EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1202 PM EDT WED JUN 19 2013

VALID 12Z SAT JUN 22 2013 - 12Z WED JUN 26 2013

...PATTERN OVERVIEW...

THE PREVAILING PATTERN DEPICTED BY THE LATEST GUIDANCE IS
CHARACTERIZED BY A PERSISTENT POSITIVE MID-LEVEL HEIGHT ANOMALY
ACROSS WESTERN CANADA WITH A CORRESPONDING NEGATIVE MID-LEVEL
HEIGHT ANOMALY OVER EASTERN CANADA...ALTHOUGH WITH GREATER
VARIABILITY.  THE EFFECT OF THE NEARLY STEADY-STATE PATTERN ACROSS
CANADA IS TO FORCE MOST OF THE STRONG FLOW ACROSS THE NORTHERN
PACIFIC SOUTHWARD INTO THE PACIFIC NORTHWEST.  MEANWHILE...THESE
ADJUSTMENTS FAVOR GRADUAL DEVELOPMENT AND STRENGTHENING OF THE
SUBTROPICAL RIDGE OVER THE SOUTHERN STATES THAT SLOWLY
RETROGRADES. 

...GUIDANCE EVALUATION/UNCERTAINTY/PREFERENCES...

MODEL AGREEMENT CONCERNING THE SHORTWAVE FLOW AND DAY-TO-DAY
DETAILS IS REASONABLY GOOD THROUGH DAY 4/SUN..RESULTING IN
SLIGHTLY ABOVE AVERAGE CONFIDENCE.  HOWEVER...EVIDENCE OF
INCREASING UNCERTAINTY DEVELOPS FROM DAY 5/MON...ONWARD DUE TO THE
GROWING SOLUTION SPREAD OF THE SHORTWAVE PATTERN AND MESOSCALE
UNCERTAINTIES RESULTING FROM CONVECTION.  THEREFORE...CONFIDENCE
FALLS BELOW AVERAGE FOR DAYS 6-7/TUE-WED. 

MODEL TRENDS AND CLUSTERING SUPPORT MOST OF THE RATIONALE FOR
MODEL CHOICES THROUGH DAY 4...WITH THE 06Z GFS SUPPORTING THE 00Z
ECMWF AND 00Z ECMWF ENSEMBLE MEAN...PARTICULARLY WITH GREATER
AMPLIFICATION OF THE SHORTWAVE TROUGH REACHING THE WEST COAST. 
MEANWHILE...
MODEL CHOICES ARE LESS CLEAR ACROSS THE SOUTHEAST...WHERE SUBTLE
DIFFERENCES IN THE THERMAL/MOISTURE PROFILES AND MODEL PHYSICS
ACCOUNT FOR MOST OF THE SENSIBLE WEATHER DIFFERENCES.  THE 00-06Z
GFS ARE CONSIDERABLY WETTER INLAND COMPARED TO THE
ECMWF/UKMET...RESULTING IN PART FROM COLLIDING MOISTURE PLUMES
ALONG A RESIDUAL FRONTAL BOUNDARY AND A TROPICAL WAVE ENTERING THE
EASTERN CARIBBEAN.  GIVEN THAT RISING HEIGHTS ARE ANTICIPATED
DURING THIS PERIOD AND CONCERNS THAT THE CONVECTIVE
PARAMETRIZATION OF THE GFS MAY BE TOO AGGRESSIVE...THE PREFERENCE
IS FOR A ECMWF/UKMET-LIKE SCENARIO THAT MAINTAINS MOST OF THE
PRECIPITATION ALONG THE SOUTHEAST COAST.

BY DAY 5/MON...THE GFS AND GEFS MEMBERS POSSIBLY SHOW THEIR
OCCASIONAL FAST BIAS IN ALLOWING LEADING PORTIONS OF THE LONGWAVE
TROUGH FORMING ACROSS THE WEST TO EJECT EASTWARD INTO THE BUILDING
UPPER RIDGE.  GIVEN GROWING UNCERTAINTY FROM THIS PERIOD
ONWARD...AND SUPPORT FOR SLOWER EJECTION PROVIDED BY THE 06Z
PARALLEL GFS...THE PREFERENCE FROM DAY 5 ONWARD IS FOR A SOLUTION
CLOSEST TO THE SLOWER ECMWF ENSEMBLE MEAN...BUT WITH BELOW AVERAGE
CONFIDENCE.  SIMILAR PREFERENCES WERE CHOSEN ACROSS THE CENTRAL
AND EAST...BUT WITH LESS CONFIDENCE DUE TO MESOSCALE UNCERTAINTIES
WHICH MAY INFLUENCE THE FRONTAL POSITIONS.

...SENSIBLE WEATHER HIGHLIGHTS...

WET CONDITIONS ARE LIKELY ACROSS THE PACIFIC NORTHWEST POSSIBLY
INTO NORTHERN OR CENTRAL CALIFORNIA.  MODELS GENERALLY AGREE IN
ALLOWING THE DEEP MOISTURE PLUME SOUTHEAST OF CHINA OT REACH
OREGON OR NORTHERN CALIFORNIA BY DAYS 6-7/TUE-WED...WITH
PRECIPITABLE WATER VALUES POSSIBLY REACHING 1.75 INCHES.  PERIODIC
PRECIPITATION IS ALSO LIKELY ALONG A SLOWLY MODULATING FRONTAL
ZONE EXTENDING FROM THE CENTRAL PLAINS TOWARD THE OHIO VALLY AND
NORTHEAST...WHILE LESS UNCERTAIN RAINFALL IS EXPECTED OVER PARTS
OFF THE SOUTHEAST EARLIER IN THE PERIOD.  ABOVE NORMAL
TEMPERATURES SHOULD ALSO CONTINUE ACROSS THE CENTRAL AND SOUTHERN
STATES...PARTICULARLY IN THE SOUTHWEST.

JAMES