107  
FXUS02 KWBC 241600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1159 AM EDT WED OCT 24 2018  
 
VALID 12Z SAT OCT 27 2018 - 12Z WED OCT 31 2018  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT AND PREFERENCES
 
 
GENERAL AGREEMENT WITH LARGE SCALE FEATURES WILL PROVIDE SOMEWHAT  
BETTER THAN AVERAGE CONFIDENCE IN THE OVERALL MEAN PATTERN  
EVOLUTION BUT EMBEDDED MEDIUM TO SMALLER SCALE SHORTWAVE FEATURES  
WILL LEAD TO UNCERTAINTY IN SOME FORECAST DETAILS.  
 
INTO THE LATEST CYCLE OF GUIDANCE RECENT TRENDS ARE CONTINUING FOR  
SYSTEMS WITHIN/SUPPORTED BY THE EASTERN U.S. MEAN TROUGH ALOFT.  
CONSENSUS MAINTAINS FASTER PROGRESSION OF THE STORM SYSTEM  
AFFECTING AREAS NEAR THE EAST COAST THIS WEEKEND IN RESPONSE TO  
AMPLIFYING TRENDS FOR UPSTREAM ENERGY DIGGING INTO THE EASTERN  
TROUGH. IMPORTANCE OF SMALLER SCALE DETAILS ALOFT FOR THE EAST  
COAST STORM IS HIGHLIGHTED BY THE SIGNIFICANT DIVERGENCE OF  
INDIVIDUAL ENSEMBLE MEMBER SURFACE LOWS THAT ARISES AFTER EARLY  
SAT. THE AMPLIFYING TREND FOR THE UPSTREAM ENERGY IS STEADILY  
INCREASING THE POTENTIAL FOR MEANINGFUL SYSTEM DEVELOPMENT ALONG  
THE NORTHERN HALF OF THE EAST COAST AROUND DAYS 5-6 MON-TUE.  
HOWEVER THERE IS A FAIR AMOUNT OF SPREAD AMONG MODELS/ENSEMBLE  
MEMBERS REGARDING HOW THE ENERGY ALOFT WILL ULTIMATELY EVOLVE,  
WITH THE SURFACE LOW TRACK/STRENGTH QUITE SENSITIVE TO THOSE  
SPECIFICS.  
 
MEANWHILE EVOLUTION OVER THE EASTERN PACIFIC AND WESTERN U.S. WILL  
LEAD TO A SIGNIFICANT PATTERN CHANGE, WITH AN INITIAL EASTERN  
PACIFIC TROUGH EVENTUALLY AMPLIFYING/ELONGATING OVER THE WEST AS A  
RIDGE STEADILY BUILDS OVER THE EASTERN PACIFIC. INTERESTINGLY,  
OVER THE PAST COUPLE DAYS THE OPERATIONAL GFS RUNS HAVE LED THE  
OPERATIONAL ECMWF IN REFLECTING THE AMPLITUDE AND SHARPNESS OF THE  
UPPER TROUGH WHILE THE AMONG THE MEANS THE GEFS HAS BEEN TRENDING  
TOWARD THE ECMWF MEAN THAT HAD AN EARLIER REFLECTION OF THE TROUGH  
FROM THE EASTERN PACIFIC INTO THE WEST. AS A WHOLE CONSENSUS HAS  
BEEN TRENDING MORE AMPLIFIED/ELONGATED WITH THE TROUGH, AND  
TELECONNECTIONS RELATIVE TO THE UPSTREAM RIDGE'S POSITIVE HEIGHT  
ANOMALY CENTER SUPPORT THE NORTHEAST-SOUTHWEST ORIENTED TROUGH  
SEEN IN MOST GUIDANCE BY LATE IN THE PERIOD. FROM THE  
TELECONNECTION/MULTI-DAY MEAN PERSPECTIVE BY DAY 7 WED EITHER AN  
OPEN TROUGH (GFS, ENSEMBLE MEANS) OR EMBEDDED CLOSED LOW (00Z  
ECMWF/CMC) ARE PLAUSIBLE. THUS AT THAT TIME AN EVEN BLEND AMONG  
MODEL/ENSEMBLE SOLUTIONS WOULD BE REASONABLE--INDICATING SOME  
SEPARATION IN FLOW BUT NOT TO THE POINT OF HAVING A CLOSED LOW.  
 
THE EVOLVING PACIFIC/WESTERN U.S. PATTERN WILL EVENTUALLY FAVOR A  
FLATTENING OF FLOW ALOFT OVER THE EASTERN U.S. AROUND THE END OF  
THE EXTENDED PERIOD THE QUESTION FOR TIMING THE DEPARTURE OF THE  
EASTERN TROUGH ALOFT WILL BE THE RELATIVE INFLUENCE OF THE  
UPSTREAM PATTERN THAT WILL BECOME INCREASINGLY FAVORABLE FOR THE  
TROUGH'S PROGRESSION, VERSUS RATE OF WEAKENING OF STRONG ATLANTIC  
RIDGING THAT DURING MOST OF THE PERIOD FAVORS THE AMPLIFIED  
EASTERN TROUGH.  
 
THE FORECAST UPDATE STARTED WITH AN OPERATIONAL MODEL BLEND AND  
MINOR DETAIL ENHANCEMENT EARLY IN THE PERIOD FOLLOWED BY A GRADUAL  
TREND TOWARD AN EVEN MODEL/ENSEMBLE MEAN BLEND BY DAY 7 WED GIVEN  
WESTERN U.S. PREFERENCES AND INCREASING DETAIL UNCERTAINTY OVER  
THE EAST.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE INITIAL SYSTEM ALONG THE EAST COAST WILL BRING AN AREA OF  
SIGNIFICANT PRECIPITATION TO THE NORTHEAST THIS WEEKEND.  
INLAND/HIGHER ELEVATION LOCATIONS MAY SEE SOME SNOW FROM THIS  
EVENT. MEANWHILE A TIGHT PRESSURE GRADIENT BETWEEN THE SURFACE  
LOW AND RETREATING HIGH PRESSURE TO THE NORTH WILL LEAD TO STRONG  
WINDS ON THE NORTH SIDE OF THE SYSTEM. THE NEXT SYSTEM SHOULD  
SPREAD AN AREA OF RAIN ACROSS PARTS OF THE NORTHERN PLAINS/MIDWEST  
THIS WEEKEND AND THEN THE APPALACHIANS AND EAST COAST STATES BY  
THE FIRST HALF OF NEXT WEEK. DETAILS OF SYSTEM EVOLUTION WILL  
DETERMINE THE DEGREE OF SNOW POTENTIAL OVER THE HIGHER TERRAIN.  
IF SURFACE DEVELOPMENT IS SUFFICIENTLY STRONG THEN THERE WOULD BE  
ANOTHER EPISODE OF BREEZY TO WINDY CONDITIONS OVER THE NORTHEAST  
TUE-WED.  
 
A FRONTAL SYSTEM NEARING THE PACIFIC NORTHWEST DURING THE WEEKEND  
AND A PERIOD OF ONSHORE FLOW WILL BRING A PERIOD OF FOCUSED  
PRECIPITATION TO THE COASTAL RANGES, CASCADES, AND EVENTUALLY TO A  
LESSER EXTENT NORTHERN ROCKIES. ENERGY ROUNDING THE STRENGTHENING  
EASTERN PACIFIC RIDGE ALOFT MAY SUPPORT ADDITIONAL ACTIVITY TOWARD  
MIDWEEK BUT AT THIS TIME IT IS UNCERTAIN WHETHER IT WILL EXTEND  
FAR ENOUGH SOUTH TO REACH THE PACIFIC NORTHWEST. EXPECT MOSTLY  
RAIN EXCEPT FOR PERHAPS A LITTLE SNOW AT HIGHEST ELEVATIONS. RAIN  
AND HIGHER ELEVATION SNOW WILL LIKELY REACH THE FOUR CORNERS  
STATES BY TUE-WED AS THE UPPER LEVEL TROUGH PROGRESSES INLAND.  
 
MUCH OF THE WESTERN THIRD OF THE LOWER 48 WILL BE QUITE WARM  
DURING SAT-MON WITH SOME LOCATIONS SEEING ONE OR MORE DAYS OF  
HIGHS AND/OR LOWS 10-20F ABOVE NORMAL. ARRIVAL OF UPPER TROUGHING  
WILL THEN BRING A RAPID COOLING TREND WITH HIGHS AT LEAST 5-10F  
BELOW NORMAL FROM THE GREAT BASIN/SOUTHWEST INTO CENTRAL ROCKIES  
BY NEXT WED. MUCH OF THE EASTERN U.S. WILL SEE BELOW NORMAL  
TEMPERATURES MOST OF THE PERIOD WITH ONLY GRADUAL MODERATION FROM  
WEST TO EAST BY NEXT TUE-WED. BEST POTENTIAL FOR HIGHS 10F OR  
MORE BELOW NORMAL WILL EXTEND FROM THE OHIO/TENNESSEE VALLEYS INTO  
THE APPALACHIANS AND NORTHEAST. CLOUDS/WIND WILL TEND TO KEEP  
MORNING LOWS CLOSER TO NORMAL.  
 
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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