EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1059 AM EST THU FEB 22 2018 VALID 12Z SUN FEB 25 2018 - 12Z THU MAR 01 2018 DUE TO TECHNICAL DIFFICULTIES, SOME PRODUCTS MAY NOT BE AVAILABLE OR UPDATED. PLEASE CHECK ISSUANCE TIMES. WE APOLOGIZE FOR ANY INCONVENIENCE. ...PATTERN OVERVIEW... MID-LATITUDE FLOW SHOULD EXPERIENCE SOME DEGREE OF TRANSITION WITH WEAKENING OF THE PERSISTENT POSITIVE HEIGHT ANOMALY OVER THE WESTERN ATLANTIC AND CORRESPONDING SHIFT OF RIDGE EMPHASIS INTO THE GULF OF MEXICO. HOWEVER THE STRONG AND PERSISTENT MEAN RIDGE OVER THE EAST-CENTRAL PACIFIC WILL CONTINUE TO FAVOR A LONG TERM TROUGH NEAR THE WEST COAST OR 120W LONGITUDE. INDIVIDUAL SHORTWAVES EMBEDDED WITHIN THIS WEST COAST MEAN TROUGH WILL HAVE THE POTENTIAL TO BRING VARIOUS IMPACTS TO PARTS OF THE WEST AND THEN TO LOCATIONS DOWNSTREAM AS THE FEATURES EJECT DOWNSTREAM. EXPECT A LEADING VIGOROUS SYSTEM TO BE NEAR THE UPPER GREAT LAKES AT THE START OF THE FORECAST EARLY SUN. SPECIFICS FOR TRAILING SHORTWAVES DIGGING INTO THE WEST COAST MEAN TROUGH BECOME MORE UNCERTAIN TOWARD THE MIDDLE OF NEXT WEEK. ...MODEL GUIDANCE/UNCERTAINTY ASSESSMENT/PREFERENCES... A COMPROMISE INVOLVING MOSTLY THE 06Z GFS/00Z ECMWF PROVIDES A REASONABLE STARTING POINT FOR THE UPPER GREAT LAKES SYSTEM AS OF EARLY DAY 3 SUN. ENSEMBLES GENERALLY REFLECT TRAITS OF THEIR PARENT MODEL, WITH MOST GEFS MEMBERS TENDING TO BE A LITTLE SLOWER THAN ECMWF MEMBERS WHILE CMC MEMBERS POPULATE THE SOUTHERN PART OF THE TRACK SPREAD WITH RECENT OPERATIONAL CMC RUNS LIKEWISE FARTHER SOUTH THAN GFS/ECMWF RUNS. STRENGTH HAS VARIED CONSIDERABLY IN RECENT DAYS. GENERAL MODEL TRENDS WERE DEEPER GOING INTO YESTERDAY BUT HAVE RELAXED SOMEWHAT SINCE THEN, INCLUDING THE 06Z GFS RELATIVE TO THE 00Z RUN. THE GFS HAS FAIRLY CONSISTENTLY BEEN ON THE DEEP SIDE OF THE ENVELOPE. GUIDANCE IS NOW BETTER CLUSTERED FOR THE WEAK WAVE LIKELY TO DEVELOP NEAR AND TRACK AWAY FROM THE NEW ENGLAND COAST. FROM MON ONWARD THERE IS MEANINGFUL SPREAD FOR SPECIFICS ALONG THE U.S.-CANADIAN BORDER FROM THE NORTHERN PLAINS EASTWARD BUT THE MOST PROMINENT UNCERTAINTY INVOLVES FLOW AROUND THE NORTHERN PERIPHERY OF THE EAST-CENTRAL PACIFIC RIDGE AND DOWNSTREAM EFFECTS OF FLOW DETAILS. THERE IS A SPLIT IN GUIDANCE OVER PROGRESSION OF LEADING ENERGY REACHING THE BERING SEA AROUND SUN, WITH THE GFS/CMC AND THEIR MEANS BRINGING IT INTO WESTERN NORTH AMERICA BY AROUND TUE WHILE THE ECMWF/ECMWF MAINTAIN A ROBUST LEADING RIDGE. THEN THE ECMWF/ECMWF MEAN/CMC BECOME MUCH MORE AMPLIFIED WITH TRAILING ENERGY REACHING THE NORTHEAST PACIFIC BY LATE IN THE PERIOD. BY DAY 7 THU THESE DIFFERENCES LEAD TO A RANGE OF SOLUTIONS FROM AN OFFSHORE CLOSED LOW (ECMWF/CMC, AND BARELY OFFSHORE IN THEIR MEANS) TO AN OPEN TROUGH ALONG THE COAST. TELECONNECTIONS RELATIVE TO THE EAST-PACIFIC MEAN RIDGE FAVOR AN INTERMEDIATE SOLUTION THAT LEANS A BIT AWAY FROM THE FARTHEST WEST SIDE OF THE SPREAD (ESPECIALLY THE CMC AND TO SOME EXTENT ECMWF) LATE IN THE PERIOD. CONFIDENCE IS LOWER ON EJECTION OF EARLY-MID WEEK WESTERN TROUGH ENERGY AS APPROACH OF UPSTREAM FLOW COULD WELL ENCOURAGE SOMEWHAT EARLIER DEPARTURE FROM THE WEST THAN FORECAST BY THE ECMWF/CMC. INTERESTINGLY THE 06Z GFS HAS TRENDED SLOWER WITH THIS EJECTION VERSUS THE 00Z RUN BUT THE 06Z GEFS MEAN HAS TRENDED FASTER. THE 00Z GEFS MEAN COMPARES BETTER TO THE OTHER 00Z MEANS FOR POSSIBLE LOW PRESSURE NEAR THE MS VALLEY BY EARLY DAY 7 THU. LATEST FORECAST UPDATE INCORPORATES 06Z GFS/00Z ECMWF GUIDANCE PLUS A SMALL 00Z CMC WEIGHT AT THE START OF THE PERIOD, FOLLOWED BY GRADUALLY INCREASING 00Z GEFS/ECMWF ENSEMBLE MEAN COMPONENTS SUCH THAT THE DAY 7 THU FORECAST LEANS 80 PERCENT TO THE MEANS. ...WEATHER HIGHLIGHTS/THREATS... STRONG LOW PRESSURE REACHING THE UPPER GREAT LAKES BY SUN AND CONTINUING NORTHEASTWARD SHOULD BRING AN AXIS OF WINTRY PRECIP TO THE NORTH/WEST OF THE LOW TRACK. SOME SNOW IS ALSO POSSIBLE OVER PARTS OF NEW ENGLAND WITH A WAVE TRACKING OFFSHORE POSSIBLY HELPING TO KEEP COOL AIR OVER THE REGION. SOME AREAS MAY SEE FAIRLY STRONG WINDS AS WELL. THE COLD FRONT TRIALING FROM THIS SYSTEM SPREAD RAINFALL ALONG THE EAST COAST AND ACROSS THE SOUTH. MOISTURE/RAINFALL OF VARYING INTENSITY MAY LINGER OVER THE SOUTH INTO MON. NORTHEAST PACIFIC TROUGH ENERGY DIGGING INTO THE WEST LATE WEEKEND ONWARD, POSSIBLY CLOSING OFF AN UPPER LOW, WILL LIKELY FOCUS A PERIOD OF LOCALLY MODERATE/HEAVY RAIN AND HIGHER ELEVATION SNOW OVER WESTERN OR/WA WITH HIGHEST TOTALS OVER THE CASCADES. A PORTION OF THIS MOISTURE MAY EXTEND INTO THE ROCKIES. EXPECT PRECIP TO SPREAD SOUTHWARD INTO CA/GREAT BASIN EARLY NEXT WEEK WHILE ASSOCIATED COLD AIR SIGNIFICANTLY LOWERS SNOW LEVELS, WITH MOISTURE THEN CONTINUING INTO THE SOUTHWEST/SOUTHERN ROCKIES. EVENTUAL EJECTION OF THIS SYSTEM SHOULD ENCOURAGE LOW PRESSURE TO DEVELOP OVER THE CENTRAL U.S. BY AROUND MIDWEEK WITH LEADING RETURN FLOW SUPPORTING ANOTHER EPISODE OF POTENTIALLY HEAVY RAINFALL FROM THE ARKLATEX REGION EASTWARD/NORTHEASTWARD. THERE IS A FAIR DEGREE OF UNCERTAINTY OVER SPECIFICS OF THIS EVENT THOUGH. HYDROLOGIC ISSUES OVER THIS REGION MAY BE AN ONGOING CONCERN GIVEN PRIOR HEAVY/EXCESSIVE RAINFALL. RELOADING OF THE MEAN TROUGH ALOFT TOWARD WED-THU SHOULD SPREAD ADDITIONAL AREAS OF RAIN/HIGHER ELEVATION SNOW OVER PARTS OF THE WEST BUT UNCERTAINTY IN SPECIFICS OF FLOW ALOFT MAKE IT DIFFICULT AT THIS TIME TO DETERMINE COVERAGE/INTENSITY/TIMING. THE EASTERN HALF OF THE COUNTRY SHOULD CONTINUE TO SEE ABOVE NORMAL TEMPS FOR MOST OF THE PERIOD. EXPECT GREATEST ANOMALIES FOR MIN TEMPS OVER THE APPALACHIANS/EAST COAST SUN-MON WHERE PLUS 20-30F ANOMALIES MAY EXCEED DAILY RECORDS. POTENTIAL SURGE OF MOISTURE AHEAD OF A DEVELOPING CENTRAL U.S. SYSTEM BY WED-THU MAY LEAD TO ANOTHER PERIOD OF VERY WARM MIN TEMPS. EXCEPT FOR MAX TEMPS ALONG THE EAST COAST ON SUN, DAYTIME HIGHS OVER THE EAST WILL BE MORE MODERATE BUT STILL GENERALLY ABOVE NORMAL. MEANWHILE THE PERSISTENCE OF UPPER TROUGHING WILL KEEP TEMPS BELOW TO WELL BELOW AVERAGE ACROSS MUCH OF THE WEST WITH THE COLD AIR EXTENDING THROUGH THE NORTHERN PLAINS. THE NORTHERN HIGH PLAINS SHOULD SEE READINGS 15-20F BELOW NORMAL MOST DAYS AND THE AREA ENCOMPASSING OR/NORTHERN CA/GREAT BASIN LOOKS TO BE ANOTHER AREA WITH MULTIPLE DAYS 10F OR MORE BELOW NORMAL. RAUSCH