Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
245 PM EDT Fri Sep 21 2018

Valid Sep 21/1200 UTC thru Sep 25/0000 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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...Overall preference for the entire CONUS...
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Preference: Blend of 12Z GFS, 12Z UKMET and 12Z ECMWF
Confidence: Average

The models are overall in very good agreement with the evolution
of the northern stream flow across the CONUS. The initial
shortwave trough over the Great Lakes region quickly advances
through southeast Canada through tonight with a trailing cold
front that crosses the Northeast U.S. and settles well south down
across the TN Valley and toward the southern Plains through
Saturday. The guidance then shows a new upper trough gradually
crossing the Pacific Northwest through Saturday and Sunday, before
then ejecting and amplifying out toward the central and northern
Plains on Monday. The height falls will drive a new cold front
across the Intermountain West and across the Plains, as an area of
low pressure lifts up toward the Upper MS Valley by the end of the
period. The guidance shows minimal deterministic spread with this
evolution and there is good ensemble clustering.

Regarding the closed mid-level low center over the southern
Plains, the 12Z CMC is a bit more out of tolerance compared to the
otherwise well clustered 12Z NAM/GFS/UKMET/ECMWF solutions. All of
the models though do tend to agree in gradually shearing the
energy off to the northeast through the weekend and early next
week across the Lower MS and TN Valleys and also the central
Appalachians. The CMC though tends to shear the energy more
quickly to the northeast and is also a bit of a weaker outlier
overall. At the surface there are some differences of note in the
short-term, as the 12Z NAM, 12Z ECMWF and 12Z UKMET all tend to
have a deeper surface low compared to the 12Z GFS and 12Z CMC, The
12Z HREF components led by the ARW and NMMB though tend to favor
the relatively weaker solutions. Given the spread with this system
at the surface over the next 24 hours, a compromise in between
camps will be preferred.

Based on the latest model clustering, trends and ensemble
guidance, a blend of the 12Z GFS, 12Z UKMET and 12Z ECMWF will be
preferred.


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

Orrison