930  
FXUS02 KWBC 140656  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
156 AM EST WED NOV 14 2018  
 
VALID 12Z SAT NOV 17 2018 - 12Z WED NOV 21 2018  
   
..OVERVIEW
 
 
THE LATEST UPDATE TO THE FORECAST MAINTAINS FAIRLY GOOD  
CONTINUITY, WITH CONSENSUS IDEAS THROUGH THE 12Z/18Z CYCLES NOT  
SHOWING DRAMATIC CHANGES AND LATER PERIOD SPREAD SIMILAR TO THAT  
FROM THE PAST DAY. CONTINUE TO EXPECT BROAD MEAN TROUGHING  
DOWNSTREAM FROM A RIDGE THAT BUILDS INTO WESTERN NORTH AMERICA  
DURING THE WEEKEND. WITH TYPICAL DIFFERENCES/VARIABILITY IN THE  
DETAILS, THERE IS STILL AGREEMENT IN PRINCIPLE THAT UPSTREAM  
EASTERN PACIFIC TROUGH ENERGY WILL SPLIT WITH SOME PASSING THROUGH  
THE WESTERN CANADA PART OF THE RIDGE AND THE REST FORMING AN  
UNDERCUTTING TROUGH/LOW THAT SHOULD PROGRESS INTO THE SOUTHWEST  
U.S./NORTHWESTERN MEXICO BY NEXT TUE-WED. A FAIR DEGREE OF  
UNCERTAINTY PERSISTS WITH THE SPECIFICS OF THE NEXT PACIFIC TROUGH  
FORECAST TO APPROACH AND POSSIBLY REACH THE WEST COAST BY MIDWEEK.  
 
   
..GUIDANCE/UNCERTAINTY ASSESSMENT
 
 
THE FORECAST APPROACH FOR THIS UPDATE WAS SIMILAR TO RECENT CYCLES  
WITH A CONSENSUS MODEL BLEND USED EARLY IN THE PERIOD, AS THE 18Z  
GFS AND 12Z FV3 GFS/ECMWF/UKMET/CMC DISPLAYING SOME COMPARABLE  
ASPECTS DEPENDING ON THE REGION/FEATURE. THEN THE FORECAST  
EVENTUALLY TRENDED TOWARD ABOUT A 2/3 TILT TO THE 18Z GFS/GEFS  
MEAN AND 12Z FV3 GFS RELATIVE TO THE 12Z ECMWF/ECMWF MEAN. THE  
KEY WEAKNESSES IN THE 12Z GFS APPEARED TO BE A MORE RAPID THAN  
CONSENSUS BREAKDOWN OF THE WESTERN RIDGE BY DAY 4 SUN AND A FASTER  
THAN CONSENSUS PROGRESSION OF THE SOUTHERN STREAM TROUGH/UPPER  
LOW. THE 18Z GFS WAS ON THE SHARP SIDE WITH THE UPPER TROUGH  
CROSSING THE GREAT LAKES/NORTHEAST SUN-MON LEADING TO SURFACE LOW  
PRESSURE TO THE EAST/NORTHEAST OF MAINE WRAPPING UP MUCH MORE  
STRONGLY THAN IN OTHER SOLUTIONS, FAVORING SUFFICIENTLY LOW WEIGHT  
TO DOWNPLAY THIS ASPECT OF THE MODEL RUN.  
 
THE FULL RANGE OF MODEL/ENSEMBLE SOLUTIONS OVER THE EASTERN  
PACIFIC/WESTERN NORTH AMERICA WOULD SUGGEST A MODERATE OR HIGHER  
DEGREE OF UNCERTAINTY IN FORECAST SPECIFICS ALREADY BY DAY 4 SUN,  
AS SOME LINGERING MEMBERS DID NOT SHOW THE FLOW SEPARATION WHICH  
NEARLY ALL OPERATIONAL RUNS HAVE DISPLAYED RECENTLY. THE  
OPERATIONAL SCENARIO IS MOST LIKELY GIVEN ITS GENERAL CONSISTENCY  
IN RECENT RUNS (MINUS TRACK AND TIMING DIFFERENCES FOR THE  
SOUTHERN LOW) BUT THE MODEST TREND OVER THE PAST DAY TOWARD A  
LITTLE MORE INITIAL ENERGY STAYING IN THE NORTHERN STREAM  
HIGHLIGHTS THE ONGOING POTENTIAL FOR FURTHER CHANGES IN SOME  
DETAILS. THEN BY THE SECOND HALF OF THE PERIOD THE OVERALL  
SOLUTION SPREAD HAS REMAINED SIMILAR TO YESTERDAY WITH THE 12Z  
ECMWF/ECMWF MEAN STILL BRINGING UPSTREAM HEIGHT FALLS INTO THE  
WEST FASTER THAN THE GFS/GEFS MEAN. THROUGH THE TIME OF FORECAST  
PREPARATION, MULTI-DAY TRENDS AND SUPPORT FROM THE CMC/CMC MEAN  
(WHICH AT TIMES OVER RECENT WEEKS HAD ACTUALLY BEEN TOO  
FLAT/PROGRESSIVE WHEN OTHER GUIDANCE WAS INDICATING AMPLIFIED  
PATTERNS) FAVORED LEANING MORE TOWARD THE SLOWER GFS/GEFS  
SCENARIO. AT LEAST 1/3 INCLUSION OF THE ECMWF IDEA WAS STILL  
REASONABLE AS UPSTREAM NORTH PACIFIC POSITIVE HEIGHT ANOMALIES  
TELECONNECTED TO TROUGHING THAT COULD BE A LITTLE EAST OF THE  
GFS/GEFS. NEW 00Z GUIDANCE THAT HAS ARRIVED SO FAR IS SUGGESTING  
AT LEAST A MORE EVEN BALANCE OF THE FASTER/SLOWER IDEAS AS THE CMC  
MADE A DRAMATIC CHANGE TO BEING BETWEEN THE GFS AND 12Z ECMWF  
WHILE EXTRAPOLATION OF THE 00Z UKMET WOULD LEAD TO A FARTHER EAST  
TROUGH THAN THE GFS AS WELL.  
 
ELSEWHERE THERE WAS GOOD CONTINUITY/CLUSTERING WITH THE INITIAL  
COLD FRONT PUSHING SOUTHWARD/EASTWARD AHEAD OF HIGH PRESSURE  
REACHING THE NORTHERN-CENTRAL PLAINS DURING THE WEEKEND. SPREAD  
AND VARIABILITY SO FAR YIELD LOWER CONFIDENCE IN SPECIFICS OF A  
FRONT AND EMBEDDED WAVE(S) REACHING THE NORTHERN TIER STATES BY  
EARLY NEXT WEEK AND PERHAPS PERSISTING OVER OR NEAR THE GREAT  
LAKES/NORTHEAST THEREAFTER.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
EXPECT THE FORECAST PATTERN DURING THE SAT-WED PERIOD TO YIELD  
FAIRLY LOW COVERAGE OF MEANINGFUL PRECIPITATION ACROSS THE LOWER  
48 STATES. SOME RAIN WILL LIKELY DEVELOP OVER PARTS OF SOUTHERN  
TEXAS BY LATE SUN IN ASSOCIATION WITH A WESTERN GULF COAST SURFACE  
TROUGH AND APPROACH OF THE COLD FRONT PUSHING SOUTHWARD THROUGH  
THE PLAINS. RAINFALL COVERAGE/INTENSITY MAY INCREASE OVER THE  
SOUTHERN PLAINS AND VICINITY TOWARD TUE-WED AS THE SOUTHERN STREAM  
UPPER SYSTEM APPROACHES. ALSO BY NEXT TUE-WED THE WEST COAST  
STATES SHOULD SEE SOME INCREASE IN MOISTURE BUT WITH CONSIDERABLE  
UNCERTAINTY REGARDING THE TIMING/COVERAGE/MAGNITUDE OF ASSOCIATED  
PRECIPITATION. FARTHER EASTWARD THE NORTH-CENTRAL ROCKIES/HIGH  
PLAINS WILL SEE SOME EARLY-PERIOD SNOW GENERATED BY SHORTWAVE  
ENERGY ALOFT AND LOW LEVEL UPSLOPE FLOW. SOME OF THE MOISTURE MAY  
EXTEND EASTWARD AS LIGHT RAIN/SNOW. THE GREAT LAKES REGION WILL  
SEE PERIODS OF LAKE EFFECT SNOW WITH THIS ACTIVITY TENDING TO  
FAVOR NORTHERN AND EASTERN AREAS. THE WAVY FRONT POTENTIALLY  
SETTLING OVER THE AREA NEXT WEEK MAY PROMOTE SOME SYNOPTIC-SCALE  
SNOW AS WELL.  
 
OVER THE COURSE OF THE PERIOD THE EMPHASIS FOR COLDEST ANOMALIES  
(SOME HIGHS AT LEAST 15-20F BELOW NORMAL) SHOULD SHIFT FROM THE  
CENTRAL U.S. DURING THE WEEKEND INTO THE NORTHEAST BY TUE-WED.  
BEHIND THE COLD AIR THERE WILL BE A MODERATING TREND THAT  
PROGRESSES FROM THE PLAINS INTO EAST-CENTRAL U.S. ANTICIPATE  
WARMEST HIGHS RELATIVE TO NORMAL OVER AND NEAR NORTHERN  
CALIFORNIA/SOUTHERN OREGON DURING THE FIRST HALF OF THE PERIOD  
WITH SOME PLUS 10-15F ANOMALIES.  
 
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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