EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
253 PM EDT TUE MAY 05 2009
 
VALID 12Z FRI MAY 08 2009 - 12Z TUE MAY 12 2009
 
THE LATEST DETERMINISTIC AND ENSEMBLE SOLUTIONS MAINTAIN
REASONABLE CONTINUITY THROUGH THE FIRST HALF OF THE MEDIUM RANGE
PERIOD. A DEEP VORTEX OVER THE CANADIAN ARCHIPELAGO HELPS MAINTAIN
A LOW AMPLITUDE TROUGH OVER THE NORTHEAST FOR MOST OF THE
PERIOD...WHILE THE FLOW ACROSS THE PACIFIC UNDER THE BASE OF A
POSITIVE ANOMALY NEAR FAR EASTERN ASIA RESULTS IN A CUTOFF LOW IN
THE NORTHEAST PACIFIC THAT EVENTUALLY EJECTS TOWARD THE PACIFIC
NORTHWEST OR SOUTHWEST CANADA LATE IN THE PERIOD. THROUGH THE
MID-LATITUDES...A QUASI-ZONAL SOUTHERN STREAM CONTINUES TO MOVE
THROUGH THE LOWER 48.

INTERESTINGLY...TELECONNECTIONS MOST FAVOR THE 00Z GEFS AND 00Z
ECMWF ENSEMBLE MEANS...SHOWING A SLOWER PROGRESSION WITH THE
SYSTEM /OR A WEAKER SYSTEM/ APPROACHING THE WESTERN US/CANADIAN
BORDER EARLY NEXT WEEK.  THE DIFFERENCES BETWEEN THE 12Z GFS/12Z
ECMWF AND THEIR ENSEMBLE MEANS ARE STRIKING...WITH THE OPERATIONAL
GUIDANCE SHOWING A STRONGER/QUICKER MOVING SYSTEM THAN THEIR
MEANS.  THE 00Z MULTI-MODEL ENSEMBLE SPAGHETTI DIAGRAMS USING THE
5520 METER HEIGHT LINE DEVOLVE INTO CHAOS ACROSS THE NORTHEAST
PACIFIC/PACIFIC NORTHWEST AFTER SUNDAY...AS THE GEFS MEMBERS FOCUS
ON A SECOND MORE OFFSHORE UPPER CYCLONE WHILE THE ECMWF MEMBERS
FOCUS ON THE FIRST SYSTEM...AND EVEN THEN SHOW SIGNIFICANT TIMING
DIFFERENCES.  EVEN THOUGH THE OPERATIONAL ECMWF AND GFS SOLUTIONS
HAVE CONVERGED WITH THIS SYSTEM...WE ARE NOT SOLD ON THEIR IDEA
QUITE YET UNTIL THEIR ENSEMBLE MEANS SHOW A MORE DISTINCT SYSTEM
AND THEIR ENSEMBLE MEMBERS SHOW A HINT OF CLUSTERING.  

CONSIDERING THE UNCERTAINTY...NO CHANGES WERE MADE TO THE MORNING
PREFERENCE.  EARLY ON...A 50/50 BLEND OF THE 00Z OR 06Z GFS/00Z
ECMWF WAS A REASONABLE STARTING POINT FOR CREATING THE
PRESSURES/FRONTS INTO SATURDAY.  THEREAFTER...A BLEND OF THE 00Z
ECMWF AND ITS ENSEMBLE MEAN /WHICH IS CLOSE TO THE 00Z GEFS MEAN/
ACCOUNTS FOR THE PROGRESSION DIFFERENCE WITH THE SYSTEM MOVING
FROM THE NORTHEAST PACIFIC INTO WEST-CENTRAL NORTH AMERICA WHICH
ALSO KEEPS MORE IN LINE WITH THE TELECONNECTION-FAVORED PATTERN OF
TROUGHING LINGERING OFFSHORE WESTERN CANADA.  THIS PATTERN KEEPS
THE MID-MISSISSIPPI VALLEY AND MID-SOUTH/SOUTHERN APPALACHIANS WET
THROUGH THE PERIOD.

ROTH/JAMES