577  
FXUS02 KWBC 251555  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1055 AM EST SAT NOV 25 2017  
 
VALID 12Z TUE NOV 28 2017 - 12Z SAT DEC 02 2017  
   
..PATTERN ASSESSMENT
 
 
CONFIDENCE IN THE MEDIUM RANGE FORECAST IS AVERAGE WITH RESPECT TO  
REGIONAL DETAILS BUT PERHAPS ABOVE AVERAGE ON THE LARGE END OF THE  
SYNOPTIC SCALE...OWING TO THE FACT THAT THE ENSEMBLE MEANS SHOW  
GOOD AGREEMENT AND AS EACH OPERATIONAL MODEL SOLUTION MOVES AWAY  
FROM THE CONSENSUS AT SOME TIME INTO THE FORECAST...IT DOES SO IN  
A MANNER CONSISTENT WITH KNOWN BIASES. THAT IS TO SAY THE GFS  
TENDS TO ERR ON THE FAST SIDE AND INDICATES GREATER PHASING AT  
TIMES. THE ECMWF TENDS TO ERR ON THE SLOWER AND OVER-DEVELOPED  
SIDE.  
 
EXPECT A PROGRESSIVE WAVE TRAIN TO BECOME ESTABLISHED ALL THE WAY  
FROM THE WESTERN PACIFIC ACROSS NORTH AMERICA...THEN SLOWING AND  
SPLITTING AROUND A PERSISTENT BLOCK OVER THE MIDDLE ATLANTIC  
OCEAN. TIME OF YEAR AND STRENGTH OF THE OVERALL POLE TO EQUATOR  
GRADIENTS OFFER POTENTIAL FOR EMBEDDED NOTEWORTHY OR EVEN STRONG  
LOWS TO SPIN UP WITHIN THE PROGRESSIVE FLOW...ALTHOUGH FOR NOW THE  
DETERMINISTIC AND ENSEMBLE GUIDANCE SUGGESTS CYCLOGENESIS WILL FOR  
THE MOST PART REMAIN NORTH OF THE LOWER 48 STATES THROUGH NEXT  
WEEK. EVEN AT THAT...THE STORM TRACK ACROSS CANADA REMAINS FAIRLY  
TEPID EXCEPT FOR A DEEP LOW LIFTING TOWARD HUDSON BAY ON DAY 3.  
 
SOUTHERN ROCKIES/PLAINS...THE EASTERN-MOST TROUGH OF INTEREST TO  
BEGIN DAY 3 IS FORECAST TO MOVE THROUGH THE CENTRAL AND EASTERN  
U.S. WITH BOTH A NORTHERN AND SOUTHERN STREAM COMPONENT. ONE OF  
THE MORE SIGNIFICANT TRENDS HAS BEEN TOWARD A FASTER MOVING  
SOUTHERN STREAM TROUGH WITH EMBEDDED CLOSED LOW...WITH THE 00Z  
ECMWF ENSEMBLE MEMBERS HAVING JUMPED WHOLE-HEARTEDLY TOWARD THE  
FASTER...MORE GFS-LIKE SOLUTION. PERHAPS WITHIN THE PROGRESSIVE  
PATTERN THAT IS SETTING UP THIS MAKES SENSE...BUT SOME CAUTION IS  
ADVISED AS THE CANADIAN AND SEVERAL OF ITS ENSEMBLE MEMBERS STILL  
HANG BACK WITH A SLOWER SOLUTION...AND THE MODELS SOMETIMES MAKE A  
JUMP IN THE DAY 4-5 PERIOD ONLY TO LATER RETURN TO WHERE THEY WERE  
BEFORE. STILL...GIVEN THE OVERWHELMING CLUSTERING OF GEFS/EC  
ENSEMBLE MEMBERS AND MOST OF THE OPERATIONAL MODELS FROM THE 00Z  
CYCLE...WE DID SCOOT THE SYSTEM ALONG FASTER THAN THE PREVIOUS  
FORECAST. ITS STRENGTH AND LACK OF PHASING...PERHAPS OWING TO  
QUICKER MOTION...ALSO KEEPS IT SUPPRESSED TO A MORE SOUTHERLY  
LATITUDE AND IT SHEARS OUT MORE SLOWLY OVER THE SOUTHEAST STATES.  
WE MAY NOT HAVE SEEN THE LAST OF THE TRENDS WITH THIS ONE...BUT  
BEING SO MILD...AT LEAST THERE ARE NO WINTER WEATHER IMPACTS TO BE  
CONCERNED ABOUT EAST OF THE ROCKIES.  
 
WESTERN U.S...THE UPSTREAM TROUGH APPROACHING THE WEST COAST ON  
TUE ALSO PRESENTS SOME CHALLENGING DETAILS TO ITS EVOLUTION. A  
SOUTHERN STREAM PORTION WILL LIKELY CLOSE OFF A CIRCULATION  
OFFSHORE OF SOUTHERN CALIFORNIA...AS MORE AND MORE ENSEMBLE  
MEMBERS ARE INDICATING THIS BY DAY 5 TO 6. AS THE NORTHERN STREAM  
PORTION HEADS INTO THE WESTERN U.S. THERE ARE SPEED AND DEPTH  
ISSUES. THE OPERATIONAL ECMWF JOINS THE ENSEMBLE MEANS WITH A  
FASTER SOLUTION...BUT THE 00Z CANADIAN...AND ESPECIALLY 00Z UKMET  
AND 00Z/06Z GFS HOLD PRONOUNCED TROUGHING BACK OVER THE ROCKIES  
THROUGH DAY 5 IN AN ENTIRELY PLAUSIBLE FASHION GIVEN THE SPLITTING  
ENERGY AND BROAD CYCLONIC NATURE OF THE NORTHERN STREAM FLOW OVER  
CENTRAL NORTH AMERICA AT THAT TIME. THE ONLY THING THAT CAN BE  
SAID WITH A LITTLE MORE CONFIDENCE IS THAT THE GFS APPEARED TOO  
FAST WITH THE TROUGHING THAT IT EXTENDED WELL DOWNSTREAM TOWARD  
QUEBEC ON DAY 5.  
 
HOWEVER THE MODELS MAY DIFFER IN GETTING THERE...THEY ALL BRING  
THE NEXT TROUGH IN THE PACIFIC WAVE TRAIN TOWARD THE WEST COAST BY  
DAY 7...LIKELY ABSORBING THE CLOSED LOW OFF CALIFORNIA TO YIELD A  
FULL LATITUDE TROUGH. IN TERMS OF BOTH DEPTH AND POSITION THE  
ECMWF MEAN RUNS HAVE PROVIDED THE MOST CONSISTENT EVOLUTION FOR  
THESE LATER PERIODS ALONG THE WEST COAST...ALTHOUGH IN THE MORE  
RECENT CYCLES THERE WAS GOOD SUPPORT FROM THE GEFS AND ESPECIALLY  
NAEFS MEANS.  
 
   
..MODEL BLEND CHOICES
 
 
WPC USED WHAT ROUGHLY EQUATES TO A 50/25/25 PERCENT BLEND OF  
ECMWF/UKMET/GFS THROUGH EARLY DAY 4. WE THEN TRANSITION QUICKLY  
AWAY FROM THE GFS GIVEN ITS NORTHERN STREAM SPEED ISSUES AND OTHER  
LESS DESIRABLE TRAITS TO ITS SOLUTION OVER ALASKA. WE MOVED AWAY  
FROM THE UKMET GIVEN ALSO ITS SLOWER SOLUTION THROUGH THE ROCKIES  
ON DAY 5...WHILE ACKNOWLEDGING THERE IS STILL A LOT OF UNCERTAINTY  
TO THE DETAILS OF THAT SYSTEM. FROM LATE DAY 4 ONWARD OUR BLEND  
CONSISTED OF ECMWF / ECMWF ENSEMBLE MEAN / NAEFS MEAN...GRADUALLY  
DIALING DOWN THE OPERATIONAL RUN AT LATER TIMES AND LEANING  
SLIGHTLY MORE IN FAVOR OF THE MORE CONSISTENT ECMWF ENSEMBLE MEAN  
BY DAY 7.  
 
   
..WEATHER HIGHLIGHTS
 
 
TELECONNECTIONS RELATIVE TO THE PACIFIC/WEST COAST PATTERN AS WELL  
AS ATLANTIC RIDGE SUGGEST THERE SHOULD BE A TENDENCY TOWARD ABOVE  
NORMAL HEIGHTS ALOFT OVER THE EASTERN U.S. THERE WILL BE DECENT  
COVERAGE OF WELL ABOVE NORMAL TEMPS OVER THE PLAINS/EAST-CENTRAL  
U.S. ON TUESDAY WITH HIGHEST ANOMALIES OF PLUS 15-25F POSSIBLE FOR  
MINS THAT MORNING. PROGRESSION OF THE INITIAL GREAT LAKES-PLAINS  
FRONT WILL LOWER TEMPS TO MODESTLY ABOVE NORMAL LEVELS OVER MOST  
OF THE EAST AND THEN THE NEXT FRONT REACHING THE EAST BY THU-FRI  
SHOULD BRING READINGS DOWN TO A FEW DEGREES F EITHER SIDE OF  
NORMAL. HOWEVER SOME PLUS 10-15F ANOMALIES WILL LIKELY PERSIST  
OVER/NEAR THE NORTHERN PLAINS THROUGH THE PERIOD. NEAR TO SLIGHTLY  
ABOVE NORMAL TEMPS SHOULD TREND A LITTLE BELOW NORMAL OVER THE  
WEST COAST STATES BY NEXT SAT AS UPPER TROUGHING ARRIVES.  
 
EXPECT FAIRLY LOW COVERAGE OF SIGNIFICANT PRECIP ACROSS THE LOWER  
48 DURING THE PERIOD. THE FEATURE EMERGING FROM THE FOUR CORNERS  
REGION MAY PRODUCE A BRIEF PERIOD OF SNOW OVER THE ROCKIES  
INITIALLY...FOLLOWED BY RAINFALL ACROSS PARTS OF THE PLAINS/MS  
VALLEY. LACK OF AGREEMENT ON TIMING/TRACK OF THIS SYSTEM KEEPS  
CONFIDENCE LOW FOR INTENSITY AND PRECISE LOCATION OF THIS RAIN. A  
LEADING FRONT CROSSING THE NORTHEAST AROUND MIDWEEK SHOULD BE  
FAIRLY DRY EXCEPT FOR EXTREME NORTHERN AREAS. THE NEXT FRONT  
ARRIVING INTO THE NORTHWEST ON TUESDAY WILL BRING A PERIOD OF  
RAIN/SNOW WITH BEST FOCUS OVER THE FAVORED TERRAIN OF WESTERN  
WA/OR AND THE NORTHERN ROCKIES. PRECIP COVERAGE SHOULD EXPAND WITH  
THE FRONT AS IT CROSSES THE EAST LATER IN THE WEEK. ANY SNOW OVER  
THE EAST SHOULD BE CONFINED TO FAR NORTHERN AREAS/HIGH ELEVATIONS.  
DURING THE LATTER HALF OF THE PERIOD EXPECT THE UPPER TROUGH  
NEARING THE WEST COAST TO BRING AN INCREASE OF MOISTURE TO THE  
REGION. SHOULD THE ABSORBED LOW OFF CALIFORNIA PAN OUT PER THE  
ECMWF SOLUTION A SUB-TROPICAL MOISTURE PLUME COULD SUPPORT A ROUND  
OF RAIN INTO SOUTHERN CALIFORNIA...BUT CONFIDENCE IS CURRENTLY  
LESS THAN DESIRED FOR THE DAY 7 FORECAST.  
 
BURKE/RAUSCH  
 

 
 
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