282  
FXUS02 KWBC 231904  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
203 PM EST TUE NOV 23 2021  
 
VALID 12Z FRI NOV 26 2021 - 12Z TUE NOV 30 2021  
   
..OVERVIEW
 
 
A RELATIVELY STAGNANT UPPER-LEVEL PATTERN IS EXPECTED IN THE  
MEDIUM RANGE PERIOD, WITH MEAN TROUGHING FOR THE EASTERN HALF OF  
THE U.S. AND QUASI-ZONAL FLOW SHIFTING TO RIDGING BY THE WEEKEND  
ACROSS MUCH OF THE WEST, EXCEPT FOR A SOUTHERN STREAM LOW  
GRADUALLY MAKING ITS WAY ACROSS NORTHERN MEXICO AND THE SOUTHWEST  
AS IT WEAKENS. THIS PATTERN SHOULD BE RELATIVELY DRY ACROSS MUCH  
OF THE CONTIGUOUS U.S., WITH THE EXCEPTION OF WAVES OF  
PRECIPITATION OVER THE PACIFIC NORTHWEST THROUGH THE PERIOD, AS  
WELL AS SOME PRECIPITATION COMING IN ACROSS EASTERN TROUGH,  
INCLUDING A ROUND OF MODERATE TO HEAVY LAKE EFFECT AND INTERIOR  
NORTHEAST SNOW FRIDAY INTO SATURDAY.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
GUIDANCE CONTINUES TO SHOW ABOVE AVERAGE AGREEMENT ON THE LARGE  
SCALE FLOW PATTERN DESCRIBED ABOVE PARTICULARLY THROUGH THE  
WEEKEND, AND A MULTI-MODEL DETERMINISTIC BLEND OF THE 00Z ECMWF,  
CMC, UKMET, AND 00Z/06Z GFS WAS USED EARLY IN THE PERIOD. GREATER  
DIFFERENCES ARISE WITH ENERGY DIGGING THROUGH THE WESTERN SIDE OF  
THE TROUGH SERVING TO RELOAD IT. THE FIRST SHORTWAVE WITH  
SIGNIFICANT DIFFERENCES BETWEEN MODELS SHOWS UP IN THE GREAT LAKES  
AROUND SUNDAY, WITH ANOTHER POSSIBLY DIVING TOWARD THE NORTHERN  
PLAINS AROUND MONDAY OR TUESDAY. THE LATTER SHORTWAVE SPINS UP A  
CLOSED 500MB LOW ACROSS THE GREAT LAKES REGION IN THE 00Z ECMWF BY  
TUESDAY; A HINT OF THIS IS SHOWN BY THE 00Z GFS AND A WEAKER  
VERSION IS SHOWN BY THE NEW 12Z ECMWF. ENSEMBLE GUIDANCE SHOWS A  
BIT OF A SPLIT WITH GEFS MEMBERS HAVING MORE RIDGING ACROSS THE  
NORTH-CENTRAL U.S. AND EC ENSEMBLE MEMBERS SHOWING MORE TROUGHING.  
SO UNCERTAINTY IS STILL HIGH WITH THESE FEATURES. AT THE SURFACE,  
THERE IS STILL A POSSIBILITY OF A SURFACE LOW TO SPIN UP AND  
DEEPEN OVER THE ATLANTIC MONDAY INTO TUESDAY, BUT THE TIMING AND  
STRENGTH OF THE LOW VARY. ENSEMBLE MEANS ARE WEAKER, AS EXPECTED,  
WITH THE LOW BUT DO PROVIDE A NICE MIDDLE GROUND SOLUTION AT THIS  
TIME, PARTICULARLY THE 00Z GEFS AND EC MEANS THAT WERE RIGHT ON  
TOP OF EACH OTHER. INCOMING 12Z GUIDANCE APPEARS IT MAY BE MORE  
CLUSTERED WITH THIS LOW. THE WPC FORECAST FOR THESE FEATURES  
SHIFTED FROM A DETERMINISTIC BLEND EARLY ON TOWARD UTILIZING MORE  
OF THE GEFS AND EC MEAN SOLUTIONS AS THE PERIOD PROGRESSED.  
 
SOME DIFFERENCES BETWEEN MODELS ALSO REMAIN WITH THE TRACK AND  
TIMING OF THE SOUTHERN STREAM MID-UPPER LOW TRACKING NORTHEASTWARD  
FROM BAJA CALIFORNIA INTO THE SOUTHWEST. 00/06Z GFS RUNS LINGER A  
CLOSED LOW LONGER THAN OTHER GUIDANCE, BUT ALL GUIDANCE HAS A  
GENERAL WEAKENING TREND AS THE PERIOD PROGRESSES. ADDITIONALLY,  
THE PRECISE ORIENTATION OF AN ATMOSPHERIC RIVER EVENT THIS WEEKEND  
INTO BRITISH COLUMBIA AND THE PACIFIC NORTHWEST JUST NORTH OF THE  
LOW AMPLITUDE RIDGING REMAINS IN QUESTION--GFS HAVE GENERALLY KEPT  
THE GREATER INTEGRATED WATER VAPOR TRANSPORT CONFINED TO WESTERN  
CANADA, WHILE THE 00Z ECMWF BRINGS IT INTO THE PACIFIC NORTHWEST.  
THIS WILL HAVE AN IMPACT ON HOW MUCH RAIN IMPACTS THE OLYMPICS AND  
NORTHERN CASCADES VERSUS THE BULK OF THE PRECIPITATION STAYING  
NORTH IN BRITISH COLUMBIA, INCREASING UNCERTAINTY. FOR THESE  
FEATURES, A MULTI-MODEL BLEND SUFFICED TO MINIMIZE DIFFERENCES AND  
PRODUCED A MIDDLE GROUND SOLUTION AT THIS POINT, AND WILL CONTINUE  
TO MONITOR.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
AS THE PERIOD BEGINS FRIDAY, A COLD FRONT EXITING THE EAST COAST  
WILL BRING WESTERLY FLOW TO THE GREAT LAKES REGION AND THE  
NORTHEAST, FAVORABLE CONDITIONS FOR LAKE EFFECT AND INTERIOR  
NORTHEAST (PARTICULARLY HIGHER ELEVATION) SNOW THROUGH SATURDAY.  
MODERATE TO LOCALLY HEAVY SNOW AMOUNTS DOWNWIND OF LAKE ONTARIO,  
THE ADIRONDACKS, AND NORTHERN NEW ENGLAND ARE POSSIBLE. MEANWHILE,  
A PLUME OF MOISTURE/ATMOSPHERIC RIVER COMING INTO THE PACIFIC  
NORTHWEST COULD BRING ANOTHER ROUND OF MODERATE TO LOCALLY HEAVY  
RAINS TO PARTS OF WESTERN WASHINGTON, WITH THE HEAVIEST TOTALS  
LIKELY ALONG FAVORABLE TERRAIN. LIGHTER PRECIPITATION SHOULD SPILL  
OVER INTO NORTHERN PARTS OF THE ROCKIES AS WELL THIS WEEKEND BUT  
QUICKLY DRY OUT EARLY NEXT WEEK. THEN ANOTHER ROUND OF  
PRECIPITATION IS FORECAST FOR THE PACIFIC NORTHWEST BY AROUND  
TUESDAY. THE REMAINDER OF THE COUNTRY SHOULD REMAIN FAIRLY DRY  
THROUGH THE WEEKEND AND INTO NEXT WEEK. A WEAK SYSTEM MOVING  
THROUGH THE OHIO VALLEY INTO THE EAST MAY BRING SOME LIGHT  
PRECIPITATION TO THE REGION BUT NOTHING LOOKS PARTICULARLY  
IMPACTFUL AT THIS TIME.  
 
A BUILDING TROUGH OVER THE EAST SHOULD KEEP TEMPERATURES BELOW  
NORMAL THROUGH THE ENTIRE FIVE DAY PERIOD WITH THE GREATEST  
ANOMALIES (-10 TO -15F) LIKELY ON FRIDAY INTO SATURDAY FROM SOUTH  
TEXAS/LOWER MISSISSIPPI VALLEY TO THE OHIO VALLEY AND  
MID-ATLANTIC. MEANWHILE, AMPLIFIED RIDGING WILL PROMOTE A WARMING  
TREND ACROSS THE WEST AND INTO THE PLAINS WITH SEVERAL DAYS OF +10  
TO +20 ANOMALIES (LOCALLY HIGHER) LIKELY, ESPECIALLY ACROSS PARTS  
OF THE NORTHERN AND CENTRAL HIGH PLAINS SUNDAY AND MONDAY.  
 
TATE/SANTORELLI  
 
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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