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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0635Z Sep 21, 2014)
 
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EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
235 AM EDT SUN SEP 21 2014

VALID 12Z WED SEP 24 2014 - 12Z SUN SEP 28 2014

...OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES...

ESPECIALLY WITH THE ARRIVAL OF THE 00Z UKMET/CMC IT APPEARS
GUIDANCE IS FINALLY DEVELOPING SOME DEGREE OF CONSENSUS WITH THE
AMPLIFIED ERN PAC TROUGH APPROACHING/MOVING INTO THE WEST DURING
THE PERIOD.  DOWNSTREAM THERE CONTINUES TO BE GOOD AGREEMENT UPON
RATHER HIGH HGTS IN GENERAL WITH A CORE OF POSITIVE HGT ANOMALIES
CENTERED OVER ONTARIO... THOUGH SMALLER SCALE WEAKNESSES TYPICALLY
MORE DIFFICULT TO RESOLVE MORE THAN 2-3 DAYS OUT IN TIME WILL HAVE
SOME INFLUENCE ON SENSIBLE WEATHER OVER PARTS OF THE CNTRL-ERN
STATES.

FOR ABOUT THE FIRST HALF OF THE FCST THERE IS SUFFICIENTLY LOW
SPREAD WITH THE ERN PAC TROUGH AND LACK OF CONFIDENCE WITH
SPECIFICS OF WEAKNESSES ALOFT OVER THE CNTRL/ERN STATES TO
RECOMMEND A CONSENSUS BLEND THAT IS PRIMARILY WEIGHTED TOWARD THE
18Z GFS/12Z ECMWF DAY 3 WED AND INCORPORATES HIGHER AMTS OF 18Z
GEFS/12Z ECMWF MEAN GUIDANCE WITH TIME INTO DAY 5 FRI.

BY DAYS 6-7 SAT-SUN THE PRIMARY FCST ISSUES INVOLVE THE TIMING OF
THE ERN PAC/WRN CONUS TROUGH AND THE DEGREE OF FLOW SEPARATION
WITHIN THIS TROUGH.  AMONG LATEST GUIDANCE THE GFS/GEFS RUNS ARE
SOMEWHAT ON THE FAST SIDE OF THE SPREAD AND PROBABLY WORTH
ADJUSTING SOMEWHAT SLOWER IN LIGHT OF BIASES AND AGREED UPON
RIDGING DOWNSTREAM... BUT STILL MERIT SOME WEIGHTING AS THEY HAVE
BEEN MORE STABLE OVER THE PAST COUPLE DAYS.  THE 00Z CMC ALSO
CATCHES UP TO THE 00Z GEFS MEAN BY DAY 7 SUN.  ON THE OTHER HAND
THE 12Z ECMWF LINGERS ON THE SLOWER SIDE.  MEANWHILE OPERATIONAL
MODELS HAVE BEEN INDICATING SOMEWHAT GREATER POTENTIAL FOR AN UPR
LOW TO CLOSE OFF WITHIN THE TROUGH VERSUS THE TYPICALLY MORE
PHASED ENSEMBLE MEANS.  AT SOME FCST HRS RECENT GEFS MEANS HAVE
DEPICTED GREATER SEPARATION THAN THE 12Z ECMWF MEAN.  ULTIMATELY
FAVORED AN EVEN WEIGHTING OF THE 18Z GEFS/12Z ECMWF MEANS TO
ACCOUNT FOR TIMING/SEPARATION CONSIDERATIONS.  FARTHER EWD THIS
BLEND HELPS TO DOWNPLAY SMALLER SCALE FEATURES THAT WILL HAVE EVEN
LOWER PREDICTABILITY THAN DURING DAYS 3-5.


...SENSIBLE WEATHER HIGHLIGHTS...

THE ERN PAC TROUGH MOVING INTO THE WEST SHOULD BRING A PRONOUNCED
COOLING TREND TO THE REGION WITH MSTR SPREADING INLAND AFTER
FOCUSING A NARROW BAND OF ENHANCED RNFL ALONG THE PAC NW/NRN CA
COAST ON WED.  AHEAD OF THIS SYSTEM... PCPN ASSOC WITH A WEAKNESS
ALOFT OVER THE PLAINS SHOULD TREND LIGHTER/MORE SCT WITH TIME. 
THE NRN TIER SHOULD SEE WELL ABOVE NORMAL TEMPS IN THE STRONG SRLY
FLOW BETWEEN THE SLOWLY ADVANCING WRN SYSTEM AND SFC HIGH PRESSURE
MEANDERING OVER THE NORTHEAST/GRTLKS... WITH MULTIPLE DAYS OF PLUS
10-20F ANOMALIES FOR MIN/MAX READINGS FROM THE NRN ROCKIES INTO
THE UPR MS VLY.  THE PRECISE EVOLUTION OF A WEAKNESS ALOFT OVER
THE SE/APLCHNS/MID ATLC WILL INFLUENCE THE NWD/INLAND EXTENT OF
RNFL... SOME LOCALLY HVY... EXPECTED NEAR THE SERN COAST AND PSBLY
INTO THE MID ATLC.  LOW PREDICTABILITY OF SUCH A WEAK/SMALL SCALE
FEATURE ARGUES FOR AN INTERMEDIATE SOLN BETWEEN THE FARTHEST
NWD/NWWD EXTENT OF THE 12Z ECMWF/ECMWF MEAN VERSUS THE SUPPRESSED
GFS.  ALSO EXPECT MOIST ELY LOW LVL FLOW TO SUPPORT PERIODS OF
RNFL OVER FL AND PORTIONS OF THE GULF COAST.  DEPENDING ON EXACT
EVOLUTION ALOFT MSTR ALONG COASTAL AREAS MAY TRY TO SPREAD A
LITTLE FARTHER INLAND OVER THE SOUTHEAST LATE IN THE PERIOD.

RAUSCH