EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 300 AM EDT MON OCT 16 2017 VALID 12Z THU OCT 19 2017 - 12Z MON OCT 23 2017 ...A COOLING AND STORMY WEATHER PATTERN TO DEVELOP OVER THE WRN TO CENTRAL... ...PATTERN AND GUIDANCE/UNCERTAINTY ASSESSMENT... IT REMAINS THE CASE THAT MODELS AND ENSEMBLES SHOW A PATTERN TRANSITION TO ONE WITH AN AMPLIFIED TROUGH MOVING ONSHORE OF THE WEST COAST LATER THIS WEEK THAT PROGRESSES INLAND TO THE CENTRAL US NEXT WEEKEND. THE NRN PORTION OF THIS TROUGH IS EXPECTED TO WORK OVER THE E-CENTRAL U.S. INTO ABOUT A WEEK...BUT VARIED GUIDANCE SOLUTIONS SHOW POTENTIAL FOR FLOW SEPARATION TRENDING TO A CLOSED TROUGH/LOW SOMEWHERE NEAR THE SRN PLAINS BY DAY 7. DOWNSTREAM FROM THESE FEATURES...A WARMING HIGH AMPLITUDE ERN US RIDGE BUILDS THIS WEEK. THERE IS GENERALLY GOOD AGREEMENT WITH ABOVE NORMAL CONFIDENCE AND CONTINUITY FOR THE OVERALL FLOW PATTERN THROUGH MUCH OF THE NEXT WEEK. HOWEVER...FORECAST SPREAD AND UNCERTAINTY DOES INCREASE WITH THE EXTENT OF FLOW SEPARATION AND POTENTIAL CLOSED LOW DEVELOPMENT IN VICINITY OF THE SRN PLAINS IN ABOUT A WEEK THAT WOULD PRESENT AN EMERGING LOCAL HEAVY RAINFALL AND RUNOFF RISK. WPC SURFACE FRONTS/PRESSURES AND 500 MB PROGS...QPF...AND SENSIBLE WEATHER GRIDS WERE PRIMARILY DERIVED FROM A COMPOSITE BLEND OF WPC CONTINUITY AND REASONABLY WELL CLUSTERED AND COMPATABLE GUIDANCE FROM THE 18/00 UTC GFS/GEFS MEAN...THE 12 UTC NAEFS MEAN...AND THE 12 UTC ECMWF/ECMWF ENSEMBLE MEAN. THE DETERMINISTIC CANADIAN WAS AGAIN NOT USED IN THE WPC FORECAST BLEND AS A FASTER OUTLIER THAT IS COUNTERED BY SLOWER CANADIAN ENSEMBLES THAT ARE NOW MORE IN LINE WITH THE WPC COMPOSITE BLEND THAN YESTERDAYS RUNS. ...SENSIBLE WEATHER HIGHLIGHTS/THREATS... EXPECT A RATHER STORMY/WET PERIOD FOR THE NWRN US/CA WITH LESSER AMOUNTS OF PRECIP SPREADING INLAND ACROSS THE WEST...SUPPORTED BY SYNOPTIC SCALE ASCENT IN THE VICINITY OF THE UPPER JET AUGMENTED BY TOPOGRAPHIC LIFT. A SERIES OF EMBEDDED SHORTWAVES TAP AVAILABLE PACIFIC MOISTURE TO PRODUCE WIDESPREAD PRECIPITATION INCLUDING SOME HEAVY TERRAIN ENHANCED PRECIPITATION IN THE PACIFIC NORTHWEST...ESPECIALLY THE RANGES OF WA STATE. POST-FRONTAL COLD SURGES DOWN THROUGH THE WRN STATES WILL LEAD TO LIKELY MOUNTAIN SNOWS WITH LOWERING SNOW LEVELS. THE UPPER TROUGH MOVING INTO THE PLAINS NEXT WEEKEND INTO EARLY NEXT WEEK DRIVES A COLD FRONT ACROSS THE REGION. EAST OF THE FRONT...MOISTURE RETURN GRADUALLY INCREASES. THE INCREASE IN MOISTURE AND STRENGTH OF THE LOW LEVEL JET MAY FUEL RAINFALL/CONVECTIVE POTENTIAL EXPANSION LATER SAT INTO SUN OVER THE CENTRAL TO SRN PLAINS AND MID MS VALLEY. A BIGGER THREAT FOR HEAVIER RAINFALL MAY BE INTO NEXT MON OVER THE SRN PLAINS/LOWER MS VALLEY. MOST OF THE ERN THIRD OF THE COUNTRY WILL BE DRIER THAN NORMAL UNDER THE INFLUENCE OF HIGH PRESSURE. HOWEVER...PERSISTING MOISTURE AND COASTAL CONVERGENCE PRODUCES THE LIKELIHOOD OF SHOWERS ALONG THE EAST COAST OF FLORIDA...WITH AN INVERTED TROUGH PRODUCING SHOWERS ALONG THE TEXAS/LA COASTS. THE UPPER RIDGE LEADS TO WARMER THAN NORMAL CONDITIONS ACROSS MOST OF THE ERN US...INCLUDING HIGH TEMPERATURES 10-15 DEGREES ABOVE NORMAL IN THE UPPER MS VALLEY...GREAT LAKES...AND OH VALLEY LATE THIS WEEK INTO THE WEEKEND...WHEN THE ANOMALOUS WARMTH SPREADS INTO THE APPALACHIANS AND NORTHEAST. SCHICHTEL