440  
FXUS02 KWBC 200651  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
150 AM EST TUE NOV 20 2018  
 
VALID 12Z FRI NOV 23 2018 - 12Z TUE NOV 27 2018  
   
..OVERVIEW
 
 
A VERY STRONG CLOSED HIGH NORTH OF ENGLAND IS FORECAST TO  
RETROGRADE WESTWARD ACROSS SOUTHERN GREENLAND LATER THIS WEEK AND  
SETTLE OVER NORTHERN QUEBEC, FAVORING BLOCKED TROUGHING OVER THE  
EASTERN THIRD OF THE LOWER 48. IN THE WEST, TROUGHING WILL PASS  
THROUGH LATE THIS WEEK AND BE REPLACED BY RIDGING SUN-MON AHEAD OF  
ANOTHER SYSTEM NEXT TUESDAY.  
 
   
..GUIDANCE/UNCERTAINTY ASSESSMENT
 
 
MODELS AND ENSEMBLES CONTINUE TO PERFORM ADEQUATELY ENOUGH THAT A  
DETERMINISTIC BLEND SUFFICED FOR THE FIRST COUPLE DAYS OF THE  
PERIOD THOUGH THE 12Z UKMET WAS MOST DIFFERENT (QUITE STRONG) FROM  
THE REST AFTER FRIDAY AND WAS DROPPED FROM THE BLEND. TIMING THE  
SYSTEMS FRI-SAT HAS BEEN MOST PROBLEMATIC OVER THE PAST FEW DAYS  
THUS A BLENDED/CONSENSUS STARTING POINT REMAINED THE BEST OPTION.  
BY SATURDAY, LOW PRESSURE WILL DEVELOP AND LIFT ALONG THE  
SOUTHEAST COAST BUT THE MODELS CONTINUE TO DIVERGE IN HOW TO TRACK  
THE SYSTEM. PREFERRED TO FOLLOW THE ECMWF ENSEMBLE MEAN EARLY NEXT  
WEEK AS IT HAS BEEN MORE STABLE THAN THE GEFS MEAN, TAKING THE LOW  
OFF THE DELMARVA EARLY SUNDAY AND THEN EASTWARD AS THE INCOMING  
RIDGING FROM THE NORTH BLOCKS ITS CONTINUED NORTHEASTWARD MOVEMENT  
(DISCOUNTED THE MUCH FATHER NORTH 12Z ECMWF PER THIS REASON). TO  
THE WEST, SECOND TROUGH AND THEN UPPER LOW WILL DIP INTO THE  
MID-MISSISSIPPI VALLEY SUNDAY AND LIFT INTO THE GREAT LAKES,  
TAKING ANOTHER SYSTEM NORTHEASTWARD ON THE WEST SIDE OF THE  
APPALACHIANS. BY NEXT MON/TUE, WITH THE UPPER RIDGING FIRMLY IN  
PLACE OVER CANADA, THIS SYSTEM SHOULD HAVE NO CHOICE BUT TO SCOOT  
EASTWARD AND STRETCH OUT E-W OVER THE NORTHEAST WITH THE TRIPLE  
POINT LOW LIKELY TAKING OVER FROM ABOUT THE DELMARVA NORTHWARD.  
THIS COULD BE A SIGNIFICANT 1-2 PUNCH FOR MUCH OF THE EAST THIS  
WEEKEND AND EARLY NEXT WEEK.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
CONTINUED WET FOR THE WEST AS THE SECOND OF TWO SYSTEMS (THE FIRST  
IN THE SHORT RANGE) MOVES THROUGH ON FRIDAY/SATURDAY WITH PLENTY  
OF SNOW AT HIGHER ELEVATIONS. ADDITIONAL PRECIPITATION WILL BE  
LIKELY NEXT WEEK AHEAD OF THE NEXT PACIFIC SYSTEM, BUT FAVORED  
OVER COASTAL AREAS OF WA/OR BY THEN. IN THE EAST, LEAD SYSTEM ON  
FRIDAY WILL TAP THE GULF FOR MOISTURE AND EVEN AS THE NORTHERN  
FRONT WEAKENS INTO THE GREAT LAKES, PRECIPITATION WILL EXPAND  
EASTWARD AS THE GULF SURFACE LOW ORGANIZES. RAIN WILL SPREAD  
NORTHEASTWARD WITH SOME WINTRY WEATHER ON THE NORTH AND NORTHEAST  
SIDE AS THE COLD AIRMASS RETREATS (MODELS REALLY STRUGGLE WITH  
THIS EVOLUTION). HEAVIEST RAIN MAY BE ALONG THE I-81/I-95 CORRIDOR  
FROM SC UP TO THE MID-ATLANTIC TO THE NORTHWEST OF THE SURFACE LOW  
TRACK. WILL GET ONLY A BRIEF RESPITE AHEAD OF THE NEXT SYSTEM  
LIFTING INTO THE LAKES WHICH HAS THE POTENTIAL TO BE SLOWER AND  
STRONGER THAN THE PREVIOUS SYSTEM AS IT GETS BLOCKED FROM A  
TYPICAL EXIT THROUGH SOUTHEASTERN CANADA.  
 
TEMPERATURES WILL MODERATE IN THE EAST AFTER A COLD BLACK FRIDAY  
BUT A COLD FRONT OUT OF CANADA WILL BRING IN COLDER AIR AGAIN NEXT  
WEEK WITH A LARGE AREA OF MAX TEMP NEGATIVE ANOMALIES (5-15F BELOW  
AVERAGE) FROM THE ROCKIES EASTWARD.  
 
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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