783  
FXUS02 KWBC 171827  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
126 PM EST FRI DEC 17 2021  
 
VALID 12Z MON DEC 20 2021 - 12Z FRI DEC 24 2021  
 
 
1830 UTC UPDATE...  
 
NO MAJOR CHANGES WERE NEEDED FOR TODAYS UPDATE TO THE WPC MEDIUM  
RANGE PROGS. THE 00Z/06Z SUITE OF MODELS REMAINED FAIRLY  
CONSISTENT TO THEIR PREVIOUS RUNS AND SO ONLY MINOR CHANGES WERE  
MADE TO FRONTAL STRUCTURES AND PLACEMENTS FROM THE OVERNIGHT  
SHIFT. THE FORECAST WAS GOOD ENOUGH FOR AN OPERATIONAL MODEL BLEND  
THROUGH ABOUT DAY 5, QUICKLY INCREASING INCORPORATION OF THE MEANS  
THEREAFTER TO ACCOUNT FOR FORECAST VARIABILITY AND INSTABILITY,  
PARTICULARLY REGARDING DETAILS AND EVOLUTION OF WESTERN U.S.  
TROUGH ENERGIES. THE 00Z ECMWF WAS STILL CONSIDERABLY FASTER ON  
DAY 7, WITH A TROUGH MAKING IT ALL THE WAY INTO THE PLAINS, WHILE  
THE BETTER CONSENSUS FAVORED SOMETHING MORE BACK TOWARDS THE WEST  
COAST. FOR THIS REASON, LIMITED ECMWF WAS USED FOR DAYS 6 AND  
ESPECIALLY 7. SEE THE PREVIOUS DISCUSSION BELOW FOR ADDITIONAL  
DETAILS.  
 
SANTORELLI  
 
PREVIOUS DISCUSSION BELOW ISSUED AT 0658 UTC...  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE MAJORITY OF GUIDANCE SUGGESTS THAT AN AMPLIFIED MEAN TROUGH  
ALOFT OVER THE FAR EASTERN PACIFIC WILL BE THE DOMINANT FEATURE  
DURING THE PERIOD, BRINGING MULTIPLE DAYS OF PRECIPITATION TO  
PARTS OF THE WEST COAST STATES WITH LESSER AMOUNTS FARTHER INLAND.  
TO THE EAST A SOUTHERN STREAM SHORTWAVE TRACKING EASTWARD FROM  
THE SOUTHERN PLAINS MONDAY ONWARD WILL SUPPORT A WESTERN GULF INTO  
EASTERN ATLANTIC SYSTEM SPREADING RAIN ACROSS THE GULF COAST AND  
SOUTHEAST. EXPECT MOSTLY LIGHT PRECIPITATION WITH ONE OR MORE  
NORTHERN TIER WAVES/FRONTS CARRIED ALONG BY THE PROGRESSIVE MEAN  
FLOW. THE LARGE SCALE PATTERN WILL LIKELY FAVOR WELL ABOVE NORMAL  
TEMPERATURES OVER PARTS OF THE CENTRAL/SOUTHERN PLAINS BUT BELOW  
NORMAL TEMPERATURES OVER THE EXTREME NORTHERN PLAINS.  
 
FOR ABOUT THE FIRST HALF OF THE PERIOD A COMPOSITE OF 12Z/18Z  
OPERATIONAL MODELS REPRESENTED THE MOST COMMON IDEAS OF GUIDANCE,  
THOUGH LINGERING UNCERTAINTIES EXIST IN MULTIPLE PLACES. LATEST  
SOLUTIONS HAVE ADJUSTED A BIT FASTER THAN PREVIOUS FORECAST FOR  
THE SOUTHERN TIER SYSTEM WHILE THE MODELS ARE STILL IN THE PROCESS  
OF RESOLVING SPECIFICS OF A COUPLE SHORTWAVES WITHIN THE SOUTHERN  
CANADA/NORTHERN TIER U.S. STREAM. THERE IS DECENT AGREEMENT IN  
PRINCIPLE FOR THE UPPER LOW OFF THE WEST COAST, WITH LATEST TRENDS  
ADJUSTING THE FEATURE A LITTLE MORE OFFSHORE CORRESPONDING WITH  
THE UPPER RIDGE OVER THE NORTHWEST/INTERIOR WEST BEING STRONGER.  
 
AROUND MIDWEEK A STRONG BUNDLE OF ENERGY DIVING SOUTH FROM ALASKA  
BEGINS TO ADD TO THE FORECAST COMPLEXITY, WITH RECENT INDIVIDUAL  
MODEL RUNS AND ENSEMBLE MEMBERS SHOWING CONSIDERABLE SPREAD FOR  
WHAT HAPPENS. POSSIBILITIES INCLUDE THIS ENERGY MERGING WITH THE  
INITIAL UPPER LOW, FORMING A NEW UPPER LOW WHILE EJECTING ENERGY  
FROM THE INITIAL LOW EASTWARD, OR ITSELF BECOMING A MERE  
PROGRESSIVE SHORTWAVE WHILE THE FIRST UPPER LOW REMAINS OFFSHORE  
(12Z AND NEW 00Z ECMWF). LATEST ENSEMBLE MEANS HAVE BEEN  
PROVIDING A GOOD MIDDLE GROUND AMONG THESE VARIOUS OPTIONS,  
KEEPING AN UPPER TROUGH AXIS CLOSE TO THE WEST COAST INTO DAY 7  
FRIDAY--A REASONABLE IDEA BASED ON TELECONNECTIONS RELATIVE TO THE  
UPSTREAM RIDGE'S POSITIVE HEIGHT ANOMALY. THUS THE FORECAST  
TRANSITIONED TOWARD ABOUT HALF ENSEMBLE MEANS (18Z GEFS/12Z ECENS)  
WHILE SPLITTING THE LINGERING GFS INPUT BETWEEN ITS 12Z/18Z RUNS  
AND EVENTUALLY SWITCHING ECMWF INPUT TO THE 00Z/16 RUN THAT WAS  
SLOWER WITH THE WEST COAST TROUGH (AND KEEPING A LITTLE 12Z CMC).  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
THE UPPER LOW INITIALLY OFFSHORE THE WEST COAST AND THEN DIGGING  
TROUGH ENERGY FROM ALASKA SHOULD PROVIDE FAVORABLE INGREDIENTS FOR  
PERIODS OF MODERATE TO HEAVY PRECIPITATION OVER PORTIONS OF THE  
WEST COAST STATES WITH LESSER AMOUNTS EXTENDING INLAND THROUGH THE  
GREAT BASIN AND ROCKIES. CURRENTLY NORTHERN AND CENTRAL  
CALIFORNIA HAVE THE BEST POTENTIAL FOR SEEING THE HIGHEST FIVE-DAY  
TOTALS, AIDED BY SLOW MOTION OF THE INITIAL OFFSHORE SYSTEM, A  
DEVELOPING MOISTURE PLUME, AND FAVORED SOUTHERN/WESTERN FACING  
TERRAIN ENHANCEMENT. SOME GUIDANCE SHOWS SUFFICIENTLY HEAVY  
RAINFALL AT LOWER ELEVATIONS THAT THERE COULD BE SOME LOCAL  
RUNOFF/FLOODING ISSUES WHILE SNOW WOULD BE HEAVY OVER SIERRA  
NEVADA/SHASTA AREAS. CONFIDENCE IN THE DETAILS OF COVERAGE AND  
INTENSITY OF PRECIPITATION AFTER MIDWEEK DECREASES THOUGH.  
 
ELSEWHERE, LOW PRESSURE TRACKING FROM THE WESTERN GULF OF MEXICO  
THROUGH FLORIDA AND INTO THE ATLANTIC WILL SPREAD RAIN ACROSS  
PORTIONS OF THE SOUTH/SOUTHEAST. SOME LOCATIONS OVER THE  
SOUTHEAST/FLORIDA COULD SEE MODERATE TO LOCALLY HEAVY RAINFALL BUT  
THERE IS STILL A FAIR AMOUNT OF UNCERTAINTY FOR INTENSITY AT ANY  
PARTICULAR LOCATION AS WELL AS THE NORTHWARD EXTENT OF MOISTURE.  
ONE OR MORE NORTHERN TIER WAVES/FRONTS MAY SPREAD MOSTLY LIGHT  
SNOW FROM THE EXTREME NORTHERN PLAINS THROUGH THE GREAT LAKES AND  
NEW ENGLAND.  
 
UPPER RIDGING OVER THE WEST-CENTRAL U.S. WILL SUPPORT PERSISTENTLY  
WARM TEMPERATURES OVER THE SOUTHERN TWO-THIRDS OF THE PLAINS NEXT  
WEEK. EXPECT THE COVERAGE OF PLUS 10-20F ANOMALIES FOR HIGHS TO  
EXPAND DURING THE FIRST HALF OF THE WEEK AND REMAIN BROAD THROUGH  
FRIDAY. A FEW LOCATIONS MAY SEE HIGHS EVEN GREATER THAN 20F ABOVE  
NORMAL. ON THE OTHER HAND THE EXTREME NORTHERN PLAINS SHOULD SEE  
BELOW NORMAL TEMPERATURES. MONDAY SHOULD BE THE COLDEST DAY WITH  
HIGHS 10-15F OR SO BELOW NORMAL OVER PARTS OF MONTANA AND NORTH  
DAKOTA.  
 
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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