Prognostic Meteorological Discussion
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954
FXUS10 KWNH 201854
PMDHMD

MODEL DIAGNOSTIC DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
253 PM EDT WED SEP 20 2017

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

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

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


...OCCLUDED LOW OVER SOUTH-CENTRAL CANADA TODAY...
...ASSOCIATED COLD FRONT OVER THE UPPER MIDWEST DIMINISHING THIS
EVENING...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: GENERAL MODEL CONSENSUS
CONFIDENCE: ABOVE AVERAGE

EXCELLENT AGREEMENT AMONG 12Z MODELS ON OCCLUDED LOW OVER THE
CANADIAN PRAIRIES WITH ASSOCIATED COLD FRONT OVER WISCONSIN AS OF
19Z WHICH WILL CONTINUE TO DIMINISH BY THE TIME IT REACHES
MICHIGAN THIS EVENING.


...WESTERN CONUS TROUGH AMPLIFICATION AND MOVEMENT THROUGH
SATURDAY...
...SURFACE LEE CYCLOGENESIS BEGINNING THURSDAY...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: NON-12Z NAM BLEND
CONFIDENCE: ABOVE AVERAGE

19Z UPDATE: MOVEMENT OF THE TROUGH OUT WEST BY LATE SATURDAY IS
SIMILAR BETWEEN THE 12Z ECMWF/CMC WHICH ARE FARTHER WEST AND THE
12Z GFS/UKMET WHICH ARE SLIGHTLY EAST. THE 12Z NAM IS FARTHER EAST
AND STILL WEAKENS THE ENERGY IN THE TROUGH. THEREFORE A BLEND OF
THE NON-NAM GUIDANCE IS STILL PREFERRED.

DETERMINISTIC MODELS STILL AGREE ON INITIAL SHORTWAVE DIGGING AND
AMPLIFYING THE LONG WAVE TROUGH OVER THE WEST THROUGH THURSDAY
NIGHT WITH A SECONDARY SHORTWAVE ROUNDING THE TROUGH FRIDAY AND
FRIDAY NIGHT. THE INITIAL WAVE PACKET STRETCHES WEST FROM THE
OREGON COAST THIS MORNING AND WILL TRACK ACROSS CALIFORNIA AND
NEVADA THROUGH THURSDAY NIGHT. CONSENSUS IS FOR A CLOSED MID-LEVEL
LOW TO FORM BY FRIDAY OVER IDAHO/NEVADA WITH AN ASSOCIATED LEE
SURFACE LOW CYCLOGENESIS EAST OF THE COLORADO ROCKIES LATE FRIDAY.
THE 12Z NAM IS AN OUTLIER WITH A QUICKER ADVECTION OF THIS LOW
NORTHEAST AS IT OPENS BACK INTO A WAVE.

THE SECOND SHORTWAVE PACKET REVOLVES AROUND THE WESTERN PERIPHERY
OF DEVELOPING LOW...WITH EACH DETERMINISTIC MODEL TAKING THE
RELATED MID-LEVEL VORTICITY MAXIMUM JUST SOUTH OF CALIFORNIA
FRIDAY NIGHT. EASTWARD PROGRESS OF THIS LOW/TROUGH HAS TRENDED
SLOWER AMONG THE MODELS THOUGH THE 12Z GFS IS STILL SLIGHTLY
FARTHER EAST THAN THE 00Z ECMWF/UKMET/CMC. THE 12Z NAM IS STILL
FARTHER EAST THAN THE REST. WILL SEE HOW THE 12Z NON-AMERICAN
MODELS TREND.

WILL MAINTAIN A NON-NAM BLEND AS THE PREFERENCE DUE TO SLIGHT
DIFFERENCES COMPARED TO THE REST OF THE MODELS.


...TROPICAL STORM JOSE...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: GFS/ECMWF/UKMET CONSENSUS CLOSEST TO THE 15Z NHC
ADVISORY
CONFIDENCE: REFERENCE NHC PRODUCTS

19Z UPDATE: 12Z MODELS IN EXCELLENT AGREEMENT WITH EASTWARD MOTION
THROUGH 00Z/22 BEFORE TURNING WEST. THE 12Z CMC MOVES THE SYSTEM
FARTHEST WEST WITH THE LARGEST SHIFT OF ANY MODEL SINCE 00Z.

GOOD AGREEMENT AMONG MODELS THROUGH 00Z/22 AS JOSE DRIFTS EAST
FROM THE NORTHERN MID-ATLANTIC COAST...THOUGH THE 00Z CMC HAS MORE
EAST MOVEMENT THAN THE REST. JOSE IS FORECAST TO BECOME
POST-TROPICAL AT THIS POINT AND VARIABILITY AMONG THE MODELS
INCREASES AS TO HOW FAR WEST THE SYSTEM MOVES THROUGH THE REST OF
THE FORECAST PERIOD...00Z/24. A CONSENSUS OF GFS/ECMWF/UKMET IS
CLOSEST TO THE 15Z NHC FORECAST POSITIONS. PLEASE REFER TO THE NHC
FORECAST FOR THE LATEST INFORMATION ON THIS SYSTEM.


...WEAK UPPER LEVEL TROUGH EXTENDING SOUTHWEST FROM JOSE...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: NON-12Z CMC MODEL CONSENSUS
CONFIDENCE: SLIGHTLY ABOVE AVERAGE

19Z UPDATE: THE 12Z CMC EXTENDS THE RIDGE OVER MAINE FARTHER EAST
TO THE NORTH OF JOSE. GIVEN THE RUN TO RUN VARIABILITY WOULD
PREFER A NON-CMC BLEND.

MODELS ARE IN FAIRLY GOOD AGREEMENT THAT A MID-LEVEL TROUGH
PERSISTS OVER THE SOUTHEASTERN CONUS THROUGH FRIDAY. THE POSITION
OF A RIDGE OVER THE MIDWEST IS ALSO IN FAIRLY GOOD AGREEMENT AMONG
THE MODELS THROUGH SATURDAY. ONLY SLIGHT DIFFERENCES ARE APPARENT
IN THIS RIDGE/TROUGH SYSTEM SO A GENERAL MODEL CONSENSUS IS
PREFERRED.


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

JACKSON/CHENARD

$$





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