553  
FXUS02 KWBC 110558  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1257 AM EST SAT DEC 11 2021  
 
VALID 12Z TUE DEC 14 2021 - 12Z SAT DEC 18 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 AND AN ASSOCIATED LARGE ENHANCED WIND FIELD, WINTRY  
PRECIPITATION WILL SPREAD ALONG THE NORTH SIDE OF THE LOW TRACK  
ACROSS THE NORTHERN PLAINS AND UPPER MIDWEST/UPPER GREAT LAKES AND  
ERODING HIGH PRESSURE DAMMED NORTHERN NEW ENGLAND. MEANWHILE  
UNDERNEATH, 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 TRAILING COLD FRONT.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
MODEL AND ENSEMBLE FORECAST CLUSTERING AND SPREAD THROUGH MOST OF  
THE MEDIUM RANGE REMAINS 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/UKMET/CANADIAN AND GEFS/ECMWF  
ENSEMBLE MEANS DAYS 3-6 ALONG WITH WPC CONTINUITY AND THE NATIONAL  
BLEND OF MODELS (NBM). FORECAST SPREAD INCREASES A BIT MORE  
RAPIDILY INTO DAY 7, BUT THE GEFS/ECMWF ENSEMBLE MEANS REMAIN A  
STILL COMPATIBLE FORECAST OPTION ALONG WITH THE NBM.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
MOIST ONSHORE FLOW WITH AN ATMOSPHERIC RIVER WILL FUEL WIDESPREAD  
PRECIPITATION IN THE WESTERN U.S. EARLY NEXT WEEK. HEAVIEST  
PRECIPITATION WILL SPREAD SOUTHWARD ACROSS CENTRAL TO SOUTHERN  
CALIFORNIA TUE AND OUT ACROSS OVER THE GREAT BASIN, 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. 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.  
DOWNSTREAM, DEEP MIDWEEK NORTH-CENTRAL U.S. CYCLOGENESIS AND  
SUBSEQUENT NOTHEASTWARD TRACK INTO CANADA WILL SPREAD SNOW ACROSS  
THE NORTHERN PLAINS AND UPPER MIDWEST/UPPER GREAT LAKES. SNOWS  
WILL COMBINE WITH ENHANCED WINDS AROUND THE LARGE CIRCULATION AND  
THE HEAVIEST WRAP-AROUND FOCUS MAY SETUP OVER THE UPPER MIDWEST.  
 
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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