454
FXUS02 KWBC 041839
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
238 PM EDT TUE AUG 04 2009
VALID 12Z FRI AUG 07 2009 - 12Z TUE AUG 11 2009
MEDIUM RANGE GUIDANCE IS FAIRLY AGREEABLE IN PRINCIPLE WITH THE
LARGE SCALE EVOLUTION THRU THE FCST PERIOD. EXPECT THE TROF OVER
THE WRN CONUS AS OF EARLY FRI TO EJECT NORTHEASTWARD WHILE AN
UPPER LOW NEAR THE ERN ALEUTIANS ON FRI TRACKS SEWD INTO THE NERN
PACIFIC. THE TROF ASSOC WITH THIS FEATURE SHOULD APPROACH THE
WEST COAST BY THE LATTER HALF OF THE PERIOD. MEANWHILE MOST SOLNS
ANTICIPATE THE BUILDING OF AN ERN CONUS RIDGE INTO THE WEEKEND...
WITH SOME WEAKENING/RETROGRESSION BY EARLY NEXT WEEK IN RESPONSE
TO UPSTREAM FLOW.
THE LARGEST SCALE UNCERTAINTY TODAY INVOLVES THE TIMING OF THE
NERN PAC UPPER LOW/TROF SUN-TUE. THE SIMILAR 00Z-06Z GFS AND GEFS
MEAN RUNS ARE SUFFICIENTLY FAST TO BRING THE AXIS OF THE OVERALL
TROF TO THE WEST COAST BY DAY 7 TUE WHILE THE 00Z ECMWF/EC
ENSEMBLE MEAN/CANADIAN/NOGAPS KEEP THE MEAN TROF WELL OFF THE WEST
COAST THRU NEXT TUE. BY DAY 5 MON THE UKMET BRINGS MORE ENERGY TO
THE WEST COAST THAN THE ECMWF CLUSTER BUT KEEPS THE UPPER LOW
FARTHER NWD THAN GFS-BASED GUIDANCE. THE UKMET SOLN RESEMBLES THE
12Z/03 ECMWF. GIVEN AN ENVELOPE OF SOLNS IN WHICH GFS/GEFS SOLNS
ARE FASTEST... SLOWER TIMING THAN THE GFS IS USUALLY BEST.
HOWEVER WITH THE 00Z ECMWF SLOWER THAN THE PREVIOUS THREE ECMWF
RUNS... THE CANADIAN ENSEMBLE MEAN OFFERING A COMPROMISE TROF AXIS
BY DAY 7... AND SOME SPREAD WITH THE EXPECTED POSN OF THE CORE OF
NEGATIVE HGT ANOMALIES OVER ERN CANADA... AT LEAST A MODESTLY
FASTER ADJUSTMENT FROM THE 00Z ECMWF SEEMS WARRANTED.
AHEAD OF THIS EVOLUTION THERE IS TYPICAL SPREAD WITH DETAILS OF
THE TROF EJECTING FROM THE WEST DURING THE FIRST HALF OF THE
PERIOD. THE 06Z GFS LEANS TO THE FAST EDGE OF GUIDANCE WITH THE
REMAINING SHRTWV BY EARLY DAY 5 SUN. ASIDE FROM THIS POSSIBLE
TIMING ERROR... THE 06Z GFS IS A VAST IMPROVEMENT OVER ERN CANADA
AND NERN CONUS VERSUS THE 00Z GFS DAYS 5-7 SUN-TUE. THE 00Z GFS
HAD SHOWN ENOUGH AMPLITUDE OVER ERN CANADA TO ALLOW FOR PHASING
WITH THE EJECTING WRN CONUS ENERGY... AN EVOLUTION THAT HAD LITTLE
SUPPORT FROM OTHER MODELS OR GEFS/ECMWF/CMC ENSEMBLES. SOME
UNCERTAINTY REMAINS WITH THE EXACT DETAILS/POSN OF A WAVY FRONT
THAT SHOULD GENERALLY LIFT NEWD OVER THE ERN CONUS. A SOLN
WEIGHTED MORE IN THE ECMWF DIRECTION APPEARS REASONABLE GIVEN SOME
QUESTION MARKS IN GFS FLOW UPSTREAM.
12Z UPDATE... THE 12Z GFS/GEFS MEAN ARE STILL FAIRLY QUICK TO
BRING HGT FALLS TO THE WEST COAST BUT EVENTUALLY THE GFS TRENDS
TOWARD THE 00Z ECMWF CLUSTER WITH RESPECT TO HOLDING THE NERN PAC
UPPER LOW CENTER FARTHER NWWD THAN 06Z/00Z GFS RUNS. BY DAY 6 MON
THE UKMET PATTERN OVER THE NERN PAC/WEST COAST IS A TAD SE OF THE
12Z GFS. THE GEFS MEAN ESSENTIALLY BECOMES A BLEND OF
LATEST/PREVIOUS RUNS BY LATE IN THE FCST. BY DAY 6 MON THE 12Z
CANADIAN SHOWS A COMPROMISE TIMING WITH THE ERN PAC TROF... BUT IS
A FAST OUTLIER WITH THE EJECTING WRN TROF DOWNSTREAM. THE UKMET
GENERALLY SUPPORTS THE ECMWF/12Z GFS WITH THE EJECTING WRN TROF.
IN SUMMARY THE 12Z GUIDANCE MAINTAIN REASONABLE SUPPORT FOR AN
ECMWF-TYPE SOLN OVER THE ERN TWO THIRDS OF THE CONUS... AND A
COMPROMISE WITH THE NERN PAC TROF APPROACHING THE WEST COAST.
BASED ON THE ABOVE REASONING THE DAYS 3-4 FRI-SAT FCST MAINTAINS A
SOLN CLOSE TO THE 00Z ECMWF. DAYS 5-7 SUN-TUE FOLLOW A 50/20/30
BLEND OF THE 00Z ECMWF/EC ENSEMBLE MEAN/GEFS MEAN TO NUDGE THE
NERN PAC TROF TIMING TOWARD AN INTERMEDIATE SOLN AND TO ACCOUNT
FOR TYPICAL LATE PERIOD DETAIL UNCERTAINTY.
REGIONAL HIGHLIGHTS...
..WEST
AFTER A COOL AND UNSETTLED START TO THE FCST PERIOD FRI...
DEPARTURE OF AN UPPER TROF FROM THE WEST SHOULD YIELD A DRIER
TREND WITH TEMPS GRADUALLY RECOVERING TO NEAR OR SLIGHTLY ABOVE
NORMAL READINGS. PERIODS OF LGT RNFL MAY PERSIST OVER THE EXTREME
NORTHWEST DEPENDING ON THE EWD/SEWD PROGRESS OF A NERN PAC TROF
WHOSE TIMING REMAINS MODERATELY UNCERTAIN... AND SOME DIURNAL
CONVECTION IS POSSIBLE OVER/NEAR THE SRN ROCKIES.
..CENTRAL/EAST
FLOW AHEAD OF THE SYSTEM LIFTING NEWD FROM THE WEST SHOULD PULL
WARMER THAN NORMAL TEMPS NEWD ACROSS MUCH OF THE ERN CONUS THRU
THE WEEKEND. LOW PRESSURE TRACKING NEWD FROM THE NRN PLAINS SAT
ONWARD WILL BRING A COLD FRONT ACROSS NRN AREAS BUT TEMPS ACROSS
MOST OF THE SRN HALF OF THE CNTRL-ERN CONUS SHOULD REMAIN AT OR
ABOVE NORMAL THRU NEXT TUE. BEST ORGANIZED CONVECTION/RAINFALL
SHOULD BE FROM THE NRN PLAINS INTO THE GRTLKS WITH THE LOW
PRESSURE/COLD FRONT... AND WITH DIURNAL ACTIVITY NEAR THE GULF
COAST AND OVER THE SOUTHEAST.
RAUSCH
A
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