EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 419 AM EDT SAT MAY 20 2017 VALID 12Z TUE MAY 23 2017 - 12Z SAT MAY 27 2017 ...PATTERN OVERVIEW AND MODEL PREFERENCES/UNCERTAINTY... AN AMPLIFIED FLOW PATTERN WILL DOMINATE ACROSS THE CONUS THROUGH MOST OF THE MEDIUM RANGE PERIOD, WITH UPPER LEVEL RIDGING ACROSS THE WESTERN U.S. AND TROUGHING OVER THE EASTERN PART OF THE NATION. SIMILAR TO YESTERDAY, THE MAIN UNCERTAINTY EXISTS WITH PACIFIC ENERGY BREAKING DOWN THE NORTHERN EXTENT OF THE WESTERN U.S. RIDGE. MODELS HAVE TRENDED TOWARD A MORE AMPLIFIED WAVE MOVING INTO THE RIDGE FOR THE MIDDLE OF THE WEEK. FARTHER EAST, MODELS SHOW RELATIVELY GOOD LARGE SCALE AGREEMENT WITH RESPECT TO AN ANOMALOUS TROUGH/UPPER LOW DEEPENING ACROSS THE EASTERN HALF OF THE CONUS. INDIVIDUAL SHORTWAVE PERTURBATIONS WITHIN THE FLOW AROUND THE MEAN TROUGH ARE MORE DIFFICULT TO ASCERTAIN. THE WPC FORECAST WAS INITIALLY BASED ON A MULTI-MODEL BLEND THROUGH DAY 4, INCORPORATING COMPONENTS OF THE OPERATIONAL GFS/ECMWF/UKMET AS WELL AS THE 12Z EC MEAN AND GEFS MEAN. THE CMC WAS NOT USED SINCE IT WAS MORE AGGRESSIVE WITH HEIGHT FALLS OVER THE PACIFIC NORTHWEST BEGINNING AS EARLY AS TUESDAY. FOR THE SECOND HALF OF THE FORECAST PERIOD, THE FORECAST WAS WEIGHTED MORE HEAVILY TOWARD ENSEMBLE MEAN COMPONENTS. ...WEATHER HIGHLIGHTS... GIVEN THE PERSISTENCE OF UPPER LEVEL TROUGHING ACROSS THE CENTRAL AND EASTERN U.S, TEMPERATURES IN THESE AREAS SHOULD GENERALLY REMAIN 5 TO 15 DEGREES BELOW AVERAGE FOR MOST OF THE UPCOMING WEEK. UPPER LEVEL RIDGING ALONG THE WEST COAST WILL RESULT IN SOME OF THE WARMEST TEMPERATURES ACROSS THE CONTINENTAL U.S., WITH HIGHS 10 TO 20 DEGREES ABOVE NORMAL, EQUATING TO HIGHS NEAR 90 DEGREES IN THE INTERIOR PACIFIC NORTHWEST AND PERHAPS APPROACHING 100 DEGREES OR ACROSS THE CALIFORNIA INTERIOR VALLEYS AND ESPECIALLY THE DESERT SOUTHWEST ON TUESDAY. SLIGHTLY COOLER READINGS ARE EXPECTED TO ENSUE LATER IN THE WEAK FOR THIS REGION. ACROSS THE EASTERN HALF OF THE U.S., MULTIPLE ROUNDS OF SHOWERS AND THUNDERSTORMS ARE LIKELY ALONG AND AHEAD OF AN ADVANCING COLD FRONT, WITH THE HEAVIEST RAINFALL POTENTIAL EXPECTED FROM THE CENTRAL GULF COAST TO THE SOUTHEAST COAST. THE PRIMARY FRONTAL BOUNDARY SHOULD BE PROGRESSIVE, BUT AMPLE MOISTURE AND INSTABILITY AVAILABLE FOR CONVECTION MAY SUPPORT THE POTENTIAL FOR HEAVY RAINFALL ACROSS THESE AREAS ON TUESDAY. FARTHER NORTH, FORCING WILL BE STRONGER IN THE VICINITY OF THE SURFACE AND UPPER LEVEL LOW, BUT MOISTURE WILL BE SOMEWHAT MORE LIMITED OVER THE GREAT LAKES REGION, AND SCATTERED AREAS OF CONVECTION ARE EXPECTED. A SECOND WAVE OF LOW PRESSURE IS FORECAST TO DEVELOP ALONG THE FRONT ACROSS THE LOWER GREAT LAKES ON WEDNESDAY, WHICH SHOULD BRING A ROUND OF WIDESPREAD SHOWERS AND THUNDERSTORMS TO THE MID-ATLANTIC AND NORTHEAST THROUGH THURSDAY. HAMRICK/RYAN