085  
FXUS02 KWBC 081732  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1231 PM EST WED DEC 08 2021  
 
VALID 12Z SAT DEC 11 2021 - 12Z WED DEC 15 2021  
 
...LOCALLY HEAVY RAIN POSSIBLE PARTICULARLY IN THE SOUTHERN  
APPALACHIANS SATURDAY WITH RECORD-BREAKING WARMTH ALONG THE  
EASTERN SEABOARD AHEAD OF A STRONG COLD FRONT...  
 
...AN ATMOSPHERIC RIVER WILL SPREAD SOUTHWARD ALONG THE WEST COAST  
THIS WEEKEND INTO EARLY NEXT WEEK BRINGING COASTAL RAIN AND HEAVY  
MOUNTAIN SNOW...  
   
..OVERVIEW
 
 
A GENERALLY PROGRESSIVE AND ACTIVE WEATHER PATTERN IS FORECAST TO  
OCCUR THROUGH THE WEEKEND, AS A MID-UPPER TROUGH PUSHES FROM THE  
CENTRAL U.S. SATURDAY TOWARD THE EAST BY SUNDAY, WHILE A SURFACE  
LOW AND POTENT COLD FRONT TRACK AHEAD OF IT. RECORD-SETTING WARM  
TEMPERATURES AND MODERATE TO LOCALLY HEAVY PRECIPITATION ARE  
LIKELY IN THE EAST 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. THE OVERALL PATTERN IS EXPECTED  
TO STAGNATE MONDAY AND TUESDAY WITH UPPER RIDGING BUILDING INTO  
MUCH OF THE LOWER 48 TO THE EAST OF THE PACIFIC TROUGH, BEFORE THE  
TROUGH BEGINS TO MAKE ITS WAY EASTWARD AROUND MIDWEEK.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
MODEL GUIDANCE FOR THE 00/06Z CYCLE CONTINUES TO SHOW VERY GOOD  
AGREEMENT THROUGH MOST OF THE PERIOD WITH THE PATTERN DESCRIBED  
ABOVE. AFTER SOME PREVIOUS WAFFLING, GUIDANCE HAS BEEN CONSISTENT  
FOR A COUPLE OF CYCLES NOW SHOWING A MORE PHASED (NO  
NORTHERN/SOUTHERN STREAM SEPARATION) TROUGH IN THE EAST OVER THE  
WEEKEND, CAUSING A FASTER TRACK OF THE TROUGH AND ASSOCIATED COLD  
FRONT, SO THIS TREND WAS MAINTAINED IN THE WPC FORECAST. FARTHER  
WEST, THERE REMAIN SOME MODEL DIFFERENCES WITH THE EASTERN PACIFIC  
TROUGH REGARDING THE DISTRIBUTION OF ENERGY AND THE POTENTIAL FOR  
A CLOSED LOW TO DEVELOP (ESPECIALLY WITH GFS RUNS) WITHIN IT. BUT  
OVERALL GUIDANCE AGREES IN A SLOW TRACK OF THE TROUGH AXIS  
SOUTHWARD BRINGING PERSISTENT MOIST SOUTHWESTERLY FLOW INTO THE  
WEST COAST. THERE ARE INCREASING TIMING DIFFERENCES AMONG  
DETERMINISTIC AND ENSEMBLE GUIDANCE WITH THE TROUGH EVENTUALLY  
PUSHING EASTWARD TUESDAY AND WEDNESDAY, ALONG WITH DISTRIBUTION OF  
SHORTWAVES WITHIN IT. THE CMC IS THE QUICKEST TO EJECT THIS TROUGH  
INTO THE SOUTHWEST U.S. NEXT WEDNESDAY, THE ECMWF SLOWEST (STILL  
BACK ALONG THE COAST), WITH THE GFS SITTING IN BETWEEN. SPAGHETTI  
DIAGRAMS OF VARIOUS ENSEMBLE SOLUTIONS SUGGEST THE DETERMINISTIC  
CMC MAY BE TOO QUICK, AND THE BETTER CONSENSUS IS SOMEWHERE AROUND  
THE ECMWF/GFS (WHICH IS ALSO CONSISTENT WITH THE ENSEMBLE MEANS).  
 
THE WPC FORECAST UTILIZED A MULTI-MODEL DETERMINISTIC BLEND OF THE  
00/06Z GUIDANCE EARLY IN THE PERIOD, AND WAS ABLE TO HOLD ONTO A  
MAJORITY DETERMINISTIC MODELS BLEND THROUGH DAY 6 (WITHOUT THE CMC  
DUE TO REASONS ABOVE). DAY 7, LEANED MORE ON THE ENSEMBLE MEANS  
DUE TO TROUGHING TIMING ISSUES IN THE WEST. THIS MAINTAINED  
EXCELLENT CONTINUITY WITH THE PREVIOUS WPC FORECAST AS WELL.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
A STRONG COLD FRONT WILL TRACK THROUGH THE EASTERN THIRD OF THE  
U.S. ON SATURDAY INTO SUNDAY 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 FOR  
PORTIONS OF THE SOUTHEAST, TENNESSEE AND OHIO VALLEYS, AND  
APPALACHIANS, WHICH COULD CAUSE LOCALIZED FLOODING/FLASH FLOODING  
ISSUES ESPECIALLY IN THE MORE SENSITIVE TERRAIN OF THE SOUTHERN  
APPALACHIANS. WITH A QUICKER UPPER TROUGH AND COLD FRONTAL TRACK,  
MOISTURE WILL LIKELY CLEAR OUT OF THE EASTERN SEABOARD BY SUNDAY,  
AND WITH HIGH PRESSURE IN ITS WAKE, DRY CONDITIONS ARE LIKELY IN  
THE CENTRAL AND EASTERN U.S. FOR THE FIRST PART OF NEXT WEEK OTHER  
THAN SOME POSSIBILITY OF INCREASING SHOWERS IN THE SOUTH-CENTRAL  
U.S. AND GREAT LAKES BY MONDAY AND TUESDAY. MEANWHILE, PERSISTENT  
MOIST ONSHORE FLOW WITH A WEAK ATMOSPHERIC RIVER IS LIKELY  
INITIALLY IN THE PACIFIC NORTHWEST AND NORTHERN ROCKIES SATURDAY,  
AND THEN WILL 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, AND SPREADING TO THE CENTRAL GREAT BASIN AND ARIZONA BY  
TUESDAY. LOCALIZED FLASH FLOODING COULD BE A CONCERN, ESPECIALLY  
FOR SENSITIVE BURN SCAR AREAS IN NORTHERN CALIFORNIA.  
 
AHEAD OF THE TROUGH AND COLD FRONT, MILD TO WARM TEMPERATURES ARE  
FORECAST FOR THE EASTERN THIRD OF THE U.S. ON SATURDAY. DOZENS OF  
DAILY RECORD HIGH MAXIMUM AND MINIMUM TEMPERATURES COULD BE SET  
FROM THE CENTRAL GULF COAST TO NORTHEAST WITH TEMPERATURES 20-30F  
ABOVE NORMAL. TEMPERATURES SHOULD MODERATE TO NEAR NORMAL BEHIND,  
BUT BY EARLY NEXT WEEK AS RIDGING BUILDS OVER THE REGION,  
TEMPERATURES ARE EXPECTED TO WARM TO 10-20F ABOVE NORMAL ON SUNDAY  
IN THE CENTRAL U.S., WITH WARMTH EXPANDING INTO THE EASTERN U.S.  
AS THE WORKWEEK BEGINS, AND WARMING FURTHER TO POSSIBLY 30+F ABOVE  
AVERAGE IN PARTS OF THE CENTRAL PLAINS AND MISSISSIPPI VALLEY BY  
NEXT WEDNESDAY.  
 
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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