HPC Preliminary Graphics & Discussion
Posted by cypresstx on 8/23/2012, 12:04 pm

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER COLLEGE PARK MD
956 AM EDT THU AUG 23 2012

VALID 12Z MON AUG 27 2012 - 12Z THU AUG 30 2012

THE PRIMARY ISSUES DURING THE MEDIUM RANGE PERIOD INVOLVE THE
DETAILS OF FLOW WITHIN AN EXPECTED NERN PAC/PAC NW COAST MEAN
TROF... AND DOWNSTREAM IMPLICATIONS OVER THE ERN CONUS WITH IMPACT
ON THE EVENTUAL TRACK OF T.S. ISAAC WHICH IS FCST TO REACH HURCN
STRENGTH BY THE TIME IT REACHES THE ERN GULF OF MEXICO BY THE
FIRST HALF OF NEXT WEEK.  CONSULT LATEST NHC
ADVISORIES/DISCUSSIONS FOR FURTHER INFO REGARDING THE FCST OF T.S.
ISAAC.

FROM THE NERN PAC INTO WRN NOAM... 00Z/06Z GFS RUNS AND THE 00Z
GEFS MEAN ARE ON THE SLOW SIDE OF THE FULL GUIDANCE SPREAD WITH A
CLOSED LOW FCST TO BE OFFSHORE THE PAC NW COAST AS OF EARLY DAY 3
SUN.  ON THE OTHER HAND THE 00Z ECMWF/ECMWF MEAN ARE ON THE FAST
SIDE OF THE SPREAD TO EJECT THE FEATURE INLAND WITH THE UKMET
SOMEWHAT SLOWER AND THE CANADIAN SLOWER YET... BUT STILL FASTER
THAN GFS/GEFS MEAN SOLNS.  THE BIG QUESTION MARK IS THE EXTENT TO
WHICH UPSTREAM ENERGY FEEDS INTO THE MEAN TROF WITH THE CANADIAN
ENSEMBLE MEAN ADDING TO A MODERATE MAJORITY INDICATING THAT THIS
UPSTREAM FLOW WILL SERVE TO EJECT THE INITIAL CLOSED LOW FASTER
THAN THE GFS CLUSTER.  CONFIDENCE IN ONE SCENARIO OR THE OTHER IS
NOT PARTICULARLY HIGH SO A COMPROMISE APPROACH IS FAVORED.

AS FOR ERN NOAM IMPACT OF THE NERN PAC/WEST COAST DETAILS... THE
SLOWER EJECTING GFS CLUSTER OVER THE ERN PAC GENERALLY LEADS TO
SLOWER/MORE AMPLIFIED TROFFING OVER ERN NOAM AND FASTER NWD/NEWD
PROGRESSION OF ISAAC AFTER DAY 5 TUE.  THE 06Z GFS IS FARTHEST WWD
WITH THE ERN PAC CLOSED LOW AND MOST AMPLIFIED WITH ERN NOAM
TROFFING/FASTEST TO LIFT ISAAC NEWD.  THE 00Z GFS IS LESS EXTREME.
SOLNS THAT ARE FASTER TO EJECT ERN PAC ENERGY INTO NOAM ARE
LIKEWISE MORE PROGRESSIVE OVER ERN NOAM SO THAT ISAAC IS NOT
PULLED AS FAR NEWD.  INTERESTINGLY THE GEFS MEAN IS PART OF THE
SLOWER CLUSTER UPSTREAM AND PULLS OFF A PIECE OF ENERGY FROM ISAAC
THAT TRACKS IN A MANNER SIMILAR TO THE 06Z GFS... BUT ALSO LEAVES
BEHIND A REFLECTION OVER THE SOUTHEAST THRU DAY 7.  OVERALL A
MODERATELY SLOW PROGRESSION OF ISAAC APPEARS TO BE FAVORED BASED
ON COMPROMISE FLOW TO THE W AND N.

DAYS 3-5 SUN-TUE USE A BLEND OF HALF 00Z ECMWF MEAN AND THE
REMAINDER 00Z GFS/GEFS MEAN TO REFLECT A COMPROMISE APPROACH WITH
DETAILS OF ERN PAC THRU SRN CANADA FLOW... WITH THE LATTER ALSO
HELPING TO REDUCE NEEDED ADJUSTMENTS TO ACCOMMODATE THE NHC TRACK
FOR ISAAC.  THE DESIRED COMPROMISE IS BEST DEFINED BY A SIMPLER
BLEND OF THE 00Z GEFS/ECMWF MEANS FOR DAYS 6-7 WED-THU.  THE DAYS
6-7 TRACK FOR ISAAC MAINTAINS REASONABLE CONTINUITY WITH EARLY
PRELIM ISSUANCE... REACHING THE CNTRL AL/GA BORDER BY EARLY NEXT
THU.  AFTN ISSUANCE WILL REFLECT THE 17Z NHC/HPC COORDINATED
TRACK.

RAUSCH








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HPC Preliminary Graphics & Discussion - cypresstx, 8/23/2012, 12:04 pm
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