714  
FXUS02 KWBC 180700  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
159 AM EST THU NOV 18 2021  
 
VALID 12Z SUN NOV 21 2021 - 12Z THU NOV 25 2021  
 
...POTENTIAL CONTINUES FOR A STRONG COLD FRONT AND SIGNIFICANT  
STORM TO AFFECT THE EAST NEXT WEEK...  
   
..OVERVIEW
 
 
LATEST MODEL AND ENSEMBLE GUIDANCE REMAINS SIMILAR FOR THE  
EVOLVING LARGE SCALE PATTERN THROUGH THANKSGIVING WHILE DISPLAYING  
TYPICAL SPREAD AND RUN-TO-RUN VARIABILITY FOR THE DETAILS. A  
PACIFIC UPPER RIDGE MOVING INTO THE WEST/PLAINS DURING THE FIRST  
PART OF THE CALENDAR WEEK WILL SUPPORT AN AMPLIFYING EASTERN U.S.  
TROUGH WHOSE AXIS SHOULD REACH NEAR THE EAST COAST BY EARLY  
TUESDAY. IT IS STILL UP FOR DEBATE WHERE ONE OR MORE EMBEDDED LOWS  
MAY CLOSE OFF OVER THE GREAT LAKES OR SOUTHERN CANADA EARLY IN THE  
WEEK BUT THERE IS A MORE COMMON THEME OF AN UPPER LOW OVER OR JUST  
OFFSHORE NEW ENGLAND BY WEDNESDAY. THEREAFTER, HOW QUICKLY THE  
UPPER LOW DEPARTS WILL DEPEND ON THE STRENGTH AND WESTWARD EXTENT  
OF NORTH ATLANTIC UPPER RIDGING. THE EVOLUTION ALOFT AND  
POTENTIALLY COMPLEX SURFACE LOW PRESSURE/FRONTAL EVOLUTION MAY  
BRING A HEAVY RAIN THREAT TO NORTHERN NEW ENGLAND IN THE WARM  
SECTOR AND A PERIOD OF WELL BELOW NORMAL TEMPERATURES, BRISK TO  
STRONG WINDS, AND LAKE EFFECT/TERRAIN ENHANCED SNOW BEHIND THE  
STRONG COLD FRONT. CONTINUE TO MONITOR FORECASTS AS DETAILS MAY  
CHANGE. FARTHER WEST, THE COMBINATION OF A PACIFIC SHORTWAVE  
REACHING THE WEST BY TUESDAY AND AN OPENING UPPER LOW INITIALLY  
SOUTHWEST OF CALIFORNIA SHOULD YIELD A POSITIVELY TILTED UPPER  
TROUGH ALIGNED FROM THE NORTHERN PLAINS INTO NORTHWESTERN MEXICO  
BY THANKSGIVING. THESE FEATURES AND ENERGY ROUNDING THE UPSTREAM  
RIDGE MAY PRODUCE PERIODS OF PRECIPITATION OVER THE PACIFIC  
NORTHWEST WHILE RAIN MAY EXPAND/INTENSIFY ALONG AND NORTH OF THE  
WESTERN GULF COAST ON THANKSGIVING AS GULF MOISTURE INTERACTS WITH  
A PLAINS FRONT.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
FOR THE EVOLUTION ACROSS EASTERN NORTH AMERICA, OVER RECENT DAYS  
THE ENSEMBLE MEANS (AND MODEL RUNS CLOSEST TO THEM IN PRINCIPLE)  
HAVE PROVIDED A FAIRLY STEADY ANCHOR FOR THE GENERAL FORECAST WITH  
FAIRLY MODEST ADJUSTMENTS OVER CONSECUTIVE RUNS. LATEST GEFS/ECMWF  
MEANS HAVE ACHIEVED SIMILAR SOLUTIONS FOR THE EAST COAST UPPER  
TROUGH BY EARLY TUESDAY BY WAY OF THE ECMWF MEAN NUDGING FASTER  
AND GEFS MEAN SOMEWHAT SLOWER. BOTH THE OPERATIONAL MODELS AND  
MEANS HAVE RECENTLY SHOWN A TREND FOR A SHARPER/DEEPER TROUGH IN  
RESPONSE TO A SLOWER/STRONGER TROUGH REACHING THE WEST AND  
STRONGER INTERVENING RIDGE. MODELS HAVE BEEN WAFFLING ON IF/WHERE  
AN EMBEDDED UPPER LOW MAY FORM BETWEEN SUNDAY AND EARLY TUESDAY.  
HOWEVER THERE IS A DECENT CONSENSUS THAT AN UPPER LOW SHOULD REACH  
ALONG OR JUST OFFSHORE THE MID-ATLANTIC OR NEW ENGLAND COAST BY  
LATER TUESDAY OR WEDNESDAY. THUS FAR ANY STRAY SOUTHERN SOLUTIONS  
FOR THIS UPPER LOW (SOME ECMWF RUNS, 18Z GFS) HAVE REVERTED BACK  
TO THE NORTH TOWARD THE LARGER CLUSTERING IN SUBSEQUENT RUNS. A  
BUILDING UPPER RIDGE OVER THE NORTH ATLANTIC AND GREENLAND POSES  
THE RISK OF SLOWING DOWN THE SYSTEM FOR A PERIOD OF TIME. LATEST  
GFS RUNS HAVE STARTED TO SHOW THIS AND THE NEW 00Z ECMWF HAS  
TRENDED MUCH SLOWER LATE IN THE PERIOD. AT THE SURFACE, THERE IS  
REASONABLE CONTINUITY WITH LOW PRESSURE CONSOLIDATING OVER THE  
UPPER GREAT LAKES/SOUTHEASTERN CANADA SUNDAY-MONDAY. AFTER THAT  
THE FORECAST GETS MORE COMPLICATED AS SOME MODELS SHOW A FRONTAL  
WAVE TRACKING NORTHWARD OVER THE WESTERN ATLANTIC WHILE ONE OR  
MORE OTHER WAVES EVOLVE OVER OR REACH AREAS FROM OVER/EAST OF NEW  
ENGLAND INTO THE CANADIAN MARITIMES, IN RESPONSE TO THE UPPER  
DYNAMICS. ENOUGH UPPER RIDGING TO THE NORTH COULD KEEP THIS SYSTEM  
NEAR NEW ENGLAND THROUGH OR BEYOND MIDWEEK. MANY OF THESE DETAILS  
HAVE LOWER PREDICTABILITY SEVERAL DAYS OUT IN TIME, ESPECIALLY  
SINCE THE SUPPORTING ENERGY IS STILL OVER OR NEAR ALASKA AT THIS  
TIME.  
 
THE 12Z/18Z GFS, 12Z ECMWF/CMC, AND THE ENSEMBLE MEANS PROVIDED A  
REASONABLE CLUSTER OF GUIDANCE FOR THE UPPER TROUGH REACHING THE  
WEST BY TUESDAY (AS MENTIONED, A LITTLE DEEPER/SLOWER THAN SEEN  
YESTERDAY; UKMET RUNS HAVE BEEN SLOW) AND THE EASTERN PACIFIC  
UPPER LOW OPENING UP TO THE SOUTH, AS WELL AS FOR THE OVERALL  
TROUGH THAT CONTINUES EASTWARD THROUGH THE REST OF THE PERIOD.  
HOWEVER BY THANKSGIVING THERE IS A FAIR DEGREE OF UNCERTAINTY OVER  
HOW ENERGY WILL BE DISTRIBUTED WITHIN THE TROUGH. ALSO NEW 00Z  
RUNS ADD OTHER OPTIONS, SUCH AS A STRONGER UPSTREAM SHORTWAVE  
REACHING THE WEST IN THE GFS AND MUCH MORE FLOW SEPARATION IN THE  
CMC.  
 
BASED ON GUIDANCE COMPARISONS THROUGH 12Z/18Z, THE UPDATED  
FORECAST INCORPORATED THE 12Z/18Z GFS, 12Z ECMWF, AND SOME 12Z CMC  
FOR THE FIRST HALF OF THE PERIOD AND THEN TRANSITION TO A  
MODEL/ENSEMBLE MEAN BLEND. GFS INPUT TILTED MORE TOWARD THE 12Z  
RUN ONCE THE 18Z VERSION STRAYED SOUTH OF CONSENSUS WITH THE UPPER  
LOW NEAR THE EAST COAST, WHILE THE 12Z GEFS MEAN PROVIDED A  
SLIGHTLY STRONGER SOLUTION THAN THE 18Z RUN FOR THE SYSTEM NEAR  
NEW ENGLAND.  
 
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
CONSOLIDATING NORTHERN TIER U.S./SOUTHERN CANADA LOW PRESSURE WILL  
BRING A LEADING FRONT AND STRONGER TRAILING FRONT INTO AND THROUGH  
THE EAST EARLY IN THE CALENDAR WEEK. PRECIPITATION (MOSTLY RAIN)  
WILL DEVELOP ALONG AND EAST OF THE MISSISSIPPI VALLEY ON SUNDAY.  
MOST RAIN SHOULD BE LIGHT TO MODERATE BUT LOCALLY MORE INTENSE  
ACTIVITY MAY BE POSSIBLE OVER PARTS OF THE OHIO/TENNESSEE VALLEYS.  
AS THE FRONT AND POSSIBLE WAVES HEAD INTO NEW ENGLAND, RECENT  
TRENDS HAVE LOWERED PRECIPITATION TOTALS SOMEWHAT OVER  
WESTERN/SOUTHERN AREAS BUT MAINTAIN THE POTENTIAL FOR SOME HEAVY  
RAINFALL OVER MAINE, AIDED BY ATLANTIC INFLOW. SNOW IS STILL MOST  
LIKELY 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. MEANWHILE CONFIDENCE IS HIGHER IN  
A PERIOD OF BRISK TO STRONG WINDS FROM THE UPPER MIDWEST THROUGH  
THE EAST SUNDAY INTO TUESDAY AND THEN POSSIBLY LINGERING OVER THE  
NORTHEAST THEREAFTER. ACROSS THE NORTHERN TIER THE COMBINATION OF  
LOW TEMPERATURES AND WIND WOULD PRODUCE COLD WIND CHILLS.  
 
THE PACIFIC NORTHWEST INTO NORTHERN ROCKIES SHOULD SEE MOSTLY  
LIGHT TO PERHAPS LOCALLY MODERATE PRECIPITATION DURING THE FIRST  
HALF OF NEXT WEEK WITH A SHORTWAVE/COLD FRONT 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 OTHER PARTS OF THE WEST BY  
TUESDAY/WEDNESDAY, BUT STILL WITH MODEST CONFIDENCE FOR  
PRECIPITATION COVERAGE/AMOUNTS. AROUND THANKSGIVING THE WESTERN  
FRONT MAY REACH FAR ENOUGH INTO THE PLAINS TO BEGIN INTERACTING  
WITH GULF MOISTURE TO PRODUCE AN EXPANDING AREA OF RAIN ALONG AND  
NORTH OF THE WESTERN GULF COAST. SOME OF THIS RAIN MAY BECOME  
HEAVY.  
 
THE STRONG COLD FRONT CROSSING THE EAST EARLY IN THE WEEK WILL  
BRING WELL BELOW NORMAL TEMPERATURES THAT SHOULD BE MOST  
PRONOUNCED BETWEEN MONDAY AND EARLY WEDNESDAY WHEN THERE SHOULD BE  
DECENT COVERAGE OF HIGHS/MORNING LOWS 10-15F BELOW NORMAL, WITH  
SOME POCKETS OF HIGHS POSSIBLY 15-20F BELOW NORMAL OVER SOUTHERN  
AREAS. TEMPERATURES MAY FALL BELOW FREEZING RATHER FAR INTO THE  
SOUTH. READINGS SHOULD REBOUND TOWARD NORMAL BY THANKSGIVING.  
PORTIONS OF THE ROCKIES/PLAINS WILL SEE A BRIEF WARM PERIOD EARLY  
IN THE WEEK WITH SOME PLUS 10-15F OR SO ANOMALIES. A COLD FRONT  
MOVING FROM THE WEST INTO THE PLAINS DURING TUESDAY THROUGH  
THANKSGIVING WILL BRING MODERATELY BELOW NORMAL HIGHS IN ITS WAKE.  
 
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