621  
FXUS02 KWBC 160659  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
158 AM EST TUE NOV 16 2021  
 
VALID 12Z FRI NOV 19 2021 - 12Z TUE NOV 23 2021  
   
..OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE OVERALL FORECAST IS CONSISTENT IN ADVERTISING A TRANSITION  
FROM RELATIVELY FLAT AND PROGRESSIVE MEAN FLOW LATE THIS WEEK TO A  
MORE AMPLIFIED PATTERN DURING THE WEEKEND AND EARLY NEXT WEEK AS A  
DEEPENING UPPER TROUGH HEADS INTO THE EAST DOWNSTREAM FROM A RIDGE  
THAT MOVES INTO THE WEST BY MONDAY. THE UPPER TROUGH AND A  
POSSIBLE EMBEDDED LOW MAY SUPPORT POTENTIALLY SIGNIFICANT LOW  
PRESSURE THAT WOULD AFFECT PORTIONS OF THE EAST, AND REQUIRES  
MONITORING GIVEN THE BUSY THANKSGIVING HOLIDAY TRAVEL WEEK.  
MEANWHILE IT LOOKS INCREASINGLY LIKELY THAT INITIALLY WEAK  
SOUTHERN STREAM PACIFIC ENERGY WILL COLLECT INTO AN UPPER LOW TO  
THE WEST/SOUTH OF CALIFORNIA. THIS FEATURE MAY APPROACH OR REACH  
THE SOUTHWESTERN U.S. BY NEXT TUESDAY. THE EXPECTED PATTERN WILL  
PRODUCE TWO SURGES OF CHILLY AIR OVER THE EASTERN HALF OF THE  
COUNTRY, ONE DURING FRIDAY-SATURDAY AND ANOTHER EARLY NEXT WEEK,  
WHILE MAINTAINING MOSTLY ABOVE NORMAL TEMPERATURES FROM THE WEST  
INTO PARTS OF THE HIGH PLAINS.  
 
AFTER A LOT OF SPREAD AND VARIABILITY OVER RECENT DAYS, MODEL  
CLUSTERING IS DECENT BUT STILL NOT IDEAL FOR THE SYSTEM EXPECTED  
TO REACH JUST INLAND OVER THE PACIFIC NORTHWEST AT THE START OF  
THE PERIOD EARLY FRIDAY. 12Z/18Z GFS RUNS AND THE 12Z ECMWF/CMC  
PROVIDED THE MOST COMPARABLE SOLUTIONS THROUGH THE 18Z CYCLE WHILE  
THE 12Z UKMET WAS WEAK AND SUPPRESSED. THE NEW 00Z UKMET IS CLOSER  
TO THE OTHER MODELS BUT THE 00Z ECMWF HAS NOW BECOME THE  
WEAK/SOUTH EXTREME.  
 
DETAIL DIFFERENCES WITH THIS SYSTEM'S ENERGY AS WELL AS FOR  
UPSTREAM FLOW DIGGING INTO THE AMPLIFYING MEAN TROUGH MAINTAIN A  
FAIR DEGREE OF UNCERTAINTY FOR THE SPECIFICS OF ONE OR MORE  
SURFACE LOWS AND FRONTAL SYSTEMS OVER THE CENTRAL AND EASTERN U.S.  
FROM SATURDAY THROUGH TUESDAY. THIS HAS SUPPORTED A MODEL/ENSEMBLE  
MEAN BLEND APPROACH TO TONE DOWN DIFFERENCES AND RUN-TO-RUN  
VARIABILITY SEEN IN THE OPERATIONAL MODELS. AS FOR POTENTIALLY  
STRONGER DEVELOPMENT NEAR THE EAST COAST BY DAY 7 TUESDAY, THUS  
FAR THERE HAS BEEN A DECENT SIGNAL FOR A STORM SYSTEM BUT WITHIN A  
BROAD AREA ENCOMPASSING THE GREAT LAKES TO MID-ATLANTIC THROUGH  
THE NORTHEAST, CANADIAN MARITIMES, AND WESTERN ATLANTIC. AT THIS  
TIME AN INTERMEDIATE SURFACE LOW POSITION JUST OFF SOUTHEASTERN  
NEW ENGLAND AS OF 12Z TUESDAY PROVIDES A REASONABLE STARTING POINT  
RELATIVE TO THE MODELS/ENSEMBLE MEANS. THIS IS SOMEWHAT CLOSER TO  
THE ECMWF MEAN THAT HAS BEEN FAIRLY STABLE OVER ITS PAST THREE  
RUNS IN CONTRAST TO THE GEFS MEAN THAT HAS TRENDED SOUTHWARD SINCE  
YESTERDAY (BUT HAS BEEN STEADY NEAR THE SOUTHERN TIP OF NOVA  
SCOTIA IN THE PAST 12 HOURS OF RUNS). THE 12Z CMC MEAN WAS WELL  
OFFSHORE WITH THE SURFACE LOW BUT THE NEW RUN HAS TRENDED FARTHER  
NORTHWEST FOR EARLY TUESDAY. THE SOUTHERN TRACK OF THE 12Z ECMWF  
DEEP UPPER LOW NEAR THE EAST COAST LED TO SOME 500MB HEIGHT  
ANOMALIES OF 5-6 STANDARD DEVIATIONS BELOW NORMAL WHILE THE  
ANOMALIES IN OTHER MODELS ARE SOMEWHAT LESS EXTREME. THE NEW 00Z  
ECMWF IS A TAD WEAKER AS OF EARLY TUESDAY.  
 
QUESTION MARKS UPSTREAM LATE IN THE PERIOD INVOLVE THE NEXT  
PACIFIC SHORTWAVE SCHEDULED TO ARRIVE INTO THE WEST AROUND LATE  
MONDAY-TUESDAY, ALONG WITH WHAT BECOMES OF THE UPPER LOW EXPECTED  
TO FORM OFFSHORE FROM THE SOUTHWESTERN U.S. CONSENSUS INDICATES  
THE 12Z CMC WAS TOO AMPLIFIED WITH THE NORTHERN STREAM SHORTWAVE  
AND INDEED THE NEW 00Z RUN HAS TONED IT DOWN SOMEWHAT. PRIOR  
GFS/CMC RUNS WERE FASTEST TO EJECT THE UPPER LOW ENERGY INTO THE  
WEST AND THE NEW 00Z RUNS HAVE TRENDED SLOWER.  
 
BEST MODEL CLUSTERING FAVORED USE OF THE 12Z ECMWF/CMC AND 18Z GFS  
AS THE BASIS OF THE UPDATED FORECAST EARLY IN THE PERIOD. ONGOING  
DETAIL UNCERTAINTIES WITH SHORTWAVES DIGGING INTO THE EVENTUAL  
EASTERN U.S. TROUGH AND EVENTUAL STRAYING OF THE 12Z CMC FROM  
CONSENSUS RECOMMENDED A TRANSITION TO A MIX OF THE 18Z GEFS/12Z  
ECMWF MEANS WITH LINGERING INPUT OF THE 12Z ECMWF/18Z GFS.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
THE SYSTEM FORECAST TO REACH THE NORTHWEST BY EARLY FRIDAY SHOULD  
SPREAD LIGHT TO MODERATE RAIN/MOUNTAIN SNOW FROM THE NORTHERN HALF  
OF THE WEST COAST THROUGH THE NORTHERN ROCKIES. AFTER FRIDAY MUCH  
OF THE WEST WILL SEE MULTIPLE DRY DAYS ASIDE FROM SOME MOISTURE  
RETURNING TO THE NORTHERN PACIFIC NORTHWEST SUNDAY ONWARD AND  
POSSIBLY SOME SCATTERED PRECIPITATION REACHING SOUTHERN PARTS OF  
THE WEST BY TUESDAY AHEAD OF THE UPPER LOW THAT FORMS SOUTHWEST OF  
CALIFORNIA. THE EASTERN GREAT LAKES COULD SEE SOME LINGERING LAKE  
EFFECT ACTIVITY ON FRIDAY. EXPECT PRECIPITATION TO EXPAND IN  
COVERAGE OVER THE EASTERN HALF OF THE COUNTRY FROM SUNDAY ONWARD  
IN ASSOCIATION WITH ONE OR MORE SURFACE LOWS/FRONTAL SYSTEMS. IT  
IS TOO EARLY TO RESOLVE DETAILED EFFECTS FROM LOW PRESSURE THAT  
MAY BE NEAR THE EAST COAST BY NEXT TUESDAY BUT SIGNIFICANT  
RAIN/SNOW AND STRONG WINDS COULD BE POSSIBLE. AT THIS TIME THE  
RELATIVELY HIGHER POTENTIAL FOR MEANINGFUL SNOW EXTENDS FROM THE  
CENTRAL APPALACHIANS THROUGH INTERIOR NEW ENGLAND. THE BROAD  
CYCLONIC CIRCULATION AND COLD AIR MAY PRODUCE SOME LAKE  
EFFECT/ENHANCEMENT AS WELL. EASTERLY FLOW FROM LATE WEEK THROUGH  
THE WEEKEND MAY PROMOTE PERIODS OF RAIN OVER THE SOUTHERN/EASTERN  
PARTS OF THE FLORIDA PENINSULA.  
 
MUCH OF THE EASTERN U.S. WILL SEE BELOW NORMAL TEMPERATURES FRIDAY  
INTO SATURDAY WITH SOME HIGHS 10F OR MORE BELOW NORMAL ON FRIDAY.  
ANOTHER SURGE OF CHILLY AIR WILL LIKELY BRING HIGHS OVER THE EAST  
BACK DOWN TO 5-15F BELOW NORMAL NEXT MONDAY-TUESDAY. ANOMALIES  
COULD TREND COLDER DEPENDING ON THE ULTIMATE DEPTH OF THE UPPER  
TROUGH CROSSING THE REGION. ON THE OTHER HAND THE CENTRAL/SOUTHERN  
HIGH PLAINS MAY SEE HIGHS UP TO 10-15F ABOVE NORMAL FRIDAY INTO  
SATURDAY AND AGAIN NEXT TUESDAY. EXPECT MODERATELY ABOVE NORMAL  
TEMPERATURES OVER MANY AREAS FROM THE ROCKIES TO THE WEST COAST  
THROUGH THE PERIOD.  
 
RAUSCH  
 
ADDITIONAL 3-7 DAY HAZARD INFORMATION CAN BE FOUND ON THE WPC  
MEDIUM RANGE HAZARDS OUTLOOK CHART AT:  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP  
 
WPC MEDIUM RANGE 500MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, WINTER WEATHER OUTLOOK PROBABILITIES  
AND HEAT INDICES ARE 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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