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FXUS02 KWNH 240644
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
242 AM EDT FRI MAY 24 2013

VALID 12Z MON MAY 27 2013 - 12Z FRI MAY 31 2013

...OVERVIEW AND PREFERENCES...

MOST GUIDANCE AGREES FAIRLY WELL THAT A STRENGTHENING RIDGE WILL
BUILD INTO THE EAST WHILE VARYING DEGREES OF MEAN TROUGHING SHOULD
PERSIST OVER THE WEST.  THE WRN TROUGH IS GENERALLY FCST TO
BROADEN WITH TIME UNDER THE INFLUENCE OF A STRONG PACIFIC JET
WHOSE AXIS IS MOST LIKELY TO BECOME DIRECTED TOWARD THE CNTRL WEST
COAST BY THE LATTER HALF OF NEXT WEEK.  FOR SOME DETAILS
LATEST/RECENT RUNS OF OPERATIONAL MODELS SHOW ENOUGH VARIABILITY
BETWEEN EACH OTHER AND CONSECUTIVE RUNS TO FAVOR LEANING TOWARD
THE 18Z GEFS/12Z ECMWF MEANS AS THE PRIMARY BASIS FOR THE DAYS 3-7
MON-FRI FCST.


...GUIDANCE EVALUATION...

OVER THE ERN PAC/WRN CONUS... ONE SIGNIFICANT DIFF THAT DEVELOPS
IS WITH INITIAL ERN PAC ENERGY THAT MOVES INTO THE WEST MON-TUE.
SO FAR THE 12Z ECMWF HAS MINIMAL SUPPORT FOR ITS IDEA OF PULLING
OFF A CLOSED LOW THAT EXTENDS INTO NWRN MEXICO... WITH REMAINING
OPERATIONAL MODELS AND MOST ENSEMBLES FAVORING MUCH LESS FLOW
SEPARATION.  THE 00Z GFS/GEFS MEAN/UKMET/CMC FURTHER SUPPORT PRIOR
CONSENSUS THAT LEANED AWAY FROM THE 12Z ECMWF.  BY THE LATTER HALF
OF THE PERIOD THERE IS SOME SPREAD WITH THE STRENGTH OF ENERGY
REACHING THE NRN PLAINS AS WELL AS FOR DETAILS WITHIN THE AXIS OF
LOWEST MID LVL HGTS FCST TO EXTEND FROM THE ALEUTIANS/NRN PAC
TOWARD THE WRN US-CANADIAN BORDER.  LATEST TRENDS/CLUSTERING
RECOMMEND A WEAKER MID LVL REFLECTION OVER THE NRN PLAINS THAN THE
12Z GFS BY DAY 6 THU... WHILE THE 12Z GFS/CMC ARE FARTHER SWD THAN
THE GUIDANCE AVG WITH THEIR CLOSED LOWS OFF THE WEST COAST LATE IN
THE PERIOD.  BY LATE NEXT WEEK THE 00Z GFS IS NOW OUT OF PHASE
WITH CONSENSUS OFF THE WEST COAST WHILE THE 00Z CMC HAS TRENDED
FASTER/WEAKER WITH ITS MID LVL SYSTEM.  THESE DIFFS AND RUN TO RUN
CHANGES MAINTAIN SUPPORT FOR AN ENSEMBLE MEAN APPROACH UNTIL
OPERATIONAL SOLNS STABILIZE.

FARTHER EWD THE 12Z ECMWF ALSO DEVIATES SIGNIFICANTLY FROM
CONSENSUS BY LATER IN THE PERIOD... BRINGING A TROUGH THROUGH THE
NORTHEAST IN CONTRAST TO FLATTER FLOW IN THE ENSEMBLE MEANS AND
EVEN A RIDGE IN SOME GFS/CMC RUNS.  CONFIDENCE IN ANY PARTICULAR
SOLN IS NOT ESPECIALLY HIGH GIVEN DEPENDENCE ON AN UNCERTAIN TRACK
OF A COMPACT CLOSED LOW INITIALLY JUST N OF HUDSON BAY AS WELL AS
DETAILS OF WRN CONUS EVOLUTION.  THE MODERATE ORIENTATION OF NERN
CONUS FLOW IN THE ENSEMBLE MEANS APPEARS REASONABLE IN LIGHT OF
THE LARGE SCALE TREND TOWARD BROADER TROUGHING UPSTREAM BY THE
LATTER HALF OF THE PERIOD.


...SENSIBLE WEATHER HIGHLIGHTS...

ENERGY ALOFT MOVING INTO THE WEST MAY BE AMPLIFIED ENOUGH TO BRING
SOME UNSEASONABLY LATE PCPN INTO PARTS OF SRN CA AND THE SOUTHWEST
AROUND MON-WED.  SRN PARTS OF THE WEST SHOULD TREND DRIER AFTER
THAT TIME WHILE THE MEAN PATTERN WILL KEEP UNSETTLED CONDS OVER
THE NRN HALF OF THE WEST AND INTO THE PLAINS/UPR MS VLY FOR MOST
OF THE PERIOD.  MULTIPLE EPISODES OF CONVECTION ARE ALSO PSBL OVER
THE SRN HIGH PLAINS.  MEANWHILE EXPECT A WARM FRONT LIFTING NEWD
FROM THE PLAINS/MS VLY TO SERVE AS A FOCUS FOR CONVECTION DURING
MON-WED.  TEMPS OVER THE WEST SHOULD BE BELOW NORMAL ON MOST DAYS
WHILE THE ERN HALF OF THE CONUS WILL SEE A CONSIDERABLE WARMING
TREND WITH SOME LOCATIONS PSBLY SEEING HIGHS AT LEAST 10 F ABOVE
NORMAL BY LATE NEXT WEEK.

RAUSCH

$$
National Weather Service Weather Forecast Office
