Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1255 AM EDT Sat Apr 28 2018

Valid Apr 28/0000 UTC thru May 01/1200 UTC

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

00Z NAM/GFS evaluation...with preliminary preferences
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...Evolving eastern CONUS trough/closed low, with phasing and
consolidation of several shortwaves over the Great Lakes and
Northeast through Sunday...
...Surface low near the New England/Canadian border today/Sunday...
...Offshore surface low near Nova Scotia Sunday morning...
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Preference: General model blend
Confidence: Slightly above average

Convergence in model cycles has occurred since yesterday such that
now, the models show similarly with only minor strength/placement
differences remaining with features impacting the sensible weather
across the Northeast. Given rather tight ensemble clustering, a
general model blend is recommended to smooth out smaller scale
differences.


...Closed low over the Pacific Northwest today, opening into a
trough over the Intermountain West by Monday...
...Lee cyclogenesis over the Plains by Monday with attendant cold
front...
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Preference: Blend of the 00Z GFS and 12Z ECMWF
Confidence: Slightly below average

The 00Z NAM/GFS are slightly quicker with the leading edge of the
mid-level trough axis nearing the northern Plains Sunday evening
but the complexity of the individual vorticity maxima within the
trough opening up proves challenging. Ensemble scatter low plots
emphasize a "southern" surface low over North Dakota Monday
morning, which agrees with the 12Z ECMWF/UKMET/CMC, but the
GFS/NAM have trended the focus to a "northern" surface low over
southern Saskatchewan. All of the deterministic models show both
surface lows, it is just the difference in which one is more
dominant.

For now, a blended approach is recommended, which is best
represented by the 00Z GFS and 12Z ECMWF.


...Shortwave digging from the Aleutians tonight to off the
California coast by Monday...
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Preference: Blend of the 00Z GFS, 12Z ECMWF, 12Z CMC
Confidence: Average

The differences noted in the guidance relate mostly to timing,
with the 00Z NAM on the faster end of the spectrum and the 12Z
ECMWF slowest. Given the 00Z GFS is slightly faster than the
middle ground, a blend of the 00Z GFS, 12Z ECMWF and 12Z CMC
appears to give a timing/strength nearest to the ensemble means.
The 12Z UKMET is stronger in the mid-levels and only weakly
supported in the ensemble guidance, so it is not preferred given
no other deterministic guidance fits its evolution.

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

Otto