EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 151 AM EST WED JAN 18 2017 VALID 12Z SAT JAN 21 2017 - 12Z WED JAN 25 2017 ...OVERVIEW... LITTLE HAS CHANGED WITH THE EXPECTATION THAT STRONGLY POSITIVE HGT ANOMALIES/RIDGING ALOFT OVER ERN CANADA WILL ENCOURAGE VIGOROUS ERN PAC SYSTEMS TO PROGRESS INTO/ACROSS THE LOWER 48. BEHIND A COMPACT SYSTEM LIFTING NWD OVER THE NRN PLAINS DURING THE WEEKEND... THE EXTENDED FCST WILL FOCUS ON TWO DOMINANT FEATURES. THE FIRST SHOULD EMERGE FROM THE WEST EARLY IN THE PERIOD LEADING TO A PSBL SEVERE WEATHER EVENT ALONG THE GULF COAST REGION DURING THE WEEKEND AND THEN BRING STRONG ONSHORE FLOW AND AREAS OF HVY RNFL TO THE EAST COAST STATES. THE SECOND WILL LIKELY AFFECT THE WEST COAST WITH HVY RNFL/STRONG WINDS BY SUN AND THEN BRING LOW PRESSURE INTO THE PLAINS/MS VLY BY TUE-WED WITH A BAND OF SNOW TO THE NW OF THE LOW TRACK. THE WEST SHOULD SEE A PERIOD OF DRIER WEATHER BEHIND THIS SYSTEM. ...GUIDANCE EVALUATION AND WEATHER HIGHLIGHTS/THREATS... EXISTING SPREAD IN THE GUIDANCE THROUGH THE 12Z/18Z CYCLES FAVORED ONLY MODEST ADJUSTMENTS IN THE FCST. INDIVIDUAL SOLNS COMPARED BETTER OR WORSE TO THE CONSENSUS OR AVG DEPENDING ON THE AREA... REQUIRING VARIOUS WEIGHTINGS OF RECENT GFS/ECMWF RUNS AND THEIR ENSEMBLE MEANS WITH A LITTLE 12Z CMC INCLUDED EARLY AS WELL. GUIDANCE HAS REMAINED FAIRLY WELL CLUSTERED AS THE EARLY SAT FOUR CORNERS SYSTEM REACHES THE PLAINS BY EARLY SUN. THEN GUIDANCE DIVERGES OVER THE EAST WITH LITTLE REDUCTION IN SPREAD OVER 24 HRS AGO. GFS/GEFS MEAN RUNS HAVE BEEN WAFFLING BTWN THE AVG OF ECMWF/ECMWF MEAN RUNS AND A FARTHER SWD TRACK THAT SOME CMC RUNS HAVE BEEN SHOWING AS WELL. INTERESTINGLY THE CORRESPONDING CMC MEAN IS MORE SIMILAR TO RECENT ECMWF RUNS. BEYOND THE GENERAL LATITUDE ISSUE ARE THE DETAILS OF HOW ENERGY/SFC LOW PRES MAY SPLIT. THE SMALL SCALE OF THIS ASPECT OF EVOLUTION SUGGESTS LOW PREDICTABILITY 5-7 DAYS OUT IN TIME BUT SPECIFICS WILL BE VERY IMPORTANT FOR SENSIBLE WEATHER IMPACTS OVER THE NRN MID ATLC/NEW ENGLAND. CURRENTLY THE BALANCE OF GUIDANCE APPEARS TO LEAN SLIGHTLY IN THE DIRECTION OF THE PAST COUPLE OF ECMWF RUNS THROUGH THE 12Z CYCLE WITH A SOMEWHAT MORE OFFSHORE WAVE AS IN THE ECMWF MEANS ALSO A POSSIBILITY. AS THIS SYSTEM CROSSES THE CNTRL-ERN STATES IT WILL BE ACCOMPANIED BY A BROAD SHIELD OF PCPN WITH HEAVIEST RNFL EXPECTED FROM THE ERN HALF OF THE GULF COAST NEWD ALONG THE APLCHNS/EAST COAST. CONSULT SPC OUTLOOKS FOR LATEST INFO REGARDING POTENTIAL SEVERE WEATHER ALONG THE GULF COAST DURING THE WEEKEND. RAIN OVER THE EAST COAST STATES WILL BE ENHANCED BY STRONG ATLC INFLOW. THUS FAR THERE HAS BEEN ENOUGH UNCERTAINTY OVER EXACT SYSTEM EVOLUTION TO REDUCE CONFIDENCE IN PINPOINTING LOCATION/TIMING/STRENGTH OF HIGHEST WIND SPEEDS. WITH LESSER TOTALS THERE MAY ALSO BE A RELATIVE MAXIMUM FROM THE S-CNTRL PLAINS INTO THE LWR OH VLY. PCPN TYPE STILL LOOKS TO BE NEARLY ALL RAIN WITH ANY WINTRY WEATHER CONFINED TO NEW ENGLAND... AND PSBLY LOCALLY WITH THE UPR LOW. FOR THE SECOND SYSTEM GUIDANCE IS STILL IN THE PROCESS OF TRYING TO RESOLVE SFC DETAILS OFF THE WEST COAST BY SAT NIGHT-SUN... FAVORING AN INTERMEDIATE SOLN WHILE AWAITING BETTER CLUSTERING. 12Z/18Z GFS RUNS MAY BE SOMEWHAT QUICK TO BRING LEADING HGT FALLS INTO THE NORTHWEST BUT OTHERWISE THERE IS REASONABLE AGREEMENT AMONG 12Z/18Z SOLNS FOR THE OVERALL TROUGH ALOFT UNTIL THE 12Z ECMWF/ECMWF MEAN TREND FASTER AFTER ABOUT LATE MON. THIS REFLECTS A FASTER TREND FROM THE ROCKIES THROUGH MS VLY AND IS A SWITCH FROM YDAY WHEN GFS RUNS WERE ON THE FASTER SIDE. WITH THE TIMING OF GFS/ECMWF CLUSTERS THE REVERSE OF HISTORICAL BIASES A COMPROMISE APPEARS BEST. THE NEW 00Z CYCLE ADDS NO CLARITY AS THE GFS TRENDED FASTER BUT THE GEFS MEAN/CMC REMAIN A LITTLE FARTHER BACK. THE MOST PRONOUNCED EFFECTS OF THIS SYSTEM ALONG THE WEST COAST SHOULD BE IN THE SAT NIGHT-MON TIME FRAME. HEAVIEST PCPN SHOULD FIRST ARRIVE AT THE CNTRL WEST COAST AND THEN CONTINUE SWD WITH TIME. FAVORED COASTAL LOCATIONS AND SIERRA NEVADA RANGE MAY SEE UP TO SEVERAL INCHES LIQUID. STRONGEST WINDS WILL LIKELY AFFECT THE CNTRL-NRN WEST COAST AROUND SAT NIGHT-SUN. EXPECT AN AREA OF MOSTLY SNOW TO CROSS MUCH OF THE WEST SUN INTO EARLY NEXT WEEK WITH RAIN OVER THE EXTREME SOUTH. AS THIS SYSTEM TRACKS NEWD ACROSS THE PLAINS THERE WILL LIKELY BE A BAND OF SNOW THE NW OF THE LOW TRACK. HIGHS WILL BE BELOW NORMAL OVER MUCH OF THE WEST DURING THE PERIOD WITH SOME AREAS MORE THAN 10F COLDER THAN AVG ON ONE OR MORE DAYS. MOST OF THE REMAINDER OF THE LOWER 48 SHOULD BE ABOVE TO MUCH ABOVE NORMAL ASIDE FROM THE FL PENINSULA AFTER A WARM WEEKEND AND THE PLAINS WHICH WILL BEGIN TO SEE SOME OF THE COOLER AIR MOVE IN FROM THE WEST BY TUE-WED. THE FCST IS CONSISTENT WITH THE UPR MS VLY REGION SEEING MULTIPLE DAYS OF MORNING LOWS 30-40F ABOVE NORMAL WITH SOME RECORD WARM MINS. RAUSCH