847  
FXUS02 KWBC 170713  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
212 AM EST WED NOV 17 2021  
 
VALID 12Z SAT NOV 20 2021 - 12Z WED NOV 24 2021  
 
...POTENTIAL CONTINUES FOR A STRONG COLD FRONT AND SIGNIFICANT  
STORM TO AFFECT THE EAST EARLY NEXT WEEK...  
   
..OVERVIEW
 
 
THE EXPECTED LARGE SCALE PATTERN EVOLUTION FROM THE WEEKEND INTO  
THE MIDDLE OF NEXT WEEK REMAINS ON TRACK. GUIDANCE CONSENSUS SHOWS  
FLAT MEAN FLOW AS OF EARLY SATURDAY STEADILY BECOMING MORE  
AMPLIFIED AS A DEEPENING UPPER TROUGH HEADS INTO THE EASTERN U.S.,  
DOWNSTREAM FROM A PACIFIC UPPER RIDGE THAT REACHES WESTERN CANADA  
AND THE NORTHWESTERN U.S. BY MONDAY. WITHIN THE EASTERN TROUGH  
MOST MODELS CONTINUE TO SHOW AN UPPER LOW CLOSING OFF AND TAKING A  
PATH SOMEWHERE WITHIN AN AREA FROM THE GREAT LAKES/SOUTHERN CANADA  
INTO NEW ENGLAND, SUPPORTING FAIRLY STRONG SOUTHERN CANADA INTO  
NORTHEAST U.S. LOW PRESSURE WHICH COULD UNDERGO SOME REDEVELOPMENT  
DURING THE OVERALL SYSTEM'S LIFE SPAN. THIS STORM REMAINS A  
PROMINENT WEATHER FOCUS DUE TO ITS TIMING RIGHT BEFORE  
THANKSGIVING BUT IT WILL LIKELY STILL TAKE A WHILE TO RESOLVE THE  
DETAILS. MEANWHILE THE MODELS AND ENSEMBLES STILL HAVE AN UPPER  
LOW DEVELOPING SOUTHWEST/SOUTH OF CALIFORNIA DURING THE WEEKEND  
AND EARLY NEXT WEEK. THIS FEATURE SHOULD OPEN UP BY WEDNESDAY AS A  
PACIFIC SHORTWAVE REACHES THE WEST BY TUESDAY AND ADDITIONAL  
ENERGY COULD ARRIVE THEREAFTER. NORTHERN PLAINS INTO WESTERN U.S.  
TROUGHING SHOULD AMPLIFY AT LEAST A LITTLE AROUND MIDWEEK AS  
ANOTHER UPPER RIDGE BUILDS OVER THE EASTERN PACIFIC. EASTERN NORTH  
AMERICA LOW PRESSURE AND ITS TRAILING FRONT WILL USHER IN A BRIEF  
COLD SPELL OVER THE EAST DURING THE FIRST HALF OF NEXT WEEK WHILE  
THE WEST AND PARTS OF THE HIGH PLAINS WILL SEE VARIABLE BUT  
GENERALLY ABOVE NORMAL TEMPERATURES THROUGH TUESDAY.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
RELATIVE TO TYPICAL GUIDANCE SPREAD AND ERROR FOR FORECASTS 5-7  
DAYS OUT IN TIME, MOST MODELS AND ENSEMBLE MEANS WERE FAIRLY WELL  
CLUSTERED FOR THE EASTERN NORTH AMERICA EVOLUTION AS OF THE  
12Z/18Z CYCLES. A SOMEWHAT LESS EXTREME/NORTHWARD TREND IN ECMWF  
UPPER LOW TRACK, SIX-HOURLY GFS RUNS SHUFFLING AROUND THE GUIDANCE  
AVERAGE, AND A DEEPER TREND IN THE ENSEMBLE MEANS (LEADING TO AT  
LEAST AN IMPLIED UPPER LOW IF NOT A CLOSED ONE SUCH AS DEPICTED IN  
THE NEW 00Z GEFS MEAN) LED TO A FORECAST UPPER LOW TRACK ACROSS  
THE UPPER/EASTERN GREAT LAKES AND INTO NEW ENGLAND. THIS EVOLUTION  
REINFORCES A RECENT STRONGER TREND FOR LOW PRESSURE BETWEEN THE  
GREAT LAKES AND JAMES BAY AS OF EARLY MONDAY AND A SLIGHT  
NORTHWARD ADJUSTMENT FROM CONTINUITY FOR SECONDARY DEVELOPMENT  
OVER/NEAR NEW ENGLAND BY TUESDAY. CMC DETAILS HAVE DIFFERED  
SOMEWHAT. TAKEN AS A WHOLE, RECENT GFS/ECMWF/CMC RUNS ILLUSTRATE  
THE POSSIBILITIES OF REDEVELOPMENT AND/OR LOOPING OF LOW PRESSURE.  
THE NEW 00Z ECMWF PROVIDES A SIGNIFICANTLY DIFFERENT ADJUSTMENT  
THAT WOULD LESSEN EFFECTS OVER THE NORTHEAST. IT IS IMPORTANT TO  
NOTE THAT THE UPPER DYNAMICS SUPPORTING THIS SYSTEM ARE STILL  
WITHIN FLOW TO THE NORTH/WEST OF AN UPPER LOW OVER ALASKA AND  
HISTORICALLY THE MODELS CAN HAVE DIFFICULTY IN RESOLVING IMPORTANT  
DETAILS OF AMPLIFYING SHORTWAVES FROM HIGHER LATITUDES UNTIL MUCH  
CLOSER TO AN EVENT.  
 
THE GFS/ECMWF/UKMET ALONG WITH THE GEFS/ECMWF MEANS ARE PROVIDING  
THE MOST COMMON SOLUTION FOR THE UPPER LOW THAT FORMS WEST/SOUTH  
OF CALIFORNIA, WITH SOME OPENING/PROGRESSION LIKELY TO OCCUR AFTER  
EARLY TUESDAY. INCLUDING THE NEW 00Z RUN, THE CMC HAS BEEN ON THE  
EASTERN SIDE OF THE GUIDANCE SPREAD. A GENERAL MODEL/MEAN AVERAGE  
IS REASONABLE AND CONSISTENT FOR THE SHORTWAVE REACHING THE  
NORTHWEST AND THEN THE PLAINS TUESDAY-WEDNESDAY AND FOR  
DOWNPLAYING UNCERTAIN SHORTWAVE DETAILS UPSTREAM (SUCH AS THE  
STRONGER 12Z ECMWF FEATURE NEARING THE WEST COAST EARLY WEDNESDAY,  
ELIMINATED IN THE NEW 00Z RUN).  
 
BASED ON THE 12Z/18Z GUIDANCE COMPARISONS, THE UPDATED FORECAST  
STARTED WITH THE 12Z/18Z GFS AND 12Z ECMWF ALONG WITH SOME 12Z  
UKMET FOR ABOUT THE FIRST HALF OF THE PERIOD AND THEN TRANSITIONED  
TO A BLEND OF THOSE MODELS ALONG WITH THE 18Z GEFS/12Z ECMWF MEANS.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
THE START OF THE WEEKEND SHOULD BE DRY OVER MUCH OF THE COUNTRY  
ASIDE FROM RAINFALL PERSISTING ALONG THE EASTERN FLORIDA PENINSULA  
AND SOME LIGHT SNOW OVER PARTS OF THE NORTHERN/CENTRAL ROCKIES.  
EXPECT PRECIPITATION TO DEVELOP ALONG AND EAST OF THE MISSISSIPPI  
VALLEY ON SUNDAY AHEAD OF A LEADING WAVY PLAINS FRONT AND TRAILING  
STRONG COLD FRONT (ANCHORED BY NORTHERN TIER U.S./SOUTHERN CANADA  
LOW PRESSURE) THAT CATCHES UP. MOST RAIN SHOULD BE LIGHT TO  
MODERATE GIVEN THE FRONTAL PROGRESSION BUT LOCALLY MORE INTENSE  
ACTIVITY MAY BE POSSIBLE. HIGHEST TOTALS FOR SUNDAY-SUNDAY NIGHT  
SHOULD BE IN THE OHIO/TENNESSEE VALLEYS. FROM MONDAY ONWARD THE  
MOST LIKELY EVOLUTION OF THIS SYSTEM WOULD BRING MORE OF AN  
ENHANCED RAINFALL THREAT TO NEW ENGLAND, AHEAD OF THE COLD  
FRONT/SECONDARY LOW DEVELOPMENT. THE BEST SNOW POTENTIAL WOULD BE  
IN THE COLD AIR BEHIND THE SYSTEM, TO THE LEE OF THE GREAT LAKES  
AND ALONG WESTWARD-FACING TERRAIN FROM THE CENTRAL APPALACHIANS  
NORTHEASTWARD. THERE IS STILL ENOUGH UNCERTAINTY IN SYSTEM  
EVOLUTION TO ALLOW FOR OTHER POSSIBILITIES REGARDING PRECIPITATION  
TYPES AND AMOUNTS. CONFIDENCE IS SOMEWHAT HIGHER IN THE POTENTIAL  
FOR A PERIOD OF BRISK TO STRONG WINDS BEHIND THE COLD FRONT  
SWEEPING THROUGH THE PLAINS INTO THE EAST, WITH SUCH WINDS  
CONTINUING OVER THE GREAT LAKES AND MID-ATLANTIC/NORTHEAST INTO AT  
LEAST TUESDAY. THE PACIFIC NORTHWEST INTO NORTHERN ROCKIES MAY SEE  
MOSTLY LIGHT PRECIPITATION DURING THE FIRST HALF OF NEXT WEEK WITH  
A SHORTWAVE ARRIVING FROM THE PACIFIC. THE COMBINATION OF THIS  
ENERGY ALOFT AND THE OPENING/PROGRESSION OF THE UPPER LOW  
SOUTHWEST OF CALIFORNIA MAY SPREAD SOME MOISTURE INTO PARTS OF THE  
WEST BY TUESDAY/WEDNESDAY, BUT WITH LOW CONFIDENCE FOR  
PRECIPITATION COVERAGE/AMOUNTS AT THIS TIME.  
 
THE EAST WILL SEE CHILLY TEMPERATURES (MOSTLY 5-10F BELOW NORMAL)  
LINGERING INTO THE EARLY WEEKEND. THE NEXT SURGE OF COLDER AIR  
WILL SPREAD ACROSS THE EASTERN HALF OF THE LOWER 48 MONDAY-TUESDAY  
WITH THE BROADEST COVERAGE OF HIGHS 10-15F BELOW NORMAL EXPECTED  
ON TUESDAY. SOME LOCATIONS MAY ALSO SEE MORNING LOWS 10F OR SO  
BELOW NORMAL. THE WEST INTO THE CENTRAL/SOUTHERN HIGH PLAINS WILL  
SEE MOSTLY ABOVE NORMAL TEMPERATURES THROUGH TUESDAY WITH THE  
WARMEST ANOMALIES OF PLUS 10-15F FOR HIGHS LIKELY OVER THE HIGH  
PLAINS ON SATURDAY (LINGERING INTO SUNDAY NEAR THE RIO GRANDE) AND  
AGAIN ON TUESDAY. THE UPPER TROUGH OVER THE WEST BY WEDNESDAY WILL  
LIKELY BRING HIGHS DOWN TO NEAR OR SLIGHTLY BELOW NORMAL LEVELS AT  
THAT TIME.  
 
RAUSCH  
 
ADDITIONAL 3-7 DAY HAZARD INFORMATION CAN BE FOUND ON THE WPC  
MEDIUM RANGE HAZARDS OUTLOOK CHART AT:  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP  
 
WPC MEDIUM RANGE 500MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, WINTER WEATHER OUTLOOK PROBABILITIES  
AND HEAT INDICES ARE 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  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page