Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1221 PM EDT Sat Apr 28 2018

Valid Apr 28/1200 UTC thru May 02/0000 UTC

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

12Z Model Evaluation...with Preliminary Preferences and Confidence
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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 over New England through Monday...
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Preference: General model blend
Confidence: Above average

Model mass fields are quite similar for this system, with minor
differences growing on Tuesday as the low begins to depart the
Northeast. However, the model agreement appears to be sufficient
for a preference of a general model blend.


...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 00Z ECMWF; 12Z NAM; 00Z CMC
Confidence: Average

The two shortwaves being discussed in the western CONUS will
interact as the pattern begins to shift and the downstream ridge
over the central and eastern CONUS begins to expand. This initial
shortwave will be pushing into the northern Plains as the second
shortwave is digging into the west coast, and thus will be moving
into an expanding ridge. As a result, the preference is to not
favor the more amplified model forecasts -- in this case the 12Z
GFS. Meanwhile, the 00Z UKMET shows a much weaker shortwave which
does not emerge into the Plains the way that other models are
indicating, and it also amplifies the ridge a bit more over the
central CONUS. The result is much higher heights than the other
models over the central CONUS. The preference is for a blend of
the remaining models: the ECMWF, NAM and CMC.


...Shortwave digging from the Aleutians to California by Monday
with low possibly closing off aloft on Tuesday...
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Preference: Blend of 12Z GFS; 00Z ECMWF; 12Z NAM
Confidence: Average

The GFS, NAM, and ECMWF offer relatively similar forecasts of the
shortwave digging into the Southwest early next week. They are
also the most consistent with the GEFS and ECMWF Ensemble Means.
As mentioned with the previous system, the 00Z UKMET seems to have
a much stronger response with the downstream ridge, and is not
favored for the evolution of the western CONUS trough. The 00Z CMC
begins spreading the height falls further east as compared to the
other models, and is far more compact with the shortwave as it
reaches California.


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

Lamers