362  
FXUS02 KWBC 021453  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
953 AM EST SAT DEC 02 2017  
 
VALID 12Z TUE DEC 05 2017 - 12Z SAT DEC 09 2017  
   
..CANADIAN COLD TO VISIT THE CENTRAL/EASTERN US NEXT WEEK
 
 
15 UTC UPDATE...  
 
MODEL SPREAD HAS REDUCED SUBSTANTIALLY COMPARED TO THIS TIME  
YESTERDAY, WITH FORECAST CONFIDENCE RELATIVELY HIGH DURING THE  
FIRST COUPLE DAYS OF THE MEDIUM RANGE. DEEP CYCLONE ACROSS THE  
UPPER GREAT LAKES TUE MORNING EXPECTED TO LIFT NORTH-NORTHEASTWARD  
INTO CANADA THROUGH DAYS 3-5. CONFIDENCE IS HIGH ON INITIAL POLAR  
FRONT SURGING THROUGH THE CENTRAL/EASTERN U.S. IN THE WAKE OF THIS  
CYCLONE, WITH SECONDARY ARCTIC FRONT FOLLOWING QUICKLY ON ITS  
HEELS. BUILDING RIDGE ALONG THE WEST COAST WILL KEEP THE PATTERN  
AMPLIFIED, WITH DEEP TROUGHING EXPECTED TO PERSIST ACROSS THE EAST  
INTO NEXT WEEKEND. SHORTWAVE ENERGY ORIGINATING IN THE ARCTIC  
QUICKLY DIVING SOUTHWARD AND AMPLIFYING INTO THE TROUGH WILL  
LIKELY RESULT IN THE POTENTIAL FOR A FAST-MOVING CLIPPER SYSTEM TO  
CROSS THE CENTRAL/EASTERN STATES THU-SAT, ALTHOUGH PREDICTABILITY  
WITH SUCH FEATURES AT DAYS 5-7, AND THUS CONFIDENCE IN THE  
DETAILS, REMAINS LOW AT THIS TIME. THE GFS AND ECMWF CONTINUE TO  
HINT AT SOME POTENTIAL FOR A WAVE ALONG THE FRONT OFF THE EAST  
COAST BY LATE NEXT WEEK, WITH THE 06Z GFS BRINGING THE WAVE CLOSE  
ENOUGH TO THE COAST TO SPREAD SOME PRECIPITATION ACROSS COASTAL  
AREAS OF THE MID-ATLANTIC/NORTHEAST. THE ECMWF LIES WITHIN A  
RATHER MODEST MODEL/ENSEMBLE MEMBER CONSENSUS, KEEPING ANY LOW  
FARTHER OFFSHORE WITH LESS IMPACTS EXCEPT PERHAPS SOME LIGHT SNOW  
FOR EASTERN NEW ENGLAND. THE WPC FORECAST WAS BASED HEAVILY ON A  
BLEND OF THE 00Z ECMWF AND 00Z/06Z GFS DURING DAYS 3-5, WITH  
INCREASED 00Z ECENS/NAEFS WEIGHTING DURING DAYS 6-7.  
 
RYAN  
 
PREVIOUS DISCUSSION (ISSUED AT 0513 UTC)...  
   
..OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES
 
 
AN IMPRESSIVE RIDGE/TROUGH PATTERN WILL BE FIRMLY IN PLACE OVER  
THE LOWER 48 NEXT WEEK AS COLDER CANADIAN AIR SPILLS DOWN INTO THE  
CENTRAL/EASTERN STATES WHILE THE WEST ENJOYS A MILD AND DRY  
STRETCH. THE DETERMINISTIC MODELS AGREE WITH THE MULTI-CENTER  
ENSEMBLE CONSENSUS ON THE SYNOPTIC PATTERN THROUGH NEXT WEEK BUT  
WITH EXPECTED DIFFERENCES IN SOME DETAILS (E.G. SYSTEM TIMING AND  
TRACK AS WELL AS TEMPERATURE GRADIENT ACROSS THE PLAINS). A BLEND  
OF THE RECENT GFS/ECMWF/UKMET/CANADIAN SERVED AS A GOOD STARTING  
POINT FOR THE TUE-THU FORECAST AS A DEEP SYSTEM LIFTS THROUGH  
ONTARIO AND DRAGS A COLD FRONT AND REINFORCING ARCTIC FRONT  
THROUGH THE CENTRAL/EASTERN US. THEREAFTER, THE FORECAST DIFFERS  
AMONG THE GUIDANCE ON HOW THE INDIVIDUAL SHORTWAVES RACE THROUGH  
THE TROUGH IN THE EAST, EACH WITH A DIFFERENT TRACK/INTENSITY THAT  
MODULATES THE COLD/MILD DEMARCATION LINE AND THE SNOW/RAIN LINE  
AND AMOUNTS OF EACH. SINCE THESE QUICKLY MOVING SYSTEMS HAVE  
LITTLE PREDICTABILITY AT THESE TIME RANGES, OPTED TO TREND QUICKLY  
TOWARD A MAJORITY ENSEMBLE MEAN WEIGHTING FOR NEXT FRI/SAT. THIS  
WILL TAKE A MODEST CLIPPER THROUGH THE EAST WHILE THE LEAD FRONT  
LINGERS OFFSHORE, POSSIBLY PULLING A SFC WAVE CLOSE TO THE  
NORTHEAST COAST LATER IN THE WEEK OR NEXT WEEKEND.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
MILD TEMPERATURES AHEAD OF THE LEAD FRONT ON TUESDAY (ABOUT 10-20  
DEGREES ABOVE AVERAGE) WILL BE REPLACED BY MODIFIED ARCTIC AIR  
WITH TEMPERATURES ABOUT 5-15 DEGREES BELOW AVERAGE. ALONG AND WEST  
OF THE ROCKIES, TEMPERATURES WILL MODERATE TO ABOUT 5-10 DEGREES  
ABOVE AVERAGE AS STRONG HIGH PRESSURE REMAINS ANCHORED OVER THE  
GREAT BASIN. THIS WOULD SUPPORT THE SANTA ANA WIND EVENT FOR MUCH  
OF THE WEEK IN SOUTHERN CALIFORNIA THOUGH TEMPERATURES AROUND LOS  
ANGELES MAY ONLY BE NEAR 80. PLEASE CONSULT THE LATEST SPC FIRE  
WEATHER OUTLOOK AS COASTAL SOUTHERN CALIFORNIA MAY REMAIN IN  
CRITICAL FIRE WEATHER CONDITIONS.  
 
PRECIPITATION WILL FOCUS ALONG/AHEAD OF THE LEAD FRONT IN THE EAST  
BUT THEN DECREASE IN COVERAGE AS COLDER/DRIER AIR TAKES HOLD. THE  
EXCEPTION WILL BE AROUND THE GREAT LAKES WHERE LAKE-EFFECT SNOW  
WILL BE A DAILY OCCURRENCE. TOTAL ACCUMULATIONS MAY BE SUBSTANTIAL  
FOR SOME FAVORED AREAS. ADDITIONAL LIGHT SNOW WILL BE POSSIBLE  
WITH CLIPPER SYSTEMS DIVING OUT OF CANADA ANYWHERE FROM THE HIGH  
PLAINS AND UPPER MIDWEST INTO THE OHIO/TN VALLEY AND INTO THE  
APPALACHIANS. LATE NEXT WEEK, THE LEAD FRONT MAY LINGER CLOSE  
ENOUGH TO THROW SOME PRECIPITATION BACK TO THE NORTHWEST SHOULD A  
SURFACE WAVE DEVELOP ALONG IT AND MOVE CLOSE ENOUGH TO NEW  
ENGLAND. THIS WOULD MEAN A THREAT OF AT LEAST SOME RAIN/SNOW TO  
THE MID-ATLANTIC AND NORTHEAST LATE NEXT WEEK IN AN OTHERWISE DRY  
PATTERN.  
 
FRACASSO  
 

 
 
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