EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
239 AM EDT SAT MAY 25 2013

VALID 12Z TUE MAY 28 2013 - 12Z SAT JUN 01 2013

...OVERVIEW AND PREFERENCES...

GUIDANCE SHOWS REASONABLE AGREEMENT IN THE LARGE SCALE... WITH A
BROAD MEAN TROUGH COVERING THE WRN STATES AND A STRENGTHENING
RIDGE MOVING INTO THE EAST.  HOWEVER THERE IS CONTINUED
UNCERTAINTY FOR SOME DETAILS WITHIN THE MEAN TROUGH AS WELL AS FOR
THE AMPLITUDE/ORIENTATION OF ERN CANADA/NERN CONUS FLOW ALOFT BY
THE LATTER HALF OF THE FCST.  OPERATIONAL MODEL RUNS STILL DISPLAY
ENOUGH SCATTER WITH VARIOUS FEATURES TO RECOMMEND EMPHASIZING THE
ENSEMBLE MEANS THROUGH THE MEDIUM RANGE PERIOD.  PREFER A 70/30
BLEND OF THE 12Z ECMWF/12Z GEFS MEANS DAYS 3-5 TUE-THU BASED ON
REASONABLE AGREEMENT OF THEIR SOLNS AND LONG TERM VERIFICATION. 
THE 12Z ECMWF MEAN IS USED EXCLUSIVELY AFTER EARLY THU FOR ITS
INTERMEDIATE SOLN WITH NEW ENGLAND FRONTAL POSN ALTHOUGH THE 12Z
GEFS MEAN REMAINS AN ACCEPTABLE BLEND COMPONENT OVER THE WRN 2/3
OF THE CONUS.


...GUIDANCE EVALUATION...

THE MAJORITY OF MODELS/ENSEMBLES EXPECT LEADING SHRTWV ENERGY
REACHING THE WEST AS OF EARLY TUE TO CONTINUE INTO THE PLAINS
WHILE UPSTREAM ENERGY FLOWING INTO THE NORTHWEST CATCHES UP TO THE
LEADING ENERGY.  THE BEST CLUSTERING OF SOLNS HAS THESE TWO
BUNDLES OF ENERGY CONSOLIDATING INTO A CLOSED SYSTEM ALOFT
OVER/NEAR THE NRN PLAINS BY DAY 6 FRI... SUPPORTING A FAIRLY
VIGOROUS SFC LOW.  THIS SYSTEM SHOULD BEGIN TO PROGRESS EWD BY
NEXT SAT.  OPERATIONAL ECMWF RUNS CONTINUE TO LEAN ON THE
SLOWER/MORE AMPLIFIED SIDE OF GUIDANCE WITH THE LEADING SHRTWV
REACHING THE HIGH PLAINS BY THU.  ECMWF TRENDS HAVE BEEN GRADUALLY
FASTER OVER THE PAST COUPLE DAYS.  THE 18Z GFS/GEFS MEAN ARE ON
THE FASTER SIDE OF THE SPREAD WITH UPSTREAM ENERGY AND FLATTER
WITH LEADING ENERGY... WITH THE 12Z GFS/GEFS MEAN CLOSER TO THE
OVERALL GUIDANCE MEAN.  SUFFICIENT CONTINUITY EXISTS TO HAVE ABOVE
AVERAGE CONFIDENCE IN THE IDEA OF SFC LOW DEVELOPMENT OVER/NEAR
THE NRN PLAINS AROUND THU-FRI.  THE UNCERTAINTY IN HOW MULTIPLE
PIECES OF SHRTWV ENERGY WILL INTERACT LEADS TO A DETERMINISTIC
FCST NOT MUCH STRONGER THAN THE ENSEMBLE MEANS FOR THE SFC LOW...
AND NOT AS WRAPPED UP AS THE 12Z ECMWF/CMC.  MEANWHILE STRENGTH OF
THE DOWNSTREAM RIDGE AND OCCASIONALLY FAST GFS/GEFS BIASES WOULD
FAVOR LEANING A LITTLE SLOWER THAN RECENT GFS RUNS AND 18Z GEFS
MEAN DURING FRI-SAT.  ULTIMATELY CANNOT FULLY DISCOUNT ANY SOLN AT
THIS TIME IN LIGHT OF TYPICAL DAYS 6-7 FCST ERRORS.

BY NEXT SAT THE LATEST GEFS/ECMWF/CMC MEANS INDICATE THAT FAST/LOW
AMPLITUDE PAC FLOW SHOULD BEGIN TO AMPLIFY IN RESPONSE TO
TROUGHING S OF THE ALEUTIANS... LEADING TO A SHARPER TROUGH NEAR
THE WEST COAST.  THE 12Z GFS OFFERS THE CLOSEST MATCH TO THE MEANS
FOR THIS PART OF THE FCST.  THE 12Z ECMWF BRINGS ITS ERN PAC RIDGE
EWD OF CONSENSUS WHILE THE 18Z GFS SOLN OF STALLING A CLOSED LOW
OFF THE PAC NW COAST WED ONWARD DOES NOT COMPARE WELL TO MOST
OTHER SOLNS EITHER.

FARTHER EWD THE MOST SIGNIFICANT ISSUE INVOLVES AMPLITUDE OF ERN
CANADA/NERN CONUS FLOW ALOFT DURING THE LATTER HALF OF THE PERIOD
AND RESULTING SFC FRONT POSN.  THE PAST THREE ECMWF RUNS HAVE BEEN
FAIRLY AGGRESSIVE WITH TROUGH AMPLITUDE THUS BRINGING A SFC FRONT
RATHER FAR SWD.  ON THE OTHER HAND GFS/GEFS RUNS KEEP HGTS A LOT
HIGHER WITH A MUCH FARTHER NWD FRONTAL ZONE. TELECONNECTIONS
RELATIVE TO THE WRN TROUGH AND AN ATLC RIDGE IN THE D+5 MEANS
SUGGEST LOWER NERN CONUS HGTS THAN FCST BY GFS/GEFS SOLNS BUT NOT
CONCLUSIVELY TO THE EXTENT OF THE ECMWF.  THIS LEAVES THE 12Z
ECMWF MEAN AS THE BEST OPTION FOR FRONTAL PLACEMENT AT THIS TIME.


...SENSIBLE WEATHER HIGHLIGHTS...

THE MEAN TROUGH ALOFT OVER THE WEST WILL SUPPORT UNSETTLED WEATHER
OVER THE WRN-CNTRL STATES... AND BELOW NORMAL TEMPS OVER THE WEST
EVENTUALLY REACHING THE NRN PLAINS.  POTENTIAL STILL EXISTS FOR
SOME WRN PCPN TO EXTEND UNSEASONABLY FAR SWD.  THE SRN HALF OR 2/3
OF THE WEST SHOULD SEE A DRYING TREND BY FRI-SAT.  HGT FALLS ALOFT
REACHING THE CNTRL/SRN PLAINS MID-PERIOD AND MID LVL/SFC
DEVELOPMENT FARTHER NWD LATE IN THE WEEK MAY SUPPORT MULTIPLE
AREAS OF MDT-HVY RNFL OVER/NEAR THE PLAINS.  THE WARM FRONT FCST
TO LIFT ACROSS THE NERN QUADRANT OF THE LOWER 48 TUE ONWARD IS
ALSO EXPECTED TO BE CONVECTIVELY ACTIVE.  AFTER PASSAGE OF THE
WARM FRONT EXPECT PERSISTENTLY ABOVE TO WELL ABOVE NORMAL TEMPS
OVER MUCH OF THE EAST ALBEIT WITH SOME UNCERTAINTY OVER THE
NORTHEAST.  FINALLY EXPECT A WET PATTERN OVER SRN/ERN PARTS OF THE
FL PENINSULA DUE TO ELY FLOW ON THE SRN SIDE OF ATLC/ERN CONUS SFC
HIGH PRESSURE.

RAUSCH