Prognostic Meteorological Discussion
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FXUS10 KWNH 211833
PMDHMD

MODEL DIAGNOSTIC DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
132 PM 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

MODELS DO NOT SHOW SIGNIFICANT DIFFERENCES WITH RESPECT TO THIS
FEATURE.


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

PRELIMINARY PREFERENCE: 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 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 ECMWF AND CMC
SEEM TO BE ON THE LEAST AMPLIFIED END OF THE SPECTRUM WITH THIS
FEATURE. THE RESULT OF THESE UPPER-LEVEL DIFFERENCES IS A STRONGER
SURFACE LOW FARTHER SOUTH IN THE GFS...RATHER THAN THE NORTHERN
SURFACE LOW BEING STRONGER IN THE ECMWF/UKMET/CMC...AND THE NAM
SOMEWHERE IN BETWEEN THE TWO CAMPS. ADDITIONALLY...THE PARALLEL
GFS SOLUTION IS CLOSER TO THE ECMWF/UKMET.


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

PRELIMINARY PREFERENCE: ECMWF/GFS/UKMET/CMC BLEND
CONFIDENCE: AVERAGE

THE NAM APPEARS TO BE MORE SLOWER/MORE AMPLIFIED RELATIVE TO THE
GFS AND 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 12Z UKMET HAS
MAINTAINED A SIMILAR SOLUTION TO THAT RECOMMENDED PREVIOUSLY FOR
THIS SYSTEM...WHILE THE ECMWF HAS TRENDED TOWARD A SOMEWHAT
SLOWER/MORE AMPLIFIED SOLUTION RELATIVE TO ITS PREVIOUS
RUN...RESULTING IN A SLOWER AND DEEPER SURFACE LOW. GIVEN THE
TRENDS IN THE ECMWF...CAN NOT EXCLUDE THE GFS FROM A REASONABLE
RANGE OF SPREAD.


...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: NAM/CMC/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 THE UKMET ON THE
FAST END OF THE SPREAD AND THE GFS ON THE SLOW SIDE.


...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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