Prognostic Meteorological Discussion
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FXUS10 KWNH 181647
PMDHMD

MODEL DIAGNOSTIC DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1147 AM EST MON DEC 18 2017

VALID DEC 18/1200 UTC THRU DEC 22/0000 UTC

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

12Z NAM/GFS EVALUATION...WITH PRELIMINARY PREFERENCES
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

...CLOSED LOW CURRENTLY OVER THE DESERT SOUTHWEST/NW
MEXICO...EJECTING OUT ACROSS THE SOUTHERN PLAINS ON TUE AND
WEAKENING ACROSS THE EAST COAST LATE WED...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: GENERAL MODEL BLEND
CONFIDENCE: SLIGHTLY ABOVE AVERAGE

ONLY MINOR DIFFERENCES IN TIMING/AMPLITUDE ARE SEEN WITH THIS
FEATURE. THE 00Z ECMWF IS A BIT STRONGER WITH THE 500 MB CORE OF
THE MID-LEVEL LOW WHILE THE 12Z GFS IS SLIGHTLY WEAKER AND THE 00Z
UKMET/CMC A BIT FARTHER SOUTH WITH THE MAIN VORTICITY MAX CROSSING
THE MS VALLEY. OVERALL...A GENERAL MODEL BLEND IS RECOMMENDED TO
IRON OUT THESE MINOR SMALLER SCALE DIFFERENCES.


...BROAD/LOW AMPLITUDE MID-UPPER LEVEL TROUGH CROSSING THE GREAT
LAKES/NORTHEAST TUE/WED...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: GENERAL MODEL BLEND
CONFIDENCE: ABOVE AVERAGE

THE 12Z NAM IS ON THE DEEPER SIDE WITH THE MID-LEVEL CORE OF THE
TROUGH EARLY WED MORNING AS THE SYSTEM BEGINS TO EXIT ACROSS THE
CANADIAN MARITIMES. HOWEVER...DIFFERENCES ACROSS THE NORTHEAST
U.S. ARE MINOR SO A GENERAL MODEL BLEND IS RECOMMENDED HERE.


...UPPER TROUGH AXIS CROSSING THE PACIFIC NORTHWEST TUE NIGHT AND
REACHING THE FOUR CORNERS ON THU...
...SURFACE CYCLONE CROSSING THE CENTRAL PLAINS ON THU...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: BLEND OF THE 00Z ECMWF/UKMET
CONFIDENCE: SLIGHTLY BELOW AVERAGE

THERE IS A FAIR AMOUNT OF SPREAD IN THE LATEST ENSEMBLE GUIDANCE
AND WHILE TRENDS SHOW A LESSENING IN THAT SPREAD...DIFFERENCES ARE
STILL SIGNIFICANT WHICH LOWERS CONFIDENCE.

THE 12Z NAM IS UNIQUE REGARDING THE LATEST DETERMINISTIC MODELS
WITH MORE OF AN OPEN...POSITIVELY TILTED TROUGH AXIS FROM THE FOUR
CORNERS INTO THE N-CNTRL U.S. BY 00Z/22 WHEREAS THE REMAINING
DETERMINISTIC GUIDANCE SHOWS MORE ENERGY HANGING BACK TO THE
SOUTHWEST. INITIALLY ZONAL FLOW OVER THE CONUS MAY FAVOR A FASTER
SOLUTION LIKE THE 12Z GFS...AND THE ENSEMBLE SPAGHETTI PLOTS SHOW
THE GFS AS REASONABLE...BUT THE LATEST ENSEMBLE MEANS ARE CLOSER
TO A BLEND OF THE 00Z ECMWF/UKMET. GIVEN AGREEMENT TO THE ENSEMBLE
MEANS AND NO DISCERNIBLE SLOWER/FASTER TREND IN THE GUIDANCE
CONCERNING PROGRESSION OF THE TROUGH AXIS...STAYING CLOSEST TO THE
ENSEMBLE MEANS IS CONSIDERED BEST FOR THIS SYSTEM.

REGARDING THE SURFACE LOW...THE LATEST ENSEMBLE SCATTER LOW PLOTS
SUPPORT A BLEND BETWEEN THE 12Z NAM/12Z GFS AND 00Z ECMWF BY
00Z/22 WITH THE 00Z UKMET/CMC FARTHER BACK TO THE WEST.


...UPPER TROUGH AXIS NEARING THE PACIFIC NORTHWEST FROM THE NORTH
ON THU...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: 12Z NAM...12Z GFS...00Z ECMWF BLEND
CONFIDENCE: AVERAGE

THE ENSEMBLE MEANS SUPPORT A TIMING CLOSER TO THAT OF THE 12Z
NAM/12Z GFS/00Z ECMWF WITH THE 00Z UKMET/CMC FASTER TO BRING THE
UPPER TROUGH INTO THE PACIFIC NORTHWEST LATE THU.


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

OTTO



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