Prognostic Meteorological Discussion
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000
FXUS10 KWNH 270450
PMDHMD

MODEL DIAGNOSTIC DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1249 AM EDT THU JUL 27 2017

VALID JUL 27/0000 UTC THRU JUL 30/1200 UTC

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

...AN EVALUATION OF THE 00Z NAM/GFS ALONG WITH PRELIMINARY
PREFERENCES/FORECAST CONFIDENCE...


...PERSISTENT VORTICITY CENTER LINGERING ALONG THE GA/SC COAST...

PRELIMINARY PREFERENCE: GENERAL MODEL COMPROMISE
FORECAST CONFIDENCE: SLIGHTLY ABOVE AVERAGE

WATER VAPOR IMAGERY SHOWED A BAND OF VORTICITY SPINNING OFF THE GA
COAST...GENERALLY POSITIONED WITHIN A VERY HIGH PRECIPITABLE WATER
ENVIRONMENT...OVER 2.25 INCHES BASED ON BLENDED-TPW IMAGERY. GIVEN
NO SYNOPTIC FEATURE NEARBY TO STEER IT AWAY...IT DOES LINGER UNTIL
ABOUT FRIDAY MORNING. THEREAFTER...THE SOUTHERN EXTENT OF HEIGHT
FALLS FROM THE APPROACHING UPPER TROUGH SHOULD BE ENOUGH TO SHEAR
THE SYSTEM AND PUSH IT OUT TO SEA. GIVEN DECENT MODEL
AGREEMENT...A GENERAL MODEL COMPROMISE IS WARRANTED.


...WEAK MID-LEVEL HEIGHT FALLS GRAZING NEW ENGLAND THURSDAY
EVENING...
...ADDITIONAL UPPER LOW CENTER PASSING THROUGH EASTERN CANADA ON
SATURDAY...

PRELIMINARY PREFERENCE: A NON-12Z CMC MODEL COMPROMISE
FORECAST CONFIDENCE: AVERAGE

AMPLIFIED FLOW ACROSS EASTERN CANADA WILL DRAW A PAIR OF SYSTEMS
IN THE VICINITY OF NORTHERN NEW ENGLAND THE NEXT COUPLE OF DAYS.
WHILE SOMEWHAT SUBTLE IN NATURE...THE 12Z CMC IS A HAIR QUICKER
WITH THE INITIAL HEIGHT FALLS MOVING TOWARD NOVA SCOTIA LATE
THURSDAY. LOOKING FARTHER UPSTREAM...THE FOLLOWING SYSTEM HAS
QUITE A BIT MORE MODEL SPREAD TO CONTEND WITH. THE ENSEMBLE MEANS
ARE WELL CLUSTERED BY 500-MB HEIGHT COMPARISONS WITH MOST
DETERMINISTIC SOLUTIONS REASONABLY IN LINE WITH THEM. THE
EXCEPTION CONTINUES TO BE THE 12Z CMC WHICH IS MORE
AMPLIFIED...PARTICULARLY WHEN ANALYZING THE 570-MB ENSEMBLE
SPAGHETTI PLOTS. GIVEN THIS ASSESSMENT...A NON-12Z CMC MODEL
COMPROMISE IS RECOMMENDED HERE.


...INITIAL SHORTWAVE PUSHING INTO NORTHERN MN BY 27/1800Z...
...GRADUAL EXPANSION TO A BROAD CLOSED LOW AFFECTING THE EASTERN
U.S. BY FRIDAY...
...DEEP SURFACE LOW TRACKING FROM THE CENTRAL APPALACHIANS TOWARD
THE DELMARVA...

PRELIMINARY PREFERENCE: BLEND OF THE 00Z GFS/12Z ECMWF
FORECAST CONFIDENCE: SLIGHTLY BELOW AVERAGE

ATOP A RIDGE ANCHORING THE CENTER OF THE COUNTRY...A PROGRESSIVE
SHORTWAVE WILL PUSH THROUGH NORTHERN MN BY 27/1800Z WHILE EVOLVING
INTO A MUCH MORE PRONOUNCED SYSTEM WHILE CROSSING THE GREAT LAKES
ON FRIDAY. THE 12Z CMC IS A HAIR AHEAD OF THE CONSENSUS AT THIS
POINT WITH DIFFERENCES INCREASING FARTHER INTO THE FORECAST. AN
IMPRESSIVE SURFACE CYCLONE DEVELOPS OVER THE MID-ATLANTIC AND INTO
THE COASTAL WATERS WITH MANY SOLUTIONS FAVORING A SUB-1000 MB LOW
BY FRIDAY NIGHT. ENSEMBLE SCATTER LOW PLOTS SHOW THE BEST
CLUSTERING AMONG THE 12Z GEFS/ECMWF ENSEMBLE MEMBERS WHILE THE CMC
SOLUTIONS ARE OFF TO THE EAST...SIMILAR TO ITS PARENT
DETERMINISTIC RUN. THE 00Z NAM ALSO SEEMS TO BE OFF TO THE RACES
AT A CERTAIN POINT OF THE FORECAST AS WELL. BY SATURDAY
EVENING...A PAIR OF SURFACE LOWS ARE NOTED WHILE THE ENSEMBLE
MEANS TRY TO AVERAGE THE TWO WHICH MAKES IT A LESS REALISTIC
OPTION AT THIS TIME. WHILE THE 12Z ECMWF IS QUICKER THAN THE 00Z
GFS...WILL TAKE AN AVERAGE OF THE TWO AT THIS POINT WITH SLIGHTLY
BELOW AVERAGE CONFIDENCE.


...FEATURE INITIALLY ALONG THE NORTHERN BORDER OF
CA/NV...GRADUALLY SHEARING IN TIME...

PRELIMINARY PREFERENCE: A NON-12Z UKMET MODEL CONSENSUS
FORECAST CONFIDENCE: SLIGHTLY ABOVE AVERAGE

A MODEST SHORTWAVE SPINNING OVER THE FAR NORTHEASTERN CORNER OF CA
SHOULD CONTINUE TO MARCH EASTWARD INTO THE CENTRAL GREAT BASIN.
GRADUAL SHEARING IS EXPECTED AS THE WAVE MOVES DOWNSTREAM TOWARD
THE NORTHERN EXTENT OF A PROMINENT RIDGE. THE 12Z UKMET SEEMS TO
BUILD THE RIDGE BACK TOO QUICKLY...ESSENTIALLY WIPING OUT THIS
SHORTWAVE. WILL DISMISS THIS UKMET SOLUTION AND FAVOR THE
REMAINING CONSENSUS.


...PERSISTENT MEAN UPPER TROUGH SITTING OVER THE NORTHEASTERN
PACIFIC...

PRELIMINARY PREFERENCE: BLEND OF THE 12Z ECMWF/ECMWF ENSEMBLE MEAN
FORECAST CONFIDENCE: SLIGHTLY ABOVE AVERAGE

A LONGWAVE TROUGH...BEING A QUASI-PERMANENT FEATURE OF
SORTS...WILL ANCHOR THE NORTHEASTERN PACIFIC THROUGHOUT MUCH OF
THE FORECAST. THE INITIAL TROUGH PASSING THROUGH THE REGION WILL
GRAZE THE PACIFIC NORTHWEST WITH NO IDENTIFIABLE MODEL
DIFFERENCES. MEAN RIDGING SHOULD BUILD FOR SATURDAY BEFORE A MORE
PRONOUNCED LONGWAVE TROUGH SWINGS IN OFF THE NORTHEAST PACIFIC.
MULTI-DAY ENSEMBLE TRENDS SHOW A QUICKER PROGRESSION THAN A DAY OR
TWO AGO WITH THE ECMWF MEMBERS TENDING TO BE THE MOST CONSISTENT
OVERALL. WHILE A BIT QUICKER THAN THE CONSENSUS...WILL LEAN
CLOSELY ON THIS TREND AND FAVOR A COMBINATION OF THE 12Z
ECMWF/ECMWF ENSEMBLE MEAN.


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

RUBIN-OSTER

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