556  
FXUS02 KWBC 291600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1059 AM EST THU NOV 29 2018  
 
VALID 12Z SUN DEC 02 2018 - 12Z THU DEC 06 2018  
   
..OVERVIEW
 
 
MEAN TROUGHING ALOFT EXPECTED TO COVER MOST OF THE WESTERN 2/3 OF  
THE LOWER 48 LATE IN THE WEEKEND WILL GRADUALLY SHIFT EASTWARD AS  
ENERGY WITHIN AN INITIAL EAST-CENTRAL PACIFIC TROUGH UNDERCUTS AN  
EASTERN PACIFIC RIDGE. THE RESULTING SOUTHERN STREAM  
TROUGH/EMBEDDED UPPER LOW SHOULD APPROACH CALIFORNIA BY MIDWEEK  
WHILE THE LEADING RIDGE MOVES INTO THE WEST. MEANWHILE MOST  
GUIDANCE SHOWS THE NORTHERN PART OF THE EASTERN PACIFIC RIDGE  
CLOSING OFF A HIGH FOR A TIME, COMPLICATING THE DOWNSTREAM  
FORECAST TO SOME DEGREE, BUT GOOD CONSENSUS EXISTS TOWARD THE  
ESTABLISHMENT OF CANADIAN FLOW ALOFT EXTENDING INTO THE NORTHERN  
HALF OF THE CENTRAL-EASTERN U.S. BY TUE-THU. DETAILS OF  
INDIVIDUAL SHORTWAVES/ASSOCIATED SURFACE FRONTS WITHIN THIS  
CANADIAN FLOW WILL HAVE LOWER PREDICTABILITY THOUGH.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
AFTER SHOWING CONSIDERABLE DIVERGENCE OVER THE PAST COUPLE DAYS,  
SOLUTIONS HAVE COME INTO MUCH BETTER AGREEMENT FOR THE SYSTEM  
FORECAST TO BE OVER THE MIDWEST/GREAT LAKES REGION AS OF EARLY DAY  
3 SUN. CURRENT CONSENSUS APPEARS TO BE SOMEWHAT CLOSER TO PRIOR  
SLOWER ECMWF/CMC RUNS VERSUS THE FASTER GFS. THERE IS STILL SOME  
COMPLEXITY TO SYSTEM EVOLUTION AS DYNAMIC SUPPORT RAPIDLY  
TRANSITIONS FROM A CENTRAL PLAINS/MIDWEST UPPER LOW TO SEPARATE  
EASTERN CANADA/UPPER GREAT LAKES ENERGY THAT ACCELERATES BEST  
DEFINED SURFACE LOW PRESSURE INTO THE CANADIAN MARITIMES BY DAY 4  
MON.  
 
THERE IS PERSISTENT GUIDANCE SPREAD REGARDING SPECIFICS OF THE  
EASTERN PACIFIC RIDGE ALOFT/POSSIBLE CLOSING OF AN UPPER HIGH NEAR  
THE ALASKA PANHANDLE, AND CORRESPONDING INFLUENCE ON SHORTWAVE  
ENERGY EMERGING FROM THE WEST. THIS CONTINUES THE SIGNIFICANT  
WAFFLING THAT GUIDANCE HAS EXHIBITED REGARDING WHETHER MEANINGFUL  
SURFACE DEVELOPMENT/PRECIPITATION WILL OCCUR OVER THE EASTERN  
STATES AROUND TUE-WED. IN RECENT MODEL RUNS (ESPECIALLY THE  
LATEST 00Z VERSION) THE ECMWF HAS BEEN TRENDING BACK TOWARD SOME  
HINT OF AT LEAST A SUPPRESSED SURFACE WAVE AFTER COMPLETELY WIPING  
OUT ANY LOW PRESSURE FOR A COUPLE DAYS. ON THE OTHER HAND GFS  
RUNS HAD BEEN THE MOST PERSISTENT IN HOLDING ONTO A  
STRONGER/SLOWER SHORTWAVE AND GREATER SURFACE DEVELOPMENT (LIKE  
THE CURRENT 00Z CMC) BUT THE 06Z GFS HAS TRENDED FLAT/PROGRESSIVE  
LIKE SOME EARLIER ECMWF RUNS. THE HISTORY OF GUIDANCE THUS FAR  
AND BROAD/ILL-CLUSTERED ENSEMBLE MEMBERS FOR POSSIBLE SURFACE LOW  
PRESSURE AS OF EARLY TUE FAVOR AN INTERMEDIATE SOLUTION, WITH THE  
FULL ARRAY OF GUIDANCE SUGGESTING ENOUGH SHORTWAVE ENERGY ALOFT TO  
PROMOTE AT LEAST A MODEST SURFACE REFLECTION ON TUE BEFORE  
STRONGER DEVELOPMENT OCCURS OVER THE WESTERN ATLANTIC BY WED.  
 
FOR THE SOUTHERN STREAM UPPER LOW/TROUGH APPROACHING CALIFORNIA,  
THE MAJORITY CONSENSUS/AVERAGE HAS PROVIDED FAIRLY GOOD CONTINUITY  
THUS FAR. THE CMC CONTINUES TO BE A PROMINENT EXTREME RELATIVE TO  
MOST OTHER SOLUTIONS, HOLDING THE FEATURE FARTHER WESTWARD BY THE  
LATTER HALF OF THE PERIOD. EVEN THE CMC ENSEMBLE MEAN IS MUCH  
CLOSER TO THE OTHER MODELS/MEANS SO CONFIDENCE IN THE 00Z CMC IS  
QUITE LOW.  
 
THE UPDATED FORECAST STARTED WITH PRIMARY EMPHASIS ON THE 00Z FV3  
GFS/ECMWF WITH LESSER INPUT OF SOME ASPECTS OF THE UKMET/CMC FOR  
DAYS 3-5 SUN-TUE. THE BLEND INCORPORATED ABOUT HALF TOTAL WEIGHT  
OF THE 06Z GEFS/00Z ECMWF MEANS FOR DAYS 6-7 WED-THU WHILE KEEPING  
SOME FV3 GFS/ECMWF INFLUENCE--BUT A LITTLE LESS OF THE FV3 GFS AS  
ITS DEPICTION OF THE ARRIVING EASTERN PACIFIC FEATURE DIFFERED  
FROM CONSENSUS SOMEWHAT.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
THE STORM SYSTEM TRACKING OUT OF THE MIDWEST/GREAT LAKES AFTER  
EARLY SUN WILL BRING AN AXIS OF SNOW TO THE NORTHWEST OF ITS TRACK  
AS WELL AS NORTHERN NEW ENGLAND. NEW ENGLAND MAY SEE A CHANGEOVER  
DEPENDING ON HOW QUICKLY WARMER AIR ARRIVES. LINGERING  
MOISTURE/WAVINESS ALONG THE TRAILING FRONT OVER THE SOUTHEAST MAY  
GIVE THAT REGION AREAS OF ENHANCED RAINFALL EARLY IN THE WEEK. IT  
WILL TAKE ADDITIONAL TIME TO SEE IF INGREDIENTS MAY COMBINE  
SUFFICIENTLY TO SUPPORT ANY STRONG CONVECTION. MEANWHILE COLD  
FLOW AROUND THE DEPARTING STORM SHOULD PRODUCE PERIODS OF LAKE  
EFFECT SNOW. CENTRAL/NORTHERN PARTS OF THE EAST MAY SEE SOME  
RAIN/SNOW TUE-WED BUT WITH CONTINUED SIGNIFICANT UNCERTAINTY  
REGARDING COVERAGE/INTENSITY. MOISTURE AHEAD OF THE SYSTEM  
APPROACHING CALIFORNIA SHOULD REACH THE WEST COAST DURING THE  
FIRST HALF OF THE WEEK AND THEN EXTEND INTO THE SOUTHWEST BY THU.  
EXPECT THE BEST POTENTIAL FOR GREATEST RAINFALL/HIGH ELEVATION  
SNOW TOTALS WITH THIS SYSTEM TO EXTEND FROM CALIFORNIA INTO  
ARIZONA. AHEAD OF THIS SYSTEM, INITIAL UPPER TROUGHING WILL  
PROMOTE AREAS OF RAIN/MOUNTAIN SNOW PRIMARILY FOCUSED OVER THE  
FOUR CORNERS STATES ON SUN.  
 
FORECAST PATTERN EVOLUTION WILL LEAD TO INCREASING COVERAGE OF  
BELOW NORMAL TEMPERATURES ACROSS THE LOWER 48 DURING THE PERIOD.  
THE EAST WILL SEE WELL ABOVE AVERAGE READINGS IN THE WARM SECTOR  
OF THE INITIAL MIDWEST/GREAT LAKES SYSTEM, WITH MIN TEMPS  
POTENTIALLY EXCEEDING CLIMATOLOGY BY AT LEAST 20F OR MORE AT SOME  
LOCATIONS SUN INTO MON. FARTHER WEST, HIGH TEMPERATURES 10-20F  
BELOW NORMAL OVER THE CENTRAL/SOUTHERN PARTS OF THE WEST INTO THE  
CENTRAL HIGH PLAINS WILL EXPAND EASTWARD WITH TIME. ONE OR MORE  
FRONTS DROPPING SOUTH FROM CANADA MAY PROVIDE REINFORCING SURGES  
OF COLD AIR DURING THE LATTER HALF OF THE FORECAST PERIOD.  
 
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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