Prognostic Meteorological Discussion
Issued by NWS

000
FXUS10 KWNH 180657
PMDHMD

MODEL DIAGNOSTIC DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
257 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 MODEL EVALUATION INCLUDING MODEL PREFERENCES
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
NAM/GFS INITIALIZATION ERRORS DO NOT APPEAR TO DEGRADE THEIR SHORT
RANGE FORECASTS.


DEEP CYCLONE DROPPING TOWARDS THE PACIFIC NORTHWEST...
PREFERENCE:  00Z ECMWF
CONFIDENCE:  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
ACROSS THE NORTHERN PORTION OF THE WESTERN HEMISPHERE, AND THE
SYSTEM APPEARS TO ACHIEVE THIS LATITUDE DURING THE EARLY PORTION
OF THE MEDIUM RANGE PERIOD.  WITH THE CHANGES SEEN IN THE 00Z
UKMET/00Z ECMWF, THE GUIDANCE IS SHOWING INCREASED CONVERGENCE
ALOFT.  DETAIL ISSUES BETWEEN THE NAM/ECMWF/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, MOST SUPPORTIVE OF THE 00Z
ECMWF SOLUTION.  AS SUCH, THE 00Z ECMWF, BOTH AT THE SURFACE AND
ALOFT, IS PREFERRED WITH ABOVE AVERAGE CONFIDENCE.


TROUGH/DEEP CYCLONE MOVING FROM THE WEST INTO THE PLAINS...
PREFERENCE:  COMPROMISE OF THE 00Z GFS/00Z 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.  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 00Z NAM SOLUTION.  THE 00Z UKMET BECOMES
THE STRONGEST ALOFT, WHICH FORCES ITS SURFACE LOW NORTH OF THE
OTHER GUIDANCE.  A COMPROMISE OF THE 00Z GFS/00Z 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 WITH
THIS SYSTEM 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
$$