Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
305 AM EDT Sat Apr 27 2019

Valid Apr 27/0000 UTC thru Apr 30/1200 UTC

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

00Z Model Evaluation...with Preferences and Confidence
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...Overall Pattern Across the CONUS...
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Preference: General model blend (non-NAM northern Plains Sun/Mon;
non-GFS/NAM southwest US Tue)
Confidence: Slightly above average

07z update: With the rest of the 00z guidance now available, the
most significant model differences are on Day 3 with the southwest
US closed low as it opens up and eventually moves into the central
Plains. The ECMWF is still advertising a slower solution which
agrees with the CMC and UKMET. The GFS/NAM are faster outliers and
is not favored at this time. Otherwise, across the CONUS and
particularly the northern stream systems, the same preferences
remain (general model blend with less weight toward the NAM on
Sun/Mon).

---previous discussion---
Fairly fast zonal northern stream flow will gradually transition
to trough west, ridge east across the CONUS through the forecast
period, setting the stage for an increasingly active weather
pattern with several systems to watch.

First, the deep shortwave trough over the Northeast US is expected
to move off the coast with its associated precipitation exiting by
the end of Saturday. Quick on its heels is a compact shortwave
trough moving through the central/northern Plains into the Upper
Midwest. A tight baroclinic zone with a surface low over Iowa will
produce a swath of precipitation. In the last 12-24 hours, there
has been a minor but consistent shift southward with the low track
and thus precipitation axis, with the low now tracking through
southern Iowa into central Illinois. This is seen in the latest
NAM but also hi-res guidance.

Another shortwave dives down through the Pacific Northwest on
Sunday into Monday. This features is a bit stronger compared to
the one moving through currently and is likely to close off at 500
mb over Montana. Model mass fields are fairly similar through 48
hours with the track/strength of the shortwave trough. At the
surface, there are more differences, with the NAM being the
furthest northwest while the ECMWF/GFS/UKMET are fairly similar
with its position at 18z Sunday. A non-NAM blend is preferred
here.

Finally, a closed low off Baja California is expected to track
across the southwest US early next week before opening up in the
southwest flow over the central Plains. The typical fast bias in
the GFS is seen here with the shortwave ejecting out into the
central Plains 6-12 hours ahead of the other models, where is
pretty good agreement seen in the ECMWF/UKMET/CMC. The NAM is fast
as well, but not quite as much as the GFS. Here, a general model
blend can be applied, which should slow down the faster GFS.


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

Taylor