EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 248 AM EDT THU OCT 26 2017 VALID 12Z SUN OCT 29 2017 - 12Z THU NOV 02 2017 ...POTENTIAL HEAVY RAINFALL EVENT FOR THE NORTHEASTERN U.S. SUNDAY AND MONDAY... ...OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES... WITH THE DEPARTURE OF AN INITIALLY SHARP/AMPLIFIED EASTERN U.S. TROUGH, GUIDANCE CONTINUES TO SHOW A GENERAL BROADENING TREND FOR CYCLONIC MEAN FLOW ACROSS THE LOWER 48 AS THE MEAN RIDGE OVER THE WEST COAST/EASTERN PACIFIC RETROGRADES WITH TIME. THIS RETROGRESSION IS REPRESENTED IN DAILY FORECASTS BY THE COLLAPSE OF A WESTERN RIDGE ALREADY IN PROGRESS AS OF EARLY SUN FOLLOWED BY THE BUILDING OF A STRONG NORTHEAST PACIFIC RIDGE AND THEN FINALLY INCREASING DOMINANCE OF AN UPSTREAM RIDGE OVER/NEAR ALASKA. THE UNCERTAIN DETAILS OF THIS LATTER RIDGE PRODUCE A CONSIDERABLE PORTION OF MODEL/ENSEMBLE SPREAD BY DAYS 6-7 WED-THU. BY LATE IN THE PERIOD THIS PATTERN EVOLUTION MAY INCREASE POTENTIAL FOR ANY SURGES OF COLD AIR REACHING THE NORTHERN TIER STATES TO HAVE SOME CONNECTION TO ALASKAN/NORTHERN CANADA HIGH PRESSURE VERSUS PREVIOUS HIGHS MIGRATING ACROSS THE NORTH PACIFIC. TELECONNECTION-FAVORED FLOW AT THAT TIME ALSO HIGHLIGHTS THE NORTHERN TIER AS A LIKELY FOCUS FOR BELOW NORMAL TEMPS. APPROXIMATELY THE FIRST HALF OF THE FORECAST PERIOD INCORPORATES IDEAS FROM THE VARIOUS 12Z/18Z OPERATIONAL MODELS. THE FINER DETAILS WILL TAKE MORE TIME TO RESOLVE BUT GUIDANCE IS QUITE AGREEABLE IN PRINCIPLE ON THE IDEA OF EASTERN U.S. TROUGH ENERGY BECOMING NEGATIVELY TILTED AS IT LIFTS NORTHEASTWARD AHEAD OF ANOTHER VIGOROUS SYSTEM DROPPING INTO THE NORTHERN TIER. THERE IS STILL A DECENT SIGNAL THAT THE EAST COAST SYSTEM WILL INCORPORATE LOW PRESSURE/MOISTURE EMERGING FROM THE NORTHWEST CARIBBEAN ULTIMATELY LEADING TO A STRONG/DEEPENING SURFACE LOW TRACKING TOWARD/INTO THE NORTHEAST. AS OF 12Z MON MOST MODELS SHOW A DEPTH IN THE UPPER 970'S/LOW 980'S MB WHICH COULD CHALLENGE OCTOBER SEA LEVEL PRESSURE RECORDS AT SOME LOCATIONS OVER THE NORTHEAST. DEEPENING TRENDS IN THE ENSEMBLE MEANS SUPPORT AN OPERATIONAL MODEL COMPROMISE AT THIS TIME. RECENT FASTER TRENDS IN MOST GUIDANCE FOR THE UPSTREAM SYSTEM WOULD SUGGEST ANY FUTURE ADJUSTMENTS NEAR THE EAST COAST MAY BE TOWARD THE FASTER SIDE OF THE SPREAD. TRENDS WITH THE TRAILING SYSTEM ALSO LEAD TO A FASTER SOUTHWARD PROGRESSION OF THE COLD FRONT OVER THE WEST/PLAINS. AFTER EARLY MON THE ANCHORING SURFACE LOW REACHING THE UPPER GREAT LAKES MAY BECOME ABSORBED INTO THE SOUTHERN PERIPHERY OF THE LEADING SYSTEM'S BROAD CIRCULATION. ALONG THE WEST COAST THERE ARE A COUPLE SHORTWAVE FEATURES FOR WHICH LATEST GUIDANCE SEEMS TO HAVE ACQUIRED GREATER DEFINITION ALBEIT WITH SOME SPREAD FOR SPECIFICS. CONFIDENCE WITH THESE FEATURES IS ON THE LOWER SIDE OF THE SPECTRUM GIVEN THEIR SMALL SCALE. THE LATTER HALF OF THE FORECAST BECOMES DOMINATED BY HOW TROUGH ENERGY EVOLVES DOWNSTREAM FROM THE STRENGTHENING ALASKA RIDGE. THUS FAR THE ECMWF MEAN HAS BEEN THE MOST STABLE AND PERSISTENT GUIDANCE SOURCE IN SHOWING THE DEVELOPMENT OF A TROUGH EXTENDING FROM CANADA INTO THE NORTHEAST PACIFIC, JUST OFF THE PACIFIC NORTHWEST COAST. GEFS MEANS CAUGHT ONTO THIS EVOLUTION IN THE 00Z/25 RUN. OPERATIONAL MODELS HAVE VARIED DETAILS DEPENDING ON CHARACTER OF THE UPSTREAM RIDGE. TELECONNECTIONS RELATIVE TO THE STRONG POSITIVE HEIGHT ANOMALY CENTER NEAR THE SOUTHWEST CORNER OF MAINLAND ALASKA IN LATEST D+8 MULTI-DAY MEANS WOULD FAVOR A PATTERN CLOSEST TO THE 18Z GEFS/12Z ECMWF ENSEMBLE MEANS. THE 18Z GFS APPEARS TO BE ONE OF THE LESS LIKELY SOLUTIONS AS IT HAS UPSTREAM FLOW CUTTING THROUGH THE ALASKA RIDGE. DEEPER NORTHEAST PACIFIC TROUGHING IN 12Z GFS/ECMWF RUNS AND NEW 00Z GFS ALONG WITH SOME SHORTWAVE RIDGING OVER THE WEST-CENTRAL U.S. IS WITHIN THE REALM OF POSSIBILITY BUT LOW ENOUGH IN CONFIDENCE THAT FAR OUT IN TIME TO FAVOR STAYING CLOSER TO THE MEANS. AS FOR WESTERN CANADA LOW PRESSURE TRACKING ALONG/NORTH OF THE U.S.-CANADIAN BORDER TUE-THU, A COMPROMISE AMONG THE 18Z GEFS MEAN/12Z ECMWF AND FARTHER SOUTH 12Z ECMWF MEAN-00Z/25 ECMWF LOOKS REASONABLE. COMBINING THESE CONSIDERATIONS LEADS TO TRENDING THE FORECAST TOWARD A BLEND OF THOSE SOLUTIONS FROM LATE TUE ONWARD. ...WEATHER/THREATS HIGHLIGHTS... GUIDANCE CONTINUES TO TARGET PARTS OF THE NORTHEAST FOR A PERIOD OF VERY INTENSE RAINFALL ALONG WITH STRONG WINDS SUN-MON AS TROPICAL MOISTURE/LOW PRESSURE INTERACT WITH VIGOROUS DYNAMICS ALOFT TO BRING A DEEP LOW PRESSURE SYSTEM ACROSS THE REGION. THIS SYSTEM WILL LIKELY AFFECT SOME AREAS THAT RECEIVED SIGNIFICANT RAINFALL IN THE PRECEDING EVENT. MEANWHILE THE GREAT LAKES WILL SEE PERIODS OF LAKE EFFECT/LAKE ENHANCED RAIN AND SNOW WITH CHILLY CYCLONIC MEAN FLOW IN PLACE AND SUPPORT FROM ONE OR TWO SYNOPTIC SYSTEMS. EXPECT COVERAGE OF RAINFALL TO INCREASE FROM THE SOUTHERN PLAINS NORTHEASTWARD BY TUE-WED AS LOW LEVEL FLOW FROM THE GULF INTERSECTS THE FRONT THAT SHOULD PUSH SOUTHEAST THROUGH THE PLAINS AND THEN STALL/LIFT BACK AS A WARM FRONT. PARTS OF THE ROCKIES AND NEARBY AREAS MAY SEE ONE OR MORE PERIODS OF MOSTLY LIGHT RAIN OR SNOW DEPENDING ON ELEVATION. MINUS 10-20F ANOMALIES FOR TEMPS OVER THE SOUTH AND EAST TO START THE PERIOD SUN SHOULD MODERATE THEREAFTER. THE COLD FRONT DROPPING SOUTH FROM THE NORTHERN PLAINS SUN ONWARD WILL BRING SIMILARLY COOL ANOMALIES ACROSS THE PLAINS DURING MON-TUE. THE LEADING EDGE OF POTENTIALLY COLDER AIR MAY REACH THE NORTHERN PLAINS BY NEXT THU. OVER THE WEST, HIGHS OF 10-20F ABOVE NORMAL SHOULD BE MOST COMMON ON SUN THEN DECREASE IN COVERAGE MON ON THE WAY TO READINGS GENERALLY WITHIN A FEW DEGREES OF NORMAL FOR THE REST OF THE PERIOD. RAUSCH