175
FXUS02 KWBC 311905
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
305 PM EDT MON AUG 31 2009
VALID 12Z THU SEP 03 2009 - 12Z MON SEP 07 2009
THE APEX OF THE RIDGE ACROSS NORTHWEST CANADA EARLY IN THE PERIOD
AND A QUASI-ZONAL FLOW ACROSS THE MID-LATITUDES OF THE PACIFIC SET
THE STAGE FOR A PAIR OF SYSTEMS TO MOVE ASHORE NORTH AMERICA NEAR
THE WESTERN US/CANADIAN BORDER. THE MODELS GENERALLY AGREE ON
THIS SCENARIO. OUT EAST...PARTICULARLY ACROSS THE
NORTHEAST...DIFFERENCES SHOW UP IN THE GUIDANCE AS THE 00Z ECMWF
STRAYS FROM THE OTHER DETERMINISTIC AND ENSEMBLE GUIDANCE WITH ITS
RIDGING ACROSS NEW ENGLAND EARLY NEXT WEEK. THE 00Z ECMWF IS ALSO
OUT OF TOLERANCE WITH THE REMAINDER OF THE GUIDANCE NEAR ALASKA.
WHILE THE 12Z ECMWF IMPROVES THE SITUATION OVER THE NORTHEAST...IT
BECOMES QUITE THE OUTLIER OUT WEST NEXT MONDAY...SIMILAR TO THE
30/12Z GFS BUT DELAYED BY ONE DAY. EARLY ON...WHEN THE
DIFFERENCES ARE MINIMAL...A 00Z ECMWF/00Z CANADIAN SOLUTION WAS
USED. AFTER SATURDAY MORNING...A 00Z CANADIAN/06Z GFS SOLUTION
WAS USED TO ACCOUNT FOR THE EXPECTATION OF TROUGHING OVER NEW
ENGLAND AND ATLANTIC CANADA. THIS PATTERN LOOKS WET FOR THE
SOUTHWEST...PACIFIC NORTHWEST...MIDWEST...LOWER MISSISSIPPI
VALLEY...AND THE SOUTHEAST. INCREASING SPREAD OUT WEST LATE IN
THE PERIOD LOWERS CONFIDENCE...BUT IT DID NOT CHANGE OUR SOLUTION
FROM THE MORNING CONTINUITY.
SOUTHWEST/CENTRAL ROCKIES...
AS HURRICANE JIMENA MOVES UP THE BAJA CALIFORNIA PENINSULA...A
MONSOON SURGE IS EXPECTED TO MOVE UP THE GULF OF CALIFORNIA/SEA OF
CORTEZ TO ITS EAST...ENHANCING RAINFALL TO THE NORTHEAST OF THE
SYSTEM ALONG THE COAST OF WESTERN MAINLAND MEXICO BEFORE MOVING
INTO ARIZONA THURSDAY AFTERNOON/EVENING. THE ECMWF HAS TRENDED
SERIOUSLY TO THE RIGHT WITH ITS TRACK OVER ITS PAST DAY OF
RUNS...NOW TAKING ITS MID-LEVEL REMNANT INTO SOUTHERN TEXAS EARLY
NEXT WEEK RATHER THAN WEST ALONG 20N INTO THE OPEN PACIFIC. THE
NORTHEAST TREND IN THE NHC GUIDANCE LED TO A ONE DAY ACCELERATION
OF THE ANTICIPATED ARRIVAL OF THIS MOISTURE FROM THIS TIME
YESTERDAY. THUNDERSTORMS SHOULD FIRE EACH AFTERNOON AND
EVENING...WITH THE MOISTURE FEED EXPECTED TO BE DIRECTED INTO THE
CENTRAL ROCKIES BY THIS WEEKEND. AREAL AVERAGE AMOUNTS OF 0.50
INCH OR MORE PER DAY OVER AND DOWNWIND OF TOPOGRAPHY IN ARIZONA
CAN BE EXPECTED FROM THURSDAY INTO THIS WEEKEND...WITH LESSER
AMOUNTS DOWNSTREAM ALONG THE CONTINENTAL DIVIDE OF THE CENTRAL
ROCKIES.
WESTERN SUBTROPICAL ATLANTIC...
THIS MODEL CHOICE DOES NOT EXTEND TO THE TROPICAL DISTURBANCE NEAR
15.5N 53.5W...WHERE A COMBINATION OF CONTINUITY AND THE 00Z
MULTI-MODEL ENSEMBLE SPREAD WERE UTILIZED TO RECURVE THE LOW
OFFSHORE FLORIDA/THE BAHAMAS JUST AHEAD OF THE STATIONARY FRONT
AND MINIMIZE POSSIBLE ERROR. THERE IS SIGNIFICANT SPREAD BOTH IN
THE 00Z MULTI-MODEL ENSEMBLE MEMBERS AND THE DETERMINISTIC
GUIDANCE AS TO WHERE THIS SYSTEM GOES AND HOW STRONG IT WILL
BE...WITH THE 00Z CANADIAN ON THE NORTHEAST SIDE OF THE SPREAD AND
THE 00Z ECMWF/06Z GFS ON THE WEAK SIDE OF THE SPREAD...MOVING A
TROPICAL WAVE THROUGH THE FLORIDA STRAITS. HOW MUCH OF A SURFACE
LOW EXISTS OFFSHORE FLORIDA AND THE BAHAMAS IS QUESTIONABLE LATE
IN THE PERIOD...AS THE UPPER PATTERN IN THE 00Z ECMWF/06Z GFS/00Z
CANADIAN BECOMES LESS FAVORABLE NORTH OF 20N LATITUDE AND WEST OF
60W LONGITUDE. THE CURRENTLY AGREED UPON POINTS ARE SIMILAR TO
PREVIOUS PROGS AND RELATIVELY CLOSE TO THE 12Z ECMWF RUN...WHICH
FINALLY HAS A SURFACE LOW TO FOLLOW OVER THE NEXT WEEK.
ROTH
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