790
FXUS02 KWBC 281855
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
254 PM EDT FRI AUG 28 2009
VALID 12Z MON AUG 31 2009 - 12Z FRI SEP 04 2009
THE MODELS AND ENSEMBLES OFFER BELOW NORMAL MEDIUM RANGE SOLUTION
SPREAD/LARGER SCALE FLOW UNCERTAINTY OVER NORTH AMERICA FOR DAYS
4/5 AND TO A LESSER EXTENT DAYS 6/7 FORECAST PERIOD. THE REGIME IS
HIGHLIGHTED BY SLOWLY PROGRESSIVE AND AMPLIFIED FLOW COMPRISED OF
MEAN MID-UPPER LEVEL TROUGHS OVER THE ERN PAC AND ERN US THAT
SANDWICH A WEST-CENTRAL NOAM MEAN RIDGE.
IN THIS PATTERN...THE HPC MEDIUM RANGE PROGS WERE PRIMARILY
DERIVED FROM A 50-50 BLEND OF THE 00 UTC GFS AND 00 UTC ECMWF.
HOWEVER OVER THE NEXT 3-5 DAYS...ONE POTENTIAL FORECAST ISSUE
DESPITE GENERAL GUIDANCE SUPPORT CONCERNS THE TRACK OF SMALLER
SCALE AND SENSIBLE WEATHER FOCUSING CLOSED LOW ENERGY ACROSS THE
NWRN US AND INTO THE N-CENTRAL STATE THROUGH A WRN NOAM MEAN
LONGWAVE RIDGE POSITION. WATER VAPOR IMAGERY SHOWS A COMPLICATED
LEAD-IN SHORTER RANGE FLOW DYNAMIC THAT INCLUDES THE MOIST
REMNANTS OF IGNACIO OFF THE US WEST COAST AND THE FARTHER NORTH
ACROSS THE NE PAC THE EXTRATROPICALLY PHASED REMNANTS OF
EX-TYPHOON VAMCO. ALL THINGS CONSIDERED...HPC GUIDANCE WAS
MANUALLY ADJUSTED TO FAVOR A SLIGHTLY WEAKER INLAND HEIGHT FALLS
INFULTRATION WITHIN THE MEAN MID-UPPER RIDGE...BUT WITH SLIGHTLY
MORE MOISTURE AVAILABILITY.
BY DAYS 6-7...MODELS AND ENSEMBLES DO DIFFER MORE IN ERNEST WITH
RESPECT TO BOTH THE PROGRESSION OF AMPLIFIED ERN PACIFIC MID-UPPER
LEVEL TROUGH ENERGY INTO THE WRN US AND DOWNSTREAM WRN US RIDGE
AND LEAD ERN US MID-UPPER LEVEL PROGRESSION/KICK. PREFER A
COMPROMISE SOLUTION PROGRESSION CLOSE TO THAT PROVIDED AGAIN BY A
50-50 BLEND OF THE GENERALLY MORE PROGRESSIVE 00 UTC GFS AND LESS
PROGRESSIVE 00 UTC ECMWF. THIS HPC SOLUTION IS LESS PROGRESSIVE
THAN PRIOR HPC CONTINUITY BUT HAS SUPPORT CONSIDERING THE
AMPLIFIED NATUREE OF THE FLOW AND A COMPOSITE OF 00 UTC
GFS/ECMWF/CMC/NAEFS ENSEMBLE MEMBERS. CONTINUITY HERE IS NOW MORE
OF AN ISSUE AS THE 12 UTC GFS AND ECMWF OFFER A MORE PROGRESSIVE
SOLUTION INTO THE NWRN US IN CONTRAST TO THE STILL SUPPORTIVE AND
LESS PROGRESSIVE 12 UTC UKMET. OPTED TO MAINTAIN HPC CONTINUITY
FOR THE FINAL PROGS AND REVISIT WITH UPCOMING 00 UTC GUIDANCE TO
SEE IF THE FASTER RECENT GUIDANCE TREND IS MAINTAINED.
MUSHER/SCHICHTEL
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