065  
FXUS02 KWBC 141553  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1052 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 THE 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 INTO SOUTHERN CALIFORNIA. A STRONG JET AIMED AT THE  
NORTHWEST U.S./EXTREME SOUTHWESTERN CANADA WILL PROMOTE A  
CONTINUED 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 DETERMINISTIC MODEL RUNS FROM THE 00Z/06Z CYCLES TO  
REPRESENT THE MOST COMMON IDEAS OF GUIDANCE AND YIELD ONLY MODEST  
DETAIL ADJUSTMENTS FROM CONTINUITY. BY WED SOME SPREAD ARISES WITH  
SYSTEMS ALONG/NORTH OF THE CANADIAN BORDER (ESPECIALLY WITH THE  
UKMET) AND OFF THE WEST COAST BUT A CONSENSUS/BLEND OFFERED A GOOD  
STARTING POINT. THIS MODULATED WELL THE DIFFERENCES BETWEEN THE  
INITIALLY QUICKER GFS RUNS WITH THE THEN QUICKER ECMWF/CANADIAN  
RUNS.  
 
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 (AROUND 145W) 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 FULL ARRAY OF GUIDANCE AND ESTABLISHED  
PROGRESSION OF THE PATTERN SUPPORT THE IDEA OF SOME SHORTWAVE  
ENERGY REACHING WELL INLAND BY DAY 7 FRI INTO THE NORTHWEST BUT  
THE EXPECTED STRENGTH OF THE WESTERN RIDGE (AT LEAST S CALIFORNIA  
INTO THE SOUTHWEST) AS OF THU WOULD FAVOR LEANING AWAY FROM THE  
FLATTER ECMWF MEAN (SUBJECT TO AVERAGING 50 FORECASTS OF THE  
TROUGH/RIDGE COUPLET OVER VARIOUS LONGITUDES YIELDING AN UNLIKELY  
ZONAL FLOW). UTILIZED A MODEL/MEAN BLEND BY NEXT THU/FRI THAT  
MAINTAINED AT LEAST THE SOUTHERN PORTION OF THE WESTERN RIDGE (VIA  
THE 06Z GEFS MEAN AND PREVIOUS FORECAST) AND ALLOWED SOME NORTHERN  
STREAM HEIGHT FALLS ACROSS THE PAC NW, WHICH SEEMED TO BE A  
REASONABLE STARTING POINT. OVER THE EAST THE BLEND SUGGESTED MORE  
NORTHERN STREAM DOMINANCE WITH EVENTUAL SEPARATION FROM ENERGY  
THAT DESCENDS INTO MEXICO. THIS WOULD TAKE THE SURFACE LOW OUT OF  
THE NORTHERN GULF THURSDAY AND UP THROUGH THE MID-ATLANTIC FRIDAY  
AS A NORTHERN STREAM FRONT BARRELS INTO NEW ENGLAND FROM THE GREAT  
LAKES.  
 
   
..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 WITH SIGNIFICANT  
UPSLOPE-ENHANCED PRECIPITATION IN FAVORED AREAS. 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 BRIEF  
LAKE-ENHANCED SNOW IS ALSO POSSIBLE WITH/BEHIND THE FRONTAL  
PASSAGE MONDAY. 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 A MORE  
MODEST EPISODE OF LAKE EFFECT SNOW IN ITS WAKE. THE DEGREE OF  
INTERACTION WITH GULF MOISTURE IS QUITE UNCERTAIN BUT THE  
POTENTIAL EXISTS FOR THE INFUSION OF GULF/ATLANTIC MOISTURE UP THE  
COAST LATE NEXT WEEK INTO/ALONG THE EAST COAST.  
 
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. DUE TO RELATIVELY COLD AVERAGE  
TEMPERATURES FOR MID/LATE DECEMBER, ACTUAL MINS ARE FORECAST TO BE  
IN THE 20S (AVERAGE IN THE SINGLE DIGITS) WITH MAXES IN THE  
30S/40S.  
 
FRACASSO/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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