050  
FXUS02 KWBC 071832  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
132 PM EST TUE DEC 07 2021  
 
VALID 12Z FRI DEC 10 2021 - 12Z TUE DEC 14 2021  
 
...ACCUMULATING SNOWFALL LIKELY FOR THE NORTHERN PLAINS TO  
MIDWEST/UPPER GREAT LAKES WITH HEAVY RAIN ACROSS PARTS OF THE EAST  
CENTRAL U.S. AHEAD OF THE COLD FRONT FRIDAY INTO SATURDAY...  
 
...ATMOSPHERIC RIVER TO SPREAD SOUTHWARD ALONG THE WEST COAST LATE  
THIS WEEK INTO EARLY NEXT WEEK BRINGING HEAVY COASTAL RAIN AND  
MOUNTAIN SNOW...  
   
..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. MEANWHILE, A SLOW-MOVING AND RELOADING TROUGH IN THE  
EASTERN PACIFIC WILL DIRECT A PERSISTENT AND MOIST INFLOW  
(ATMOSPHERIC RIVER) TO THE WEST COAST, WITH HEAVY COASTAL RAIN AND  
MOUNTAIN SNOWS LIKELY THAT SHOULD SPREAD SOUTHWARD WITH TIME.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE LATEST 00Z/06Z GUIDANCE CONTINUES TO SHOW VERY GOOD AGREEMENT  
THROUGH MUCH OF THE PERIOD AND A GENERAL DETERMINISTIC MODEL BLEND  
SUFFICED, ESPECIALLY FOR THE EARLY PART OF THE PERIOD. THE LATEST  
CYCLE OF MODELS SEEMS TO HAVE TRENDED TOWARDS A MORE PROGRESSIVE  
AND ELONGATED TROUGH THROUGH THE EAST. THE MAIN EXCEPTION TO THIS  
IS THE UKMET WHICH CONTINUES TO SHOW THE POTENTIAL FOR STREAM  
SEPERATION WITH THE TROUGH, LEAVING BEHIND A COMPACT CLOSED LOW  
OVER THE SOUTHEAST SUNDAY-MONDAY. THERE ARE MANY OF THE EC  
ENSEMBLE MEMBERS (AND THE MEAN) WHICH ALSO SUPPORTS THIS SCENARIO  
TOO (THOUGH THE GEFS, NAEFS, AND CMCE MEANS ARE MORE PROGRESSIVE).  
THUS, THERE REMAINS PLENTY OF UNCERTAINTY REGARDING THE LATE  
WEEKEND EVOLUTION OF THIS TROUGH, WHICH OF COURSE WOULD IMPACT HOW  
QUICKLY PRECIPITATION MOVES THROUGH AND OVERALL FLOOD CONCERNS.  
THE WPC FORECAST FOR TODAY LEANS TOWARDS THE BETTER CONSENSUS OF A  
PROGRESSIVE, INTACT TROUGH FROM THE DETERMINISTIC GFS, ECMWF, AND  
CMC ALONG WITH THE GEFS AND NAEFS MEANS.  
 
OUT WEST, THE GUIDANCE ALSO SHOWS GOOD SYNOPTIC AGREEMENT  
REGARDING THE TROUGH DROPPING SOUTH ALONG THE WEST COAST AND A  
POTENTIAL ATMOSPHERIC RIVER EVENT FOR THE PACIFIC NORTHWEST AND  
CALIFORNIA. SOME QUESTIONS IN THE DETAILS REMAIN HOWEVER, AND  
THERE ARE SOME TIMING DIFFERENCES WITH THE TROUGH AXIS ENTERING  
THE WEST COAST BY DAY 7. INCORPORATING MORE OF THE ENSEMBLE MEANS  
BY THE END OF THE PERIOD HELPED MITIGATE THESE DIFFERENCES.  
 
OVERALL, THIS UPDATE TO THE WPC MEDIUM RANGE FORECAST REMAINED  
FAIRLY CONSISTENT WITH THE OVERNIGHT PACKAGE. THE ONLY NOTABLE  
CHANGES WERE REGARDING A FASTER COLD FRONT THROUGH THE EAST, GIVEN  
THE CURRENT MODEL TREND.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
THE SURFACE LOW PRESSURE SYSTEM TRACKING NORTHEASTWARD INTO THE  
MIDWEST/GREAT LAKES FRIDAY TO EARLY SATURDAY WILL LIKELY SPREAD  
SNOW ON ITS 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 AND ESPECIALLY INTO SATURDAY. 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. GUSTY WINDS, ESPECIALLY ACROSS  
PORTION OF THE SOUTHERN HIGH PLAINS ON FRIDAY, MAY ACCOMPANY THE  
LOW CONSISTENT WITH A TIGHT PRESSURE GRADIENT. 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 IN THE CASCADES, SIERRA  
NEVADA, AND NORTHERN ROCKIES.  
 
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 AND POTENTIALLY RECORD-SETTING 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 MUCH OF THE  
LOWER 48 BY EARLY NEXT WEEK, TEMPERATURES SHOULD RETURN TO 10-20F  
ABOVE NORMAL ACROSS THE CENTRAL U.S. BY SUNDAY AND LASTING/WARMING  
FURTHER THROUGH TUESDAY.  
 
SANTORELLI/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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