502  
FXUS02 KWBC 061953  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
252 PM EST MON DEC 06 2021  
 
VALID 12Z THU DEC 09 2021 - 12Z MON DEC 13 2021  
 
...BACK-TO-BACK HEAVY PRECIPITATION EXPECTED ACROSS THE PACIFIC  
NORTHWEST...  
   
..OVERVIEW
 
 
A GENERALLY PROGRESSIVE PATTERN IS EXPECTED TO PERSIST THROUGH THE  
MEDIUM RANGE PERIOD WITH A TENDENCY FOR A RIDGE TO BECOME MORE  
DOMINANT ACROSS THE SOUTH BY EARLY NEXT WEEK. A COUPLE OF  
SYNOPTIC SCALE SYSTEMS MOVING THROUGH THIS PROGRESSIVE PATTERN  
WILL BE OF CONCERN. A LOW PRESSURE SYSTEM FORECAST TO TRACK  
THROUGH THE CENTRAL PLAINS TO THE GREAT LAKES WILL LIKELY BRING A  
SWATH OF WINTRY WEATHER ACROSS THE INTERIOR WESTERN U.S. LATE WEEK  
TO THE GREAT LAKES BY THE WEEKEND, WHILE HEAVY RAIN IS POSSIBLE ON  
SATURDAY ACROSS THE TENNESSEE VALLEY AHEAD OF A COLD FRONT WITH  
TIMING UNCERTAINTIES. OVER THE PACIFIC NORTHWEST, THE FIRST WAVE  
OF HEAVY MOUNTAIN SNOWS LATE THIS WEEK WILL LIKELY BE FOLLOWED BY  
ANOTHER WAVE OF MOISTURE WHICH HAS THE POTENTIAL TO DELIVER  
HEAVIER SNOWFALL AMOUNTS FOR THE HIGHER ELEVATIONS.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE LATEST MODEL GUIDANCE THIS MORNING EXHIBITS REASONABLY GOOD  
AGREEMENT WITH THE OVERALL LARGE-SCALE PATTERN DESCRIBED ABOVE,  
WITH INCREASING UNCERTAINTIES ACROSS THE SOUTH BY EARLY NEXT WEEK.  
FOR THE LOW PRESSURE SYSTEM FORECAST TO DEVELOP OVER THE CENTRAL  
PLAINS ON FRIDAY, MODELS HAVE GENERALLY INCREASED THE AMPLITUDE OF  
THE SHORTWAVE DIPPING INTO THE WESTERN U.S., RESULTING IN A  
STRONGER/DEEPER LOW TO DEVELOP WITH A SLIGHTLY FASTER FORWARD  
MOTION. THERE HAS BEEN A TENDENCY TO PUSH THE ASSOCIATED COLD  
FRONT FASTER TOWARD THE EAST THROUGH THE EASTERN U.S. DURING THE  
WEEKEND AS WELL. BOTH THE GFS AND THE ECMWF HAVE ATTEMPTED TO  
DEVELOP A FRONTAL WAVE OVER THE SOUTHERN APPALACHIANS DURING THE  
WEEKEND IN SOME PREVIOUS RUNS ALTHOUGH THEIR ENSEMBLE MEANS DO NOT  
SUPPORT SUCH DEVELOPMENT. BY EARLY NEXT WEEK, ENSEMBLE MEANS  
GENERALLY SUPPORT THE ESTABLISHMENT OF A RIDGE ACROSS THE SOUTH.  
HOWEVER, THE LATEST (12Z) ECMWF DEVELOPS AN ANOMALOUS UPPER LOW  
OVER THE DEEP SOUTH AMID SIGNIFICANT AMPLIFICATION OF THE FRONTAL  
WAVE UNDER INCREASINGLY AMPLIFIED UPPER-LEVEL PATTERN. THIS  
SCENARIO WILL BE MONITORED IN THE FUTURE MODEL GUIDANCE FOR EARLY  
NEXT WEEK.  
 
OVER THE PACIFIC NORTHWEST, HEAVY MOUNTAIN SNOWS LATE THIS WEEK  
WILL LIKELY BE FOLLOWED BY THE NEXT SURGE OF MOISTURE AHEAD OF THE  
NEXT PACIFIC FRONT. MODELS HAVE GENERALLY SHOWN A PROGRESSIVELY  
FASTER/EARLIER ARRIVAL OF THIS ROUND OF MOISTURE BY THE WEEKEND.  
 
THE WPC MEDIUM-RANGE FORECASTS WERE BASED ON 40% FROM THE 00Z  
ECMWF AND EC MEAN, 40% FROM THE 00Z/06Z GFS AND THE 06Z GEFS, AS  
WELL AS 20% FROM THE 00Z CMC/CMC MEAN. MORE OF THE ENSEMBLE MEANS  
WERE INCLUDED FOR DAYS 6 AND 7.  
   
..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.  
HIGHEST 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 A LOW PRESSURE SYSTEM DEVELOPS OVER THE CENTRAL  
PLAINS ON FRIDAY, SNOW MOVING THROUGH THE CENTRAL ROCKIES WILL  
EXIT INTO THE THE NORTHERN PLAINS TOWARD THE UPPER MIDWEST. BY  
FRIDAY NIGHT, THE SNOW AND MIXED PRECIPITATION SHOULD SPREAD INTO  
THE GREAT LAKES WITH MODERATE TO LOCALLY HEAVY AMOUNTS POSSIBLE.  
SOME WINTRY MIX IS ALSO POSSIBLE ACROSS INTERIOR NEW ENGLAND  
FRIDAY NIGHT/SATURDAY MORNING. FARTHER SOUTH, RAIN WITH EMBEDDED  
THUNDERSTORMS ARE EXPECTED TO EXTEND SOUTHWESTWARD INTO THE DEEP  
SOUTH, WITH HEAVY RAINFALL POSSIBLE OVER THE TENNESSEE VALLEY ON  
SATURDAY. WHILE THE COLD FRONT IS CURRENTLY FORECAST TO CLEAR THE  
EAST COAST BY EARLY NEXT WEEK, THERE IS SOME POTENTIAL FOR AN  
UPPER LOW TO FORM OVER THE DEEP SOUTH WHICH WOULD SIGNIFICANTLY  
CHANGE THE WEATHER OUTLOOK.  
 
OUT WEST, THE FIRST WAVE OF HEAVY MOUNTAIN SNOWS LATE THIS WEEK  
WILL LIKELY BE FOLLOWED BY THE ARRIVAL OF THE NEXT WAVE OF  
MOISTURE BY THE WEEKEND. IT APPEARS THAT THIS RENEWED ATMOSPHERIC  
RIVER HAS THE POTENTIAL TO DELIVER HEAVIER SNOWFALL AMOUNTS FOR  
THE HIGHER ELEVATIONS ACROSS THE PACIFIC NORTHWEST.  
 
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 FROM PORTIONS  
OF TEXAS TO 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 ASSUMED TO BUILD 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.  
 
KONG/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  
 
HAZARDS:  
- HEAVY RAIN ACROSS PORTIONS OF THE LOWER MISSISSIPPI VALLEY, THE  
CENTRAL APPALACHIANS, THE TENNESSEE VALLEY, THE SOUTHERN  
APPALACHIANS, THE GREAT LAKES, AND THE OHIO VALLEY, FRI-SAT, DEC  
10-DEC 11.  
- HEAVY SNOW ACROSS PORTIONS OF THE PACIFIC NORTHWEST AND THE  
NORTHERN GREAT BASIN, THU-SUN, DEC 9-DEC 12.  
- HEAVY SNOW ACROSS PORTIONS OF THE CENTRAL PLAINS, THE CENTRAL  
ROCKIES, THE CENTRAL GREAT BASIN, THE SOUTHERN ROCKIES,  
CALIFORNIA, AND THE SOUTHWEST, THU, DEC 9.  
- HEAVY SNOW ACROSS PORTIONS OF THE NORTHERN ROCKIES AND THE  
NORTHERN GREAT BASIN, SAT-SUN, DEC 11-DEC 12.  
- HEAVY SNOW ACROSS PORTIONS OF THE GREAT LAKES, THE UPPER  
MISSISSIPPI VALLEY, AND THE NORTHERN PLAINS, FRI, DEC 10.  
- HIGH WINDS ACROSS PORTIONS OF THE SOUTHERN ROCKIES AND THE  
SOUTHERN PLAINS, FRI, DEC 10.  
 
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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