135  
FXUS02 KWBC 130658  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
157 AM EST TUE NOV 13 2018  
 
VALID 12Z FRI NOV 16 2018 - 12Z TUE NOV 20 2018  
   
..OVERVIEW
 
 
LATEST GUIDANCE IS FAIRLY CONSISTENT WITH THE OVERALL PATTERN OF A  
RIDGE BUILDING INTO WESTERN NORTH AMERICA WHILE BROAD MEAN  
TROUGHING PREVAILS DOWNSTREAM OVER THE REST OF THE CONTINENT.  
THERE CONTINUES TO BE SOME DAY-TO-DAY TRENDING FOR THE CHARACTER  
OF THE WESTERN RIDGE AS WELL AS THE SHORTWAVE ENERGY TO ITS EAST.  
FARTHER UPSTREAM THERE IS GENERAL AGREEMENT THAT FLOW WITHIN A  
PACIFIC TROUGH NEAR 150W AS OF EARLY DAY 3 FRI WILL SPLIT, WITH  
NORTHERN ENERGY GOING AROUND THE NORTHERN SIDE OF THE RIDGE WHILE  
THE REST FORMS A CLOSED LOW THAT MAY TRACK TOWARD CALIFORNIA.  
HOWEVER THERE IS A FAIR AMOUNT OF SPREAD AND VARIABILITY WITH THE  
EXACT TRACK/TIMING OF THIS FEATURE. IN A MINORITY OF GUIDANCE  
THERE IS THE POSSIBILITY THAT FLOW FARTHER UPSTREAM COULD BEGIN TO  
HAVE SOME INFLUENCE ON THE WEST COAST PATTERN BY THE END OF THE  
PERIOD.  
 
   
..GUIDANCE/UNCERTAINTY ASSESSMENT
 
 
EARLY IN THE PERIOD THE MOST PROMINENT SYSTEM SHOULD BE TRACKING  
NORTHEASTWARD FROM NEAR THE MID-ATLANTIC COAST. OVER RECENT DAYS  
GUIDANCE HAS HAD CONSIDERABLE DIFFICULTY IN RESOLVING THE TIMING  
OF THE UPPER LOW/TROUGH ENERGY AND SPECIFICS OF SURFACE LOW  
PRESSURE. THE MODEL/ENSEMBLE ENVELOPE HAS NARROWED CONSIDERABLY  
OVER THE PAST 24 HOURS TO PROVIDE SOME INCREASE OF FORECAST  
CONFIDENCE, WITH AN EMPHASIS ON A 12Z ECMWF/18Z GFS AVERAGE  
PROVIDING A REASONABLE INTERMEDIATE SOLUTION. LATEST UKMET RUNS  
HAVE BEEN ON THE FAST SIDE OF THE SPREAD (FASTER THAN THE GFS/GEFS  
MEAN) WHILE CMC RUNS HAVE SUPPORTED THE SLOWER ECMWF.  
 
OVER THE WESTERN HALF OF THE COUNTRY THERE ARE STILL SOME EVOLVING  
TRENDS. THE MOST NOTABLE FROM THE PAST 24 HOURS INVOLVE A MORE  
WESTWARD ELONGATION OF SHORTWAVE ENERGY DROPPING OUT OF  
SOUTHWESTERN CANADA IN RESPONSE TO THE MORE CLOSED OR NEARLY  
CLOSED NATURE OF THE UPPER RIDGE BUILDING INTO THE NORTHWEST. THE  
12Z ECMWF WAS THE MOST EXTREME IN CLOSING OFF THE HIGH ENOUGH TO  
PULL SOME OF THE SHORTWAVE ENERGY ALL THE WAY BEYOND THE  
CALIFORNIA COAST. REMAINING GUIDANCE PLUS THE OCCASIONAL  
HISTORICAL TENDENCY FOR THE ECMWF TO OVERDO RETROGRESSION OF  
ENERGY UNDER UPPER HIGHS RECOMMEND LEANING AWAY FROM THIS ASPECT  
OF THE 12Z ECMWF. CHANGES ALOFT LEAD TO A STRONGER PUSH OF COOLER  
SOUTHWARD ACROSS THE INTERMOUNTAIN WEST.  
 
WELL OFFSHORE THE WEST COAST THE RECENT/LATEST OPERATIONAL MODEL  
RUNS HAVE FAIRLY WELL LATCHED ONTO THE IDEA OF SEPARATING TROUGH  
ENERGY FORMING A LOWER LATITUDE CLOSED LOW THAT MAY TRACK TOWARD  
CALIFORNIA. HOWEVER THERE IS STILL A LOT OF SPREAD FOR THE  
PRECISE TRACK AND TIMING OF THIS LOW. AT LEAST INTO DAY 6 MON THE  
12Z-18Z GFS/12Z FV3 GFS AND 12Z ECMWF/CMC PROVIDED A FAIRLY ROBUST  
CLUSTER THAT REFLECTS THE EXPECTED PATTERN BETTER THAN THE LESS  
DEFINED ENSEMBLE MEANS. WHILE NOT THE CASE IN RECENT ECMWF RUNS  
(ADDRESSED NEXT), REMAINING MODELS HAVE TENDED TO EXHIBIT SLOWER  
TRENDS OVER RECENT DAYS AND THE NEW 00Z CMC/UKMET SUGGEST THE  
UPPER LOW COULD BE SLOWER YET. MEANWHILE BOTH THE ECMWF AND ECMWF  
MEAN ARE SIGNIFICANTLY FASTER TO BRING UPSTREAM TROUGH ENERGY INTO  
THE PICTURE LEADING TO GREATER HEIGHT FALLS OVER THE NORTHWEST  
LATE IN THE PERIOD. WITH MULTI-DAY TRENDS HAVING FAVORED STRONGER  
AND MORE PERSISTENT NORTHWEST RIDGING, WOULD PREFER TO LEAN MORE  
TOWARD THE GFS/GEFS VERSUS THE ECMWF/ECMWF MEAN. THE NEW 00Z  
GFS/GEFS/CMC SOLUTIONS FURTHER SUGGEST A LOWER PROBABILITY FOR THE  
12Z ECMWF/ECMWF MEAN SCENARIO.  
 
THERE IS DECENT CONSENSUS SHOWING STRONG HIGH PRESSURE TRACKING  
FROM WESTERN CANADA INTO THE CENTRAL/EASTERN U.S., BEHIND A COLD  
FRONT THAT PROGRESSES SOUTH AND EAST FROM THE NORTHERN TIER  
STATES. TYPICALLY LOW PREDICTABILITY FOR SHORTWAVE DETAILS 6-7  
DAYS OUT IN TIME LOWERS CONFIDENCE FOR FRONTAL/SURFACE LOW  
PRESSURE DETAILS NEAR THE CANADIAN BORDER BY NEXT MON-TUE.  
 
THE FORECAST BLEND BASED ON GUIDANCE THROUGH THE 18Z CYCLE STARTED  
WITH AN OPERATIONAL MODEL BLEND FOR DAYS 3-4 FRI-SAT WITH GREATER  
EMPHASIS ON THE 12-18Z GFS/12Z ECMWF RELATIVE TO THE CMC/UKMET.  
THEN WITH TIME THE INITIAL BLEND TRANSITIONED TOWARD A GREATER  
PROPORTION OF GFS/FV3 GFS AND GEFS RELATIVE TO THE ECMWF/ECMWF  
MEAN.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE SYSTEM TRACKING OFF THE NORTHEAST COAST ON FRI WILL LIKELY  
PRODUCE SOME MEANINGFUL WINTRY PRECIPITATION OVER PARTS OF THE  
NORTHEAST. BEST POTENTIAL EXISTS OVER INTERIOR LOCATIONS WHILE  
CLOSER TO THE COAST THERE WILL BE GREATER SENSITIVITY TO THE EXACT  
TRACK OF THE SURFACE LOW. A FAIRLY WEAK SYSTEM CROSSING GREAT  
LAKES EARLY IN THE PERIOD WILL BRING SOME LIGHT PRECIPITATION TO  
THE REGION. THEN TRAILING COLD LOW LEVEL FLOW WILL SUPPORT ONE OR  
MORE EPISODES OF LAKE EFFECT SNOW. EARLY NEXT WEEK A FRONT AND  
POSSIBLE EMBEDDED WAVINESS MAY BRING SOME PRECIPITATION TO PARTS  
OF THE GREAT LAKES REGION BUT CONFIDENCE IN SPECIFICS IS LOW AT  
THIS TIME. OVER/NEAR THE NORTHERN-CENTRAL ROCKIES AND HIGH PLAINS  
EXPECT A PERIOD OF SNOW FROM LATE WEEK INTO THE WEEKEND IN  
ASSOCIATION WITH SHORTWAVE ENERGY ALOFT AND ENHANCEMENT FROM LOW  
LEVEL UPSLOPE FLOW. GREATEST ACCUMULATIONS WITH THIS ACTIVITY  
SHOULD BE OVER NORTHERN AREAS. ASIDE FROM THE SEEMINGLY LESS  
LIKELY ECMWF SCENARIO, RECENT DRIER TRENDS SUGGEST THE WEST COAST  
STATES SHOULD SEE LITTLE TO NO PRECIPITATION FOR MOST OF THE  
PERIOD. PARTS OF THE SOUTHEASTERN HALF OF TEXAS MAY SEE PERIODS  
OF RAIN BY THE LATTER HALF OF THE PERIOD WITH SOME ASSISTANCE FROM  
A PERSISTENT SURFACE TROUGH ALONG THE MEXICO/SOUTHERN TEXAS COAST.  
HIGHEST TOTALS SHOULD BE OVER EXTREME SOUTHERN LOCATIONS.  
 
STRONG HIGH PRESSURE DROPPING INTO THE PLAINS WILL BRING COLD  
TEMPERATURES OF AT LEAST 10-20F BELOW NORMAL FOR HIGHS INTO MANY  
PARTS OF THE PLAINS/MISSISSIPPI VALLEY DURING THE WEEKEND.  
ANOMALIES SHOULD BE A LITTLE LESS EXTREME OVER THE SOUTH AND EAST  
BY THE FIRST PART OF NEXT WEEK BUT SOME AREAS WILL LIKELY STILL BE  
10-15F OR MORE BELOW NORMAL. EXPECT THE WEST COAST STATES TO SEE  
ABOVE NORMAL TEMPERATURES THROUGH AT LEAST SUN WITH SOME LOCALIZED  
ANOMALIES EXCEEDING PLUS 10F FOR HIGHS. WARMTH SHOULD CONTINUE TO  
SOME EXTENT EARLY NEXT WEEK.  
 
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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