433  
FXUS02 KWBC 170500  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1159 PM EST FRI NOV 16 2018  
 
VALID 12Z TUE NOV 20 2018 - 12Z SAT NOV 24 2018  
   
..OVERVIEW
 
 
UPPER PATTERN WILL BE IN A STATE OF FLUX OVER THE LOWER 48 NEXT  
WEEK AS EASTERN TROUGHING LIFTS OUT AND A COUPLE TROUGHS PUSH  
INTO/THROUGH THE WEST. THIS WILL GUIDE A LEAD AND THEN TRAILING  
FRONT INTO CALIFORNIA ON WEDNESDAY AND THEN WA/OR/CA ON FRIDAY,  
RESPECTIVELY.  
 
   
..GUIDANCE/UNCERTAINTY ASSESSMENT
 
 
FORECAST FOR TUE-WED WAS FORMED FROM A MULTI-MODEL DETERMINISTIC  
BLEND OF THE RECENT 12Z/18Z GUIDANCE, WHICH MAINTAINED GOOD  
CONTINUITY TO START. THE MODELS/ENSEMBLES HAVE REALLY STRUGGLED  
WITH BOTH THE NORTHERN STREAM FLOW ACROSS SOUTHERN CANADA AND  
ESPECIALLY OVER THE PACIFIC (BOTH MID AND HIGH LATITUDES). RATHER  
THAN BEING A QUESTION OF QUICKER VS SLOWER WITH A MAIN TROUGH AND  
SECONDARY WAVE DEVELOPMENT, THE GUIDANCE HAS NOW SHIFTED TOWARD A  
LEAD SYSTEM ON PACE WITH THE QUICKER SOLUTIONS FROM 24 HRS AGO  
(ECMWF-LED GUIDANCE) FOLLOWED BY ANOTHER SYSTEM DIVING OUT OF THE  
HIGHER LATITUDES AS A KICKER. THE GFS/GEFS HAVE SWUNG MUCH FASTER  
THAN JUST 24-36 HRS AGO WITH THE LEAD SYSTEM IN THE WEST AND WAS  
MUCH QUICKER IN THE NORTHEAST/SOUTHEASTERN CANADA THAT IT WAS  
QUITE SUSPECT. THE 12Z ECMWF/CANADIAN/UKMET WERE PREFERRED OVER  
MUCH OF THE CONUS FROM WED ONWARD TRENDING TOWARD A MAJORITY ECMWF  
ENSEMBLE MEAN WEIGHT BY NEXT SATURDAY. CONFIDENCE REMAINS NO  
BETTER THAN AVERAGE DUE TO THE UNSTABLE FORECASTS. THIS RESULTED  
IN CHANGES TO THE ONGOING FORECAST BUT WITH THE SAME RESULT IN THE  
WEST -- A MUCH WETTER PERIOD AND COOLER TEMPERATURES. ADJUSTED THE  
DOWNSTREAM FLOW ACCORDINGLY, WHICH WAS OVERALL QUICKER WITH THE  
LEAD FRONT MOVING THROUGH THE PLAINS ON FRIDAY.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THANKSGIVING SUMMARY (THU 11/22) - CHILLY IN THE NORTHEAST WITH  
MANY AREAS STAYING BELOW FREEZING. WET ALONG THE WEST COAST INTO  
THE GREAT BASIN AND ROCKIES WITH NEAR AVERAGE TEMPERATURES AND  
HIGHER ELEVATION SNOW. MILDER AND DRY FOR THE HIGH PLAINS WITH  
40S/50S COMMON. RAIN CHANCE FOR THE GULF COAST FROM TEXAS EASTWARD  
THROUGH MUCH OF FLORIDA TO THE NORTH OF A FRONT IN THE FLORIDA  
STRAITS.  
 
RATHER QUIET TO START DOMINATED BY NW FLOW OUT OF CENTRAL CANADA  
EAST OF 100W. PESKY STATIONARY BOUNDARY AND INVERTED TROUGH IN THE  
WESTERN GULF WILL KEEP A HEAVY RAINFALL THREAT FOR ESPECIALLY THE  
COAST WHERE INSTABILITY IS HIGHEST. THAT WILL CONTINUE THROUGH THE  
WEEK WITH PERHAPS SOME ENHANCEMENT BY THE INCOMING SOUTHERN STREAM  
SHORTWAVE ON WEDNESDAY.  
 
THE ENTIRE WEST COAST WILL SEE AN INCREASE IN FIRST CLOUDS THEN  
RAIN/ELEVATION SNOW BY WEDNESDAY. FOCUS APPEARS TO BE AROUND  
NORTHERN CALIFORNIA AND SOUTHWESTERN OREGON AS THE UPPER TROUGH  
SPLITS N-S. MULTI-DAY RAINFALL AMOUNTS MAY BE IN THE 3-6 INCH  
RANGE OVER FAVORED AREAS AND EVEN ABOUT AND INCH TO AN INCH AND A  
HALF IN THE CENTRAL VALLEY. SNOW LEVELS WILL FALL THROUGH THE  
PERIOD WITH SEVERAL FEET OF SNOW LIKELY AT THE HIGHEST ELEVATIONS.  
 
BELOW AVERAGE TEMPERATURES IN THE GREAT LAKES/NORTHEAST WILL BE  
SLOW TO EASE BUT WILL TREND BACK TOWARD TYPICAL LATE NOVEMBER  
VALUES BY NEXT WEEKEND. RECORD COLD VALUES ARE POSSIBLE ON TUESDAY  
AND MAYBE WEDNESDAY. TEMPERATURES IN THE WEST WILL BE ABOVE  
AVERAGE BY ABOUT 5-10 DEGREES BUT WILL MOVE EAST AS TROUGHING  
MOVES INLAND. THE PLAINS AND UPPER MIDWEST WILL SEE MILDER  
TEMPERATURES WEDNESDAY-SATURDAY, GENERALLY ABOVE FREEZING FOR  
AFTERNOON HIGHS.  
 
FRACASSO  
 
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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