382  
FXUS02 KWBC 180700  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
159 AM EST SAT DEC 18 2021  
 
VALID 12Z TUE DEC 21 2021 - 12Z SAT DEC 25 2021  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
FROM TUESDAY INTO CHRISTMAS DAY/SATURDAY, MOST GUIDANCE CONTINUES  
TO SHOW THAT IMPULSES ROUNDING AN IMPRESSIVELY STRONG CENTRAL  
PACIFIC UPPER RIDGE SHOULD PROVIDE PERIODIC REINFORCEMENT OF A  
DEEP UPPER TROUGH ALIGNED JUST OFF THE WEST COAST. THIS TROUGH  
WILL LIKELY CONTAIN ONE OR MORE UPPER LOWS, AND A PERSISTENT FLOW  
OF MOISTURE AHEAD OF THE TROUGH AXIS WILL LIKELY PRODUCE A  
MULTI-DAY PERIOD OF SIGNIFICANT PRECIPITATION OVER PARTS OF THE  
WEST COAST (NORTHERN/CENTRAL CALIFORNIA IN PARTICULAR) WITH  
MEANINGFUL AMOUNTS ALSO EXTENDING ACROSS THE WEST INTO THE  
ROCKIES. SOUTHERN CANADA/NORTHERN TIER U.S. FLOW WILL CARRY ALONG  
A COUPLE WAVES WITH MODEST PRECIPITATION BEFORE LOWER 48 FLOW  
BECOMES MORE DOMINATED BY WHAT ENERGY MAY EJECT FROM THE EASTERN  
PACIFIC TROUGH. FARTHER SOUTH, A SOUTHERN STREAM SYSTEM TRACKING  
FROM NORTHERN FLORIDA OR VICINITY INTO THE ATLANTIC MAY BRING SOME  
RAIN TO THE SOUTHEAST EARLY IN THE FORECAST PERIOD. EXPECT A  
BROADENING AREA OF ABOVE NORMAL TEMPERATURES OVER THE SOUTHERN  
TWO-THIRDS OF THE CENTRAL U.S. WHILE THE EXTREME NORTHERN PLAINS  
WILL TEND TO SEE BELOW NORMAL TEMPERATURES.  
 
FOR THE EASTERN PACIFIC UPPER TROUGH THE 12Z/18Z MODELS AND MEANS  
HAVE IMPROVED THEIR RELATIVE CLUSTERING THANKS TO THE 12Z ECMWF  
FINALLY ADJUSTING TO A MORE COMPATIBLE SOLUTION VERSUS THE  
PREVIOUS COUPLE RUNS THAT WERE QUESTIONABLY PROGRESSIVE WITH GULF  
OF ALASKA ENERGY THAT DIGS SOUTHWARD TUESDAY-WEDNESDAY. THE  
CURRENT CONSENSUS FOR THE MEAN TROUGH ALOFT, AS WELL AS THE BEST  
PRECIPITATION FOCUS OVER NORTHERN/CENTRAL CALIFORNIA, COMPARE WELL  
TO TELECONNECTIONS RELATIVE TO THE UPSTREAM RIDGE'S POSITIVE  
HEIGHT ANOMALY CENTER. HOWEVER FOR SPECIFIC DAY-TO-DAY DETAILS  
THERE IS STILL A FAIR AMOUNT OF ENSEMBLE SPREAD AND COMPLEXITY  
OVER INTERACTION AMONG FEATURES AND THEIR EXACT POSITION. THIS  
INCLUDES NOT ONLY THE INITIAL OFFSHORE UPPER LOW AND THE  
AFOREMENTIONED GULF OF ALASKA ENERGY BUT YET ANOTHER STRONG BUNDLE  
OF ENERGY THAT MAY DROP INTO THE MEAN TROUGH LATE IN THE WEEK. IN  
THE NEW 00Z MODEL RUNS THE CMC HAS STRAYED SIGNIFICANTLY FOR SOME  
DETAILS BUT THE GFS/UKMET REMAIN FAIRLY CLOSE TO CONSENSUS. THE  
ECMWF HAS GONE BACK TO A FASTER SCENARIO.  
 
THE PROGRESSIVE NATURE OF FLOW TO THE EAST OF THE ROCKIES MAY  
CONTINUE TO CAUSE SOME TROUBLE FOR RESOLVING SPECIFICS OF  
INDIVIDUAL WAVES/FRONTS. RECENTLY THERE HAS BEEN IMPROVED  
DEFINITION OF A NORTHERN PLAINS WAVE THAT TRACKS ACROSS THE UPPER  
GREAT LAKES AND SOUTHEASTERN CANADA TUESDAY ONWARD BUT  
CONTINUITY/CLUSTERING HAVE BEEN LACKING FOR TRAILING FEATURES  
WHOSE DYNAMIC SUPPORT ARE INCREASINGLY LIKELY TO COME FROM ENERGY  
THAT EJECTS FROM THE EASTERN PACIFIC UPPER TROUGH. MEANWHILE  
AGREEMENT AND TO SOME DEGREE CONTINUITY CONTINUE TO BE POOR FOR  
THE PRECISE STRUCTURE, TIMING, AND TRACK OF THE SOUTHERN STREAM  
SYSTEM EXPECTED TO BE OVER OR NEAR NORTHERN FLORIDA AS OF EARLY  
TUESDAY AND CONTINUE INTO THE ATLANTIC THEREAFTER. NEW 00Z MODEL  
CLUSTERING HAS GENERALLY GRAVITATED TO THE SLOWER SIDE OF THE  
PRIOR ENVELOPE, CLOSEST TO RECENT GFS RUNS.  
 
A BLEND OF 12Z/18Z MODELS FROM TUESDAY INTO EARLY WEDNESDAY  
ELIMINATED THE UKMET AFTER THE LATTER TIME DUE TO STRAYING FAST  
WITH THE WESTERN ATLANTIC SYSTEM AND GRADUALLY TRENDED TOWARD A  
LATE-PERIOD SOLUTION OF 40 PERCENT TOTAL 18Z GEFS/12Z ECMWF MEANS  
WITH REMAINING INPUT COMING FROM THE 18Z GFS AND 12Z ECMWF/CMC.  
 
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
EXPECT THE WESTERN U.S. TO SEE A BROADENING AREA OF RAIN AND  
MOUNTAIN SNOW DOWNSTREAM FROM THE PERSISTENT UPPER TROUGH JUST  
OFFSHORE THE WEST COAST, WITH SOME OF THIS ACTIVITY LIKELY TO BE  
MODERATE TO HEAVY. THERE ARE STILL QUESTION MARKS FOR SOME OF THE  
DETAILS DUE TO UNCERTAINTY OVER INDIVIDUAL SYSTEMS AND THEIR  
INTERACTION, BUT GUIDANCE SIGNALS AND TELECONNECTIONS FOR THE MEAN  
PATTERN ARE CONSISTENT IN SHOWING NORTHERN AND CENTRAL CALIFORNIA  
AS HAVING THE HIGHEST PRECIPITATION POTENTIAL THROUGH THE  
PERIOD--INCLUDING HEAVY SNOW OVER SIERRA NEVADA/SHASTA AREAS.  
THIS REGION SHOULD SEE THE MOST CONSISTENT MOISTURE FLOW AND  
EXPERIENCE SOUTHERN/WESTERN FACING TERRAIN ENHANCEMENT. IT MAY  
TAKE INTO THE SHORTER-RANGE TIME FRAME TO ASSES THE LOCATION AND  
INTENSITY OF LOWER ELEVATION RAINFALL WITH RESPECT TO LOCAL  
RUNOFF/FLOODING ISSUES. EFFECTS FROM THE MULTI-DAY ACCUMULATION  
OF PRECIPITATION WILL ALSO REQUIRE MONITORING EVEN IF IT IS NOT  
INTENSE ON ANY PARTICULAR DAY. SOME MOISTURE MAY EXTEND AS FAR  
SOUTH AS SOUTHERN CALIFORNIA AND ARIZONA BY MID-LATE WEEK, WHILE  
THE INTERIOR WEST AND ROCKIES SHOULD SEE MEANINGFUL PRECIPITATION  
AS WELL.  
 
LOW PRESSURE TRACKING FROM OVER OR NEAR NORTHERN FLORIDA INTO THE  
ATLANTIC WILL SPREAD RAIN ACROSS PORTIONS OF THE SOUTHEAST ON  
TUESDAY AND POSSIBLY INTO WEDNESDAY. SOME LOCATIONS OVER THE  
SOUTHEAST/FLORIDA COULD SEE MODERATE TO LOCALLY HEAVY RAINFALL.  
SOME UNCERTAINTY PERSISTS FOR THE INTENSITY AND NORTHWARD EXTENT  
OF RAINFALL THOUGH LATEST TRENDS ARE INCREASING THE POTENTIAL FOR  
MEANINGFUL TOTALS OVER PARTS OF THE EXTREME SOUTHEAST. A LEADING  
NORTHERN TIER SYSTEM SHOULD SPREAD MOSTLY LIGHT SNOW FROM THE  
EXTREME NORTHERN PLAINS EASTWARD THROUGH THE GREAT LAKES AND NEW  
ENGLAND DURING TUESDAY-WEDNESDAY. THE SYSTEM'S ACCOMPANYING FRONT  
SHOULD BE RELATIVELY DRY. ANY TRAILING FEATURES COULD PRODUCE  
SOME PRECIPITATION BUT AGAIN LIKELY WITH FAIRLY LIGHT AMOUNTS.  
 
 
ABOVE NORMAL TEMPERATURES FOCUSED OVER THE CENTRAL/SOUTH-CENTRAL  
HIGH PLAINS ON TUESDAY WILL STEADILY EXPAND EASTWARD ACROSS THE  
SOUTHERN TWO-THIRDS OF THE CENTRAL U.S. WITH TIME AND GRADUALLY  
BECOME MORE ANOMALOUS. THROUGH MIDWEEK HIGHS IN THIS REGION WILL  
TEND TO BE 10-20F OR SLIGHTLY MORE ABOVE NORMAL. TOWARD THE END  
OF THE WEEK THERE SHOULD BE MORE LOCATIONS WITH HIGHS 20-30F ABOVE  
NORMAL AND SOME RECORD HIGHS COULD BE CHALLENGED OVER THE SOUTHERN  
PLAINS BY CHRISTMAS DAY/SATURDAY. ON THE OTHER EXTREME, THE FAR  
NORTHERN PLAINS SHOULD SEE BELOW NORMAL TEMPERATURES EXCEPT FOR  
PERHAPS BEING NEAR NORMAL WEDNESDAY. A STRONGER PUSH OF COLD AIR  
MAY REACH MONTANA AND THE DAKOTAS NEXT WEEKEND. THE UNSETTLED  
PATTERN OVER THE WEST SHOULD LEAD TO NEAR OR ABOVE NORMAL LOWS AND  
NEAR TO BELOW NORMAL HIGHS.  
 
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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