327
FXUS02 KWBC 011828
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
227 PM EDT THU OCT 01 2009
VALID 12Z SUN OCT 04 2009 - 12Z THU OCT 08 2009
AS OF DAY 3 SUN THERE IS GOOD AGREEMENT UPON A TROF EXTENDING FROM
THE ERN BERING SEA SWD... A RIDGE OVER THE NERN PAC... AND CLOSED
LOWS OVER THE WRN CONUS AND UPR GRTLKS/ONTARIO. HOWEVER MODEL AND
ENSEMBLE SPREAD ALONG WITH MODEL RUN DISCONTINUITIES REGARDING THE
HANDLING OF ENERGY WITHIN THE ERN BERING SEA/NRN PAC TROF INCREASE
RAPIDLY AFTER DAY 3. THESE DIFFS INFLUENCE THE SHAPE OF THE
DOWNSTREAM RIDGE... AND IN TURN THE TIMING AND STRENGTH OF WRN
CONUS ENERGY THAT IS EXPECTED TO EJECT NEWD AND SUPPORT LOW
PRESSURE TRACKING INTO ERN CANADA. THERE IS ALSO THE QUESTION OF
HOW MUCH PHASING OCCURS BETWEEN NRN STREAM FLOW AND THE MEAN TROF
OVER THE WEST.
AT THE VERY LEAST THE PAST COUPLE ECMWF RUNS AND 00Z
UKMET/CANADIAN INDICATE THAT A MORE SUBSTANTIAL PORTION OF INITIAL
WRN CONUS ENERGY WILL BE EJECTED THAN FCST BY THE 00Z/06Z GFS.
THRU DAY 5 TUE THE 00Z GFS IS ON THE FAST EDGE OF THE
MODEL/ENSEMBLE SPREAD WITH ITS SFC WAVE REACHING THE GRTLKS WHILE
THE 06Z GFS COMPARES SOMEWHAT BETTER TO THE GUIDANCE SPREAD. BY
DAYS 6-7 DIFFS WITH NRN STREAM FLOW LEAD THE GFS RUNS TO BRING
FAIRLY STRONG LOW PRESSURE INTO THE NORTHEAST OR MID ATLC WHILE
MOST REMAINING GUIDANCE CLUSTERS TOWARD TWO FAVORED AREAS OF LOW
PRESSURE... ONE WELL AHEAD OF THE GFS LOWS AND ANOTHER FARTHER
NWWD. OVERALL THE 06Z GFS/00Z ECMWF AND TO A LESSER DEGREE AN
AVERAGE OF ENSEMBLE SOLNS PROVIDE A FAIR ACCOUNT FOR PREFERENCES
EARLY IN THE FCST... FOLLOWED BY AN AVERAGE OF ENSEMBLE MEANS.
MEANWHILE THERE IS FAIR AGREEMENT UPON RESIDUAL WRN CONUS ENERGY
FORMING ANOTHER CLOSED LOW IN THE VICINITY OF CA BY AROUND DAY 6
WED. AN AVERAGE OF LATEST MODEL/ENSEMBLE GUIDANCE SHOULD PROVIDE
THE BEST STARTING POINT FOR THIS FEATURE IN LIGHT OF UNCERTAINTY
WITH NEARBY NRN STREAM FLOW.
RELATIVE TO THE NERN PAC RIDGE... TELECONNECTIONS INDICATE THAT
THE DEGREE OF PHASING BETWEEN NRN STREAM FLOW AND THE WRN CONUS
TROF IS FAIRLY SENSITIVE TO THE EXACT POSITION OF THE POSITIVE HGT
ANOMALY CENTER OVER THE EXTREME NERN PAC... AND THUS CONFIDENCE IS
LOW WITH THIS PART OF THE FCST. BY THE D+8 TIME FRAME AN UPSTREAM
NRN PAC TROF APPEARS TO GAIN PROMINENCE WHICH COMBINED WITH A
POSSIBLE SLIGHTLY EWD NUDGE OF THE NERN PAC RIDGE SHOULD FAVOR A
MODERATE ERN NOAM TROF WITH A CORE OF NEGATIVE ANOMALIES OVER AND
TO THE N/E OF THE UPR GRTLKS WITH FLAT TO SLIGHTLY CYCLONIC FLOW
FARTHER SWD. THIS LATE PERIOD TELECONNECTION RELATIONSHIP WOULD
ARGUE THAT BY DAY 7 THU THE CANADIAN IS TOO DEEP WITH ITS ERN
CONUS TROF WHILE THE 06Z GFS AND 00Z ECMWF/ECMWF ENSEMBLE MEAN
COULD BE A TAD STRONG WITH THE SERN RIDGE BUT NOT TO AN EXTENT AS
TO PRECLUDE SOME WEIGHTING IN A FCST BLEND.
12Z UPDATE... THE UKMET/CANADIAN OFFER SOME DETAIL/TIMING DIFFS
BUT ROUGHLY SUPPORT THE 00Z ECMWF/ECMWF MEAN IDEA OF A WELL
DEFINED WAVE REACHING NEAR THE WRN-CNTRL GRTLKS BY LATE TUE OR
EARLY TUE NIGHT. THE 12Z GEFS MEAN IS WEAKER AND IN THE SLOWER
HALF OF THE CLUSTERING BUT OTHERWISE SUPPORTS THIS IDEA. THIS
LEAVES THE WEAKER/FASTER 12Z GFS AS A RELATIVE OUTLIER. VARIOUS
FEATURES UPSTREAM AND OVER THE WEST SHOW CONTINUED SPREAD BUT FALL
WITHIN THE PREVIOUSLY DEFINED SOLN SPREAD.
DAYS 3-4 SUN-MON START WITH A BLEND THAT INCORPORATES SLIGHTLY
MORE WEIGHTING OF THE 06Z GFS/00Z ECMWF THAN THE 12Z GEFS/00Z
ECMWF ENSEMBLE MEANS... AS THE 06Z GFS COMPARES BETTER TO
REMAINING GUIDANCE THAN THE 12Z RUN. AS UNCERTAINTY RAPIDLY
INCREASES THEREAFTER... THE PREFERRED STARTING POINT THEN
TRANSITIONS TO MAJORITY WEIGHTING OF THE 12Z GEFS/00Z ECMWF MEANS.
THE RESULTING FCST REFLECTS A MODERATE FORM OF THE BEST
CLUSTERING OF GUIDANCE WITH LOW PRESSURE FCST TO TRACK NEWD FROM
THE PLAINS THRU THE GRTLKS AND PROVIDES A REASONABLE INTERMEDIATE
SOLN WITH UPSTREAM SYSTEMS.
REGIONAL HIGHLIGHTS...
..WEST
EXPECT BELOW TO MUCH BELOW NORMAL TEMPS OVER MOST OF THE WEST
DURING THE PERIOD. COLDEST DAYTIME READINGS OF 10-20 F BELOW
AVERAGE SHOULD OCCUR IN THE SUN-TUE TIME FRAME FROM PORTIONS OF
CA/OR THRU THE NRN HALF OF THE ROCKIES. NIGHTTIME LOWS SHOULD BE
LESS EXTREME. RAIN AND HIGHER ELEV SNOW SHOULD GRADUALLY DIMINISH
SUN-TUE OVER THE N-CNTRL INTERIOR WEST AND NRN-CNTRL ROCKIES.
ANOTHER AREA OF PCPN MAY DEVELOP OVER PARTS OF THE SOUTHWEST
MID-LATE WEEK.
..CENTRAL/EAST
AREAS FROM THE SRN HALF OF THE PLAINS INTO THE LOWER OH VALLEY MAY
SEE ONE OR MORE PERIODS OF LOCALLY MDT/HVY RNFL DURING THE
PERIOD... AIDED BY THE COMBINATION OF LOW LVL GULF INFLOW AND
HIGHER LVL FLOW CARRYING MOISTURE FROM T.D. EIGHTEEN E NEWD INTO
THE REGION. MEANWHILE AN ORGANIZED AREA OF RNFL MAY PROGRESS FROM
THE NRN PLAINS THRU THE NORTHEAST DURING THE PERIOD... BUT AT THIS
TIME CONFIDENCE IS LOW WITH RESPECT TO TIMING/COVERAGE/INTENSITY.
DAYTIME TEMPS WILL BE BELOW TO WELL BELOW NORMAL OVER THE NRN
PLAINS. ELSEWHERE READINGS SHOULD GENERALLY BE MORE MODERATE...
WITH ABOVE NORMAL TEMPS POSSIBLE OVER PORTIONS OF THE SOUTH BY
TUE.
RAUSCH
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