969  
FXUS02 KWBC 210652  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
152 AM EST WED NOV 21 2018  
 
VALID 12Z SAT NOV 24 2018 - 12Z WED NOV 28 2018  
   
..OVERVIEW
 
 
STRONG POSITIVE HEIGHT ANOMALIES VIA A CLOSED HIGH CURRENTLY 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  
AND 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
 
 
THE 18Z GFS/GEFS AND 12Z ECMWF/ECMWF ENSEMBLE MEAN (AND 12Z  
FV3-GFS) OFFER GOOD CLUSTERING THROUGH THE PERIOD. THE  
CANADIAN/UKMET WERE EACH DISPLACED AWAY FROM THE CONSENSUS AND  
GIVEN NO WEIGHT AFTER SATURDAY. CONTINUE TO FAVOR A WAVY PATTERN  
TRENDING TOWARD BLOCKED FLOW IN THE EAST WITH SLOWING SYSTEMS AS  
TROUGHING GETS RELOADED INTO THE GREAT LAKES REGION. 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
 
 
LEAD SYSTEM IN THE SOUTHEAST ON SATURDAY WILL RIDE ALONG THE COAST  
AND TURN EAST OFF THE DELMARVA SUNDAY SPREADING RAIN AND MAYBE  
SOME ISOLATED WINTRY PRECIPITATION THROUGH THE MID-ATLANTIC INTO  
PARTS OF THE NORTHEAST. NORTHERN EXTENT AND INTENSITY (AND PTYPE)  
OF THE PRECIPITATION SHIELD REMAINS IN QUESTION DUE TO UNCERTAINTY  
WITH 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.  
 
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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