Prognostic Meteorological Discussion
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FXUS10 KWNH 211625
PMDHMD

MODEL DIAGNOSTIC DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1124 AM EST SUN DEC 21 2014

VALID DEC 21/1200 UTC THRU DEC 25/0000 UTC

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

12Z NAM/GFS EVALUATION AND PRELIMINARY MODEL PREFERENCES

MODEL INITIALIZATION ERRORS ARE DESCRIBED UNDER THE RELEVANT
SYSTEM HEADER.

...UPPER-LEVEL TROUGH INITIALLY OVER THE NORTHEAST...

PRELIMINARY PREFERENCE: GENERAL MODEL COMPROMISE
CONFIDENCE: ABOVE AVERAGE

THE NAM AND GFS ARE COMPARABLE TO THE GENERAL MODEL CONSENSUS WITH
RESPECT TO THIS FEATURE.


...LOW-PRESSURE SYSTEM MOVING FROM THE NORTHERN ROCKIES INTO THE
NORTHERN PLAINS TODAY...

PRELIMINARY PREFERENCE: 00Z ECMWF/UKMET/CMC BLEND
CONFIDENCE: BELOW AVERAGE

THE NAM AND GFS BECOME MORE AMPLIFIED WITH THE UPPER-LEVEL
SHORTWAVE ACROSS THE NORTHERN PLAINS BY EARLY MONDAY RELATIVE TO
THE 00Z NON-NCEP GUIDANCE. THE PARALLEL GFS IS LESS AMPLIFIED THAN
THE OPERATIONAL VERSION AT THIS TIME...LENDING SOME CREDENCE TO
THE NON-NCEP CAMP OF SOLUTIONS. THE DIFFERENCES APPEAR TO STEM
FROM THE FAST-MOVING LOW-AMPLITUDE SHORTWAVE MOVING ONSHORE IN THE
NORTHWEST THIS MORNING. THE NAM (AND THE GFS TO A LESSER EXTENT)
INITIALIZED 500 HPA HEIGHTS A LITTLE TOO LOW IN THE VICINITY OF
THE FEATURES THIS MORNING. THIS COULD IMPACT ITS RESULTANT
SOLUTION WITH A MORE AMPLIFIED TROUGH BY THE TIME THE SHORTWAVE
REACHES THE NORTHERN PLAINS. ON THE OTHER HAND...THE 00Z ECMWF
SEEMS TO BE ON TH LEAST AMPLIFIED END OF THE SPECTRUM WITH THIS
FEATURE. THE RESULT OF THESE UPPER-LEVEL DIFFERENCES ARE A
STRONGER SURFACE LOW FARTHER SOUTH...RATHER THAN THE NORTHERN
SURFACE LOW BEING STRONGER IN THE 00Z ECMWF/UKMET/CMC...AND THE
NAM SOMEWHERE IN BETWEEN THE TWO CAMPS. ADDITIONALLY...THE
PARALLEL GFS SOLUTION IS CLOSER TO THE 00Z ECMWF/UKMET.


...WAVE OF LOW PRESSURE DEVELOPING IN THE NORTHWEST GULF OF MEXICO
TUESDAY AFTERNOON...

PRELIMINARY PREFERENCE: 00Z ECMWF/UKMET/CMC BLEND
CONFIDENCE: AVERAGE

THE NAM APPEARS TO BE MORE SLOWER/MORE AMPLIFIED RELATIVE TO THE
GFS AND 00Z NON-NCEP GUIDANCE WITH A SHORTWAVE MOVING ONSHORE IN
BRITISH COLUMBIA EARLY MONDAY MORNING...WHICH EVENTUALLY AMPLIFIES
AS IT RAPIDLY MOVES SOUTHWARD ACROSS THE ROCKIES MONDAY NIGHT AND
INTO THE SOUTHERN PLAINS TUESDAY AS IT ROUNDS THE BASE OF THE
LARGE-SCALE TROUGH. AS A RESULT...THE NAM IS SHARPER WITH THE
LARGE-SCALE TROUGH AND EVEN CLOSES OFF A LOW AT 500 HPA OVER THE
SOUTHERN PLAINS EARLY WEDNESDAY. THESE DIFFERENCES TRANSLATE TO
THE SURFACE AS WELL...WITH THE NAM LAGGING BEHIND THE GFS AND
NON-NCEP MODELS WITH THE PROGRESSION OF THE SURFACE LOW. THE GFS
DEMONSTRATES SOME OF THE SAME TENDENCIES OBSERVED IN THE
NAM...ALBEIT TO A LESSER EXTENT. THE 12Z PARALLEL GFS SOLUTION
BECOMES A SIGNIFICANT OUTLIER BY LATE TUESDAY WITH AN ANOMALOUSLY
DEEP LOW AT THE SURFACE AND UPPER-LEVELS. THE 06Z RUN OF THE
PARALLEL GFS LOOKED MUCH MORE LIKE THE 00Z ECMWF/UKMET/CMC.
THEREFORE...WILL RECOMMEND A BLEND OF THE NON-NCEP GUIDANCE.


...STATIONARY FRONTAL BOUNDARY AND WEAK WAVES OF LOW PRESSURE
AFFECTING THE SOUTHEASTERN U.S...

PRELIMINARY PREFERENCE: GENERAL MODEL COMPROMISE
CONFIDENCE: ABOVE AVERAGE

MODEL DIFFERENCES WITH RESPECT TO THE SURFACE STATIONARY FRONT AND
THE WEAK WAVES OF LOW PRESSURE MOVE ALONG IT ARE RELATIVELY SMALL
AND NO SIGNIFICANT OUTLIERS ARE NOTED.


...LOW-PRESSURE SYSTEM REACHING THE PACIFIC NORTHWEST TUESDAY INTO
WEDNESDAY...

PRELIMINARY PREFERENCE: GFS/NAM/00Z ECMWF BLEND
CONFIDENCE:

ON TUESDAY AS A DEEP PACIFIC LOW PRESSURE SYSTEM MOVES INTO
BRITISH COLUMBIA...THE GFS AND NAM SOLUTIONS BOTH LIE WITHIN A
REASONABLE RANGE OF SPREAD AND NEITHER CAN BE DISCARDED AT THIS
TIME. MODEL SPREAD BECOMES LARGER WITH A SECOND SHORTWAVE THAT
REACHES THE PACIFIC NORTHWEST ON WEDNESDAY. WITH THIS FEATURE AS
WELL THE GFS AND NAM LIE WITHIN THE MIDDLE OF THE SPREAD OF
SOLUTIONS...WITH THE 00Z UKMET AS A FAST OUTLIER.


...MODEL TREND GRAPHICS AT
www.wpc.ncep.noaa.gov/html/model2.shtml...
...500 MB FORECASTS AT
www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml...


RYAN

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