747  
FXUS02 KWBC 170700  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
159 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 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. OVERALL NORTHERN  
PLAINS INTO WESTERN U.S. TROUGHING SHOULD AMPLIFY AT LEAST A  
LITTLE AROUND MIDWEEK AS ANOTHER UPPER RIDGE BUILDS OVER THE  
EASTERN PACIFIC. THE NORTHEAST STORM 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) HAVE 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 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).  
 
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 PLAINS FRONT AND TRAILING  
STRONG COLD FRONT (ANCHORED BY SOUTHERN CANADA LOW 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 IN  
WESTWARD-FACING TERRAIN FROM THE CENTRAL APPALACHIANS  
NORTHEASTWARD. THERE IS STILL ENOUGH UNCERTAINTY IN SYSTEM  
EVOLUTION TO ALLOW FOR OTHER POSSIBILITIES REGARDING PRECIPITATION  
TYPES/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 DURING SUNDAY-MONDAY AND  
CONTINUING OVER THE GREAT LAKES AND MID-ATLANTIC/NORTHEAST THROUGH  
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 BROADER 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  
 

 
 
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