773  
FXUS02 KWBC 011859  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
158 PM EST WED DEC 01 2021  
 
VALID 12Z SAT DEC 04 2021 - 12Z WED DEC 08 2021  
   
..OVERVIEW
 
 
QUASI-ZONAL UPPER FLOW EXPECTED TO BE IN PLACE TO BEGIN THE  
WEEKEND ACROSS THE NORTHERN TIER STATES SHOULD AMPLIFY INTO A  
BROAD MEAN TROUGH THEREAFTER AS AN UPPER RIDGE STRENGTHENS OVER  
THE EASTERN PACIFIC. WITHIN THIS PATTERN A WEAK LEADING  
SHORTWAVE/SURFACE SYSTEM WILL REACH THE EAST COAST EARLY IN THE  
WEEKEND, FOLLOWED BY TWO MORE SIGNIFICANT STORM SYSTEMS--ONE  
TRACKING FROM THE NORTHERN PLAINS INTO SOUTHEASTERN  
CANADA/NORTHEAST U.S. FROM THE WEEKEND THROUGH MONDAY AND THE  
SECOND TRACKING FROM THE WEST THROUGH THE PLAINS AND INTO THE OHIO  
VALLEY/MID-SOUTH MONDAY-WEDNESDAY.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE RELATIVELY GREATER PROPORTION OF UNCERTAINTY IN THE FORECAST  
STILL SEEMS TO EMANATE FROM QUESTION MARKS WITHIN MULTIPLE STREAMS  
OF AN UPPER TROUGH OVER THE EAST-CENTRAL PACIFIC. LEADING ENERGY  
REACHING THE NORTHWEST BY EARLY SATURDAY AND THE ASSOCIATED  
SURFACE REFLECTION HAVE TRENDED CLOSER TOGETHER IN THE PAST 24  
HOURS, THOUGH THE NEW 12Z GFS RUN IS ALREADY A BIT MORE AMPLIFIED  
THAN CONSENSUS AT THAT TIME. THEN AS THIS ENERGY AMPLIFIES  
DOWNSTREAM TO PRODUCE A PLAINS INTO EASTERN U.S./CANADA LOW AND  
FRONTAL SYSTEM, RECENT ECMWF/CMC TRENDS HAVE BEEN MORE IN THE  
DIRECTION OF, THOUGH NOT COMPLETELY TO, THE DEEP AND NORTHWEST  
GFS. OVER THE PAST DAY THE UKMET HAS BEEN VERY INCONSISTENT WITH  
THE 00Z RUN QUITE SUPPRESSED FOR A TIME VERSUS 12Z RUNS THAT HAVE  
BEEN MORE LIKE SOME GFS RUNS.  
 
THE CHARACTER OF THE NEXT SHORTWAVE NEARING THE NORTHWEST EARLY  
NEXT WEEK WILL DEPEND ON HOW MUCH IF ANY INTERACTION THERE MAY BE  
BETWEEN NORTHERN STREAM ENERGY JUST SOUTH OF ALASKA AND A SEPARATE  
UPPER LOW FARTHER SOUTH, WITH NO CLEAR CONSENSUS EVIDENT YET FOR  
EXACTLY WHAT WILL HAPPEN. DETAIL DIFFERENCES AT THIS "DECISION  
POINT" ULTIMATELY LEAD TO MEANINGFUL AMPLITUDE AND TIMING  
DIFFERENCES AS THE ENERGY DROPS INTO THE BROAD MEAN TROUGH OVER  
THE LOWER 48. ADDITIONAL WEAKER ENERGY COULD DROP DOWN BEHIND  
THIS SHORTWAVE AS WELL. ON THE POSITIVE SIDE THE MORE EXTREME  
SOLUTIONS FROM SOME EARLIER MODEL RUNS HAVE MODERATED SOMEWHAT,  
LEADING TO SOMEWHAT BETTER COMPARISONS AMONG THE MODELS AND MEANS  
ON AVERAGE BY DAYS 6-7 TUESDAY-WEDNESDAY. THIS TENDENCY HAS ALSO  
IMPROVED CONSENSUS TOWARD LONGER PERSISTENCE OF THE UPPER LOW OVER  
OR NEAR BAJA CALIFORNIA.  
 
THE FIRST HALF OF THE FORECAST INCORPORATED THE 00Z/06Z GFS AND  
00Z ECMWF/CMC, WITH THE UKMET EXCLUDED DUE TO POOR COMPARISON TO  
OTHER GUIDANCE FOR THE SYSTEM REACHING THE PLAINS DURING THE  
WEEKEND. A BLEND OF THE AFOREMENTIONED MODELS ALONG WITH THE 06Z  
GEFS/00Z ECMWF MEANS REPRESENTED THE PATTERN WELL WHILE ACCOUNTING  
FOR LOWER CONFIDENCE DETAIL SPECIFICS AFTER EARLY MONDAY.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
A WEAK LEADING SYSTEM MAY BRUSH PARTS OF THE UPPER GREAT LAKES AND  
NORTHEAST WITH LIGHT PRECIPITATION (MOSTLY SNOW) ON SATURDAY.  
SOLUTIONS ARE GRADUALLY COMING CLOSER TOGETHER FOR THE NEXT  
STRONGER SYSTEM FROM THE WEEKEND INTO MONDAY, WITH SOME POTENTIAL  
FOR A BAND OF MEANINGFUL SNOW FROM PARTS OF NORTH DAKOTA INTO THE  
UPPER GREAT LAKES. PRECIPITATION TYPE AND INTENSITY OVER THE  
EASTERN GREAT LAKES AND NEW ENGLAND WILL DEPEND ON THE EXACT LOW  
TRACK. MOISTURE INCREASING AHEAD OF THE COLD FRONT SHOULD YIELD  
SCATTERED SHOWERS AND A FEW THUNDERSTORMS FROM THE DEEP SOUTH TO  
THE OHIO VALLEY OVER THE WEEKEND. THERE COULD BE SOME LOCALLY  
MODERATE TO HEAVY RAIN EMBEDDED WITHIN THIS AREA BUT WITH FAIRLY  
LOW CONFIDENCE IN EXACTLY WHERE AT THIS TIME. THE SHORTWAVE AND  
COLD FRONT FORECAST TO REACH THE PACIFIC NORTHWEST BY SUNDAY NIGHT  
AND THROUGH MONDAY SHOULD PRODUCE A PERIOD OF MODERATE TO LOCALLY  
HEAVY RAIN AND MOUNTAIN SNOWS OVER WESTERN WASHINGTON AND OREGON,  
WITH SOME OF THIS MOISTURE REACHING THE NORTHERN ROCKIES BY  
TUESDAY. HEAVY SNOW WILL BE POSSIBLE FOR THE HIGHER ELEVATIONS OF  
NORTHERN IDAHO, WESTERN MONTANA, AND WESTERN WYOMING. SOME  
MOISTURE WILL EXTEND FARTHER SOUTH OVER FAVORED TERRAIN IN THE  
GREAT BASIN AND CENTRAL ROCKIES. BY NEXT TUESDAY-WEDNESDAY,  
PLAINS INTO EASTERN U.S. LOW PRESSURE AND ASSOCIATED FRONTS ARE  
LIKELY TO GENERATE ANOTHER EPISODE OF SHOWERS AND SOME STORMS WITH  
HIGHEST TOTALS BETWEEN THE OHIO VALLEY AND LOWER MISSISSIPPI  
VALLEY. AT LEAST SOME PRECIPITATION SHOULD BE IN THE FORM OF SNOW  
TO THE NORTH OF THIS RAIN AREA.  
 
EXPECT TEMPERATURES TO BE ABOVE NORMAL FOR EARLY DECEMBER ACROSS  
MOST OF THE NATION THROUGH EARLY NEXT WEEK. THE GREATEST POSITIVE  
ANOMALIES SHOULD BE OVER THE NORTH-CENTRAL HIGH PLAINS AND  
VICINITY ON SATURDAY WITH HIGHS 15-20F OR SO ABOVE AVERAGE,  
FOLLOWED BY ANOTHER EPISODE OF SIMILAR ANOMALIES OVER PARTS OF  
TEXAS AND OKLAHOMA ON TUESDAY AHEAD OF THE NEXT COLD FRONT. ON  
THE OTHER END OF THE SPECTRUM, COLD WEATHER WILL LIKELY CONTINUE  
ACROSS THE NORTHEAST U.S. WITH HIGHS RUNNING UP TO 10 DEGREES  
BELOW NORMAL THROUGH THE WEEKEND WITH A COLD UPPER LEVEL TROUGH  
OVERHEAD. BEHIND THE SUNDAY-MONDAY STORM SYSTEM AND NORTH OF THE  
NEXT ONE, AREAS ALONG THE EXTREME NORTHERN TIER FROM THE PLAINS  
INTO NEW ENGLAND MAY SEE MODERATELY BELOW NORMAL HIGHS FROM MONDAY  
INTO WEDNESDAY. NO MAJOR ARCTIC AIRMASS INTRUSIONS ARE EXPECTED  
DURING THIS FORECAST PERIOD THOUGH.  
 
RAUSCH/HAMRICK  
 
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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