222  
FXUS02 KWBC 040710  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
210 AM EST SAT DEC 04 2021  
 
VALID 12Z TUE DEC 07 2021 - 12Z SAT DEC 11 2021  
 
...NOTABLE SNOW IN THE INTERIOR NORTHEAST IS CURRENTLY EXPECTED  
WEDNESDAY, WITH WINTRY IMPACTS POTENTIALLY REACHING SOUTHWARD INTO  
THE I-95 CORRIDOR AND POSSIBLY THE MID-ATLANTIC STATES, WHILE  
LOCALLY HEAVY RAIN MAY OCCUR OVER THE SOUTHEAST...  
 
...CONSIDERABLE MOUNTAIN SNOWS ARE LIKELY ACROSS THE CASCADES,  
SIERRA NEVADA, INTERMOUNTAIN WEST, AND ROCKIES DURING THE LATTER  
HALF OF NEXT WEEK...  
   
..OVERVIEW
 
 
EXPECT A BROAD MEAN TROUGH ALOFT TO PREVAIL ACROSS MUCH OF THE  
LOWER 48 NEXT WEEK, WITH AN AXIS GENERALLY EXTENDING FROM  
CENTRAL/EAST-CENTRAL CANADA INTO THE WESTERN U.S. EMBEDDED  
SHORTWAVES WILL PUSH LOW PRESSURE/FRONTAL SYSTEMS ACROSS THE  
COUNTRY, LEADING TO A SOMEWHAT MORE ACTIVE WEATHER PATTERN THAN  
HAS BEEN THE CASE LATELY. IN PARTICULAR, PARTS OF THE WEST SHOULD  
SEE MEANINGFUL PRECIPITATION WHILE THE INTENSITY/COVERAGE OF  
RAINFALL SHOULD INCREASE OVER THE SOUTHEASTERN QUADRANT OF THE  
COUNTRY AND SOME IMPACTFUL SNOW MAY FALL OVER NORTHERN PARTS OF  
THE EAST.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
AFTER MULTIPLE DAYS OF GFS RUNS HAD BEEN SIGNIFICANTLY STRONGER  
WITH SHORTWAVE ENERGY COMING INTO THE WEST DAY 3/TUESDAY, THE 18Z  
AND 00Z GFS HAVE COME IN WEAKER WITH THE FEATURE, MORE IN LINE  
WITH OTHER GUIDANCE. SO THERE SEEMS TO BE AN INCREASING CONSENSUS  
WITH THIS SHORTWAVE, BUT UNCERTAINTY IS STILL SOMEWHAT HIGH GIVEN  
THE ENERGY STEMS FROM THE LESS PREDICTABLE HIGH LATITUDES. THE  
UNCERTAINTY IN THE DETAILS OF THE FORECAST CONTINUES WEDNESDAY  
ONWARD DESPITE REASONABLE CONSENSUS FOR THE OVERALL PATTERN.  
DIFFERENCES IN THE HANDLING OF THIS SHORTWAVE ENERGY AS WELL AS  
NORTHERN STREAM/GREAT LAKES ENERGY LEAD TO EVOLUTION AND  
TRACK/TIMING UNCERTAINTY FOR AN AREA OF LOW PRESSURE FROM THE  
EASTERN U.S. INTO THE WESTERN ATLANTIC WEDNESDAY-THURSDAY. OVERALL  
THE LOW PRESSURE SYSTEM CONTINUED ITS OVERALL TREND MORE  
SUPPRESSED, BUT WITH CONSIDERABLE VARIABILITY IN ITS TRACK AND  
TIMING WITH THE 12/18Z MODEL CYCLE AND THE INCOMING 00Z CYCLE,  
WHICH LEADS TO UNCERTAINTY WITH PRECIPITATION AMOUNTS ACROSS THE  
MID-ATLANTIC AND NORTHEAST.  
 
OVERALL THERE IS FAIRLY GOOD AGREEMENT ON ANOTHER BUNDLE OF  
PACIFIC ENERGY COMING INTO NORTH AMERICA WEDNESDAY, WITH LESS  
AMPLIFICATION THAN ITS PREDECESSOR, PRODUCING SOUTHERN CANADA LOW  
PRESSURE THAT ANCHORS A FRONT REACHING INTO THE WEST/PLAINS/GREAT  
LAKES. THE 12Z ECMWF APPEARS TO BE THE OUTLIER WITH THIS SURFACE  
LOW, FASTER/FARTHER EAST COMPARED TO CONSENSUS AND THE PREVIOUS  
MODEL CYCLES. THEN, ENERGY IS YET AGAIN EXPECTED TO DIG SOUTHWARD  
AND AMPLIFY TROUGHING IN THE WEST BY FRIDAY, THOUGH THE DEGREE OF  
AMPLIFICATION DIFFERS AMONG GUIDANCE, WITH THE 12Z CMC DEPTH A BIT  
ON THE MORE EXTREME SIDE.  
 
MODEL PREFERENCES LED TO STARTING WITH A COMPOSITE OF 12Z/18Z  
OPERATIONAL MODELS FOR ABOUT THE FIRST HALF OF THE PERIOD. THEN  
THE FORECAST BLEND TRANSITIONED TO A BLEND OF THE OPERATIONAL  
12/18Z GFS AND 12Z ECMWF AND THE 18Z GEFS/12Z ECMWF MEAN.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
GULF MOISTURE SHOULD INCREASE BY TUESDAY INTO WEDNESDAY AHEAD OF  
FRONTAL SYSTEMS AND CAUSE MODERATE TO POSSIBLY LOCALLY HEAVY  
RAINFALL FROM THE LOWER MISSISSIPPI VALLEY INTO THE TENNESSEE  
VALLEY AND SOUTHEAST. MOISTURE SPREADING FARTHER NORTH INTO COOLER  
AIR SHOULD LEAD TO SNOW ACROSS THE NORTHERN LATITUDES, WITH THE  
HIGHEST PROBABILITIES OF NOTABLE SNOW ACROSS THE NORTHEAST. THERE  
IS STILL CONSIDERABLE UNCERTAINTY OVER HOW LOW PRESSURE FROM THE  
EASTERN U.S. INTO THE WESTERN ATLANTIC WILL EVOLVE/TRACK, SO  
CONFIDENCE IS FAIRLY LOW FOR EXACTLY HOW MUCH PRECIPITATION MAY  
FALL OVER THE NORTHEAST, AND HOW FAR SOUTH SIGNIFICANT SNOW  
REACHES. SOME RAIN COULD CONTINUE LATE NEXT WEEK NEAR A WAVY FRONT  
LINGERING OVER THE EXTREME SOUTHEAST.  
 
FAIRLY LIGHT PRECIPITATION IS FORECAST FOR THE PACIFIC NORTHWEST  
AND THE FOUR CORNERS STATES ON TUESDAY, BEFORE PRECIPITATION  
CHANCES INCREASE FROM MIDWEEK ONWARD. AS SHORTWAVE ENERGY AND  
TROUGHING DROP SOUTHWARD THROUGH THE WEST, PRECIPITATION WILL  
SPREAD FROM THE PACIFIC NORTHWEST AND NORTHERN ROCKIES WEDNESDAY  
SOUTHWARD TO CALIFORNIA INTO THE GREAT BASIN TO CENTRAL ROCKIES  
THURSDAY AND FRIDAY. GENERALLY EXPECT HIGHER ELEVATION SNOW AND  
LOWER ELEVATION RAIN, WITH SNOW LEVELS LIKELY TO DECLINE LATER IN  
THE WEEK AS UPPER TROUGHING AMPLIFIES OVER THE WEST. PARTS OF  
SOUTHERN CALIFORNIA MAY SEE ITS FIRST RAINFALL IN OVER A MONTH.  
 
CHILLY TEMPERATURES ARE LIKELY ACROSS THE FAR NORTH-CENTRAL U.S.  
ON TUESDAY GIVEN SNOW COVER FROM SUNDAY-MONDAY'S SYSTEM, WITH  
TEMPERATURES 10-20F BELOW NORMAL. OTHER THAN THAT, TEMPERATURES  
SHOULD BE WITHIN ABOUT 10 DEGREES OF NORMAL IN MOST PLACES ACROSS  
THE LOWER 48 TUESDAY AND WEDNESDAY BEFORE A WARMUP BY THURSDAY  
THROUGH SATURDAY FROM THE PLAINS EASTWARD, WITH 10-20F ABOVE  
AVERAGE TEMPERATURES. COOLER TEMPERATURES MAY SPREAD INTO THE WEST  
LATE IN THE WEEK (HIGHS 5-15F OR SO BELOW NORMAL BY FRIDAY AND  
SATURDAY) WITH THE POTENTIAL FOR UPPER TROUGHING TO AMPLIFY OVER  
THE REGION.  
 
TATE/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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