515  
FXUS02 KWBC 230700  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
300 AM EDT TUE OCT 23 2018  
 
VALID 12Z FRI OCT 26 2018 - 12Z TUE OCT 30 2018  
   
..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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