456  
FXUS02 KWBC 151543  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1042 AM EST THU NOV 15 2018  
 
VALID 12Z SUN NOV 18 2018 - 12Z THU NOV 22 2018  
   
..OVERVIEW
 
 
UPPER-LEVEL RIDGING ACROSS WESTERN NORTH AMERICA WILL BE SLOW TO  
WEAKEN/BREAK DOWN DURING THE EXTENDED PERIOD, WHICH WILL SUPPORT A  
CONTINUED DOWNSTREAM MEAN TROUGH ACROSS THE GREAT LAKES AND  
NORTHEAST. POLAR SHORTWAVE ENERGY WILL TRAVERSE THE MEAN TROUGH  
THROUGH MUCH OF THE PERIOD, WITH SOME POTENTIAL FOR PHASING  
INTERACTIONS WITH SOUTHERN STREAM/PACIFIC ENERGY UNDERCUTTING THE  
WESTERN RIDGE AND CROSSING THE CONUS SOUTHERN TIER. THE POSITION  
OF THE MEAN TROUGH WILL DELAY ANY POTENTIAL PHASING INTERACTIONS  
SUCH THAT ANY DEVELOPING COASTAL LOW PRESSURE SYSTEMS WILL LIKELY  
BE QUICKLY PULLING AWAY FROM THE NORTHEASTERN U.S. BEFORE  
SIGNIFICANT DEEPENING BEGINS.  
 
   
..GUIDANCE/UNCERTAINTY ASSESSMENT
 
 
MODEL CONSENSUS WAS SUFFICIENT DURING DAYS 3-5 (SUN-TUE) TO BASE  
THE FORECAST ON A MULTI-MODEL DETERMINISTIC BLEND (INCLUDING THE  
ECMWF/GFS/FV3/UKMET). DIFFERENCES AMONG THESE SOLUTIONS WERE  
RELATIVELY SMALL WITH RESPECT TO SHORTWAVE ENERGY ROUNDING THE  
BASE OF THE MEAN TROUGH ACROSS THE GREAT LAKES DURING THAT TIME  
PERIOD, AND WITH AN ELONGATED/TRAILING SHORTWAVE EXTENDING BACK  
INTO THE PLAINS AND FOUR CORNERS REGIONS. AS THESE FEATURES  
PARTIALLY PHASE, AN AREA OF LOW PRESSURES IS EXPECTED TO DEVELOP  
ALONG THE NORTHEAST U.S. COAST WED NIGHT/THU MORNING, WHICH WILL  
VERY QUICKLY MOVE NORTHEASTWARD INTO CANADA. BY DAY 5 (TUE)  
ANOTHER SHORTWAVE CROSSING THE GREAT LAKES AND A TRAILING FRONTAL  
BOUNDARY WILL DEVELOP YET ANOTHER SURFACE WAVE WHICH IS EXPECTED  
TO QUICKLY MOVE NORTHEASTWARD INTO CANADA BY DAY 6 (WED). FROM DAY  
6 ONWARD, MODELS CONTINUE TO SHOW SOME SIGNIFICANT DIFFERENCES,  
ESPECIALLY WITH RESPECT TO ANY POTENTIAL WEAKENING OF THE WESTERN  
RIDGE AND THE ARRIVAL OF HEIGHT FALLS IN THE WEST ASSOCIATED WITH  
PACIFIC SHORTWAVE ENERGY. THE ECMWF WAS REJECTED DURING THIS TIME  
PERIOD YESTERDAY AS IT WAS BY FAR THE FASTEST SOLUTION. A GENERAL  
TREND TOWARD A SLOWER RIDGE BREAKDOWN/SLOWER ARRIVAL OF HEIGHT  
FALLS WAS OBSERVED ACROSS THE MODEL/ENSEMBLE SUITE, AND DESPITE A  
SIGNIFICANT TREND IN THAT DIRECTION, THE ECMWF REMAINS AMONG THE  
FASTER SOLUTIONS TO SPREAD HEIGHT FALLS INTO THE WEST BY THE  
MIDDLE OF NEXT WEEK. ENSEMBLE MEANS (ESPECIALLY THE GEFS MEAN)  
HAVE REMAINED MUCH MORE STABLE THAN DETERMINISTIC SOLUTIONS. AT  
THIS TIME, THE SLOWER SOLUTIONS WILL CONTINUE TO BE FAVORED.  
WEIGHTING OF ENSEMBLE MEANS WAS BOOSTED SUBSTANTIALLY BY DAYS 6-7,  
WITH A BIT MORE WEIGHT TOWARD THE GEFS MEAN RELATIVE TO THE ECENS  
MEAN, WITH VERY LITTLE EMPHASIS ON THE DETERMINISTIC ECMWF BY THAT  
TIME.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE TREND TOWARD A SLOWER ARRIVAL OF SIGNIFICANT HEIGHT FALLS IN  
THE WESTERN U.S. APPEARS LIKELY TO DELAY ANY CHANCES OF  
PRECIPITATION ACROSS THE WEST COAST STATES UNTIL AT LEAST TUE  
NIGHT/WED. LOCATIONS NEAR THE CENTRAL AND NORTHERN WEST COAST  
SHOULD HAVE THE BEST POTENTIAL FOR SEEING SOME RAIN AND POSSIBLY  
HIGH ELEVATION SNOW. A COUPLE SOUTHERN STREAM SHORTWAVES ALONG  
WITH A REGION OF POTENTIAL ISENTROPIC UPGLIDE NORTH OF A GULF OF  
MEXICO FRONTAL BOUNDARY WILL SUPPORT AN EXPANSION OF RAINFALL  
ACROSS THE WESTERN GULF COAST DURING THE PERIOD, WITH THE  
POTENTIAL FOR SOME AREAS OF HEAVY RAINFALL, ESPECIALLY CLOSE TO  
THE COAST WHERE INSTABILITY WILL BE BEST. PERIODS OF LAKE EFFECT  
SNOW SHOULD BE MOST FAVORED OVER NORTHERN AND EASTERN PORTIONS OF  
THE GREAT LAKES. SOME OF THE SNOW OVER THIS AREA MAY ALSO BE  
FOCUSED NEAR ONE OR MORE FRONTAL BOUNDARIES/EMBEDDED WAVES.  
 
DURING SUN-MON EXPECT COLDEST TEMPERATURE ANOMALIES OF 10-20F OR  
SO BELOW NORMAL TO BE MOST COMMON FROM THE SOUTHERN PLAINS INTO  
MIDWEST/GREAT LAKES. THEN AS THE CORE OF UPPER TROUGHING ADJUSTS  
INTO THE NORTHEAST, THAT REGION SHOULD SEE COLDEST READINGS VERSUS  
NORMAL TUE-THU, 10-25F BELOW NORMAL. SUCH ANOMALIES MAY CHALLENGE  
DAILY RECORD VALUES AT SOME LOCATIONS. CONFIDENCE REMAINS ABOVE  
AVERAGE FOR PLUS 5-15F HIGH TEMPERATURE ANOMALIES ALONG/INLAND  
FROM THE CENTRAL WEST COAST THROUGH AT LEAST THE FIRST PART OF  
NEXT WEEK, AFTER THAT TIME, FORECAST CONFIDENCE DECREASES GIVEN  
THE UNCERTAINTY REGARDING THE ARRIVAL OF HEIGHT FALLS FROM THE  
PACIFIC.  
 
RYAN  
 
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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