EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 212 AM EDT FRI APR 17 2015 VALID 12Z MON APR 20 2015 - 12Z FRI APR 24 2015 ...UNCERTAINTY ASSESSMENT/MODEL PREFERENCES... BLENDED THE 16/12Z ECENS...NAEFS AND GEFS AT A 50/30/20 RATIO---TO HANDLE THE 'VAGARIES' OF THE FLOW PATTERN---ALONG THE WEST COAST OF NORTH AMERICA. FOR THE EAST AND CENTRAL UNITED STATES---THIS BLEND REASONABLY HANDLES THE EAST-NORTHEASTWARD MIGRATION OF A CUTOFF 500MB LOW FROM THE WEST CENTRAL HIGH PLAINS TO THE CENTRAL GREAT LAKES AT DAY 4. AND ITS GRADUAL 'DECAY' AND SHEARED APPEARANCE ACROSS THE ST LAWRENCE VALLEY AND NORTHERN NEW ENGLAND---DAY 5 THROUGH DAY 7---WILL BE A 'PLACE-HOLDER' FOR NOW---BUT EXPECT SOME ALTERATIONS FOR THE NEW ENGLAND AND CANADIAN MARITIME LOW TRACK FORECASTS IN THE FUTURE. IN THE WEST...MUCH OF THE MODEL SPREAD AND CONTENTION ORIGINATES WEST OF HAIDA GWAII AROUND DAY 5---AND CONCERNS THE LONGITUDE WHERE---AND STRUCTURE OF---ORGANIZED ENERGY AND ITS TRANSFER DOWNSTREAM FROM THE GULF OF ALASKA TO THE LOWER 48'S WEST COAST. WHAT ENERGY THAT DOES MIGRATE EASTWARD ALONG 50N---WILL ENCOUNTER A BROAD UPPER-LEVEL RIDGE---ANCHORED ALONG THE CANADIAN ROCKIES AND QUICKLY DIG SOUTHWARD ALONG 125W-130W. AT THIS POINT...CANNOT TELL IF MID-LEVEL ENERGY PINCHES OFF INTO A CLOSED LOW...SINGLE OPEN WAVE...OR A SERIES OF SHORTWAVES. THE GFS...ECMWF AND CANADIAN SEEM TO BE IN GENERAL AGREEMENT AT THE DAY3-DAY 4 TIME FRAME---WITH A LEAD IMPULSE DIGGING SOUTH-SOUTHEASTWARD ALONG THE CENTRAL AND SOUTHERN COAST LINE OF CALIFORNIA. BUT THAT IS AS FAR AS THE DETERMINISTIC GUIDANCE COULD REASONABLY BE COUNTED UPON (TO DETERMINE COLD ADVECTION)---AND DEFERRED TO THE BLENDED MEANS AS BROAD TROUGHING BEGINS TO TAKE SHAPE BETWEEN 120W-125W FOR DAYS 6-7. IN THE EAST...THE 16/12Z DETERMINISTIC GFS/ECMWF WERE 'USE-ABLE' INTO DAY 4---BEFORE THE RATE AT WHICH NORTHERN STREAM ENERGY ORIGINATING IN CENTRAL CANADA---BEGINS TO ALTER THE SPECIFIC TIMING OF NOTABLE SHORTWAVES---AND PRECIPITATION GENERATION (P-TYPE). THE CYCLONIC FLOW DOES OFFER SOME COLD ADVECTION NO DOUBT---BUT ALSO RAISES QUESTIONS. FIRST QUESTION---CAN THE CYCLONIC FLOW THAT APPEARS TO BE SHEARING THROUGH TIME---GENERATE AND SUSTAIN A SERIES DEEPENING SURFACE WAVES ALONG ITS BASE? AND QUESTION TWO...CAN SOUTHERN STREAM ENERGY 'PHASE' THE PATTERN ALONG THE EAST COAST (BETWEEN CAPE HATTERAS AND THE GULF OF MAINE) BEYOND DAY 5---AS THE 16/12Z ECMWF AND CANADIAN CURRENTLY SUGGEST? BY NEXT WEDNESDAY---THIS PATTERN AND SCENARIO (FOR MORE WINTER WEATHER?) SHOULD BECOME MUCH CLEARER THAN IT SITS RIGHT NOW. ...SENSIBLE WEATHER HIGHLIGHTS... THE LOWER 48 WILL GRADUALLY GET AWAY FROM THE SPLIT-FLOW REGIME CURRENTLY IN PLACE AS THE CUTOFF LOW OVER EASTERN COLORADO SLOWLY DRIFTS OFF INTO THE GREAT LAKES. EXPECT A WET PATTERN FOR DAYS 3-4 FOR THE MIDWEST---THE LOWER HALF OF THE MISSOURI VALLEY...THE ENTIRE MISSISSIPPI VALLEY AND WESTERN OHIO VALLEY. BY DAY 5...AS THE CUTOFF SYSTEM BEGINS TO STALL INVOF WESTERN ONTARIO---THE PRECIPITATION BEGINS TO SHIFT NORTHEASTWARD---IN EARNEST---ACROSS THE APPALACHIANS AND THE EAST COAST. TEMPERATURES BENEATH THE SLOW-MOVING UPPER-LEVEL LOW SHOULD BE 10-15F BELOW NORMAL FOR THE LAST TEN DAYS OF APRIL (IE GREAT LAKES AND MIDWEST IN GENERAL). IN THE WEST---ISOLATED TO SCATTERED PRECIPITATION APPEARS TO BE MORE OF A PRE-FRONTAL CHALLENGE AND CONVECTIVE IN NATURE FROM NORTHERN CALIFORNIA TO EASTERN WASHINGTON---WITH A SOUTHERLY MID-LEVEL FLOW PRECEDING A SLOW-MOVING BUT 'SHALLOW' COLD FRONT---DAYS 4-7. THE 'SHALLOW' FRONT---THE RESULT OF A DIGGING TROUGH AXIS ALONG THE WEST COAST---PULLING MARITIME PACIFIC AIR SOUTHWARD RATHER THAN EASTWARD---PREVENTING A DEEPENING OF THE COLD ADVECTION INTO THE NORTHERN GREAT BASIN AND EAST OF THE CASCADES. VOJTESAK