619  
FXUS02 KWBC 291851  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
150 PM EST MON NOV 29 2021  
 
VALID 12Z THU DEC 02 2021 - 12Z MON DEC 06 2021  
   
..OVERVIEW
 
 
THE OVERALL PATTERN WILL BE FEATURED BY AN UPPER-LEVEL RIDGE FROM  
THE EASTERN PACIFIC TO THE INTERMOUNTAIN WEST, AND QUASI-ZONAL  
FLOW TO LOW AMPLITUDE TROUGHING FROM THE PLAINS TO THE EAST COAST  
WITH MULTIPLE SHORTWAVE PERTURBATIONS PIVOTING AROUND A SYNOPTIC  
SCALE TROUGH OVER SOUTHEAST CANADA. THIS MAY ACT TO AMPLIFY THE  
EASTERN U.S. TROUGHING SLIGHTLY THROUGH THE PERIOD. THIS WILL  
INCLUDE AN ORGANIZED LOW PRESSURE SYSTEM TRACKING ACROSS  
SOUTHEASTERN CANADA THROUGH FRIDAY WITH A TRAILING FRONT FOLLOWED  
BY A WEAKER LOW THIS WEEKEND, LEADING TO ROUNDS OF MAINLY LIGHT  
PRECIPITATION ACROSS THE GREAT LAKES AND NORTHEASTERN U.S. BY THE  
WEEKEND, SHOWERS AND SOME STORMS ARE LIKELY TO DEVELOP AHEAD OF A  
DEVELOPING STORM SYSTEM FROM THE GULF COAST TO THE OHIO VALLEY.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE 00Z/06Z GUIDANCE SUITE CONTINUES TO SHOW DECENT SYNOPTIC SCALE  
AGREEMENT THROUGH FRIDAY WITH AN UPPER RIDGE OVER THE SOUTHWESTERN  
U.S. AND A BROAD TROUGH OVER THE NORTHEAST U.S. THE ONLY NOTABLE  
DIFFERENCE IS THE 06Z/12Z GFS CONTINUE TO BE MUCH SLOWER THAN THE  
REST OF THE GUIDANCE ON THURSDAY/DAY 3 WITH ORGANIZED LOW PRESSURE  
ACROSS THE GREAT LAKES, BUT EVENTUALLY CATCH UP BY DAY 4. AS THE  
FLOW PATTERN BECOMES MORE QUASI-ZONAL INTO THE WEEKEND, MODEL  
SPREAD INCREASES SOME WITH RESPECT TO THE NEXT SHORTWAVE TROUGH  
ENTERING THE NORTHWESTERN U.S., AS WELL AS A WEAK SOUTHERN STREAM  
UPPER LOW DEVELOPING OVER NORTHERN MEXICO. ENOUGH RUN-TO-RUN MODEL  
VARIATIONS CONTINUES TO SUGGEST A BELOW AVERAGE FORECAST  
CONFIDENCE FOR THE SECOND HALF OF THE PERIOD. THE CMC CONTINUES TO  
BE THE MOST PROBLEMATIC AS IT BECOMES OUT OF PHASE ALONG THE WEST  
COAST FOR DAYS 6-7. AT THE END OF THE PERIOD, MUCH OF THE GUIDANCE  
SUGGESTS AMPLIFICATION OF THE EASTERN U.S. TROUGH MAY LEAD TO LOW  
PRESSURE DEVELOPMENT NEAR THE SOUTHEAST/MID-ATLANTIC ON DAY 7, AND  
POSSIBLY TRACK NORTHWARD ALONG THE NORTHEAST COAST IMMEDIATELY  
FOLLOWING THE END OF THE MEDIUM RANGE FORECAST PERIOD (12/6 AT  
12Z). CONFIDENCE IS LOW IN THE SPECIFICS FOR IMPACTS ACROSS THE  
NORTHEAST AMIDST ENOUGH TIMING AND INTENSITY UNCERTAINTIES IN THE  
INDIVIDUAL SOLUTIONS.  
 
THE WPC FORECAST WAS PRIMARILY BASED ON THE DETERMINISTIC GUIDANCE  
BLEND THROUGH FRIDAY. THEREAFTER, INCREASED WEIGHTING TOWARDS THE  
ENSEMBLE MEANS EVENTUALLY GETTING TO A 50/50 BLEND OF ENSEMBLES  
WITH THE DETERMINISTIC ECMWF AND GFS.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
WEATHER SHOULD BE RELATIVELY QUIET IN TERMS OF RAINFALL OR SNOW  
RELATED HAZARDS WITH NOT MUCH IN THE WAY OF HEAVY PRECIPITATION  
CURRENTLY EXPECTED THROUGH SATURDAY. ROUNDS OF RELATIVELY LIGHT  
PRECIPITATION ARE FORECAST FOR THE MIDWEST, GREAT LAKES, AND  
NORTHEAST WITH A PROGRESSIVE STORM TRACK OVER THE AREA. AMOUNTS  
SHOULD BE MOST SUBSTANTIAL IN FAVORED LAKE EFFECT AND POSSIBLY  
UPSLOPE AREAS, PARTICULARLY DOWNWIND OF LAKE ONTARIO INTO THE  
ADIRONDACKS. THEN MOISTURE IS SET TO INCREASE ACROSS THE SOUTHERN  
PLAINS AND LOWER MISSISSIPPI VALLEY LATE FRIDAY AND INTO THE  
WEEKEND, WHICH COULD LEAD TO SHOWERS AND THUNDERSTORMS BECOMING  
MORE WIDESPREAD THERE AND THEN EXTENDING NORTHEASTWARD AS THE  
SURFACE LOW OVER THE GREAT LAKES BECOMES BETTER ORGANIZED. A  
SHORTWAVE INTO THE NORTHWEST EARLY NEXT WEEK COULD BRING ANOTHER  
ROUND OF MODERATE TO LOCALLY HEAVY RAIN AND MOUNTAIN SNOWS TO  
PARTS OF WESTERN WASHINGTON AND OREGON.  
 
TEMPERATURES ARE EXPECTED TO BE ABOVE NORMAL FOR THIS TIME OF YEAR  
ACROSS MOST OF THE NATION THROUGH NEXT WEEKEND, WITH THE GREATEST  
POSITIVE ANOMALIES EXPECTED FOR THE CENTRAL AND NORTHERN PLAINS  
WHERE BOTH HIGHS AND LOWS ARE FORECAST TO BE 15 TO 25+ DEGREES  
ABOVE AVERAGE FOR LATE IN THE WEEK. A FEW LOCATIONS MAY APPROACH  
DAILY MAXIMUM AND MINIMUM TEMPERATURE RECORDS WITH THESE MILD  
TEMPERATURES. BY THE WEEKEND, TEMPERATURES ARE CURRENTLY FORECAST  
TO MODERATE SOMEWHAT, BUT THIS IS SUBJECT TO CHANGE GIVEN FORECAST  
UNCERTAINTIES. NO ARCTIC AIRMASS INTRUSIONS ARE EXPECTED DURING  
THIS FORECAST PERIOD.  
 
SANTORELLI/HAMRICK  
 
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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