Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1158 AM EDT Sat Aug 17 2019

Valid Aug 17/1200 UTC thru Aug 21/0000 UTC

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

12Z Model Evaluation...Including Preferences and Confidence
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...Overall Pattern Across the CONUS...
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Preference: Non-UKMET blend
Confidence: Slightly above average

Broad global scale cyclonic trof base covers the northern half of
the CONUS with a few smaller scale shortwaves progressing through
the Great Lakes and one starting to tip into the Northern Plains
today.  Additionally, return tropical flow along western periphery
of Atlantic subtropical high continues to converge along
persistent deep moisture confluence axis from Western Gulf to NC
Coast and north of Bermuda.  Eventually, the northern Plains
system will continue to amplify in south with frontal zone across
the northern Tier while supporting increased moisture and ascent
in advance of it across the Midwest into the Northeast Late Sunday
through Tuesday.   In this trof's wake, the flow will flatten and
lift into Canada with a large 594+ dm ridge expanding from the
Southwest into the Central Plains by the end of the forecast
period.

In the tropical confluence stream across the Southeast, a few weak
surface waves will translate along it.  The first currently near
the VA/NC border just off shore, will continue to slide east with
all but the GFS closing the system off along 40N.  Still the GFS
does depict the wave just a bit weaker.   The second currently
near Charleston SC, has all guidance suggesting a slightly
stronger wave but the UKMET suggests a very strong compact wave.
Given the confluence axis (shape/orientation/etc) and necessity
for convective upscale growth (which is not expected to the
magnitude needed), would prefer a non-UKMET blend.  

In the northern stream, as the shortwave sharpens/depresses into
the northern tier by Sunday, model agreement is strong, but as the
wave passes north-south and swings slightly negative tilt (while
filling/weakening though), timing and placement of the surface
wave/frontal zone start to displace.  This is particularly noted
with the UKMET which is further east and stronger, possibly
falling into a negative bias noted with convective upscale growth,
as it slides through the Great Lakes.  This is fairly minor but
does show the greatest variance and its removal would help the
overall blend.   There are other placement/depth issues with the
inner core of the wave well north of the border up into Hudson
Bay, but otherwise a non-UKMET blend will suffice at slightly
above average confidence.


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

Gallina