255  
FXUS02 KWBC 151801  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
100 PM EST WED DEC 15 2021  
 
VALID 12Z SAT DEC 18 2021 - 12Z WED DEC 22 2021  
 
18Z UPDATE: THE 00/06Z DETERMINISTIC GUIDANCE AND ENSEMBLES ARE  
IN VERY GOOD SYNOPTIC SCALE AGREEMENT ON SATURDAY ACROSS THE  
CONTINENTAL U.S., ALTHOUGH THE 00/12Z UKMET DIVERGES FROM THE  
MODEL CONSENSUS BY SUNDAY ACROSS SOUTHERN CANADA WITH A MUCH  
FASTER SHORTWAVE WHICH BECOMES OUT OF PHASE BY MONDAY ACROSS THE  
NORTHERN GREAT LAKES AND THE NORTHEAST. THIS LEADS TO A SURFACE  
LOW WELL TO THE EAST OF THE OTHER GUIDANCE, AND THUS NOT FAVORED.  
BESIDES THAT, MODEL AGREEMENT REMAINS DECENT THROUGH TUESDAY  
BEFORE MORE NOTEWORTHY DIFFERENCES BECOME APPARENT ACROSS THE  
NORTHEAST PACIFIC, WITH THE 06/12Z GFS MUCH STRONGER WITH AN UPPER  
LOW DROPPING SOUTH FROM THE GULF OF ALASKA BY NEXT WEDNESDAY. ONE  
OF THE MAIN CHANGES FROM THE OVERNIGHT FORECAST ISSUANCE IS HIGHER  
QPF TOTALS ACROSS CENTRAL/NORTHERN CALIFORNIA FOR THE STORM SYSTEM  
EARLY NEXT WEEK, WITH THE POTENTIAL FOR AN ATMOSPHERIC RIVER  
EVENT. THE WPC FRONTS AND PRESSURES FORECAST WAS PRIMARILY  
DERIVED WITH A GFS/ECMWF/CMC BLEND THROUGH MONDAY, FOLLOWED BY  
INCREASED USE OF THE GEFS/ECENS MEANS FOR TUESDAY AND WEDNESDAY.  
THE PREVIOUS DISCUSSION IS APPENDED BELOW FOR REFERENCE. /HAMRICK  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
MODELS AND ENSEMBLES REMAIN IN GOOD AGREEMENT THIS WEEKEND INTO  
MONDAY IN A PATTERN WITH ABOVE AVERAGE FORECAST CONFIDENCE AND  
CONTINUITY. FORECAST SPREAD AND CONTINUITY ISSUES BECOME  
INCREASINGLY EVIDENT WITH THE TIMING OF EMBEDDED WEATHER FEATURES  
AND STREAM INTERACTIONS NEXT WEEK. THE WPC MEDIUM RANGE PRODUCT  
SUITE WAS PRIMARILY COMPRISED OF A BLEND OF BEST CLUSTERED  
SOLUTIONS FROM THE 18 UTC GFS/GEFS MEAN AND THE 12 UTC ECWMF/ECMWF  
ENSEMBLE MEAN. BLEND WEIGHTING FAVORS THE MODELS THIS WEEKEND  
BEFORE TRANSITION TO MAINLY THE COMPATIBLE ENSEMBLE MEANS NEXT  
WEEK AMID GROWING SYSTEM UNCERTAINTY AND RUN TO RUN VARIANCES.  
WPC PREFERRED GUIDANCE AND LATEST 00 UTC RUNS OVERALL HAVE  
SUPPRESSED SOUTHWARD TO OFF THE SOUTHEAST A THREAT FOR COASTAL LOW  
GENESIS EARLY-MID NEXT WEEK.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
A DEVELOPING FRONTAL LOW HAS THE POTENTIAL TO BRING A SWATH OF  
PLOW-ABLE SNOW AS ENHANCED BY TERRAIN FROM THE UPPER OH VALLEY  
THROUGH ESPECIALLY THE INTERIOR NORTHEAST THIS WEEKEND. AN  
ENHANCED RAINFALL FOCUS WITH THE TRAILING FRONT WILL MEANWHILE  
SINK ACROSS THE GULF COAST AND SOUTHEAST AS POST-FRONTAL COOLING  
HIGH PRESSURE BUILDS ACROSS THE PLAINS AND TO THE EAST. GUIDANCE  
HAS BEEN INCONSISTENT WITH THE PROSPECT OF SUBSEQUENT FRONTAL WAVE  
DEVELOPMENTS AND COASTAL LOW GENESIS EARLY NEXT WEEK UP THE EAST  
COAST IN PART FROM NORTHERN STREAM ENERGIES AND/OR WITH LATEST  
GUIDANCE TRENDS NOW FAVORING EJECTION OF A SEPARATED SOUTHERN  
STREAM TROUGH THAT LEADS TO SOUTHWARD SUPPRESSED WET LOWS FROM THE  
GULF TO OFFSHORE THE SOUTHEAST.  
 
WELL UPSTREAM, DYNAMIC BUT PROGRESSIVE UPPER TROUGH AND SURFACE  
SYSTEM PASSAGE WILL BUILD MOISTURE TO SUPPORT MODERATE TO TERRAIN  
ENHANCED HEAVY PRECIPITATION OVER THE NORTHWEST THIS WEEKEND.  
AMPLIFIED UPPER TROUGH DIGGING OVER THE EASTERN PACIFIC EARLY-MID  
NEXT WEEK IN THE WAKE OF THIS SYSTEM OFFERS POTENTIAL TO FOCUS  
INGREDIENTS FOR MODERATE TO HEAVY PRECIPITATION INTO NORTH-CENTRAL  
CA WITH A SLOW SYSTEM APPROACHING.  
 
SCHICHTEL/HAMRICK  
 
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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