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000
FXUS10 KWNH 180421
PMDHMD

MODEL DIAGNOSTIC DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1220 AM EDT SAT MAY 18 2013

VALID MAY 18/0000 UTC THRU MAY 21/1200 UTC

...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR
INGEST...


00Z NAM/GFS EVALUATION WITH PRELIMINARY MODEL PREFERENCES
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
NAM/GFS INITIALIZATION ERRORS DO NOT APPEAR TO DEGRADE THEIR SHORT
RANGE FORECASTS.


DEEP CYCLONE DROPPING TOWARDS THE PACIFIC NORTHWEST...
PREFERENCE:  NON-UKMET COMPROMISE
CONFIDENCE:  SLIGHTLY ABOVE AVERAGE

A MID-LEVEL POSITIVE HEIGHT ANOMALY NEAR 50N 155W FAVORS TROUGHING
JUST OFFSHORE THE PACIFIC NORTHWEST MONDAY INTO TUESDAY, THOUGH IT
DOES NOT REVEAL EXACTLY HOW FAR SOUTH THIS MID-LEVEL CYCLONE
SHOULD END UP.  THE GENERAL FLOW PATTERN OVER THE NEXT SEVERAL
DAYS APPEARS TO FAVOR CLOSED CYCLONES NEAR THE 40TH PARALLEL --
WHICH WOULD FAVOR THE UKMET -- IF IT WASN`T FOR THE FACT THAT THE
SYSTEM KEEPS DROPPING SOUTHWARD DURING THE EARLY PORTION OF THE
MEDIUM RANGE PERIOD.  THE GUIDANCE AGREES WITH THIS IDEA, THOUGH
THE 12Z UKMET DROPS THE SYSTEM QUICKER TO THE SOUTH THAN SHOWN ON
THE OTHER GUIDANCE.  IT SHOWED THIS IDEA IN ITS 16/12Z RUN BEFORE
ELIMINATING IT ON ITS 00Z RUN, SO THE UKMET HAS NOT SHOWN THE
GREATEST CONSISTENCY WITH THIS SYSTEM, MAKING ITS CURRENT SOLUTION
QUESTIONABLE.  DETAIL ISSUES BETWEEN THE 00Z NAM/12Z ECMWF AND 00Z
GFS CAMPS LEAD TO DIFFERENT SURFACE LOW SOLUTIONS OFFSHORE THE
NORTHWESTERN UNITED STATES MONDAY NIGHT, WITH MINIMAL 12Z GLOBAL
ENSEMBLE LOW CLUSTERING SPLITTING THE DIFFERENCE.  AS SUCH, A
COMPROMISE OF THE NON-UKMET GUIDANCE, BOTH AT THE SURFACE AND
ALOFT, IS PREFERRED WITH SLIGHTLY ABOVE AVERAGE CONFIDENCE.


TROUGH/DEEP CYCLONE MOVING FROM THE WEST INTO THE PLAINS...
PREFERENCE:  COMPROMISE OF THE 00Z GFS/12Z ECMWF
CONFIDENCE:  ABOVE AVERAGE

THE NAM ROTATES STRONGER SHORTWAVE ENERGY AROUND THE SOUTHEAST
SIDE OF THIS SYSTEM SUNDAY NIGHT/MONDAY THAN SEEN ON THE OTHER
GUIDANCE, WHILE THE UKMET SHOWS IT ROTATING AROUND THE DEEP
CYCLONE`S EAST SIDE, WHICH LEADS TO A FASTER SURFACE LOW
PROGRESSION.  IN GENERAL, A STRENGTHENING NORTHERN STREAM RIDGE IN
SOUTH-CENTRAL CANADA SHOULD BE ALLOWING THIS BROAD SYSTEM TO
STRENGTHEN UNDER ITS BASE.  SINCE THE OVERALL SYSTEM IS SO BROAD,
ANY SHORTWAVES ORBITING THE SYSTEM SHOULD BE LESS OBVIOUS/WEAKER
THAN WHAT IS SHOWN ON THE 12Z UKMET/00Z NAM SOLUTIONS.  A
COMPROMISE OF THE 00Z GFS/12Z ECMWF TAKES INTO ACCOUNT THE
UNCERTAINTY REGARDING THE SURFACE LOW PLACEMENT YET REMAINS CLOSE
TO THE 12Z GLOBAL ENSEMBLE LOW CLUSTERING.  THIS COMPROMISE IS
PREFERRED WITH ABOVE AVERAGE CONFIDENCE DUE TO ITS AMPLIFIED
NATURE AND RELATIVELY SLOW PROGRESSION.


SHORTWAVE MOVING THROUGH THE SOUTHEAST...
PREFERENCE:  NON-NAM COMPROMISE
CONFIDENCE:  AVERAGE

LIKE YESTERDAY, THE 00Z NAM IS THE SLOWEST TO PROGRESS THIS SYSTEM
EASTWARD.  THIS TIME, THE NAM APPEARS TO BE INTERNALLY
INCONSISTENT AS IT HAS A STRONGER SHORTWAVE MOVING AROUND THE
SOUTH TO SOUTHEAST PERIPHERY OF A DEVELOPING DEEP CYCLONE IN THE
NORTHERN PLAINS, WHICH SHOULD LEAD TO A FASTER PROGRESSION RATHER
THAN A SLOWER ONE.  SINCE THE NAM HAS A LONG-STANDING BIAS OF
BECOMING TOO SLOW WITH SYSTEM PROGRESSION IN THE WESTERLIES, THE
PREFERENCE IS FOR A NON-NAM COMPROMISE HERE WITH AVERAGE
CONFIDENCE CONSIDERING ITS PROGRESSIVE NATURE.


...MODEL TREND GRAPHICS AT
WWW.WPC.NCEP.NOAA.GOV/HTML/MODEL2.SHTML...
...500 MB FORECASTS AT WWW.WPC.NCEP.NOAA.GOV/H5PREF/H5PREF.SHTML...

ROTH
$$