028  
FXUS02 KWBC 191915  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
214 PM EST SUN DEC 19 2021  
 
VALID 12Z WED DEC 22 2021 - 12Z SUN DEC 26 2021  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE UPPER-LEVEL PATTERN CONTINUES TO FAVOR A SLOWER EVOLUTION  
THROUGH THE END OF THE WEEKEND. DURING THIS TIME, A VERY STRONG  
RIDGE WILL BE ANCHORED OVER CENTRAL/EAST-CENTRAL PACIFIC OCEAN.  
THERE WILL BE IMPULSES TRACKING THROUGH THE FLOW ON THE NORTH SIDE  
OF THIS RIDGE, WHICH HELP REINFORCE A MEAN TROUGH OVER THE EASTERN  
PACIFIC THAT DRIFTS NEAR/ALONG THE WEST COAST. FAST FLOW WILL  
PREVAIL DOWNSTREAM, BETWEEN LOW HEIGHTS OVER CENTRAL CANADA AND AN  
UPPER HIGH FORECAST TO MIGRATE EASTWARD ACROSS MEXICO AND THE  
SOUTHERN GULF OF MEXICO. THIS PATTERN WILL FAVOR MULTIPLE DAYS OF  
WIDESPREAD PRECIPITATION OVER THE WEST, WITH THE SIERRA NEVADA  
LIKELY TO SEE THE HIGHEST FIVE-DAY TOTALS. WELL-ABOVE NORMAL  
TEMPERATURES ARE EXPECTED ACROSS MUCH OF THE SOUTHERN PLAINS  
BEFORE SPREADING TO THE EAST LATER IN THE EXTENDED PERIOD. IN  
CONTRAST, TEMPERATURE WILL TREND COLDER ACROSS THE NORTHERN TIER  
OF THE COUNTRY. SYSTEMS TO THE EAST OF THE ROCKIES WILL INITIALLY  
HAVE DYNAMIC SUPPORT FROM SOUTHERN CANADA/NORTHERN TIER U.S. FLOW  
AND THEN BECOME MORE DEPENDENT ON FEATURES EJECTING FROM THE WEST  
COAST TROUGH.  
 
EARLY IN THE PERIOD OVER THE EAST, CONSENSUS HAS BEEN STABLE IN  
RECENT RUNS FOR THE SYSTEM CROSSING SOUTHEASTERN CANADA AND  
NORTHEASTERN U.S. AROUND MIDWEEK. HOWEVER GUIDANCE IS STILL  
HAVING SOME DIFFICULTIES IN RESOLVING THE DETAILS OF THE SYSTEM  
EXPECTED TO BE JUST OFF THE NORTH CAROLINA COAST EARLY WEDNESDAY  
AND CONTINUE OVER THE ATLANTIC THEREAFTER, FAVORING AN  
INTERMEDIATE APPROACH. THE WPC BLEND BEGAN WITH A MIX OF THE 00Z  
ECWMF/CMC/UKMET/GFS(AND 06Z) AND WITH TIME INCREASED WEIGHTING OF  
BOTH THE EC ENSEMBLE AND GEFS MEANS WHILE DECREASING THE USE OF  
THE CMC AND UKMET.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
LOWER ELEVATION RAIN AND MID-HIGH ELEVATION SNOW CAN BE EXPECTED  
FOR A BROAD AREA OF THE WESTERN STATES AS A PERSISTENT FLOW OF  
MOISTURE IS DIRECTED ONSHORE AHEAD OF AN EASTERN PACIFIC/WEST  
COAST UPPER TROUGH. THERE WILL BE PERIODS WHERE THE PRECIPITATION  
WILL BE MODERATE OR HEAVY WHICH MAY INCREASE SOIL SATURATION AND  
INCREASE SENSITIVITY TO LOCAL RUNOFF; HOWEVER GIVEN SOME OF THE  
UNCERTAINTY ON TIMING AND INTERACTIONS, THE EXACT LOCATION AND  
INTENSITY OF LOWER ELEVATION RAINFALL WITH RESPECT TO LOCAL  
RUNOFF/FLOODING ISSUES OVER CALIFORNIA. PARTS OF NORTHERN/CENTRAL  
CALIFORNIA IS EXPECTED TO HAVE SOME OF THE HIGHEST ACCUMULATIONS,  
ESPECIALLY HEAVY SNOW IN THE SIERRA NEVADA. THESE SNOWS MAY HAVE  
HIGH LIQUID EQUIVALENTS IN THE FAVORED UPSLOPE AREAS OF THE SIERRA  
NEVADA RANGE. SOME MOISTURE SHOULD EXTEND AS FAR SOUTH AS SOUTHERN  
CALIFORNIA AND ARIZONA AFTER MIDWEEK WHILE FAVORED TERRAIN OVER  
THE GREAT BASIN AND NORTHERN/CENTRAL ROCKIES WILL SEE MEANINGFUL  
PRECIPITATION AS WELL. TEMPERATURES RANGING 15 TO 25 DEGREES F  
WARMER THAN SEASONAL AVERAGE WILL BE PRESENT OVER MUCH OF THE  
SOUTHERN PLAINS WILL SPREAD INTO THE MISSISSIPPI VALLEY LATER IN  
THE PERIOD - A FEW RECORD HIGHS MAY BE POSSIBLE OVER TEXAS. A WARM  
UP OF 10 TO 20 DEGREES F ABOVE THE MEAN MAY REACH THE EAST BY THE  
WEEKEND.  
 
LOW PRESSURE TRACKING AWAY FROM NEAR THE NORTH CAROLINA COAST MAY  
BE ACCOMPANIED BY A LITTLE RAIN EARLY WEDNESDAY WHILE PASSAGE OF A  
SOUTHEASTERN CANADA SYSTEM AND A WEAK UPSTREAM FEATURE MAY PRODUCE  
LIGHT SNOW OVER THE UPPER GREAT LAKES AND NORTHERN NEW ENGLAND. A  
MORE ORGANIZED SYSTEM MAY SPREAD SNOW AND RAIN SPREAD FROM THE  
NORTHERN HIGH PLAINS TO THE NORTHEAST. THE FAR NORTHERN PORTION OF  
THE PLAINS WILL HAVE TEMPERATURES PLUMMET- WITH 15 TO 30 DEGREES  
F BELOW NORMAL OVER PARTS OF MONTANA AND NORTH DAKOTA. THE  
UNSETTLED PATTERN OVER THE WEST SHOULD LEAD TO NEAR OR ABOVE  
NORMAL LOWS AND NEAR TO BELOW NORMAL HIGHS THROUGH THE ENTIRE  
WEEK. WESTERN HIGHS SHOULD GRADUALLY TREND COOLER WITH TIME AS  
THE AXIS OF THE EASTERN PACIFIC UPPER TROUGH REACHES AT LEAST AS  
FAR AS THE WEST COAST.  
 
CAMPBELL/RAUSCH  
 
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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