Prognostic Meteorological Discussion
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000
FXUS10 KWNH 201858
PMDHMD

MODEL DIAGNOSTIC DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
257 PM EDT SAT SEP 20 2014

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 AND PREFERENCES
==============================================
NAM/GFS INITIALIZATION ERRORS DO NOT APPEAR TO
SIGNIFICANTLY AFFECT THEIR LARGE SCALE OUTPUT.
==============================================

...EASTERN U.S...INCLUDING A COASTAL LOW...

PREFERENCE DAYS 1-2: BLEND OF 12Z NAM/GFS/ECMWF
PREFERENCE DAY 3: 12Z GEFS MEAN
CONFIDENCE: AVERAGE

MODELS ARE NOW SIMILAR IN HANDLING THE TRACK AND TIMING OF COASTAL
LOW PRESSURE AS IT PARALLELS THE ATLANTIC COAST BEFORE BEING
ABSORBED AHEAD OF A SYNOPTIC LOW/FRONT NEAR NEW ENGLAND. DAY 3
PRESENTS SOME INTERESTING QUESTIONS...THOUGH. THE 00Z CANADIAN
STOOD OUT AS A STRONG OUTLIER AMONG THE OPERATIONAL
MODELS...DEEPENING A BACKSIDE SHORTWAVE OVER THE
MID-ATLANTIC...AND PULLING THE OFFSHORE FRONTAL ZONE BACK TOWARD
THE SHORE. THERE ACTUALLY WAS SOME SUPPORT FROM THE 00Z ECMWF
ENSEMBLE...IN WHICH 10 OF THE 50 MEMBERS PRODUCED A SIMILAR
SCENARIO...PREDICTING A PRONOUNCED FRONTAL WAVE AND ASSOCIATED
PRECIPITATION AFFECTING THE MID ATLANTIC ON DAY 3. THE 12Z
MODELS...NAMELY THE ECMWF/UKMET/CANADIAN SEEM TO SUPPORT THE TREND
TOWARD A DEEPER SHORTWAVE DIVING INTO THE EASTERN/SOUTHEASTERN
U.S. ON DAY 3...BUT THEY PRODUCE A MORE TEPID RESPONSE IN THE LOW
LEVELS...ALLOWING THE LOW LEVEL FRONTAL ZONE TO REMAIN WELL
OFFSHORE THROUGH WEDNESDAY MORNING. THIS MAKES SENSE...AS IT WOULD
BE UNEXPECTED TO SEE A STRONG SURFACE RESPONSE TO A MEDIUM
WAVELENGTH TROUGH FOLLOWING IMMEDIATELY IN THE WAKE OF THE BROAD
CYCLONE THAT WILL OCCUR HERE THROUGH MONDAY.

THE NAM/GFS/ECMWF CONTINUE TO SHOW GOOD CONTINUITY AND AGREEMENT
THROUGH DAY 2. AFTER THAT TIME WE MOVE OUR PREFERENCE TOWARD THE
12Z GEFS MEAN...REPRESENTING A COMPROMISE BETWEEN THE PROGRESSIVE
AND LOW-AMPLITUDE TRAILING WAVE IN THE NAM/GFS AND THE SHARPER
TRAILING WAVE IN THE ECMWF/UKMET.


...SHEAR AXIS TRAPPED IN THE UPPER RIDGE BUILDING OVER THE
SOUTHERN PLAINS...

PREFERENCE: BLEND OF THE OPERATIONAL MODELS
CONFIDENCE: AVERAGE

CONVECTIVE FLARE-UPS AND CONVECTIVELY INDUCED MID LEVEL
CIRCULATIONS HAD DIMINISHED IN BREADTH SINCE FRIDAY MORNING. THE
REMNANTS OF FORMER TROPICAL CYCLONE ODILE WERE...THEREFORE...SEEN
ONLY AS A WEAK 700 MB SHEAR AXIS...AND ONE THAT BECOMES LESS
PRONOUNCED WITH TIME. A RESIDUAL PLUME OF RICH MOISTURE WILL
CONTINUE TO SUPPORT PRECIPITATION...ESPECIALLY IN THE HIGH PLAINS
OF WEST TEXAS AND EASTERN NEW MEXICO. MODEL PRECIPITATION OUTPUT
IS SIMILAR ENOUGH TO RECOMMEND A CONSENSUS BLEND...BUT THE NAM MAY
BE TOO WET FARTHER EAST ACROSS TEXAS BY DAY 3...WHEN THE BUILDING
UPPER LEVEL RIDGE WILL TEND TO SUPPRESS ORGANIZED CONVECTION.


...SYSTEM TRACKING FROM CALIFORNIA TO THE NORTHERN PLAINS...

PREFERENCE: ANY OF THE OPERATIONAL MODELS
CONFIDENCE: ABOVE AVERAGE

THE OPERATIONAL MODELS SHOW STRONG AGREEMENT AT THE SURFACE AND
ALOFT.


...DEEP TROUGH LEANING INTO THE PACIFIC NORTHWEST...

PREFERENCE: ANY OF THE OPERATIONAL MODELS...LESS WEIGHT ON THE 12Z
CANADIAN
CONFIDENCE: ABOVE AVERAGE

A LEAD SHORTWAVE WILL BRING HEIGHT FALLS AND THE ONSET OF LIGHT
PRECIPITATION WEST OF THE WA/OR CASCADES ON MONDAY. THE WAVE WILL
PUSH A WEAK BOUNDARY INTO THE NORTHERN ROCKIES MON/TUE. A DEEPER
LONGWAVE TROUGH WILL APPROACH THE COAST BY LATE TUESDAY. MODEL
AGREEMENT IS STRONG...ASIDE FROM THE CANADIAN WHICH HAS SHOWN MORE
RUN TO RUN VARIABILITY WITH RESPECT TO SHORTWAVE TIMING AND
STRENGTH.

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

BURKE
$$





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