EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1127 AM EDT TUE APR 21 2015 VALID 12Z FRI APR 24 2015 - 12Z TUE APR 28 2015 ...OVERVIEW... ACROSS THE EASTERN HALF OF THE NATION...A LARGE AND SLOW MOVING UPPER LOW ACROSS MAINE INTO THE CANADIAN MARITIMES WILL DOMINATE THE UPPER FLOW AND INFLUENCE THE MOVEMENT AND INTENSITY OF OTHER SMALLER SCALE SYSTEMS THAT INTERACT WITH THIS FEATURE THROUGHOUT THE DAY 3 TO DAY 7 FORECAST/FRIDAY THROUGH TUESDAY. IN PARTICULAR...A SIGNIFICANT TROUGH WILL MOVE EASTWARD OUT OF THE SOUTHWEST STATES ON FRIDAY AND BEGIN TO SHEAR WITHIN THE BROAD UPPER CONFLUENT UPPER FLOW DICTATED BY THE PRESENCE OF THE ANOMALOUS UPPER LOW NEAR MAINE AND THE UPPER RIDGE OVER THE EASTERN GULF/CARIBBEAN. AS A RESULT...THIS SYSTEM WILL TEND TO TRACK EASTWARD ACROSS THE LOWER TO MID MISSISSIPPI VALLEY ON SATURDAY TO OFF THE SOUTHEAST TO SOUTH MIDDLE ATLANTIC COAST BY SUNDAY. BEHIND THIS SYSTEM...HIGH PRESSURE BLEEDING SOUTHWARD FROM CENTRAL CANADA WILL INFLUENCE THE WEATHER ACROSS THE NORTHERN PLAINS TO MIDWEST AND OHIO VALLEY ON FRIDAY AND SATURDAY AND THEN MOVE SOUTHWARD TOWARD THE GULF BY SUNDAY/DAY 5. ANOTHER UPPER TROUGH WILL MOVE IN BEHIND THE SOUTHWEST SYSTEM ALONG THE WEST COAST ON DAY 3 AND THEN BEGIN TO MOVE EASTWARD IN THE NEXT COUPLE OF DAYS. ONE PORTION OF THIS TROUGH MAY REMAIN AS A MORE NORTHERLY COMPONENT AND MOVE EAST WHILE THERE ALSO MAY BE A SOUTHERN COMPONENT THAT COULD HANG BACK...OR NOT...OVER THE SOUTHWEST AS THE WEEKEND COMES TO AN END. THE EVOLUTION OF THIS SYSTEM EASTWARD ON DAYS 6 TO 7 IS OF COURSE LESS CERTAIN BUT THE MEANS SHOW A SLOW EASTWARD TREND POSSIBLY REACHING THE EASTERN THIRD OF THE COUNTRY BY DAY 7/TUESDAY. ...MODEL PREFERENCES... IN GENERAL...THERE ARE NO MAJOR DIFFERENCES ON DAYS 3 THROUGH 5 BETWEEN THE GEFS/EUROPEAN/NAEFS ENSEMBLE MEANS WITH RESPECT TO THE SPEED/AMPLITUDE OF THE MAJOR TROUGH/RIDGE SYSTEMS. THIS IS EVEN TRUE WHEN COMPARED TO MOST OF THE OVERNIGHT OPERATIONAL MODEL RUNS...ESPECIALLY ON DAYS 3 AND 4...WHEN THE 00Z/06Z GFS AND 00Z ECMWF SHOW SMALL RELATIVELY DIFFERENCES ESPECIALLY COMPARED WITH THE ENSEMBLE MEANS. THE 00Z UKMET WAS DI SCARDED BECAUSE IT SEEMED TO HAVE MORE SIGNIFICANT DIFFERENCES THAN THE OTHERS WHILE THE 00Z CANADIAN COULD HAVE FIGURED INTO THE BLENDS SINCE IT ALSO SEEMED TO FIT THE OTHER GENERAL PATTERNS. BY DAY 6...NOT SURPRISINGLY...THE MEANS START SHOWING SOME SPREAD BUT GENERALLY IN JUST RELATIVELY SMALL SHIFTS IN WAVELENGTH AND SPEED ON HOW THE DUAL TROUGH SYSTEM ACROSS THE WEST MOVES EASTWARD WITH MORE AMPLITUDE IN THE ECMWF MEANS AND A SLIGHTLY MORE SHEARED APPEARANCE OF THE SOUTHWEST TROUGH IN THE GEFS/NAEFS. BY THIS TIME...IT APPEARS THAT THE OPERATIONAL GFS MAY HAVE MORE OF AN ADVANTAGE OVER THE ECMWF SINCE THE ECMWF IS MORE AMPLIFIED WITH THE TROUGH OVER THE MIDDLE OF THE US AND SLOWER AND MORE AMPLIFIED OVER THE WEST...WHILE THE MEANS APPEAR TO BE FASTER WITH ANOTHER TROUGH SETTING UP OVER THE EASTERN PACIFIC WHILE THE OPERATIONAL ECMWF HAS IT CONSIDERABLY WEST. AT THIS POINT...THE AMPLIFICATION OF THE OPERATIONAL ECMWF ON DAYS 6 AND 7 APPEARS TO BE A RELATIVE OUTLIER WHEN COMPARED TO THE MEANS...INDICATING LOWER PROBABILITIES. AS A RESULT...THE ECMWF RESULTS IN A PRETTY STRONG CYCLONE IN THE EAST ON DAY 7 WHILE THE GFS/MEANS APPEAR TO SUGGEST SOMETHING POSSIBLY MORE SUPPRESSED AND WEAKER. AN INTERMEDIATE SOLUTION SLIGHTLY FAVORING THE GFS OVER THE ECMWF WAS USED AS A TEMPLATE FOR DAY 7/TUESDAY. ...SENSIBLE WEATHER HIGHLIGHTS... THE BLOCKING UPPER LOW OVER SOUTHEAST CANADA DURING THE PERIOD SUGGESTS BELOW NORMAL TEMPERATURES ACROSS MUCH OF THE EAST THROUGH THE PERIOD. THE FIRST SHORTWAVE TROUGH LOOKS LIKE IT WILL BE CAPABLE OF SOME SEVERE WEATHER IN THE SOUTHERN PLAINS/GULF ON DAY 3 AND A RAIN THREAD MOVING ACROSS SOUTHERN AND CENTRAL PLAINS ON FRIDAY AND SPREADING EASTWARD ON SATURDAY/DAY 2. EVEN WITH THE TROUGH WEAKENING AS IT COMES EAST ON DAY 4/SATURDAY...RAIN AND RELATIVELY COOL TEMPERATURES COULD FALL ACROSS THE OHIO AND TENNESSEE VALLEYS WHILE ANOTHER CHANCE OF THUNDERSTORMS AND POSSIBLE SEVERE WEATHER CONTINUES ACROSS THE GULF/SOUTHEAST AS WELL. BY SUNDAY/DAY 6...THE SYSTEM EXITS THE EAST COAST WITH CLEARING POSSIBLE ACROSS MUCH OF THE EAST BUT THE NEXT SYSTEM FROM THE SOUTHWEST BEGINS TO MOVE EASTWARD TOWARD THE SOUTHERN PLAINS. WHILE THE EVOLUTION OF THIS NEXT SYSTEM IS STILL QUITE UNCERTAIN...THERE APPEARS TO BE A TREND TOWARD RAIN/THUNDERSTORMS DEVELOPING AGAIN ACROSS THE SOUTHERN PLAINS ON SUNDAY/DAY 5 AND THEN MOVING TOWARD THE LOWER AND MID MISSISSIPPI VALLEY ON MONDAY/DAY 6 AND ACROSS THE SOUTHEAST/MID ATLANTIC ON TUESDAY/DAY 7. ACROSS THE WEST...THE COMBINATION OF PROGRESSIVE TROUGHS AND THE MOVEMENT OF ANOTHER UPPER LOW TOWARD THE WESTERN US/CANADIAN BORDER WILL TEND TO KEEP THE UPPER RIDGE FROM AMPLIFYING SO RELATIVELY SEASONABLE WEATHER IS LIKELY WITH A FRONT MAKING IT EASTWARD ACROSS THE WEST ON DAYS 3 TO 5/FRIDAY TO SUNDAY WHILE ANOTHER FRONT REACHES THE NORTHWEST COAST ON DAY 5/SUNDAY THAT MOVES ACROSS THE ROCKIES BY TUESDAY/DAY 7. KOCIN