184  
FXUS02 KWBC 140657  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
155 AM EST FRI DEC 14 2018  
 
VALID 12Z MON DEC 17 2018 - 12Z FRI DEC 21 2018  
   
..SIGNIFICANT PRECIPITATION LIKELY OVER THE NORTHWEST
 
 
   
..OVERVIEW
 
 
OVERALL EXPECT FLOW ACROSS THE LOWER 48 TO BE FAIRLY PROGRESSIVE  
BUT IT WILL BE AMPLIFIED OVER SOME AREAS FOR AT LEAST A PORTION OF  
NEXT WEEK. DURING THE EARLY-MID WEEK TIME FRAME STRONG DYNAMICS  
DIGGING ACROSS NEW ENGLAND WILL SERVE TO PULL LOW PRESSURE  
INITIALLY OFF THE NEW ENGLAND COAST INTO THE CANADIAN MARITIMES.  
MEANWHILE SHORTWAVE ENERGY ENTERING THE WEST EARLY MON WILL  
QUICKLY SEPARATE WITH THE NORTHERN PART CONTINUING ALONG THE  
CANADIAN BORDER AS THE REST AMPLIFIES INTO THE SOUTHWEST/SOUTHERN  
ROCKIES-PLAINS AND NORTHERN MEXICO DUE TO A RIDGE BUILDING  
UPSTREAM. A STRONG JET AIMED AT THE NORTHWEST U.S./EXTREME  
SOUTHWESTERN CANADA WILL PROMOTE A WET PATTERN OVER THAT REGION.  
BY NEXT THU-FRI THERE IS INCREASING UNCERTAINTY OVER DETAILS OF  
FLOW ALOFT FROM THE EASTERN PACIFIC EASTWARD--RAPIDLY LOWERING  
CONFIDENCE IN SOME ASPECTS OF THE SENSIBLE WEATHER FORECAST.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
FOR THE DAYS 3-5 MON-WED PERIOD THE FORECAST BLEND WAS ABLE TO  
EMPLOY OPERATIONAL MODEL RUNS FROM THE 12Z/18Z CYCLES TO REPRESENT  
THE MOST COMMON IDEAS OF GUIDANCE AND YIELD ONLY MODEST DETAIL  
ADJUSTMENTS FROM CONTINUITY. BY WED THERE IS SOME SPREAD THAT  
ARISES WITH SYSTEMS ALONG/NORTH OF THE CANADIAN BORDER, IN  
PARTICULAR THE NEW 00Z UKMET BECOMING QUITE FAST WITH THE LEADING  
FEATURE CROSSING ONTARIO AND NEW 00Z GFS STRAYING A BIT FASTER  
THAN CONSENSUS WITH THE TRAILING SYSTEM. THERE IS ALSO SOME  
SPREAD WITH FRONTAL WAVINESS OFF THE WEST COAST.  
 
BY THU-FRI SPAGHETTI PLOTS OF THE FULL RANGE OF MODEL/ENSEMBLE  
SOLUTIONS DESCEND INTO CHAOS WITH WIDE DIFFERENCES IN HOW EASTERN  
PACIFIC TROUGH ENERGY MAY SPLIT AND HOW QUICKLY/STRONGLY THE  
NORTHERN PART OF THIS ENERGY MAY PROGRESS AROUND THE PERIPHERY OF  
THE UPPER RIDGE THAT BUILDS OVER THE WEST BY MIDWEEK. THESE  
ISSUES ALSO LEAD TO A WIDENING ARRAY OF POSSIBLE SOLUTIONS FOR THE  
CHARACTER OF UPPER TROUGHING EXPECTED TO AMPLIFY INTO THE EASTERN  
STATES AS WELL AS HOW MUCH INTERACTION THERE MAY BE WITH THE  
TROUGH FORECAST TO BE OVER THE SOUTHERN PLAINS INTO MEXICO AS OF  
WED. VERY LOOSELY THERE ARE TWO CLUSTERS, GFS/GEFS RUNS THAT WANT  
TO HOLD BACK MOST OF THE PACIFIC ENERGY INTO DAY 7 FRI VERSUS THE  
ECMWF/CMC AND THEIR MEANS THAT ARE QUICKER/STRONGER TO VARYING  
DEGREES. THE ECMWF/ECMWF MEAN ARE NOTICEABLY STRONGER WITH THEIR  
SHORTWAVE ENERGY THOUGH, AND INTERESTINGLY THE 18Z FV3 GFS IS IN  
THIS CLUSTER TOO. THE FULL ARRAY OF GUIDANCE AND ESTABLISHED  
PROGRESSION OF THE PATTERN SUPPORT THE IDEA OF SOME SHORTWAVE  
ENERGY REACHING WELL INLAND BY DAY 7 FRI BUT THE EXPECTED STRENGTH  
OF THE WESTERN RIDGE AS OF DAY 6 THU WOULD FAVOR LEANING AWAY FROM  
THE MOST AMPLIFIED ECMWF/ECMWF MEAN. A MODEL/MEAN BLEND THAT  
EMPHASIZES THE ENSEMBLE MEANS BY LATE IN THE PERIOD YIELDS A  
RATHER FLAT SHORTWAVE WHICH SEEMS TO BE A REASONABLE STARTING  
POINT. OVER THE EAST THE BLEND SUGGESTS MORE NORTHERN STREAM  
DOMINANCE WITH EVENTUAL SEPARATION FROM ENERGY THAT DESCENDS INTO  
MEXICO.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
THE MOST CONFIDENT FOCUS FOR HEAVY PRECIPITATION DURING THE PERIOD  
WILL BE OVER THE PACIFIC NORTHWEST AND MAY AT TIMES EXTEND INTO  
EXTREME NORTHERN CALIFORNIA. THE AMPLITUDE OF UPSTREAM PACIFIC  
TROUGHING ALOFT MAY PROMOTE INCORPORATION OF SOME LOWER LATITUDE  
MOISTURE TO ENHANCE RAINFALL AND HIGHER ELEVATION SNOW. PERIODS  
OF STRONG WINDS ARE POSSIBLE AS WELL. IN LESS EXTREME FASHION  
SOME OF THIS MOISTURE WILL REACH INTO THE NORTHERN ROCKIES.  
FARTHER SOUTH, SEPARATING SHORTWAVE ENERGY ALOFT MAY BRING  
RELATIVELY LIGHT PRECIPITATION FROM CALIFORNIA AND THE GREAT BASIN  
TO THE SOUTHERN ROCKIES EARLY IN THE WEEK. THEN CONFIDENCE IS  
BELOW AVERAGE REGARDING COVERAGE/INTENSITY OF RAINFALL THAT MAY  
SPREAD EASTWARD FROM TEXAS WED ONWARD.  
 
EARLY NEXT WEEK STRONG DYNAMICS WILL INTENSIFY A WESTERN ATLANTIC  
STORM THAT TRACKS INTO THE CANADIAN MARITIMES, LEADING TO A PERIOD  
OF FAIRLY STRONG WINDS OVER THE NORTHEAST. SOME LAKE EFFECT SNOW  
IS ALSO POSSIBLE. LOW PRESSURE CURRENTLY EXPECTED TO TRACK INTO  
THE GREAT LAKES AND SOUTHEAST CANADA/NEW ENGLAND (ALONG WITH ITS  
ASSOCIATED FRONTS) LATE IN THE WEEK SHOULD BRING AT LEAST  
LIGHT-MODERATE PRECIPITATION TO PARTS OF THE EAST WITH ANOTHER  
EPISODE OF LAKE EFFECT SNOW IN ITS WAKE. THE DEGREE OF  
INTERACTION WITH GULF MOISTURE IS QUITE UNCERTAIN.  
 
STRONG DYNAMICS ALOFT/LEADING COLD FRONT WILL BRING A BRIEF PERIOD  
OF BELOW NORMAL TEMPERATURES TO THE NORTHEAST, ESPECIALLY AROUND  
TUE. OTHERWISE MUCH OF THE LOWER 48 WILL SEE NEAR TO ABOVE NORMAL  
READINGS MOST DAYS WITH THE PLAINS LIKELY TO SEE THE MOST EXTREME  
WARM ANOMALIES, PLUS 15-25F OVER SOME LOCATIONS--ESPECIALLY OVER  
NORTHERN AREAS FOR MORNING LOWS.  
 
RAUSCH  
 
WPC MEDIUM RANGE 500 MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, WINTER WEATHER OUTLOOK PROBABILITIES  
AND HEAT INDEXES ARE FOUND 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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