Prognostic Meteorological Discussion
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000
FXUS10 KWNH 211857
PMDHMD

MODEL DIAGNOSTIC DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
156 PM EST TUE NOV 21 2017

VALID NOV 21/1200 UTC THRU NOV 25/0000 UTC

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

12Z MODEL EVALUATION WITH FINAL PREFERENCES
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
NAM/GFS INITIALIZATION ERRORS DO NOT SEEM TO DEGRADE THEIR SHORT
RANGE FORECASTS.


...UPPER TROUGH AXIS CURRENTLY OVER THE GREAT LAKES...MOVING TO
THE NORTHWEST ON WED...
...SURFACE LOWS TRACKING ALONG AND OFFSHORE OF THE NEW ENGLAND
COAST ON WED...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: 12Z ECMWF / 12Z GFS / 12Z UKMET BLEND
CONFIDENCE: ABOVE AVERAGE

ENSEMBLE SCATTER LOW PLOT TRENDS HAVE BEEN TO SHIFT THE GULF OF
MAINE LOW ALONG AT 18Z/22 - 00Z/23 TOWARD THE SOUTH WITH THE
LATEST 12Z MODEL CONSENSUS APPEARING IN REASONABLE AGREEMENT WITH
THIS FIRST LOW.

A SECOND LOW SHOWS A BIT MORE SPREAD...FORECAST TO TRACK EAST OF
THE 40N/70W BENCHMARK LATE WED. THE MORE PROGRESSIVE UPPER TROUGH
SHOWN IN THE 12Z NAM AND 12Z CMC DO NOT FIT WELL TO THE LATEST
ENSEMBLE SPAGHETTI HEIGHT CHARTS AND THE 12Z ECMWF/GFS  AND NOW
THE 12Z UKMET SHOW TOWARD THE DEEPER SIDE. GIVEN TRENDS AND THE
LATEST ENSEMBLE CLUSTERING...A BLEND OF THE 12Z GFS/ECMWF/UKMET
APPEARS BEST AT THIS TIME.


...UPPER TROUGH AXIS CROSSING THE PACIFIC NORTHWEST ON THU AND
REACHING THE N-CNTRL U.S. BY LATE FRI...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: 12Z GFS/ECMWF/CMC BLEND
CONFIDENCE: AVERAGE

DIFFERENCES WITH THIS SYSTEM ARISE FROM HOW THE MODELS HANDLE
INTERACTIONS WITH SHORTWAVE ENERGY TRACKING NORTH OF THE MAIN
TROUGH AXIS. ENSEMBLES SHOW THE NAM AS A NEAR OUTLIER WITH THE
DEPTH/PLACEMENT OF THE TROUGH AXIS BY FRI AFTERNOON AND THE 12Z
CMC AS MUCH FASTER THROUGH FRI. A BLEND OF THE 12Z GFS...12Z
ECMWF...12Z UKMET IS CLOSEST TO THE ENSEMBLE CONSENSUS THROUGH
00Z/25.


...BROAD MID-LEVEL TROUGH AXIS CLIPPING THE GREAT LAKES AND
NORTHEAST THU/FRI...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: GENERAL MODEL BLEND
CONFIDENCE: ABOVE AVERAGE

THE MODELS SHOW SIMILARLY WITH THIS SYSTEM.


...SLOW MOVING MID-UPPER LEVEL TROUGH ALONG THE GULF COAST...
...SURFACE LOWS IN THE EASTERN GULF OF MEXICO/NEAR FLORIDA...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: NON 12Z NAM BLEND
CONFIDENCE: SLIGHTLY BELOW AVERAGE

STARTING THURSDAY MORNING...THE NAM STARTS DIVERGING FROM THE
OTHER GUIDANCE IN BECOMING DEEPER/SLOWER ALOFT WITH THIS SYSTEM.
ENSEMBLE SPREAD IS FAIRLY LARGE WHICH LOWERS CONFIDENCE A BIT
LOWER THAN NORMAL. THE LATEST ENSEMBLE SCATTER LOW PLOTS SHOW THE
00Z UKMET/CMC ON THE WESTERN SIDE OF THE SPREAD WITH THE SURFACE
LOW AS IT NEARS THE SOUTHEAST COAST ON FRI BUT THE 12Z UKMET/CMC
TRENDED AWAY FROM THE COAST INTO THE BETTER ENSEMBLE LOW
CLUSTERING. THE 12Z ECMWF TRENDED SLOWER LIKE THE 12Z NAM BUT IT
IS NOT AS DEEP. THEREFORE...A NON 12Z NAM BLEND APPEARS REASONABLE
FOR THIS SYSTEM BUT DETAIL DIFFERENCES REMAIN LARGER. FOR QPF
CONSIDERATIONS...SEE OUR QUANTITATIVE PRECIPITATION DISCUSSION
(QPFPFD).


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

OTTO

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