAN EARLY IN THE PERIOD AND ADHERES CLOSE TO THE 00Z ECMWF MEAN FROM DAY 5 SUN ONWARD. THE FAVORED BLEND EARLY IN THE PERIOD ACCOUNTS FOR INTERMEDIATE SHORT RANGE PREFS REGARDING SYSTEMS OVER/NEAR THE NRN PLAINS AND NEW ENGLAND AS OF EARLY FRI. FROM SUN ONWARD SOLNS INCREASINGLY DIVERGE OVER THE ERN 2/3 OF THE LOWER 48. THESE DIFFS ARISE FROM A COMBINATION OF GFS-BASED GUIDANCE BEING SOMEWHAT FASTER TO AMPLIFY WRN U.S. TROUGHING AND ALSO MORE AMPLIFIED WITH SRN CANADA/NRN TIER CONUS FLOW. BY THE END OF THE PERIOD MODELS/ENSEMBLE MEANS SHOW WELL ABOVE AVERAGE AGREEMENT WITH THE OVERALL ERN PAC/WRN NOAM PATTERN BUT ARE QUITE DIFFERENT OVER THE EAST. THE PAST 1-2 DAYS OF ENSEMBLE MEAN RUNS SHOW A PRONOUNCED TREND TOWARD THE DEEP WRN UPR LOW SEEN IN MOST OPERATIONAL MODEL RUNS. BY THE D+8 TIME FRAME GUIDANCE IS SIMILAR WITH A STRONG POSITIVE HGT ANOMALY JUST OFF THE ALASKA PANHANDLE. TELECONNECTIONS RELATIVE TO THIS FEATURE SHOW A POSITIVELY TILTED MEAN TROUGH WITH LOW AMPLITUDE FLOW DOWNSTREAM. THIS RELATIONSHIP AT LEAST OFFERS SUPPORT FOR SOMEWHAT FASTER ERN U.S. TIMING THAN THE OPERATIONAL 00Z ECMWF BY LATE IN THE PERIOD. THE ECMWF MEAN PROVIDES AN INTERMEDIATE SOLN REASONABLY CLOSE TO WPC CONTINUITY WHILE AWAITING IMPROVED GUIDANCE CLUSTERING. ...SENSIBLE WEATHER HIGHLIGHTS/THREATS... OVER THE WEST... EARLY IN THE PERIOD EXPECT AN AREA OF UNSETTLED WEATHER FROM THE NORTHWEST INTO THE NRN-CNTRL ROCKIES BEHIND A COUPLE FRONTS DROPPING SWD/SEWD OVER THE WEST/PLAINS. BEST POTENTIAL FOR MEANINGFUL SNOW SHOULD BE OVER PORTIONS OF WY/CO. AS THE ERN PAC/WRN U.S. PATTERN AMPLIFIES MSTR SHOULD MOVE INTO THE NORTHWEST DURING THE WEEKEND AND SPREAD SEWD WITH TIME. THERE WILL LIKELY BE SOME AREAS OF LOCALLY ENHANCED PCPN WITH SPECIFICS DEPENDING ON EXACT TRACK OF THE FCST UPR LOW... AND OVER SOME AREAS DISTRIBUTED DIFFERENTLY FROM CLIMATOLOGY. EARLY IN THE PERIOD TEMPS SHOULD BE FAIRLY CHILLY OVER THE NRN-CNTRL ROCKIES BUT NEAR TO ABOVE NORMAL WWD. THEN EXPECT DAYTIME HIGHS IN PARTICULAR TO TREND COOLER IN THE VICINITY OF THE DEVELOPING UPR TROUGH/LOW. FROM THE PLAINS EWD... LEADING SYSTEM INITIALLY NEAR THE EAST COAST WILL PROVIDE ONE AREA OF ENHANCED PCPN LATE THIS WEEK WITH SOME WINTRY WEATHER PSBL IN EXTREME NRN AREAS. TRAILING NRN TIER LOW/FRONT SHOULD GENERATE LIGHTER PCPN OF VARIOUS TYPES FROM THE NRN PLAINS EWD/SEWD. UNCERTAINTY WITH DETAILS AND TIMING OF EVOLUTION SFC/ALOFT SUN-TUE REDUCES CONFIDENCE IN PINPOINTING BEST PCPN THREATS AT THAT TIME. BASED ON THE FULL RANGE OF GUIDANCE THE HIGHEST PROBABILITY OF SIGNIFICANT RNFL SHOULD BE OVER THE CNTRL-ERN GULF COAST AND SOUTHEAST AS THERE IS REASONABLE AGREEMENT ON A PERIOD OF LOW LVL GULF INFLOW INTERACTING WITH THE TAIL END OF THE FRONT ASSOC WITH THE EAST COAST SYSTEM FROM FRI. HIGH UNCERTAINTY WITH STRENGTH/TIMING/TRACK OF SFC LOW PRES FARTHER NWD YIELDS LOWER CONFIDENCE FOR PCPN AMTS OVER CNTRL-NRN AREAS. SLOW/DEEP ECMWF SCENARIO WITH THE SFC SYSTEM WOULD GENERATE HVY AMTS WHILE THE FAST GFS SCENARIO WOULD PRODUCE MUCH LIGHTER TOTALS. THE NWRN PART OF THE PCPN SHIELD MAY CONTAIN SOME SNOW. CONSULT LATEST SPC OUTLOOKS FOR INFO REGARDING ANY POTENTIAL FOR STRONG/SEVERE CONVECTION. RAUSCH