EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 219 AM EDT THU APR 23 2015 VALID 12Z SUN APR 26 2015 - 12Z THU APR 30 2015 ...OVERVIEW... A SERIES OF PACIFIC TROUGHS WILL MAKE THEIR WAY ACROSS THE LOWER 48 THIS FORECAST PERIOD---SETTING UP A WET FORECAST FOR THE SOUTHERN ROCKIES...PLAINS AND SOUTHEAST. ...MODEL PREFERENCES/UNCERTAINTY ASSESSMENT... THE 22/12Z ECENS-NAEFS-GEFS MEANS PROVIDED A VERY REASONABLE MASS FIELD FORECAST BASELINE---AND THE DETERMINISTIC ECMWF...GFS AND CANADIAN WERE BEGINNING TO ESTABLISH BETTER CONTINUITY OVER SOUTH CENTRAL CANADA. THE GUIDANCE HAS RESOLVED THE STORM TRACK---TAKING THE PACIFIC SYSTEM NORTHEASTWARD TOWARDS THE ARCTIC CIRCLE (ALONG 100W LONGITUDE) RATHER THAN SENDING IT EASTWARD INTO THE DAKOTAS (DAY 3-4). THE AMPLIFICATION OF LONGWAVE RIDGE ACROSS THE WEST COAST AND NORTHERN ROCKIES BEHIND THAT SYSTEM---NOW GIVES THE 'NEW' PACIFIC JET 'ROOM' TO DIG SOUTHEASTWARD ALONG THE CONTINENTAL DIVIDE AND BEGIN THE PROCESS OF CARVING OUT THE BASE OF A 'NEW' TROUGH AXIS---EAST OF THE MISSISSIPPI RIVER ALONG 30N LATITUDE FROM 90W TO 80W LONGITUDE---BETWEEN TUESDAY AND THURSDAY OF NEXT WEEK. EMBEDDED WITHIN THIS 'NEW' TROUGH---WILL BE AN ACTIVE FOUR CORNERS CLOSED 500MB LOW. THE FORECAST CHALLENGE WILL BE THE MIGRATION OF THIS SYSTEM ACROSS THE SOUTHERN PLAINS---AND ITS INTENSIFICATION DOWNSTREAM ACROSS THE SOUTHEAST AND SOUTHERN MID-ATLANTIC REGION...PRIOR TO ITS EXIT INTO THE WESTERN ATLANTIC OCEAN. A KEY COMPONENT OF THE SYSTEM'S INTENSIFICATION WILL BE THE INCORPORATION OF SOUTHEASTWARD-MOVING SHORTWAVE ENERGY AND AT WHAT LATITUDE/LONGITUDE THIS NORTHERN STREAM ENERGY WILL 'PHASE' WITH THE CUTOFF SYSTEM. PRIOR TO ITS PHASING---A SERIES OF WEAKER FAST-MOVING SHORTWAVES ARE ANTICIPATED TO EJECT EASTWARD ACROSS THE PIEDMONT AND CONSOLIDATE IN THE WESTERN ATLANTIC WITH A STAGNANT CYCLONIC FLOW INVOF THE CANADIAN MARITIMES. ...SENSIBLE WEATHER HIGHLIGHTS... VERY WET 4-DAY PERIOD FOR THE SOUTHEAST QUARTER OF THE NATION...INCLUDING FLORIDA. MOST RECENT MODEL QPF GUIDANCE CENTERS THE HEAVIEST RAINFALL AMOUNTS ALONG THE BROAD BASE OF THE EASTERN TROUGH---ALONG/SOUTH OF THE I-10 CORRIDOR FROM SOUTHWEST LOUISIANA TO NORTHEAST FLORIDA INCLUDING A LARGE PORTION OF CENTRAL AND SOUTH FLORIDA. A SECONDARY PRECIPITATION MAXIMA IS EXPECTED ALONG A 'ARC' FROM SOUTHEAST COLORADO INTO SOUTH CENTRAL OKLAHOMA AND EAST CENTRAL TEXAS.