278  
FXUS02 KWBC 150659  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
159 AM EST THU NOV 15 2018  
 
VALID 12Z SUN NOV 18 2018 - 12Z THU NOV 22 2018  
   
..OVERVIEW
 
 
UPPER RIDGING FORECAST TO BUILD INTO WESTERN NORTH AMERICA FROM  
THE LATE WEEKEND INTO EARLY NEXT WEEK WILL PROMOTE DOWNSTREAM MEAN  
TROUGHING, LEADING TO WARMTH OVER PARTS OF THE WEST AND WELL BELOW  
NORMAL TEMPERATURES OVER CENTRAL AND EASTERN STATES. AS THE  
STRONGEST ENERGY WITHIN THE MEAN TROUGH REACHES THE NORTHEAST AND  
EASTERN CANADA THERE WILL BE POTENTIAL FOR CYCLOGENESIS FROM NEAR  
THE NORTHEAST U.S. COAST INTO THE CANADIAN MARITIMES. THERE IS  
DECENT CONTINUITY IN PRINCIPLE REGARDING UNDERCUTTING PACIFIC  
TROUGH/UPPER LOW ENERGY NEARING THE SOUTHWESTERN U.S./NORTHWESTERN  
MEXICO BY TUE WITH CONTINUED PROGRESSION THEREAFTER. HOWEVER  
THERE IS A LOT OF SPREAD/UNCERTAINTY OVER THE CHARACTER OF THE  
NEXT TROUGH REACHING THE EASTERN PACIFIC WITH POSSIBLE INFLUENCE  
FROM FLOW EVEN FARTHER WEST. THESE ISSUES BEGIN TO AFFECT THE  
WESTERN U.S. FORECAST BY LATER MON-TUE AND THEN THE  
WEST-CENTRAL/CENTRAL U.S. BY WED-THU. ASIDE FROM TYPICALLY LOW  
PREDICTABILITY SHORTWAVE DETAILS THERE IS DECENT CONSENSUS FOR THE  
EASTERN MEAN TROUGH THROUGH AT LEAST WED.  
 
   
..GUIDANCE/UNCERTAINTY ASSESSMENT
 
 
OVER THE EASTERN PACIFIC/WESTERN U.S., THE OVERALL CONSENSUS FOR  
LEADING SOUTHERN STREAM SHORTWAVE/EMBEDDED UPPER LOW NEARING  
CALIFORNIA AND NORTHWESTERN MEXICO AS OF EARLY TUE IS CONSISTENT  
THOUGH INDIVIDUAL SOLUTIONS CONTINUE TO VARY FOR EXACT TIMING AS  
WELL AS TRACK OF THE UPPER LOW CENTER. THIS FAVORS A MULTI-MODEL  
COMPROMISE FOR AT LEAST THE FIRST HALF OF THE PERIOD. THE  
MAJORITY OF GUIDANCE SUGGESTS THIS FEATURE WILL WEAKEN AS IT  
PROGRESSES ACROSS THE SOUTHERN U.S. THROUGH THU THE OCCASIONAL  
STRAY SOLUTION HOLDS ONTO A STRONGER DEPICTION TO THE END OF THE  
PERIOD. BEHIND THIS FEATURE THE MODELS AND ENSEMBLES CONTINUE TO  
SHOW SIGNIFICANT DIVERGENCE FOR IMPORTANT FEATURES. THE PAST  
COUPLE ECMWF RUNS HAVE BEEN SURPRISINGLY FAST/STRONG WITH  
AMPLIFYING EASTERN PACIFIC SHORTWAVE ENERGY THAT THE MODEL  
ULTIMATELY BRINGS THROUGH THE WESTERN MEAN RIDGE AND INTO THE  
PLAINS BY LATE IN THE PERIOD. THIS EVOLUTION IS LIKELY AIDED BY  
AN UPSTREAM SHORTWAVE ALSO STRONGER THAN IN OTHER GUIDANCE. THE  
12Z ECMWF MEAN HAD A MUCH MORE SUBDUED HINT OF THE ECMWF IDEAS AND  
PROVIDED THE BEST FORECAST INPUT FOR REPRESENTING THE MINORITY  
PROBABILITY OF THE ECMWF SCENARIO. ON THE OTHER EXTREME THE GFS  
RUNS SINCE AT LEAST THE 12Z CYCLE HAVE BEEN SEPARATING THE TROUGH  
IN A MANNER NOT SEEN IN OTHER GUIDANCE--LEADING TO DRAMATIC  
DIFFERENCES IN DOWNSTREAM FLOW. LIKE THE ECMWF MEAN RELATIVE TO  
ITS OPERATIONAL RUN, THE GEFS MEANS HAVE BEEN HINTING AT SOME  
SLOWNESS/SEPARATION (BUT TO A GREATER DEGREE IN THE NEW 00Z RUN).  
THE 12Z/18Z FV3 GFS RUNS CLUSTERED MORE SIMILARLY WITH THE 12Z  
CMC/CMC MEAN AND ECMWF MEAN, WITH THIS INTERMEDIATE GROUP OF  
SOLUTIONS PROVIDING THE FOUNDATION FOR THE FORECAST THAT MAINTAINS  
BETTER THAN AVERAGE CONTINUITY IN SPITE OF THE WIDE SOLUTION  
ENVELOPE.  
 
FARTHER EASTWARD A MODEL BLEND REPRESENTS CONSENSUS IDEAS WELL FOR  
A COLD UPPER TROUGH CROSSING EASTERN CANADA AND NORTHEASTERN  
QUARTER OF THE CONUS DURING THE FIRST HALF OF THE PERIOD ALONG  
WITH THE LEADING SURFACE FRONT. DETAILS OF POTENTIAL LOW PRESSURE  
DEVELOPMENT FROM NEAR THE NORTHEASTERN U.S. COAST INTO THE  
CANADIAN MARITIMES MON-TUE WILL BE SENSITIVE TO SPECIFICS OF  
ENERGY ALOFT. AT THIS TIME GFS RUNS ARE THE WEAKEST WITH THIS  
EVOLUTION. BEHIND THIS SYSTEM THERE IS A REASONABLE SIGNAL FOR A  
WAVY FRONT PERSISTING OVER AND WEST-NORTHWEST FROM THE UPPER GREAT  
LAKES, BUT CONFIDENCE IN DETAILS IS LOW GIVEN COMBINED INFLUENCE  
FROM CANADIAN FLOW AS WELL AS THE INCREASING UNCERTAINTY TO THE  
WEST.  
 
BASED ON THE CURRENT ARRAY OF GUIDANCE AND CONTINUITY, THE DAYS  
3-4 SUN-MON PART OF THE FORECAST INCORPORATED IDEAS FROM THE 18Z  
GFS/FV3 GFS ALONG WITH THE 12Z ECMWF/UKMET/CMC. DAY 5 TUE WAS THE  
TRANSITION FOR REPLACING THE ECMWF WITH THE ECMWF MEAN AND  
REMOVING THE 18Z GFS. THE REST OF THE FORECAST USED ABOUT 1/3  
ECMWF MEAN WITH THE REMAINDER CONSISTING OF THE 18Z FV3 GFS/12Z  
CMC AND 18Z GEFS MEAN/12Z NAEFS MEAN.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
OVER THE WEST THERE IS DECENT CONFIDENCE IN DRY WEATHER FROM SUN  
THROUGH ABOUT MON NIGHT. AFTER THAT TIME THE INCREASING  
DIVERGENCE IN SOLUTIONS FOR FLOW ALOFT LEAD TO LOWER THAN AVERAGE  
CONFIDENCE FOR THE TIMING/COVERAGE/MAGNITUDE OF PRECIPITATION.  
LOCATIONS NEAR THE CENTRAL AND NORTHERN WEST COAST SHOULD HAVE THE  
BEST POTENTIAL FOR SEEING SOME RAIN AND POSSIBLY HIGH ELEVATION  
SNOW. THE SOUTHERN STREAM TROUGH/SHORTWAVE NEARING CALIFORNIA BY  
EARLY TUE SHOULD GENERATE LITTLE IF ANY RAINFALL. AS THIS FEATURE  
CONTINUES EASTWARD IT SHOULD LEAD TO A LATE-PERIOD EXPANSION OF  
RAINFALL THAT WILL LIKELY TEND TO FOCUS OVER/NEAR SOUTHERN TEXAS  
DURING EARLY NEXT WEEK. THE SOUTHERN TEXAS ACTIVITY WILL BE AIDED  
BY A COLD FRONT PUSHING SOUTHWARD THROUGH THE AREA AND A  
PERSISTENT WESTERN GULF COAST SURFACE TROUGH. SOME POCKETS OF  
HEAVIER RAINFALL WILL BE POSSIBLE WITHIN THIS AREA OF MOISTURE.  
PERIODS OF LAKE EFFECT SNOW SHOULD BE MOST FAVORED OVER NORTHERN  
AND EASTERN PORTIONS OF THE GREAT LAKES. SOME OF THE SNOW OVER  
THIS AREA MAY ALSO BE FOCUSED NEAR ONE OR MORE FRONTAL  
BOUNDARIES/EMBEDDED WAVES.  
 
DURING SUN-MON EXPECT COLDEST TEMPERATURE ANOMALIES OF 10-20F OR  
SO BELOW NORMAL TO BE MOST COMMON FROM THE SOUTHERN PLAINS INTO  
MIDWEST/GREAT LAKES. THEN AS THE CORE OF UPPER TROUGHING ADJUSTS  
INTO THE NORTHEAST, THAT REGION SHOULD SEE COLDEST READINGS VERSUS  
NORMAL TUE-THU, 10-25F BELOW NORMAL. SUCH ANOMALIES MAY CHALLENGE  
DAILY RECORD VALUES AT SOME LOCATIONS. CONFIDENCE REMAINS ABOVE  
AVERAGE FOR PLUS 5-15F HIGH TEMPERATURE ANOMALIES ALONG/INLAND  
FROM THE CENTRAL WEST COAST DURING THE FIRST PART OF NEXT WEEK.  
THEN THERE IS GREATER THAN AVERAGE UNCERTAINTY IN THE TEMPERATURE  
FORECAST OVER THE WEST GIVEN THE DIVERGENCE IN GUIDANCE ALOFT.  
 
RAUSCH  
 
WPC MEDIUM RANGE 500 MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, WINTER WEATHER OUTLOOK PROBABILITIES  
AND HEAT INDEXES ARE FOUND AT:  
 
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML  
 

 
 
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