EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 940 AM EDT THU MAR 15 2018 VALID 12Z SUN MAR 18 2018 - 12Z THU MAR 22 2018 PATTERN OVERVIEW ~~~~~~~~~~~~~~~~ A STRONG POSITIVE ANOMALY SOUTH OF THE ALEUTIANS EXTENDS NORTHWEST WITH TIME INTO THE BERING SEA WITH TIME WHICH RESULTS IN INCREASING MERIDIONAL FLOW NEAR NORTH AMERICA. WHAT STARTS OUT AS A BLOCKY PATTERN IN THE SOUTHERN STREAM EVOLVES INTO AN AMPLIFIED TROUGH/RIDGE/TROUGH PATTERN ACROSS THE EASTERNMOST PACIFIC/PLAINS/EAST BY NEXT THURSDAY. IT IS THE EVOLUTION OF THE UPPER PATTERN EAST OF THE ROCKIES THAT IS THE MOST PROBLEMATIC PORTION OF THE FORECAST AS THREE MID-LEVEL SYSTEMS COULD INTERACT. BINARY AND TRINARY/FUJIWARA-LIKE INTERACTIONS ARE NOTORIOUSLY DIFFICULT TO FORECAST AND KEEP LEADING TO TRACK SHIFTS TO THE SURFACE CYCLONE APPROACHING THE APPALACHIANS AND ASSOCIATED LOW DEVELOPMENT ALONG THE LEADING PORTION OF THE DOWNSTREAM WEDGE NEAR THE EAST COAST. MODEL ASSESSMENT/PREFERENCES ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ THE LATEST GUIDANCE MAINTAINS GOOD AGREEMENT FOR THE FORECAST EVOLUTION OF LARGE SCALE FLOW DURING THE PERIOD. GUIDANCE REMAINS WELL CLUSTERED FOR THE EASTERN PACIFIC UPPER LOW, WITH GENERAL TRENDS BEING SOMEWHAT SLOWER. A DETERMINISTIC AND ENSEMBLE CONSENSUS PROVIDES A GOOD REPRESENTATION OF THE EASTERN PACIFIC/WESTERN U.S. PATTERN THROUGH THE PERIOD. A COMPROMISE OF THE 06Z GFS/00Z ECMWF/00Z CANADIAN WAS FAVORED EARLY BEFORE SWITCHING TO A 06Z GFS/00Z ECMWF/00Z ECMWF ENSEMBLE MEAN/00Z NAEFS MEAN LATE IN THE PERIOD TO ACCOUNT FOR UNCERTAINTY. GUIDANCE HAS MAINTAINED A DECENT OVERALL SIGNAL FOR THE SYSTEM EXPECTED TO EMERGE FROM THE WEST AND TRACK ACROSS THE CENTRAL LATITUDES OF THE PLAINS/EASTERN U.S. WITH COASTAL/OFFSHORE DEVELOPMENT LIKELY AROUND TUESDAY, WITH SOME SLOWING LATE IN THE PERIOD WHICH APPEARS TO BE PARTIALLY DUE TO THE SLOWING OF THE SYSTEM APPROACHING THE WEST COAST. A COMPROMISE OF THE AVAILABLE DETERMINISTIC GUIDANCE FELL NEAR THE MIDDLE OF THE 00Z GLOBAL ENSEMBLE ENVELOPE FOR THE 00Z-06Z CYCLES OF DATA. THE 00Z ECMWF EVENTUALLY BECAME AN OUTLIER BY HOLDING BACK THE UPPER SYSTEM RELATIVE TO CONSENSUS THUS MAINTAINING GREAT LAKES LOW PRESSURE FOR A LONGER DURATION THAN OTHER SOLUTIONS. THE ONLY MEMORABLE OCCASIONS WHERE A NORTH TO NORTHWEST EXTRATROPICAL CYCLONE TRACK FROM THE MID-SOUTH/CENTRAL APPALACHIANS TO MICHIGAN HAVE BEEN NOTED HAVE BEEN WITH POWERFUL CYCLONES (LIKE THE LATE NOVEMBER 1950 STORM AND THE JANUARY 1978 CLEVELAND SUPERBOMB) -- THE ECMWF DOES NOT DEPICT A SYSTEM ANYWHERE NEAR AS STRONG AND ITS UPPER PATTERN DOES NOT APPEAR REMOTELY SIMILAR. AT THE TIME RANGE IT DOES SO -- WEDNESDAY INTO NEXT THURSDAY -- THE OPERATIONAL ECMWF HAS RELATIVELY LITTLE ENSEMBLE SUPPORT SO DID NOT CONSIDER ITS SOLUTION USEFUL AFTER EARLY NEXT WEEK. THERE IS SIGNIFICANT ENSEMBLE SPREAD WITH THE EXPECTED COASTAL LOW, BUT AT LEAST THE RESULTING MEANS/06Z GFS ARE RELATIVELY CLOSE. ONCE THE SYSTEM REACHES THE EAST THE DETAILS OF NORTHERN STREAM INVOLVEMENT CONTINUE TO BE A SIGNIFICANT UNCERTAINTY. RUN-TO-RUN AND DAY-TO-DAY CHANGES IN THESE DETAILS WILL LIKELY PESTER THE GUIDANCE EVEN WITHIN THE SHORT RANGE/DAYS 1-3 PERIOD AND PRODUCE SOME TIMING AND LATITUDE ADJUSTMENTS FOR THE SURFACE SYSTEM IN FUTURE CYCLES. FOR NOW, A COMPROMISE OF THE 00Z UKMET/00Z CANADIAN/00Z ECMWF/06Z GFS WAS FAVORED FROM SUNDAY INTO TUESDAY BEFORE SWITCHING MORE TOWARDS A 06Z GFS/00Z ECMWF ENSEMBLE MEAN/00Z NAEFS MEAN SOLUTION THEREAFTER. THE 06Z GEFS MEAN WAS A REASONABLE ALTERNATIVE. WEATHER HIGHLIGHTS/THREATS ~~~~~~~~~~~~~~~~~~~~~~~~~~ EXPECT MOST INTERIOR WEST/ROCKIES PRECIP ASSOCIATED WITH THE INITIAL UPPER TROUGH/LOW OVER THE WEST TO CONCLUDE BY THE END OF THE WEEKEND. THERE WILL BE A BRIEF PERIOD OF RELATIVELY DRY CONDITIONS OVER THE WEST BEFORE FLOW AHEAD OF THE EASTERN PACIFIC UPPER LOW WILL BRING INCREASING MOISTURE INTO THE WESTERN U.S. FROM ABOUT TUE ONWARD. CURRENTLY THE BEST SIGNAL FOR GREATEST RAINFALL/HIGHER ELEVATION SNOW TOTALS IS OVER FAVORED TERRAIN IN THE SIERRA NEVADA RANGE. THERE IS ALSO POTENTIAL FOR ONE OR MORE PERIODS OF FOCUSED MODERATE TO HEAVY PRECIP ALONG THE COASTAL RANGES FROM NORTHERN THROUGH SOUTH-CENTRAL CA. SOME OF THIS MOISTURE WILL STREAM INTO THE INTERIOR WEST/ROCKIES. CHILLY DAYTIME HIGHS OVER THE WEST WILL BECOME LESS EXTREME AFTER SUN-MON WHILE INCOMING CLOUDS/MOISTURE THEREAFTER SHOULD STEADILY RAISE MIN TEMPS TO ABOVE NORMAL BY WED-THU. THE SYSTEM TRACKING OUT OF THE CENTRAL PLAINS WILL SPREAD A BROAD PRECIP SHIELD ACROSS THE CENTRAL/EASTERN U.S. THE BEST POTENTIAL FOR MEANINGFUL SNOW CONTINUES TO EXTEND FROM THE NORTHERN PLAINS INTO THE NORTHEAST. PRECIP TYPE OVER SOME LOCATIONS FROM THE MIDWEST, OHIO VALLEY, AND MID ATLANTIC REGIONS REMAIN SENSITIVE TO THE EXACT TRACK OF LOW PRESSURE. SOME LOCALLY MODERATE/HEAVY RAINFALL WILL BE POSSIBLE ACROSS THE SOUTHERN TIER. ONCE THE SYSTEM'S ENERGY TRANSFERS TO THE EAST COAST THERE IS STILL A FAIR DEGREE OF UNCERTAINTY OVER TIMING/STRENGTH AND THUS ASSOCIATED WIND/PRECIP EFFECTS. FOR SUN-MON THE NORTHEAST WILL BE QUITE CHILLY WHILE WELL ABOVE NORMAL TEMPS WILL PREVAIL ACROSS THE SOUTHERN TIER, WITH SOME MINUS OR PLUS 10-20F ANOMALIES RESPECTIVELY. BELOW NORMAL TEMPS WILL THEN SPREAD THROUGH THE EASTERN HALF OF THE NATION TUE-THU BEHIND THE PLAINS TO ATLANTIC LOW PRESSURE. THE NORTHERN PLAINS SHOULD REMAIN QUITE COOL THROUGH THE PERIOD WHILE THE CENTRAL-SOUTHERN HIGH PLAINS WILL START THE PERIOD WARM, FALL BACK TO NORMAL, THEN LIKELY BECOME QUITE WARM AGAIN BY NEXT THU. ROTH/RAUSCH