136  
FXUS02 KWBC 131848  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
147 PM EST MON DEC 13 2021  
 
VALID 12Z THU DEC 16 2021 - 12Z MON DEC 20 2021  
   
..OVERVIEW
 
 
A FAIRLY PROGRESSIVE WEATHER PATTERN IS EXPECTED IN THE MEDIUM  
RANGE PERIOD ESPECIALLY IN THE NORTHERN STREAM, AS AN INITIAL DEEP  
SURFACE LOW LIFTS QUICKLY NORTHWARD INTO EASTERN CANADA, THOUGH  
ITS TRAILING FRONT COULD STALL ACROSS PARTS OF THE CENTRAL AND  
EASTERN U.S. AND PRODUCE HEAVY RAINFALL LATE THIS WEEK. A SERIES  
OF TROUGHS WILL MAKE THEIR WAY ACROSS THE NORTHERN TIER OF THE  
U.S. DURING THE PERIOD, WHILE A SOUTHERN STREAM TROUGH MAY  
SEPARATE FROM THE NORTHERN PREVAILING FLOW AND MOVE SLOWLY ACROSS  
THE SOUTHWEST/NORTHERN MEXICO THIS WEEKEND. RIDGING OVER THE  
SOUTHEAST QUADRANT OF THE COUNTRY SHOULD GET SUPPRESSED EARLY NEXT  
WEEK AND BRING TEMPERATURES CLOSER TO NORMAL AFTER A MILD/WARM  
SPELL.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
DETERMINISTIC AND ENSEMBLE MODELS FOR THE 00/06Z CYCLE REMAIN  
FAIRLY WELL CLUSTERED WITH THE OVERALL PATTERN DESCRIBED ABOVE.  
EARLY IN THE PERIOD, GFS RUNS WERE SLIGHTLY WEST OF CONSENSUS  
(INCLUDING THE 00Z ECMWF/UKMET/CMC) WITH THEIR POSITIONS OF THE  
SURFACE LOW NORTH OF THE GREAT LAKES EARLY THURSDAY, SO THOSE WERE  
NOT WEIGHTED AS HEAVILY. BUT OVERALL A BLEND OF THE DETERMINISTIC  
GUIDANCE WORKED WELL FOR THE WPC FORECAST, WITH EMPHASIS  
ESPECIALLY ON THE 00Z ECMWF. BY THE WEEKEND, THERE ARE SOME  
INITIALLY MINOR DIFFERENCES WITH THE AFOREMENTIONED POSSIBILITY OF  
STREAM SEPARATION IN TERMS OF TIMING AND WHETHER AN EMBEDDED  
CLOSED LOW MAY FORM, AND WITH STRENGTH OF ENERGY WITHIN NORTHERN  
STREAM SHORTWAVE TROUGHS AS WELL. NO PARTICULAR TREND WAS SEEN  
WITH THE SOUTHERN STREAM TROUGH AS MODELS ARE WAFFLING WITH THE  
DETAILS. IN THE NORTHERN STREAM, THE 00Z CMC WAS NOTABLY STRONGER  
THAN CONSENSUS WITH ANOTHER SHORTWAVE TRACKING THROUGH THE  
MIDWEST/GREAT LAKES BY DAY 5/SATURDAY, SO WEIGHTED IT LESS BY THAT  
POINT. FARTHER UPSTREAM, ANOTHER ENERGETIC SHORTWAVE LOOKS TO DIVE  
INTO THE PACIFIC NORTHWEST OVER THE WEEKEND, BUT QUESTIONS REMAIN  
WITH ITS STRENGTH AND HOW FAR SOUTH IT MAY DIVE AND INTERACT WITH  
THE SOUTHERN STREAM FEATURE BY MONDAY. THUS BY DAY 6/7, THE WPC  
PRESSURES/FRONTS FORECAST USED ABOUT HALF ENSEMBLE MEAN GUIDANCE  
(FROM THE 06Z GEFS AND 00Z EC) TO TEMPER THE INDIVIDUAL MODEL  
DIFFERENCES. THIS WAS IN GOOD AGREEMENT WITH THE PREVIOUS WPC  
FORECAST PRODUCTS.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
A POTENT UPPER TROUGH ALONG WITH A STRONG SURFACE LOW WILL LIFT  
QUICKLY NORTHEASTWARD THROUGH THE NORTH-CENTRAL U.S. WED INTO THU,  
WITH SOME LINGERING SNOW AND HIGH WINDS POSSIBLE IN THE UPPER  
GREAT LAKES REGION THU. SOUTHEAST OF THE DEEP LOW TRACK, UPPER  
RIDGING SHOULD SUPPORT SOME RECORD WARM TEMPERATURES AHEAD OF THE  
COLD FRONT, WITH POSITIVE ANOMALIES UPWARDS TO 15-25F SHIFTING  
FROM THE EAST-CENTRAL U.S. TO THE EAST THU-SAT. THE TRAILING  
PORTION OF THE FRONT WILL MEANWHILE STALL ACROSS THE SOUTH, WITH  
AN AXIS OF ENHANCED RAINFALL AND POTENTIAL FOR LOCAL RUNOFF AND  
FLOODING/FLASH FLOODING ISSUES FROM THE SOUTHERN PLAINS THROUGH  
THE MID-SOUTH/OHIO VALLEY WITH RETURN FLOW FROM THE GULF OF  
MEXICO. A DEVELOPING FRONTAL WAVE MAY ALSO BRING A SWATH OF WINTRY  
PRECIPITATION FROM THE GREAT LAKES THROUGH NORTHERN NEW ENGLAND  
INTO THE WEEKEND AS THE ENHANCED RAINFALL AXIS DROPS SOUTHWARD  
ACROSS THE GULF COAST AND SOUTHEAST AS COOLING HIGH PRESSURE  
BUILDS ACROSS THE PLAINS AND TO THE EAST.  
 
UPSTREAM, A MOISTURE SURGE WITH MODERATE PRECIPITATION WILL WORK  
INTO CALIFORNIA AND ACROSS THE NORTHWEST THU AHEAD OF AN  
APPROACHING PACIFIC SYSTEM. THIS SYSTEM WILL GRADUALLY WEAKEN INTO  
FRIDAY. MOISTURE FROM ANOTHER PACIFIC SYSTEM MAY THEN IMPACT THE  
PACIFIC NORTHWEST WITH MODERATE TO POSSIBLY LOCALLY HEAVY  
PRECIPITATION AMOUNTS FRI INTO THE WEEKEND.  
 
TATE/SCHICHTEL  
 
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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