647  
FXUS02 KWBC 210700  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
159 AM EST TUE DEC 21 2021  
 
VALID 12Z FRI DEC 24 2021 - 12Z TUE DEC 28 2021  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
GUIDANCE CONTINUES TO SHOW AN IMPRESSIVELY STRONG AND PERSISTENT  
RIDGE OVER AND NORTH OF THE EAST-CENTRAL PACIFIC HELPING TO HOLD  
AN AMPLIFIED UPPER TROUGH IN PLACE OVER OR SLIGHTLY INLAND FROM  
THE WEST COAST. THIS UPPER TROUGH WILL SUPPORT AN EXTENDED PERIOD  
OF PRECIPITATION AND MOSTLY BELOW NORMAL HIGH TEMPERATURES OVER A  
BROAD PORTION OF THE WEST. THE TROUGH'S SHAPE WILL VARY FROM DAY  
TO DAY DEPENDING ON INDIVIDUAL SHORTWAVES DIGGING INTO THE TROUGH,  
AND FEATURES EJECTING FROM THE TROUGH WILL PERIODICALLY SUPPORT  
WAVES/SYSTEMS TO THE EAST OF THE ROCKIES. ASSOCIATED FRONTS WILL  
SEPARATE A PRONOUNCED TEMPERATURE CONTRAST, BETWEEN WELL ABOVE  
NORMAL READINGS OVER AND EAST OF THE SOUTHERN TWO-THIRDS OF THE  
PLAINS AND MUCH BELOW NORMAL TEMPERATURES BECOMING ESTABLISHED  
OVER THE FAR NORTHERN PLAINS.  
 
THE PRIMARY FORECAST PROBLEMS INVOLVE THE SPECIFICS OF INDIVIDUAL  
SHORTWAVES WITHIN THE WESTERN TROUGH AND THEN OVER EASTERN U.S.  
HOW A CONFLICT OF LARGE SCALE PATTERN TELECONNECTIONS GETS  
RESOLVED WITH RESPECT TO EJECTING FEATURES--NAMELY THE  
PACIFIC/WESTERN U.S. PATTERN FAVORING ABOVE NORMAL UPPER HEIGHTS  
OVER THE EAST VERSUS THE RIDGING/POSITIVE HEIGHT ANOMALIES  
NORTHEAST OF HUDSON BAY FAVORING MODERATELY BELOW NORMAL EASTERN  
U.S. HEIGHTS.  
 
AS FOR THE FIRST SYSTEM OF INTEREST, THERE HAS BEEN DECENT  
AGREEMENT IN PRINCIPLE THAT FAST-MOVING SHORTWAVE ENERGY EMERGING  
OVER THE ROCKIES/PLAINS EARLY FRIDAY WILL SUPPORT A PLAINS-MIDWEST  
THROUGH NORTHEAST SYSTEM FRIDAY AND CHRISTMAS DAY/SATURDAY.  
GUIDANCE HAS HAD SOME DIFFICULTY IN RESOLVING THE DETAILS DUE IN  
PART TO THE BLOCKY NATURE OF FLOW OVER EASTERN CANADA. NEW 00Z  
RUNS HAD SHOWED SOME ADDED IMPROVEMENT IN CLUSTERING FOR  
SPECIFICS, BUT THE ECMWF HAS CHANGED CONSIDERABLY WITH SHORTWAVE  
DETAILS NEAR THE WEST COAST--ULTIMATELY LEADING TO A MAJOR  
CONTINUITY CHANGE FARTHER EAST.  
 
ATTENTION THEN TURNS TO THE NEXT BUNDLE OF ENERGY AMPLIFYING NEAR  
THE WEST COAST AROUND THE END OF THE WEEK AND SEPARATE SHORTWAVE  
ENERGY LINGERING OFFSHORE TO THE SOUTH (LINGERING ENERGY FROM AN  
UPPER LOW NOW OVER THE EASTERN PACIFIC). THE NORTHERN STREAM  
SHORTWAVE HAS REASONABLE AGREEMENT BUT THERE ARE IMPORTANT DETAIL  
DIFFERENCES WITH THE SOUTHERN ENERGY--LEADING TO SIGNIFICANT  
ISSUES FOR THE PRECIPITATION FORECAST OVER THE SOUTHWESTERN U.S.  
THE COMBINATION OF THESE FEATURES SHOULD PROGRESS ACROSS THE WEST  
AND THEN SUPPORT ONE OR MORE SURFACE LOWS/FRONTS FROM THE PLAINS  
EASTWARD SUNDAY ONWARD. BY MONDAY-TUESDAY INDIVIDUAL MODEL RUNS  
AND ENSEMBLE MEMBERS INCREASINGLY DIVERGE FOR DETAILS AT THE  
SURFACE AND ALOFT EAST OF THE ROCKIES, PERHAPS REFLECTING THE  
COMPETING TELECONNECTION TENDENCIES OVER THE EAST. THESE  
DIFFERENCES, AS WELL AS DECREASING CONFIDENCE FOR DETAILS WITHIN  
THE WESTERN TROUGH LATE IN THE PERIOD, FAVOR TRENDING THE INITIAL  
12Z/18Z OPERATIONAL MODEL BLEND USED EARLY IN THE FORECAST TOWARD  
AT LEAST TWO-THIRDS WEIGHT OF THE ENSEMBLE MEANS (18Z GEFS AND 12Z  
ECENS/CMCENS) BY DAY 7 TUESDAY.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
SHORTWAVES DIGGING INTO THE PERSISTENT UPPER TROUGH ALIGNED NEAR  
THE WEST COAST WILL PERIODICALLY ENHANCE THE FLOW OF MOISTURE INTO  
THE WEST, WITH AREAS OF FOCUSED/TERRAIN-ENHANCED PRECIPITATION  
EXTENDING FROM THE COAST INTO THE ROCKIES. EXPECT EPISODES OF  
MODERATE TO HEAVY PRECIPITATION FROM THE CASCADES AND COASTAL  
AREAS OF THE PACIFIC NORTHWEST INTO CALIFORNIA, WHICH WILL  
INCREASE THE SOIL SATURATION AND SENSITIVITY TO LOCAL RUNOFF.  
HOWEVER DURING THE EXTENDED TIME FRAME UNCERTAINTY ON TIMING AND  
INTERACTIONS OF SHORTWAVES WILL TEMPER CONFIDENCE IN RESOLVING THE  
EXACT LOCATION AND INTENSITY OF LOWER ELEVATION RAINFALL WITH  
RESPECT TO LOCAL RUNOFF/FLOODING ISSUES. HIGHEST  
PRECIPITATION/LIQUID EQUIVALENTS SHOULD EXTEND FROM THE OREGON  
CASCADES INTO NORTHWEST CALIFORNIA AND THE SIERRA NEVADA.  
MEANINGFUL PRECIPITATION WILL ALSO BE LIKELY ACROSS THE HIGHEST  
TERRAIN OF THE GREAT BASIN AND THE CENTRAL/NORTHERN ROCKIES.  
AREAS FROM SOUTHERN CALIFORNIA THROUGH ARIZONA AND THE SOUTHERN  
GREAT BASIN/SOUTH-CENTRAL ROCKIES MAY ALSO SEE A PERIOD OF  
ENHANCED PRECIPITATION LATE THIS WEEK AND/OR INTO THE WEEKEND BUT  
SPECIFICS ARE VERY SENSITIVE TO UPPER FLOW DETAILS FOR WHICH  
MODELS DO NOT YET AGREE. COLDER AIR FILTERING INTO THE NORTHWEST  
MAY LEAD TO VERY LOW SNOW LEVELS BY NEXT SUNDAY-TUESDAY WHILE  
OTHER PARTS OF THE WEST MAY ALSO SEE DECLINING SNOW LEVELS AS THE  
UPPER TROUGH AXIS NUDGES EASTWARD.  
 
AN AREA OF PRECIPITATION WILL ACCOMPANY A SYSTEM CROSSING THE  
NORTHEASTERN QUADRANT OF THE LOWER 48 FRIDAY INTO CHRISTMAS  
DAY/SATURDAY. THE BEST POTENTIAL FOR WINTRY WEATHER WILL EXTEND  
FROM THE UPPER GREAT LAKES INTO NEW ENGLAND. ANOTHER AREA OF  
PRECIPITATION MAY SPREAD ACROSS NORTHERN TIER AREAS AND PARTS OF  
THE EAST SUNDAY ONWARD. DETAILS OF THIS PRECIP AREA ARE STILL  
AMBIGUOUS THOUGH ONCE AGAIN ANY WINTRY WEATHER SHOULD BE CONFINED  
TO NORTHERN LATITUDES.  
 
THE SOUTHERN HALF OF THE PLAINS WILL BE AT THE CENTER OF THE  
GREATEST WARM TEMPERATURE ANOMALIES DURING THE PERIOD WITH DECENT  
COVERAGE OF MAX/MIN READINGS 20-30F ABOVE NORMAL. SUCH  
TEMPERATURES MAY CHALLENGE DAILY RECORDS. SOME OF THIS WARMTH  
WILL EXTEND FARTHER NORTHEAST/EAST AS DETERMINED BY SYSTEM/FRONT  
PROGRESSION AND POSITION. ANOMALIES SHOULD BECOME SOMEWHAT MORE  
SUPPRESSED AND LESS EXTREME BY NEXT TUESDAY. MEANWHILE  
TEMPERATURES OF 20-40F BELOW NORMAL WILL SETTLE OVER THE EXTREME  
NORTHERN PLAINS (PRIMARILY MONTANA AND NORTH DAKOTA) FROM THE  
WEEKEND ONWARD. SOME GRADUAL BROADENING OF THE WESTERN UPPER  
TROUGH SHOULD BRING A GRADUAL COOLING TREND TO THE REGION WITH  
SOME HIGHS 10F OR MORE BELOW NORMAL.  
 
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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