EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1133 AM EDT THU OCT 26 2017 VALID 12Z SUN OCT 29 2017 - 12Z THU NOV 02 2017 ...HEAVY RAIN AND GUSTY WINDS LIKELY FOR THE NORTHEASTERN U.S. SUNDAY AND MONDAY... ...OVERVIEW... ANOMALOUS RIDGING ACROSS THE NORTHEASTERN PACIFIC IS EXPECTED TO PERSIST THROUGH THE MEDIUM RANGE AND EXPAND NORTH ACROSS ALASKA TOWARD THE NORTH POLE. THE RESULT WILL BE A HIGHLY BLOCKED NORTHERN STREAM FLOW FROM THE NORTH PACIFIC/ALASKA INTO CANADA, AND A STRONGLY NEGATIVE PNA PATTERN. THE RESULT WILL BE PERSISTENT TROUGHING ACROSS THE CENTRAL/EASTERN U.S. AND A RELATIVELY ACTIVE STORM TRACK FROM THE GREAT LAKES TO THE NORTHEAST. RAPID CYCLOGENESIS IS EXPECTED TO OCCUR OFF THE EASTERN SEABOARD SUN-MON AS A TROPICAL DISTURBANCE UNDERGOING EXTRATROPICAL TRANSITION ENTERS A REGION OF HIGHLY FAVORABLE UPPER-LEVEL DYNAMICS, PRODUCING A POTENTIAL HEAVY RAIN AND WIND EVENT FOR PORTIONS OF THE NORTHEAST. ...GUIDANCE/UNCERTAINTY ASSESSMENT AND WEATHER THREATS/HIGHLIGHTS... MODELS SHOW RELATIVELY GOOD LARGE SCALE AGREEMENT ACROSS THE CONUS THROUGH THE MEDIUM RANGE, WITH THE UPSTREAM BLOCKING RIDGE ACROSS THE NORTH PACIFIC PROMOTING CONTINUED TROUGHING ACROSS THE CENTRAL AND EASTERN STATES. PARTIAL PHASING BETWEEN A SHORTWAVE MOVING FROM THE OH VALLEY TO THE MID-ATLANTIC/NORTHEAST AND ANOTHER CROSSING THE UPPER MIDWEST/GREAT LAKES DAYS 3-4 (SUN-MON) IS EXPECTED TO RESULT IN RAPID CYCLOGENESIS ALONG THE MID-ATLANTIC COAST. AFTER TRENDING TOWARD A GREATER DEGREE OF PHASING YESTERDAY, MODEL SOLUTIONS SEEM TO HAVE STABILIZED, WITH A CONSENSUS BUILDING FOR RELATIVELY STRONG/RAPID SURFACE LOW DEVELOPMENT. A TROPICAL DISTURBANCE CURRENTLY ACROSS THE WESTERN CARIBBEAN/CENTRAL AMERICA COAST IS EXPECTED TO MOVE QUICKLY NORTHWARD OFF THE SOUTHEAST U.S. COAST ON DAY 3 BEFORE MOVING ENTERING THE REGION OF RAPID EXTRATROPICAL CYCLOGENESIS. THE RESULTANT EXTRATROPICAL CYCLONE IS FORECAST BY ALL GUIDANCE TO TRACK INTO THE NORTHEAST BY MONDAY. DETERMINISTIC MODEL DIFFERENCES ARE MINOR WITH RESPECT TO TIMING/LOW TRACK, AND A RANGE OF DEPTHS IS SHOWN BY DETERMINISTIC MODELS WHEN THE SYSTEM REACHES PEAK INTENSITY ACROSS NEW ENGLAND MONDAY MORNING, LIKELY IN THE 970-985 MB RANGE (THE 00Z ECMWF WAS THE DEEPEST SHOWING A 962 MB LOW AT 12Z MON). IF THE SYSTEM IS ABLE TO MAINTAIN AT LEAST A PARTIAL WARM CORE OR WARM SECLUSION (DUE TO ITS TROPICAL ORIGIN) THIS WOULD LIKELY TIP THE PEAK INTENSITY TOWARD A DEEPER SOLUTION AS SHOWN BY THE 00Z ECMWF. IMPRESSIVE UPPER DYNAMICS ASSOCIATED WITH THE DEEPENING CYCLONE AND ANOMALOUS TROPICAL MOISTURE TRANSPORTED NORTHWARD INTO THE SYSTEM ARE EXPECTED TO PRODUCE A HEAVY RAINFALL AND GUSTY WIND EVENT SUNDAY AND MONDAY FROM PORTIONS OF THE MID-ATLANTIC TO THE NORTHEAST, BEFORE THE SYSTEM LIFTS QUICKLY NORTH AND OCCLUDES EAST OF HUDSON BAY BY DAY 5 (TUE). HEAVY RAIN MAY FALL OVER SOME AREAS RECEIVING ADDITIONAL HEAVY RAINS DURING THE SHORT RANGE, INCREASING THE POTENTIAL FOR FLOODING IN SOME AREAS. SUFFICIENT AGREEMENT WAS NOTED AMONG DETERMINISTIC GUIDANCE THAT A MULTI-MODEL BLEND INCLUDING THE 00Z ECMWF/CMC AND 06Z GFS FORMED THE BASIS FOR THE WPC FORECAST DURING DAYS 3-5. IN THE WAKE OF THIS SYSTEM, ADDITIONAL SHORTWAVES WILL REINFORCE THE TROUGH ACROSS THE CENTRAL/EASTERN U.S, KEEPING BELOW AVERAGE TEMPERATURES IN PLACE. AT THE SAME TIME, THE BUILDING RIDGE ACROSS THE HIGH LATITUDES OF THE PACIFIC WILL SUPPORT SOME DEGREE OF ARCTIC AIR TRANSPORT SOUTHWARD ACROSS CENTRAL CANADA AND INTO THE NORTH CENTRAL U.S. A VIGOROUS SHORTWAVE IS SHOWN BY THE MAJORITY OF GUIDANCE REACHING THE NORTHERN PLAINS BY WED-THU, PUSHING A SURFACE COLD FRONT INTO THE CENTRAL U.S. A COMPLEX FLOW PATTERN EVOLUTION INVOLVING THE EXACT ORIENTATION OF THE BLOCKING RIDGE AND THE DOWNSTREAM SHORTWAVE ENERGY ACROSS CANADA RENDERS THE DEGREE OF COLD AIR TRANSPORT SOUTHWARD INTO THE NORTHERN CONUS UNCERTAIN AT THIS TIME, BUT THERE REMAINS SOME POTENTIAL FOR ARCTIC HIGH PRESSURE TO WORK ITS WAY SOUTHWARD INTO THE NORTHERN PLAINS/MIDWEST BY THE MIDDLE OF NEXT WEEK. GIVEN GENERAL LARGE SCALE AGREEMENT BUT INCREASING DETERMINISTIC SPREAD ON THE SPECIFICS, THE WPC FORECAST WAS TRENDED HEAVILY TOWARD ENSEMBLE MEANS (00Z ECENS/NAEFS/06Z GEFS) DURING DAYS 6-7 (WED-THU). RYAN