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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0635Z Sep 29, 2014)
 
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EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
235 AM EDT MON SEP 29 2014

VALID 12Z THU OCT 02 2014 - 12Z MON OCT 06 2014

...OVERVIEW...

A MEAN TROUGH WITH A SLIGHT NEGATIVE TILT WILL SETTLE INTO THE
EAST... CONSISTING OF A TROUGH MOVING EWD FROM THE PLAINS AND
REINFORCED BY UPSTREAM ENERGY.  THERE IS DECENT CLUSTERING AMONG
LATEST GFS/ECMWF RUNS AND THEIR ENSEMBLE MEANS BUT SOME DETAILS OF
THE FCST HAVE SHOWN SOME TRENDING OVER THE PAST COUPLE DAYS WITH
THE LATEST UPDATE REFLECTING A CONTINUATION OF THESE TRENDS. 
THERE ARE ALSO ONGOING UNCERTAINTIES IN PRECISE SHRTWV DETAILS
THAT WILL AFFECT SENSIBLE WEATHER AT SOME LOCATIONS.


...GUIDANCE EVALUATION/PREFERENCES...

THE 18Z GFS/GEFS MEAN AND 12Z ECMWF/ECMWF MEAN ARE SUFFICIENTLY
SIMILAR IN PRINCIPLE INTO DAY 5 SAT FOR A BLEND OF THEIR SOLNS TO
PROVIDE THE MOST LIKELY SCENARIO FOR THAT TIME FRAME.  FOR THE
REMAINDER OF THE PERIOD THE FCST ADJUSTS TO A BLEND OF HALF ECMWF
MEAN AND THE REMAINDER 18Z GEFS/12Z NAEFS MEANS TO EMPHASIZE
AGREEMENT IN THE LARGER SCALE AS DIVERGENCE IN OPERATIONAL DETAILS
INCREASES.

REGARDING THE TWO PRIMARY TRENDS SEEN WITH THE TROUGH MOVING INTO
THE EAST... THE FIRST INVOLVES THE EVOLUTION OF INITIAL PLAINS
ENERGY/UPSTREAM FLOW AMPLIFYING INTO THE UPR MS VLY GRTLKS. 
GUIDANCE HAS ADJUSTED THESE TWO PIECES OF ENERGY SUCH THAT LOW
PRESSURE THAT HAD BEEN FCST TO STRENGTHEN NEAR THE UPR GRTLKS BY
EARLY FRI HAS TRENDED FASTER/NWD.  THE SECOND TREND HAS BEEN FOR
FASTER TIMING OF THE TRAILING COLD FRONT... IN RESPONSE TO
CONSENSUS BECOMING DEEPER WITH THE UPR TROUGH CORE AND THUS
ACCELERATING LEADING HGT FALLS ALOFT.  WILL ACCEPT THE 12Z/18Z
GUIDANCE CLUSTER SINCE IT REFLECTS A CONTINUATION OF PRIOR TRENDS.
 A FEW MODEL RUNS HAVE BEEN EVEN FASTER DUE TO SHRTWV DIFFS
UPSTREAM BUT WOULD LIKE TO ADJUST ONLY AS FAST AS LATEST CONSENSUS
SINCE THE DETAILS INVOLVED ARE ON A SCALE THAT LEADS TO AVG
PREDICTABILITY AT BEST.  THE 00Z GFS/GEFS MEAN/UKMET COMPARE
ACCEPTABLY TO ESTABLISHED CONSENSUS INTO THE WEEKEND.

FURTHER DETAIL QUESTIONS COME INTO THE PICTURE BY DAYS 6-7 SUN-MON
AS FAST PAC FLOW STREAMS INTO SRN CANADA AND THE NRN TIER STATES. 
18Z-12Z GEFS AND 12Z ECMWF MEANS AGREE THAT THIS FLOW SHOULD
FLATTEN THE NRN PERIPHERY OF THE WRN RIDGE A LITTLE AND SUPPORT A
WEAK SFC SYSTEM REACHING THE GRTLKS.  THE FAST/LOW AMPLITUDE
NATURE OF EMBEDDED ENERGY WILL LIKELY LEAD TO SOME VARIABILITY IN
GUIDANCE OVER COMING DAYS.

FASTER TRENDS WITH THE TROUGH REACHING THE ERN STATES INCREASE THE
LIKELIHOOD THAT THE UPR LOW NEAR THE NERN COAST AS OF EARLY DAY 3
THU AND ASSOC WRN ATLC SFC SYSTEM SHOULD PROGRESS GRADUALLY
EWD/NEWD WITH TIME.  PREFERRED CONSENSUS PROVIDES REASONABLE
CONTINUITY EARLY IN THE PERIOD BEFORE TRENDING FASTER IN RESPONSE
TO THE QUICKER UPSTREAM TROUGH.


...SENSIBLE WEATHER HIGHLIGHTS...

THE FRONT CROSSING THE ERN STATES WILL BE ACCOMPANIED BY A BROAD
SHIELD OF RNFL FROM LATE THIS WEEK INTO THE FIRST PART OF THE
WEEKEND.  BEST COMBINATION OF MSTR AND UPR DYNAMICS/PSBL SFC
WAVINESS SHOULD LEAD TO HIGHEST TOTALS FROM THE MID-LWR MS VLY
INTO THE GRTLKS... WITH SOME MSTR INPUT FROM GULF INFLOW... AND
THEN NEW ENGLAND WHERE FLOW FROM THE ATLC MAY ADD SOME MSTR.  RNFL
MAY BE FAIRLY INTENSE AT SOME LOCATIONS BUT TOTAL AMTS WILL BE
SENSITIVE TO FRONTAL TIMING.  CONSULT SPC OUTLOOKS FOR LATEST INFO
REGARDING SEVERE POTENTIAL.  BEHIND THE FRONT MOST OF THE LOWER 48
SHOULD BE DRY.  EXCEPTIONS INCLUDE A PERIOD OF SHOWERY WEATHER
OVER THE GRTLKS UNDER THE CORE OF THE UPR TROUGH FOLLOWED BY LGT
ACTIVITY WITH AN UPSTREAM SYSTEM... AND PARTS OF THE NRN PAC NW
WHICH WILL BE ON THE VERY SRN FRINGE OF MOIST PAC FLOW STREAMING
INTO WRN CANADA.  BELOW NORMAL TEMPS OVER THE INTERIOR WEST/PLAINS
EARLY IN THE PERIOD WILL SPREAD EWD AND REPLACE THE WARM
READINGS... ESPECIALLY FOR MIN TEMPS... EXPECTED OVER THE EAST
LATE THIS WEEK.  ON THE OTHER HAND THE RIDGE ALOFT BUILDING INTO
THE WEST AND DEVELOPMENT OF DOWNSLOPE FLOW E OF THE ROCKIES SHOULD
SPREAD WARMER THAN AVG TEMPS FROM THE WEST INTO THE CNTRL-SRN
PLAINS.

RAUSCH