568  
FXUS02 KWBC 031900  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
159 PM EST FRI DEC 03 2021  
 
VALID 12Z MON DEC 06 2021 - 12Z FRI DEC 10 2021  
   
..OVERVIEW
 
 
EXPECT A BROAD MEAN TROUGH ALOFT TO PREVAIL ACROSS MUCH OF THE  
LOWER 48 NEXT WEEK, WITH AN AXIS GENERALLY EXTENDING FROM  
CENTRAL/EAST-CENTRAL CANADA INTO THE WESTERN U.S. EMBEDDED  
SHORTWAVES WILL PUSH LOW PRESSURE/FRONTAL SYSTEMS ACROSS THE  
COUNTRY, LEADING TO A SOMEWHAT MORE ACTIVE WEATHER PATTERN THAN  
HAS BEEN THE CASE LATELY. IN PARTICULAR, PARTS OF THE WEST SHOULD  
SEE A COUPLE EPISODES OF MEANINGFUL PRECIPITATION WHILE THE  
INTENSITY/COVERAGE OF RAINFALL SHOULD INCREASE OVER THE  
SOUTHEASTERN QUADRANT OF THE COUNTRY AND SOME SNOW MAY FALL OVER  
NORTHERN PARTS OF THE EAST.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
GUIDANCE CONTINUES TO SHOW UNUSUALLY GREAT SPREAD FOR THE COMPACT  
SYSTEM FORECAST TO APPROACH THE PACIFIC NORTHWEST BY EARLY MONDAY.  
GFS/ECMWF RUNS AND THEIR MEANS HAVE CONVERGED FOR TRACK/TIMING BUT  
GFS RUNS HAVE BEEN ON THE DEEP SIDE OF THE SPREAD (EVEN WITH THE  
UPPER SHORTWAVE BECOMING LESS CLOSED IN THE 12Z RUN) WHILE THE  
LATEST UKMET/CMC ARE SLOWER AND WEAKER/ELONGATED DUE TO A WEAKER  
SHORTWAVE. THE PAST COUPLE GEFS RUNS SHOW EVEN MORE SPREAD AMONG  
INDIVIDUAL MEMBERS, WITH THE WEAK AND STRONG EXTREMES AT LEAST  
20-30 MB APART. THE RELATIVELY SMALL SCALE OF THE SHORTWAVE KEEPS  
PREDICTABILITY UNUSUALLY LOW FOR A DAY 3 FORECAST, GFS/ECMWF  
TIMING AND ECMWF DEPTH LOOKING LIKE THE BEST INTERMEDIATE APPROACH  
AT THIS TIME. ON THE OTHER HAND THE MODELS AND ENSEMBLES ARE MORE  
AGREEABLE FOR FOR THE STRONG SYSTEM TRACKING INTO EASTERN CANADA  
AND THE TRAILING FRONT THAT CROSSES THE EASTERN U.S., ALBEIT WITH  
SOME ONGOING NUDGES FOR DETAILS.  
 
IN SPITE OF THE ISSUES WITH SHORTWAVE ENERGY INITIALLY HEADING  
INTO THE NORTHWEST, SOLUTIONS HAVE TRENDED AT LEAST A CLOSER  
TOGETHER THAN IN PREVIOUS DAYS FOR WHAT HAPPENS DOWNSTREAM.  
CONSENSUS NOW SHOWS ENOUGH AMPLIFICATION INTO THE WEST TO SUPPORT  
FAIRLY QUICK EJECTION OF THE LEADING UPPER LOW OVER BAJA  
CALIFORNIA AS OF EARLY MONDAY BUT THEN DECENT PROGRESSION  
THEREAFTER. HOWEVER MEANINGFUL DIFFERENCES STILL ARISE OVER THE  
EAST, LEADING TO EVOLUTION AND TRACK/TIMING UNCERTAINTY FOR AN  
AREA OF LOW PRESSURE FROM THE EASTERN U.S. INTO THE WESTERN  
ATLANTIC WEDNESDAY-THURSDAY. ONE MULTI-DAY TREND OF NOTE IS THAT  
THE FORECAST SURFACE PATTERN OVER THE EAST VALID EARLY WEDNESDAY  
HAS GENERALLY BEEN TRENDING MORE SUPPRESSED (HIGHER PRESSURES).  
 
THERE IS FAIRLY GOOD AGREEMENT ON ANOTHER BUNDLE OF PACIFIC ENERGY  
COMING INTO NORTH AMERICA WEDNESDAY, WITH LESS AMPLIFICATION THAN  
ITS PREDECESSOR, PRODUCING SOUTHERN CANADA LOW PRESSURE THAT  
ANCHORS A FRONT REACHING INTO THE WEST/PLAINS/GREAT LAKES. A  
TRAILING FEATURE SHOULD AMPLIFY MORE INTO THE WEST BY LATE IN THE  
WEEK WITH A MODEL/MEAN BLEND GENERALLY LOOKING GOOD FOR THIS  
FEATURE THOUGH THE 12Z CMC DEPTH IS A BIT ON THE MORE EXTREME  
SIDE.  
 
MODEL PREFERENCES LED TO STARTING WITH A COMPOSITE OF 00Z/06Z  
OPERATIONAL MODELS FOR ABOUT THE FIRST HALF OF THE PERIOD. THEN  
THE FORECAST BLEND TRANSITIONED TO A BLEND OPERATIONAL GUIDANCE  
AND THE 06Z GEFS/00Z ECMWF MEAN.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
A RELATIVELY STRONG SURFACE LOW PRESSURE SYSTEM TRACKING AWAY FROM  
THE GREAT LAKES ON MONDAY SHOULD SUPPORT SNOW OVER THE REGION  
ALONG WITH A BROADER AREA OF STRONG WINDS OVER THE NORTHEASTERN  
QUADRANT OF THE NATION IN THE TIGHT PRESSURE GRADIENT BEHIND THE  
LOW. CONDITIONS WILL BE FAVORABLE FOR LAKE EFFECT SNOW BEHIND THE  
SYSTEM THROUGH TUESDAY. ADDITIONALLY, EXPECT RAIN TO FALL ALONG  
THE TRAILING COLD FRONT ASSOCIATED WITH THE LOW. BEST POTENTIAL  
FOR MEANINGFUL TOTALS SHOULD EXTEND FROM THE OHIO VALLEY TO  
SOUTH-CENTRAL U.S. BUT AMOUNTS ARE LIKELY TO BE TAPERING DOWN  
SOMEWHAT COMPARED TO SUNDAY-SUNDAY NIGHT. HOWEVER, GULF MOISTURE  
SHOULD INCREASE AGAIN BY TUESDAY INTO WEDNESDAY AND CAUSE MODERATE  
TO POSSIBLY LOCALLY HEAVY RAINFALL FROM THE LOWER MISSISSIPPI  
VALLEY INTO THE TENNESSEE VALLEY AND SOUTHEAST. MOISTURE SPREADING  
FARTHER NORTH INTO COOLER AIR COULD LEAD TO SNOW ACROSS THE  
NORTHERN LATITUDES, WITH RELATIVELY HIGHER PROBABILITIES FROM THE  
GREAT LAKES INTO NORTHERN NEW ENGLAND. THERE IS STILL CONSIDERABLE  
UNCERTAINTY OVER HOW LOW PRESSURE FROM THE EASTERN U.S. INTO THE  
WESTERN ATLANTIC WILL EVOLVE/TRACK, SO CONFIDENCE IS FAIRLY LOW  
FOR EXACTLY HOW MUCH PRECIPITATION MAY FALL OVER THE NORTHEAST.  
SOME RAIN COULD CONTINUE NEAR A WAVY FRONT LINGERING OVER THE  
EXTREME SOUTHEAST.  
 
A COUPLE OF SHORTWAVES MOVING INTO/ACROSS THE WEST WILL BRING  
INCREASING PRECIPITATION CHANCES THERE, WITH FAVORED WINDWARD  
TERRAIN SEEING THE HIGHEST AMOUNTS. THE PACIFIC NORTHWEST INTO  
MUCH OF THE INTERMOUNTAIN WEST AND ROCKIES SHOULD SEE  
PRECIPITATION MONDAY AND CLEARING OUT BY TUESDAY WHILE CHANCES  
RAMP UP IN THE SOUTHWEST. SOME LOCATIONS IN THE NORTHWEST MAY SEE  
FAIRLY LOW SNOW LEVELS WITH THE START OF THE MONDAY EVENT. THEN  
THE NEXT ROUND IS LIKELY FOR THE NORTHWEST ON WEDNESDAY AND  
SPREADS INTO CALIFORNIA EASTWARD BY THURSDAY AS ENERGY ALOFT DROPS  
SOUTHWARD. GENERALLY EXPECT HIGHER ELEVATION SNOW AND LOWER  
ELEVATION RAIN, WITH SNOW LEVELS LIKELY TO DECLINE LATER IN THE  
WEEK AS UPPER TROUGHING AMPLIFIES OVER THE WEST.  
 
CHILLY TEMPERATURES ARE LIKELY ACROSS THE FAR NORTH-CENTRAL U.S.  
ON MONDAY AND TUESDAY GIVEN SNOW COVER FROM SUNDAY'S SYSTEM, WITH  
TEMPERATURES 10-20F BELOW NORMAL. MEANWHILE THE EASTERN SEABOARD  
CAN EXPECT TEMPERATURES ABOUT 10-20F ABOVE NORMAL ON MONDAY AHEAD  
OF A COLD FRONT THAT WILL BRING TEMPERATURES DOWN TO NEAR AVERAGE  
BY TUESDAY. TEMPERATURES SHOULD BE WITHIN ABOUT 10 DEGREES OF  
NORMAL IN MOST PLACES ACROSS THE LOWER 48 TUESDAY AND WEDNESDAY,  
BEFORE A WARMUP BY THURSDAY AND FRIDAY FROM THE PLAINS EASTWARD,  
WITH 10-15F ABOVE AVERAGE TEMPERATURES. COOLER TEMPERATURES MAY  
SPREAD INTO THE WEST LATE IN THE WEEK (HIGHS 5-10F OR SO BELOW  
NORMAL BY FRIDAY) WITH THE POTENTIAL FOR UPPER TROUGHING TO  
AMPLIFY OVER THE REGION.  
 
RAUSCH/TATE  
 
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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