450  
FXUS02 KWBC 070708  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
208 AM EST TUE DEC 07 2021  
 
VALID 12Z FRI DEC 10 2021 - 12Z TUE DEC 14 2021  
 
...WINTER WEATHER IS LIKELY FOR THE NORTHERN PLAINS TO  
MIDWEST/UPPER GREAT LAKES BEHIND A SURFACE LOW FRIDAY, WHILE HEAVY  
RAIN IS EXPECTED IN THE EAST-CENTRAL U.S. ALONG ITS ASSOCIATED  
COLD FRONT...  
 
...HEAVY PRECIPITATION IS FORECAST TO SPREAD SOUTHWARD ALONG THE  
WEST COAST LATE THIS WEEK INTO EARLY NEXT WEEK...  
   
..OVERVIEW
 
 
A GENERALLY PROGRESSIVE AND ACTIVE WEATHER PATTERN IS FORECAST TO  
OCCUR THROUGH EARLY NEXT WEEK, AS A MID-UPPER TROUGH PUSHES FROM  
THE INTERMOUNTAIN WEST EARLY FRIDAY TOWARD THE EAST BY SUNDAY,  
WHILE A SURFACE LOW AND POTENT COLD FRONT TRACK AHEAD OF IT. A  
SWATH OF WINTRY WEATHER IS LIKELY TO SPREAD FROM THE CENTRAL  
ROCKIES INTO NORTHERN PORTIONS OF THE PLAINS TO THE MIDWEST/UPPER  
GREAT LAKES ON FRIDAY BEHIND THE LOW, WHILE HEAVY RAIN IS POSSIBLE  
LATE THIS WEEK ACROSS THE EAST-CENTRAL U.S. ALONG AND AHEAD OF THE  
COLD FRONT WITH SOME TIMING UNCERTAINTIES. MEANWHILE, SLOW-MOVING  
AND RELOADING TROUGHING IN THE EASTERN PACIFIC WILL GIVE  
PERSISTENT MOIST INFLOW TO THE WEST COAST, WITH HEAVY MOUNTAIN  
SNOWS LIKELY THAT SHOULD SPREAD SOUTHWARD AS THE PERIOD PROGRESSES.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE MEDIUM RANGE PERIOD BEGINS FRIDAY WITH QUITE GOOD MODEL  
AGREEMENT WITH THE OVERALL LARGE-SCALE PATTERN, DESPITE SOME MINOR  
DIFFERENCES IN SHORTWAVE ENERGY WITHIN THE MAIN TROUGH WITH ITS  
AXIS TRACKING THROUGH WESTERN AND CENTRAL PARTS OF THE U.S.  
THROUGH THE END OF THE WEEK. THE TRACK OF THE SURFACE LOW PRESSURE  
SYSTEM MOVING QUICKLY FROM THE CENTRAL HIGH PLAINS INTO THE UPPER  
GREAT LAKES HAS ALSO BEEN IN GOOD AGREEMENT FOR A FEW MODEL CYCLES  
NOW, THOUGH THE TIMING OF THE COLD FRONT EXTENDING SOUTHWARD FROM  
THE LOW REMAINS SOMEWHAT VARIABLE. A MULTI-MODEL DETERMINISTIC  
BLEND WAS ABLE TO BE USED EARLY IN THE PERIOD.  
 
BY SUNDAY, DIFFERENCES BEGIN TO ARISE IN THE EAST THAT AMPLIFY  
MONDAY-TUESDAY, PARTICULARLY WITH WHETHER OR NOT NORTHERN AND  
SOUTHERN STREAMS WILL SEPARATE DEPENDING PRIMARILY ON DISTRIBUTION  
OF ENERGY WITHIN THE TROUGH. THIS IS HIGHLY UNCERTAIN AND HAS HAD  
QUITE A BIT OF RUN-TO-RUN VARIABILITY WITHIN AND BETWEEN MODEL  
GUIDANCE. WITH THE 12/18Z (YESTERDAY) MODEL CYCLE, WHICH THE WPC  
FORECAST IS BASED ON, TRENDS SEEMED TO BE FOR MORE SEPARATION OF  
THE STREAMS AND A SOUTHERN STREAM LOW THAT IS SLOWER TO EXIT THE  
SOUTHEAST. THIS WAS SHOWN BY THE 12Z ECMWF, CMC, AND UKMET  
(THROUGH ITS PERIOD), WITH THE CMC AND EC ENSEMBLE MEANS ALSO  
SHOWING STREAM SEPARATION. THE GFS AND GEFS RUNS APPEARED TO BE  
BASICALLY ALONE IN THEIR MORE PHASED AND FASTER TRACK OF THE  
TROUGH EASTWARD INTO THE ATLANTIC. CONSIDERING TYPICAL BIASES OF  
THE MODEL SUITES, THE WPC FORECAST WAS BASED ON AN INTERMEDIATE  
SOLUTION BUT SLIGHTLY FAVORING THE SLOWER SIDE OF THE  
GUIDANCE--MAINLY A BLEND OF THE EC AND CMC ENSEMBLE MEANS WITH  
SOME COMPONENT OF THE IN BETWEEN DETERMINISTIC CMC, AND NOT  
FAVORING THE 12Z ECMWF THAT WAS SLOWER THAN BASICALLY EVERY  
ENSEMBLE MEMBER BUT ALSO NOT FAVORING THE FAST GFS RUNS. HOWEVER,  
THE INCOMING 00Z DETERMINISTIC MODELS INCLUDING THE CMC AND ECMWF  
ARE SHOWING FASTER MOVEMENT OF A MORE PHASED TROUGH OFFSHORE,  
INDICATING HIGH UNCERTAINTY CONTINUES. EXPECT FORECAST CHANGES  
WITH THIS FEATURE AS MODELS STILL VARY.  
 
UPPER TROUGHING IS FORECAST TO DROP SOUTHWARD FROM ALASKA ACROSS  
THE EASTERN PACIFIC OVER THE WEEKEND AND INTO EARLY NEXT WEEK.  
WHILE MODEL GUIDANCE AGREES UPON THE EXISTENCE OF THIS FEATURE AND  
ITS SLOW DRIFT SOUTHWARD, THERE IS PLENTY OF VARIETY IN HOW  
AMPLIFIED IT COULD BE AT ANY GIVEN TIME, WHICH AFFECTS THE MOIST  
ONSHORE FLOW AXIS INTO THE WEST COAST AND THE EXTENT/AMOUNTS/TYPE  
OF PRECIPITATION. AT THIS POINT, A SIMILAR MODEL BLEND PREVIOUSLY  
DESCRIBED WORKED FOR THIS FEATURE AS WELL--WITH DETERMINISTIC  
MODELS EARLY ON QUICKLY TRANSITIONING TO A BLEND HEAVY ON THE EC  
AND CMC ENSEMBLE MEANS. IN BETWEEN THESE TROUGHS, MODEL AGREEMENT  
IS REASONABLY GOOD FOR UPPER RIDGING TO BUILD IN ACROSS THE  
CENTRAL U.S.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
THE SURFACE LOW PRESSURE SYSTEM TRACKING NORTHEASTWARD INTO THE  
MIDWEST/GREAT LAKES FRIDAY TO EARLY SATURDAY WILL LIKELY SPREAD  
SNOW ON THE BACKSIDE FROM THE CENTRAL ROCKIES INTO NORTHERN  
PORTIONS OF THE PLAINS AND INTO THE MIDWEST, WITH MODERATE TO  
LOCALLY HEAVY SNOW POSSIBLE. THE TRAILING COLD FRONT ASSOCIATED  
WITH THIS LOW WILL TRACK THROUGH THE EASTERN HALF OF THE U.S. AND  
SPREAD WIDESPREAD RAIN WITH EMBEDDED THUNDERSTORMS ALONG AND AHEAD  
OF IT. THERE IS POTENTIAL FOR HEAVY RAIN AMOUNTS WITH  
QUICK-HITTING HIGH RAINFALL RATES THAT COULD CAUSE FLOODING/FLASH  
FLOODING ISSUES FOR PORTIONS OF THE LOWER MISSISSIPPI, TENNESSEE,  
AND OHIO VALLEYS ON FRIDAY INTO EARLY SUNDAY. THEN, HOW MUCH RAIN  
FALLS ALONG THE EASTERN SEABOARD WILL DEPEND ON THE TIMING OF THE  
UPPER TROUGH AND COLD FRONT EXITING INTO THE ATLANTIC, WHICH  
REMAINS UNCERTAIN. MEANWHILE, PERSISTENT ONSHORE FLOW IS LIKELY  
INITIALLY IN THE PACIFIC NORTHWEST AND NORTHERN ROCKIES AND THEN  
COULD TRACK SOUTH INTO CALIFORNIA BY EARLY NEXT WEEK. ENHANCED  
TOTALS ARE LIKELY ALONG FAVORED TERRAIN, WITH HEAVY MOUNTAIN SNOW.  
 
AHEAD OF THE TROUGH AND COLD FRONT, MILD TO WARM TEMPERATURES ARE  
FORECAST TO BE IN PLACE IN THE CENTRAL U.S. ON 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 BUILDS ACROSS THE LOWER 48 BY EARLY NEXT WEEK,  
TEMPERATURES SHOULD RETURN TO 10-20F ABOVE NORMAL ACROSS MUCH OF  
THE CENTRAL U.S. BY SUNDAY AND LASTING/WARMING FURTHER THOUGH  
TUESDAY.  
 
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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