300  
FXUS02 KWBC 220705  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
304 AM EDT MON OCT 22 2018  
 
VALID 12Z THU OCT 25 2018 - 12Z MON OCT 29 2018  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT/PREFERENCES
 
 
THE SYNOPTIC-SCALE PATTERN AHEAD WILL INITIALLY FEATURE A SPRAWLED  
OUT UPPER TROUGH EXTENDING FROM THE CANADIAN MARITIMES BACK UP  
TOWARD NORTHERN NUNAVUT. THE SOUTHERN EXTENSION OF THESE LOWER  
HEIGHTS SHOULD GRADUALLY GAIN LATITUDE IN RESPONSE TO THE NEXT  
SYSTEM TAKING SHAPE ACROSS THE CENTRAL U.S. A PHASING OF STREAMS  
CONTINUES TO BE ADVERTISED WHICH SPAWNS A WAVE OF LOW PRESSURE  
TRAVERSING THE NORTHERN GULF OF MEXICO ON THURSDAY/FRIDAY BEFORE  
THE PARENT SHORTWAVE ATTAINS A NEGATIVE TILT. DURING THE WEEKEND,  
A STRENGTHENING COASTAL LOW WILL LIKELY LIFT POLEWARD IN THE  
VICINITY OF THE GULF STREAM WITH POTENTIAL IMPACTS TO THE EASTERN  
SEABOARD. FARTHER UPSTREAM, QUASI-ZONAL FLOW SETS UP OVER THE  
NORTHEASTERN PACIFIC THROUGH FRIDAY WHICH FAVORS A SERIES OF  
PERTURBATIONS TO TRACK INTO THE PACIFIC NORTHWEST AND BRITISH  
COLUMBIA COASTS. A BROAD POSITIVE HEIGHT ANOMALY EMERGES FOR THE  
WEEKEND ACROSS THE WESTERN U.S. WHILE MIGRATORY SHORTWAVES  
TRAVERSE THE TOP OF THE RIDGE WITH A TRACK FROM BRITISH  
COLUMBIA/ALBERTA TOWARD THE NORTH-CENTRAL U.S. MANY SOLUTIONS  
SUPPORT SIGNIFICANT AMPLIFICATION WITH A COUPLE OF THESE WAVES AS  
THEY MOVE TOWARD THE MISSISSIPPI RIVER VALLEY OVER THE WEEKEND.  
 
RECENT GUIDANCE IS INITIALLY IN DECENT AGREEMENT WITH MUCH OF THE  
UNCERTAINTY LOOMING AROUND THE PHASING SHORTWAVES AND EVENTUAL LOW  
PRESSURE DEVELOPMENT/PROGRESSION. ENSEMBLE LOW PLOTS DEPICT  
CONTINUED SCATTER ALTHOUGH THIS IS LIKELY AN ARTIFACT OF A PAIR OF  
WAVES INITIALLY IN THE PICTURE. OPERATIONAL MODELS CONTINUE TO  
WAVER WITH THE ULTIMATE TIMING OF A DEEPENING SURFACE LOW OFF THE  
SOUTHEASTERN U.S. COAST FRIDAY EVENING. ON THE WESTERN SIDE OF THE  
SPREAD ARE THE 00Z UKMET AS WELL AS THE 00Z/12Z ECMWF RUNS WHILE  
THE GFS SOLUTIONS AND THEIR ENSEMBLES STAY A BIT FARTHER OFFSHORE.  
GIVEN THE RUN-TO-RUN VARIABILITY AND SPREAD SHOWN IN THE ENSEMBLE  
LOW PLOTS, CONFIDENCE STILL REMAINS ON THE LOWER SIDE WITH  
TIMING/POSITION DETAILS. MOVING WESTWARD, HEIGHT FALLS DIPPING  
DOWN FROM SASKATCHEWAN WILL LEAD TO ANOTHER SURGE OF COLDER AIR TO  
THE MISSISSIPPI RIVER VALLEY REGION WITH THE SHORTWAVE LIKELY  
JOINING THE DOWNSTREAM MEAN TROUGH. ENSEMBLE SPAGHETTI PLOTS SHOW  
NUMEROUS SPATIAL DIFFERENCES WHICH SOMEWHAT HINGES ON THE TIMING  
OF THE LEAD IMPULSE. FINALLY, ACROSS THE EASTERN PACIFIC AND  
ADJACENT WESTERN COAST OF NORTH AMERICA, ENSEMBLE MEANS ARE AT  
ODDS WITH ONE ANOTHER BY NEXT MONDAY, OCTOBER 29. THE 12Z ECMWF  
ENSEMBLE MEAN HAS TRENDED TOWARD A LONGWAVE TROUGH IN THIS REGION  
WHILE THE 18Z GEFS/12Z NAEFS MEANS FAVOR QUASI-ZONAL TO WEAK  
RIDGING. ENSEMBLE SPAGHETTI PLOTS REFLECT THESE DIFFERENCES WITH  
POSSIBLE CHANGES LIKELY IN FUTURE MODEL CYCLES.  
 
REGARDING THE PREFERENCE, UTILIZED A COMBINATION OF THE 18Z/12Z  
GFS AND 12Z ECMWF WITH THEIR RESPECTIVE ENSEMBLE MEANS (INCLUDING  
THE 12Z NAEFS) THROUGHOUT THE PERIOD. WHILE INITIALLY WEIGHTING 80  
PERCENT OF THESE OPERATIONAL RUNS INTO THE MIX ON DAY 3/THURSDAY,  
INCREMENTALLY DECREASED THEIR INFLUENCES THROUGH THE WEEKEND  
BEFORE A FULL ENSEMBLE APPROACH BY THE FOLLOWING WEEK.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE MOST ACTIVE PRECIPITATION PATTERNS WILL FOCUS ALONG THE  
CENTRAL/EASTERN GULF COAST AND ALONG THE EASTERN SEABOARD AS WELL  
AS BACK OVER THE PACIFIC NORTHWEST. ACROSS THE FORMER REGIONS, THE  
MENTIONED SURFACE LOW AND ACCOMPANYING TROPICAL MOISTURE FLUXES  
WILL MAINTAIN A WET PERIOD FROM SOUTHERN LOUISIANA EASTWARD INTO  
THE SOUTHEASTERN U.S. COAST. AS THE COASTAL CYCLONE DEEPENS OVER  
THE GULF STREAM, COMMA-HEAD PRECIPITATION SHOULD EXTEND BACK INTO  
THE EASTERN SEABOARD WITH IMPACTS DRIVEN BY THE PROXIMITY OF THE  
WAVE TO THE COAST. THERE MAY EVEN BE SOME WINTRY IMPLICATIONS OVER  
THE NORTHEASTERN U.S. INTERIOR MOUNTAINS. BACK OVER THE  
NORTHWESTERN U.S., PERSISTENT ONSHORE FLOW WILL ALLOW FOR MULTIPLE  
DAYS OF OROGRAPHICALLY DRIVEN PRECIPITATION. THE USUAL FOCUS  
SHOULD BE OVER THE UPSLOPE TERRAIN OF THE OLYMPICS AND WASHINGTON  
CASCADES. CONSIDERING THE TEMPERATURE FORECAST, BELOW AVERAGE  
TEMPERATURES ARE IN THE FORECAST FROM THE GREAT PLAINS EASTWARD  
GIVEN THE PERSISTENT OF TROUGHING. SOME OF THE LARGEST DEPARTURES,  
ON THE ORDER OF 10 TO 15 DEGREES, WILL CONGREGATE ALONG MUCH OF  
THE EASTERN SEABOARD GIVEN THE EXTENDED PERIOD OF CLOUD  
COVER/PRECIPITATION AS WELL AS POST-FRONTAL COLD ADVECTION.  
MEANWHILE, MILDER WEATHER WILL PREVAIL OVER SECTIONS OF THE WEST  
COAST INTO THE INTERMOUNTAIN WEST GIVEN THE INFLUENCE OF PACIFIC  
AIR MASSES ON THE FORECAST. IF THE GEFS/NAEFS ENSEMBLE SOLUTIONS  
WERE TO VERIFY, THIS MILD WEATHER COULD EXTEND INTO THE FOLLOWING  
WEEK.  
 
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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