616  
FXUS02 KWBC 121901  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
200 PM EST SUN DEC 12 2021  
 
VALID 12Z WED DEC 15 2021 - 12Z SUN DEC 19 2021  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
GUIDANCE CLUSTERING AND FORECAST SPREAD/PREDICTABILITY THROUGH  
MEDIUM RANGE REMAINS BETTER THAN NORMAL FOR THE MAJORITY OF THE  
SYNOPTIC SYSTEMS OF CONCERN. NEVERTHELESS, THE CMC AND CMC MEAN  
HAVE BEEN TAKING THE NEXT PACIFIC SYSTEM ON A TRACK FARTHER NORTH  
TOWARD THE PACIFIC NORTHWEST THAN THE GFS AND THE MORE RECENT  
ECMWF RUNS. THE MOST RECENT (12Z) GFS AND THE ECMWF RUNS NUDGED  
THE LOW TRACK FARTHER NORTH THOUGH. BY NEXT WEEKEND, MODELS SHOW  
SOME SPREAD ON HOW AMPLIFIED THE UPPER TROUGH ASSOCIATED WITH THIS  
SYSTEM WILL GET AS IT DIPS NEAR THE SOUTHWESTERN BORDER OF THE  
U.S. MEANWHILE, MODELS ARE SHOWING BETTER AGREEMENT/DEFINITION ON  
A FRONTAL WAVE TO DEVELOP QUICKLY NEAR/SOUTH OF THE GREAT LAKES  
NEXT WEEKEND AND THEN TRACK RAPIDLY DURING NEXT WEEKEND.  
 
THE WPC MEDIUM RANGE FORECAST PRODUCTS WERE BASED ON A BLEND OF  
40% FROM THE 06Z GFS/GEFS, 40% FROM THE 00Z ECMWF/EC MEAN AND 20%  
FROM THE 00Z CMC/CMC MEAN, WITH MORE WEIGHTS TOWARD THE ENSEMBLE  
MEAN STARTING ON DAY 6. THIS BLEND YIELDED A SOLUTION QUITE  
COMPATIBLE WITH THE PREVIOUS WPC FORECAST PACKAGE AND SEEMS WELL  
SUPPORTED BY GEFS/NAEFS/ECMWF ENSEMBLES.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
A VIGOROUS UPPER TROUGH WILL LIFT NORTHEASTWARD FROM THE SOUTHWEST  
MIDWEEK, BRINGING WIDESPREAD MOUNTAIN SNOW/RAIN THROUGH THE GREAT  
BASIN AND NORTHERN/CENTRAL ROCKIES. AS THE SYSTEM LIFTS THROUGH  
THE PLAINS WED-THU, RAPID CYCLOGENESIS ACROSS THE NORTH-CENTRAL  
U.S. WILL LEAD TO AN EXPANSIVE AREA OF STRONG WINDS, TOGETHER WITH  
A SWATH OF RAIN/SNOW THROUGH THE NORTH SIDE OF THE LOW TRACK FROM  
THE NORTHERN PLAINS AND UPPER MIDWEST TO THE UPPER GREAT LAKES.  
AHEAD OF THE DEEP CYCLONE, AN UPPER RIDGE WILL BUILD OVER THE  
CENTRAL U.S. WHERE ANOMALOUS WARMTH WILL BE MOST PRONOUNCED  
MIDWEEK WITH RECORD HIGH TEMPERATURES UPWARDS TO 30-40F ABOVE  
NORMAL. MODEST LEAD RAINS THAT SHOULD FALL ON TOP OF RECENT UPPER  
MIDWEST SNOWS TOGETHER WITH THE RECORD WARMTH COULD LEAD TO  
FLOODING ISSUES ON WEDNESDAY. ANOMALOUS WARMTH AHEAD OF THE COLD  
FRONT, WITH POSITIVE ANOMALIES UPWARDS TO 15-25F, WILL REACH THE  
EAST LATE WEEK. THE TRAILING PORTION OF THE FRONT WILL LIKELY  
STALL ACROSS THE DEEP SOUTH LATE THIS WEEK, WITH AN AXIS OF  
ENHANCED RAINFALL FORECAST TO SET UP FROM THE SOUTHERN PLAINS  
THROUGH THE MID-SOUTH/OH VALLEY. A FRONTAL WAVE DEVELOPING  
NEAR/SOUTH OF THE GREAT LAKES BY NEXT WEEKEND SHOULD BRING A SWATH  
OF WINTRY PRECIPITATION MOST LIKELY ACROSS NORTHERN NEW ENGLAND.  
MEANWHILE, THE RAINFALL AXIS ACROSS THE SOUTH WILL TEND TO PUSH  
SOUTHWARD TOWARD THE GULF COAST REGION DURING THE WEEKEND AS A  
HIGH PRESSURE SYSTEM BUILDS ACROSS THE PLAINS TOWARD THE EAST  
COAST.  
 
UPSTREAM ON THE HEELS OF THE AFOREMENTIONED LEAD STORM, ANOTHER  
MOISTURE SURGE ACCOMPANIED WITH MODERATE TO HEAVY PRECIPITATION IS  
FORECAST TO REACH NORTHERN CALIFORNIA AND OREGON WED/THU AHEAD OF  
THE NEXT PACIFIC SYSTEM DESPITE SOME UNCERTAINTY WITH THE LOW  
TRACK. MODELS SHOW GENERAL AGREEMENT THAT THIS SYSTEM WILL  
GRADUALLY WEAKEN AS IT GRADUALLY DIPS INTO SOUTHERN CALIFORNIA BY  
FRIDAY. BY NEXT WEEKEND, GUIDANCE SIGNAL IS ALSO GROWING THAT  
MOISTURE FROM YET ANOTHER PACIFIC SYSTEM WILL MODESTLY IMPACT THE  
PACIFIC NORTHWEST.  
 
KONG/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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