Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1216 PM EDT Sun Sep 23 2018

Valid Sep 23/1200 UTC thru Sep 27/0000 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 the 12Z GFS and 00Z ECMWF
Confidence: Slightly above average

The models and ensemble means are indicating a transition from a
quasi-zonal flow pattern across the northern tier of the nation to
a significantly more amplified pattern by Tuesday and Wednesday as
strong height rises/ridging building offshore the West Coast and
nosing up toward the Gulf of AK will allow the Pacific Northwest
trough at the beginning of the forecast period to dig southeast
and amplify out across the Plains. Additional northern stream
shortwave energy dropping southeast across western Canada will
reinforce the height falls out across the Plains by the end of the
period and lead the way for a fairly deep longwave trough over the
central U.S. by Wednesday. Meanwhile, downstream subtropical
ridging is expected to be in place across the Southeast and
offshore the U.S. East Coast. The guidance is in good agreement on
the details of the large scale evolution.

Regarding the lingering mid level low currently over the southern
Plains and lower MS Valley, this system should gradually weaken
and begin to dampen out over the next couple of days as it
degenerates into multiple shortwave impulses that shear out off to
the northeast across the TN Valley and toward the central
Appalachians. Meanwhile, the models show only modest timing and
amplitude differences with respect to the wave of low pressure
that should lift up across the Upper Midwest tonight and into
Monday, with the 00Z UKMET being the fastest solution, and the 00Z
CMC gradually becoming the slowest. On Tuesday and Wednesday, the
12Z NAM becomes a bit too fast it appears to advance the front
across the middle MS and OH Valleys with the 00Z UKMET becoming
the slowest solution. All of the models drop the front down across
the southern Plains and lower MS Valley by the end of the period,
but the 00Z CMC drops this portion of the front farthest down to
the south with the other models are clustered a little farther
north.

Regarding the disturbance approaching the Carolinas by midweek,
the 00Z UKMET is a stronger solution compared to the model
consensus and also tracks this system farthest west which is
similar to its previous runs. The consensus of the guidance though
outside of the UKMET is that this energy should remain off the
East Coast and begin to recurve by the end of the period out ahead
of the cold front approaching the East Coast by late Wednesday.

Based on the latest model clustering, ensemble guidance and
trends, the GFS and ECMWF reflect generally the model consensus
and offer a more preferred cluster at this time, so a blend of
these solutions 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