Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1234 AM EDT Thu Aug 01 2019

Valid Aug 01/0000 UTC thru Aug 04/1200 UTC

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

00Z NAM/GFS Model Evaluation and Preferences/Confidence
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...Overview of the CONUS features...
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Preference: General model blend
Confidence: Average

The main forecast issue for this time period is the short wave
energy riding over the top of the mid level ridge that moves
slowly westward across the Four Corners area through Day 2. The
00Z NAM was further west with the short wave energy (as was to
some extent the 00z GFS) into the Central Plains and Mid MS Valley
through 03/00z. The 00z NAM/GFS remain further south and west with
the track of the short wave energy and surface frontal position in
proximity of the mid level ridge into Day 3 across the Lower MS
Valley into the Southern Plains.

By contrast, the non-NCEP guidance (ECMWF/CMC/UKMET) were slower
with the westward movement of the mid level ridge (which impacts
the longitudinal placement of the short wave energy riding along
it). The difference in short wave tracks has implications for the
placement of heaviest rainfall. It is possible that the 00z NCEP
guidance could be catching onto the southwestward trend more
quickly than the non-NCEP guidance, but at this point it is
unclear whether the 00z non-NCEP guidance will follow suite.
Because of this, a general model blend is preferred with the short
wave energy and frontal position, especially for Days 2 and 3 from
the Central Plains into the Southern Plains.

Over the Northeast and Quebec, there is generally good model
agreement with the evolution of the mid level closed low, so a
general model blend is preferred here as well, as operational
models cluster well with the ensemble means.

There is some model spread with the timing of the positively
tilted long wave trough extending across British Columbia into the
Pacific Northwest. Initially, there is good model as the closed
mid level over Vancouver Island crosses western Canada into Day 2.
After that time, as short wave energy drops from AK and northwest
Canada into the trough position, the 00z NAM/GFS are slower with
the developing positively tilted long wave trough that stretches
across British Columbia into the Pacific Northwest. Since the
impact with respect to sensible weather for the Pacific Northwest
is minimal, a general blend is preferred here.

Finally, a surface trough or weak surface wave off the southwest
FL coast from late Day 1 into early Day 2 becomes involved with an
area of disturbed with over the Bahamas, and the surface
reflection then tracks off the Southeast United States coast
during Day 3. There is some longitudinal differences with the
track of the surface feature off the SC/NC during Day 3, but at
this point, a general model blend is preferred.

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

Hayes