890  
FXUS02 KWBC 061900  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
200 PM EST MON DEC 06 2021  
 
VALID 12Z THU DEC 09 2021 - 12Z MON DEC 13 2021  
 
...BACK-TO-BACK HEAVY MOUNTAIN SNOWS EXPECTED ACROSS THE CASCADES,  
SIERRA NEVADA, INTERMOUNTAIN WEST, AND ROCKIES...  
   
..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 OVER INCREASING AMPLITUDE OF THE UPPER-LEVEL PATTERN.  
   
..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.  
 
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  
 
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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