993
FXUS02 KWBC 271845
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
244 PM EDT MON JUL 27 2009
VALID 12Z THU JUL 30 2009 - 12Z MON AUG 03 2009
THE STRONG POSITIVE 500 MB HEIGHT ANOMALY CENTERED ALONG THE WEST
COAST OF CANADA UP TO THE ALASKA BORDER TELECONNECTS WITH A HIGH
AMPLITUDE TROUGH EXTENDING FROM SOUTH CENTRAL CANADA INTO THE
UPPER MS VALLEY/UPPER GREAT LAKES AND THE MODELS FOLLOW THIS IDEA
WELL. UNCERTAINTY CONTINUES IN THE POLAR JET OVER WESTERN
CANADA....WHERE MODELS AMONGST THEMSELVES AND FROM RUN-TO-RUN WITH
VARIOUS SHORTWAVES AND THEIR TIMING. THE SLOW MOVEMENT AND
AMPLIFIED NATURE OF THE LARGER SCALE FEATURES GIVES US ABOVE
AVERAGE CONFIDENCE IN THE OVERALL SYNOPTIC PATTERN DESPITE THE
SHORTWAVES AND THEIR TIMING DIFFERENCES. THE PRELIMINARY HPC
FRONTS/PRESSURES MAINTAIN REASONABLE CONTINUITY AND ARE BASED UPON
THE 00Z CANADIAN WHICH HAD THE BEST OVERALL SUPPORT AND HAS BEEN
ONE OF THE BETTER PERFORMING PIECES OF GUIDANCE THIS SUMMER...WITH
SOME 00Z GEFS MEAN BLENDED IN LATE IN THE PERIOD AND FURTHER
REFINEMENTS IN SURFACE LOW POSITIONS TOWARDS 00Z MULTI-MODEL
ENSEMBLE CLUSTERING IN ORDER TO ACCOUNT FOR THE
UNCERTAINTY/MINIMIZE ERROR IN THE FORECAST.
WEST...
THE 00Z/06Z GFS MOVES THE MID TO UPPER LEVEL CLOSED LOW OFF THE
WEST COAST EASTWARD INTO CALIFORNIA FASTER THAN THE OTHER
MODELS/ENSEMBLE MEANS. GIVEN THE SLOWLY PROGRESSIVE HIGH
AMPLITUDE RIDGE OVER THE INTERMOUNTAIN WEST/GREAT BASIN WHICH IS
LIKELY TO ACT AS A BLOCK TO ITS FORWARD MOTION...THE FASTEST
SOLUTION OF THE 06Z GFS WAS IGNORED HERE. BECAUSE OF DIFFERENCES
ACROSS ALASKA...THE 00Z ECMWF SLIGHTLY RETROGRADED THIS
FEATURE...WHICH IS QUITE DIFFERENT THAN ITS MORE PROGRESSIVE
SOLUTIONS FROM THREE DAYS AGO AND A PROBLEM IT FIXED IN ITS MOST
RECENT 12Z RUN. THE 00Z CANADIAN/00Z GEFS MEAN COMPROMISE
SOLUTION...WHICH IS SIMILAR TO THE 12Z ECMWF...APPEARS BEST HERE.
GIVEN THE AMPLIFIED RIDGE OVER THE WESTERN UNITED STATES AND
CANADA...THE NATION WEST OF THE ROCKIES SHOULD BE DRY AND
HOT...WITH THE LARGER ANOMALIES OVER THE PACIFIC NORTHWEST.
TROUGH IN THE MIDWEST/GREAT LAKES...
THE 00Z ECMWF CONTINUES SHOWING SIGNIFICANT RUN TO RUN CHANGES IN
SHORTWAVE TIMING FOR EACH WAVE ROTATING AROUND THE MEAN LOW NEAR
JAMES BAY/NORTHERN ONTARIO...WHICH APPEARS TO BE CAUSED BY CHANGES
IN ITS PATTERN NEAR ALASKA AS WELL. THE PERMUTATION THIS TIME WAS
TO MOVE THE LOW-MID LEVEL WAVE FASTER ACROSS THE UPPER MS VALLEY
SAT 01 AUG/GREAT LAKES SUN 02 AUG...QUEBEC MONDAY 03 AUG...MAKING
THE 00Z ECMWF AN OUTLIER SOLUTION IN BOTH STRENGTH/TIMING OF THIS
FEATURE. THE 12Z ECMWF HAS TRENDED A BIT TOWARDS THE PREVIOUS
PREFERENCE ALOFT...SO NO CHANGES WERE MADE HERE. THE 06Z GFS USES
GRIDSCALE FEEDBACK TO MOVE AN EVER-STRENGTHENING LOW ACROSS THE
GREAT LAKES INTO SOUTHEAST CANADA DURING THE EARLY PORTION OF THE
PERIOD...WHICH HAD NO SUPPORT AMONGST THE 00Z ENSEMBLE MEMBERS.
THE 00Z CANADIAN SHOWED THE BEST AGREEMENT WITH THE 00Z
MULTI-MODEL ENSEMBLE MEMBERS AND IS PREFERRED WITH DETAILS WITHIN
THIS TROUGH...BOTH AT THE SURFACE AND ALOFT. THE ANOMALOUSLY DEEP
MIDWESTERN TROUGH ALOFT WILL FAVOR COOLER THAN NORMAL TEMPERATURES
IN THE UPPER MISSISSIPPI VALLEY/NORTHERN PLAINS AND PERIODS OF
ACTIVE CONVECTION/HEAVY RAINFALL AROUND ITS SOUTHERN AND EASTERN
PERIPHERY FROM THE SOUTHERN PLAINS THROUGH THE MID-SOUTH INTO THE
MIDWEST AND MID-ATLANTIC STATES THROUGH NEW ENGLAND. THE HEAVIEST
RAINS OVER THE NEXT FIVE DAYS APPEAR TO BE FOCUSED ACROSS SOUTHERN
ARKANSAS AND THE RIVER RIVER VALLEY OF THE SOUTH...WHERE LOCAL
AMOUNTS OF 10-15 INCHES OF RAINFALL ARE POSSIBLE.
ROTH/PETERSEN
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page