EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 257 AM EDT MON MAR 21 2016 VALID 12Z THU MAR 24 2016 - 12Z MON MAR 28 2016 ...GUIDANCE/UNCERTAINTY ASSESSMENT AND FLOW PATTERN HIGHLIGHTS/THREATS... THE WPC MEDIUM RANGE PRODUCT SUITE WAS DERIVED FROM A BLEND OF REASONABLY COMPATABLE 12 UTC GEFS/NAEFS AND ECMWF ENSEMBLE MEANS...LEANING MORE ON THE LATTER FOR BEST WPC CONTINUITY. PREFER AN ENSEMBLE MEAN FORECAST APPROACH TO AVOID DETERMINISTIC MODEL RUN TO RUN VARIANCE WITH STREAM EMBEDDED SYSTEM AMPLITUDE/TIMING DIFFERENCES. THE BLEND SMOOTHES SMALLER SCALE DIFFERENCES THAT OFFER BELOW NORMAL FORECAST PREDICTABILITY. LEAD SPRINGTIME WARMING AND POST-SYSTEM COOLING IS EXPECTED WITH MID-UPPER LEVEL TROUGH AND ASSOCIATED SURFACE LOW/SYSTEM PROGRESSION IN SEPARATED MID-LATITUIDE STREAM FLOW THROUGH THE S-CENTRAL/E-CENTRAL US THU AND ERN US FRI. THIS IS IN A PATTERN WITH UNCERTAIN MULTI-STREAM INTERACTIONS THAT EFFECT SYSTEM AMPLITUDES/TIMINGS AND NRN STREAM BASED LOWER ATMOSPHERIC COOLING SURGES THAT SPILL SEWD FROM CANADA. GUIDANCE CONTINUES TO TREND TOWARD PROGRESSION IN THIS FLOW AND THAT REMAINS REASONABLE CONSIDERING REDUCED WAVELENGTH SPACING BETWEEN AN EXTENDED SERIES OF SYSTEMS. THE MORE RECENT 00 UTC GFS/ECMWF CONTINUE TO GENERALLY REFLECT THIS OVERALL FLOW SCENARIO. A MAIN LEAD LOW AND SURFACE FRONTAL SYSTEM WILL FOCUS A WIDESPREAD EAST-CENTRAL TO ERN US PCPN SWATH FUELED BY DEEPENING GULF OF MEXICO THEN ATLANTIC MOISTURE RETURN. IN THIS PATTERN EXPECT A THREAT OF HEAVY LATE SEASON OVERRUNNING SNOW WITH SOME ICE FOR PARTS OF THE UPPER MIDWEST/GREAT LAKES AND NORTHEAST ON THE COLDER NRN PERIPHERY OF THE OVERALL PCPN SHIELD. CLIPPER LOW PASSAGE INTO/ACROSS THE US NRN TIER FRI-SUN MAY LAY DOWN A COOLED AIRMASS AND SUPPORT SOME MODEST SNOWS AGAIN ACROSS THIS AREA. MEANWHILE...IMPULSE ENERGIES ALOFT DIGGING SEWD BACK INTO THE WRN US FRI/SAT SUPPORTS SNOW AS ENHANCED BY TERRAIN OVER THE N-CENTRAL GREAT BASIN THEN ROCKIES. EJECTING ENERGIES DOWNSTREAM SUPPORT MOISTURE/PCPN RETURN OVER THE S-CENTRAL PLAINS AND WITH LEAD INFLOW OVER AN OLD/STALLED FRONT INTO THE GULF COAST AND SERN US STATES ENHANCING A HEAVY RAIN THREAT OVER THE WEEKEND THEN UP THE EAST COAST INTO EARLY NEXT WEEK. SCHICHTEL