Prognostic Meteorological Discussion
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000
FXUS10 KWNH 201835
PMDHMD

MODEL DIAGNOSTIC DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
134 PM EST MON NOV 20 2017

VALID NOV 20/1200 UTC THRU NOV 24/0000 UTC

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

12Z MODEL EVALUATION WITH FINAL PREFERENCES AND CONFIDENCE
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


...EVOLVING DEEP CYCLONE OVER THE EASTERN PACIFIC AND JUST
OFFSHORE OF THE PACIFIC NORTHWEST THROUGH TUE NGT...
...OPENING INTO WAVE AND EJECTING INTO THE NORTHWEST WED/THU...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: GENERAL MODEL BLEND
CONFIDENCE: SLIGHTLY ABOVE AVERAGE

DESPITE SOME COMPLEXITIES WITH DETAILS SUCH AS EXACT PLACEMENT OF
SURFACE LOWS AND HOW THOSE ARE DRIVEN BY EMBEDDED SHORTWAVES
ROTATING AROUND A LARGER UPPER LEVEL LOW...THE MODELS ARE IN
FAIRLY GOOD AGREEMENT WITH THEIR MASS FIELDS AND THE BIG PICTURE
WITH THIS SYSTEM. NORMALIZED STANDARD DEVIATION OF 00Z ECMWF
ENSEMBLE SPREAD OVER WESTERN NORTH AMERICA TUESDAY INTO WEDNESDAY
IS BELOW NORMAL FOR 500MB HEIGHTS AND MSLP...WITH GREATER SPREAD
OFFSHORE. THUS THE DETAIL DIFFERENCES WOULD LIKELY HAVE A GREATER
FORECAST IMPACT OFFSHORE...BUT SENSIBLE WEATHER IN THE PACIFIC
NORTHWEST DOES NOT APPEAR TO BE SUBSTANTIALLY AFFECTED BY THESE
DIFFERENCES. THEREFORE...A GENERAL MODEL BLEND IS PREFERRED AT
THIS TIME.

18Z UPDATE: NO CHANGE TO THE PRELIMINARY PREFERENCE. MODEL
DIFFERENCES REMAIN RELATIVELY SMALL WITH THIS SYSTEM ON THE 12Z
MODEL CYCLE. THEREFORE A GENERAL MODEL BLEND IS STILL PREFERRED.


...SHORTWAVE DIGGING THROUGH THE GREAT LAKES THROUGH TUESDAY...
...BECOMING NEGATIVE TILT OVER THE NORTHEAST ON WEDNESDAY WITH
SURFACE CYCLOGENESIS JUST OFFSHORE...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: 40 PERCENT ECMWF ENSEMBLE MEAN
            20 PERCENT EACH 00Z ECMWF / 00Z UKMET / 00Z CMC
CONFIDENCE: SLIGHTLY BELOW AVERAGE

MORE SUBSTANTIAL DIFFERENCES EXIST WITH THIS SYSTEM...AND APPEAR
TO BE LARGELY ROOTED IN THE SPEED OF THE SHORTWAVE DIGGING THROUGH
THE GREAT LAKES. THE 12Z NAM AND GFS SHOW THE PRIMARY MID-LEVEL
VORT MAX ASSOCIATED WITH THE WAVE SHIFTING THROUGH THE REGION MUCH
FASTER...REACHING MAINE AND BECOMING INCREASINGLY NEGATIVELY
TILTED BY 22/18Z. AT THE SAME TIME...THE 00Z ECMWF...CMC...AND
UKMET SHOW THE VORT MAX CLOSER TO UPSTATE NEW YORK WITH A SLOWER
PROGRESSION OVERALL. THIS DOES AFFECT THE FORECAST AS THE FASTER
MODELS INDICATE THE SURFACE TROUGH AND LOW(S) FURTHER OFFSHORE AND
DOWNSTREAM FROM THE NORTHEASTERN CONUS. MEANWHILE...THE SLOWER
MODELS SHOW A SLIGHTLY STRONGER SURFACE WAVE...WITH THE SURFACE
TROUGH AND LOW(S) TUCKED IN CLOSER TO THE NORTHEAST. THIS IS ALSO
EVIDENT IN ENSEMBLE SENSITIVITY PLOTS...WITH A STRONG RELATIONSHIP
BETWEEN A SLOWER DIGGING WAVE LEADING TO LOWER SURFACE PRESSURES
OFF THE NORTHEAST.

00Z HIGHER PROBABILITY ENSEMBLE CLUSTERS (GEFS AND ECMWF) TEND TO
ALIGN WITH THE SLOWER WAVE...WITH THE TWO CLUSTERS THAT ARE
COMPRISED OF THE GREATEST NUMBER OF ENSEMBLE MEMBERS IN GOOD
AGREEMENT WITH THAT SOLUTION. FOR THIS REASON...THE PREFERENCE IS
FOR THE SLOWER SOLUTION AT THIS TIME. WITHIN THAT CAMP OF
MODELS...THE ECMWF...UKMET...AND CMC DO SHOW QUITE DIFFERENT
SURFACE LOW LOCATIONS AND INTENSITIES. THE OPERATIONAL 00Z ECMWF
MAY BE TOO INTENSE WITH ITS SURFACE LOW...AS IT IS AT LEAST 10MB
DEEPER THAN ANY OTHER GLOBAL MODEL. HOWEVER...WE CANNOT DISCOUNT
THAT IT MAY BE MORE ACCURATE WITH RESPECT TO LOCATION. FOR THIS
REASON...PREFER A BLEND OF THE CMC...UKMET...AND ECMWF TO
INCORPORATE THESE DIFFERENCES AND THE INHERENT UNCERTAINTY...WITH
A LARGE COMPONENT OF THE ECMWF ENSEMBLE MEAN INCLUDED AS WELL.

18Z UPDATE: NO CHANGE TO THE PRELIMINARY PREFERENCE. THE 12Z
ECMWF...UKMET...AND CMC CONTINUE TO REPRESENT A SLOWER SOLUTION
WITH THE WAVE DIGGING THROUGH THE GREAT LAKES AND THUS THE PRIOR
REASONING HAS NOT CHANGED APPRECIABLY AT THIS TIME.


...BROAD UPPER LEVEL TROUGH OVER THE GULF OF MEXICO...
...REINFORCING SHORTWAVE DIGGING FROM THE ROCKIES...
...POSSIBLE WEAK LOW-LEVEL CYCLOGENESIS BY THURSDAY...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: 12Z ECMWF / 12Z GFS / 12Z CMC
CONFIDENCE: SLIGHTLY ABOVE AVERAGE

THE 00Z CMC AND UKMET SHOW HIGHER HEIGHTS THROUGH MUCH OF THE GULF
OF MEXICO WITH THIS TROUGH...AND LOWER HEIGHTS IN THE INTERIOR
SOUTHEAST...BY THURSDAY. THIS IS DUE TO A STRONGER SECONDARY
DIGGING WAVE AND RESULTS IN SURFACE CYCLOGENESIS FURTHER NORTHEAST
OFF THE SOUTHEAST ATLANTIC COAST. MEANWHILE...THE 12Z NAM SHOWS A
SLIGHTLY DEEPER TROUGH OVERALL WITH THE TROUGH AXIS ALIGNED
FURTHER WEST OF MODEL CONSENSUS. THE 12Z GFS AND 00Z ECMWF ARE
MORE CLOSELY ALIGNED WITH THEIR ENSEMBLE MEANS (12Z GEFS AND 00Z
ECMWF ENS) WHICH ALSO SEEM TO BE THE MORE LIKELY SOLUTIONS. THESE
GENERATE A BROAD WEAK BAROCLINIC LOW IN THE GULF OF MEXICO BY
THURSDAY. THEREFORE...GIVEN THE SUPPORT FROM ENSEMBLES...THE
PREFERENCE IS FOR A BLEND OF THE ECMWF AND GFS.

18Z UPDATE: ADD THE CMC TO THE BLEND. THE 12Z UKMET CONTINUES TO
SHOW A STRONGER SECONDARY WAVE OVER THE INTERIOR SOUTHEAST BY
THURSDAY...AND THE UKMET AND 12Z NAM SHOW STRONGER SURFACE LOW
DEVELOPMENT OFF THE SOUTHEAST ATLANTIC COAST (WITH LIMITED SUPPORT
FROM OTHER MODELS). THE 12Z UKMET IS ALSO THE DEEPEST WITH THE
SURFACE LOW OVER THE GULF OF MEXICO...AND ONE OF THE MODELS WITH A
STRONGER UPSTREAM SURFACE RIDGE...WHICH MAY RESULT IN STRONGER
LOW-LEVEL FLOW ACROSS THE REGION. THE 12Z CMC HAS MOVED INTO
BETTER AGREEMENT WITH THE GFS AND ECMWF OVER BOTH THE GULF OF
MEXICO AND THE INTERIOR SOUTHEAST...AND THUS IT HAS BEEN ADDED TO
THE PREFERENCE.


www.wpc.ncep.noaa.gov/html/model2.shtml
www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml

LAMERS

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