EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 148 AM EST THU DEC 04 2014 VALID 12Z SUN DEC 07 2014 - 12Z THU DEC 11 2014 ...HEAVY PCPN FOR THE PACIFIC NW AND AN ERN US COASTAL LOW THREAT... ...OVERVIEW... A FAST-MOVING WESTERLY FLOW ALOFT WILL PREVAIL ACROSS A LARGE PORTION OF THE MID-LATITUDE PACIFIC DURING THIS MEDIUM RANGE PERIOD. UNTIL DAY 5...AND DOWNSTREAM OF THE CONTINENTAL DIVIDE---THE FLOW CONTAINS A COOL LOW-AMPLITUDE NORTHERN STREAM AND ACTIVE SOUTHERN STREAM. AT THE SURFACE...THE PREVAILING WEATHER ACROSS THE GREAT LAKES...NORTHEAST AND COASTAL MID-ATLANTIC REGION WILL BE DOMINATED BY STRONG HIGH PRESSURE. ...UNCERTAINTY ASSESSMENT... THE DETERMINISTIC MODEL SPREAD CONTINUES TO FOLLOW A CONSISTENT THEME---POTENTIAL PHASING OF THE NORTHERN AND SOUTHERN AIR STREAMS---EAST OF THE ROCKIES---AND ABOVE A STRONG SURFACE RIDGE ANCHORED IN SOUTHEAST CANADA AND NEW ENGLAND. MUCH OF THE LOW-LEVEL FLOW ALONG THE EAST COAST WILL BE A GRADIENT WIND (EAST TO EAST-NORTHEAST)---WHICH IS COOL AND MOIST. IN A SPLIT-FLOW PATTERN---DISLODGING THIS SURFACE RIDGE FROM THE SOUTH AND A SOUTHERN STREAM FLOW---IS BETTER SUITED FOR ENHANCING LIFT AND CREATING WARM ADVECTION WHEN THE SOUTHERN STREAM IS AMPLIFIED. TO THIS POINT---THE SOUTHERN STREAM LOOKS TO BE A FAST BUT FLAT FLOW DURING THIS MEDIUM RANGE PERIOD---DIRECTING A SERIES OF WAVES OFF THE EAST COAST---BETWEEN THE OUTER BANKS AND DELMARVA. WARM ADVECTION APPEARS TO BE VERY LIMITED WITH EITHER SCENARIO. IN THE LOW-AMPLITUDE NORTHERN STREAM FLOW---THE AMPLIFICATION APPEARS TO DEVELOP IN THE CENTRAL ROCKIES THEN DIG DOWNSTREAM OVER THE LOWER OHIO VALLEY AND CENTRAL APPALACHIANS. IT IS CONCEIVABLE---BUT THE JET STREAK WILL ACTUALLY SERVE TO FORM A DRY SLOT---AND PROVIDE EVEN MORE SEPARATION BETWEEN NORTHERN AND SOUTHERN AIR STREAMS...NOT PHASE THEM OFF THE EAST COAST. AND THIS IS WHERE THE DETERMINISTIC RUNS DIFFER IN PRINCIPLE---AND PROVIDE ALL THE ENSEMBLE SPREAD. IF THE DETERMINISTIC RUNS FAVOR A PORTION OF NORTH AMERICA FOR PHASING---IT'S INVOF ATLANTIC CANADA. UNTIL THE SURFACE RIDGE CAN BE COMPLETELY ERODED...THE AIRMASS MAY BE COLD ENOUGH FOR WINTER WEATHER ACROSS INTERIOR NEW ENGLAND...WITH A NORTHERN STREAM IMPULSE PRODUCING THE BULK OF THE LIFT. ...MODEL PREFERENCES... THE 3/12Z ECENS...NAEFS AND GEFS PROVIDED THE BASELINE FOR THE MASS FIELD FORECASTS ACROSS THE LOWER 48. THE KEY FEATURES OF THE PATTERN WILL ORIGINATE IN THE PACIFIC---WITH A VERY LENGTHY MID-LATITUDE 200MB-300MB JET AXIS---STRETCHING ACROSS THE ENTIRE PACIFIC ALONG 40N LATITUDE. THIS AIR CURRENT IS FORECAST TO CONTAIN WINDS EXCEEDING 130+ KT AND WILL PRODUCE A WELL-DEFINED WAVE TRAIN...DIRECTING A SERIES OF STRONG WARM FRONTS INTO THE PACIFIC NORTHWEST AND NORTHERN CALIFORNIA. THE MILD PACIFIC AIRMASS SURGES INTO SOUTH CENTRAL CANADA AND THE NORTH CENTRAL PLAINS BY THE END OF THE PERIOD. ...SENSIBLE WEATHER HIGHLIGHTS... ADDITIONAL PRECIPITATION IS EXPECTED FOR THE NORTH COAST OF CALIFORNIA...WITH PERIODS OF HIGH WINDS FOR COASTAL SECTIONS OF THE WEST COAST FROM CAPE MENDOCINO TO CAPE FLATTERY. ENSEMBLE-BASED PRECIPITATION AMOUNTS TARGET THE KING RANGE...AND OREGON/WASHINGTON COASTS WITH OVER 5 INCHES OF RAINFALL THIS 96-HOUR PERIOD. THE OLYMPICS AND VOLCANIC PEAKS OF THE CASCADES---CLOSER TO 10 INCHES OF PRECIPITATION. THE SOUTHERN STREAM ENERGY AND MOISTURE IS EXPECTED TO DELIVER 1/2 TO AN INCH OF PRECIPITATION FOR THE SOUTHERN PLAINS...SOUTHEAST AND PIEDMONT THIS MEDIUM RANGE PERIOD. A COOL AND MOIST EAST TO NORTHEAST FLOW IS EXPECTED TO PERSIST ACROSS COASTAL NEW ENGLAND AND THE NORTHERN MID-ATLANTIC STATES UNTIL DAY 4. VOJTESAK