879  
FXUS02 KWBC 101904  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
203 PM EST FRI DEC 10 2021  
 
VALID 12Z MON DEC 13 2021 - 12Z FRI DEC 17 2021  
 
...ATMOSPHERIC RIVER WILL SPREAD SOUTHWARD ALONG THE WEST COAST BY  
EARLY NEXT WEEK BRINGING A THREAT OF HEAVY RAIN AND HEAVY MOUNTAIN  
SNOW...  
 
...RECORD WARMTH TO SPREAD FROM THE CENTRAL U.S. THROUGH THE  
MID-ATLANTIC/NORTHEAST TUESDAY-THURSDAY...  
 
   
..OVERVIEW
 
 
AMPLIFIED UPPER TROUGHING WILL DIG THROUGH THE WEST EARLY NEXT  
WEEK. THIS WILL PROMOTE INCREASING AND WIDESPREAD RAINS WITH LOCAL  
RUNOFF ISSUES AND TERRAIN ENHANCING HEAVY SNOWS OVER MUCH OF THE  
WEST/ROCKIES, FIRST ALONG THE COAST THEN PROGRESSING SOUTHWARD AND  
EASTWARD, AS AN ATMOSPHERIC RIVER EVENT ENSUES. AS THE MAIN SYSTEM  
LIFTS NORTHEASTWARD THROUGH THE PLAINS NEXT WED-THU AND SPAWNS  
CYCLOGENESIS, WINTRY PRECIPITATION WILL SPREAD ALONG THE NORTH  
SIDE OF THE LOW TRACK. DOWNSTREAM, UPPER RIDGING WILL BUILD FROM  
THE SOUTH-CENTRAL U.S. TO THE EAST. WARM SECTOR RECORD WARMTH WILL  
INCLUDE TEMPERATURES UPWARDS TO 20-35F ABOVE NORMAL TUE-THU  
SPREADING FROM THE CENTRAL U.S. TO THE MID-ATLANTIC/NORTHEAST  
PRIOR TO THE ARRIVAL OF GENERALLY MODEST RAINFALL FOCUSED ALONG A  
COLD FRONT.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
MODEL AND ENSEMBLE FORECAST CLUSTERING AND SPREAD THROUGH MEDIUM  
RANGE TIME SCALE IS MUCH BETTER THAN NORMAL, BOLSTERING FORECAST  
CONFIDENCE. THE WPC MEDIUM RANGE PRODUCT SUITE WAS PRIMAILRY  
DERIVED FROM A SEEMINGLY REASONABLE AND CONSISTENT COMPOSITE BLEND  
OF THE GFS/ECMWF/CANADIAN AND GEFS/ECMWF ENSEMBLE MEANS DAYS 3-7  
ALONG WITH WPC CONTINUITY AND THE NATIONAL BLEND OF MODELS.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
MOIST ONSHORE FLOW WITH AN ATMOSPHERIC RIVER WILL CAUSE WIDESPREAD  
PRECIPITATION IN THE WESTERN U.S. EARLY NEXT WEEK. HEAVIEST  
PRECIPITATION WILL SPREAD SOUTHWARD ACROSS CENTRAL AND SOUTHERN  
CALIFORNIA MON-TUE AND EASTWARD OVER THE GREAT BASIN AND FOUR  
CORNERS STATES AND ROCKIES THROUGH MIDWEEK. ENHANCED TOTALS ARE  
LIKELY ALONG FAVORED TERRAIN, WITH HEAVY MOUNTAIN SNOW ESPECIALLY  
IN THE SIERRA NEVADA AND THEN THE CENTRAL GREAT BASIN AND ROCKIES.  
HEAVY COASTAL TO INTERIOR RAINS ARE EXPECTED IN CALIFORNIA, EVEN  
DOWN INTO SOUTHERN CA. SOME LOCALLY HIGHER RAINFALL RATES MAY  
CAUSE SOME FLOODING ISSUES, ESPECIALLY FOR SENSITIVE BURN SCAR  
AREAS. TEMPERATURES WILL BECOME BELOW NORMAL IN THE WEST  
UNDERNEATH THE UPPER TROUGH, WITH TEMPERATURES 10-15F BELOW  
AVERAGE FOR PARTS OF CALIFORNIA AND NEVADA NEXT WEEK.  
 
KEBEDE/SCHICHTEL  
 
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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