Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1239 AM EDT Sun Apr 01 2018

Valid Apr 01/0000 UTC thru Apr 04/1200 UTC

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

00Z Model Evaluation...Including Model Preferences and Confidence
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...Surface low lifting through the St. Lawrence Seaway with a
trailing cold front sweeping across the Northeast...
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Preference: General model blend
Confidence: Above average

The models are in excellent agreement with this system and a
general model blend is preferred with high confidence.


...Shortwave pushing into the Central Plains this morning and
moving quickly downstream to the Mid Atlantic by tonight...
...Weak surface low developing in the lee of the Appalachians...
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Preference: 12Z ECMWF, 12Z ECMWF Ensemble Mean, 00Z NAM
Confidence: Average

There is still a decent amount of variability with the model
forecasts for this system, as subtle details make a difference in
the sensible weather impacts. The 00Z GFS continues to be on the
fast end of model spread with the surface low and the associated
shortwave aloft. The ECMWF has been very consistent over the past
several model cycles with timing and placement, it matches closely
with its ensemble mean, and the other global models are reasonably
close to it. Therefore, the preference is to lean toward the 12Z
ECMWF, with the EC ENS Mean and 00Z NAM being fairly close as well.


...Upper level trough digging into the Northwest today and
reaching the Upper Midwest by Tuesday...
...Surface cyclogenesis over the Plains Monday with a deepening
low moving into the Great Lakes Tuesday...
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Preference: 00Z GFS
Confidence: Slightly below average

Model spread with this system continues to be relatively high,
particularly beyond 03.06Z (Monday Night), with a variety of
surface low tracks from the Plains into the Great Lakes. Based on
the first 24 hours of the forecast, two models seem like less
likely scenarios -- the 12Z CMC and 00Z NAM. The CMC has a weaker
upper level low along the coast of British Columbia than the other
models, with substantial (positive) height deviations downstream
over the western CONUS. The 00Z NAM, on the other hand, has a
stronger upper level low initially, and digs this stronger wave
faster to the southeast. Its timing over the Great Lakes ends up
being faster, and the wave aloft is stronger as well.

The 12Z ECMWF continues to be on the slower end of model
forecasts, and this is a typical bias. Comparing the operational
run to the ensemble members, it is lagging a significant cluster
of ensemble members that is closer to the 00Z GFS. Given the
trends from the ECMWF ensemble, and that the GFS is roughly a
median solution in terms of timing, the preference is to lean
toward the GFS. The 12Z UKMET was also faster than the ECMWF, but
the trajectory of the surface low track was further south than any
other model, so the GFS seemed more realistic in that regard as
well.


...Low-amplitude shortwaves approaching the Pacific Northwest by
Tuesday Night and Wednesday Morning...
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Preference: 18Z GEFS Mean, 12Z ECMWF Ensemble Mean
Confidence: Below Average

Spaghetti diagrams of mid-upper level heights over the eastern
Pacific show little in the way of a coherent signal for timing and
strength of individual shortwaves. However, there is broad
agreement that low-amplitude shortwaves embedded in the relatively
zonal flow will begin to approach the Pacific Northwest by
Wednesday with an increase in precipitation chances. With the
limited signal, the preference is to lean toward ensemble means.


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

Lamers