Prognostic Meteorological Discussion
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FXUS10 KWNH 171835
PMDHMD

MODEL DIAGNOSTIC DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
134 PM EST SUN DEC 17 2017

VALID DEC 17/1200 UTC THRU DEC 21/0000 UTC

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

12Z MODEL EVALUATION...WITH LATEST PREFERENCES AND CONFIDENCE
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

...DEAMPLIFYING SHORTWAVE OVER LOWER/MID MS AND OH VALLEY TODAY...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: GENERAL MODEL BLEND
CONFIDENCE: ABOVE AVERAGE

THE GUIDANCE IS IN VERY GOOD AGREEMENT WITH THE DETAILS OF THIS
ENERGY SHEARING OUT OVER THE NEXT 12 TO 24 HOURS. A GENERAL MODEL
BLEND WILL BE PREFERRED.


...CLOSED LOW OVER THE DESERT SOUTHWEST/NW MEXICO...
...EJECTING OUT ACROSS THE SOUTHERN PLAINS ON TUESDAY...
...SHEARING OUT ACROSS THE LOWER MS/TN VALLEYS ON WEDNESDAY...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: BLEND OF THE 12Z GFS/12Z UKMET AND 12Z ECMWF
CONFIDENCE: SLIGHTLY ABOVE AVERAGE

THE MODELS ARE IN VERY GOOD AGREEMENT TAKING THE UPPER LOW OVER
THE DESERT SOUTHWEST AND FAR NORTHWEST PORTION OF MEXICO OFF TO
THE EAST OVER THE NEXT COUPLE OF DAYS. THE ENERGY WILL EJECT OUT
ACROSS THE SOUTHERN PLAINS THROUGH TUESDAY AND THEN BEGIN TO SHEAR
OUT DOWNSTREAM OVER THE LOWER MS/TN VALLEYS THROUGH WEDNESDAY. THE
HEIGHT FALLS WILL INDUCE AN AREA OF LOW PRESSURE ACROSS THE
SOUTHEAST TX COASTAL PLAIN BY LATE TUESDAY WHICH WILL THEN MOVE
QUICKLY DOWNSTREAM ACROSS THE TN VALLEY AND INTERIOR SOUTHEAST ON
WEDNESDAY. THE 12Z NAM APPEARS TO BE A STRONG OUTLIER WITH THE
EVOLUTION OF THIS SYSTEM BY 36 HOURS AND THIS REMAINS THE CASE AS
IT ADVANCES DOWNSTREAM ACROSS THE SOUTH AND ULTIMATELY LEADS TO A
SURFACE LOW TRACKING A BIT NORTH OF THE MODEL CONSENSUS. THE 12Z
CMC FOR ITS PART APPEARS TO BE STILL A TAD TOO SLOW WITH IT ENERGY
PROPAGATION FROM 60 HOURS ONWARD AND ALSO APPEARS TO GENERALLY BE
TOO WEAK AND TOO FAR SOUTH WITH ITS LOW TRACK AND FRONTAL
PLACEMENT. A COMPROMISE OF THE BETTER CLUSTERED 12Z GFS/12Z UKMET
AND 12Z ECMWF WILL BE PREFERRED.


...SHORTWAVE TRAVERSING THE NORTHERN PLAINS TODAY...
...ENERGY DAMPENING OUT OVER THE GREAT LAKES/NORTHEAST BY MONDAY...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

PREFERENCE: GENERAL MODEL BLEND
CONFIDENCE: ABOVE AVERAGE

THE MODELS ARE IN GOOD AGREEMENT WITH THE DETAILS OF THIS SYSTEM
AS IT SHEARS OUT OVER THE NEXT 24 HOURS OR SO. A WEAK AREA OF LOW
PRESSURE OVER THE MIDWEST WILL CROSS THE LOWER GREAT LAKES AND
NORTHEAST THROUGH MONDAY BEFORE DAMPENING OUT. A GENERAL MODEL
BLEND WILL BE PREFERRED.


...AMPLIFYING NORTHERN STREAM TROUGH OVER SOUTHERN CANADA MONDAY...
...ENERGY DIGGING ACROSS THE MIDWEST/GREAT LAKES BY TUESDAY...
...TROUGHING OVER SOUTHEAST CANADA/NORTHEAST U.S. BY WEDNESDAY...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: GENERAL MODEL BLEND...THROUGH 60 HOURS
            BLEND OF THE 12Z GFS AND 12Z ECMWF...AFTER 60 HOURS
CONFIDENCE: ABOVE AVERAGE

THE MODELS GRADUALLY AMPLIFY A NORTHERN STREAM LONGWAVE TROUGH
ACROSS AREAS OF SOUTHERN CANADA AND THE NORTHERN U.S. WITH IMPACTS
ACROSS THE NORTHERN PLAINS/MIDWEST AND GREAT LAKES THROUGH MONDAY
AND TUESDAY. A STRONG COLD FRONT WILL ACCOMPANY THESE HEIGHT
FALLS. THE MODELS ARE IN RATHER GOOD AGREEMENT WITH THE LARGE
SCALE DETAILS OF THE TROUGH AND THE COLD FRONT. THE 12Z NAM
BECOMES A DEEPER OUTLIER IN TIME WITH THE LARGER SCALE TROUGH
THOUGH...MAINLY AFTER 60 HOURS...OVER SOUTHEAST CANADA. THE 12Z
UKMET ALSO TENDS TO PIVOT ENERGY A BIT MORE AGGRESSIVELY OFFSHORE
OF NEWFOUNDLAND BY THE END OF THE PERIOD COMPARED TO THE REMAINING
GUIDANCE. WILL SUGGEST A GENERAL MODEL BLEND THROUGH ABOUT 60
HOURS...AND THEN A BLEND OF THE WELL CLUSTERED 12Z GFS AND 12Z
ECMWF THEREAFTER WHICH IS ALSO SUPPORTED BY THE 12Z CMC..


...DEEP UPPER TROUGH/CLOSED LOW CROSSING THE GULF OF AK MONDAY...
...ENERGY DIGGING SHARPLY INTO THE PACIFIC NORTHWEST ON TUESDAY...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: BLEND OF THE 12Z GFS AND 12Z ECMWF...THROUGH 60 HOURS
            BLEND OF THE 12Z UKMET AND 12Z ECMWF...AFTER 60 HOURS
CONFIDENCE: ABOVE AVERAGE

THE MODELS TAKE A NORTHERN STREAM SHORTWAVE TROUGH OVER THE GULF
OF AK AND AMPLIFY IT SOUTHEAST TOWARD THE PACIFIC NORTHWEST ON
TUESDAY WHICH ALLOWS FOR SOME STRONG HEIGHT FALLS TO ARRIVE IN
CONJUNCTION WITH A DEEPENING AREA OF LOW PRESSURE. THIS LOW
PRESSURE WAVE MOVES ACROSS NORTHWEST WA BY LATE TUESDAY. LOW
PRESSURE AND A RATHER STRONG COLD FRONT WILL THEN TRAVERSE THE
INTERMOUNTAIN WEST THROUGH EARLY WEDNESDAY. LOW PRESSURE WILL BE
REDEVELOPING OUT ACROSS THE HIGH PLAINS BY LATE WEDNESDAY ALONG
WITH A STRENGTHENING WARM FRONT NEAR PARTS OF THE NORTH-CENTRAL
PLAINS AND MIDWEST. THE 12Z NAM IS A BIT OF A DEEPER OUTLIER WITH
THE UPPER LOW THAT DIGS IN ACROSS THE NORTHWEST U.S. TUESDAY AND
WEDNESDAY AND GRADUALLY APPEARS TO BE A BIT TOO SLOW. THE
REMAINING GUIDANCE IS A TAD FASTER WITH THE 12Z ECMWF THE MOST
PROGRESSIVE...HOWEVER THE 12Z UKMET AND 12Z CMC ARE CLUSTERED
RATHER CLOSE TO THE ECMWF SOLUTION. THE 12Z GFS IS A LITTLE SLOWER
THAN THIS CONSENSUS. AT THE SURFACE...THE 12Z UKMET APPEARS TO BE
A BIT TOO STRONG WITH THE LOW PRESSURE THAT IMPACTS WESTERN WA AND
VANCOUVER ISLAND. THERE ARE SOME MODEST TIMING DIFFERENCES WITH
THE EVOLUTION OF LOW PRESSURE OVER THE HIGH PLAINS...WITH THE
ECMWF A BIT QUICKER TO CONSOLIDATE LOW PRESSURE OVER THE CENTRAL
HIGH PLAINS VERSUS THE SLIGHTLY SLOWER GFS...BUT THE UKMET AND CMC
ARE CLOSER TO THE ECMWF SOLUTION. THE 00Z ECENS MEAN AND 12Z GEFS
MEAN BOTH SUGGEST THE GFS IS A TAD TOO SLOW AND ARE MORE
SUPPORTIVE OF THE ECMWF. SO WILL SUGGEST A BLEND OF THE GFS AND
ECMWF THROUGH ABOUT 60 HOURS BASED ON EXCELLENT MODEL
CLUSTERING...BUT THEN A BLEND OF THE UKMET AND ECMWF THEREAFTER
SINCE THE GFS APPEARS A LITTLE TOO SLOW.


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

ORRISON



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