Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1248 PM EDT Sun Sep 16 2018

Valid Sep 16/1200 UTC thru Sep 20/0000 UTC

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

12Z model evaluation with preferences and forecast confidence
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...Shortwaves along the US/Canadian border/Florence moving into
the Mid-Atlantic/Northeast...
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Preference: 12z NAM and 00z ECMWF/UKMET blend
Confidence: Slightly above average

Current setup depicts compact wave skirting the Central
US/Canadian border with associated surface wave in NDAK, slightly
attached to a weak but shearing vort center in the lower MO River
Valley..  The main upstream core of the wave is translating
through the southern Canadian Rockies.   The lead shortwave and
surface low quickly shift eastward across SE Canada... the 00z
UKMET is generally a bit north, deeper and therefore slower to the
main consensus but does not appear to have much affect to the
sensible weather.  The main wave will also advance and over-top
the ridging in front of the remnants of Florence over the Ohio
Valley, and will help to elongate/stretch the mid-level of
Florence across the Mid-Atlantic and Northeast.  There is strong
agreement with the placement of best moisture convergence/flux
across the Northeast to have good confidence in a blend for QPF,
but the mass fields are a bit more complicated as the 12z GFS has
slowed relative the lead wave and loads the bulk of the sheared
shortwave energy in the Mid-Atlantic (eventually spurring a
surface wave south of Long Island.  While plausible, this even
even slower and south of the 00z ECMWF to have lower confidence in
it compared to the remaining guidance.  The CMC is the key outlier
though as it is very quick to the lift out the wave, and is very
flat/weak with the expected stronger, northern stream wave.   As
such would favor something closer to the 12z NAM and 00z
ECMWF/UKMET for these 3 systems. 

...Southwest US/Tropical EPAC...
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Preference: Non-NAM blend
Confidence: Slightly above average

The 12z NAM is a clear outlier, especially in the Southwest where
it is the only guidance member to develop and rapidly deepen an
EPAC cyclone.  While not affecting the CONUS by Day 3, directly,
the 12z NAM takes the shortwave over the Big Bend of TX and the TC
and tightens a stronger gradient flow and moisture flux into the
Southwest.  Here, a slower more subtle enhancement of the monsoon
is preferred in a non-NAM blend.


...Longwave trof over West/Intermountain West...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: 12z GFS/06z GEFS & 00z CMC/ECENS mean
Confidence: Below average

The West Coast/Intermountain West will be dealing with a
transitional period, with a complicated larger scale trof
advancing ashore late Mon into Tuesday.  The large scale trof is a
combination a combination of three streams/shortwave features, and
each model supports an evolution slightly different.  While the
larger scale trof is generally agreed upon, the internal features
lead to low confidence preferences.   The northern stream, just
north of the Canadian border is generally a zonal/sheared axis
between the Arctic cold/jet streak shifting over Central Canada;
embedded are small compact waves, with a consolidating closed low
feature off Vancouver island by Wed/Thurs.   The mid-stream is
favored by the 12z GFS/00z CMC which are generally slower across
OR/N CA into the Snake River Valley/WY Rockies... compared to a
faster but similar depth UKMET/NAM.  The 00z ECMWF is very weak
through this portion of the larger scale wave, favoring a much
stronger base through S CA.  The orientation of the waves
internally leads to a flattening/negative tilt with the NAM/UKMET
which is not favored in the longer term cycle to cycle ensemble
spaghetti analysis,  a more latitudinal stacked/phased wave with
the GFS/CMC and a flat washed out wave in the ECMWF.  The ECMWF,
is generally faster and much more flat than the bulk of 00z ECENS
members, suggesting a bit more uncertainty and lower preference. 
As such a 12z GFS and 00z CMC/ECENS mean solution is favored in
the West/Rockies but a low confidence.  


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

Gallina