035  
FXUS02 KWBC 211609  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1109 AM EST WED NOV 21 2018  
 
VALID 12Z SAT NOV 24 2018 - 12Z WED NOV 28 2018  
   
..OVERVIEW
 
 
A BLOCKING PATTERN WILL BE FAVORABLE THROUGH THE COURSE OF THE  
EXTENDED PERIODS AS A CLOSED HIGH RETROGRADES FROM ENGLAND TO  
NORTHERN QUEBEC. THE EASTERN THIRD OF THE U.S. WILL STAY IN A  
TROUGH WITH ELONGATED NEGATIVE HEIGHT ANOMALIES ACROSS THE  
MID-LATITUDES OF THE ATLANTIC. IN THE WEST, RIDGING WILL ATTEMPT  
TO REASSERT ITSELF ON SUNDAY AHEAD OF ANOTHER SYSTEM OR TWO NEXT  
TUESDAY/WEDNESDAY.  
 
   
..GUIDANCE/UNCERTAINTY ASSESSMENT
 
 
MODEL GUIDANCE IS PRETTY WELL CLUSTERED ON SATURDAY AT THE  
BEGINNING OF THE EXTENDED FORECAST BUT NOTICEABLE DEPARTURES ARISE  
OVER NORTH-CENTRAL CONUS AS EARLY MONDAY. THE LATEST RUNS OF THE  
CANADIAN/UKMET HAVE CONTINUED TO BE SOMEWHAT DISPLACED FROM THE  
GFS/ECWMF/ENSEMBLE MEANS. THE 06Z GFS HAS SLOWED RESULTING IN THE  
EAST COAST SYSTEM STAYING CLOSER TO THE COAST AND NOT RACING OUT  
INTO ATLANTIC, WHICH HAS BEEN THE PREFERRED TRACK OF LATE. IT WAS  
MORE FAVORABLE THAN ITS 00Z CYCLE THROUGH MONDAY, HOWEVER, IT  
QUICKLY DIVERGES FROM THE CLUSTER ON TUESDAY MAKING IT AN  
UNREALISTIC SOLUTION.  
 
AFTER A COUPLE QUIETER DAYS IN THE WEST SUN/MON, TROUGHING WILL  
ATTEMPT TO PUSH INTO THE RE-ESTABLISHED RIDGE BUT PREFER TO KEEP  
THE RAIN THREAT TO MAINLY COASTAL AREAS OF WA/OR AS AN UPPER LOW  
OVER THE EASTERN ALEUTIANS SHOULD BE THE MAIN PLAYER IN KEEPING  
THE DOWNSTREAM RIDGE MOSTLY IN TACT. THIS WOULD STILL ALLOW SOME  
WEAKENING FRONTS INTO THE PAC NW BUT LIKELY NOT INTO MUCH OF  
CA/NV.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
A SYSTEM LIFTING THROUGH THE SOUTHEAST AND MID-ATLANTIC STATES  
WILL SPREAD PRECIPITATION UP THE COAST TO NORTHEAST... WITH SOME  
ISOLATED WINTRY PRECIPITATION THROUGH THE MID-ATLANTIC INTO PARTS  
OF THE NORTHEAST. THERE CONTINUES TO BE SOME UNCERTAINTY ON HOW  
FAR NORTH, PRECIPITATION TYPE AND ITS INTENSITY WILL BE DUE TO HOW  
THE WEAKENING SYSTEM THROUGH THE GREAT LAKES INTERACTS. STILL  
FAVOR THE I-81/95 CORRIDORS BETWEEN NC AND PA/NJ AND MAYBE  
SOUTHEASTER NY FOR THE HEAVIEST RAIN (OVER AN INCH TO AN INCH AND  
A HALF POSSIBLE, PRIMARILY ON SATURDAY EVENING INTO SUNDAY  
MORNING).  
 
POTENT SHORTWAVE IN THE WEST ON SATURDAY AND ATTENDANT SURFACE  
FRONT WILL TAKE THE WIDESPREAD RAIN/SNOW EASTWARD THROUGH THE  
PLAINS INTO THE MID-MISSISSIPPI VALLEY WHERE PRECIPITATION AHEAD  
OF THE FRONT WILL EXPAND IN COVERAGE AS GULF MOISTURE GETS DRAWN  
NORTHWARD. COLD FRONT OUT OF CANADA TO THE NORTH WILL PROVIDE  
ENOUGH COLD AIR TO SUPPORT SNOW OR A MIXED BAG OF PRECIP TO THE  
NORTH/NORTHWEST OF THE SURFACE LOW SUNDAY FROM NE/SD EASTWARD TO  
THE CENTRAL GREAT LAKES. AS THAT LOW LIFTS TOWARD LAKE ERIE BY  
EARLY MONDAY IT WILL GET BLOCKED FROM CONTINUING NORTHEASTWARD AND  
INSTEAD LIKELY WRAP ITS TRIPLE POINT LOW OUT THROUGH SOUTHEASTERN  
NEW ENGLAND AS ITS FRONT PUSHES OFF THE COAST. THIS PIVOT COULD  
YIELD A SIGNIFICANT RAIN AND INLAND SNOW EVENT FOR THE NORTHEAST  
DEPENDING ON THE EVOLUTION OF THE SYSTEM.  
 
TEMPERATURES WILL GENERALLY BE BELOW AVERAGE OVER MOST OF THE  
LOWER 48 BY ABOUT 5-10F. A COUPLE BRIEF EXCEPTIONS WOULD BE ACROSS  
THE SOUTHERN PLAINS SATURDAY AHEAD OF THE WESTERN SYSTEM AND  
SOUTHERN FLORIDA UNTIL ABOUT TUESDAY WHEN A COLD FRONT SHOULD  
SETTLE INTO THE FLORIDA STRAITS.  
 
CAMPBELL/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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