Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1246 PM EDT Fri Oct 05 2018

Valid Oct 05/1200 UTC thru Oct 09/0000 UTC

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

Preliminary 12Z Model Evaluation...with latest preferences and
confidence
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...Overall Pattern Across the CONUS...
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Preference: 05/12Z GFS/ECMWF blend...through 60 hrs
            05/00Z ECMWF/ECENS mean/GEFS mean...after 60 hrs
Confidence: Slightly above average

The guidance indicates a northern stream shortwave crossing the
Pacific Northwest over the next couple of days, with the energy
quickly crossing the northern Plains and upper MS Valley through
Saturday. This will drive a wave of low pressure across the
Midwest and Great Lakes region and then across northern New
England on Sunday as the shortwave energy traverses southeast
Canada. A cold front will cross all of these areas, but the front
will then stall out by Sunday back to the southwest across the
middle MS Valley and the central Plains region. The guidance is
generally in agreement with this scenario, but the 12Z NAM
continued its idea of a stronger shortwave trough and thinking is
that it still may end up being a bit too strong.  This left the
05/00Z CMC on the slow side of the envelope of solutions qirh its
surface low crossing the Great Lakes.

In the wake of the initial northern stream shortwave, a secondary
shortwave will cross the Pacific Northwest by later this evening
or overnight hours and then amplify sharply down across the Great
Basin this weekend, which will lead to a highly amplified large
scale pattern across the CONUS Sunday and Monday. This will
feature a strong longwave trough over the Western U.S. and a
strong subtropical ridge across the Eastern U.S., and this will be
a major reason for the aforementioned front becoming stalled out
over the central Plains and Midwest. Unfortunately, this will also
set the stage for a multi-day and high-impact rainfall event
across this part of the country.

The models agree very well with the details of the large scale
pattern through the end of the Day 2 period (08/00Z).  by the end
of the period, although the NAM still tends to drop its height
falls a bit farther south over the Four Corners region compared to
the global models. Additionally, there are some concerns also that
the NAM may be a bit too robust with a surface wave and associated
mid level shortwave impulse lifting northeast up across the
Midwest by Monday with the deep layer southwest flow across the
region. The latest GFS showed a subtle shift to the south and a
movement towards a somewhat slower solution which puts it in
better agreement with the overnight GEFS mean.

Based on the latest model clustering and ensemble guidance, a
05/00Z ECMWF and 05/12Z GFS blend will be preferred for the mass
fields across the CONUS.  However...the ECMWF and the latest
ensemble means are a viable cluster late on Day 3.


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

Bann