689  
FXUS02 KWBC 181821  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
120 PM EST SAT DEC 18 2021  
 
VALID 12Z TUE DEC 21 2021 - 12Z SAT DEC 25 2021  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
FROM TUESDAY INTO CHRISTMAS DAY/SATURDAY, MOST GUIDANCE CONTINUES  
TO SHOW THAT IMPULSES ROUNDING AN IMPRESSIVELY STRONG CENTRAL  
PACIFIC UPPER RIDGE SHOULD PROVIDE PERIODIC REINFORCEMENT OF A  
DEEP UPPER TROUGH ALIGNED JUST OFF THE WEST COAST. THIS TROUGH  
WILL LIKELY CONTAIN ONE OR MORE UPPER LOWS, AND A PERSISTENT FLOW  
OF MOISTURE AHEAD OF THE TROUGH AXIS WILL LIKELY PRODUCE A  
MULTI-DAY PERIOD OF SIGNIFICANT PRECIPITATION OVER PARTS OF THE  
WEST COAST (NORTHERN/CENTRAL CALIFORNIA IN PARTICULAR) WITH  
MEANINGFUL AMOUNTS ALSO EXTENDING INTO THE ROCKIES. SOUTHERN  
CANADA/NORTHERN TIER U.S. FLOW WILL CARRY ALONG A COUPLE WAVES  
WITH MODEST PRECIPITATION BEFORE LOWER 48 FLOW BECOMES MORE  
DOMINATED BY WHAT ENERGY MAY EJECT FROM THE EASTERN PACIFIC  
TROUGH. FARTHER SOUTH, A SOUTHERN STREAM SYSTEM TRACKING FROM  
NORTHERN FLORIDA OR VICINITY INTO THE ATLANTIC MAY BRING SOME RAIN  
TO THE SOUTHEAST EARLY IN THE FORECAST PERIOD. EXPECT A BROADENING  
AREA OF ABOVE NORMAL TEMPERATURES OVER THE SOUTHERN TWO-THIRDS OF  
THE CENTRAL U.S. WHILE THE EXTREME NORTHERN PLAINS WILL TEND TO  
SEE BELOW NORMAL TEMPERATURES.  
 
THE EASTERN PACIFIC UPPER TROUGH DOES SHOW BETTER AGREEMENT ON THE  
LARGE SCALE COMPARED TO PREVIOUS DAYS, BUT DAY TO DAY DETAILS  
REGARDING FEATURE INTERACTIONS AND TIMING CONTINUE TO BE  
PROBLEMATIC, ESPECIALLY BEYOND DAY 4. THE 00Z CMC AT THIS TIME IS  
THE OUTLIER AS IT SEPARATES ENERGY DROPPING SOUTH FROM THE GULF OF  
ALASKA AND ACCELERATES IT MUCH FASTER INTO THE WESTERN U.S. THAN  
THE OTHER MODELS. THE 00Z ECMWF IS ALSO A LITTLE FASTER WITH THE  
TROUGHING (MORE PHASED) WHILE THE 06Z GFS IS THE MOST AMPLIFIED  
AND THUS SLOWEST. THE ENSEMBLE MEANS WOULD FAVOR A SOLUTION MORE  
IN BETWEEN THE GFS AND ECMWF WHICH SEEMS REASONABLE AT THIS TIME.  
 
TROUBLE CONTINUES FURTHER DOWNSTREAM WITH THE DETAILS OF  
INDIVIDUAL WAVES AND FRONTS MOVING THROUGH THE NORTHERN PLAINS AND  
UPPER GREAT LAKES, THE DYNAMICS OF SOME LIKELY DEPENDENT ON ENERGY  
EJECTING FROM THE EASTERN PACIFIC UPPER TROUGH. MEANWHILE, LARGE  
SCALE AGREEMENT IS DECENT FOR THE SOUTHERN STREAM LOW MOVING  
ACROSS NORTHERN FLORIDA OR THE SOUTHEAST INTO THE WESTERN ATLANTIC  
WITH SOME TIMING AND STRUCTURE QUESTIONS REMAINING.  
 
THE UPDATED WPC MEDIUM RANGE PROGS FOR TODAY USED A BLEND OF THE  
OPERATIONAL MODELS (WITH MORE EMPHASIS TOWARDS THE GFS/ECMWF OVER  
THE CMC/UKMET) FOR DAYS 3-4. AFTER THAT, INCREASINGLY ADDED MORE  
WEIGHTING TO THE ENSEMBLE MEANS, ELIMINATING THE CMC DUE TO  
PROBLEMS DISCUSSED ABOVE. KEPT SOME DEGREE OF ECMWF AND GFS JUST  
FOR A LITTLE ADDED SYSTEM DEFINITION. THIS ALLOWED FOR FAIRLY GOOD  
CONTINUITY WITH THE PREVIOUS SHIFT WITH NO MAJOR CHANGES OR  
ADJUSTMENTS NEEDED.  
 
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
EXPECT THE WESTERN U.S. TO SEE A BROADENING AREA OF RAIN AND  
MOUNTAIN SNOW DOWNSTREAM FROM THE PERSISTENT UPPER TROUGH JUST  
OFFSHORE THE WEST COAST, WITH SOME OF THIS ACTIVITY LIKELY TO BE  
MODERATE TO HEAVY. THERE ARE STILL QUESTION MARKS FOR SOME OF THE  
DETAILS DUE TO UNCERTAINTY OVER INDIVIDUAL SYSTEMS AND THEIR  
INTERACTION, BUT GUIDANCE SIGNALS AND TELECONNECTIONS FOR THE MEAN  
PATTERN ARE CONSISTENT IN SHOWING NORTHERN AND CENTRAL CALIFORNIA  
AS HAVING THE HIGHEST PRECIPITATION POTENTIAL THROUGH THE  
PERIOD--INCLUDING HEAVY SNOW OVER SIERRA NEVADA/SHASTA AREAS.  
THIS REGION SHOULD SEE THE MOST CONSISTENT MOISTURE FLOW AND  
EXPERIENCE SOUTHERN/WESTERN FACING TERRAIN ENHANCEMENT. IT MAY  
TAKE INTO THE SHORTER-RANGE TIME FRAME TO ASSES THE LOCATION AND  
INTENSITY OF LOWER ELEVATION RAINFALL WITH RESPECT TO LOCAL  
RUNOFF/FLOODING ISSUES. EFFECTS FROM THE MULTI-DAY ACCUMULATION  
OF PRECIPITATION WILL ALSO REQUIRE MONITORING EVEN IF IT IS NOT  
INTENSE ON ANY PARTICULAR DAY. SOME MOISTURE MAY EXTEND AS FAR  
SOUTH AS SOUTHERN CALIFORNIA AND ARIZONA BY MID-LATE WEEK, WHILE  
THE INTERIOR WEST AND ROCKIES SHOULD SEE MEANINGFUL PRECIPITATION  
AS WELL.  
 
LOW PRESSURE TRACKING FROM OVER OR NEAR NORTHERN FLORIDA INTO THE  
ATLANTIC WILL SPREAD RAIN ACROSS PORTIONS OF THE SOUTHEAST ON  
TUESDAY AND POSSIBLY INTO WEDNESDAY. SOME LOCATIONS OVER THE  
SOUTHEAST/FLORIDA COULD SEE MODERATE TO LOCALLY HEAVY RAINFALL.  
SOME UNCERTAINTY PERSISTS FOR THE INTENSITY AND NORTHWARD EXTENT  
OF RAINFALL THOUGH LATEST TRENDS ARE INCREASING THE POTENTIAL FOR  
MEANINGFUL TOTALS OVER PARTS OF THE EXTREME SOUTHEAST. A LEADING  
NORTHERN TIER SYSTEM SHOULD SPREAD MOSTLY LIGHT SNOW FROM THE  
EXTREME NORTHERN PLAINS EASTWARD THROUGH THE GREAT LAKES AND NEW  
ENGLAND DURING TUESDAY-WEDNESDAY. THE SYSTEM'S ACCOMPANYING FRONT  
SHOULD BE RELATIVELY DRY. ANY TRAILING FEATURES COULD PRODUCE  
SOME PRECIPITATION BUT AGAIN LIKELY WITH FAIRLY LIGHT AMOUNTS.  
 
 
ABOVE NORMAL TEMPERATURES FOCUSED OVER THE CENTRAL/SOUTH-CENTRAL  
HIGH PLAINS ON TUESDAY WILL STEADILY EXPAND EASTWARD ACROSS THE  
SOUTHERN TWO-THIRDS OF THE CENTRAL U.S. WITH TIME AND GRADUALLY  
BECOME MORE ANOMALOUS. THROUGH MIDWEEK HIGHS IN THIS REGION WILL  
TEND TO BE 10-20F OR SLIGHTLY MORE ABOVE NORMAL. TOWARD THE END  
OF THE WEEK THERE SHOULD BE MORE LOCATIONS WITH HIGHS 20-30F ABOVE  
NORMAL AND SOME RECORD HIGHS COULD BE CHALLENGED OVER THE SOUTHERN  
PLAINS BY CHRISTMAS DAY/SATURDAY. ON THE OTHER EXTREME, THE FAR  
NORTHERN PLAINS SHOULD SEE BELOW NORMAL TEMPERATURES EXCEPT FOR  
PERHAPS BEING NEAR NORMAL WEDNESDAY IN BETWEEN SYSTEMS. A  
STRONGER PUSH OF COLD AIR MAY REACH MONTANA AND THE DAKOTAS NEXT  
WEEKEND. THE UNSETTLED PATTERN OVER THE WEST SHOULD LEAD TO NEAR  
OR ABOVE NORMAL LOWS AND NEAR TO BELOW NORMAL HIGHS THROUGH THE  
ENTIRE WEEK.  
 
SANTORELLI/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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