426  
FXUS02 KWBC 190659  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
158 AM EST SUN DEC 19 2021  
 
VALID 12Z WED DEC 22 2021 - 12Z SUN DEC 26 2021  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
LATEST MODELS AND ENSEMBLES CONTINUE TO SHOW ONLY SLOW EVOLUTION  
OF THE UPPER PATTERN THROUGH THE WEDNESDAY-SUNDAY PERIOD.  
IMPULSES FLOWING AROUND THE NORTH SIDE OF A VERY STRONG  
CENTRAL/EAST-CENTRAL PACIFIC UPPER RIDGE SHOULD REINFORCE A MEAN  
TROUGH WHOSE AXIS SHOULD GRADUALLY DRIFT FROM THE EASTERN PACIFIC  
TO THE WEST COAST OR VERY SLIGHTLY INLAND. FAST FLOW WILL PREVAIL  
DOWNSTREAM, BETWEEN LOW HEIGHTS OVER CENTRAL CANADA AND AN UPPER  
HIGH FORECAST TO MIGRATE EASTWARD ACROSS MEXICO AND THE SOUTHERN  
GULF OF MEXICO. THIS PATTERN WILL FAVOR MULTIPLE DAYS OF  
WIDESPREAD PRECIPITATION OVER THE WEST WITH THE SIERRA NEVADA  
LIKELY TO SEE THE HIGHEST FIVE-DAY TOTALS. SYSTEMS TO THE EAST OF  
THE ROCKIES WILL INITIALLY HAVE DYNAMIC SUPPORT FROM SOUTHERN  
CANADA/NORTHERN TIER U.S. FLOW AND THEN BECOME MORE DEPENDENT ON  
FEATURES EJECTING FROM THE WEST COAST TROUGH. MUCH ABOVE NORMAL  
TEMPERATURES WILL PREVAIL OVER THE SOUTHERN HALF TO TWO-THIRDS OF  
THE PLAINS WITH SOME OF THIS WARMTH EXTENDING EASTWARD WITH TIME.  
MEANWHILE THE FAR NORTHERN PLAINS SHOULD SEE A PRONOUNCED COLDER  
TREND BY NEXT WEEKEND.  
 
GUIDANCE THROUGH THE 12Z/18Z CYCLES STILL DISPLAYED SOME IMPORTANT  
DETAIL DIFFERENCES FOR FEATURES WITHIN THE EASTERN PACIFIC/WEST  
COAST MEAN TROUGH WITH EVENTUAL IMPLICATIONS ON THE FORECAST  
ACROSS THE LOWER 48. DURING MID-LATE WEEK THE PRIMARY ISSUE HAS  
BEEN THE CHARACTER OF DIGGING ENERGY THAT REPLACES AN INITIAL  
SHORT-TERM UPPER LOW. GFS RUNS HAD BEEN EAGER TO CLOSE OFF  
ANOTHER OFFSHORE UPPER LOW WHOSE ENERGY TAKES LONGER TO EJECT  
INLAND, THUS LEADING TO A SLOWER SYSTEM REACHING THE EAST BY NEXT  
WEEKEND. ASIDE FROM ONE ECMWF RUN, THAT MODEL HAS TENDED TO BE  
MORE OPEN WITH THE SHORTWAVE WITH FASTER EJECTION--BUT GENERALLY  
NOT AS FAST AS LATEST CMC RUNS. ENSEMBLE MEANS HAVE BEEN  
PROVIDING A GOOD MIDDLE GROUND AMONG THESE OPTIONS. BY NEXT  
WEEKEND THE MEANS END UP GRAVITATING TOWARD THE 12Z ECMWF IDEA OF  
A WELL-DEFINED SYSTEM REACHING THE GREAT LAKES ON CHRISTMAS  
DAY/SATURDAY AND THE CANADIAN MARITIMES ON SUNDAY. THE FAST  
NATURE OF THE MEAN FLOW OVER MOST OF THE LOWER 48 WOULD SEEM TO  
FAVOR THIS IDEA VERSUS THE SLOWER 12Z/18Z GFS AS WELL. THE NEW  
00Z GFS HAS MADE A SIGNIFICANT ADJUSTMENT TOWARD THE MAJORITY  
CLUSTER, NOW SHOWING MORE NORTH-SOUTH ELONGATION OF THE MID-LATE  
WEEK ENERGY INSTEAD OF A SLOW UPPER LOW AND THEN FASTER EJECTION  
THEREAFTER. LATER IN THE PERIOD THE GFS/ECMWF AND THEIR MEANS  
COMPARE FAIRLY WELL FOR THE WEST COAST TROUGH AS ADDITIONAL ENERGY  
REINFORCES THE FEATURE, WITH DIFFERENCES WELL WITHIN TYPICAL  
ERROR/GUIDANCE SPREAD FOR FORECASTS THAT FAR OUT IN TIME.  
 
EARLY IN THE PERIOD OVER THE EAST, CONSENSUS HAS BEEN STABLE IN  
RECENT RUNS FOR THE SYSTEM CROSSING SOUTHEASTERN CANADA AND  
NORTHEASTERN U.S. AROUND MIDWEEK. HOWEVER GUIDANCE IS STILL  
HAVING SOME DIFFICULTIES IN RESOLVING THE DETAILS OF THE SYSTEM  
EXPECTED TO BE JUST OFF THE NORTH CAROLINA COAST EARLY WEDNESDAY  
AND CONTINUE OVER THE ATLANTIC THEREAFTER, FAVORING AN  
INTERMEDIATE APPROACH.  
 
THE OPERATIONAL MODEL BLEND USED TO START THE FORECAST FOR  
WEDNESDAY BEGAN TO INCORPORATE THE 18Z GEFS/12Z ECMWF MEANS BY DAY  
5 FRIDAY AND THEN REACHED 60 PERCENT TOTAL ENSEMBLE WEIGHT BY DAY  
7 SUNDAY. THE FORECAST EXCLUDED THE CMC BY THE LATTER HALF OF THE  
PERIOD AND 12Z/18Z GFS WEIGHT WAS KEPT LOW ENOUGH TO MINIMIZE  
INFLUENCE OF ITS SLOWER HANDLING OF LATE WEEK WEST COAST ENERGY.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
A BROAD AREA OF THE WESTERN U.S. WILL SEE RAIN AND MOUNTAIN SNOW  
DURING THE PERIOD, SUPPORTED BY A PERSISTENT FLOW OF MOISTURE  
AHEAD OF AN EASTERN PACIFIC/WEST COAST UPPER TROUGH. SOME OF THIS  
ACTIVITY WILL LIKELY BE MODERATE TO HEAVY THOUGH IT WILL TAKE  
ADDITIONAL TIME TO RESOLVE THE FINER DETAILS DUE TO UNCERTAINTY  
OVER INDIVIDUAL SYSTEMS AND THEIR INTERACTION. HOWEVER GUIDANCE  
SIGNALS AND TELECONNECTIONS FOR THE MEAN PATTERN STILL SUGGEST  
THAT NORTHERN AND CENTRAL CALIFORNIA SHOULD HAVE THE HIGHEST  
PRECIPITATION POTENTIAL OVER THE FIVE-DAY PERIOD--INCLUDING HEAVY  
SNOW OVER SIERRA NEVADA/SHASTA AREAS. THE GREATEST LIQUID  
EQUIVALENT TOTALS SHOULD BE OVER FAVORED TERRAIN ALONG THE SIERRA  
NEVADA. IT MAY STILL TAKE INTO THE SHORTER-RANGE TIME FRAME TO  
ASSES THE LOCATION AND INTENSITY OF LOWER ELEVATION RAINFALL WITH  
RESPECT TO LOCAL RUNOFF/FLOODING ISSUES OVER CALIFORNIA. EFFECTS  
FROM THE MULTI-DAY ACCUMULATION OF PRECIPITATION WILL ALSO REQUIRE  
MONITORING EVEN IF IT IS NOT INTENSE ON ANY PARTICULAR DAY. SOME  
MOISTURE SHOULD EXTEND AS FAR SOUTH AS SOUTHERN CALIFORNIA AND  
ARIZONA AFTER MIDWEEK WHILE FAVORED TERRAIN OVER THE GREAT BASIN  
AND NORTHERN/CENTRAL ROCKIES WILL SEE MEANINGFUL PRECIPITATION AS  
WELL.  
 
LOW PRESSURE TRACKING AWAY FROM NEAR THE NORTH CAROLINA COAST MAY  
BE ACCOMPANIED BY A LITTLE RAIN EARLY WEDNESDAY WHILE PASSAGE OF A  
SOUTHEASTERN CANADA SYSTEM AND A WEAK UPSTREAM FEATURE MAY PRODUCE  
LIGHT SNOW OVER THE UPPER GREAT LAKES AND NORTHERN NEW ENGLAND.  
LATEST GUIDANCE IS SIGNALING A MORE ORGANIZED SYSTEM COULD PRODUCE  
SNOW OR RAIN DEPENDING ON LATITUDE BETWEEN THE NORTHERN PLAINS AND  
GREAT LAKES/NORTHEAST, THOUGH DIFFERENCES IN EVOLUTION KEEP  
CONFIDENCE LOW FOR RESOLVING COVERAGE AND INTENSITY OF  
PRECIPITATION AT THIS TIME.  
 
ABOVE NORMAL TEMPERATURES OVER THE SOUTHERN TWO-THIRDS OF THE HIGH  
PLAINS ON WEDNESDAY WILL EXPAND EASTWARD ACROSS THE PLAINS AND  
INTO THE MISSISSIPPI VALLEY LATER IN THE WEEK, WITH INCREASING  
COVERAGE OF HIGHS AT LEAST 15-25F ABOVE NORMAL. SUCH READINGS  
WILL LIKELY PERSIST OVER THE SOUTHERN HALF OF THE  
PLAINS/MISSISSIPPI VALLEY THROUGH NEXT WEEKEND. A FEW RECORD  
HIGHS MAY BE POSSIBLE OVER TEXAS DURING THIS WARM PERIOD. SOME OF  
THIS WARMTH SHOULD ALSO REACH THE EAST WITH HIGHS REACHING 10-20F  
ABOVE NORMAL BY THE END OF THE WEEK OR NEXT WEEKEND. ON THE OTHER  
HAND THE FAR NORTHERN PLAINS SHOULD SEE A PRONOUNCED COLDER TREND  
DURING THE WEEKEND AS TEMPERATURES DROP TO 15-30F BELOW NORMAL  
OVER PARTS OF MONTANA AND NORTH DAKOTA. THE UNSETTLED PATTERN  
OVER THE WEST SHOULD LEAD TO NEAR OR ABOVE NORMAL LOWS AND NEAR TO  
BELOW NORMAL HIGHS THROUGH THE ENTIRE WEEK. WESTERN HIGHS SHOULD  
GRADUALLY TREND COOLER WITH TIME AS THE AXIS OF THE EASTERN  
PACIFIC UPPER TROUGH REACHES AT LEAST AS FAR AS THE WEST COAST.  
 
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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