218  
FXUS02 KWBC 100657  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
257 AM EDT WED OCT 10 2018  
 
VALID 12Z SAT OCT 13 2018 - 12Z WED OCT 17 2018  
 
...SERGIO MOISTURE/RAINS CONTINUE NORTHEASTWARD THROUGH THE  
SOUTHERN PLAINS...  
 
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
LATEST GUIDANCE AGREES WELL WITH EACH OTHER AND CONTINUITY FOR THE  
OVERALL EVOLUTION OF THE MEAN PATTERN, FEATURING THE EVENTUAL  
ESTABLISHMENT OF MEAN TROUGHING OVER THE EASTERN HALF OF NORTH  
AMERICA AS AN EASTERN PACIFIC RIDGE GRADUALLY BUILDS INTO THE  
NORTHWESTERN U.S./WESTERN CANADA WHILE A MODEST MEAN LOW ALOFT  
MEANDERS OVER THE VICINITY OF THE SOUTHWEST U.S./EXTREME NORTHWEST  
MEXICO. BY DAY 7 WED THE MAJORITY OF GUIDANCE SHOWS THE WESTERN  
U.S. PATTERN TAKING ON THE SHAPE OF A REX BLOCK CONFIGURATION.  
WHILE CONSENSUS/CONTINUITY SUGGEST ABOVE AVERAGE CONFIDENCE FOR  
THE LARGE SCALE PATTERN FORECAST, THERE ARE SOME SIGNIFICANT  
UNCERTAINTIES EMBEDDED WITHIN.  
 
ALREADY DURING THE FIRST HALF OF THE PERIOD THERE IS A LOT OF  
MODEL/ENSEMBLE SPREAD AND RUN-TO-RUN VARIABILITY FOR THE EVOLUTION  
OF ENERGY WITHIN THE AMPLIFYING UPPER TROUGH THAT DROPS INTO THE  
NORTHERN/WESTERN U.S. 12Z UKMET AND CMC/CMC ENSEMBLES WERE MOST  
AGGRESSIVE IN THE AMOUNT OF ENERGY THAT IS ULTIMATELY DEPOSITED  
OVER THE WEST--WHILE LEAVING A FLATTER SHORTWAVE TO PROGRESS  
ACROSS THE EASTERN HALF OF THE LOWER 48. ON THE OTHER HAND THE  
12Z/18Z GFS RUNS KEPT ALMOST ALL ENERGY WITHIN THE NORTHERN STREAM  
LEADING TO A MORE AMPLIFIED EASTERN U.S. SHORTWAVE, WHILE THE PAST  
COUPLE ECMWF RUNS HAVE BEEN SOMEWHAT CLOSER TO THE GFS. ENOUGH  
GEFS MEMBERS CONCENTRATED LESS IN THE NORTHERN STREAM FOR THE MEAN  
TO SUPPORT AN INTERMEDIATE SOLUTION RATHER THAN THE OPERATIONAL  
GFS RUNS. TELECONNECTIONS RELATIVE TO THE CORE OF POSITIVE HEIGHT  
ANOMALIES EXPECTED OFF THE ALASKA PANHANDLE EARLY IN THE PERIOD  
FAVOR SOME SOUTHWESTWARD ELONGATION OF SHORTWAVE ENERGY REACHING  
THE WEST BUT LIKELY TO A LESSER EXTENT THAN THE UKMET/CMC. THE  
UNCERTAINTY IN SHORTWAVE ENERGY DISTRIBUTION ULTIMATELY LEADS TO  
INCREASING SPREAD FOR THE CHARACTER OF LOW PRESSURE TRACKING  
ACROSS THE GREAT LAKES-NORTHEAST AND/OR ADJACENT CANADA, TIMING OF  
THE TRAILING COLD FRONT, AND THE LEADING SERGIO MOISTURE STREAMING  
ACROSS THE PLAINS AND BEYOND.  
 
GUIDANCE IS SLOWLY COMING INTO BETTER AGREEMENT FOR THE COMPACT  
UPPER LOW THAT SHOULD START THE PERIOD OFF THE SOUTHERN CALIFORNIA  
COAST. THE GFS AND SOME OTHER MODELS PICKED UP ON THIS FEATURE  
EARLIER THAN THE ECMWF (WHICH HAD BEEN MORE OPEN AND PROGRESSIVE  
UNTIL THE 12Z RUN), THOUGH SPORADIC GFS RUNS LIKE THE 12Z VERSION  
HAVE BEEN A BIT ON THE SLOW SIDE OF THE ENVELOPE. WHAT ULTIMATELY  
HAPPENS WITH THIS LOW WILL DEPEND ON HOW MUCH MID-LATITUDE ENERGY  
AMPLIFIES INTO THE WEST, WITH THE GFS/ECMWF AND THEIR MEANS  
CURRENTLY PROVIDING A MAJORITY SCENARIO THAT KEEPS A MODEST UPPER  
LOW OVER/NEAR THE SOUTHWEST FOR MUCH OF THE PERIOD.  
 
CONTINUITY HAS HELD UP WELL SO FAR FOR THE NEXT CLIPPER IN THE  
SERIES, MOST LIKELY BRUSHING THE NORTHERN STATES DURING THE LATTER  
HALF OF THE PERIOD. A MODEL/ENSEMBLE BLEND WOULD PROVIDE A  
REASONABLE STARTING POINT FOR THIS FEATURE.  
 
THE UPDATED FORECAST INCORPORATED VARIOUS ELEMENTS OF THE 12Z  
ECMWF/18Z GFS/12Z CMC IN ORDER OF HIGHER TO LOWER WEIGHT EARLY IN  
THE PERIOD WITH A MODEST 18Z GEFS/12Z ECMWF COMPONENT. CMC WEIGHT  
WAS LOW ENOUGH NOT TO HAVE SIGNIFICANT INFLUENCE ON THE WESTERN  
U.S. FORECAST. BETTER COMPARISON TO THE MEANS MID-LATE PERIOD  
ALLOWED FOR SOMEWHAT GREATER RETENTION OF THE 12Z ECMWF VERSUS 18Z  
GFS FOR THAT PART OF THE FORECAST, WHILE THE OVERALL SOLUTION  
INCREASED ENSEMBLE MEAN INFLUENCE DUE TO GOOD AGREEMENT FOR THE  
LARGE SCALE PATTERN BUT EMBEDDED DETAIL UNCERTAINTY.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
DURING THE WEEKEND THERE IS REASONABLE CONSENSUS IN PRINCIPLE THAT  
SERGIO'S MOISTURE WILL PRODUCE A BAND OF LOCALLY HEAVY RAINFALL  
OVER THE SOUTHERN PLAINS--AIDED BY INTERACTION WITH AN APPROACHING  
COLD FRONT--BUT SOME ONGOING SPREAD FOR LATITUDE OF HIGHEST  
ACCUMULATIONS. THE MIDDLE OF THE SPREAD WOULD PLACE SUCH AN AXIS  
FROM NORTHERN TEXAS/SOUTHWEST OKLAHOMA INTO NORTHWEST ARKANSAS AND  
EXTREME SOUTHERN MISSOURI. SOME OF THIS MOISTURE MAY CONTINUE  
NORTHEASTWARD ALONG THE FRONT INTO THE OHIO VALLEY/GREAT  
LAKES/NORTHEAST BY EARLY NEXT WEEK. THE CLIPPER SYSTEM ANCHORING  
THE AFOREMENTIONED COLD FRONT WILL BRING SOME LIGHT PRECIPITATION  
TO LOCATIONS NEAR THE EASTERN HALF OF THE CANADIAN BORDER, AND THE  
NEXT CLIPPER IN THE SERIES SHOULD SPREAD ANOTHER AREA OF MOISTURE  
ACROSS THE NORTHERN TIER STATES DURING THE FIRST HALF OF NEXT  
WEEK. THE NORTHERN-CENTRAL ROCKIES SHOULD SEE ONE OR MORE PERIODS  
OF MOSTLY SNOW, WHILE PARTS OF THE SOUTHWEST INTO SOUTHERN PLAINS  
MAY SEE SOME RAIN DUE TO THE PERSISTENT UPPER LOW NEAR THE  
SOUTHWEST AND SOUTHWESTERLY FLOW DOWNSTREAM. A FRONT STALLING  
OVER THE FLORIDA PENINSULA MAY HELP TO FOCUS RAINFALL IN ITS  
VICINITY DURING THE WEEKEND.  
 
EXPECT THE ROCKIES/PLAINS TO BE THE FOCUS FOR THE MOST EXTREME  
TEMPERATURE ANOMALIES DURING THE PERIOD, WITH HIGHS OF 20-30F  
BELOW NORMAL PROGRESSING FROM THE NORTHERN ROCKIES/HIGH PLAINS ON  
SAT INTO THE SOUTHERN HIGH PLAINS BY EARLY NEXT WEEK. MIN  
TEMPERATURES SHOULD BE SOMEWHAT LESS EXTREME. IN MODIFIED FORM  
SOME OF THIS COOL AIR WILL EXTEND EASTWARD, LEADING TO NEAR OR  
BELOW NORMAL HIGHS OVER ALL BUT THE SOUTHEAST. MIN TEMPERATURES  
OVER THE EAST WILL TEND TO BE NEAR OR SOMEWHAT ABOVE NORMAL  
THOUGH. MEANWHILE THE PACIFIC NORTHWEST SHOULD SEE A WARM PERIOD  
(UP TO PLUS 10F OR SO ANOMALIES) AS UPPER RIDGING BUILDS IN, WHILE  
SOUTHERN CALIFORNIA AND THE SOUTHWEST WILL LIKELY REMAIN SOMEWHAT  
BELOW NORMAL DUE TO THE SLOW MOVING UPPER LOW IN THE VICINITY.  
 
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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