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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1600Z Dec 09, 2017)
 
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


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