442  
FXUS02 KWBC 092051  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
350 PM EST THU DEC 09 2021  
 
VALID 12Z SUN DEC 12 2021 - 12Z THU DEC 16 2021  
 
...AN ATMOSPHERIC RIVER WILL SPREAD SOUTHWARD ALONG THE WEST COAST  
EARLY NEXT WEEK BRINGING COASTAL RAIN AND HEAVY MOUNTAIN SNOW...  
 
...RECORD WARMTH LIKELY FOR MUCH OF THE CENTRAL PORTION OF THE  
LOWER 48...  
 
   
..OVERVIEW
 
 
UPPER TROUGHING IN THE EAST ON SUNDAY WILL BE REPLACED BY  
BURGEONING UPPER RIDGING AROUND MIDWEEK AS LEAD AND SUCCESSIVE  
TROUGHING DIGS THROUGH THE WEST. THIS WILL PROMOTE INCREASING AND  
WIDESPREAD PRECIPITATION THROUGH MUCH OF THE WEST, FIRST ALONG THE  
COAST THEN PROGRESSING SOUTHWARD AND EASTWARD, AS A MODEST  
ATMOSPHERIC RIVER EVENT ENSUES. TO THE EAST, STRONG UPPER RIDGING  
AND SOUTHERLY FLOW AHEAD OF THE WESTERN SYSTEM WILL BRING IN WELL  
ABOVE NORMAL TEMPERATURES TO THE CENTRAL STATES WITH DAILY RECORD  
HIGHS LIKELY FOR MANY LOCATIONS. AS THE WESTERN SYSTEM MOVES  
NORTHEASTWARD THROUGH THE NORTHERN PLAINS NEXT WED-THU, WINTRY  
PRECIPITATION MAY SPREAD ACROSS THE HIGH PLAINS ALONG THE NORTH  
SIDE OF THE SURFACE LOW TRACK.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THROUGH THE 00Z/06Z CYCLE, THE DETERMINISTIC MODELS SHOWED VERY  
GOOD AGREEMENT OVERALL WITH THE PATTERN EVOLUTION OF THE WESTERN  
TROUGH AND DOWNSTREAM RIDGE, WITH EXPECTED DETAIL DIFFERENCES. BY  
NEXT TUE-THU, UPSTREAM FLOW FROM THE GULF OF ALASKA MAY SPAWN A  
TRAILING UPPER TROUGH THAT COULD INFLUENCE WHERE ANOTHER ROUND OF  
PRECIPITATION MAY IMPACT THE WEST (LOTS OF NORTH-SOUTH SPREAD),  
AND OPTED TO TREND TOWARD THE GEFS MEAN AND ECMWF ENSEMBLE MEAN  
WHILE FAVORING THE ECMWF AND CANADIAN FOR SOME ADDED DETAILS. THE  
GFS WAS ON THE SOUTHERN END OF THE SPREAD BUT REMAINED A PLAUSIBLE  
SOLUTION. TREND HAS BEEN A BIT SLOWER BY NEXT WED-THU WITH THE  
TROUGH AND RESULTANT AREA OF LOW PRESSURE MOVING THROUGH THE  
PLAINS TO THE UPPER MIDWEST, DEPENDENT ON THE SHARPNESS OF THE  
UPPER PATTERN. THIS MAY SUPPORT A MODEST AREA OF LOW PRESSURE WITH  
SNOW ON THE NORTHERN SIDE OF THE LOW BUT RAIN FOR MANY AREAS EVEN  
INTO THE GREAT LAKES WITH MILD TEMPERATURES.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
MOIST ONSHORE FLOW WITH A WEAK ATMOSPHERIC RIVER WILL CAUSE  
WIDESPREAD PRECIPITATION IN THE WESTERN U.S. THROUGH THE FIRST  
HALF OF NEXT WEEK. PRECIPITATION IS FORECAST FOR THE PACIFIC  
NORTHWEST, NORTHERN ROCKIES, AND ESPECIALLY NORTHERN CALIFORNIA ON  
SUNDAY, WHICH WILL SPREAD SOUTHWARD INTO SOUTHERN CALIFORNIA  
MONDAY AND EASTWARD INTO THE GREAT BASIN AND FOUR CORNERS STATES  
ON TUESDAY. ENHANCED TOTALS ARE LIKELY ALONG FAVORED TERRAIN, WITH  
HEAVY MOUNTAIN SNOW ESPECIALLY IN THE SIERRA NEVADA AND PORTIONS  
OF THE ROCKIES. COASTAL RAIN IS EXPECTED IN CALIFORNIA, EVEN DOWN  
INTO SOUTHERN CA, AND SOME LOCALLY HIGHER RAINFALL RATES MAY CAUSE  
SOME FLOODING ISSUES, ESPECIALLY FOR SENSITIVE BURN SCAR AREAS.  
APPRECIABLE RAINFALL AND MOUNTAIN SNOW IS ALSO LIKELY INTO THE  
GREAT BASIN AND CENTRAL ROCKIES AS THE SYSTEM MOVES EASTWARD.  
TEMPERATURES WILL BE NEAR TO SOMEWHAT BELOW NORMAL IN THE WEST  
UNDERNEATH THE UPPER TROUGH, WITH TEMPERATURES 10-20F BELOW  
AVERAGE FOR PARTS OF CALIFORNIA AND NEVADA.  
 
FARTHER EAST, INCREASING WARMTH IN THE CENTRAL AND EASTERN U.S.  
VIA STRONG RIDGING ALOFT WILL LEAD TO TEMPERATURES ABOUT 10-20F  
ABOVE AVERAGE ON SUNDAY BUT INCREASING TO 20-35F ABOVE NORMAL BY  
WEDNESDAY. PARTS OF THE PLAINS/MISSISSIPPI VALLEY COULD SEE HIGHS  
INTO THE 70S WHICH WILL LIKELY BREAK DAILY RECORDS FOR MULTIPLE  
LOCATIONS. RECORD TEMPERATURES WILL BE POSSIBLE/LIKELY FROM THE  
SOUTHERN PLAINS TO GREAT LAKES REGION AHEAD OF THE FRONT, WITH  
TEMPERATURES 10-20F ABOVE AVERAGE FORECAST TO EXPAND INTO THE  
EAST. ANY WIDESPREAD PRECIPITATION IS NOT LIKELY IN THE  
CENTRAL/EASTERN U.S. UNTIL NEXT WEDNESDAY AS THE WESTERN SYSTEM  
LIFTS TOWARD/INTO CANADA.  
 
FRACASSO/TATE  
 
HAZARDS:  
- HEAVY PRECIPITATION FROM COASTAL WASHINGTON SOUTHWARD INTO THE  
NORTHERN HALF OF CALIFORNIA, SUN-MON, DEC 12-DEC 13.  
- HEAVY PRECIPITATION ACROSS A LARGE PORTION OF CALIFORNIA  
INCLUDING THE SIERRA NEVADA, AS WELL AS PORTIONS OF NEVADA AND  
UTAH, AND PARTS OF SOUTHWESTERN COLORADO, TUE-WED, DEC 14-DEC 15.  
- HEAVY SNOW ACROSS PORTIONS OF THE HIGHER ELEVATIONS IN IDAHO AND  
NORTHEASTERN OREGON, SUN, DEC 12.  
- HEAVY SNOW ACROSS PORTIONS OF THE NORTHERN ROCKIES, TUE, DEC 14.  
- HEAVY SNOW ACROSS PORTIONS OF THE CENTRAL ROCKIES, WED, DEC 15.  
- HIGH WINDS OVER PORTIONS OF THE NORTHERN AND CENTRAL ROCKIES,  
AND ACROSS THE NORTHERN PLAINS INTO THE UPPER MIDWEST, WED-THU,  
DEC 15-DEC 16.  
- HIGH WINDS ACROSS PORTIONS OF EASTERN WYOMING, SUN, DEC 12.  
- MUCH ABOVE NORMAL TEMPERATURES ACROSS PORTIONS OF THE CENTRAL  
PLAINS TO THE UPPER MIDWEST, TUE-WED, DEC 14-DEC 15.  
 
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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