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FXUS02 KWBC 171930
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
330 PM EDT THU SEP 17 2009
VALID 12Z SUN SEP 20 2009 - 12Z THU SEP 24 2009
AN EX SUPER TYPHOON ENTERING THE WESTERLIES AND A SHARP AMPLIFYING
RIDGE OVER THE W COAST STATES AND VANCOUVER/BRITISH COLUMBIA ARE
P0RIDGING THE BACKDROP FOR AN UNUSUALLY WIDE RANGE OF SOLUTIONS
CONCERNING THE POTENTIAL TROF OR CLOSED UPPER LOW THAT IS EXPECTED
TO DEVELOP OVER THE LOWER 48 DOWNSTREAM OF THE SHARP RIDGE DURING
THIS MEDIUM RANGE PERIOD.
MODEL AND ENSEMBLE GUIDANCE CONTINUES TO DISPLAY CONSIDERABLE
SPREAD REGARDING HOW WRN/CENTRAL NOAM TROF ENERGY WILL EVOLVE IN
RESPONSE TO A STRENGTHENING WEST COAST RIDGE. WHILE THIS SPREAD
AMONG 00Z/17 AND 12Z/17 SOLUTIONS IS CONSIDERABLE...00Z/17 GEFS
AND 00Z ECMWF ENSEMBLES DEFINED SUCH A BROAD ENVELOPE OF POTENTIAL
SOLNS THAT THERE WERE NO TRUE OUTLIERS AMONG THE 00Z/17
OPERATIONAL FCSTS.
12Z DETERMINISTIC MODELS DO LITTLE TO SWAY US FROM OUR EARLIER
THINKING ON THE POSSIBLE CLOSED LOW TO DEVELOP IN THE W CENTRAL
CONUS. THE NEW 12Z/17 CANADIAN HAS TRENDED W TOWARDS THE GFS
POSITION OF THE LOW DAYS 4-6. HOWEVER...THE WWD TREND OF THE
CANADIAN IS OFFSET BY THE SUPER-PROGRESSIVE SOLUTION OF THE UKMET.
THAT LEAD US BACK TO THE SAME LONGITUDE AS THE 00Z/17 CANADIAN
RUN.. IT SEEMS THAT THERE IS AT LEAST A 50% CHANCE OF THERE BEING
SOME KIND OF CLOSED UPPER LOW IN THE BASE OF THE TROF BY TUE.
AS MEDIUM RANGE LUCK WOULD HAVE IT...THE NEW 12Z ECMWF CAME IN
WITH ITS CLOSED LOW OVER WRN IA DAY 5 ...MIDWAY BETWEEN THE NEW
GFS AND UKMET! THRU DAYS 7... FINAL PROGS STAYED WITH A 00Z/17
CANADIAN/GFS ENSEMBLE MEAN/ECMWF ENSEMBLE MEAN BLEND....WHICH
GENERATES A FCST WHICH PROVIDES THE OPERATIONAL SOLN CORRESPONDING
BEST TO TELECONNECTIONS. EVEN THOUGH THE 00Z/17 MODEL ENSEMBLE
MEANS PLACE THE MEAN RIDGE A LITTLE WWD OF OPERATIONAL
RUNS...THEIR NOAM TROF COMPARES ACCEPTABLY TO TELECONNECTIONS.
MULTI-DAY MEANS BY MID-LATE PERIOD INDICATE A CORE OF POSITIVE HGT
ANOMALIES SOMEWHERE BETWEEN THE PAC NW AND JUST W/NW OF VANCOUVER
ISLAND. TELECONNECTIONS RELATIVE TO THIS CENTER FAVOR A
MODERATELY AMPLIFIED MEAN TROF WITH LITTLE HINT OF THE FLOW
SEPARATION DEPICTED IN THE MAJORITY OF DETERMINISTIC 00Z/17 AND
12Z/17 MODEL RUNS.THERE IS HIGH LONGITUDINAL SENSITIVITY OF THE
DOWNSTREAM TROF RELATIVE TO THE POSITIVE HEIGHT ANOMALY CENTER
POSN. TELECONNECTIONS DO NOT FAVOR AS MUCH ENERGY BEING PULLED
INTO THE SWRN CONUS OR SRN HIGH PLAINS AS DEPICTED IN THE RECENT
GFS/ECMWF DETERMINISTIC RUNS. THERE IS ALSO HIGH UNCERTAINTY
CONCERNING THE CANADIAN/NEW ENG NRN STREAM TROFS BY DAY 7 THU.
THUS WE PREFER TO LEAN AWAY FROM SOLUTIONS SHOWING A LOT OF TROF
IN ERN CANADA/NEW ENG DAY 6 OR 7 UNTIL THERE IS BETTER
CONTINUITY/CONSENSUS.
THE SENSIBLE WEATHER SCENARIO OVER THE NATION WILL BE LOWER THAN
NORMAL CONFIDENCE AND A FUNCTION OF THE BEHAVIOR OF THE TROF OR
CLOSED LOW AS IT EVOLVES OVER THE LOWER 48 STATES.
WHILE TIMING/LOCATION OF AN UPPER LOW IN PLAINS IS IN DOUBT BEYOND
DAY 4....THERE SHOULD BE ENUF DYNAMICS WHEREVER THE UPPER LOW ENDS
UP TO SUPPORT A FEW STRONG CONVECTIVE STORMS.
FLOOD
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