EXTENDED FORECAST DISCUSSION...CORRECTION TP PARAGRAPH 3 NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 256 AM EDT TUE MAR 17 2015 VALID 12Z FRI MAR 20 2015 - 12Z TUE MAR 24 2015 COLD UPPER-LEVEL TROUGH REMAINS THE KEY SYNOPTIC-SCALE FEATURE ACROSS SOUTHEAST CANADA ...THE EASTERN GREAT LAKES AND NEW ENGLAND THIS MEDIUM RANGE PERIOD. UPPER-LEVEL RIDGE IN WESTERN CANADA IS EXPECTED TO MAINTAIN ITS 'BLOCKY' NATURE OVER WESTERN CANADA---ALLOWING THE SPLIT-FLOW PATTERN TO PREVAIL ACROSS THE LOWER 48. THE KEY SYNOPTIC-SCALE FEATURE IN THE EASTERN PACIFIC WILL BE THE PRESENCE OF AN ORGANIZED UPPER-LEVEL TROUGH---SPAWNING A SERIES OF SURFACE CYCLONES AND MID-LEVEL SHORTWAVES...THAT WILL PERIODICALLY AND PROGRESSIVELY---MIGRATE INTO THE WESTERN US. ...MODEL PREFERENCES/UNCERTAINTY ASSESSMENT... THE 16/12Z ECENS/NAEFS CONTINUED TO BE THE BASELINE FOR THE MEDIUM RANGE AND WITH ENOUGH CONTINUITY IN THE 16/12Z ECMWF THROUGH THE END OF DAY 5---TO UTILIZE ITS SOLUTION TO CAPTURE SOME OF THE SUB-SYNOPTIC DETAILS EMBEDDED WITHIN THE ACTIVE SOUTHERN STREAM FROM THE ARIZONA-NEW MEXICO BORDER EASTWARD TO THE CAROLINAS. THERE HAS NOT MUCH WAVERING WITH THE BROAD-SCALE PATTERN ACROSS THE LOWER 48---GIVEN THE CLOSED 500MB LOW OVER QUEBEC...TROUGH AXIS EXTENDING SOUTHWARD TO THE CENTRAL APPALACHIANS---AND THE UPPER-LEVEL RIDGE IN WESTERN CANADA/INTERIOR ALASKA MAINTAIN THEIR QUASI-STATIONARY POSITIONS ACROSS THE NORTH AMERICAN CONTINENT. THE EASTERN PACIFIC REMAINS THE CHALLENGE----AND WILL BE THE SPAWNING GROUND FOR 'UNDERCUTTING' ENERGY AND MOISTURE EXPECTED TO MIGRATE FROM THE PACIFIC TO THE ATLANTIC. THE PACIFIC NORTHWEST RECEIVING THE BULK OF THE MID-LEVEL WARM ADVECTION PRECIPITATION AND WINDS...AND THE INTERMOUNTAIN WEST AND PLAINS---RECEIVING THE MODIFIED-PACIFIC FRONTAL PROGRESSIONS. THESE FRONTS MIGRATING DOWNSTREAM INTO THE 'CONFLUENCE ZONE' ALOFT ALONG 40N LATITUDE AND EAST OF 100W LONGITUDE---THEN OFF INTO THE WESTERN ATLANTIC WILL REQUIRE SOME SOUTHERN STREAM ENHANCEMENT TO GENERATE ANY INTERACTION WITH THE NORTHERN STREAM---LOCKED IN OVER THE NORTHEAST QUARTER OF THE NATION. BASICALLY...THE 'SET-UP' WITH THIS SPLIT-FLOW PATTERN OFFERS LITTLE IN THE WAY OF WAVE-AMPLITUDE WITHIN ITS SOUTHERN BRANCH THIS MEDIUM RANGE PERIOD. THE BETTER PROSPECTS FOR MID-LEVEL WAVE ENERGY GAINING AMPLITUDE LIES WITHIN THE NORTHERN STREAM. BUT WITH VERY LIMITED CONNECTION TO THE PACIFIC---THE BEST IT CAN MUSTER---IS A PROGRESSIVE SERIES OF RATHER ROBUST CANADIAN SURFACE HIGHS AND 'RELATIVELY' DRY FRONTAL PASSAGES. UNTIL THE CONFLUENCE ZONE ALOFT CAN LIFT NORTHWARD---THE SOUTHERN STREAM---WHICH WILL CARRY THE MORE 'DYNAMIC' MID-LEVEL WAVE ENERGY---FROM PACIFIC TO ATLANTIC OCEANS---WILL BE FOCUSING MUCH OF THE SIGNIFICANT PRECIPITATION---ALONG AND SOUTH OF 40N LATITUDE. ...SENSIBLE WEATHER HIGHLIGHTS... A FEW INCHES OF LATE-SEASON SNOWFALL THURSDAY NIGHT, FRIDAY, AND FRIDAY NIGHT IS POSSIBLE FROM THE CENTRAL APPALACHIANS THROUGH THE INTERIOR NORTHEAST. THE ACTIVE SOUTHERN JET IN THIS SPLIT-FLOW PATTERN DOWNSTREAM OF THE CONTINENTAL DIVIDE WILL FOCUS THE HIGHER CONCENTRATIONS OF PRECIPITATION ACROSS THE SOUTHERN PLAINS AND SOUTHEAST THIS MEDIUM RANGE PERIOD. AS THE MODIFIED-CANADIAN AIR SPREADS SOUTHWARD TO THE GULF COAST---THE NORTHERN GULF OF MEXICO AND FLORIDA COAST APPEAR TO BECOME BREEDING GROUNDS FOR SCATTERED CONVECTION. BELOW NORMAL TEMPERATURES FOR THE GREAT LAKES...NORTHEAST AND MID-ATLANTIC REGION. VOJTESAK