EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
250 PM EDT MON MAY 12 2008
 
VALID 12Z THU MAY 15 2008 - 12Z MON MAY 19 2008
 
THERE IS GOOD AGREEMENT AMONG MODEL/ENSEMBLE GUIDANCE UPON A
STRONG WEST COAST RIDGE THAT WILL MAINTAIN A MEAN TROF OVER THE
ERN HALF OF NORTH AMERICA.  THE PRIMARY FCST PROBLEMS WITHIN THIS
CONSENSUS MEAN FLOW INVOLVE THE EVOLUTION OF ENERGY TO THE E OF
THE RIDGE... AND LATER IN THE PERIOD HOW MUCH PACIFIC ENERGY
PROGRESSES INTO THE RIDGE OVER THE NWRN CONUS/SWRN CANADA.

CONFIDENCE IS LOW WITH TIMING OF THE SHORT RANGE SWRN CONUS/SRN
ROCKIES CLOSED LOW EJECTING NEWD DURING THE LATTER HALF OF THE
WEEK.  00Z/06Z/12Z GFS AND FRACTIONALLY SLOWER 00Z ECMWF SHOW
SIGNIFICANTLY FASTER TIMING THAN THE PAST TWO
CANADIAN/UKMET/NOGAPS RUNS... AND THUS ARE FASTER WITH AN ASSOC
WAVE CROSSING THE ERN HALF OF THE CONUS THU-FRI.  THE 12Z GFS MAY
BE A TAD OVERDONE WITH THE WWD EXTENT OF UPSTREAM ENERGY AS OF THU
BUT THE RAPID APPROACH OF THIS ENERGY IN ALL GUIDANCE AT LEAST
SEEMS TO SUGGEST POTENTIAL FOR FASTER EJECTION THAN FCST BY THE
CMC CLUSTER... EVEN IF NOT QUITE TO THE EXTENT OF THE GFS SOLNS.

THE 12Z GFS HAS TRENDED AWAY FROM 00Z/06Z GFS RUNS WHICH WERE
FAIRLY PRONOUNCED OUTLIERS WITH THE ENERGY WHICH THOSE RUNS
AMPLIFIED INTO THE SRN ROCKIES/HIGH PLAINS BY FRI.  00Z RUNS OF
THE ECMWF/UKMET/CANADIAN/NOGAPS ALL PULL THIS ENERGY FARTHER SWWD
INTO THE SWRN CONUS/NWRN MEXICO AND THUS DO NOT DEPICT THE STRONG
SRN PLAINS-SERN CONUS SHRTWV FCST BY THE GFS.  00Z ECMWF ENSEMBLES
AGREE WELL WITH THE NON-GFS CLUSTER.  GEFS/CANADIAN ENSEMBLES DO
NOT LEAN AS STRONGLY IN THAT DIRECTION... BUT STILL TEND TO
DISCOUNT THE 00Z/06Z GFS SCENARIO.  TELECONNECTIONS RELATIVE TO
THE STRONG POSITIVE HGT ANOMALY CENTER OFF THE PAC NW COAST
SUPPORT THE MAJORITY SOLN THAT BRINGS MORE ENERGY INTO THE SWRN
CONUS/NWRN MEXICO THAN THE 00Z/06Z GFS... WITH FLATTER FLOW
DOWNSTREAM.  THE 12Z UKMET HAS ADJUSTED TO THE 00Z/06Z GFS IDEA
BUT PREMATURELY FLATTENS THE WEST COAST RIDGE BY DAY 5 SAT.  

SPREAD AMONG MODEL/ENSEMBLE SOLNS INCREASES RAPIDLY BY DAYS 6-7
SUN-MON AS PACIFIC ENERGY BEGINS TO IMPACT THE WEST COAST RIDGE. 
SOME RECENT GFS RUNS... ESPECIALLY FROM 06Z AND 12Z/11... THE
12Z/11 ECMWF... AND A DECENT NUMBER OF ECMWF ENSEMBLES FROM THE
PAST DAY... SHOW CONSIDERABLY MORE ENERGY REACHING THE NWRN CONUS
COMPARED TO OTHER OPERATIONAL SOLNS AND MOST 00Z/06Z GEFS MEMBERS
ALONG WITH 00Z CANADIAN ENSEMBLES.  GIVEN MODEL TENDENCIES TO
WEAKEN STRONG RIDGES TOO QUICKLY... PREFER TO GIVE MAJORITY
WEIGHTING TO SOLNS THAT MAINTAIN A STRONGER WEST COAST RIDGE. 
WEAKER TREND OF THE 12Z GFS WITH INCOMING SHRTWV ENERGY SUPPORTS
THIS PREFERENCE.  HOWEVER FROM THE MULTI-DAY MEAN PERSPECTIVE IT
IS STILL POSSIBLE FOR A PROGRESSIVE SHRTWV TO PASS THRU THE MEAN
RIDGE SO A VERY MODEST WEIGHTING OF THE ECMWF ENSMEAN LATE IN THE
PERIOD IS STILL REASONABLE.

ABOVE CONSIDERATIONS FAVOR USING A SOLN CLOSE TO THE 00Z ECMWF AS
THE FOUNDATION FOR SIGNIFICANT FEATURES THRU DAYS 3-7 THU-MON.  TO
ACCOUNT FOR INCREASING UNCERTAINTY OVER THE NORTHWEST AND TYPICAL
SHRTWV DETAIL DIFFS OVER THE EAST LATE IN THE PERIOD... THE DAYS
6-7 FCST INCORPORATES 20 PERCENT EACH OF THE 00Z GEFS AND 00Z
ECMWF ENSEMBLE MEANS.    



REGIONAL HIGHLIGHTS...

...WEST...

STRONG RIDGE ALOFT NEAR THE WEST COAST WILL SUPPORT VERY WARM AND
DRY CONDITIONS WEST OF THE ROCKIES THRU MOST OF THE PERIOD. 
FRI-SAT HIGHS MAY APPROACH RECORD LEVELS AT A FEW LOCATIONS
ESPECIALLY THOSE WITH SHORTER PERIODS OF RECORD.  BY EARLY NEXT
WEEK THE RIDGE SHOULD WEAKEN SOMEWHAT BUT IT IS UNCERTAIN WHETHER
IT WILL WEAKEN ENOUGH TO ALLOW A LITTLE LGT RNFL TO REACH THE PAC
NW AT THAT TIME.  PRIMARILY DURING THU-THU NIGHT... CNTRL/SRN
ROCKIES SHOULD SEE LGT-MDT RNFL IN ASSOC WITH ENERGY FLOWING SWD
ON THE ERN SIDE OF THE WEST COAST.

...CENTRAL/EAST...

EXPECT BELOW TO WELL BELOW NORMAL TEMPS OVER MOST OF THE ERN HALF
OF THE COUNTRY... WITH CLOUD COVER GENERALLY SUPPORTING GREATER
NEGATIVE ANOMALIES DURING THE DAY.  ONLY EXTREME SRN AREAS SHOULD
SEE NEAR NORMAL READINGS ON AVERAGE.  LOCALLY HVY RNFL MAY AFFECT
AREAS OVER THE GULF COAST STATES INTO THE SOUTHEAST THU INTO FRI
WHILE SOMEWHAT LIGHTER AMTS PREVAIL FARTHER NWD.  FAIRLY
PERSISTENT UNSETTLED CONDS SHOULD DEVELOP OVER THE GRTLKS AND
VICINITY BY THE WEEKEND AS THE MEAN TROF OVER ERN NOAM DEEPENS.  

RAUSCH/CISCO