680  
FXUS02 KWBC 231600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1159 AM EDT TUE OCT 23 2018  
 
VALID 12Z FRI OCT 26 2018 - 12Z TUE OCT 30 2018  
 
1600 UTC UPDATE...  
 
GUIDANCE AS A WHOLE IS STILL SIMILAR IN PRINCIPLE, EMPHASIZING  
AMPLIFIED EASTERN NORTH AMERICA TROUGHING THAT PERSISTS THROUGH  
MUCH OF THE PERIOD WHILE TROUGHING MOVES INTO THE WEST BY THE  
FIRST HALF OF NEXT WEEK DOWNSTREAM FROM A BUILDING EASTERN PACIFIC  
RIDGE. THE TWO MOST SIGNIFICANT FORECAST UNCERTAINTIES INVOLVE  
SHORTWAVE DETAILS WITHIN THE EASTERN TROUGH/INFLUENCE ON  
SOUTHEAST-EAST COAST LOW PRESSURE FOLLOWED BY SPECIFICS OF UPPER  
TROUGH ENERGY MOVING INTO THE WEST. THE EASTERN U.S. FORECAST  
WILL DEPEND ON SPECIFICS OF MULTIPLE MEDIUM TO SMALLER SCALE  
SHORTWAVES, WITH SUCH SCALE AND UNCERTAIN INTERACTION TENDING TO  
YIELD LOWER THAN AVERAGE PREDICTABILITY FOR EXACT EVOLUTION AT THE  
SURFACE. OPERATIONAL MODEL RUNS HAVE YET TO LOCK ONTO DETAIL  
SPECIFICS AND INDEED LATEST RUNS SHOW SOME DIFFERENCES FROM  
PREVIOUS CYCLES. THE ONE NOTABLE LONGER-TERM TREND IS TOWARD  
SLIGHTLY FASTER PROGRESSION OF THE OVERALL UPPER TROUGH AND  
BEST-DEFINED SURFACE LOW. OVER THE WEST, THE ENSEMBLE MEANS  
PROVIDE A GOOD INTERMEDIATE STARTING POINT BY DAYS 6-7 MON-TUE  
BETWEEN THE RELATIVELY SLOWER/FLATTER 00Z ECMWF AND FASTER/MORE  
AMPLIFIED 00Z-06Z GFS. IN THAT TIME FRAME THE MODELS AND MEANS  
ARE MORE AGREEABLE FOR THE OVERALL EASTERN TROUGH ALOFT. BASED ON  
THE ARRAY OF GUIDANCE A BLEND OF LATEST OPERATIONAL SOLUTIONS  
PROVIDED A GOOD MIDDLE GROUND EARLY IN THE PERIOD GIVEN DETAIL  
UNCERTAINTIES. 06Z GEFS/00Z ECMWF MEAN WEIGHT INCREASED TO 40-70  
PERCENT OF THE BLEND BY DAYS 6-7 RESPECTIVELY, ESPECIALLY IN LIGHT  
OF PREFERENCES FOR THE LATE PERIOD WESTERN U.S. UPPER TROUGH.  
 
SERIES OF AT LEAST A COUPLE FRONTAL SYSTEMS WILL BRING EPISODES OF  
PRECIPITATION TO THE PACIFIC NORTHWEST AND NORTHERN ROCKIES WITH  
HIGHEST AMOUNTS OVER FAVORED WESTWARD-FACING TERRAIN. LOCATIONS  
FROM THE CENTRAL APPALACHIANS/MID-ATLANTIC INTO NEW ENGLAND STILL  
FACE THE POTENTIAL FOR SIGNIFICANT PRECIPITATION TOTALS FROM THE  
STORM TRACKING NEAR THE EAST COAST. TRACK/TIMING WILL INFLUENCE  
PRECISE AMOUNTS AS WELL AS WHERE WINTRY WEATHER MAY OCCUR OVER  
SOME INLAND/HIGHER ELEVATION AREAS. LATEST GUIDANCE THAT HAS  
SHIFTED SOMEWHAT LEFT OF CONTINUITY WOULD PUSH THE RAIN/SNOW LINE  
A BIT TO THE NORTHWEST. THE UPPER PATTERN WILL FAVOR ABOVE NORMAL  
TEMPERATURES OVER THE WEST AND BELOW NORMAL READINGS OVER THE  
EAST, WITH SOME LOCATIONS SEEING ANOMALIES GREATER THAN 10F FROM  
NORMAL ON EACH END OF THE SPECTRUM. THE WEST SHOULD SEE A COOLING  
TREND TOWARD NORMAL AS UPPER TROUGHING ARRIVES BY NEXT TUE.  
 
RAUSCH  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT AND PREFERENCES
 
 
MID-LATITUDE FLOW AMPLIFYING ACROSS THE MID-SOUTH WILL INGEST THE  
UPPER-LEVEL REMAINS OF WILLA WHICH AIDS IN THE DEVELOPMENT AND  
STRENGTHENING OF A SURFACE LOW LIFTING OUT OF THE NORTHEASTERN  
GULF OF MEXICO. THE NEGATIVE TILT OF THE SHORTWAVE WILL FURTHER  
AID IN THE DEEPENING PROCESS AS THE CYCLONE LIFTS POLEWARD ALONG  
THE GULF STREAM TOWARD THE OFFSHORE WATERS OF THE NORTHEASTERN  
U.S. TRAILING THIS FEATURE SHOULD BE ANOTHER DISTURBANCE TRACKING  
SOUTHEASTWARD ACROSS THE OHIO/TENNESSEE VALLEY REGIONS OVER THE  
WEEKEND. THE COMBINATION OF THESE SYSTEMS WILL MAINTAIN A BROAD  
NEGATIVE HEIGHT ANOMALY IN THE VICINITY OF THE NORTHEASTERN U.S.  
INTO EARLY NEXT WEEK. FARTHER UPSTREAM, THE PATTERN IS FORECAST TO  
REMAIN PROGRESSIVE WHICH FORCES THE MEAN UPPER RIDGE ACROSS THE  
WESTERN U.S. TO ADVANCE TOWARD THE ROCKY MOUNTAIN CHAIN BY  
MONDAY/OCTOBER 29. THIS WILL ULTIMATELY ALLOW FURTHER TROUGHING TO  
INVADE THE IMMEDIATE WEST COAST BY EARLY NEXT WEEK WHILE BROAD  
RIDGING RE-ESTABLISHES ITS PRESENCE OVER THE EASTERN PACIFIC.  
 
MULTI-DAY ENSEMBLE LOW PLOT COMPARISONS HAVE SHOWN A DECREASE IN  
SPATIAL SPREAD, PARTICULARLY AS THE 12Z ECMWF MEMBERS HAVE BEGUN  
TO CLUSTER. HOWEVER, THE ECMWF ENSEMBLE SOLUTIONS CONTINUE TO BE  
THE QUICKER AMONGST THE GLOBAL ENSEMBLE DATASETS. WHILE THE MODELS  
DISAGREE ON FORECAST TRACK, THE INTENSITY DOES SEEM TO SHUT OFF BY  
SATURDAY WITH PRESSURES EVENTUALLY RISING TOWARD SUNDAY AS THE  
WAVE LIFTS UP TOWARD COASTAL NEW ENGLAND. REGARDING THE AMPLIFIED  
FLOW PUSHING TOWARD THE TENNESSEE RIVER VALLEY, MULTI-CYCLE  
ENSEMBLE SPAGHETTI PLOT COMPARISONS HAVE SHOWN A STRENGTHENING  
TREND, PARTICULARLY WITH OPERATIONAL SOLUTIONS. UNCERTAINTY EXISTS  
IN HOW LONG TO HOLD ONTO THE NORTHEASTERN U.S. MEAN TROUGH BUT IT  
APPEARS IT SHOULD STICK AROUND UNTIL DAY 7/OCTOBER 30. FARTHER  
WEST, WHILE HEIGHT FALLS TRACK INTO THE PACIFIC NORTHWEST/BRITISH  
COLUMBIA COASTS, THE ECMWF SOLUTIONS CONTINUE TO FAVOR A SLOWER  
PROGRESSION. ENSEMBLE MEANS AFFORD A MORE STABLE APPROACH GIVEN  
THE LEVEL OF UNCERTAINTY EXHIBITED FROM RUN TO RUN. THIS APPROACH  
SUPPORTS A MEAN UPPER RIDGE ACROSS THE EASTERN PACIFIC WHILE A  
SHORTWAVE EMBEDDED WITHIN MID-LEVEL NORTHWESTERLY FLOW WILL TAKE  
SHAPE OVER THE INTERMOUNTAIN WEST NEXT TUESDAY.  
 
THE PREFERENCE FOCUSED HEAVILY ON THE OUTPUT OF THE 18Z/12Z GFS  
AND 12Z ECMWF EARLY ON BEFORE ADDING THEIR RESPECTIVE MEANS INTO  
THE PICTURE IN A RATHER LINEAR FASHION. GENERALLY REDUCED THE  
OPERATIONAL PORTION OF THE BLEND BY 20 PERCENT EACH DAY WHICH  
ALLOWED FOR A HEAVILY ENSEMBLE-BASED FORECAST FOR NEXT WEEK. WOULD  
SAY CONFIDENCE IS AROUND AVERAGE AS A WHOLE BUT STILL A LOT OF  
UNCERTAINTY REVOLVING AROUND THE COASTAL LOW ALONG THE EASTERN  
SEABOARD.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
PRIMARY HEAVY RAINFALL IMPACTS SHOULD FOCUS FROM THE COASTAL  
CAROLINAS NORTHWARD ACROSS THE MID-ATLANTIC AND INTO NEW ENGLAND.  
OF COURSE ANY DISPLACEMENT OF THE LOW FROM THE CURRENT FORECAST  
WOULD MOVE THE AREA OF HIGHER IMPACTS. GIVEN THE TIME OF YEAR,  
THIS WILL BE A LIQUID PRECIPITATION EVENT FOR MANY ALTHOUGH ENOUGH  
COLD AIR DAMMING INLAND SHOULD AFFORD A SNOWFALL THREAT OVER THE  
HIGHER TERRAIN OF NEW ENGLAND. THE OTHER REGION OF ACTIVE WEATHER  
WILL BE THE PACIFIC NORTHWEST GIVEN THE SEMI-PERMANENT ONSHORE  
FLOW SLAMMING INTO THE OLYMPICS AND CASCADES. THE TYPICAL RAIN  
SHADOW IS EVIDENT IN THE RAINFALL FORECAST WITH A SECONDARY MAXIMA  
INLAND OVER THE UPPER INTERMOUNTAIN WEST.  
 
REGARDING TEMPERATURES, AREAS EAST OF THE GREAT PLAINS WILL REMAIN  
COOL THROUGHOUT MUCH OF THE PERIOD. THIS WILL PARTICULARLY BE THE  
CASE ALONG THE EASTERN SEABOARD GIVEN THE COMBINATION OF ABUNDANT  
CLOUD COVER, HEAVY RAINFALL, AND LOWER 1000-500 DM THICKNESSES. AT  
TIMES, READINGS SHOULD SIT AROUND 10 TO 15 DEGREES BELOW  
CLIMATOLOGY SUGGESTING HIGHS IN THE 50S IN MANY LOCATIONS.  
HOWEVER, THESE SAME FACTORS SHOULD DIMINISH NOCTURNAL COOLING  
EFFECTS WITH EXPECTED LOWS AROUND AVERAGE. ACROSS THE WESTERN  
U.S., WARM CONDITIONS WILL PREVAIL HIGHER HEIGHTS SITTING OVER  
CALIFORNIA INTO THE CENTRAL GREAT BASIN. WIDESPREAD 80S ARE LIKELY  
FOR HIGHS ACROSS THE DESERT SOUTHWEST WITH EVEN A FEW POCKETS OF  
LOWER 90S POSSIBLE.  
 
RUBIN-OSTER  
 
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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