Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
116 PM EDT Sat Jul 20 2019

Valid Jul 20/1200 UTC thru Jul 24/0000 UTC

...See NOUS42 KWNO (ADMNFD) for the status of the upper air
ingest...

12Z NAM/GFS Initial Model Evaluation Including Preference and
Confidence
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...Overall Pattern Across the CONUS...
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Preference: 12Z GFS/00Z ECMWF
Confidence: Slightly Above Average

Synoptic flow across the CONUS will rapidly change through Monday
as large closed lows off the Pacific Northwest and eastern Canada
develop deep troughs as ridging builds from the Four Corners into
central Canada. The overall pattern is in excellent agreement on
the long wave pattern. Slight Differences in the strength and
positioning of shortwave troughs which dictate the depth of the
trough over the east and the timing of the associated cold front
as it pushes south.

For the blend preference, the key differences involve the speed of
the trough/cold front in the east, and speed/strength/placement
differences with shortwave axis crossing the Great Lakes Sunday
night through Monday night and associated surface low development
from the central Mid-Atlantic Monday to New England Tuesday. The
12Z NAM and 00Z UKMET are the fastest with the progression of the
cold front with a more positively tilted mid-level trough in the
east. The GFS remains more amplified with the trough in the east
than the ECMWF, but the cold frontal timing is in excellent
agreement with the ECMWF. The 00Z CMC and UKMET are the most
progressive with the surface low over the Northeast, so preference
is given to the 12Z GFS and 00Z ECMWF.

From a QPF perspective the similar frontal timing in the GFS and
ECMWF should be disrupted by mesoscale processes with organized
prefrontal thunderstorms which will likely make for an earlier
arrival of thunderstorms/cooling for some areas before the actual
front arrives. Those details may not be known until the
thunderstorms develop.

Model trends at www.wpc.noaa.gov/html/model2.html
500 mb forecasts at www.wpc.noaa.gov/h5pref/h5pref.shtml

Jackson