EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1145 AM EDT TUE APR 11 2017 VALID 12Z FRI APR 14 2017 - 12Z TUE APR 18 2017 ...PATTERN OVERVIEW... THE GENERAL PATTERN DEPICTED BY GUIDANCE CONSISTS OF A PERSISTENT MEAN UPPER TROUGH OFF THE WEST COAST, WITH SHORTWAVES EJECTING EASTWARD WHILE POSSIBLY INTERACTING OR PHASING WITH SHORTWAVE TROUGHS OVER CANADA. THE NET RESULT IS PERIODICALLY WET AND GENERALLY COOL CONDITIONS FOR CALIFORNIA AND THE NORTHWEST, WITH OCCASIONAL COLD FRONTAL PASSAGES FROM THE PLAINS EASTWARD THAT WEAKEN SOUTHWARD. PERSISTENT SOUTHERLY TO SOUTHWESTERLY FLOW ALONG WITH A LINGERING SURFACE FRONT SHOULD RESULT IN MULTIPLE ROUNDS OF CONVECTION ACROSS THE PLAINS AND PORTIONS OF THE MS VALLEY. AN UPPER RIDGE WILL PERSIST ACROSS THE SOUTHEASTERN U.S. THROUGH MUCH OF THE MEDIUM RANGE, BEFORE GRADUALLY WEAKENING BY EARLY NEXT WEEK. ...MODEL CHOICES AND CONFIDENCE... MODELS SHOW GENERALLY GOOD AGREEMENT ON THE LARGE-SCALE PATTERN ACROSS THE CONUS INITIALLY (FRI AND SAT), WITH THE EXCEPTION OF THE 00Z CANADIAN, WHICH APPEARS TO LOWER HEIGHTS ACROSS THE WESTERN U.S. TOO QUICKLY ON DAY 3 /FRI AND INTO THE PLAINS ON DAY 4 /SAT. AT THE SURFACE, THERE ARE SOME DIFFERENCES WITH RESPECT TO THE LOW TRACK ACROSS THE CANADIAN PLAINS ON DAYS 3-4, WITH THE 06Z GFS/06Z GEFS FARTHER NORTH AND SLOWER, AND THE 00Z ECMWF/00Z EC ENS MEAN FARTHER SOUTH AND QUICKER. A COMPROMISE APPROACH WAS PREFERRED AT THIS TIME. THE WPC FORECAST ON DAYS 3-4 WAS BASED ON A BLEND OF THE 06Z GFS/00Z ECMWF, WITH LESSER COMPONENTS OF THE 06Z GEFS/00Z ECMWF ENS MEANS. CONFIDENCE WAS AVERAGE TO SLIGHTLY ABOVE AVERAGE FOR DAYS 3-4. SPREAD INCREASES SIGNIFICANTLY FROM SUN ONWARD, WITH DETERMINISTIC SOLUTIONS VARYING WIDELY WITH HOW THEY HANDLE SHORTWAVE ENERGY CROSSING UPPER MIDWEST ON DAY 5 /SUN, AS WELL AS WITH RESPECT TO THE EVOLUTION OF ANOTHER TROUGH/UPPER LOW OFF THE PAC NW COAST. BOTH THE GFS AND THE ECMWF HAVE SHOWN RUN-TO-RUN DISCONTINUITIES WITH RESPECT TO THESE FEATURES. ADDITIONALLY, THE 06Z GFS/06Z GEFS MEAN HAVE TRENDED TOWARD KEEPING THE UPPER RIDGE ACROSS THE SOUTHEAST IN PLACE FOR LONGER ON DAYS 6-7 (MON-TUE). THERE IS LITTLE SUPPORT FOR THIS AMONG OTHER ENSEMBLE GUIDANCE, HOWEVER, WITH THE 00Z ECMWF/00Z EC ENS MEAN CONTINUING TO BREAK THE RIDGE DOWN QUICKER. GIVEN THE DESCRIBED SPREAD AND UNCERTAINTY INCREASING QUICKLY TOWARD THE END OF THE MEDIUM RANGE, THE WPC FORECAST TRENDED QUICKLY TOWARD AND ENSEMBLE-BASED APPROACH DURING DAYS 5-7. DAY 5 WAS THE TRANSITION DAY, WITH THE FORECAST BASED HALF ON THE 06Z GEFS MEAN/00Z EC ENS MEAN AND HALF ON THE 06Z GFS/00Z ECMWF. BY DAYS 6-7 THE FORECAST WAS BASED EXCLUSIVELY ON A BLEND OF THE 00Z EC ENS MEAN/06Z GEFS MEAN (WITH SLIGHTLY MORE WEIGHT PLACED ON THE 00Z EC ENS MEAN DUE TO SOMEWHAT BETTER RUN-TO-RUN CONSISTENCY NOTED RELATIVE TO THE GEFS). FORECAST CONFIDENCE IS BELOW AVERAGE FOR DAYS 5-7. ...SENSIBLE WEATHER IMPACTS... HEAVY RAINFALL IS POSSIBLE FROM THE CENTRAL AND SOUTHERN PLAINS TO PORTIONS OF THE GREAT LAKES, AS COLD FRONTS PROGRESS THROUGH THE REGION, INTERSECT MOISTURE/INSTABILITY RETURNING FROM THE GULF, AND MOVE TOWARD SOUTHERN EDGES OF THE POLAR JET, WHERE WEAKER WIND FIELDS MAY SUPPORT LONGER PERIODS OF HIGH RAIN RATES. AS MENTIONED ABOVE, THE WEST COAST WILL SEE PERIODS OF WET AND COOL CONDITIONS. MOISTURE FLUX INTO PORTIONS OF THE WEST COAST SHOULD INCREASE BY DAYS 6-7 AS THE SECOND UPPER TROUGH AMPLIFIES OFFSHORE AND A PLUME OF MOISTURE WITH A CONNECTION TO THE TROPICAL PACIFIC SETS UP. TEMPERATURES WILL GENERALLY BE ABOVE AVERAGE ACROSS THE EASTERN TWO-THIRDS OF THE NATION THROUGH MUCH OF THE MEDIUM RANGE AS RIDGING PERSISTS ALOFT. A COOLER AIR MASS MAY OVERSPREAD THE NORTHERN TIER, FROM THE NORTHERN PLAINS TO NORTHERN NEW ENGLAND BY SUN-MON AS A SURFACE COLD FRONT PASSES. RYAN