481  
FXUS02 KWBC 091600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1100 AM EST SAT DEC 09 2017  
 
VALID 12Z TUE DEC 12 2017 - 12Z SAT DEC 16 2017  
 
...A MIDWEST/GREAT LAKES THROUGH NORTHEASTERN U.S. HEAVY SNOW  
THREAT...  
 
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
IT REMAINS THE CASE THAT A BROAD LONGWAVE TROUGH WILL ENCOMPASS  
REGIONS EAST OF THE GREAT PLAINS. A SERIES OF EMBEDDED  
PERTURBATIONS IN THE FLOW WILL DRIVE WAVES OF LOW PRESSURE WHICH  
SHOULD BE CONDUITS TO WINTRY PRECIPITATION ANYWHERE FROM THE UPPER  
MIDWEST THROUGH THE GREAT LAKES INTO THE NERN U.S. ONE SUCH  
CYCLONE IS FORECAST TO BE IN THE PROCESS OF LIFTING AWAY FROM THE  
UPPER MID-ATLANTIC TOWARD NEW ENGLAND ON TUE. THE NEXT REINFORCING  
SHOT OF ARCTIC AIR WILL BRING ADDITIONAL LOW PRESSURE THROUGH THE  
GREAT LAKES/OH VALLEY THEN ONWARD INTO THE NRN  
MID-ATLANTIC/NORTHEAST THU-FRI. TOWARD THE END OF NEXT  
WEEK...WHILE CYCLONIC FLOW REMAINS ALOFT OVER THE ERN HALF OF THE  
NATION...THERE ARE HINTS AT THE FLOW RELAXING A BIT ACROSS THE  
MID-LATITUDES OF NORTH AMERICA. LOOKING FROM THE ROCKIES  
WESTWARD...AN UPPER RIDGE WILL REMAIN A FIXTURE IN THE FORECAST  
FOR MUCH OF THE PERIOD. 500MB HEIGHT ANOMALIES ARE AROUND 2-2.5  
STANDARD DEVIATIONS ABOVE CLIMATOLOGY ALTHOUGH A PATTERN CHANGE IS  
SUGGESTED BY GUIDANCE BY NEXT FRI/SAT.  
 
BEFORE THIS OCCURS...A SHORTWAVE TRACKING ATOP THIS RIDGE WILL  
DIVE SOUTHEASTWARD THROUGH THE CENTRAL HIGH PLAINS WHILE  
EVENTUALLY JOINING THE PARENT DOWNSTREAM TROUGH. GUIDANCE  
CONTINUES TO OFFER SHORT RANGE VARIANCE WITH THIS SYSTEM TRACK  
FROM THE NERN PACIFIC INTO WRN CANADA TUE THAT PORTENDS  
SIGNIFICANT DOWNSTREAM UNCERTAINTY THROUGH LATER NEXT WEEK. ECMWF  
CONTINUIUTY HAS BEEN EXCEPTIONALLY POOR WITH DEPICTION OF THESE  
WAVES. THE OO UTC RUN YESTERDAY SPLIT AMPLE ENERGY TO THE LEE OF  
THE RIDGE THAT DUG WELL UNDERNEATH INTO THE SOUTHWEST. THE LATEST  
00 UTC RUN INSTEAD HOLDS THE BULK OF ENERGY WITHIN A NRN STREAM  
FLOW SEWD INTO THE CENTRAL THEN ERN US. THE 06 UTC GFS AND 00 UTC  
CANADIAN SEEM A GOOD COMPROMISE IN BETWEEN THE 00 UTC GFS AND THE  
LATEST 00 UTC ECMWF FOR THIS SYSTEM INTO LATER NEXT WEEK BEFORE  
REVERTING TO 00 UTC GEFS/ECMWF ENSEMBLES BY NEXT FRI-SAT TO  
MAINTAIN A BIT MORE OVERALL FLOW AMPLITUDE GIVEN CURRENT FLOW  
AMPLITUDE. TOWARD THAT END...MID/UPPER-LEVEL HEIGHTS REBOUND  
QUICKLY ACROSS THE WESTERN STATES WITH THE EVENTUAL POTENTIAL FOR  
FEATURES OVER THE EASTERN PACIFIC TO BRING MORE SUBSTANTIAL HEIGHT  
FALLS INTO THE PACIFIC NORTHWEST BY DAY 6/7...DECEMBER 15/16.  
LOOKING DOWN INTO THE SUBTROPICAL PACIFIC...A PERSISTENT UPPER LOW  
IS FORECAST TO DRIFT FROM NORTHWESTERN MX BACK INTO THE EASTERN  
PACIFIC AS IT BECOMES FURTHER CUT OFF FROM THE MID-LATITUDE FLOW.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
GIVEN THE STAGNANT HIGH-AMPLITUDE PATTERN IN PLACE WELL INTO NEXT  
WEEK BEFORE RELAXING...PRONOUNCED TEMPERATURE ANOMALIES WILL RULE  
THE COUNTRY. HOWEVER...EXPECTED HIGHS/LOWS DO NOT SEEM TO BE  
EXTREME ENOUGH TO BREAK ANY WIDESPREAD DAILY TEMPERATURE RECORDS.  
GENERALLY SPEAKING...READINGS SHOULD BE AROUND 10 TO POSSIBLY 20  
DEGREES ABOVE CLIMATOLOGY OVER THE WESTERN HALF OF THE U.S. THE  
EXCEPTION APPEARS TO BE AREAS OF THE PACIFIC NORTHWEST/CENTRAL  
GREAT BASIN WHICH WILL LIKELY HAVE A PERSISTENT LOW-LEVEL  
TEMPERATURE INVERSION THUS KEEPING TEMPERATURES CLOSER TO AVERAGE.  
FROM THE MS RIVER VALLEY EASTWARD...THE INFLUENCE OF ARCTIC AIR IN  
THE FORECAST WILL KEEP HIGH TEMPS A WINTERY 10 TO 20 DEGREES BELOW  
AVERAGE.  
 
SNOWFALL WILL BE COMMONPLACE FROM THE UPPER MIDWEST THROUGH THE  
MIDWEST/GREAT LAKES INTO THE CENTRAL/NORTHERN APPALACHIANS...NRN  
MID-ATLANTIC AND NEW ENGLAND. A LEAD LOW WILL BRING MODERATE/HEAVY  
SNOW TO PARTS OF THE NORTHEAST TUESDAY/WEDNESDAY. SEVERAL  
ADDITIONAL LOWS WITH ASSOCIATED ORGANIZED SNOW THREATS ARE ALSO  
POSSIBLE ON THE HEELS OF THE LEAD SYSTEM FROM THE UPPER MIDWEST  
AND MIDWEST/GREAT LAKES TO THE NRN MID-ATLANTIC/NORTHEAST  
TUESDAY-FRIDAY...BUT SUPPORT ALOFT HAS BEEN QUITE UNCERTAIN FROM  
RUN TO RUN.  
PERSISTENT CYCLONIC FLOW WILL KEEP THE LAKE EFFECT TRAIN GOING IN  
THE WAKE OF THESE LOWS ALTHOUGH SOME BREAKS ARE LIKELY LOCALLY  
GIVEN SHIFTS IN THE PREVAILING LOW-LEVEL WIND DIRECTION.  
ELSEWHERE...A RETURN OF PRECIPITATION IS LIKELY LATE NEXT WEEK  
OVER THE NORTHWEST AS THE UPPER RIDGE STARTS TO BREAKDOWN AS A  
SERIES OF ILL-DEFINED SHORTWAVE IMPULSES PENETRATE INLAND.  
 
SCHICHTEL  
 

 
 
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