Prognostic Meteorological Discussion
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000
FXUS10 KWNH 200643
PMDHMD

MODEL DIAGNOSTIC DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
242 AM EDT WED SEP 20 2017

VALID SEP 20/0000 UTC THRU SEP 23/1200 UTC

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

00Z MODEL EVALUATION...WITH FINAL PREFERENCES AND CONFIDENCE
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


...OCCLUDING EXTRATROPICAL CYCLONE OVER MANITOBA AND SASKATCHEWAN
TODAY AND THURSDAY...
...ASSOCIATED COLD FRONT OVER THE NORTH CENTRAL CONUS...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: NON 00Z CMC BLEND
CONFIDENCE: ABOVE AVERAGE

UPDATE: THE 00Z CMC IS IN BETTER ALIGNMENT WITH THE OTHER
MODELS...BUT IT STILL DEVELOPS A STRONGER SECONDARY CLOSED
MID-LEVEL LOW FURTHER TO THE EAST. THEREFORE...WILL CONTINUE TO
PREFER A NON-CMC BLEND.

OVERALL MODELS ARE IN GOOD AGREEMENT ON MANY OF THE MASS FIELDS
ASSOCIATED WITH THIS OCCLUDING CYCLONE WITH THE EXCEPTION OF THE
12Z CMC. THE CANADIAN MODEL SHOWS A SECONDARY CLOSED MID-LEVEL LOW
DEVELOPING NEAR THE BASE OF THE NEGATIVELY TILTED EJECTING
SHORTWAVE OVER MANITOBA. THIS SECONDARY LOW THEN BECOMES
DOMINANT...WITH A REFLECTION AT THE SURFACE...WHICH IS SEVERAL
HUNDRED MILES EAST OF THE CONSENSUS POSITION. FOR THIS REASON...A
NON 12Z CMC BLEND IS PREFERRED FOR THIS SYSTEM.


...DIGGING SHORTWAVES INTO THE PACIFIC COAST WED/THU...
...ASSOCIATED WESTERN CONUS TROUGH AMPLIFICATION THROUGH FRI...
...SURFACE LEE CYCLOGENESIS BEGINNING LATER ON THURSDAY...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: NON 00Z NAM BLEND
CONFIDENCE: SLIGHTLY ABOVE AVERAGE

UPDATE: NO CHANGES TO THIS PREFERENCE...AS THE 00Z NAM CONTINUES
TO BE THE OUTLIER WITH THIS SYSTEM. THE 00Z CMC HAS SOME SLIGHT
DETAIL DIFFERENCES AS COMPARED TO THE GFS...UKMET...AND
ECMWF...BUT NOT SUBSTANTIAL ENOUGH TO BE DISCOUNTED.

AVAILABLE DETERMINISTIC MODELS LARGELY AGREE ON TWO DISTINCT
SHORTWAVES DIGGING INTO THE PACIFIC COAST OVER THE NEXT COUPLE
DAYS. THE INITIAL ONE ARRIVES IN WESTERN OREGON THIS MORNING AND
WRAPS INTO THE BROADER TROUGH IN THE VICINITY OF IDAHO...POSSIBLY
LEADING TO CLOSED MID-LEVEL LOW DEVELOPMENT BY LATE THURSDAY OR
FRIDAY. THE SECOND SHORTWAVE REVOLVES AROUND THE WESTERN PERIPHERY
OF THE LARGER TROUGH...EVENTUALLY SHARPLY DIGGING OUT THE BASE OF
THE TROUGH OVER CALIFORNIA AND THE GREAT BASIN.

THE 00Z NAM DIFFERS THE MOST IN HOW IT HANDLES THE INITIAL
SHORTWAVE. THE NAM SHEARS THE WAVE OUT FASTER AND IS THE ONLY
DETERMINISTIC MODEL THAT DOES NOT EVENTUALLY SHOW A CLOSED LOW AT
500MB. THIS ALSO LEADS TO HEIGHT FALLS A LITTLE FURTHER EAST...AND
AS A RESULT THE NAM TENDS TO BE COLDER IN THE VICINITY OF REGIONS
OF ACTIVE PRECIPITATION. FOR THAT REASON...THE 00Z NAM WAS
DISCOUNTED FOR THIS SYSTEM. THE OTHER MODELS SEEM IN CLOSE ENOUGH
AGREEMENT THAT A NON-00Z NAM BLEND IS THE PREFERENCE.


...TROPICAL STORM JOSE...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: 00Z GFS / 00Z UKMET CLOSEST TO THE 03Z NHC ADVISORY
CONFIDENCE: REFERENCE NHC PRODUCTS

THE 00Z GFS AND 00Z UKMET ARE CLOSEST TO THE LATEST 03Z NHC
ADVISORY TRACK ON JOSE. THE 00Z CMC IS STILL THE FURTHEST EAST
FROM THE NHC FORECAST POSITIONS. PLEASE REFER TO THE NHC FORECAST
FOR THE LATEST INFORMATION ON THIS SYSTEM.


...PV ANOMALY / VORT MAX DROPPING SOUTHEAST FROM THE OHIO RIVER
VALLEY TO THE CAROLINAS...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: NON 00Z NAM BLEND
CONFIDENCE: SLIGHTLY ABOVE AVERAGE

UPDATE: THE 00Z UKMET AND 00Z CMC HAVE COME IN MORE SUBDUED WITH
THEIR DEPICTIONS OF THIS WAVE...AND ARE IN BETTER ALIGNMENT WITH
THE GFS AND ECMWF. THEREFORE...WILL RECOMMEND A NON-NAM BLEND.

MODELS ARE IN FAIRLY GOOD AGREEMENT THAT A MID-LEVEL VORTICITY
MAXIMUM WILL DROP SOUTHEAST INTO THE CAROLINAS OVER THE NEXT DAY
OR SO. THIS SUBTLE WAVE IS TIED TO SOME CHANCES FOR RAIN IN PARTS
OF THE SOUTHEAST OVER THE NEXT FEW DAYS. RIGHT NOW THE WAVE IS NOT
ASSOCIATED WITH ANY CONCENTRATED OR DEEP CONVECTION...AND UPPER
LEVEL WINDS AROUND THE WAVE ARE FAIRLY LIGHT. THEREFORE...PREFER
THE MORE SUBDUED PORTRAYAL OF THE WAVE ON THE GFS AND ECMWF...AS
COMPARED TO THE MORE CONCENTRATED AND HIGHER VALUES OF VORTICITY
DEPICTED ON THE NAM...CMC...AND UKMET.


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

LAMERS

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