EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 255 AM EDT FRI MAY 12 2017 VALID 12Z MON MAY 15 2017 - 12Z FRI MAY 19 2017 ...PATTERN OVERVIEW... THE PATTERN WILL REMAIN QUITE AMPLIFIED OVER THE LOWER 48 NEXT WEEK. A DYNAMIC UPPER LOW DROPPING FROM BRITISH COLUMBIA INTO THE PACIFIC NORTHWEST AND CENTRAL GREAT BASIN EARLY TO MID NEXT WEEK WILL HELP MAINTAIN MEAN TROUGHING OVER THE WESTERN HALF OF THE NATION. EAST OF THE ROCKIES...AN ANOMALOUS CLOSED LOW IMPACTING THE NORTHEAST ON MONDAY SHOULD LIFT TOWARDS NEWFOUNDLAND WHILE UPPER RIDGING INITIALLY ALIGNED OVER THE PLAINS/MISSISSIPPI VALLEY SHIFTS DOWNSTREAM INTO THE EASTERN U.S.. ...GUIDANCE EVALUATION AND PREFERENCES... MODELS HAVE BEEN FAIRLY AGREEABLE AND CONSISTENT WITH THE LARGE SCALE FLOW PATTERN OVER THE CONUS NEXT WEEK...BUT THERE IS STILL SOME SPREAD THAT DEVELOPS WITH THE SMALLER SCALE DETAILS OF THE FORECAST. SOME OF THE GREATEST UNCERTAINTY EARLY IN THE PERIOD IS WITH THE TIMING AND STRENGTH OF THE UPPER LOW LIFTING OUT OF THE NORTHEAST ON MONDAY. GFS AND GEFS MEMBERS ARE CURRENTLY ON THE SLOWER/WEAKER SIDE OF MODEL SPREAD WITH THE ASSOCIATED SURFACE LOW PULLING AWAY FROM THE NEW ENGLAND COAST...WHILE THE ECWMF AND ECWMF ENSEMBLE MEMBERS REPRESENT SOME OF THE FASTER/STRONGER SOLUTIONS WITH THIS FEATURE. TOWARDS MID TO LATE NEXT WEEK...THE FOCUS FOR FORECAST UNCERTAINTY SHIFTS INTO THE GREAT PLAINS WHERE MODELS HAVE YET TO LOCK IN ON THE TIMING AND AMPLITUDE OF SHORTWAVE ENERGY EJECTING EASTWARD OUT OF THE WESTERN U.S. TROUGH ON WEDNESDAY. ALSO...THERE IS STILL SOME SPREAD THAT DEVELOPS WITH THE TIMING AND POSITION OF THE DYNAMIC UPPER LOW DROPPING OUT OF BRITISH COLUMBIA...ESPECIALLY AS IT POTENTIALLY LIFTS FROM THE CENTRAL GREAT BASIN INTO THE CENTRAL ROCKIES THURSDAY INTO FRIDAY. DESPITE THE ECWMF/ECMWF ENSEMBLE MEAN CURRENTLY BEING ON THE FASTER SIDE OF MODEL SPREAD WITH LIFTING THIS FEATURE OUT OF THE CENTRAL GREAT BASIN...THERE ARE STILL SEVERAL ECWMF ENSEMBLE MEMBERS THAT SUGGESTS THE POTENTIAL FOR A SOLUTION SIMILAR TO THE RELATIVELY SLOWER GFS/GEFS MEAN. THE WPC DAY 3-7 FORECAST WAS LARGELY BASED ON A BLEND BETWEEN THE GFS/GEFS MEAN AND ECWMF/ECMWF ENSEMBLE MEAN IN ORDER TO SMOOTH OUT THE SMALLER SCALE DETAIL DIFFERENCES...WITH MORE WEIGHT GIVEN TOWARDS THE MEANS BY THE DAY 6/7 TO ACCOUNT FOR SOME OF THE UNCERTAINTY WITH UPPER LOW LIFTING OUT OF THE CENTRAL GREAT BASIN LATE NEXT WEEK. ...WEATHER HIGHLIGHTS... HEAVY RAINS AND HIGH WINDS IMPACTING THE NORTHEAST ON MONDAY SHOULD GRADUALLY DIMINISH AS AN UPPER LOW AND ASSOCIATED SURFACE CYCLONE LIFT AWAY FROM THE REGION...AND A GENERAL WARMING TREND IS EXPECTED OVER THE EASTERN U.S. NEXT WEEK WHILE UPPER RIDGING SHIFTS EASTWARD OUT OF THE PLAINS/MISSISSIPPI VALLEY. A MEAN UPPER TROUGH SHOULD KEEP TEMPERATURES BELOW NORMAL OVER MUCH OF THE WESTERN HALF OF THE NATION NEXT WEEK...WITH ESPECIALLY COOL AND UNSETTLED CONDITIONS (INCLUDING SNOW OVER THE HIGH ELEVATIONS) EXPECTED WITH THE DYNAMIC UPPER LOW DROPPING FROM THE PACIFIC NORTHWEST TO THE CENTRAL GREAT BASIN. SHOWERS AND STORMS ARE ANTICIPATED FROM NE/IA NORTHWARD INTO THE UPPER MIDWEST/UPPER MS VALLEY WITH LOCALLY HEAVY RAINS, WHICH COULD BE EXCESSIVE, FROM MON THROUGH WED. A SURFACE DRYLINE SHOULD TRIGGER ISOLATED TO SCATTERED CONVECTION TUES AND WED ACROSS THE PLAINS WITH THE POTENTIAL FOR A HEAVIER AXIS OF QPF FROM OK/NORTH AND CENTRAL TX INTO AR AND THE SRN OH RIVER ON WED AND THURS WITH INCREASING EXCESSIVE RAIN THREAT. GERHARDT