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FXUS02 KWBC 211756
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
155 PM EDT FRI AUG 21 2009
VALID 12Z MON AUG 24 2009 - 12Z FRI AUG 28 2009
MULTI-DAY MEANS AND ENSEMBLES SHOW THE LONGWAVE PATTERN
DE-AMPLIFYING AND RETROGRADING IN THE MEDIUM RANGE PERIOD...WITH
GENERAL AGREEMENT FOR A WESTERN NORTH AMERICAN RIDGE AND EASTERN
NORTH AMERICAN TROUGH BY DAY 7. IN TERMS OF THE DETAILS...THE GFS
IS FASTEST IN LIFTING A SIGNIFICANT PORTION OF THE EASTERN CONUS
UPPER LOW INTO CANADA DAY 3...AND SLOWEST TO PROGRESS AN UPPER LOW
THROUGH WESTERN CANADA...BOTH OF WHICH HAVE THE LEAST
DETERMINISTIC AND ENSEMBLE SUPPORT. THUS...WILL BEGIN THE
PRESSURES/FRONTS WITH A BLEND OF THE ECMWF/CANADIAN DAYS
3/4...WHICH IS NEAR THE CONSENSUS. SOLUTION SPREAD INCREASES
NOTICEABLY BY DAY 5...ESPECIALLY ACROSS ALASKA/NORTHEAST PACIFIC
WHERE THE OPERATIONAL ECMWF LIES NEAR THE DEEP EDGE OF THE
GUIDANCE ENVELOPE WITH AMPLIFICATION OF A SIGNIFICANT SHORTWAVE
TROUGH...INCLUDING CONSIDERABLE RUN-TO-RUN VARIABILITY...AND SO
CONFIDENCE IN ITS SOLUTION IS LOW. RATHER...PREFER 1/3 EACH
ECMWF/CANADIAN/ECMWF ENSEMBLE MEAN STARTING DAY 5 TO LESSEN THE
AMPLITUDE OF THE OPERATIONAL ECMWF WHILE EMPHASIZING THE
RELATIVELY AGREEING CANADIAN AND ECMWF ENSEMBLE MEAN...BEFORE
TRANSITIONING TOWARD A 2/3 ECMWF ENSEMBLE MEAN TO 1/3 CANADIAN
BLEND FOR DAYS 6/7.
UPDATED MORNING HPC PRELIMS INCORPORATE MORE OF THE ENS MEANS OF
BOTH GFS AND ECMWF AND A COMBINATION LAGGED AVERAGE FORECAST OF
GFS AND ECMWF. THIS SETTLES THE DIFFERENCE BETWEEN OP 00Z GFS AND
ECMWF OVER THE PAC NORTHWEST BY DAY 7 WHILE LEAVING AN AGREED UPON
PATTERN OF A MEAN NERN PAC TROF/DOWNSTREAM NRN CONUS/CANADIAN
ROCKIES RIDGING AND RIDGING PERSISTING OVER SWRN CONUS AND A MEAN
ERN CONUS TROF WITH ITS AXIS ALONG 80W.
12Z GFS CONTS TO BE CLOSER TO ITS 00Z RUN OVER THE NERN PAC WITH
ITS CLOSER IN GLFAK LOW AND DRIVING THE DOWNSTREAM RIDGE EWD TO
NEAR 100W-105W WHILE 00Z CMC/ECMWF AND 00Z ENS MEANS OF BOTH ECMWF
AND GFS KEEP RIDGING ALONG 110W AND AGREE ON A DOWNSTREAM TROF
AXIS NEAR 80W BY DAYS 6 AND 7. NO CHANGES TO AFTN FINALS FROM
MORNING PRELIMS.
MONSOONAL CONDITIONS WILL CONTINUE OVER THE SOUTHWEST AND SRN
ROCKIES AT LEAST THRU MID WEEK WHILE SOME LIGHTER SHOWERS WILL BE
ASSOCIATED WITH COLD FRONT DROPPING SWD THRU THE PLAINS
TUES-THURS. POTENTIAL FOR SOME STRONGER CONVECTIVE ACTIVITY IN THE
CENTRAL PLAINS MID WEEK ALONG THE SRN EDGE OF THE WESTERLIES.
POSSIBLE GULF COAST SHOWERS AND STRONGER CONVECTION MAY BE
TRIGGERED BY A POTENTIAL RESIDUAL MID LEVEL LOW THAT MAY BREAK OFF
FROM THE MEAN ERN CONUS TROF WED-THURS.
EWD A TROPICAL WAVE CURRENTLY SW OF THE CAPE VERDE ISLANDS MAY
APPROACH THE BAHAMAS THURSDAY WITH SHOWERS POSSIBLY REACHING FL BY
FRIDAY. CMC CONTS TO BE VERY AGGRESSIVE WITH THIS WAVE WHILE 00Z
ECMWF ENSEMBLES SHOW A STRONG CLUSTERING OF LOWS IN THIS REGION
LATE WEEK. CHI VERTICAL VELOCITY POTENTIAL ANOMALY CHARTS STRONGLY
FAVOR THE TROPICAL EPAC FOR DEVELOPMENT THIS PERIOD WITH SOME
LESSER BUT STILL FAVORABLE CONDITIONS OVER THE CARRIBEAN/GLFMEX
AND BAHAMAS REGION. LATEST 12Z UKMET/CMC MAINTAIN CONTINUITY
INDICATING A TROPICAL WAVE OR GREATER WILL BE IN THIS REGION LATE
NEXT WEEK. SFC POSITIONS DEPICTED BY DAILY HPC/TPC COORDINATION.
JAMES/ROSENSTEIN
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