EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1125 AM EDT TUE MAR 13 2018 VALID 12Z FRI MAR 16 2018 - 12Z TUE MAR 20 2018 15 UTC UPDATE... SPLIT FLOW PATTERN LOOKS PERSISTENT ACROSS THE CONUS DURING THE MEDIUM RANGE. WITH NEGATIVE HEIGHT ANOMALIES ALONG/OFF THE WEST COAST, AND ACROSS NEW ENGLAND/EASTERN CANADA, AND FAIRLY PROGRESSIVE FLOW IN BETWEEN, FROM THE ROCKIES TO THE OH VALLEY. MODEL/ENSEMBLE SPREAD IS RELATIVELY LOW EARLY IN THE PERIOD, BUT INTERACTION OF ENERGY FROM THE SOUTHERN/NORTHERN STREAMS ACROSS THE EASTERN U.S. LATE IN THE PERIOD INCREASES UNCERTAINTY RATHER SIGNIFICANTLY BY EARLY NEXT WEEK. A MULTI-MODEL DETERMINISTIC BLEND INCLUDING THE 00Z ECMWF/UKMET/06Z GFS SERVED AS A BASIS FOR THE FORECAST DURING DAYS 3-5 (FRI-SUN). THESE SOLUTIONS SHOWED FAIRLY SMALL DIFFERENCES (PRIMARILY WITH RESPECT TO TIMING) WITH RESPECT TO SHORTWAVE ENERGY/SURFACE LOW CROSSING THE CENTRAL PLAINS/MID-MS VALLEY DURING THIS TIME FRAME. THE SOLUTIONS ARE ALSO IN GENERAL AGREEMENT THAT THE SYSTEM BECOMES MORE SUPPRESSED BY THE TIME IT REACHES THE EAST COAST, WITH THE SURFACE LOW EXPECTED TO QUICKLY MOVE WELL OFFSHORE BY DAY 5 (SUN). ADDITIONAL SHORTWAVE CROSSING THE ROCKIES ON DAY 5, ALONG WITH NORTHERN STREAM STREAM ENERGY ORIGINATING IN ARCTIC CANADA, DIVING SOUTHWARD IN THE VICINITY OF HUDSON BAY, WILL BE KEY IN WHAT EXACTLY OCCURS FROM THE OH VALLEY TO THE EASTERN U.S. DURING DAYS 6-7 (MON-TUE). THE GFS AND ECMWF HAVE BOTH SHOWN A MODERATE DEGREE OF RUN-TO-RUN VARIABILITY BY THIS TIME PERIOD, WITH RESPECT TO THE FLOW EVOLUTION ACROSS THE MIDWEST/EAST AS WELL AS WITH VIGOROUS ENERGY DIGGING SOUTHWARD OFF THE WEST COAST. A TREND WAS NOTED AMONG THE OVERNIGHT GUIDANCE TOWARD A CUTOFF LOW DIGGING FARTHER SOUTH OFF THE WEST COAST, WHICH KEEPS SOMEWHAT STRONGER RIDGING IN PLACE FROM THE ROCKIES TO THE HIGH PLAINS, AND ALLOWS ENERGY ACROSS THE EASTERN U.S. TO AMPLIFY MORE QUICKLY. ALL GUIDANCE GENERALLY SHOW A SURFACE LOW CROSSING THE CENTRAL PLAINS AND OH/TN VALLEYS MON/MON NIGHT. ONE MAJOR QUESTION IS HOW QUICKLY ENERGY TRANSFERS TO A NEW SURFACE LOW EAST OF THE APPALACHIANS. OVERNIGHT GUIDANCE TRENDED TOWARD THIS PROCESS OCCURRING QUICKER, WITH THE 06Z GFS SHOWING A SURFACE LOW NEAR THE OUTER BANKS BY 12Z ON DAY 7 (TUE). ENSEMBLE SPREAD IS QUITE A BIT LARGER THAN THE APPARENT SIMILAR TRENDS AMONG THE ECMWF/GFS WOULD SUGGEST, WITH SURFACE LOW SCATTER PLOTS FOR DAY 7 SHOWING ECENS/GEFS LOWS FROM OFF THE MID-ATLANTIC COAST TO THE MS/TN VALLEYS. THUS, WAS NOT CONFIDENT ENOUGH IN THE TREND TO ADJUST THE FORECAST COMPLETELY IN THAT DIRECTION (GIVEN THE SLOWER ENERGY TRANSFER INDICATED IN THE OVERNIGHT ISSUANCE). THUS ENSEMBLE (00Z ECENS/06Z GEFS) MEAN WEIGHTING WAS INCREASED SUBSTANTIALLY DURING THE DAY 6-7 TIME FRAME, WITH 20 PERCENT OF WPC CONTINUITY ALSO INCLUDED TO TEMPER THE CHANGES. THIS RESULTED IN A DAY 7 SURFACE LOW POSITION ACROSS THE SOUTHERN APPALACHIANS, WITH THE ENERGY TRANSFER LIKELY IN THE PROCESS OF OCCURRING AROUND THAT TIME. RYAN PREVIOUS DISCUSSION (ISSUED AT 0648 UTC)... ...PATTERN OVERVIEW/MODEL ASSESSMENT/PREFERENCES... THERE ARE PLENTY OF UNCERTAINTIES THAT REMAIN WITH DETAILS OF THE FORECAST BUT COMPARED TO 24 HOURS AGO THERE ARE SOME THEMES THAT SEEM TO BE A LITTLE MORE COMMON IN THE GUIDANCE. THIS IS PARTICULARLY THE CASE FOR UPPER TROUGH/CLOSED LOW ENERGY FORECAST TO DROP INTO THE EASTERN PACIFIC DURING THE WEEKEND AND POSSIBLY DRIFT TOWARD THE WEST COAST NEXT WEEK. THE MOST PROMINENT TREND HAS BEEN RECENT GFS/GEFS RUNS JOINING THE ECMWF/ECMWF MEAN/CMC THAT HAVE CONSISTENTLY BEEN SLOWER/MORE AMPLIFIED. THERE IS STILL MEANINGFUL SPREAD FOR THE ULTIMATE PATH OF THE UPPER LOW, WITH SUBTLE DETAIL DIFFERENCES IN SURROUNDING FLOW POTENTIALLY LEADING TO VERY DIFFERENT OUTCOMES. TELECONNECTIONS RELATIVE TO THE UPSTREAM POSITIVE HEIGHT ANOMALY CENTER SEEM TO FAVOR A MIDDLE GROUND AMONG THE CURRENT SPREAD. THERE IS GOOD AGREEMENT IN PRINCIPLE THAT THE AFOREMENTIONED AMPLIFYING TROUGH/UPPER LOW WILL HELP TO EJECT INITIAL WEST COAST TROUGH ENERGY THROUGH THE WEST AND BEYOND. EXACTLY WHAT FORM THE EJECTING TROUGH WILL TAKE IS STILL UP FOR DEBATE THOUGH, AS THERE ARE INDICATIONS THAT SOME OF THE INITIAL UPPER LOW ENERGY MAY LINGER NEAR THE PACIFIC NORTHWEST INSTEAD OF BEING CARRIED ALONG IN THE TROUGH. ALSO THERE IS THE QUESTION OF HOW ENERGY THAT DOES EJECT EASTWARD EVOLVES AND ITS EFFECT ON LOW PRESSURE THAT IS LIKELY TO TRACK EASTWARD FROM THE HIGH PLAINS DURING THE LATTER HALF OF THE PERIOD. TRENDS FOR THIS SYSTEM HAVE GENERALLY BEEN SLOWER OVER THE PAST DAY. DURING THE FIRST HALF OF THE PERIOD THE MOST NOTABLE TRENDS ARE WITH FLOW TO THE SOUTH AND EAST OF THE INITIAL WEST COAST UPPER LOW. LATEST CONSENSUS IS FOR FLOW OVER THE SOUTHWEST U.S. TO BE LESS AMPLIFIED, LEADING TO A LESS INTENSE MOISTURE FOCUS OVER THAT AREA, PLUS A CLEARER SIGNAL THAT A BUNDLE OF LEADING SHORTWAVE ENERGY (POSSIBLY A CLOSED LOW) SHOULD EMERGE OVER THE CENTRAL PLAINS AROUND FRI. FOR THIS LEADING FEATURE 00Z GFS TRENDS TOWARD RECENT ECMWF/UKMET/CMC RUNS ADD CONFIDENCE TO THE GENERAL SCENARIO ALOFT. HOWEVER THE SURFACE REFLECTION IS MORE UNCERTAIN. THE ONGOING TREND OF GUIDANCE SHOWING LONGER PERSISTENCE OF CYCLONIC MEAN FLOW ALOFT OVER THE NORTHEAST ALONG WITH OCCASIONAL TENDENCY FOR GUIDANCE TO EXTEND LOW PRESSURE TOO FAR NORTH INTO COOL AIR LEAD TO SOME SKEPTICISM OF THE MORE NORTHERN CMC/UKMET RUNS FOR THE SURFACE LOW. IN GENERAL THE NORTHEASTERN U.S. TRENDS ALOFT OVER THE PAST COUPLE DAYS HAVE LED TO A MORE SUPPRESSED FRONT OVER THE EASTERN HALF OF THE COUNTRY AHEAD OF THE SUN-TUE SYSTEM. BASED ON GUIDANCE AVAILABLE THROUGH THE 12Z/18Z CYCLES, A MODEL/ENSEMBLE MEAN BLEND--INCORPORATING VARIOUS IDEAS OF GFS/ECMWF/CMC RUNS AND 12Z GEFS/ECMWF MEANS--PROVIDED THE BEST STARTING POINT FOR THE FORECAST. CONFIDENCE AT THE TIME WAS A LITTLE LOWER WITH STRENGTH OF LEADING CENTRAL PLAINS ENERGY ALOFT ON FRI GIVEN THE GFS WAS STILL RATHER WEAK. OTHERWISE THE MODEL/MEAN BLEND PROVIDED A GOOD BALANCE OF DEFINITION VERSUS FORECAST UNCERTAINTY FOR THE SYSTEM EMERGING FROM THE WEST AND THEN TRACKING INTO THE EAST ALONG WITH THE EVOLUTION OFF THE WEST COAST. ...WEATHER HIGHLIGHTS/THREATS... THE INITIAL SYSTEM NEAR THE WEST COAST WILL SPREAD A FAIRLY EXPANSIVE SHIELD OF RAIN AND HIGHER ELEVATION SNOW ACROSS THE WEST WITH HIGHEST TOTALS LIKELY TO BE OVER THE SIERRA NEVADA RANGE LATE THIS WEEK. GUIDANCE TRENDS WITH FLOW ALOFT HAVE LED TO LIGHTER PRECIP EXPECTATIONS OVER SOUTHERN CA AND THE SOUTHWEST. EJECTION OF THE UPPER TROUGH WILL PROMOTE A DRIER TREND AT SOME LOCATIONS. ANOTHER SYSTEM EVOLVING OFFSHORE MAY BRING ANOTHER AREA OF MOISTURE TO PORTIONS OF THE WEST COAST DURING THE FIRST HALF OF NEXT WEEK. HOW MUCH MOISTURE ARRIVES AND WHEN IS STILL FAIRLY UNCERTAIN. SYSTEM OVER THE PLAINS ON FRI WILL BE ACCOMPANIED BY AN AREA OF PRECIP MOST LIKELY TO EXTEND FROM THE NORTHERN HIGH PLAINS INTO THE SOUTHERN HALF OF THE EASTERN U.S. SOME SNOW MAY BE POSSIBLE OVER THE NORTHERN HIGH PLAINS. BEHIND THIS FEATURE, WEAK SUBTROPICAL ENERGY ALOFT ALONG WITH A LINGERING SURFACE FRONT MAY GENERATE AN EPISODE OF LOCALLY MODERATE TO HEAVY RAINFALL OVER THE EXTREME SOUTHERN TIER. THEN THE SUN-TUE SYSTEM TRACKING FROM THE PLAINS INTO THE EAST WILL BRING ANOTHER PRECIP EVENT TO THE CENTRAL/EASTERN STATES WITH WINTRY WEATHER POSSIBLE IN THE NORTHERN PART OF THE MOISTURE SHIELD. THE SOUTHERN 2/3 OF THE WEST AND NORTHERN HIGH PLAINS WILL LIKELY SEE MULTIPLE DAYS OF WELL BELOW NORMAL TEMPS ESPECIALLY FOR DAYTIME HIGHS. THE NORTHERN 2/3 OF THE EAST WILL SEE CHILLY READINGS FRI-SAT AS WELL, WITH SOME MINUS 10-15F ANOMALIES IN BOTH CASES. PERSISTENCE OF CYCLONIC FLOW ALOFT OVER THE NORTHEAST WILL KEEP THAT PART OF THE COUNTRY ON THE COOL SIDE BUT WITH LESS EXTREME ANOMALIES THAN IN THE EARLY PART OF THE PERIOD. THE SOUTHERN HALF OF THE PLAINS AND AT TIMES INTO THE GULF COAST STATES SHOULD BE WARMEST ANOMALY-WISE WITH SOME DAYS 10F OR MORE ABOVE NORMAL. RAUSCH