370  
FXUS02 KWBC 221600  
PMDEPD  
 
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  
 

 
 
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