621  
FXUS02 KWBC 051901  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
200 PM EST SUN DEC 05 2021  
 
VALID 12Z WED DEC 08 2021 - 12Z SUN DEC 12 2021  
 
...SOME SNOW IS POSSIBLE ON WEDNESDAY FOR PORTIONS OF THE  
NORTHEAST AND NORTHERN MID-ATLANTIC, POTENTIALLY INCLUDING THE  
I-95 CORRIDOR...  
 
...CONSIDERABLE MOUNTAIN SNOWS ARE LIKELY ACROSS THE CASCADES,  
SIERRA NEVADA, INTERMOUNTAIN WEST, AND ROCKIES DURING THE LATTER  
HALF OF NEXT WEEK...  
   
..OVERVIEW
 
 
EXPECT THE START OF THE MEDIUM RANGE PERIOD ON WEDNESDAY TO  
FEATURE A BROAD MEAN TROUGH ACROSS MUCH OF THE LOWER 48,  
SUPPORTING AN ACTIVE WEATHER PATTERN WITH EMBEDDED SHORTWAVES  
PUSHING LOW PRESSURE/FRONTAL SYSTEMS ACROSS THE COUNTRY. ONE  
SYSTEM SHOULD BRING WIDESPREAD RAIN TO THE SOUTHEAST ON WEDNESDAY  
WHILE PARTS OF THE NORTHEAST/MID-ATLANTIC MAY SEE SNOW. BY  
THURSDAY-FRIDAY AMPLIFYING ENERGY ENTERING THE WESTERN SIDE OF THE  
MEAN TROUGH WILL DEVELOP A MORE PRONOUNCED UPPER TROUGH OVER THE  
WEST, WITH THE TROUGH LIKELY TO PROGRESS STEADILY ONWARD INTO THE  
CENTRAL AND EASTERN U.S. DURING THE WEEKEND. THIS SYSTEM WILL  
BRING THE POTENTIAL FOR MEANINGFUL PRECIPITATION AND HEAVY  
MOUNTAIN SNOWS OVER THE WEST AND THEN ORGANIZED PRECIPITATION  
SPREADING INTO THE CENTRAL AND EASTERN U.S. BY LATE WEEK THROUGH  
THE WEEKEND ALONG AND AHEAD OF A STRONG COLD FRONT. GUIDANCE IS  
ADVERTISING A CHANGE IN THE PATTERN AROUND THE END OF THE PERIOD  
AND BEYOND WITH THE NEXT TROUGH AMPLIFYING OVER THE EASTERN  
PACIFIC.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
DURING THE FIRST HALF OF THE PERIOD, THE OVERALL LARGE SCALE  
PATTERN REMAINS AGREEABLE AMONG MODEL GUIDANCE BUT EMBEDDED  
SHORTWAVE UNCERTAINTIES CONTINUE TO LOWER CONFIDENCE IN SOME  
IMPORTANT DETAILS. THE MOST NOTABLE EXAMPLE CONTINUES TO BE WITH A  
SHORTWAVE FORECAST TO BE OVER THE EAST-CENTRAL U.S. EARLY  
WEDNESDAY AND NORTHERN NEW ENGLAND/WESTERN ATLANTIC BY EARLY  
THURSDAY. OVER MULTIPLE RUNS SUBTLE LOW-PREDICTABILITY DETAIL  
DIFFERENCES HAVE LED TO VERY DIFFERENT SURFACE SOLUTIONS FOR LOW  
PRESSURE THAT TRACKS OFF THE EAST COAST ALONG WITH ASSOCIATED  
COVERAGE/INTENSITY OF PRECIPITATION AND LOCATION OF THE RAIN-SNOW  
LINE. WITHIN THE BROAD SPREAD OF GUIDANCE, RECENT GFS RUNS HAVE  
BEEN AMONG THE SLOWEST AND DEEPEST WITH THE MIDLEVEL SHORTWAVE,  
LEADING TO SURFACE LOW TRACKS FARTHER WEST CLOSER TO THE NORTHEAST  
COAST COMPARED TO CONSENSUS. THE 00Z/06Z RUNS WERE STILL ON THE  
WESTERN SIDE OF THE SPREAD IN SPITE OF SOME EASTWARD TREND  
RECENTLY, WHILE ENSEMBLE TRENDS OVER THE PAST DAY SHOW A DRAMATIC  
TRIMMING OF MEMBERS ON THE WESTERN SIDE OF THE ENVELOPE. THE NEW  
12Z GFS HAS CONTINUED ITS RECENT TREND OF GRADUAL SUPPRESSION OF  
THE TRACK AND MOISTURE SHIELD. THE GFS AND OVERALL GUIDANCE TREND  
TOWARD A MORE SUPPRESSED/OFFSHORE LOW TRACK IS LEADING TO LESS  
PRECIPITATION FORECAST TO FALL OVER THE NORTHEASTERN QUADRANT OF  
THE CONUS OVERALL, THOUGH A CORRESPONDING COLDER TREND WOULD  
SUPPORT A FARTHER SOUTH RAIN-SNOW LINE. UPSTREAM ENERGY REACHING  
THE NORTHWEST AND SOUTHWEST WEDNESDAY AND MOVING QUICKLY EAST  
THEREAFTER, ALONG WITH THE ASSOCIATED SURFACE REFLECTION, HAVE  
ALSO EXHIBITED SOME DETAIL DIFFERENCES BUT WITH LESS INFLUENCE ON  
SENSIBLE WEATHER IMPACTS. FOR THIS PART OF THE FORECAST THE  
PREFERENCE CONTINUES TO BE WITH AN INTERMEDIATE SOLUTION DERIVED  
FROM A 00Z/06Z MODEL COMPOSITE.  
 
THE GENERAL CONSENSUS REMAINS FAIRLY GOOD FOR THE UPPER TROUGH  
AMPLIFYING INTO THE WEST BY LATE IN THE WEEK AND CONTINUING  
EASTWARD THEREAFTER, BUT WITH LINGERING SPREAD FOR SPECIFICS OF  
HOW ENERGY WILL BE DISTRIBUTED WITHIN THE TROUGH AND FRONTAL WAVE  
DETAILS. TRENDS OVER THE PAST COUPLE DAYS HAVE GENERALLY BEEN  
SOMEWHAT FASTER BUT THE NEW 12Z ECMWF HAS REVERSED THAT TREND A  
BIT. MEANWHILE CMC RUNS AND SOME ENSEMBLE MEMBERS HAVE BEEN TRYING  
TO PULL OFF SOME ENERGY INTO A SLOWER SOUTHERN STREAM FEATURE,  
WITH 00Z CMC RUNS TENDING TO BE MORE EXTREME VERSUS OTHER  
MODELS/MEANS AND THE 12Z RUNS ADJUSTING EASTWARD CLOSER TO OTHER  
GUIDANCE. BEHIND THIS SYSTEM, ESSENTIALLY TWO CLUSTERS DEVELOP FOR  
THE UPPER TROUGH EXPECTED TO AMPLIFY OVER THE EASTERN PACIFIC--THE  
SOMEWHAT FASTER AND MORE AMPLIFIED GFS/GEFS MEAN VERSUS  
SLOWER/BROADER ECMWF/CMC AND THEIR ENSEMBLE MEANS. DIFFERENCES  
FOR THESE TWO MAJOR FEATURES ARE GENERALLY WITHIN THE REALM OF  
GUIDANCE ERROR FOR THE LATE EXTENDED TIME FRAME (THOUGH UNDERSTOOD  
TO BE VERY IMPORTANT FOR THE CHARACTER OF PRECIPITATION OVER THE  
PACIFIC NORTHWEST NEXT WEEKEND), LEADING TO TRANSITIONING THE  
FORECAST TOWARD A BLEND OF 00Z/06Z GFS AND 06Z GEFS RUNS ALONG  
WITH THE 00Z ECMWF/ECMWF MEAN.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
AROUND MIDWEEK WAVY FRONTS OVER THE SOUTHEAST AND LOW DEVELOPMENT  
INTO THE WESTERN ATLANTIC WILL PRODUCE RAINFALL OVER THE SOUTHEAST  
AND SPREAD SOME MOISTURE INTO COOLER AIR FARTHER NORTH WHERE  
PRECIPITATION MAY FALL AS SNOW. MODELS ARE STILL HAVING DIFFICULTY  
IN RESOLVING IMPORTANT SUBTLE DETAILS THAT WILL DETERMINE  
PRECIPITATION COVERAGE, TYPE, AND AMOUNTS. RECENT TRENDS FOR A  
MORE MORE SUPPRESSED SURFACE LOW TRACK ARE BRINGING THE POTENTIAL  
RAIN-SNOW LINE GRADUALLY SOUTHWARD OVER THE MID-ATLANTIC BUT ALSO  
LOWERING THE AMOUNT OF SNOW THAT MAY FALL TO THE NORTH OVER NEW  
ENGLAND. OVERALL THE PROBABILITIES FOR SIGNIFICANT AMOUNTS OF SNOW  
HAVE DECLINED IN THE PAST DAY.  
 
AS SHORTWAVE ENERGY AND UPPER TROUGHING DROP SOUTHWARD THROUGH THE  
WEST, PRECIPITATION WILL SPREAD FROM THE PACIFIC NORTHWEST AND  
NORTHERN ROCKIES WEDNESDAY SOUTHWARD TO CALIFORNIA INTO THE GREAT  
BASIN TO CENTRAL ROCKIES THURSDAY AND FRIDAY. FAVORED TERRAIN IN  
ARIZONA COULD ALSO SEE SOME ENHANCED ACTIVITY. 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. PARTS OF SOUTHERN CALIFORNIA MAY SEE THE FIRST  
RAINFALL IN OVER A MONTH. THEN AS THE WESTERN UPPER TROUGH  
APPROACHES/PUSHES INTO THE PLAINS TOWARD THE END OF THE WEEK,  
RAINFALL MAY BECOME MODERATE TO HEAVY OVER PARTS OF THE  
EAST-CENTRAL U.S. BY NEXT SATURDAY WITH THE AID OF STRENGTHENING  
GULF INFLOW AHEAD OF A WAVY FRONT. THE CHARACTER OF ONE OR MORE  
FRONTAL WAVES WILL DETERMINE PRECIPITATION AMOUNTS OVER THE  
EASTERN HALF OF THE COUNTRY AND POSSIBLE WINTER WEATHER IMPACTS  
FROM THE PLAINS THROUGH THE MIDWEST/GREAT LAKES.  
 
STRONG FLOW AHEAD OF A DEVELOPING EASTERN PACIFIC UPPER TROUGH MAY  
BRING AN ATMOSPHERIC RIVER EVENT INTO THE PACIFIC NORTHWEST  
STARTING AROUND FRIDAY NIGHT OR SATURDAY AND CONTINUING THROUGH  
THE WEEKEND. THERE IS STILL A FAIR DEGREE OF UNCERTAINTY OVER THE  
PROGRESSION/SOUTHWARD EXTENT OF MOISTURE BUT THIS EVENT REQUIRES  
MONITORING DUE TO THE POTENTIAL FOR SOME AREAS TO SEE HEAVY  
PRECIPITATION.  
 
TEMPERATURES WEDNESDAY WILL BE ABOUT 10-15F ABOVE AVERAGE FOR THE  
HIGH PLAINS, AND EXPECT TEMPERATURES TO TREND EVEN WARMER ACROSS  
THE CENTRAL AND EVENTUALLY EASTERN U.S. AS THE PATTERN AMPLIFIES.  
BY FRIDAY TEMPERATURES IN THE SOUTHERN PLAINS AND THE LOWER/MIDDLE  
MISSISSIPPI VALLEY SHOULD REACH 15-25F ABOVE NORMAL, WITH HIGHS  
NEARING OR EXCEEDING 80F IN PARTS OF TEXAS INTO THE LOWER  
MISSISSIPPI VALLEY, WHICH COULD APPROACH OR REACH DAILY RECORDS.  
AFTER A CHILLY START WEDNESDAY, THE EASTERN U.S. WILL SEE A  
PRONOUNCED WARMING TREND AS HIGHS 15-25F AND LOWS 15-30F ABOVE  
NORMAL REACH INTO THE EASTERN U.S. FOR THE WEEKEND. BEHIND THE  
FRONT, THE WEST SHOULD SEE A COUPLE DAYS OF HIGHS 5-15F BELOW  
NORMAL LATE IN THE WEEK BUT THIS COOL AIR WILL MODIFY CLOSER TO  
NORMAL BY THE TIME IT REACHES THE PLAINS. THE UPCOMING CHANGE IN  
THE PATTERN TOWARD THE END OF THE PERIOD SHOULD RETURN  
TEMPERATURES TO 10-20F ABOVE NORMAL ACROSS THE NORTHERN TWO-THIRDS  
OF THE PLAINS BY NEXT SUNDAY.  
 
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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