079  
FXUS02 KWBC 041900  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
200 PM EST SAT DEC 04 2021  
 
VALID 12Z TUE DEC 07 2021 - 12Z SAT DEC 11 2021  
 
...NOTABLE SNOW POSSIBLE IN THE INTERIOR NORTHEAST AROUND  
WEDNESDAY, WITH WINTRY IMPACTS POTENTIALLY REACHING SOUTHWARD INTO  
THE I-95 CORRIDOR/NORTHERN MID-ATLANTIC, 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
 
 
LATEST GUIDANCE MAINTAINS A BROAD MEAN TROUGH ALOFT 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 MORE ACTIVE WEATHER PATTERN THAN HAS BEEN  
THE CASE LATELY. IN PARTICULAR, PARTS OF THE WEST SHOULD SEE  
MEANINGFUL PRECIPITATION--POSSIBLY EXTENDING INTO PARTS OF THE  
PLAINS LATE IN THE WEEK--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
 
 
EMBEDDED SHORTWAVE UNCERTAINTIES CONTINUE TO LOWER CONFIDENCE IN  
SOME IMPORTANT DETAILS WITHIN A MUCH MORE AGREEABLE MEAN PATTERN.  
IN TERMS OF THE COMBINED GUIDANCE SPREAD AND ASSOCIATED IMPACTS OF  
THE VARIED SOLUTIONS, ONE OF THE GREATEST ISSUES IN LATEST CYCLES  
HAS BEEN WITH THE ULTIMATE EVOLUTION OF SHORTWAVE ENERGY THAT  
PROGRESSES FROM THE NORTHERN PLAINS/WESTERN U.S. AS OF EARLY  
TUESDAY INTO THE EAST BY LATE WEDNESDAY/EARLY THURSDAY. THERE IS A  
CLEAR SIGNAL FOR THE SHORTWAVE'S EXISTENCE BUT SUBTLE  
LOW-PREDICTABILITY DIFFERENCES LEAD TO VERY DIFFERENT SURFACE  
SOLUTIONS FOR LOW PRESSURE THAT TRACKS OFF THE EAST COAST ALONG  
WITH ASSOCIATED COVERAGE/INTENSITY OF PRECIPITATION AND LOCATION  
OF THE RAIN-SNOW LINE. LATEST GFS RUNS HAVE BEEN ON THE STRONG  
SIDE OF THE SPREAD WITH THE SHORTWAVE AND ON THE DEEP/WEST SIDE AT  
THE SURFACE BY EARLY THURSDAY WHILE THE 00Z ECMWF/ECMWF MEAN ARE  
ON THE WEAK/EASTERN SIDE. LOW CONFIDENCE WITH FINE DETAILS FOR  
THIS FEATURE WOULD FAVOR AN INTERMEDIATE SOLUTION FOR STRENGTH AND  
TRACK, CLOSEST TO A DEEPER VERSION OF THE 06Z GEFS MEAN.  
 
ONE UPSTREAM UNCERTAINTY INVOLVES A BUNDLE OF ENERGY THAT  
ORIGINALLY COMES FROM AN UPPER LOW OVER THE MID-LOWER LATITUDES OF  
THE PACIFIC, WITH DIFFERENCES OVER WHETHER IT DROPS AROUND THE  
EAST/SOUTHEAST SIDE OF THE PACIFIC UPPER RIDGE (ECMWF/UKMET/CMC)  
OR PROGRESSES ALONG WITHIN THE LOWER 48 MEAN TROUGH (GFS). THERE  
IS ALSO SOME SPREAD FOR DETAILS OF NORTHERN STREAM PACIFIC ENERGY  
THAT SHOULD REACH WESTERN NORTH AMERICA BY WEDNESDAY AND SUPPORT  
SOUTHERN CANADA LOW PRESSURE WITH TRAILING FRONT THAT CROSSES THE  
WESTERN/CENTRAL U.S. AT LEAST RELATIVE TO TYPICAL BEHAVIOR FOR 6-7  
DAYS OUT IN TIME, MODELS AND ENSEMBLE MEANS ARE GENERALLY BETTER  
BEHAVED THAN AVERAGE FOR THE UPPER TROUGH THAT SHOULD DIG INTO THE  
WEST LATE IN THE WEEK AND THEN PUSH INTO THE PLAINS. HOWEVER  
INDIVIDUAL RUNS/ENSEMBLE MEMBERS DO STILL VARY CONSIDERABLY FOR  
EXACTLY HOW ENERGY WILL BE DISTRIBUTED WITHIN THE OVERALL TROUGH  
AND BY NEXT SATURDAY FOR DETAILS OF CORRESPONDING LOW PRESSURE  
ALONG A MEAN FRONT WITHIN A CORRIDOR BETWEEN THE GREAT LAKES AND  
SOUTHERN PLAINS.  
 
GUIDANCE COMPARISONS/PREFERENCES LED TO STARTING THE FORECAST WITH  
A 00Z/06Z OPERATIONAL MODEL COMPOSITE FOR DAYS 3-4  
TUESDAY-WEDNESDAY, FOLLOWED BY SOMEWHAT EARLIER INPUT OF 06Z  
GEFS/00Z ECMWF MEANS THAN AVERAGE BY THURSDAY (30 PERCENT TOTAL)  
AND THEN 50-60 PERCENT MEANS FOR FRIDAY-SATURDAY.  
   
..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 PRODUCE SNOW ACROSS THE NORTHERN LATITUDES, WITH THE  
HIGHEST PROBABILITIES OF MEANINGFUL 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.  
 
EXPECT FAIRLY LIGHT PRECIPITATION OVER THE PACIFIC NORTHWEST AND  
THE FOUR CORNERS STATES ON TUESDAY, BEFORE PRECIPITATION CHANCES  
AND AMOUNTS 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. FAVORED TERRAIN IN ARIZONA COULD ALSO SEE  
SOME ENHANCED ACTIVITY. 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 THE FIRST RAINFALL IN OVER A MONTH.  
THEN AS THE WESTERN UPPER TROUGH APPROACHES/PUSHES INTO THE PLAINS  
TOWARD THE END OF THE WEEK, RAINFALL MAY BECOME MODERATE TO HEAVY  
OVER PARTS OF THE EAST-CENTRAL U.S. BY NEXT SATURDAY WITH THE AID  
OF STRENGTHENING GULF INFLOW AHEAD OF A WAVY FRONT. THE CHARACTER  
OF ONE OR MORE FRONTAL WAVES WILL DETERMINE PRECIPITATION AMOUNTS  
AND POSSIBLE WINTER WEATHER IMPACTS OVER THE PLAINS AND MIDWEST.  
 
CHILLY TEMPERATURES ARE LIKELY ACROSS THE FAR NORTH-CENTRAL U.S.  
INTO THE GREAT LAKES ON TUESDAY GIVEN SNOW COVER FROM  
SUNDAY-MONDAY'S SYSTEM, WITH TEMPERATURES 10-20F BELOW NORMAL.  
HIGHS OF 10F OR SO BELOW NORMAL COULD REACH AS FAR SOUTH AS THE  
OHIO VALLEY. THE NEGATIVE ANOMALIES WILL MODERATE AS THE AIRMASS  
CONTINUES INTO THE NORTHEAST U.S. ON WEDNESDAY. FARTHER UPSTREAM,  
THE WEST WILL SEE MOSTLY ABOVE NORMAL READINGS THROUGH MIDWEEK  
(SOME POCKETS OF PLUS 10-15F ANOMALIES) WITH MORE PRONOUNCED  
WARMING LIKELY OVER THE PLAINS ON THURSDAY AND PROGRESSING INTO  
THE EAST BY NEXT WEEKEND. THERE SHOULD BE FAIRLY BROAD COVERAGE OF  
PLUS 10-20F ANOMALIES FOR HIGHS/MORNING LOWS, AND IN ADDITION  
HIGHS OVER PARTS OF THE SOUTHERN PLAINS ON FRIDAY PLUS MORNING  
LOWS FROM THE MISSISSIPPI VALLEY NORTHEASTWARD ON SATURDAY MAY  
REACH 20-25F ABOVE NORMAL. A FEW PLACES IN THE FAR SOUTHERN TIER  
(PRIMARILY TEXAS) COULD APPROACH DAILY RECORD HIGHS. COOLER  
TEMPERATURES SHOULD SPREAD INTO THE WEST AND EVENTUALLY HIGH  
PLAINS LATE IN THE WEEK (HIGHS 5-15F OR SO BELOW NORMAL BY FRIDAY  
AND SATURDAY) UNDER AN AMPLIFYING UPPER TROUGH.  
 
RAUSCH/TATE  
 
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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