793  
FXUS02 KWBC 030655  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
154 AM EST WED JAN 03 2018  
 
VALID 12Z SAT JAN 06 2018 - 12Z WED JAN 10 2018  
   
..OVERVIEW
 
 
WITH THE DEPARTURE OF THE COLD/DEEP EASTERN U.S. TROUGH DURING THE  
WEEKEND, THE PRIMARY FORECAST PROBLEMS REVOLVE AROUND TWO PACIFIC  
TROUGHS FOR WHICH MODELS AND ENSEMBLES ARE HAVING DIFFICULTY IN  
RESOLVING THE SPECIFICS OF EMBEDDED ENERGY. THE FIRST NEAR THE  
WEST COAST AS OF EARLY SAT WILL PROGRESS EASTWARD AND BRING A  
FRONTAL SYSTEM ACROSS THE CENTRAL-EASTERN STATES FROM THE WEEKEND  
INTO EARLY NEXT WEEK, WITH DETAILS OF POTENTIALLY INTERACTING  
CANADIAN FLOW ALSO A QUESTION MARK. THE NEXT EAST PACIFIC TROUGH  
SHOULD AMPLIFY AROUND MON-TUE, WITH DIFFERENCES AFFECTING THE  
WESTERN STATES.  
 
   
..GUIDANCE ASSESSMENT AND PREFERENCES
 
 
OVER RECENT RUNS THE ENSEMBLE MEANS HAVE PROVIDED THE MOST STABLE  
FORECAST OF THE UPPER TROUGH EXPECTED TO BE NEAR THE WEST COAST AS  
OF 12Z SAT. INCLUDING NEW 00Z RUNS, INDIVIDUAL MODELS CONTINUE TO  
BE ALL OVER THE PLACE WITH RESPECT TO WHERE THE STRONGEST  
CONCENTRATION OF ENERGY WITHIN THE OVERALL TROUGH WILL BE  
(ANYWHERE BETWEEN OVER/OFFSHORE THE PACIFIC NORTHWEST AND WESTERN  
CANADA) AND ENSEMBLE MEMBERS SHOW MORE SCATTER THAN USUAL FOR A  
DAY 3 FORECAST AS WELL. THIS SPREAD LEADS TO DOWNSTREAM  
UNCERTAINTY FOR A FRONTAL SYSTEM WHICH AT LEAST HAS SOME AGREEMENT  
IN PRINCIPLE, BUT WITH A VARIETY OF POSSIBILITIES DEPICTED FOR  
PARENT LOW PRESSURE AND POSSIBLE FRONTAL WAVE(S). DURING DAYS 3-4  
SAT-SUN PRIMARY WEIGHTING OF THE 18Z GFS/12Z ECMWF/12Z UKMET  
PROVIDED A CONSENSUS APPROACH WITH DECENT SUPPORT FROM THE MEANS.  
TOWARD DAY 5 PREFERENCE SHIFTED A MORE EVEN MODEL/MEAN BLEND WITH  
ENOUGH MODEL SIGNAL TO ADD A WEAK TRAILING FRONTAL WAVE WHILE  
MAINTAINING GREAT LAKES/SOUTHERN CANADA LOW PRESSURE WHICH HAS  
BEEN THE CONSENSUS ENSEMBLE MEAN TRACK IN RECENT DAYS. THE 12Z  
RUN OF THE GEFS MEAN WAS MORE DEFINED WITH THE SURFACE LOW THAN  
THE 18Z VERSION SO THE FORMER PROVIDED THE GEFS COMPONENT OF THE  
BLEND. BEYOND THE ISSUES WITH INITIAL SHORTWAVE ENERGY, THE  
CHARACTER OF HIGH LATITUDE FLOW DOWNSTREAM FROM ALASKA/YUKON  
RIDGING PROVIDES ADDED UNCERTAINTY. CONSENSUS SUGGESTS THAT THE  
12Z ECMWF MAY BE UNDERDONE WITH EVENTUAL AMPLITUDE OF THIS FLOW  
FROM CANADA BY THE LATTER HALF OF THE PERIOD WHILE SOME GFS RUNS  
HAVE BEEN ON THE DEEP SIDE OF THE SPREAD. PRESENCE OF THE  
UPSTREAM RIDGE IN GENERAL SUPPORTS AT LEAST MODERATE AMPLITUDE BUT  
THERE SEEMS TO BE A FAIRLY RAPID REBUILDING CYCLE WITH THE RIDGE,  
PERHAPS LIMITING DOWNSTREAM TROUGH AMPLITUDE TO SOME DEGREE. THUS  
THE ENSEMBLE MEANS APPEAR REASONABLE AS THE BEST STARTING POINT  
FOR THAT ASPECT OF THE FORECAST.  
 
THE NEXT TROUGH APPROACHING THE WEST COAST DISPLAYS SOME SIMILAR  
PROBLEMS AS WITH ITS PREDECESSOR, AS GUIDANCE VARIES WITH THE  
DEGREE TO WHICH EMBEDDED FLOW SEPARATES (POSSIBLY CLOSING OFF A  
FAIRLY DEEP LOW) ALONG WITH WHERE SEPARATION OCCURS AND TIMING OF  
CONSTITUENT PARTS OF THE TROUGH. OVER THE PAST DAY THE  
GFS/ECMWF/CMC HAVE BUILT UP SOME DEGREE OF CONTINUITY IN PULLING  
OFF A CLOSED LOW OFFSHORE WITH GFS RUNS HOLDING THE UPPER LOW  
FARTHEST WESTWARD. AS IS TYPICALLY THE CASE IN THESE SITUATIONS,  
HOW MUCH SHORTWAVE ENERGY CONTINUES ALONG IN THE NORTHERN STREAM  
INTO THE NORTHWEST U.S. IS VERY UNCERTAIN. GIVEN THE ENSEMBLE  
SPREAD, THE RESULTING MEANS DISPLAY A MORE OPEN/PHASED TROUGH THAN  
THE MODELS. RECENT GEFS MEANS DO SHOW MORE SOUTHWEST ELONGATION  
IN REFLECTING SOME THE PARENT MODEL'S IDEAS. GEFS MEANS MAY BE  
SOMEWHAT WEAK WITH THE OVERALL TROUGH. MEANWHILE THE 12Z  
ECMWF/CMC MEANS ARE THE FASTEST WITH TROUGH PROGRESSION. THIS WAS  
ALSO THE CASE A FEW DAYS AGO WHEN THE SHORT-TERM EAST PACIFIC  
FEATURES WERE IN THE EXTENDED FORECAST, AND THE FAST SOLUTIONS DID  
NOT VERIFY WELL. GIVEN THE SIMILARITY IN THE OVERALL PATTERN AND  
THE FAIRLY STRONG AMPLIFICATION/SHARPENING SEEN IN THE TROUGH IN  
THE SUN-TUE TIME FRAME, THE ECMWF/CMC MEANS MAY BE TOO FAST.  
ADDING UP THESE CONSIDERATIONS, PREFERRED A SOLUTION DEPICTING AN  
OPEN TROUGH BUT WITH SOME SEPARATION, AND TIMING THAT WAS MOST  
COMPARABLE TO A SUBDUED COMPROMISE AMONG RECENT ECMWF/CMC RUNS.  
EVOLUTION OF THIS TROUGH WILL ALSO DICTATE HOW LINGERING UPPER LOW  
ENERGY NEAR 25N 135W AS OF EARLY SAT WILL EJECT NORTHEAST TOWARD  
THE WEST COAST, WITH A COMPROMISE AGAIN FAVORED.  
 
AFTER THE EARLY PERIOD OPERATIONAL MODEL BLEND AND DAY 5  
MODEL/MEAN SOLUTION, THE FORECAST FOR THE REST OF THE PERIOD USED  
COMPONENTS FROM THE 18Z GFS/12Z GEFS MEAN ALONG WITH THE 12Z AND  
OLD 00Z ECMWF AS WELL AS 12Z ECMWF MEAN. THE DESIRED RESULT WAS  
ESSENTIALLY AN OPEN WAVE VERSION OF ECMWF RUNS NEAR THE WEST COAST  
WHILE MAINTAINING REASONABLE CONSENSUS/CONTINUITY FOR THE FRONT  
DEPARTING THE EAST COAST AND UPSTREAM FLOW THAT SHOULD SUPPORT A  
TRAILING FRONT PUSHING SOUTHEAST FROM CANADA.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
INITIAL WEST COAST ENERGY WILL PRODUCE AREAS OF RAIN AND HIGH  
ELEVATION SNOW OVER FAVORED TERRAIN PRIMARILY IN NORTHERN/CENTRAL  
PARTS OF THE WEST. THEN AS THE LEADING FRONT PROGRESSES INTO AND  
BEYOND THE PLAINS, AN EXPANDING AREA OF PRECIP WILL DEVELOP/SPREAD  
ACROSS THE EASTERN HALF OF THE LOWER 48. SOME SNOW IS LIKELY OVER  
NORTHERN AREAS, A POSSIBLE TRANSITION OVER CENTRAL/NORTH-CENTRAL  
LATITUDES, AND RAIN TO THE SOUTH. THERE IS STILL SIGNIFICANT  
DISAGREEMENT ON FINER DETAILS BUT IN GENERAL GUIDANCE IS SIGNALING  
BEST POTENTIAL FOR ENHANCED PRECIP (MOSTLY RAIN) FROM THE LOWER  
HALF OF THE MS VALLEY INTO THE OH VALLEY/CENTRAL-SOUTHERN  
APPALACHIANS. SUCH PROBABILITIES ARE LOWER ALONG THE EAST COAST.  
 
EVEN WITH THE SPREAD/VARIABILITY IN GUIDANCE ALONG THE WEST COAST,  
THERE IS SOME AGREEMENT THAT ONE OR MORE WAVES OF MOISTURE SHOULD  
BRING ADDITIONAL EPISODES OF PRECIP TO THE PACIFIC  
NORTHWEST/NORTHERN ROCKIES BY THE END OF THE WEEKEND AND THE FIRST  
HALF OF NEXT WEEK. MORE UNCERTAIN ARE THE TIMING AND INTENSITY OF  
PRECIP OVER CENTRAL/SOUTHERN AREAS OF THE WEST DURING MON-WED. AT  
THIS TIME NORTHERN-CENTRAL CALIFORNIA HAVE A HIGHER PROBABILITY OF  
SEEING SOME LOCALLY MODERATE/HEAVY PRECIP RELATIVE TO OTHER  
LOCATIONS, WHILE SOUTHERN CALIFORNIA AND AREAS FARTHER INLAND TO  
THE GREAT BASIN/FOUR CORNERS STATES ALSO HAVE SOME PRECIP  
POTENTIAL.  
 
VERY COLD AIR OVER THE EAST DURING THE WEEKEND WILL LIKELY PRODUCE  
NUMEROUS DAILY RECORDS, IN PARTICULAR FOR COLD HIGHS ON SAT AND  
MORNING LOWS ON SUN. HIGHS MAY BE MORE THAN 30F BELOW NORMAL OVER  
PARTS OF THE NORTHEAST ON SAT AS UPPER TROUGHING CROSSES THE  
REGION. THE FRONT CROSSING THE CENTRAL/EASTERN STATES WILL BRING  
A DRAMATIC WARMING TREND, FOLLOWED BY ANOTHER EPISODE OF COLD AND  
BREEZY CONDITIONS BUT LESS EXTREME THAN WITH PRECEDING EVENTS.  
MUCH OF THE WEST SHOULD SEE ABOVE NORMAL TEMPS THROUGH THE PERIOD.  
EXPECT THE GREATEST ANOMALIES TO BE FOR WARM MINS, WITH DECENT  
COVERAGE OF AT LEAST PLUS 10-20F ANOMALIES. SMALLER AREAS OF  
SIMILAR ANOMALIES FOR MAX TEMPS ARE POSSIBLE AS WELL.  
 
RAUSCH  
 

 
 
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