EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 151 AM EDT WED SEP 09 2015 VALID 12Z SAT SEP 12 2015 - 12Z WED SEP 16 2015 THE 7/12Z ECENS/GEFS AND NAEFS MEANS (A 40/30/30 CONFIGURATION) CONTINUE TO DO A REASONABLE JOB WITH THE PROLIFERATION OF A BROAD TROUGH CONFIGURATION ACROSS CANADA AND THE SUBTLE CHANGES EXPECTED FOR THE LOWER 48. BASED ON THE PREVAILING NORTHERN STREAM DOMINANCE OVER ALASKA...THE NORTHEAST PACIFIC AND CENTRAL CANADA...THE MEDIUM RANGE NATIONAL FORECAST USED THESE MEANS---TO BALANCE OUT THE DIFFERENCES AMONG THE DETERMINISTIC RUNS AT VARYING TIMES AND LOCATIONS NATIONWIDE. ALONG THE EAST COAST--- FOR SEVERAL DAYS THE ECMWF/GFS HAVE HELD TO THEIR SOLUTIONS WITH THE EVACUATION OF THE RIDGE PATTERN IN THE EAST AND SOUTHEAST. FOR THE LEAD CANADIAN FRONTAL PROGRESSION THROUGH THE EASTERN SEABOARD FOR DAY 3-4---THE ECMWF ALWAYS HELD A MORE PROGRESSIVE SOLUTION ACROSS THE SOUTH AND SOUTHEAST...AND THE GFS---THE SLOWER PROGRESSION ACROSS THE SOUTHEAST. IN SOME WAYS...SLOWER HAS BEEN 'BETTER'---FOR THE MEDIUM RANGE FORECAST. BUT NOW---SLOWER MEANS MORE THINGS TO CONSIDER GOING FORWARD ACROSS THE LOWER 48 (BEYOND DAY 4). FOR DAY 3...THE 7/12Z DETERMINISTIC GUIDANCE SHOWS TWO SURFACE WAVES ALONG THE FRONT---ONE NEAR NOVA SCOTIA AND THE OTHER SOMEWHERE BETWEEN THE PIEDMONT AND DELMARVA. AND PERHAPS THE WEAK CIRCULATION OVER NORTHEAST GEORGIA WILL BECOME ONE OF THE SURFACE WAVES. ON DAY 4 (13/12Z)...THE 7/12Z DETERMINISTIC GUIDANCE IS GOING WITH A POST-FRONTAL CLOSED LOW (ECMWF/CANADIAN/UKMET) AND/OR OPEN WAVE (GFS) SOLUTION. FOR A TIME...SOME SOLUTIONS WERE INVOF TORONTO ALOFT AND NOW...INVOF LAKE ERIE. AND THIS MID-LEVEL 'SYSTEM' IS EXPECTED TO INTERACT BRIEFLY PERHAPS WITH THE FRONT ALONG THE EAST COAST? EXACTLY HOW THESE TWO FEATURES 'INTERACT' --- SEEMS TO BE WHERE THE SOLUTIONS HAVE MET A CROSSROADS. AND SINCE MUCH EARLIER SOLUTIONS ON PREVIOUS MEDIUM FORECAST DAYS (MAYBE 5-6-7 DAYS AGO) PROBABLY HAD TOO MUCH PROGRESSION TO THE FRONT TO BEGIN WITH NORTH OF 40N LATITUDE---THE NEW DAY 3-4 FORECAST PERIOD NOW HAS TO CONSIDER WHAT ASPECT OF A 'POST-IGNACIO' ENERGY PACKET---REALLY MIGHT HAVE ON THE PATTERN IN SOUTHEAST CANADA. WHERE THERE WAS NOT MUCH AGREEMENT---IS WITH HOW MUCH DOWNSTREAM RIDGING DEVELOPS OVER ATLANTIC CANADA TO ALLOW THE FRONT TO SLOW DOWN ---EVEN MORE--- BETWEEN THE OUTER BANKS AND CAPE COD THAN WHAT THE SLOWER GFS/GEFS SOLUTIONS WERE HINTING AT MUCH EARLIER ON. THE 7/12Z ECMWF/CANADIAN/UKMET ARE THE DEEPER SOLUTIONS AT 500 MBS---DOWN TO 558DM AND 564DM RESPECTIVELY. THAT'S A LITTLE DEEPER THAN WHAT WE HAVE SEEN SO FAR THIS SUMMER...YET DOWNSTREAM ALONG THE NORTHEAST COAST---THE HEIGHTS STRUGGLE TO FALL BELOW 576DM. AGAIN...WOULD LIKE TO SEE A SYSTEM PRODUCING SOME DECENT DOWNSTREAM HEIGHT FALLS---AND THIS 500MB VORTICITY/CIRCULATION CANNOT QUITE GET ENOUGH SUPPORT TO GENERATE A VERY DEEP SURFACE WAVE ALONG THE EAST COAST. HAVING SOME OF 'A POST-IGNACIO' ENERGY MIGHT HELP SOME---SO WOULD THINK THE SYSTEM DOES NOT REALLY 'PHASE' OR IF IT DOES...THE BETTER REGION FOR THE PHASING WOULD BE OVER SOUTHERN QUEBEC/WESTERN LABRADOR. IN A 'WORST CASE' SCENARIO...THE LOW-LEVEL FRONT STALLS AND THERE WILL BE A SECONDARY COLD FRONT (TIED TO THE MID-LEVEL CUTOFF/WAVE) THAT DEVELOPS OVER THE EASTERN LAKES---GETS SOME OROGRAPHIC ENHANCEMENT FROM THE APPALACHIANS---THEN SWEEPS THE STALLED FRONT OUT TO SEA LATE SUNDAY INTO MONDAY. THIS WOULD KEEP MAINE AND NORTHERN NEW ENGLAND IN THE PRECIPITATION A DAY OR SO LONGER THAN THE MID-ATLANTIC REGION. A QUICK LOOK AT THE 9/00Z GFS WOULD SUPPORT THE NOTION FOR A SECONDARY COLD FRONT AND/OR SQUALL LINE RACING THROUGH THE NORTHEAST THIS WEEKEND. ACROSS THE SOUTHERN TIER FROM CALIFORNIA TO TEXAS... THE 'SLOWER' SOLUTIONS WILL HAVE AN IMPACT ACROSS THE SOUTH---WITH THE COLD FRONT NOW TAKING A LITTLE LONGER TO REACH THE RIO GRANDE AND WESTERN GULF COAST. THE RESPONSE IS TO DRIVE LESS TROUGHING THROUGH THE CENTRAL GULF...AND A SLIGHTLY QUICKER 'REBOUND' OF THE UPPER-LEVEL RIDGE ALONG/SOUTH OF 30N. AND THE MEANS DO A NICE JOB OF ILLUSTRATING THIS TREND ON DAY 4...WHEN UPPER-LEVEL TROUGH BEGINS LIFTING OUT ACROSS THE EAST INTO ATLANTIC CANADA. THIS DEVELOPMENT ALONG/EAST OF 90W LONGITUDE WILL ALLOW THE SOUTHWESTERN GULF TO 'OPEN UP' SOME AND PERHAPS A DAY OR SO SOONER THAN PREVIOUSLY CONSIDERED. ANOTHER ASPECT OF THE FORECAST HINGES ON A WEAK 'TUTT' CELL INVOF 31N 135W AND ITS INFLUENCE ON 'LINDA' AS IT MEANDERS VERY SLOWLY EAST-NORTHEASTWARD TOWARDS THE MAINLAND DURING THE EARLY PORTION OF THE MEDIUM RANGE. WITH 'LINDA' AT A MORE NORTHERN LATITUDE AND A COUPLE OF DAYS 'OLDER' FROM ITS PROJECTED LIFE CYCLE BACK ON SEPT 4/5/6 TIME FRAME--- THE NORTHERN PERIPHERY OF ITS MOISTURE NOW COMES INTO PLAY ALONG THE WEST COAST AND LOWER COLORADO BASIN UNTIL DAY 5/6---WHEN THE NEXT NORTHERN STREAM TROUGH/SYSTEM IN THE GULF OF ALASKA BEGINS TO MIGRATE SOUTHEASTWARD INTO THE WEST CENTRAL COAST OF NORTH AMERICA. WHERE WILL 'LINDA' BE? AND WHAT WILL IT LOOK LIKE? ANOTHER REASON TO USE THE MEANS. VOJTESAK