Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
235 PM EDT Sun Apr 15 2018

Valid Apr 15/1200 UTC thru Apr 19/0000 UTC

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

12Z model evaluation and final preferences
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...Midwest cyclone becoming centered over New England by
Wednesday...
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Preference: General model blend
Confidence: Slightly Above Average

The models have converged on the upper level evolution of the
cyclone currently over Ohio valley as it tracks east. Only minor
strength/position differences remain with the upper and lower
level evolution such that a general model blend can be used to
iron out any remaining differences.

No significant changes were noted between the 12Z ECMWF/UKMET/CMC
compared to their previous 00Z cycles.


...Trough progressing Through the West, Reaching the Plains late
Tuesday...
...Surface low tracking from KS into the Ohio valley/Great Lakes
through late Wednesday...
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Preference: Blend of the 12Z ECMWF/UKMET
Confidence: Average

Ensemble spaghetti height trends showed that the faster members
along with the faster NAM, GFS and CMC from Saturday have all
trended slower for today's runs, while the slower members have
generally stayed the same over the past 3-4 cycles. The ECMWF mean
continues to look like a favorable middle ground considering
convergence in ensemble guidance...which splits the difference
between the slightly faster 00Z UKMET and slower 00Z ECMWF. The
12Z NAM and GFS continue to appear too fast lying toward the east
end of the ensemble spread through Tuesday night across the middle
of the country. The centroid of ensemble scatter low plots also
agrees well with a 00Z ECMWF/UKMET blend.

19Z update: The 12Z ECMWF/UKMET nudged a tad faster from their
previous 00Z cycles, bigger changes noted from the ECMWF, but not
to the faster degree of the 12Z NAM/GFS. The 12Z CMC remains
similar (faster) with its progression and is still considered too
fast.


...Next Pacific trough reaching the West Coast Tuesday night...
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Preference: non 12Z UKMET blend
Confidence: Average

On Wednesday, another trough axis is expected to near the West
Coast with ensemble spaghetti plots showing a modest degree of
possibilities. The slower GEFS/GFS has trended closer to the
middle, but the 12Z GFS and 12Z NAM are toward the faster/southern
side with the base of the trough, while the 00Z ECMWF/CMC are
slower/less amplified to the north valid 00Z/19. Only the 00Z
UKMET differs significantly here and has only a strong minority of
ensemble support with its faster/less amplified trough
progression.. Therefore, a blend of the GFS/ECMWF idea should work
well here, with the NAM supporting the GFS and CMC supporting the
ECMWF.

The 12Z ECMWF/UKMET/CMC all nudged in the direction of the more
aggressive 12Z NAM/GFS, but not as extreme with the 12Z UKMET
remaining the outlier of the deterministic guidance to the
north/weaker by 00Z/19. The preference will remain for a non-UKMET
blend, with better agreement in the latest 12Z cycles compared to
earlier.

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

Otto