922  
FXUS02 KWBC 060701  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
200 AM EST MON DEC 06 2021  
 
VALID 12Z THU DEC 09 2021 - 12Z MON DEC 13 2021  
 
...CONSIDERABLE MOUNTAIN SNOWS ARE LIKELY ACROSS THE CASCADES,  
SIERRA NEVADA, INTERMOUNTAIN WEST, AND ROCKIES THURSDAY INTO  
FRIDAY...  
   
..OVERVIEW
 
 
AS THE MEDIUM RANGE PERIOD BEGINS THURSDAY, A BROAD MEAN UPPER  
TROUGH CAUSING QUASI-ZONAL FLOW ACROSS MUCH OF THE COUNTRY WILL BE  
IN PLACE, SUPPORTING AN ACTIVE WEATHER PATTERN WITH EMBEDDED  
SHORTWAVES PUSHING LOW PRESSURE/FRONTAL SYSTEMS ACROSS THE  
COUNTRY. THE PATTERN IS FORECAST TO AMPLIFY LATE WEEK AS ENERGY  
ENTERING THE WESTERN SIDE OF THE MEAN TROUGH DEVELOPS 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. AS THAT TROUGH EXITS THE U.S. AROUND MONDAY, BROAD  
RIDGING SHOULD DOMINATE THE LOWER 48 AS ANOTHER EASTERN PACIFIC  
TROUGH AMPLIFIES.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE 12/18Z MODEL CYCLE CONTINUES TO BE IN REASONABLY GOOD  
AGREEMENT WITH THE OVERALL LARGE-SCALE PATTERN DESCRIBED ABOVE,  
WHILE SMALLER-SCALE SHORTWAVE UNCERTAINTIES CONTINUE TO LOWER  
CONFIDENCE IN THE DETAILS. SOME SLIGHT MODEL DIFFERENCES WERE SEEN  
WITH AMPLITUDE AND TRACK OF THE SHORTWAVE ENERGY DROPPING THROUGH  
THE WEST THURSDAY-FRIDAY SERVING TO DEEPEN THE TROUGH, WHILE THERE  
IS ALSO VARIETY IN TRACK AND TIMING OF WHEN A SOUTHERN STREAM  
SHORTWAVE WEST OF BAJA CALIFORNIA THURSDAY GETS ABSORBED LIKELY ON  
FRIDAY. THE 12Z ECMWF APPEARS TO BE A BIT OF AN OUTLIER WITH THE  
LATTER, WITH A EASTERN POSITION AND FASTER ABSORPTION COMPARED TO  
OTHER GUIDANCE AND ITS PREVIOUS RUNS, WHICH IS LIKELY A CAUSE OF  
MINOR DIFFERENCES IN THE BROAD WESTERN TROUGH FRIDAY (SLIGHTLY  
EASTWARD EXTENT AND MORE OF A NEUTRAL THAN POSITIVE TILT IN THE  
12Z ECMWF COMPARED TO OTHER GUIDANCE). FORTUNATELY THIS DOES NOT  
CAUSE ANY SIGNIFICANT ISSUES WITH THE SURFACE LOW PRESSURE  
DEEPENING IN THE LEE OF THE CENTRAL ROCKIES FRIDAY, AND THE BULK  
OF MODEL GUIDANCE IS IN OVERALL GOOD AGREEMENT WITH THE TROUGH  
(AND THE ASSOCIATED SURFACE COLD FRONT AHEAD OF IT) TRACKING  
ACROSS THE CENTRAL U.S. SATURDAY AND TOWARD THE EAST  
SUNDAY-MONDAY. DIFFERENCES WITH THE SPECIFICS OF HOW ENERGY WILL  
BE DISTRIBUTED WITHIN THE TROUGH AS WELL AS FRONTAL WAVE  
DIFFERENCES REMAIN--THOUGH NEW 00Z GUIDANCE SHOWS BETTER  
CLUSTERING WITH A SURFACE LOW IN THE GREAT LAKES EARLY SATURDAY.  
BY SUNDAY, THE 12Z CMC SEPARATES THE NORTHERN AND SOUTHERN STREAMS  
AND CLOSES OFF A MID-UPPER LOW IN THE LOWER MISSISSIPPI VALLEY,  
WHICH IS AN OUTLIER AT THIS POINT BUT NOT OUTSIDE THE REALM OF  
POSSIBILITIES.  
 
BY EARLY NEXT WEEK, TROUGHING IS FORECAST TO DEVELOP IN THE  
EASTERN PACIFIC, BUT MODELS SHOW VARIATIONS IN TIMING OF THE  
TROUGH PUSHING SOUTHWARD, WHICH IMPACTS THE PACIFIC NORTHWEST IN  
PARTICULAR. GFS RUNS AND TO SOME EXTENT THE GEFS MEANS HAVE BEEN  
CONSISTENT IN SHOWING THE TROUGHING WITH MORE OF A SOUTHWARD  
EXTENT BY SUNDAY, GIVING THE NORTHWEST SOUTHWESTERLY FLOW, WHILE  
SLOWER TROUGHING IN THE ECMWF AND CMC RUNS LEAD TO ZONAL EAST-WEST  
FLOW THERE SUNDAY. THE DIFFERENCES PROGRESS INTO MONDAY WITH HOW  
CLOSE TROUGHING WILL BE TO THE COAST. 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).  
 
THE WPC FORECAST WAS BASED ON A BLEND OF THE 12/18Z DETERMINISTIC  
GUIDANCE EARLY IN THE PERIOD, AND PHASED IN THE GEFS AND EC  
ENSEMBLE MEANS TO ABOUT 60% OF THE BLEND BY THE END OF THE PERIOD  
TO REDUCE SMALLER SCALE MODEL DIFFERENCES.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
AS THE TROUGH DEEPENS IN THE WEST, PRECIPITATION WILL SPREAD FROM  
THE PACIFIC NORTHWEST AND NORTHERN ROCKIES 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 SOMEWHAT LOW AS UPPER TROUGHING AMPLIFIES.  
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, PRECIPITATION SHOULD TRACK  
AHEAD OF A LOW PRESSURE SYSTEM AND A STRONG COLD FRONT. SOME  
WINTRY WEATHER COULD SPREAD INTO THE NORTHERN PLAINS AND  
MIDWEST/GREAT LAKES FRIDAY IN THE COLD AIR NORTH AND WEST OF THE  
LOW TRACK. THEN RAINFALL MAY BECOME MODERATE TO HEAVY OVER PARTS  
OF THE EAST-CENTRAL U.S. ESPECIALLY BY SATURDAY WITH THE AID OF  
STRENGTHENING GULF INFLOW AHEAD OF THE STRONG COLD FRONT.  
ADDITIONALLY, HEAVY PRECIPITATION CHANCES ARE FORECAST TO INCREASE  
IN THE PACIFIC NORTHWEST STARTING AROUND FRIDAY NIGHT OR SATURDAY  
AND CONTINUING THROUGH THE WEEKEND AS A POTENTIAL ATMOSPHERIC  
RIVER EVENT SETS UP, THOUGH UNCERTAINTY REMAINS WITH THE  
PROGRESSION/SOUTHWARD EXTENT OF MOISTURE, AND WILL CONTINUE TO BE  
MONITORED.  
 
AHEAD OF THE TROUGH AND COLD FRONT, MILD TO WARM TEMPERATURES ARE  
FORECAST TO BE IN PLACE IN THE CENTRAL U.S. THURSDAY AND FRIDAY,  
WITH HIGHS CURRENTLY FORECAST TO RISE INTO THE 80S IN PORTIONS OF  
TEXAS AND THE LOWER MISSISSIPPI VALLEY, WHICH COULD APPROACH OR  
SET DAILY RECORDS AS THE TEMPERATURES SHOULD BE 15-30F ABOVE  
NORMAL. THESE WARM TEMPERATURES WILL PUSH INTO THE OHIO/TENNESSEE  
VALLEYS AND EASTERN SEABOARD ON SATURDAY, WHILE TEMPERATURES  
MODERATE TO NEAR NORMAL BEHIND. THE WESTERN U.S. CAN EXPECT BELOW  
AVERAGE TEMPERATURES ESPECIALLY FRIDAY AND SATURDAY, WITH THE  
CENTRAL GREAT BASIN IN PARTICULAR SEEING TEMPERATURES 10-20F BELOW  
NORMAL. THEN, AS RIDGING BUILDS ACROSS THE LOWER 48 BY EARLY NEXT  
WEEK, TEMPERATURES SHOULD RETURN TO 10-20F ABOVE NORMAL ACROSS THE  
NORTHERN TWO-THIRDS OF THE PLAINS BY NEXT SUNDAY AND MONDAY.  
 
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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