483  
FXUS02 KWBC 271600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1200 PM EDT SAT OCT 27 2018  
 
VALID 12Z TUE OCT 30 2018 - 12Z SAT NOV 03 2018  
   
..PATTERN OVERVIEW
 
 
A MODERATELY PROGRESSIVE FLOW PATTERN ACROSS THE U.S. CONTINUING  
FROM THE SHORT RANGE PERIOD INTO THE START OF THE EXTENDED  
FORECAST WILL BECOME INCREASINGLY DOMINATED BY LARGER SCALE/MORE  
AMPLIFIED AND SLOWER TO EVOLVE FEATURES FROM THE MIDDLE OF NEXT  
WEEK ONWARD. GUIDANCE SHOWS GOOD AGREEMENT AND CONTINUITY IN  
DEPICTING THE ESTABLISHMENT OF A BROAD AND DEEP NORTH AMERICAN  
TROUGH BETWEEN STRONG RIDGES OVER THE EASTERN PACIFIC/WESTERN  
ATLANTIC. THE PACIFIC RIDGE MAY WEAKEN/BROADEN SOMEWHAT TOWARD  
FRI-SAT. THE EXPECTED FLOW PATTERN WILL SET THE STAGE FOR  
TRANSPORT OF CHILLY AIR SOUTHWARD INTO THE CENTRAL U.S. UNDER THE  
UPPER TROUGH WHILE WET CONDITIONS DEVELOP ON THE EASTERN SIDE OF  
THE TROUGH, ESPECIALLY FROM THE LOWER MISSISSIPPI VALLEY/OHIO  
VALLEY TO THE EASTERN SEABOARD.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
EARLY IN THE PERIOD THE GUIDANCE SEEMS TO BE IN THE PROCESS OF  
BECOMING MORE STABLE/AGREEABLE FOR SOUTHERN CANADA LOW PRESSURE  
AND TRAILING FRONT THAT SHOULD EXTEND INTO AND JUST WEST OF THE  
SOUTHERN PLAINS, WHILE A WEAKENING WAVY WARM FRONT ENTERS THE  
PACIFIC NORTHWEST. AN OPERATIONAL MODEL BLEND REPRESENTS THIS  
PART OF THE FORECAST WELL.  
 
FROM LATE WED ONWARD THERE IS STILL A FAIR DEGREE OF UNCERTAINTY  
REGARDING THE EXACT DETAILS OF WESTERN U.S. SHORTWAVE ENERGY THAT  
FLOWS THROUGH THE BASE OF THE DEVELOPING MEAN TROUGH AND THEN  
PROGRESSES NORTHEASTWARD--LIKELY GENERATING A FRONTAL WAVE THAT  
TRACKS SOMEWHERE BETWEEN THE MISSISSIPPI VALLEY AND APPALACHIANS.  
AS WAS THE CASE 24 HOURS AGO THE PRIMARY OPERATIONAL MODEL CLUSTER  
WOULD FAVOR A FASTER WAVE THAN MOST ENSEMBLE MEANS, THOUGH AS OF  
12Z THU THE ECMWF MEAN HAS GRAVITATED TOWARD THE OPERATIONAL IDEA.  
THE 00Z FV3 GFS WAS SLOWER LIKE THE RECENT GEFS MEANS BUT THE  
RECENTLY ARRIVING 06Z VERSION ADJUSTED TO A COMPROMISE. ENSEMBLE  
LOW PLOTS CONTINUE TO DEPICT WIDE SPREAD WITH LITTLE DISCERNIBLE  
CLUSTERING. BY DAY 5 THU PREDICTABILITY SEEMS TO BE SUFFICIENTLY  
LOW TO RECOMMEND AN INTERMEDIATE APPROACH BY WAY OF INCORPORATING  
ENOUGH WPC CONTINUITY TO NUDGE THE SURFACE WAVE A BIT WEST OF THE  
00Z ECMWF/UKMET/CMC AND 06Z GFS, ESSENTIALLY A COMPROMISE BETWEEN  
THOSE RUNS AND THE 00Z GFS.  
 
ENSEMBLE SPREAD HAS NARROWED SOMEWHAT FOR PACIFIC SHORTWAVE ENERGY  
FORECAST TO HEAD INTO/AROUND THE EASTERN PACIFIC/WEST COAST RIDGE  
AROUND DAY 6 FRI. A MAJORITY OF THIS NARROWING HAS BEEN BY  
TRIMMING AWAY THE SOUTHERN SIDE OF THE SPREAD (I.E. YESTERDAY'S  
MEMBERS SHOWING THE STRONGEST SHORTWAVES). THIS TREND AND  
OCCASIONAL TENDENCY FOR RIDGES TO HOLD ON A LITTLE LONGER THAN  
FORECAST PROVIDE SUPPORT FOR REMOVING THE 00Z CMC FROM THE  
FORECAST DURING THE LATTER HALF OF THE PERIOD AS IT HAS ONE OF THE  
STRONGER DEPICTIONS OF THIS SHORTWAVE ENERGY. OTHERWISE A  
MODEL/ENSEMBLE MEAN/CONTINUITY APPROACH LOOKS REASONABLE. BY DAY  
7 SAT ENOUGH FLATTENING DOES OCCUR TO SUPPORT A MODEST EASTWARD  
NUDGE FROM CONTINUITY FOR THE DOWNSTREAM TROUGH ALOFT, WHICH  
PUSHES AT LEAST THE NORTHERN HALF OF THE EAST COAST FRONT A LITTLE  
FARTHER EASTARD. TRAILING ENERGY WITHIN THE CORE OF THE UPPER  
TROUGH MAY REFLECT AS A COUPLE FRONTAL SYSTEMS ANCHORED BY WEAK  
LOWS.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE AMPLIFYING TROUGH ACROSS THE CENTRAL U.S. WILL FAVOR TRANSPORT  
OF GULF OF MEXICO MOISTURE AND INSTABILITY NORTHWARD INTO PORTIONS  
OF THE CENTRAL/EASTERN U.S., ALONG/AHEAD OF A WAVY COLD FRONT. THE  
RESULT WILL BE POTENTIALLY WIDESPREAD RAIN AND THUNDERSTORMS, WITH  
THE POTENTIAL FOR AREAS OF HEAVY RAINFALL FROM THE  
LOWER/MID-MISSISSIPPI VALLEY AND OHIO VALLEY TUE-WED TO THE  
MID-ATLANTIC AND NORTHEAST THU-FRI. THERE IS STILL A FAIRLY  
STRONG SIGNAL IN THE GUIDANCE FOR THIS OVERALL EVENT BUT STILL  
SIGNIFICANT UNCERTAINTY IN SPECIFICS OF THE SURFACE WAVE AND  
SUPPORTING DYNAMICS, SO IT WILL TAKE ADDITIONAL TIME TO RESOLVE  
DETAILS SUCH AS EXACT AXIS OF HEAVIEST RAINFALL. MEANWHILE THE  
PACIFIC NORTHWEST AND NORTHERN ROCKIES WILL ALSO SEE MULTIPLE  
ROUNDS OF PRECIPITATION (RAIN AND HIGH ELEVATION SNOW) OF VARYING  
INTENSITY AS WEAKENING SHORTWAVES/FRONTAL SYSTEMS TRAVERSE THE  
NORTHERN PERIPHERY OF THE PACIFIC RIDGE ALOFT. WITH LIGHTER  
AMOUNTS SOME OF THIS MOISTURE WILL LIKELY EXTEND SOUTHEASTWARD  
INTO THE CENTRAL ROCKIES AND VICINITY. ONE OR MORE WEAK  
SYSTEMS/FRONTS CROSSING THE CENTRAL/EAST-CENTRAL STATES MAY  
PRODUCE LIGHT TO LOCALLY MODERATE PRECIPITATION. SOME OF THIS  
ACTIVITY MAY FALL AS SNOW OVER NORTHERN AREAS.  
 
A FAIRLY BROAD AREA OF ABOVE AVERAGE TEMPERATURES ACROSS MUCH OF  
THE CENTRAL U.S. AS OF EARLY TUE WILL DECREASE IN COVERAGE AND  
MOVE INTO THE EAST BY MID-LATE WEEK AS THE UPPER TROUGH AMPLIFIES  
AND A SURFACE COLD FRONT SPREADS COLDER AIR ACROSS AREAS FROM THE  
HIGH PLAINS THROUGH THE MISSISSIPPI VALLEY AND EVENTUALLY CLOSE TO  
THE APPALACHIANS. EXPECT HIGH TEMPERATURES FROM 5 TO 15 DEG F  
BELOW AVERAGE FROM THE SOUTHERN HALF OF THE ROCKIES THROUGH THE  
CENTRAL U.S. BY MID-LATE WEEK. THE ROCKIES/HIGH PLAINS SHOULD  
BEGIN TO MODERATE SLIGHTLY BY NEXT WEEKEND AND THE APPROACH OF  
UPPER RIDGING WILL PROMOTE ABOVE NORMAL READINGS OVER THE WEST  
COAST STATES BY THU-SAT. MEANWHILE AHEAD OF THE PRIMARY FRONT  
HEADING INTO THE EAST, SOME AREAS WILL SEE HIGH TEMPERATURES 5 TO  
10 DEGREES ABOVE AVERAGE WITH GREATER ANOMALIES LIKELY FOR MORNING  
LOWS.  
 
RAUSCH/RYAN  
 
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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