369
FXUS02 KWBC 231841
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
241 PM EDT SUN AUG 23 2009
VALID 12Z WED AUG 26 2009 - 12Z SUN AUG 30 2009
THE MODELS AND ENSEMBLES BUILD A DEEP MID-UPPER LEVEL TROUGH OVER
THE NERN PAC AND GULF OF AK OVER THE UPCOMING WEEK THAT SHOULD
FORCE CURRENT DOWNSTREAM MID-CONTINENT RIDGING TO RETROGRADE BACK
OVER WRN NORTH AMERICA BY LATE WEEK. THIS LEADS TO SEVERAL
SIGNIFICANT GUIDANCE DIFFERENCES BY DAYS 5-7 WITH RESPECT TO THE
EXTENT THAT PACIFIC SYSTEM ENERGY PENETRATES OR ROUNDS THE
BUIDLING WRN NOAM RIDGE PRIOR TO DIGGING INTO A DOWNSTREAM
MID-UPPER LEVEL EAST-CENTRAL NOAM TROUGH POSITION.
ONE SUCH MODEL DIFFERENCE INVOLVES A MID-UPPER LEVEL TROUGH OVER
WRN CANADA DAY 3/WED AND THE EXTENT TO WHICH THE TROUGH DIGS AFTER
IT CRESTS THE MID-UPPER LEVEL RIDGE AXIS. THE 00 UTC ECMWF DIGS
THE TROUGH MORE OVER ROBUSTLY INTO THE EAST-CENTRAL US MORE THAN
MOST OTHER GUIDANCE BY DAYS 5-7. WHILE THIS STRONGER SOLUTION
APPEARS ATTAINABLE CONSIDERING THE AMPLITUDE OF THE RETROGRADING
UPSTREAM MID-UPPER LEVEL RIDGE IN MOST GUIDANCE...IT MAY BE WISE
AMID UNCERTAINTY TO SCALE BACK AMPLITUDE A BIT BY INCORPORATING
SUPPORTIVE BUT SOMEWHAT LESS AMPLIFIED ENSEMBLE GUIDANCE.
MEANWHILE...THE 00/06/12 UTC GFS RUNS OFFER A MUCH WEAKER
MID-UPPER LEVEL TROUGH OVER THE GREAT LAKES AND DOWN ACROSS THE
EAST-CENTRAL US LATE WEEK INTO NEXT WEEKEND...IN STEP WITH
REMAINING A STRONG OUTLIER OVER THE NORTHERN PLAINS BY DAY 6 WITH
A KICKER SHORTWAVE TROUGH THAT PENETRATES THE BUILDING WRN NOAM
MID-UPPER LEVEL RIDGE BY DAYS 4/5. THIS SEEMS A LESS LIKELY
SCENARIO CONSIDERING LIKELY WRN NOAM RIDGE STRENGTH IN MOST
GUIDANCE. THE EAST-CENTRAL NOAM SOLUTION IS ALSO IN BETWEEN
LATEST 12 UTC UKMET AND 12 UTC CANADIAN GUIDANCE AND GENERALLY IN
LINE WITH 12 UTC GFS ENSEMBLES AND THE 12 UTC ECMWF.
HOWEVER...THIS LATEST 12 UTC ECMWF RUN DOES NOT SEEM PLAUSIBLE
OVER THE NWRN US HEADING THROUGH NEXT WEEKEND WITH INLAND CLOSED
LOW DEVELOPMENT ALOFT THAT SEEMS EXCESSIVE IN A MEAN RIDGE
POSITION.
THUS...THE HPC PRESSURES/FRONTS AND 500 MB PROGS REMAIN PRIMARILY
DERIVED FROM A BLEND OF THE 00 UTC GFS/ECMWF DAY 4 INTO 5...BEFORE
ABANDONING THE GFS IN FAVOR OF A MORE 50/50 PERCENT BLEND OF THE
00 UTC ECMWF AND 00 UTC GFS ENSEMBLE MEAN BY NEXT WEEKEND.
JAMES/SCHICHTEL
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