EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1042 AM EST THU FEB 12 2015 VALID 12Z SUN FEB 15 2015 - 12Z THU FEB 19 2015 ...FRIGID ARCTIC BLASTS WITH STORMY WINTER WEATHER... ...GUIDANCE/UNCERTAINTY ASSESSMENT... LATEST GUIDANCE CONTINUE TO OFFER BELOW NORMAL FORECAST SPREAD INTO TUE/DAY 5. THIS LENDS PREFERENCE FOR A COMPOSITE BLEND OF COMPATABLE 06 UTC GFS AND 00 UTC ECMWF/CANADIAN MODELS AS A BASEMAP FOR WPC SURFACE FRONTS/PRESSURES AND 500 MB PROGS. THIS BLEND OF DETERMINISTIC MODEL INPUT INCORPORATES MORE SMALLER SCALE DETAIL THAN NORMAL AS CONSISTENT WITH PREDICTABILITY. FORECAST SPREAD INCREASES ENOUGH INTO DAYS 6/7 TO INCREASE UNCERTAINTY TO AT LEAST AVERAGE LEVELS SO BY THEN USED A MUCH MORE ENSEMBLE MEAN BASED FORECAST APPROACH WITH A 30/70 MIX OF THE 00 UTC ECMWF/ECMWF ENSEMBLE MEAN. THESE REPRESENT SOLUTIONS ON THE AMPLIFIED SIDE OF THE FULL ENVELOPE OF GUIDANCE. WATER VAPOR LOOPS SHOW QUITE AN AMPLIFYING UPSTREAM RIDGE PATTERN ALONG/OFF THE NOAM WEST COAST ALONG WITH POTENT/WELL SPACED ERN PACIFIC AND DATELINE LOWS FEEDING INTO THAT RIDGE. THE ECMWF IS A STARK OUTLIER AMPLIFIED SOLUTION THOUGH SO PREFER TO LIMIT INCLUSION GIVEN UNCERTAINTY AT LONGER TIME FRAMES. ...PATTERN OVERVIEW AND SENSIBLE WEATHER HIGHLIGHTS... A DANGEROUS AND HIGHLY AMPLIFIED NERN US/CANADIAN MARITIMES POLAR VORTEX SUN-MON WILL USHER IN DEEP COLD AIR ACROSS THE CENTRAL AND ERN US IN THE WAKE OF A DEEP NERN US COASTAL LOW AND TRAILING FRONT. THIS OFFERS AN AMPLE STORM THREAT FOR NEW ENGLAND IN PARTICULAR ALONG WITH A HUGE WIND/WAVE EVENT SPREAD WELL INTO THE WRN ATLANTIC. THIS COLD ONSET AIR ALSO SETS THE STAGE FOR A LARGE SCALE WINTER WEATHER THREAT EARLY-MID NEXT WEEK. MEANWHILE...MODELS AND ENSEMBLES HAVE TRENDED/COME INTO BETTER AGREEMENT TODAY IN SHOWING THE UPSTREAM WRN US RIDGE RETROGRESSION AND STRONG AMPLIFICATION OFFSHORE AND UP TOWARD ALASKA INTO EARLY NEXT WEEK IN RESPONSE TO UPSTREAM DEEP PACIFIC LOW DEVELOPMENTS. THIS SHOULD ALLOW UNSETTLING TROUGH DIGGING ALOFT WITH LOWERING HEIGHTS AND COOLING THROUGH THE WRN AND CENTRAL US INTO EARLY-MID NEXT WEEK ALONG WITH A POTENT COLD SURFACE HIGH INFUSIONS. THIS SUPPORTS AN EMERGING SW US AND S-CENTRAL GREAT BASIN/ROCKIES SNOW THREAT AIDED BY TERRAIN/UPSLOPE FETCH. WRN NOAM FLOW AMPLIFICATION ALSO LEADS TO AN INCREASINGLY FAVORABLE GULF OF MEXICO MOISTURE/WAA RETURN FLOW PATTERN MON-WED INTO/OVER AMBIENT AND SLOWLY RECEEDING LOWER LEVEL COLD AIR ACROSS THE EAST-CENTRAL US DOWNSTREAM ALONG/AHEAD OF A WAVY COLD FRONT. PENDING STREAM INTERACTION EFFECTS AND SWEATING THE IMPORTANT SMALLER SCALE DETAILS...THIS OFFERS SOME PROLONGED HEAVIER PERIODS OF SNOW AND DANGEROUS/TRANSITIONAL HEAVY ICING ACROSS A POTENTIALLY LARGE THREAT AREA IN VICINITY FROM THE S-CENTRAL PLAINS/MS VALLEY MON NEWD ACROSS THE E-CENTRAL US AND ERN/NERN US ON THE NRN PERIPHERY OF THE EXPANDING OVERALL PCPN SHIELD. WPC WEATHER/PCPN TYPE GRIDS OFFER A GRAPHICAL DEPICTION AND A MORE AMPLIFIED DAY 6/7 WPC SCENARIO LEAVES THE DOOR MORE OPEN FOR POSSIBLE TRAILING SURFACE FRONTAL WAVE DEVELOPMENTS THAT COULD ADD CONVERGENCE/PCPN THREAT. SCHICHTEL