578  
FXUS02 KWBC 020657  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
156 AM EST TUE JAN 02 2018  
 
VALID 12Z FRI JAN 05 2018 - 12Z TUE JAN 09 2018  
   
..OVERVIEW
 
 
MODELS AND ENSEMBLES AGREE ON THE IDEA THAT PROGRESSION OF EAST  
PACIFIC TROUGH ENERGY WILL LEAD TO A FLATTENING OF THE MEAN  
PATTERN AND HELP TO DISLODGE THE DEEP EASTERN U.S. UPPER TROUGH  
ASSOCIATED WITH THE CONTINUATION OF VERY COLD TEMPS OVER THE  
REGION THROUGH LATE THIS WEEK. SOME DETAILS OF THE INITIAL EAST  
PACIFIC UPPER TROUGH AS WELL AS ASSOCIATED SURFACE  
EVOLUTION/SENSIBLE WEATHER EFFECTS ACROSS THE LOWER 48 ARE STILL  
FAIRLY UNCERTAIN THOUGH. BY THE LATTER HALF OF THE PERIOD THERE  
ARE ALSO ISSUES WITH ANY SOUTHERN STREAM PACIFIC ENERGY LEFT  
BEHIND FROM THE FIRST TROUGH, WHILE AMPLIFYING/SHARPENING NORTHERN  
STREAM ENERGY WILL LIKELY DIG ANOTHER TROUGH OFF THE WEST COAST BY  
NEXT TUE.  
 
   
..GUIDANCE ASSESSMENT AND PREFERENCES
 
 
FROM THE START OF THE PERIOD ONWARD, INDIVIDUAL MODELS AND  
ENSEMBLES CONTINUE TO HAVE DIFFICULTY IN RESOLVING THE DETAILS OF  
ENERGY WITHIN THE UPPER TROUGH APPROACHING THE WEST COAST AS OF  
EARLY FRI. THE MOST COMMON THEME AMONG THE GUIDANCE IS THE  
POTENTIAL FOR AN EMBEDDED LOW TO CLOSE OFFSHORE AND THEN MOVE  
INLAND SOMEWHERE BETWEEN THE PACIFIC NORTHWEST AND BC. AMONG  
RECENT SOLUTIONS THE FAST-EJECTING 12Z GFS WAS THE ONLY ONE TO  
DIFFER FROM OTHER GUIDANCE ENOUGH TO BE CONSIDERED MUCH LOWER IN  
PROBABILITY.  
 
WITH TIME THESE EARLY PERIOD ISSUES TRANSLATE DOWNSTREAM, FIRST IN  
TERMS OF TROUGH AMPLITUDE AS THE ENERGY CROSSES THE WEST AND THEN  
FOR HOW LOW PRESSURE AND ASSOCIATED FRONTAL SYSTEM EVOLVE ACROSS  
THE EASTERN HALF OF NORTH AMERICA. FOR EXAMPLE BY EARLY DAY 6 MON  
THE 12Z ENSEMBLE SPREAD SHOWS POTENTIAL FOR ONE OR MORE SURFACE  
LOWS ANYWHERE WITHIN AN AXIS FROM EAST-CENTRAL CANADA THROUGH THE  
GREAT LAKES AND INTO THE SOUTHERN PLAINS. DIFFERENCES AMONG  
RECENT OPERATIONAL MODEL RUNS ARE CONSISTENT WITH THIS ENSEMBLE  
SPREAD. THE PAST 24 HOURS OR SO OF ENSEMBLE MEANS HAVE BEEN  
SHOWING A SURFACE LOW TRACK FROM THE UPPER GREAT LAKES/ONTARIO  
EASTWARD MON-TUE AS THE MOST LIKELY SCENARIO. THE 18Z GEFS MEAN  
STRAYED NORTH OF THAT TRACK WHILE SOME OPERATIONAL GUIDANCE HAS  
BEEN A LITTLE SOUTH, LEAVING THE 12Z ENSEMBLE MEANS AS THE BETTER  
STARTING POINT OVERALL. BY THE LATTER HALF OF THE PERIOD A  
FURTHER COMPLICATING FACTOR IS CANADIAN FLOW THAT MAY DROP SOUTH  
INTO THE NORTHERN TIER U.S. AND INTERACT WITH THE CONUS TROUGH,  
WITH RECENT OPERATIONAL GFS/ECMWF RUNS ON THE MORE AGGRESSIVE  
SIDE. THE LONG TERM MEAN PATTERN OFFERS SUPPORT FOR THE STRONGER  
AMPLIFICATION WHILE THE FULL ARRAY OF ENSEMBLES/OTHER MODELS WOULD  
SUGGEST THAT A FAIR AMOUNT OF UNCERTAINTY REMAINS.  
 
CONFIDENCE REMAINS LOW WITH THE EVOLUTION AND TRACK/TIMING OF  
SOUTHERN STREAM PACIFIC ENERGY THAT LINGERS OFFSHORE AFTER PASSAGE  
OF THE INITIAL TROUGH. MODELS/ENSEMBLES ARE MIXED FOR DETAILS OF  
THIS ENERGY. THEN WITH SOME SIMILARITIES TO THE PRECEDING TROUGH,  
AMPLIFYING ENERGY OFF THE WEST COAST EARLY NEXT WEEK HAS POTENTIAL  
SPLIT/CLOSE OFF ONE OR MORE UPPER LOWS PER OPERATIONAL GUIDANCE  
WHILE THE ENSEMBLE MEANS HAVE AN OPEN TROUGH AS WOULD BE EXPECTED  
FROM AVERAGING DIVERSE SOLUTIONS AT THAT TIME FRAME.  
 
THE EVOLUTION OF THE POWERFUL WESTERN ATLANTIC STORM TRACKING INTO  
THE CANADIAN MARITIMES BY EARLY FRI NOW FALLS WITHIN THE SHORT  
RANGE TIME FRAME. THERE IS STILL MODERATE ENSEMBLE SPREAD AS OF  
12Z FRI BUT BETTER CLUSTERING AMONG OPERATIONAL RUNS.  
 
EXCEPT FOR EARLY IN THE PERIOD WHERE BLENDING MOSTLY OPERATIONAL  
GUIDANCE PROVIDED A LITTLE BETTER BALANCE OF DETAIL/ACCOUNT FOR  
UNCERTAINTY WITH SIGNIFICANT FEATURES, THE UPDATED FORECAST  
EMPLOYED A MODEL/ENSEMBLE MEAN BLEND WITH 40-70 PERCENT INPUT FROM  
THE MEANS (LOWER END OF THAT RANGE DAYS 4-5 AND HIGHER END DAYS  
6-7) DUE TO THE LACK OF AGREEMENT AMONG THE OPERATIONAL RUNS FOR  
SPECIFICS. THIS YIELDED ONLY MODEST CONTINUITY CHANGES IN MOST  
RESPECTS.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE TIGHT GRADIENT BETWEEN THE VERY DEEP STORM SYSTEM LIFTING  
THROUGH THE CANADIAN MARITIMES AND EAST-CENTRAL U.S. HIGH PRESSURE  
WILL KEEP STRONG WINDS OVER NEW ENGLAND FRI INTO SAT, WITH BLOWING  
SNOW OVER SOME AREAS AND DANGEROUSLY COLD WIND CHILLS. EXPECT A  
BROAD AREA OF TEMPS 20-30F BELOW NORMAL OVER THE EAST LATE WEEK  
INTO THE WEEKEND WITH PARTS OF NEW ENGLAND POSSIBLY SEEING SOME  
READINGS 30-40F BELOW NORMAL AS THE CORE OF UPPER TROUGHING  
CROSSES THE REGION. DAILY RECORDS FOR COLD HIGHS SHOULD BE MORE  
NUMEROUS THAN RECORD LOWS AND LIKELY OCCUR FRI-SAT. DAILY RECORD  
COLD VALUES MAY LINGER OVER THE NORTHEAST INTO SUN. WITH TIME THE  
PROGRESSION OF UPSTREAM FLOW WILL BRING A PRONOUNCED WARMING TREND  
FROM THE CENTRAL U.S. EASTWARD. AS THIS OCCURS EXPECT AREAS OF  
LAKE EFFECT SNOW TO DIMINISH.  
 
INITIAL TROUGH ENERGY NEAR THE WEST COAST SHOULD BRING AN EARLY  
PERIOD OF RAIN AND HIGH ELEVATION SNOW FAVORING NORTHERN  
CALIFORNIA AND THE PACIFIC NORTHWEST. PRECIP WILL SPREAD INLAND  
THROUGH THE ROCKIES AS ENERGY ALOFT PROGRESSES ALONG, WITH  
NORTHERN/CENTRAL AREAS MOST LIKELY TO SEE THIS ACTIVITY. THEN THE  
LEADING SURFACE LOW/FRONTAL SYSTEM WILL ENCOURAGE DEVELOPMENT OF  
SNOW NORTH/RAIN SOUTH OVER THE EASTERN PLAINS/MS VALLEY AROUND  
EARLY SUN WITH THE MOISTURE SHIELD CONTINUING EASTWARD THEREAFTER.  
SOME PRECIP MAY BE HEAVY GIVEN DECENT LOW LEVEL FLOW FROM THE  
GULF AHEAD OF THE FRONT AND SOME SOLUTIONS INDICATING DECENT  
SURFACE LOW DEVELOPMENT. HOWEVER GUIDANCE SPREAD FOR SPECIFICS OF  
SURFACE EVOLUTION LOWERS CONFIDENCE IN WHERE HEAVIEST RAIN AND/OR  
SNOW MAY OCCUR. BEHIND THIS SYSTEM THE NORTHERN TIER/MIDWEST  
SHOULD RETURN TO 5-10F BELOW NORMAL TEMPS BY NEXT TUE. COLDER  
READINGS MAY BE POSSIBLE DEPENDING ON SPECIFICS OF FLOW ALOFT.  
 
AFTER THE EARLY PERIOD SYSTEM, THE WEST SHOULD SEE A PERIOD OF  
LIGHTER/MORE SCATTERED PRECIP BEFORE ANOTHER AREA OF MORE  
ORGANIZED MOISTURE MAY REACH THE WEST COAST TOWARD THE END OF THE  
PERIOD AS THE NEXT UPPER TROUGH APPROACHES. TIMING/LOCATION ARE  
FAIRLY UNCERTAIN IN LIGHT OF VARIED SOLUTIONS FOR SPECIFICS OF  
THIS TROUGH AS WELL AS ANY LINGERING ENERGY AHEAD OF IT. EXPECT  
MOST OF THE WEST TO SEE NEAR TO ABOVE NORMAL TEMPS DURING THE  
PERIOD WITH THE GREAT BASIN/FOUR CORNERS STATES MOST LIKELY TO SEE  
MULTIPLE DAYS WITH PLUS 10F ANOMALIES FOR HIGHS. THERE SHOULD BE  
A BROADER EXPANSE OF ABOVE NORMAL LOWS ESPECIALLY FRI-SUN.  
 
RAUSCH  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page